Photo-thermo-refractive Research Articles

Mechanisms and kinetics of short pulse laser-induced destruction of silver-containing nanoparticles in multicomponent silicate photo-thermo-refractive glass.

Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass that is commercially used for the recording of volume holographic elements and finds many applications in advanced laser systems. Refractive index decrement in this glass is observed after UV exposure followed by thermal development. This procedure also causes the appearance of Ag-containing particles that can then be optically bleached by using the second harmonic of a Nd:YAG laser. Despite the broad usage of this method, its mechanisms are still unclear. In this paper, a systematic study of the short pulse laser-induced destruction of Ag-containing particles' kinetics versus incident energy per pulse and dosage is presented. We show that no bleaching of Ag-containing particles occurs for an energy density in laser pulses below 0.1 J/cm2 while above 1 J/cm2, the efficiency of bleaching saturates. Efficiency of bleaching depends on the type of particles to be bleached (Ag, AgBr…). Using a simple model of short pulse laser interaction with nanoparticles embedded in glass, the temperature of the Ag-containing particles reached during the laser interaction is shown to be large enough to produce complete dissipation of these particles which is expected to be the main mechanism of short pulse laser-induced destruction of Ag-containing particles.

Absorption and scattering in photo-thermo-refractive glass induced by UV-exposure and thermal development

Photo-thermo-refractive (PTR) glass is a multicomponent photosensitive silicate glass that, after successive UV-exposure and thermal treatment, exhibits a refractive index change that results from the precipitation of nano-crystalline NaF. This glass is successfully used for the fabrication of holographic optical elements (volume Bragg gratings) that dramatically enhance properties of numerous laser systems and spectrometers. In this paper, induced absorption and scattering that determine efficiency of such elements were studied. It is found that the main contribution to induced absorption is produced by several types of silver containing particles having absorption bands with maxima in the blue-green region with exponential tails extending to the near IR spectral region. Evolution of all absorption bands was studied for different conditions of UV exposure and thermal development. Complex mechanisms of interconversion of silver containing particles is demonstrated as well as the fact that some of these particles can be associated with catalyzers of the nucleation process. It is also found that induced scattering obeys the classic Rayleigh law with an intensity depending on the conditions of UV exposure and thermal development. For short development times, scattering increases with dosage because of increased volume fraction of crystalline phase. For long development times, scattering decreases with dosage because of decreased size of individual crystals.

Ultimate efficiency of spectral beam combining by volume Bragg gratings

Spectral beam combining (SBC) by volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass is a powerful tool for laser applications that require higher radiance than a single laser unit can achieve. The beam-combining factor (BCF) is introduced as a tool to compare various beam-combining methods and experiments. It describes the change of radiance provided by a beam-combining system but is not affected by the initial beam quality of the combined lasers. A method of optimization of VBGs providing the maximum efficiency of SBC has been described for an arbitrary number of beams. An experiment confirming the proposed modeling for a two-beam SBC system by a single VBG has demonstrated a total combined power of 301 W with a channel separation of 0.25 nm, combining efficiency of 97%, close to diffraction limited divergence with M(2)=1.18, BCF of 0.77, and spectral radiance of 770 TW/(sr·m(2)·nm), the highest to date for SBC.

Crystal nucleation and growth kinetics of NaF in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive oxide glass. Upon heating, the UV-exposed regions of this glass undergo copious crystallization of NaF nano-crystals giving rise to a permanent, localized refractive index change. But the unexposed parts of the glass also undergo some crystallization which causes unwanted light scattering. Holographic optical elements produced from PTR glass have been used in special laser systems. In this article we report, for the first time since the invention of PTR glass about half a century ago, the steady-state nucleation rates, nucleation time-lags and crystal growth rates for UV-unexposed PTR glass estimated in a wide temperature range, from Tg~470°C up to 750°C. A self-consistent description of these data is presented in the framework of classical nucleation theory using the interfacial free energy of the critical nuclei and the effective diffusion coefficient as adjustable parameters. The diffusivity calculated from crystallization kinetics and that estimated from viscous flow via the Stokes–Einstein–Eyring equation show a decoupling phenomenon.

Influence of UV irradiation and heat treatment on the luminescence of molecular silver clusters in photo-thermo-refractive glasses

The luminescence spectra of photo-thermo-refractive (PTR) glasses containing cerium, silver, and antimony ions before and after UV irradiation and after heat treatment have been thoroughly investigated for the first time. It is shown that silver is present in the initial PTR glass in the form of ions and positively charged molecular clusters. After UV irradiation into the absorption band of cerium ions, silver partially passes to the neutral state in the form of atoms and neutral molecular clusters Ag2, Ag3, and Ag4, which exhibit bright luminescence in the visible spectral range. Subsequent heat treatment at a temperature below the glassformation temperature leads to an increase in the luminescence intensity due to the increase in the concentration of neutral molecular clusters. Heat treatment at a temperature above the glass-formation temperature leads to the formation of silver nanocrystals and luminescence quenching. It is proposed to use PTR glasses with molecular silver clusters as phosphors for converting UV radiation into the visible range for solar power engineering and white LEDs.

High-Performance Solid-State and Fiber Lasers Controlled by Volume Bragg Gratings

Abstract : The paper is a survey of properties and applications of optical elements which are volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass. These elements are narrow band filters that operate in spectral, angular and temporal domains. They provide spectral narrowing, stabilization and tuning of laser radiation; narrowing of divergence and beam steering; stretching, compression and temporal shaping of laser pulses, integration of different laser components in the same material and fabrication of monolithic solid state; and passive and active coherent combining along with high density spectral combining of fiber lasers.

Modeling of the induced refractive index kinetics in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass that exhibits refractive index change after successive UV-exposure and thermal treatment. The refractive index change was demonstrated to be associated with the precipitation of NaF nano-crystals in the glass matrix. This paper presents a systematic study of the dependence of the refractive index change on dosage of UV-exposure and thermal treatment duration and temperature in already UV-exposed and nucleated samples. It is shown using Avrami plots that the refractive index change in PTR glass is determined by a diffusion-controlled growth of particles from pre-existing nuclei while all coefficients have exponential dependence on temperature (Boltzmann law). The developed model, which includes both photo-chemistry and crystallization processes, allow describing the refractive index change dependence on dosage for a wide range of thermal treatment duration and temperature and an accuracy better than 10%.

Near-IR absorption in high-purity photothermorefractive glass and holographic optical elements: measurement and application for high-energy lasers

Volume Bragg gratings (VBGs) in photothermorefractive (PTR) glass are widely used for laser beam control including high-power laser systems. Among them, spectral beam combining based on VBGs is one of the most promising. Achieving 100+ kW of combined laser beams requires the development of PTR glass and VBGs with an extremely low absorption coefficient and therefore methods of its measurement. This paper describes the calorimetric method that was developed for measuring a low absorption coefficient in PTR glass and VBGs. It is based on transmission monitoring of the intrinsic Fabry-Perot interferometer produced by the plane-parallel surfaces of the measured optical elements when heated by high-power laser radiation. An absorption coefficient at 1085 nm as low as 5×10(-5) cm(-1) is demonstrated in pristine PTR glass while an absorption coefficient as low as 1×10(-4) cm(-1) is measured in high-efficiency reflecting Bragg gratings with highest purity. The actual level of absorption in PTR glass allows laser beam control at the 10 kW level, while the 100 kW level would require active cooling and/or decreasing the absorption in PTR Bragg gratings to a value similar to that in virgin PTR glass.

Quantitative UV–VIS spectroscopic studies of photo-thermo-refractive glasses. I. Intrinsic, bromine-related, and impurity-related UV absorption in photo-thermo-refractive glass matrices

The paper opens up a series of papers on the origin and parameters of spectral features forming the absorption of photo-thermo-refractive (PTR) glasses in the UV. Problems to be cleared for gaining further insight into the spectroscopic manifestations of species responsible for the photo-induced processes in PTR glasses are discussed. The samples of bromine-containing and bromine-free PTR glass matrices are synthesized and their absorption spectra in the 28,500 to 50,000 cm –1 region are recorded. The dispersion analysis of the spectra is conducted based on the convolution model for the complex dielectric function of glasses. The matrix electronic transitions that set the real part of the complex dielectric function and form the intrinsic absorption tail of the matrix are simulated with a series of effective oscillators. Spectral features forming the total absorption spectrum of PTR glass matrices in the 28,500 to 50,000 cm –1 region are deconvoluted. These features are (i) the intrinsic absorption tail, (ii) for the bromine-containing matrix, the low-wavenumber wing of an envelope around ~ 51,400 cm –1 covering the bromine-related spectral feature(s), (iii) Fe 2+- and Fe 3+-related impurity bands, and also (iv) a structureless absorption mostly due to the high-wavenumber wings of other impurity bands below 28,500 cm –1.

Effect of Bromine on NaF Crystallization in Photo‐Thermo‐Refractive Glass

Photo‐thermo‐refractive (PTR) glass is a Na2O–K2O–ZnO–Al2O3–SiO2 silicate optical glass which also contains fluorine, a small amount of bromine, and dopants that yield photo‐sensitivity in the UV range. PTR glass undergoes crystallization of NaF nanocrystals after UV‐exposure followed by thermal treatment, resulting in permanent refractive index change. In this study, where we explore only the thermally activated transformations in the UV‐unexposed glass, we show that bromine decreases the solubility of NaF, i.e., increases the super‐saturation of NaF thus increasing the thermodynamic driving force for crystallization. This feature causes a decrease in the maximum volume fraction of crystallized NaF with decreasing bromine content in the parent glass. The evolution of the glass transition temperature, Tg, with increasing isothermal treatment time revealed a minimum resulted from interplay between two concurring processes, liquid–liquid phase separation that led to decrease in Tg, and Br‐controlled NaF crystallization that acted in the opposite direction. In glasses with lower bromine content, fewer and larger crystals appeared and a surface‐initiated crystallization was dominant. A surface layer of F‐depleted glass imprinted residual macro‐stresses, which were not alleviated by annealing.

Continuous-wave diode-pumped laser action of Nd^3+-doped photo-thermo-refractive glass

Laser action of the photo-thermo-refractive (PTR) glass, which is the photosensitive material for holographic recording of volume Bragg gratings (VBGs), was demonstrated for the first time by introducing Nd³⁺. Nd:PTR glass has a bandwidth of 27.8 nm and 16.0 nm for luminescence and absorption, respectively. An uncoated 2 mm thick Nd:PTR element generated cw laser output of 124 mW, with a slope efficiency of 25%, by laser diode pumping. This Nd:PTR glass also performed wide bandwidth laser action at 1053.9-1063.3 nm, where the decrease of the pump-absorption efficiency was held off below 30%, even under a 3.5 nm shift of pump wavelength from its absorption center.

Photo-thermo-refractive glass co-doped with Nd 3+ as a new laser medium

Photo-thermo-refractive (PTR) glass demonstrates refractive index change after exposure to UV radiation followed by a thermal treatment that enables recording of high efficiency holographic optical elements. This work demonstrates feasibility of function of this material as a complex optical medium which posseses both photosensitive and luminescent properties and paves a way for creation of monolythic solid state lasers where resonator components can be holographically recorded inside of a laser medium. It was found, that incorporating of Nd 3+ ions in PTR glass does not affect photosensitivity required for hologram recording. It was demonstrated that emission wavelength, spectral width, and cross section of Nd 3+ luminescence in PTR glass are typical for silicate laser glasses and Nd-doped PTR glass can be considered as a promising laser medium for monolithic solid state lasers.

The effect of decoloration on the properties of volume phase holograms based on silicate photothermorefractive glass

This paper discusses the decoloration process of volume phase holograms based on photothermorefractive (PTR) glass under the action of the second harmonic of the pulsed radiation of a neodymium laser. It is shown that the photodecoloration process increases the refractive-index modulation of PTR glass and reduces its absorption in the visible region.

Liquid-Liquid Phase Separation in Photo-Thermo-Refractive Glass

Photo-thermo-refractive (PTR) glass is an optical Na–Al–Zn–K–O–F–Br silicate glass doped with Ag, Ce, Sb, and Sn that undergoes photo-thermo-induced volume crystallization of nanosized NaF responsible for localized refractive index changes. PTR glass has found numerous commercial applications, but the intricate mechanism of photo-thermo crystallization is far from being understood. In this paper, we demonstrate that, additional to crystalline phase precipitation, liquid–liquid phase separation (LLPS), i.e. amorphous droplets embedded in the matrix glass, appears concurrently over a wide range of temperatures. The immiscibility temperature is 925°C. The droplet phase is richer in SiO2, rendering the alkali-rich remaining matrix glass a lower glass transition temperature and a higher electrical conductivity than the original glass. The droplet's surface does not catalyze NaF nucleation. Although the effects of LLPS on optical properties of PTR glass are still to be explored, it could contribute to unwanted scattering losses and/or uncontrolled refractive index change. The substantial change in the original glass composition resulting from LLPS should play an important role on NaF crystallization kinetics, and therefore must be considered for an overall understanding of the crystallization mechanism underpinning the refractive index change in PTR glass.

Specific absorption spectra of cerium in multicomponent silicate glasses

Photoionization of glass requires photon energies larger than the bandgap of the glass matrix. To shift the photosensitivity of a glass to longer wavelengths, it is common to add a sensitizer, e.g. cerium. Cerium has two valence states but only the Ce 3+ is UV-photoactive. Hence, we present a spectral method that allows determining specific absorption spectra of both valent states of cerium in two different silicate glasses. Two overlapped absorption spectra, one for Ce 3+ and one for Ce 4+, were observed in soda lime silicate glass. It was found that both spectra consist from two Gaussian bands. Specific absorption of each ion was calculated on the basis of balance equations written for glasses melted under different redox conditions and used to estimate the absolute concentration of each ion. An identical analysis was performed on a photosensitive multicomponent silicate glass (photo-thermo-refractive (PTR) glass). It was found that absorption band of Ce 3+ in PTR glass is shifted to short wavelength region due to the presence of fluorine while absorption spectra of Ce 4+ are identical. The effect of UV-exposure on absorption spectra of cerium was studied. An absorption spectrum of photo-ionized Ce 3+ is not identical to that of Ce 4+: UV-exposure induces a new band resulting from Ce 3++ and electron color centers.

Generation and bleaching of intrinsic color centers in photo-thermo-refractive glass matrix

Photo-thermo-refractive (PTR) glass is a photosensitive multicomponent silicate glass that imparts refractive index change after exposure to ionizing radiation and thermal treatment. The origin of this photosensitivity is explained by thermal precipitation of sodium fluoride crystals controlled by atomic silver nucleation centers. These centers are produced by photo-reduction of silver ions resulted from photoionization (photo-oxidizing) of cerium ions. This feature of PTR glass is successfully used for high efficiency phase hologram recording. However, the mechanism of electric charge exchange (transfer) between these ions is not known and the role of intrinsic electron and hole centers was not studied. To elucidate this problem, pure PTR glass matrix with no dopants was prepared and the subsequent absorption spectra of color centers were studied after excitation of the intrinsic absorption edge of glass matrix. Color centers were bleached by thermal treatment and by optical excitation of induced absorption bands. The analysis of the structure of absorption spectra at different stages of coloration and bleaching led to the deconvolution of complex spectra to Gaussian components. Based on this analysis and comparison with simple silicate glasses, induced absorption bands were assigned to different intrinsic electron and hole centers. Interconversion of different centers is also studied.

Sodium Fluoride Solubility and Crystallization in Photo‐Thermo‐Refractive Glass

Photo‐thermo‐refractive (PTR) glass undergoes a refractive index change after UV exposure followed by heat treatment for nanocrystallization of NaF, allowing phase holograms to be permanently recorded in the glass. PTR glass was invented 60 years ago, and optical elements based on PTR glass have been produced for the past decade, but its detailed crystallization mechanism is largely unknown. Since solubility and composition of the parent glass determine the supersaturation and hence the thermodynamic driving force for crystallization, the present paper was aimed at measuring the solubility of NaF in PTR glass via estimating the equilibrium volume fraction of crystallized NaF as a function of temperature. The temperature above which all NaF originally contained in the parent glass remains dissolved in the melt, and that below which all NaF could crystallize, were also estimated. Redesigning of thermal processing parameters and composition of PTR glass, aiming at the improvement of optical elements based on this glass, can be performed in light of the results achieved.

Origin of crystallization-induced refractive index changes in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a multi-component silicate that undergoes localized refractive index decrease after UV-exposure and thermal treatment for partial crystallization. Based on this refractive index change, high efficiency volume Bragg gratings have been developed in PTR glass and have been successfully used for laser beam control. However, despite the fact that this type of glass has been widely studied and used over the last 20 years, the origin of the refractive index change upon crystallization is poorly understood. In this paper, we introduce three possible mechanisms (the precipitation of nano-sized NaF crystals and the associated local chemical changes of the glass matrix, the volumetric changes due to relaxation, and the local residual stresses) for the refractive index decrement in PTR glass and estimate the partial refractive index change due to each mechanism. Refractive index measurements are compared with high temperature XRD experiments and a general approach for the simulation of the refractive index change in PTR glass is proposed. We show that among the studied variables the residual stresses surrounding the crystals are the main responsible for the local refractive index decrement in this glass.

Method to assess the homogeneity of partially crystallized glasses: Application to a photo-thermo-refractive glass

Abstract We describe a new method for the study of both optical and crystallization homogeneity of partially crystallized glasses or glass–ceramics at different spatial scales (from 100 mm to 1 μm). The method relies on the association of different techniques, such as interferometry, optical microscopy and differential scanning calorimetry. The method was tested to probe the optical and crystallization homogeneity of both as-made and UV-exposed, and pre-nucleated samples of a photo-thermo-refractive (PTR) glass. This study demonstrates that pure UV-exposure did not lead to an improvement of the crystallization homogeneity of the glass. However, the benefit of associating UV-exposure and nucleation thermal treatment was clear. These combined treatments permit to homogenize the crystal distribution in PTR glass at millimeter and micron scale. This result is of major commercial interest.

Nonlinear photoionization and laser-induced damage in silicate glasses by infrared ultrashort laser pulses

We show that photoionization of wide band gap silicate glasses by infrared ultrashort laser pulses can occur without laser-induced damage. Two glasses are studied, fused silica and a multi-component silicate photo-thermo-refractive (PTR) glass. Experiments are performed by low numerical aperture focusing of ultrashort laser pulses (100 fsec<τ<1.5 psec) at the wavelengths 780 nm, 1430 nm, and 1550 nm. Filaments form inside both glasses and are visibly observable due to intrinsic luminescence. Keldysh’s theory of nonlinear photoionization is used to model the formation of filaments and values of about 1013 W cm−2 for the laser intensity and 1019 cm−3 for the free electron density are estimated for stable filaments to arise. Laser-induced damage is studied by the generation of a third harmonic from an interface created between a damage site and the surrounding glass matrix. It is found that third harmonic generation occurs only after several thousands of laser shots indicating that damage is not a single-shot phenomena. The ability to photoionize PTR glass without damage by ultrashort laser pulses offers a new approach for fabricating diffractive optical elements in photosensitive glass.