Ultrafast laser inscription in silver-containing glasses: structure/property dependence for improved photosensitivity and modulated waveguides for photonics applications

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N

U

IVERSITA DI ISA

P

Physics Department

Master Thesis in Physics of Matter

Ultrafast laser inscription in silver-containing glasses:

structure/property dependence for improved

photosensitivity and modulated waveguides for

photonics applications

Candidate: Laura Loi

Internal supervisor Prof. Mauro Tonelli External supervisors Prof. Lionel Canioni Prof. Yannick Petit April, 10th 2019

Academic year 2017/2018

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Acknowledgements

This study has been carried out with financial support from the French State, managed by the French National Research Agency (ANR) in the frame of “The Investments for the future” Programme IdEx Bordeaux − ANR-10- IDEX-03-02.

I would like to thank the Cluster of Excellence LAPHIA for providing me a financial support for the entire period I spent in Bordeaux.

I am thankful to Prof. Mauro Tonelli for showing interest in this work and for agreeing to supervise this report.

I would like to express my genuine gratitude to Prof. Lionel Canioni and Prof. Yannick Petit for having given me the opportunity to work in their group. Their capacity to handily communicate their profound knowledge and to guide and stimulate through meaningful discus-sions and suggestions, made it easy for a student like me, approaching the research field for the first time, to learn to be autonomous and be aware of such fascinating thematics. Thank you Yannick for having patiently supervised my work, not only during my permanence in the lab, but also during the preparation of this report.

During the six months of this internship I had the possibility to meet and to work with many different people bonded by the same, strong passion for research. I’m grateful to all of them for sharing with me their knowledge as well as their positive attitude and personality.

I would like to thank Jean-Michel Rampnoux from LOMA for providing his experience, which has been fundamental to this work.

I am grateful to Thierry Cardinal, Sylvain Danto, Alexandre Fargues and Théo Guérineau from the ICMCB for introducing me in their work. In particular, I sincerely appreciated working with Théo and his contagious enthusiasm.

I am thankful to the SLAM team: Bruno Bousquet, Inka Manek-H¨onninger, Delphine Syvilay, Wendwesen Gebremichael, Julian Guezenoc, Joyce Bou Sleiman, Alain Abou Khalil and Romain Laberdesque. I felt welcomed from day one and I enjoyed every day till the last one, inside and outside the lab. A special thank to Wende and Alain, for their help and tips during long experimental days.

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List of Figures

1.1 Schematic setup for DLW . . . 12

1.2 Schematic setup of an AOM . . . 13

1.3 LCOS-SLM chip structure . . . 14

1.4 Glass structure . . . 15

1.5 Glass transition . . . 15

1.6 Tetrahedral arrangement in glass formation . . . 16

1.7 Nonlinear processes . . . 18

1.8 Linear and mutiphoton absorption . . . 19

1.9 Types of structural modifications . . . 20

1.10 Heat accumulation during the pulse train . . . 20

1.11 Silver clusters formation and photodissiciation . . . 22

1.12 Morphology of the structures during DLW process . . . 22

1.13 HRSEM and AFM characterizations of the nanostructures . . . 23

1.14 Size of the photoinduced structures as a function of the laser irradiance . . . 24

1.15 Fluorescence intensity dependence on repetition rate . . . 25

1.16 Fluorescence intensity dependence on number of pulses . . . 25

1.17 3D optical data storage . . . 26

1.18 DLW in ribbon fibers . . . 26

1.19 Second-Harmonic Generation . . . 27

1.20 Spatial correlation between fluorescence and EFISHG patterns . . . 27

1.21 Stability of EFISHG after thermal treatment below Tg . . . 28

1.22 Third-Harmonic Generation . . . 29

1.23 Data storage using THG imaging . . . 29

1.24 Nanoparticles formation in silver-doped PZn glass by thermal treatment . . . 30

1.25 Evolution of the surface plasmon resonance . . . 30

1.26 Helical phase fronts of the vortex beam . . . 31

1.27 Structured light generation setup for an incident linearly polarised Gaussian beam 31 1.28 Intensity profiles for Gaussian, charge-2 vortex and four-lobe beams . . . 32

1.29 Experimental setup and phase holograms for generating LGl_p beams . . . 33

1.30 Tightly focused vortices for LG1₀ beam with linear, left-handed and right-handed circular polarisations . . . 33

1.31 Dependence of the refractive index change on the writing parameters . . . 34

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1.33 Phase image of the waveguide . . . 34

1.34 Mode profile of cw light injected inside the waveguide . . . 35

1.35 50 - 50 beam splitter . . . 35

1.36 Universal behavior of femtosecond direct laser writing in silver-containing glass matrices . . . 36

1.37 Nanoparticles formation in silver-doped LBG glass by femtosecond laser irradiation 37 1.38 Comparison between nanogratings formation in GPN and GPN:Ag samples . . . 37

1.39 SEM images of nanogratings in GPN and GPN:Ag samples . . . 38

2.1 Qn classification for P O4 tetrahedron . . . 40

2.2 GPx and GPy series on ternary diagram . . . 41

2.3 GPy series on ternary diagram . . . 42

2.4 Gallium configuration inter- and intra- chains . . . 43

2.5 GPy series: properties . . . 43

2.6 GPx series on ternary diagram . . . 44

2.7 GPx series: properties . . . 45

2.8 GPx series: emission spectra evolution for an excitation at 220 nm over the series 45 2.9 General configuration of a P CSA ellipsometer . . . 47

2.10 GPy and GPx: refractive indices . . . 48

2.11 Geometry of a Gaussian beam focused at the interface between two media . . . . 50

2.12 THG: experimental setup . . . 52

2.13 THG: procedure . . . 52

2.14 Cubic dependance of THG signal on laser power . . . 53

2.15 GPy - THG measurements: raw data . . . 54

2.16 GPx - THG measurements: raw data . . . 54

2.17 Fit of the oscilloscope trace . . . 56

2.18 Experimental beam waist extracted from fit . . . 56

2.19 Evolution of the interaction length over the depth . . . 57

2.20 Third-order susceptibility for GPy series . . . 57

2.21 Third-order susceptibility for GPx series . . . 57

2.22 Evolution of glass temperature T g . . . 58

2.23 GPy: Propagation of uncertainty on χ(3) measurements . . . 60

2.24 GPx: Propagation of uncertainty on χ(3) measurements . . . 60

2.25 Direct Laser Writing procedure . . . 61

2.26 Fluorescence imaging of GP 0.75N 15 − 2Ag sample . . . 62

2.27 Evolution of the DLW sensitivity over GPy and GPy series . . . 63

2.28 GPy series: Evolution of fluorescence as function irradiance for low speed . . . . 63

2.29 GPy series: Evolution of fluorescence as function irradiance for high speed . . . . 63

2.30 GPx series: Evolution of fluorescence as function irradiance for low speed . . . . 64

2.31 GPx series: Evolution of fluorescence as function of speed and irradiance . . . 64

3.1 Waveguide Bragg grating: Point-by-Point method . . . 69

3.2 Waveguide Bragg grating: Modulated burst method . . . 69

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3.4 Example of a driving square waveform . . . 71

3.5 Phase images: SID4Bio wavefront sensor . . . 73

3.6 Example of a phase image . . . 74

3.7 Example of fluorescence image of modulated structures . . . 74

3.8 Refractive index change evaluation: images processing steps. . . 75

3.9 Cross section of the modulated waveguides . . . 75

3.10 Evolution of refractive index change ∆n for different irradiances . . . 76

3.11 Evolution of refractive index change ∆n for different number of pulses . . . 76

3.12 Phase images processing: averaged OPD for waveguide with Λ = 1.45 µm . . . . 77

3.13 Phase images processing: waveguide with Λ = 0.5 µm showing no modulation . . 77

3.14 Highly resolved fluorescence picture: Λ = 1.45 µm - N.A. 0.75 . . . 78

3.17 High resolved fluorescence picture: Λ = 0.5 µm - extracted periodicity . . . 79

3.18 Experimental setup for injecting TiSa laser into modulated waveguides . . . 80

3.19 Bragg gratings numerical simulation: N identical dielectric layers . . . 81

3.20 Bragg gratings numerical simulation: double-layer single segment . . . 82

3.21 Bragg gratings numerical simulation: reflectance . . . 82

3.22 Simulated transmission and reflection spectra for Λ = 1.45 µm created with N.A. 0.75 objective . . . 84

3.23 Simulated transmission and reflection spectra for Λ = 1.45 µm created with N.A. 1.3 objective . . . 85

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List of Tables

2.1 GPy series: theoretical composition. . . 42

2.2 GPx series: theoretical composition. . . 44

2.3 Weight of the cubic term of polinomial fit. . . 53

2.4 Direct Laser Writing parameters. . . 61

3.1 P Zn − 8%Ag: Produced spatial modulations and relative Bragg wavelengths . . 72

3.2 GP N 20 − 3%Ag: Produced spatial modulations and relative Bragg wavelengths 72 3.3 Refractive index change ∆n for spatial modulation Λ = 1.40 − 1.45 µm . . . 77

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Introduction

The topics of three-dimensional material structuring can be addressed by directly focusing an intense laser source in the bulk of photosensitive media. This approach has raised an increasing interest in research over the last two decades, in order to understand the physics and dynamics of laser-matter interaction, but also to access optical functionalities from the induced material modifications. Direct Laser Writing (DLW) is based on multi-photon absorption, a nonlinear process characterised by a strong confinement on interaction voxel, which allows for the three-dimensional control of localised energy deposition and associated material modification. This phenomenon can be induced using a tightly focused laser pulse train (picosecond/femtosecond). DLW became rapidly popular thanks to its simplicity, flexibility and capacity of producing dif-ferent kinds of microstructures.

Two major aspects are deepened nowadays: on the one hand, the ability to locally and perman-ently modify the physical properties of a medium paved the way to a broad range of applications, from astro-photonics to optical communication, bio-photonics and sensing. On the other hand, the desire to meet specific requirements in such applications has urged the research for new materials exhibiting ad hoc physical properties in order to optimise the laser-matter interaction. In this framework, non-conventional glasses tailored with photoactive agents, such as silver ions, provide an interesting alternative to well-known commercial glasses, such as silicate glasses. As a matter of fact, in a photosensitive glass doped with silver ions, DLW can induce the nucleation of silver ions, leading to original properties such as clusters formation and associated linear and non linear optical properties. As a result, a new kind of index modification has been observed, namely type A, as opposed to standard type 1, type 2 and type 3 material modifications in non-doped glasses.

The work related to this manuscript is part of an ongoing research activity, carried out by the “Short-pulse Lasers: Applications and Materials” (SLAM) group from “Centre Lasers In-tenses et Applications” (CELIA) laboratory in Bordeaux, France. It is based on femtosecond laser structuring of non-conventional glasses doped with silver ions, which are optimised for photonics applications, such as perennial high-density optical data storage, photonics integrated circuits or sensors, both in bulk or fibered sample geometries.

Two main issues are addressed in this report. Firstly, in a new sodium-gallium-phosphate tern-ary diagram, the influence of the glass network on the interaction between the femtosecond laser and the material is analysed, in order to relate the photosensitivity to structural properties of the glassy matrix, depending on its composition. These developments show significant progress

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in the understanding and improvement of the silver photosensitivity under femtosecond laser irradiation, which is highly demanded to obtain more efficient laser-glass modification processes, as well as getting access to larger laser-induced optical contrasts. Secondly, having proved the ability of creating new kind of waveguiding structures, sustained by the localised creation of new silver species with larger polarisability, modulated waveguides are designed and produced. Such structures are supposed to enable selective spectral reflectivity for specific wavelengths for developing waveguide Bragg gratings. Since some inner features show dimensions of 100 − 200 nm typically, one can expect to access Bragg resonances down to visible spectral range, which is highly challenging by means of all-optical approaches, as well as highly demanded integrated optics.

The report follows this outline.

Chapter 1 summarises the main phenomena involved in the laser-glass interaction. A special attention is devoted to the process of silver cluster formation in glasses doped with metallic ions. Then, the state-of-the-art for femtosecond laser irradiation of silver-containing phosphate-based glasses is presented, analysing the effects on the optical properties and focusing the attention on innovative waveguiding structures.

Chapter 2 deals with a detailed study of the relation between structural environment and op-tical properties resulting from the interaction of the femtosecond laser and the glass. Two different series of gallo-phosphate glass compositions are taken into account. Results shown in this Chapter include i) a detailed analysis of third-harmonic generation depending on the glass composition, so as to correlate the third-order nonlinearity with reported properties, and ii) the description of the relationship between the glass composition, its structural aspect and its silver-based photosensitivity under femtosecond laser irradiation.

Finally, a single-step method for instantaneously creating periodically modulated waveguiding structures is proposed in Chapter 3. The chosen writing procedure is presented together with a first characterisation of the modulated structures. Extracted phase images of the waveguides are analysed in order to quantify the refractive index change. Moreover, the transmission sprectra of the guided light have been probed by means of a high-resolution spectrometer. Numerical simu-lations of the sought Bragg behavior in the ideal approximation of infinite planes are reported as well. The demonstration of waveguide Bragg resonance in these newly-reported silver-sustained waveguides is still awaited and under investigation in Bordeaux.

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Chapter 1 Femtosecond Direct Laser Writing in

glass

1.1 Introduction

This Chapter focuses on the main features of femtosecond direct laser writing (DLW) and its aim is twofold. On the one hand, a meaningful state of the art on DLW is provided. On the other hand, some fundamental concepts and experimental results are discussed, as they constitute the bases of the work of this thesis being detailed in Chapters 2 and 3, respectively.

The Chapter opens with a description of a general experimental setup used to perform DLW in glasses, especially in the non-conventional silver-containing glasses that will be further studied. After a brief resume of glass properties, the attention is focused on the possible modifications of the glass structure induced by the laser-matter interaction. Finally, the effects of DLW in silver-containing samples are described and the main laser-induced properties resulting in such a glass family are recalled.

In particular, it is highlighted how DLW can be suitably used in order to produce waveguides, including the novel class of waveguides based of the photo-activated chemistry of silver elements.

1.2 Femtosecond Direct Laser Writing experimental setup

The experimental setup for DLW is now presented and discussed. Its schematic representation is provided in Fig. 1.1.

Laser

The source used to perform DLW is a T-pulse 200 by Amplitude Syst`emes company. It is a Yb:KGW femtosecond oscillator operating at a central wavelength of 1030 nm, with a pulse

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Figure 1.1: Schematic setup for Femtosecond Direct Laser Writing, which includes a femtosecond laser oscillator, a continuously tuneable temporal switch (AOM), a spatial light modulation (SLM) for beam shaping, a polarization control, a high numerical aperture microscope objective and a high precision 3D translational stage (40 nm repeatability).

duration of 390 fs (Full Width at Half Maximum), a repetition rate of 9.8 MHz and an average power of 2.6 W. Laser irradiance is controlled by means of an Acousto-Optic Modulator (AOM) by AA Opto-electronic, which settles the energy pulse and the number of pulses, enabling the typical accumulation of N = 102–106 pulses with energy between 10 nJ and 120 nJ at the focus of the microscope objective.

Acousto-Optic Modulator

An AOM is a device used to control the irradiation parameters during sample processing. It can allow for several controls of a laser beam, such as: the reduction of the repetition rate of an oscillator while acting as a pulse picker, the continuously tuneable transmission of the laser down to full extinction, and the creation of temporal sequences of irradiation with the definition of pulse trains of defined duration and involved number of pulses. Typically, an AOM is a transparent crystal, or a glass, connected to a piezoelectric transducer driven by an electric signal. The transducer glued to the AOM is excited by a radio frequency (RF) source, producing sound waves in the crystal. While the sound propagates inside the medium, the change of pressure along the longitudinal wave leads to a periodic modification of the refractive index of the crystal. Therefore, an incoming laser beam can undergo diffraction if the Bragg condition is satisfied:

mλ = 2ΛsinθB=

2vssinθB

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In eq.(1.1), λ is the wavelength of the incident beam, vsis the speed of sound inside the crystal,

Λ is the wavelength of the acoustic wave, f the frequency of RF, m is the order of diffraction and θB is the Bragg angle. The frequency of the diffracted beam is shifted up or down from the laser

Figure 1.2: Schematic setup of an AOM: a transducer generates sound waves at which the laser beam can get partially or almost totally diffracted.

beam frequency by an amount equal to the frequency f of the RF acoustic wave, depending on the orientation of the AOM with respect to the incoming beam. The diffraction efficiency depends on the beam size: the larger the beam dimension is, the higher is the obtained efficiency. The acoustic waves are insured to be forward propagating only. This is achieved by positioning an acoustic absorber at the opposite facet of the crystal with respect to the piezoelectric transducer, preventing acoustic reflection of the waves back through the crystal.

Spatial Light Modulator

A Spatial Light Modulator (SLM) from HAMAMATSU company is used in order to shape the laser beam. In our case, the SLM acts as a corrective lens, focusing the beam at a chosen depth inside the sample, and mostly compensating for spherical aberrations.

A SLM is a device allowing for controlling the two-dimensional phase transverse distribution on a laser beam, which can affect its direction of propagation. Combined with other optics, one can further shape the transverse polarisation or intensity distribution.

Among all kinds of SLM, the LCOS-SLM (Liquid Crystal On Silicon-SLM) is an electrically addressed reflection type phase spatial light modulator based on liquid crystal micro-displays. The nematic liquid crystal medium, as shown in Fig. 1.3, is placed between a transparent electrode attached to the input window and a silicon substrate covered by a semiconductor backplane consisting of pixelated electrodes which are individually controllable.

The alignment of the liquid crystal molecules is provided by the semiconductor substrate: a change in the voltage applied on the electrodes results in a different orientation of the liquid crystal molecules. This tilt leads to a modification of the refractive index of the medium, and

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Figure 1.3: LCOS-SLM chip structure[1].

so the optical path length in the liquid crystal layer, allowing for modifying the phase of the entering light. Thus, the laser beam is focused at a chosen depth inside the sample using a microscope objective.

Beam focusing

In this work two different objectives with different magnifications and numerical apertures (N.A.) have been used in this project: a 20 × / N.A. 0.75 objective from Olympus, and a 100 × / N.A. 1.3 oil objective from Zeiss.

The sample is placed on a 3-axes translational stage, XPS from Newport company, which enables sample positioning within 40 nm precision. Finally, a CCD camera is used to observe the laser structuring process. Additional detection abilities can be added for in situ nonlinear metrology of the laser-matter interaction. For example, this can be achieved by performing forward collection of the second or third-harmonic generation, which will be detailed in Chapter 2.

1.3 Oxide glasses

Glasses are non-crystalline solids exhibiting the phenomenon of glass transition. At a molecular scale, glasses posses a liquid-like structure with no long range order, but mechanically behave like solids. Given their disordered atomic structure, glasses are commonly referred as amorphous materials.

Oxide glasses refer to glasses with a various possibilities of cations while anions are oxygen elements.

A glass is produced by rapidly cooling a melt below the freezing point, Tm, causing a strong

slowdown of molecular motion and an increase of relaxation time. During the cooling, the vis-cosity of the liquid starts to increase, making difficult for atomic structure to rearrange: if the

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Figure 1.4: Disordered atomic structure of a glass compared with a periodic crystalline structure.

cooling process is sufficiently fast, crystallisation can be avoided, even though the glass phase may not be the most stable thermodynamical phase.

Figure 1.5: Glass transition: isobaric relationships between a) volume and temperature; b) enthalpy and temperature for liquid, glass and solid states; and c) heat capacity dependence and temperature during the glass transition[2].

This falling out of equilibrium state occurs within a narrow range around the glass transition temperature Tg, at which liquid’s volume and enthalpy, functions of the temperature, change

abruptly, but still continuously, to a value comparable with that of a crystalline solid, as shown in graphs a) and b) of Fig. 1.5. This shows that the glass transition is not a thermodynamic transition but rather a kinetic transition.

Furthermore, Tg changes with the cooling rate: a slower cooling rate produces a glass

trans-ition at Tga, lower than Tgb, resulting from a faster cooling rate, as shown in Fig. 1.5.

The slope of enthalpy curve at constant pressure is the heat capacity cp, plotted in graph

c) in Fig. 1.5. It rapidly decreases in the transition region: heat capacity in liquids arises from rotational, translational and vibrational contribution, while in glasses only the vibrational degrees of freedom contribute to the heat capacity, since those rotational and translational are “frozen”.

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Here, it is now briefly introduced a description of the structure of oxide glasses, which are the kind of glasses that have been used in this work. From an historical point of view, a first criterion that has to be fulfilled for glass-formers is the Goldschmit’s rule:

0.2 < Rc Ra

< 0.4 (1.2)

that sets a correlation between the ability of a substance to form a glass and the ratio of its cationic Rcand anionic Ra radii.

Such criterion claims that the anions have to rearrange themselves around the smaller cations in a tetrahedral order, as shown in Fig. 1.6.

However, Goldschmit’s rule has not a general validity and it is applicable only to purely ionic

Figure 1.6: Tetrahedral arrangement for oxide glasses: SiO2-tetrahedron a), boron oxide in

trigonal b) and tetragonal c) configurations[3].

substances or substances where the ionic character of bonding dominates over the covalent. Zachariasen’s rules based on empirical observations overcome this problem, specifying additional structural requirements and classifying the oxides in:

• glass formers, (SiO2, P2O5, GeO2, AsO2 etc... ) which are able to form the glass network

and exhibit a high melting temperature;

• modifiers, which are add to reduce the structural cohesion and to modify the viscosity and the melting temperature, such as N aO2, CaO and K2O;

• intermediates, which can act as glass formers or modifiers but are not able to form the glass network by themselves (Al2O3, ZnO, Ga2O3 etc...).

The optical properties of a glass are dependent on the glass composition.

If light hits the surface of a material, it will be reflected, transmitted and absorbed. Lambert-Beer’s law describes the amount of light that is absorbed by the material:

I(z) = I0e−αz; (1.3)

where α is absorption coefficient, and z the propagation distance in the glass. Oxide glasses are typically transparent over a wide spectral range, however the addition of network modifiers can redshift the absorption edge of the UV cutoff to larger wavelengths.

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1.4 Laser-matter interaction

Propagation of light inside a medium is described by Maxwell equations: ∇ · D = ρ; ∇ × E = −∂B

∂t; ∇ × H = ∂D

∂t + J; ∇ · B = 0;

(1.4)

where E = E(r, t) and H = H(r, t) are the electric and magnetic fields, respectively, J and ρ the current density and charge density, respectively.

The magnetic induction B and the electric displacement D are described by the constitutive relations:

D = ε0E + P;

B = µ0(H + M);

(1.5) where P and M are the electric and magnetic polarisations, respectively. For a non-magnetic medium, M = 0 and B = µ0H.

Many optical properties of the material, such as refractive index n and absorption coefficient α, result from previous equations, especially from the linear electric permittivity. Indeed, at sufficiently low intensity I, these values are independent on the strength of the optical field. In the low intensity regime, the induced polarisation depends linearly on the electric field:

P = ε0χE; (1.6)

where ε0 is the permittivity of free space and χ is the linear susceptibility. The refractive index

is related to the susceptibility by the relation:

n =p1 + χ; (1.7)

or, equivalently, n2= ε, where ε = ε1+iε2is the complex linear permittivity, introduced in order

to describe both the energy propagation with refractive properties and the energy dissipation of the electromagnetic field inside the medium.

Thus, the refractive index can be written using the complex notation:

n = N + iκ; (1.8)

where the real part N in normal incidence is related to the phase velocity, and the imaginary part κ is known as extinction coefficient.

The intensity reflection coefficient R at a air-material plane interface is related to the complex optical index through

R = (N − 1)

2_{+ κ}2

(N + 1)2_{+ κ}2 (1.9)

in the case of normal incidence, and is commonly described by the Fresnel relation RF = n1− n2 n1+ n2 2 , (1.10)

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Figure 1.7: Nonlinear processes occurring when the intensity of the laser beam is sufficiently high, depending on γ representing the Keldysh parameter[4]: a) multiphoton ionisation; b) tunelling ionisation; and c) avalanche ionisation.

at normal incidence for transparent media for which κ is zero; n1∼ 1 (air) typically, and n2 = N .

It is possible to express the absorption coefficient in Eq. (1.3) as a function of κ, so that the intensity of the electromagnetic field, proportional to |E|2, is

I(z) = I0e−α(ω)z = I0e

2ωκ(ω)

c z. (1.11)

The development of lasers, intense quasi-monochromatic optical sources with high spatial co-herence, made it possible to observe physical phenomena that depend on the local light intensity. These phenomena are described by the field of nonlinear optics, since the optical properties can show a nonlinear dependence on the laser intensity.

In this regime, the linear relation in Eq. (1.6) is no longer reliable and higher orders need to be introduced:

P (E) = ε0{χ(1)E + χ(2)EE + χ(3)EEE + ...} =

= P(1)+ P(3)+ P(3)+ ... = = P(1)+ PN L;

(1.12)

where the (i) index, with i = 1, 2, 3, .., represents the order of Taylor series with the electric field E. The index i = 1 depicts the linear optical properties and superior indices depict nonlinear processes.

If the irradiance of the radiation is high enough, the laser can ionize the material, locally modifying it into an absorbing plasma. In this way, it is possible to excite electrons from valence bands to conduction bands of transparent materials by nonlinear photoionisation. The nonlinear photoionisation process can occur in two distinct regimes: the multiphoton absorption and/or the tunneling ionisation.

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Figure 1.8: Linear a) and four-photon absorption b) of a Gaussian beam focused in a glass material[5].

Multiphoton absorption (Fig. 1.7a) is the simultaneous absorption of two or more photons, so that the sum of the energies of the simultaneously absorbed photons can exceed the energy bandgap, while not been possible for one single considered photon. The energy of a single photon (namely the laser beam optical frequency, thus its wavelength) and the material bandgap settle the number of photons that are required to be simultaneously absorbed, which imposes the nonlinearity order of the process.

The ionisation rate is given by

P (I) = σkIk, (1.13)

where σkis the multi-photon absorption cross section coefficient, I is the radiation intensity and

k is the number of photon required to exceed the optical bandgap.

A comparison between linear and multiphoton absorption is presented in Fig. 1.8: in the latter case the laser energy is absorbed inside the volume of interaction.

In the case of tunneling ionisation (Fig. 1.7b), the Coulomb well that bounds the electron to the nucleus is repressed by the intense laser field, allowing for the electron to tunnel out of the bound state.

The Keldysh parameter γ determines which photoionisation process dominantly takes place in the material during the irradiation process:

γ = ω e

r

mncε0Eg

I , (1.14)

where ω is the laser frequency, c is the speed of light, n is the refractive index, ε0 is the

permit-tivity of free space, Eg is the bandgap of the material, I is the laser irradiance, and m and e are

the reduced electron mass and charge, respectively[6]. For γ 1, multiphoton absorption is the dominating process, otherwise, for γ 1, is the tunneling ionisation that dominates.

Under particular conditions, free electrons produced by multiphoton ionisation may be heated by the laser pulses causing subsequent collisions with the electrons from the valence band, resulting in the generation of multiple free electrons. This process, known as avalanche ionisation (Fig. 1.7c), can iterate and produce up to a critical density of free electrons that creates a high-density plasma with oscillation frequency equals to laser frequency. Thus, the

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material becomes more absorbing, which enhances the heating rate of the glass lattice, possibly causing local material damages.

Usual glass modifications: types 1, 2, 3

During nonlinear photoionisation, the energy of the focused laser beam is absorbed by the material, generating photoelectrons. Their kinetic energy is transferred to the lattice due to collisions, leading to a local increase in the temperature. Subsequently, depending on the laser peak intensity and repetition rate, heat diffuses, causing local modifications of the material under study.

Three different types of structural modification are generally identified:

• type 1: for moderate intensities, modifications result in smooth isotropic changes of the refractive index of the material;

• type 2: nano-structuration of the material and self-organized nano-gratings formation are observed at higher intensities, associated with moderate plasma production. This type of modification results in a non-isotropic index change, thus in form birefringence;

• type 3: for even higher intensities, micro-explosions cause disorganised damages and voids in the glass matrix, due to excessive plasma production and consequential Coulomb ex-plosions.

Figure 1.9: Types of chemical-physical mechan-isms and modifications a), and associated types of macroscopic modifications b) while increasing the intensity of the focused laser beam[5].

Figure 1.10: Simulated temperature be-havior during the pulse train for differ-ent repetition rate: for higher repetition rate a heat accumulation occurs. The black dotted line represents the melting threshold[7]_.

Glass structuring can be performed using two distinct modifications regimes: the athermal regime and thermal regime[8].

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The athermal regime is commonly achieved using a low-repetition-rate pulse train (about few kilohertz) and high pulse energies (from hundreds of nanojoules to few microjoules). The sep-aration in time between two consecutive pulses is large enough to allow for the diffusion of the heat outside the volume of interaction before the next pulse arrives. As a result, temperature does not accumulate and the shape of the modified volume is directly related to the intensity distribution of the laser beam. However, the writing speed has to be chosen sufficiently low to allow for a sufficient spatial overlap between pulses, slowing the fabrication process of complex devices.

On the contrary, as suggested by the name, the thermal regime is based on the heat accumu-lation by the means of a high-repetition rate pulse train (from hundreds of kilohertz to several megahertz) and relatively low pulse energy (about few hundreds nanojoules).

The accumulation of the heat causes local melting of the material; a simulation of the increasing local temperature during the pulse train is reported in Fig. 1.10, where the melting threshold is evidenced by the black dotted line[7].

However, it is remarkable to note that the achieved work in this thesis, thanks to femtosecond irradiation of silver-containing phosphate glasses, can be considered as an athermal material modification regime, despite the fact that a 9.8 MHz femtosecond laser oscillator was used[9].

1.5 Direct Laser Writing in silver-containing glasses

Femtosecond laser irradiation of photosensitive glasses doped with noble metals, such as silver, leads to nucleation of metallic ions and subsequently clusters formation.

The creation of silver clusters can be described using two time scales[10] [11]_{. The fastest process}

is the ionisation of the glass occurring in the time scale of a single laser pulse. The radiation is absorbed by the material, enabling the promotion of free electrons which are immediately trapped by silver ions Ag+_{, leading to the formation of silver atoms Ag}0_:

Ag++ e−→ Ag0. (1.15)

The absorbed laser energy is transferred to the lattice due to collisions, causing an increase of local temperature due to the high repetition rate of the laser. The accumulation of energy occurs in a longer time scale of a few microseconds. The heat diffusion (blue arrows in Fig. 1.11) activates the migration of silver atoms (green arrows in Fig. 1.11) towards the edge of the volume of interaction. Consequently, successive Ag0 are trapped by silver ions Ag+, allowing the formation and growth of molecular silver species labeled clusters Ag_mx+. However, the regime is considered as athermal since no type 1 modification is observed. Namely, the local temperature increase remains sufficiently limited (less than 100 ◦C in some cases) so that the local temperature does not exceed at all the glass transition temperature.

The Agx+_m species, where m is the number of atoms and x is the degree of ionisation, are getting more chemically stable while the nuclearity m increases. Such silver clusters show strong absorption bands in the UV which leads to intense fluorescence emission in the visible range. Due to the small mobility of Agx+_m , the diffusion process ends with the creation of silver clusters.

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Figure 1.11: Silver clusters formation and photodissiciation: a) free electrons, promoted during laser pulse, are trapped by Ag+ ions producing silver atoms Ag0; b) and c) Ag_mx+ are created thanks to migration of Ag0 (green arrows) due to heat diffusion (blue arrows); d) silver clusters are photodissociated in the center of the interaction voxel by the next laser pulse[12].

The following laser pulse destroys silver clusters through photodissociation process in the center of the laser beam, where the intensity is sufficiently high.

The unphotodissociated clusters lay at the periphery of the interaction voxel. The pulse-to-pulse cumulative structure, induced by DLW in silver-containing glasses, thus results in a ring-shape geometry in the transverse plane perpendicular to the laser propagation direction (Fig. 1.12). The depth of the structure is typically proportional to the confocal parameter, namely twice the Rayleigh length along the beam propagation direction, according to the formula zR = πw0/λ,

where w0 is the beam waist and λ is the wavelength of the radiation.

Figure 1.12: Morphology of the structures during DLW process: a) 3D volume of interaction; b) and c) transverse and longitudinal distribution of fluorescent silver clusters; d) and HRSEM image of the ring-shaped structure d)[13].

The ring-shaped structure has been topologically and chemically characterized by means of an atomic force microscope (AFM) and high resolution scanning electron microscope (HRSEM)[10]. As shown in Figs. 1.13c and 1.13d, the HRSEM profile of the longer section of the annular

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Figure 1.13: HRSEM and AFM characterizations of the nanostructures: B) and C) show the transverse profile of the HRSEM images related to long and short white segments in A) respect-ively; D) shows the corresponding AFM profile for the longer segment[10].

structure presents a major peak in the center and two minor peaks on the perimetry of the ring. Nevertheless, the related AFM profile shows that the contrasted signal of the structure is due to a chemical change induced by the accumulation of silver species. The center peak is due to chemical etching needed to reveal the structure.

Type A modification

A positive refractive index modification associated to the laser inscription has been observed[14]. Such modification does not fall within the aforementioned three types of structural modifications (type 1, 2 and 3). Instead, it is based on the localised laser-induced creation of different chemical silver species that correspond to silver clusters, which leads to new chemical bonds with enlarged molecular electric polarisability. This leads to an increase of the local susceptibility and, hence, associated refractive index change.

As a matter of fact, this type of refractive index modification has been revealed for the first time two years ago, and it has to be considered as a new type of modification, labeled type A modification named after Argentum.

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1.6 Laser-induced poperties in zinc-phosphate glasses

Photosensitive silver-containing glasses used in DLW belong mostly to phosphate glass family. Such materials, depending on the composition, show good chemical durability, excellent optical properties and ion exchangeability[15].

Moreover, their ability to contain relatively large amount of photosensitive agents, such as sil-ver, qualifies them as suitable photosensitive candidates for femtosecond laser irradiation and structuring.

All glasses considered in this work have been developed at Institut de Chimie de la Mati`ere Condens´ee de Bordeaux (ICMCB). Several remarkable laser-induced glass modifications have been reported mostly in a non-conventional zinc-phosphate glass doped with silver ions. The induced properties under femtosecond laser inscription are discussed here after.

1.6.1 Fluorescence

The dimension of the photoinduced structures, as shown in Fig. 1.14, is dependent on the laser irradiance: increasing the irradiance, a threshold effect occurs above 5 TW/cm2_{, and, typically,}

the radius of the ring shaped structures gets wide with increasing the laser intensity[10] [13] [12].

Figure 1.14: Size of the photoinduced structures, observed in fluorescence confocal microscopy, as a function of the laser irradiance[13].

The fluorescence intensity of the silver clusters is also related to the repetition rate of the laser beam: for high repetition rate the thermal effects are cumulative, efficiently allowing for the migration of silver ions and associated photochemistry that results in silver clusters[10] [13]. As shown in Fig. 1.15, at lower repetition rate, the relaxation time of some intermediate species is getting comparable to the laser period, preventing the efficient formation of silver clusters. Indeed, by decreasing the repetition rate of a factor of 10, keeping constant the irradiance and

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the number of pulses, the fluorescence intensity decreases of a factor of 5.

Moreover, the fluorescence intensity depends on the number of deposited pulses, Fig. 1.16.

Figure 1.15: Fluorescence intensity de-pendence on repetition rate, which is to be correlated to the lifetime of intermedi-ate species that allow for the growth of stable silver clusters. Note that this situ-ation is non intuitive, and corresponds to an athermal regime, despite the high repe-tition rates[13].

Figure 1.16: Fluorescence intensity de-pendence on number of pulses[16].

The control of the imprinted fluorescence intensity makes it possible to encode information into the material and, thus, to develop perennial 3D high-density optical data storage[16].

In order to achieve such result, a greyscale can be realised by modifying either the laser intensity or the number of pulses, by properly tuning the deposited dose so as to encode information into a chosen number of fluorescence levels. An example of data storage in presented in Fig. 1.17, showing a 4D optical data storage with 3D positioning and 1D greyscale level encoded in 16 levels.

Thanks to the high confinement of the multiphoton absorption into the volume of interaction (Fig. 1.8b), data can be stored in different plans with no cross-talk between them, as shown in Figs. 1.17c and 1.17d. Silver clusters durability assures a perennial durability to the information recorded into the material, with structured glasses that show no alteration even after more than 10 years.

All of these properties have been observed in the bulk of the silver-containing photosensible glasses. Material science effort has allowed to make such glass compositions shaped as rectan-gular fibers. It is noteworthy that the full potential under femtosecond laser irradiation was maintained in such fiber drawing. This allowed thus to create similar photoinduced fluorescent structures rectangular fibers, at a depth of 50 nm from the surface, Fig. 1.18[17].

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Figure 1.17: 3D optical data storage realised into a silver-containing glass: a) original picture (Nobel laureate C. Cohen-Tannoudji); b) recorded picture and c) decoded picture. In c) different pictures that have been recorded in different plans; d) side view of the encoded pictures with no cross-talk[16].

Figure 1.18: Direct laser writing performed 50 nm under the surface of a flat fiber[17].

1.6.2 Second-Harmonic Generation

Second-harmonic generation (SHG) is a nonlinear optical process in which a laser beam with frequency ω interacts with a nonlinear medium with a non zero second-order susceptibility, χ(2), generating radiation at 2ω frequency.

From a microscopic point of view, two photons with same energy Eγ and wavelength λ combine

to create a photon with energy 2Eγ and wavelength λ/2, Fig. 1.19.

Describing the electric field of the laser beam through the relationship

E(t) = E0e−iωt+ c.c., (1.16)

and taking into account Eq.(1.12), the nonlinear polarisation produced in the material can be written as

P(2)= 2ε0χ(2)EE∗+ (ε0χ(2)E2e−i2ωt+ c.c.), (1.17)

where c.c. indicates the complex conjugate. The contribution at twice the laser frequency is clearly highlighted.

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Figure 1.19: Second-Harmonic Generation: a) interaction and b) energy-level photonic dia-gram[18].

In a centrosymmetric material, such as glasses, SHG can not be demonstrated. Because of the inversion symmetry of the material, even terms in Eq.(1.12) cancel by symmetry while only odd terms survive.

Femtosecond direct laser writing can be remarkably used to break the centrosymmetry and,

hence, to induce second-order nonlinearity. This approach is localized with three-dimensionality[19] [20] [21]. Indeed, during DLW, the diffusion of hot free electrons and their subsequent trapping creates

a net space charge separation, resulting in the creation of a buried static electric field Edc, as

shown in Fig. 1.11.

Figure 1.20: Spatial correlation between fluorescence and EFISHG patterns, highlighting the localisation of the fluorescent clusters at the local minima of the electric potential while the static electric field stands at the edge of such minima[20].

The fluorescence double-line pattern in Fig. 1.20a is obtained by translating the sample during the femtosecond laser inscription. In this way, the intense irradiance at the beam center destroys the silver clusters in the center of the beam, whilst silver clusters ad the edges of the interaction voxel remain globally unaffected.

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Figures 1.20b and 1.20c show the four-line electric field induced second-harmonic generation (EFISHG) pattern and the overlapping of both fluorescence and EFISHG patterns.

In Figs. 1.20d, 1.20e and 1.20f, the spatial correlation between the two patterns is reported. The maxima of the fluorescence signal are located at the zero point of the EFISHG signal, demonstrating the anti-correlation between the burried electric field and silver clusters spatial distributions.

The EFISHG signal shows a remarkable stability after thermal treatment of the glass sample for temperature below the glass transition temperature Tg, as shown in Fig. 1.21[21]. Of course,

above Tg, major glass matrix rearrangements are made possible so that ion migration allows for

the cancellation of the initially trapped static electric field. Note that such a static field is very intense, up to 108−9 V/m, which is close to the material dielectric breakdown.

Figure 1.21: Stability of EFISHG after thermal treatment below Tg[21].

1.6.3 Third-Harmonic Generation

As aforementioned, in dealing with nonlinear interaction between an intense optical source and a centrosymmetric medium, such as glass, only odd terms in Eq. (1.12) are involved.

In the case of silver-containing phosphate glasses, femtosecond laser irradiation leads to the creation of fluorescent silver clusters. Such absorbing species can be exited by multi-photon process. Correlatively, by using an Ytterbium femtosecond oscillator with central wavelength at 1.03 µm, the third harmonic lies in absorption bands of such laser-created clusters. Therefore, one observes a resonant third-order polarisation, so that the cubic nonlinear polarization:

P(3)(t) = 3ε0χ(3)E3(t), (1.18)

leads to a strong contrast in THG.

From a microscopic point of view, Fig. 1.22, in the third-harmonic generation (THG) three photons of frequency ω are coherently combined in order to create a new photon with an energy three time large than that of incident photons, resulting in a new nonlinearly radiated optical field at the optical frequency 3ω.

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Figure 1.22: Third-Harmonic Generation: geometry a) and energy-level diagram[18].

Three-dimensional optical data storage has been demonstrated in a photosensible

silver-Figure 1.23: Data storage using THG imaging: matrix of parameters chosen for the experiment a), epiwhite light microscopy image of the exposed site, epifluorescence microscopy image at excitation light λ = 365 nm c), THG signal[22].

containing glass by using THG imaging[22]. This result can be achieved inducing a change in the third-order susceptibility χ(3) by means of a femtosecond laser irradiation.

Exposing the sample at different irradiance levels and number of pulses, it is possible to store information into the glass still below the refractive index modification threshold, as shown in Figs.1.23b and 1.23c.

1.6.4 Surface plasmon resonance from metal nanoparticles

The silver clusters can act as nucleation centers for larger entities, such as nanoparticles, after subsequent aggregation.

A thermal post-treatment completes the Ag+ diffusion and silver cluster growth and reduction, leading to the growth of metallic nanoparticles[23]. Note that this does not spontaneously occur in such silver-containing phosphate glasses because the glass matrix and its associated reduction-oxidation (Red-Ox) potential. However, after DLW, large-enough laser-induced silver clusters show a sufficiently different Red-Ox potential so that they can further grow under heat treatment close to the glass transition temperature.

Photo-induced linear patterns were inscribed inside the glass (Fig. 1.24a) and further a thermal treatment was performed, heating the sample above the glass transition temperature Tg. As

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shown in Fig. 1.24b, one can see a HRSEM image of the cross section of the inscribed patterns, while a magnification of the same structure in Fig. 1.24c shows homogeneously distributed nanoparticles with nanometric scale diameters and interdistances around 10 nm.

Figure 1.24: Nanoparticles formation: schematic drawn of the photoinduced pat-terns a), HRSEM image of the pattern after thermal treatment b), 100.000× magnifica-tion HRSEM (backscattering mode)[23].

Figure 1.25: Evolutions of the surface plas-mon resonance and fluorescence emission from the silver clusters as functions of the thermal development duration[23].

Moreover, after thermal treatment, a strong surface plasmon resonance can be measured from the laser induced patterns by means of micro-transmission microscopy. The evolution of the surface plasmon resonance as a function of the thermal development duration is depicted in Fig. 1.25, showing the correlative increase of the amplitude of the surface plasmon resonance and the associated decrease of fluorescence emission of silver clusters. This demonstrates the progressive growth of molecular fluorescent silver clusters into non-fluorescent metallic silver nanoparticles.

1.6.5 Material structuring with structured light

While irradiating photosensitive glasses with usual Gaussian beams, it is possible to get the original optical properties depicted here above. However, it is of interest to look for new spatial distributions of the fluorescent silver and associated linear and nonlinear optical properties. In-deed, the voxel interact constitutes the elementary brick at the micronscale, so as to structure photosensitive glasses.

In this framework, DLW with structured light, i.e. electromagnetic filed endowed with phase or polarisation singularities, in silver-containing glasses enables for the formation of silver clusters arranging in non conventional patterns.

A phase singularity is a point at which the phase of the field is undefined and the intensity of the wave is zero. Light beams with phase singularities, associated with orbital angular momentum, have helical phasefronts and are called vortex beams.

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For helical phasefronts, described by eilφ (where l is an integer), the Poynting’s vector has an azimuthal component that produces an orbital angular momentum parallel (or anti-parallel) to the beam axis, which is equivalent to l~ per photon[24]. The integer l indicates the multiplicity of folding around the singularity and it is known as topological charge.

Figure 1.26: A diagram showing a helical surface of equal phase with the Poynting vector indicated by a curved line[24].

Figure 1.27: Structured light generation setup for an incident linearly polarised Gaussian beam[25].

Many methods exist for producing optical vortices. These could be deliberately produced within optical fields and could occur in laser cavities with large Fresnel number. The natural laser mode containing an optical vortex is described by Laguerre-Gauss (LG) functions. Dif-fractive optical components could be used to transform spatially coherent, flat phase beams into beams containing optical vortices. However, an easier way for generating optical vortices consists of the use of computer-generated holograms by means of a SLM.

Moreover, switching from a linearly polarised beam to one with circular polarisation gives rise to a spin angular momentum, directed along the beam axis, of σz~ for photon, where σz = ∓1

for right-handed or left-handed circular polarisation[26]. Thus, the total angular momentum per photon is j = (l + σz)~.

Circularly polarised focused beams propagating along the optical axis of an uniaxial crystal (which is also a rotational symmetry axis) can lead to the generation of an optical vortex[27]. Figure 1.27 represents the experimental setup used to perform structured light-induced DLW[25] in a zinc-phosphate silver-containing glass. The incident linearly polarised Gaussian beam E0

is converted into circularly polarised light by a quarter-waveplate λ/4. The beam is then col-limated by a first lens L1 on a uniaxial calcite crystal slab. Then, a second lens L2 collects

the light that subsequently undergoes a second quarter-waveplate λ/4. The inhomogeneously polarised beam is separated into two orthogonal linearly polarised components (parallel Ek and

perpendicular E⊥ to the incident beam polarisation) by a polarised beam splitter (PBS). This

configuration enables the creation of a charge-2 optical vortex by using the E⊥ component, as

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is found to be independent of the sign of the topological charge of the incident vortex beam. Removing the two quarter-waveplates leads to the formation of a four-lobe intensity pattern, Figs 1.28c, 1.28 f, 1.28i and 1.28l. The dark cross is associated to π-phase steps, though the four bright spots produced identical fluorescent hollow pipes, showing the independence of DLW with respect to phase. Still, the spatial distribution of the phase of the writing beam allows controlling the intensity profile of the incident light field at the interaction voxel.

Figure 1.28: Intensity profiles of the incident and focused beams, respectively: Gaussian a), d), charge-2 vortex b), e) and four-lobe c), f). Related fluorescence patterns under top view and 3D view, respectively: single-wall hollow pipe g), j), double-wall hollow pipe h), k) and four single-wall hollow pipes i), l)[25].

Furthermore, is it possible to address the role of the polarisation state[28]. Tightly focused femtosecond LGl

p (where l = 0, 1, 2 is the topological charge and p = 0 the radial index) vortex

beams with linear, left-hand and right-hand circular polarisations were produced using phase holograms by means of SLM, as seen in Fig. 1.29. The performed DLW produced unique nested double-ring fluorescent structures with sub-diffraction-limited fluorescent inner patterns. The structured light distribution at the interaction voxel showed a clear asymmetry between left-hand and right-left-hand circular polarisations, due to parallel and anti-parallel spin and orbital momenta, as shown in Fig. 1.30. This leads to distinct intensity distribution at the voxel, at the root of distinct spatial distributions of silver clusters and correlated static electric field. In particular, anti-parallel spin and orbital momenta lead to a true zero intensity distribution in the case of a LG1_p beam, which allows for sub-diffraction inner distributions of silver clusters compatible with super-resolution direct laser structuring.

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Figure 1.29: Experimental setup a) and phase holograms b), c), d) for gen-erating LG0₀ f), LG1₀ g) and LG2₀ h) beams[28].

Figure 1.30: Intensity distributions in the focal plane for linear d), left-handed h) and right-handed circular l) polarisations for LG1₀ beam. Lateral intensity profiles along the horizontal/vertical directions marked in d), h), l) are shown in e), i) and m). Intensity distributions through the focal plane f), j), n). On-axis intensity profiles along the dashed direction g), k), o)[28].

1.6.6 Refractive index change

Based on the creation of silver clusters, a new type of positive refractive modification, labeled type A, can be achieved. Such index modifications based of silver species photochemistry were always observed to be positive. The refractive index change ∆n increases with the laser irradi-ance and number of pulses, as reported in Fig. 1.31[14]_.

This remarkable property is mainly exploited in fabricating waveguiding structures. During femtosecond laser inscription of silver-containing glass, sample motion allows for creating three-dimensional patterns. If the motion is kept along one direction perpendicular to the laser beam, a longitudinal, potentially waveguiding, structure can be created inside the glass. Polishing the lateral surfaces of the sample enables to reveal the cross section of the structure, Fig. 1.32. In this way it is possible to inject light inside the structure in order to test the waveguiding abil-ity. Preliminar waveguiding behavior has been observed by exposing first the glass to a broad

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Figure 1.31: Dependence of the refractive index change on the writing parameters[14].

field illumination with UV light[17] to reveal and localise the fluorescent structure and then by injecting an infrared continuous laser to demonstrate the waveguiding ability.

Waveguides

In this section, some results taken from bibliography and concerning the first-time injection of cw light inside the pattern of photoinduced clusters are briefly discussed.

The attention is here restrained on the main results obtained by authors in[14]: DLW was

Figure 1.32: White light (image on top) and fluorescence images of the waveguide: on the left, a high resolution confocal image of the cross section of the waveguide[14] [29]_.

Figure 1.33: a) Phase image of the waveguide, and b) correlated refractive index change ∆n profile[14].

performed in a silver-containing zinc-phosphate bulk glass, at a depth of 160 µm below the sur-face, using a Yb:KGW femtosecond laser. A series of 7 mm long straight structures was written, writing parameters were optmised in order to obtain a single mode waveguide, with appropriate size and refractive index change ∆n, as shown in Fig. 1.33.

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A cw laser at 630 nm was injected inside the waveguides and the near-field intensity profile (Fig. 1.34a-i) at the output of the glass was collected with a microscope objective and imaged with a CCD camera.

The horizontal mode profile (Fig. 1.34a-ii) is a non-standard single mode profile and can be fitted by the superposition of two Gaussian distributions.

Such elliptical waveguides (ellipticity ε = 1.7) proved to support only a single mode at λ = 630 nm.

Figure 1.34: Mode profile of cw light injected inside the waveguide: near-field intensity profile a-i), horizontal a-ii) and vertical a-iii) mode profiles; comparison with simulation b-i), b-ii), b-iii)[14]_.

Figure 1.35: a) 50 - 50 beam splitter: schematic presentation; b) fluorescence image (top view) under UV light; c) output modes; and d) normalized intensity profiles of the output modes[14]. Graphs of Fig. 1.34 confirm a good matching between experimental and simulated modes by FDTD methods. Therefore it was stated that there is a spatial correlation between the fluor-escence distribution of silver clusters and the associated refractive index change, demonstrating that such new silver chemical species do locally support the positive index modification. Based

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on this remarkable result, a 50 − 50 beam splitter has been built by sequentially writing two partially overlapping S−bended waveguides[14]. Two almost identical spots are imaged in Fig. 1.35c. The near-field intensity profile in Fig. 1.35d proves that the transmitted power is equal in both branches, within an error margin of 3.5%.

1.7 Properties in other glasses

DLW in different glass matrices doped with silver ions can lead to distinct behaviours, depending on the glass matrix ability to preferentially stabilise either only silver ions or silver ions with silver clusters in the pristine glass.

Figure 1.36: Microscope fluorescence imaging (excitation at 375 nm) of laser-induced silver clusters in three distinct glass matrices: zinc phosphate glass a), sodocalcite glass b) and boro-silicate glass c). Scale bar is 20 µm in each case[12].

In some cases, DLW leads to a similar behaviors observed in zinc-phosphate glasses. As an example, silver ions have been injected by thermal poling into two commercial glassy matrices, a microscope slide sodocalcite glass and a borosilicate glass. After femtosecond laser irradiation, silver-induced patterns have been observed[12]. Figure 1.36 shows fluorescence images of laser-induced silver clusters in the two distinct glass matrices, compared with a fluorescence image of the same laser-induced pattern in a silver-doped zinc-phosphate glass.

However, in compositions that facilitate the spontaneous creation of silver clusters, the situation can differ. Indeed, for example, annealing a pristine silver-doped lanthanum boro-germanate (LBG:Ag) enables for the direct formation of nanoparticles, without femtosecond laser inscription of a latent image made on silver clusters. Top image in Fig. 1.37 shows a LBG:Ag sample before (transparent glass) and after (colored glass) thermal treatment per-formed at temperature T ∼ Tg for two hours. The staining of the glass is the visible result of

homogeneous spatially-random precipitation of silver nanoparticles.

In germinate glasses, the glass matrix allows for the existence of silver clusters, additionally to that of silver ions, due to the lower solubility of silver oxide. Therefore, is it possible to achieve local silver nanoparticles precipitation by femtosecond laser irradiation, avoiding any thermal post-treating[30]_{. However, in this case, the interaction regime becomes thermal, leading to a}

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melted area, which significantly reduces the optical quality of the locally irradiated voxel, con-trarily to the non-thermal regime in silver-containing zinc phosphate glasses. Bottom image in Fig. 1.37 shows the same LBG:Ag in which the inscribed squared pattern is clearly recognisable: the yellow color reveals the presence of metallic nanoparticles with surface plasmon resonance.

Figure 1.37: Nanoparticles formation in silver-doped LBG glass by means of thermal (top image) treatment with temperature close to Tg and by femtosecond laser irradiation (yellow square in

bottom figure)[30].

Figure 1.38: Bright-field a), b) and fluorescence c), d) images of the photo-induced patterns in GPN and GPN:Ag samples[31].

Additionally, nanogratings inscription in silver-doped gallo-phospate glass (GPN:Ag) is a noteworthy result that proved the catalytic-like role of silver ions in enhancing and improving the nanogratings formation process[32]. Figure 1.38 shows bright field and fluorescence images of the laser induced patterns performed at the surface of GPN and silver-doped GPN (GPN:Ag)

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samples with linearly polarised light (polarisation perpendicular to sample’s translation direc-tion). The GPN sample exhibits a higher damage threshold with respect to the silver-doped GPN sample. For the latter, the fluorescence clusters occur prior to the damage threshold. Moreover, as depicted in Fig. 1.39, the nanogratings performed on GPN:Ag present a smoother

Figure 1.39: Scanning electron microscope (SEM) images (secondary electron emission mode) of the femtosecond laser-induced linear structures above the damage threshold for both the GPN and GPN:Ag irradiated samples[31].

shape than those performed on GPN. Thus, the inserted silver ions behave as an extra reser-voir of accessible electrons, more efficiently released than those from the glass matrix. This larger amount of electrons can enhance the creation of defects, explaining the lower nanograt-ing threshold in GPN:Ag with respect to GPN. Also, the ionisation of silver ions is much less destabilising for the glass network than ejecting electrons from the glass matrix itself, resulting in smoother nanogratings for laser inscription on GPN:Ag with respect to GPN.

In conclusion, the universality of the aforementioned laser-induced processes in silver-containing glass matrices is proved in several aspects. Nevertheless, additional attention needs to be focused on the structural influence of the glass network on its response under laser irradiation. Indeed, it is of crucial importance to further keep on improving the glass compositions and associated glass structures so as to enhance the photosensitivity during the laser-glass interaction and thus to induce more intense and/or more localised modifications in such tailored glasses.

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Chapter 2 Direct Laser Writing in

gallo-phosphate family

Introduction

In the last ten years, increasing attention has been paid to non-conventional glasses, whose composition can be tuned in order to fulfill specific tasks for several applications (e.g. optical components). Considering silver-doped phosphate glasses, the gallo-phosphate family represents a promising candidate in fabrication of micron-scale devices by means of DLW.

In the present Chapter, the role of the gallo-phosphate glass structure on the laser-glass inter-action is investigated. To this purpose, two different series of silver-containing sodium-gallium phosphate glasses, with the same amount of photosensitive oxide AgO2, have been produced.

Therefore, the optical properties of each sample have been studied by following the structural evolution of both series, which is characterised by a changing length of phosphate chains. Fi-nally, luminesce properties of laser-induced structures have been analysed in order to relate the glass photosensitivity to the glass network arrangement.

2.1 The glass network

Phosphate glasses generally possess low melting, softening and glass transition temperatures, facilitating their fabrication and shaping. However, the main disadvantage usually encountered with phosphate-based glasses is their chemical durability and poor water durability due to the existence of easily hydrated phosphate chains, which may restrict practical applications. The addition of glass intermediates (introduced in 1.3), such as gallium oxide Ga2O3, in N aP O3

glass matrices improves the limited chemical durability of the glass by modifying the network, in particular changing the number or the strength of chemical bonds, leading to an increase of the glass transition temperature.

The addition of Ga2O3 may also increase the refractive index. This allows for higher numerical

apertures and strong confinement of light in waveguide mode for integrated optoelectronics while maintaining high rare-earth solubilities: indeed, a high concentration of well dispersed rare-earth

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ions in glass is essential for designing shorter optical components in waveguides for photonics applications.

Moreover, the gallo-phosphate matrix has the ability to contain relatively large concentrations of photosensitive agents such as silver ions Ag+. The silver ion environment influences both the absorption feature and the luminescence property of the pristine glass. Therefore, the solubility of the silver ions in the glass matrix plays a critical role for efficient laser writing.

Figure 2.1: Qn classification for P O4 tetrahedron: no bonding Q0 a), one bond (head of the

chain) Q1 b), two bonds Q2 c) and three bonds Q3 d).

The structural network of phosphate glass family consists of tetrahedral unit of P O4, bound

up one with the other, forming chains. The length of such chains characterises the phosphate matrix. Adopting the typical classification in Qn units (Fig. 2.1), where Q represents the P O4

tetrahedron and n the number of bridging oxygen between phosphorus elements, is it possible to describe the glass network.

Long phosphate chains are composed by a large number of Q2 units; in this case, the glass is clas-sified as meta-phosphate. While the length of the chain decreases, the number of Q2decreases as well, whereas the number of Q1 units raises. Hence, the glass is arranged in the pyro-phosphate configuration. Finally, in ortho-phosphate glasses, the presence of Q0 units arises.

2.2 GPy and GPx series

Two series of silver-doped gallo-phosphate glasses (GPN:Ag) have been elaborated to investigate different structural behaviors among the ternary diagram of the Ga2O3, P2O5and N a2O oxides.

The first series corresponds to the nominal composition [(1 − y)N aP O3 − yGa2O3]0.98+

2%Ag2O, where y is the ratio of Ga2O3 with respect to P2O5 (y = 0.22, 0.28, 0.35, 0.5, 0.58 and

0.66). In this series, the [N a]/[P ] ratio is kept constant while modifying the Gallium content. The second series corresponds to [(N a2O)x − (74P2O5− 26Ga2O3)100−x]0.98+ 2%Ag2O, with

Ultrafast laser inscription in silver-containing glasses: structure/property dependence for improved photosensitivity and modulated waveguides for photonics applications

N

U

IVERSITA DI ISA

P

Physics Department

Ultrafast laser inscription in silver-containing glasses:

structure/property dependence for improved

photosensitivity and modulated waveguides for

photonics applications

Acknowledgements

Contents

List of Figures

List of Tables

Introduction

Chapter 1

Femtosecond Direct Laser Writing in

glass

1.1

Introduction

1.2

Femtosecond Direct Laser Writing experimental setup

1.3

Oxide glasses

1.4

Laser-matter interaction

1.5

Direct Laser Writing in silver-containing glasses

1.6

Laser-induced poperties in zinc-phosphate glasses

1.7

Properties in other glasses

Chapter 2

Direct Laser Writing in

gallo-phosphate family

Introduction

2.1

The glass network

2.2

GPy and GPx series