Photo-thermo-refractive Glass Research Articles

Evolution of microstructure and spectral characteristics of silver nanoparticles in photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is widely used in volume holographic devices. After photo-thermo treatment, silver nanoparticles (Ag-NPs) and sodium fluoride (NaF) crystals can precipitate in the glass matrix to realize refractive index modulation (RIM). The Ag-NPs are the foundation for subsequent precipitation of NaF crystals and significantly affect the absorption and luminescence characteristics of PTR glass. Within the nucleation temperature range of 440 °C-480 °C, the Ag-NPs formed by photo-thermo-induced nucleation are within 5 nm. Increasing nucleation heat treatment time causes an increase in the average size of Ag-NPs and the thickness of AgBr shell. Increasing the UV exposure dose significantly increases the concentration of Ag-NPs. The long nucleation time and high UV exposure doses can both increase the content of luminescent silver molecular cluster [Agm]n+, while high nucleation temperatures over 500 °C cause fluorescence quenching of PTR glass due to the formation of large Ag-NPs over 10 nm and NaF crystals. The formation of Ag-NPs effectively reduces the crystallization temperature of PTR glass. The refractive index of PTR glass can be effectively controlled by adjusting the parameters of photo-thermo-induced nucleation. Under the condition of crystallization at 550 °C for 30 min, a maximum RIM value of 1500 ppm can be achieved.

Effect of neodymium ions on the photosensitivity of photo-thermo-refractive glass

Neodymium doping of photo-thermo-refractive (PTR) glass leads to a fast non-radiative depopulation of excited states of cerium ions, which are responsible for the glass photosensitivity. The non-radiative energy transfer from cerium to neodymium ions results in a decrease in the efficiency of photoelectron generation under UV irradiation. Emission spectra and fluorescence decay curves of cerium ions under UV excitation were investigated. Absorption spectra of the glasses were obtained as well to distinguish the reabsorption processes and the non-radiative energy transfer mechanisms. It was found that as Nd2O3 concentration increases from 0 to 2 mol%, cerium ions average lifetime decreases from 44.5 to 28 ns, while the energy transfer efficiency reaches 37 %. It was also found that Förster resonance energy transfer may compete with other relaxation mechanisms and the Förster distance for the Ce3+-Nd3+ pair in PTR glass is ∼0.33 nm. This study has important implications for future research; it informs future studies on optimizing the photosensitivity of the PTR glass and choosing rare-earth activators.

Crystallization heat treatments for the fabrication of volume Bragg gratings based on photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a functional optical material with broad applications in holographic optics. In this work, we examine the effect of crystallization heat treatment on the growth and distribution of sodium fluoride (NaF) nano-crystals in PTR glass using DSC, XRD, SEM, and TEM. The optical properties of the crystallized PTR glass are analyzed using a spectrophotometer. The results indicate that the NaF nano-crystals precipitated in PTR glass matrix are in the form of polycrystalline, and the growth of which is highly dependent on temperature. The growth of NaF nano-crystals saturates during the crystalline phase transition, resulting in decreased distribution uniformity. Higher crystallization temperature and longer crystallization time lead to an increase in absorption and scattering losses in the visible band. However, the crystallized PTR glass exhibits favorable absorption and scattering loss characteristics in the near-infrared band. By adjusting the crystallization treatments within the temperature range of 530 °C–570 °C for 10–120 min, refractive index modulation (RIM) values of the transmitting volume Bragg gratings (TVBG) ranging from 50 to 350 ppm can be achieved, which allows for maximum relative diffraction efficiencies exceeding 90% at 532 nm and 1053 nm using different crystallization processes. This research enhances the understanding of NaF nano-crystal growth mechanisms and provides insights into the application of holographic optical elements based on PTR glass in both the near-infrared and visible bands.

Effect of the SiO2/Na2O ratios on the structure and crystallization properties of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a type of functional optical glass material with multi-component and multivalent particles, which is a substrate material for volume Bragg gratings (VBGs) and widely used in high power laser, holography and other applications. Recording VBGs in PTR glass involves photo-thermo-induced crystallization (UV exposure and subsequent heat treatment), while spontaneous crystallization in the unexposed area is one of the important reasons for the light scattering loss of PTR glass, which can also lead to a decrease in the refractive index modulation and absolute diffraction efficiency of the prepared VBGs. In order to optimize spontaneous crystallization of PTR glass, we analyzed the effects of variations in the SiO2/Na2O ratio on the structure and crystallization behaviors of SiO2–Na2O–Al2O3–ZnO–NaF PTR glass using Raman spectroscopy, FTIR spectroscopy, DSC, XRD, and SEM. As the molar ratio (Rm) of SiO2/Na2O increases from 3.37 to 5.38, the polymerization of the silica network structure is significantly enhanced, the crystallization activation energy of original PTR glass increases from 93.52 kJ/mol to 285.23 kJ/mol, and the crystal growth index decreases from 3.57 to 1.49. In the isothermal crystallization process, we observed that increasing the SiO2/Na2O ratio significantly decreases the grain density and homogenizes the grain distribution of sodium fluoride (NaF) crystals under long-time crystallization conditions, which contributes to maintaining the high transmittance of the crystallized PTR glass in both visible and near-infrared regions.

Effect of ultraviolet exposure and thermal treatment on NaF micro-crystallization mechanism in photo-thermo-refractive glasses

Photo-thermo-refractive (PTR) glass is a promising recording medium for volume-holographic devices. The recording processes involve a series of steps including ultraviolet (UV) exposure, nucleation, and crystallization thermal treatment. Sodium fluoride (NaF) microcrystals precipitate after crystallization, and modulate the refractive index locally of the glass matrix. In this study, to simulate the volume Bragg grating recording processes and study the effect of photothermal-induced processes on the crystallization kinetics in PTR glass, Si-Al-Na-Zn-K PTR glasses with and without Br− ions were prepared using the melt-quenching method. The UV exposure duration, thermal treatment temperature and crystallization time were regulated during subsequent treatment. During these processes, Br-free samples were used for comparison to demonstrate the potential roles of Br−ion in the nucleation and crystallization. The Ag nanoparticle (NP) nuclei surrounded by AgBr (AgF) microcrystals were characterized using UV–visible absorption spectroscopy and transmission electron microscopy, and the changes in the precipitated NaF were characterized using X-ray diffraction, Raman spectroscopy as well as XPS. The mechanism of NaF micro-crystallization is disclosed based on above experimental results. It is found the formation of Ag nanoparticle surrounded by AgBr (AgF) microcrystals inhibits the crystallization kinetics of NaF microcrystals in PTR glass.

Buried Optical Waveguide in Photo-Thermo-Refractive Glass by Ion Exchange Technology

To realize application of functional material Photo-Thermo-Refractive (PTR) glass for integrated photonic device production, PTR glass material is fabricated and employed as matrix material for planar optical waveguide manufacturing. Buried optical waveguide, with a core dimension of 8.7 μm × 9.0 μm (width by height), and its front edge located at approximately 12.5 μm underneath the glass substrate surface, is obtained by using thermal ion exchange (TIE) and subsequent field-assisted ion migration (FAIM). Transmission loss of these waveguides can be reduced to a value as low as 0.08 dB/cm at the operating wavelength of 1550 nm, and the coupling loss to standard single-mode-fiber is measured to be 0.47 dB/facet, demonstrating the feasibility of low-loss integrated photonic devices fabrication on PTR glass materials by ion exchange technology.

Development of bromine-related potentials for molecular dynamics simulations of the oxyhalide photo-thermo-refractive glass

The oxyhalide photo-thermo-refractive (PTR) glass has found various applications in optical devices. However, due to its complex compositions, researches on the structures and structure-property relationship of this kind of materials is still limited. In this work, we applied molecular dynamics (MD) simulations to investigate the structure of the classic PTR glass (excluding Ce, Ag, Sb, and Sn). A set of bromine-related potential parameters has been developed and tested by comparing the simulated crystal structures with those from experiments or ab initio calculations. The PTR glass has then been simulated by using MD simulations with the newly developed potentials. The structure information of the PTR glass, such as the pair distribution function, bond angle distribution, coordination number, and two-dimensional distributions of elements, have been calculated and compared with available experimental data from literature. In addition, the growth of the fluorine/bromine-rich regions in the PTR glass after heat treatment has also been investigated. It is found that the bromine prefers to stay around the fluorine-rich regions and form the phase boundary between the fluorine-rich phase and the oxygen-rich glass matrix. And the presence of sodium ions in the fluorine/bromine-rich regions increase the aggregation tendency of these regions thus lead to the growth of the region size. The results show that the newly developed bromine/fluorine-related potential parameters can describe the PTR glass structures thus provide a new method to help design and improve new applications.

Comprehensive study of spectroscopic and holographic properties of the chlorine-containing photo-thermo-refractive glass doped with neodymium ions

A study of the optical, spectral-luminescent and holographic properties of the chlorine-containing photo-thermo-refractive (PTR) glass doped with neodymium ions was carried out. The chlorine-containing PTR glass has been recently produced by replacing the bromine in the standard PTR glass with chlorine. The glass was doped with different concentrations of neodymium ions. The spectroscopic analysis of neodymium in the glass has been performed using the Judd-Ofelt theory based on experimental absorption and emission measurements. Spectroscopic parameters, such as the Judd-Ofelt intensity parameters, radiative and non-radiative transition probabilities, fluorescence lifetimes, stimulated emission cross-sections, and branching ratios, were obtained and found to be comparable with the parameters of the standard PTR glass doped with neodymium. The mechanism of non-radiative energy transfer was analyzed based on the lifetime kinetics measurements. It was found that cross-relaxation among neodymium ions is the dominant non-radiative energy transfer mechanism.Holographic characteristics of the chlorine-containing PTR glasses doped with neodymium have been also studied. The volume Bragg grating was recorded in the doped and undoped glasses. It was found that the UV exposure and heat treatment do not change the spectroscopic characteristics of neodymium ions in the glasses, while the neodymium ions reduce the sensitivity of the glass. The energy transfer from cerium to neodymium when irradiating using the 325-nm laser makes the hologram recording possible on the samples of low neodymium concentrations only. Studying the spectroscopic and holographic properties of the chlorine-containing PTR glass doped with neodymium ions demonstrates the potential of using such a material in the applications of photonics integration and monolithic optical devices like distributed-feedback lasers.

Effect of photo-thermo-induced nucleation on crystallization properties of SiO2–Al2O3–ZnO–Na2O photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a kind of photo-thermo-sensitive silicate glass. After photo-thermo treatment, it can precipitate sodium fluoride (NaF) crystals in the glass matrix to change the refractive index, which is widely used in volume holographic devices. The purpose of this paper is to study the effect of photo-thermo-induced nucleation on the crystallization kinetics of PTR glass, and several techniques, such as SEM, DSC, absorption spectrum, and XRD, are used to analyze the properties of the induced nucleation process. The results show that photo-thermo-induced nucleation can significantly change the crystallization ability of PTR glass and affect the size, morphology, and volume fraction of precipitated NaF crystals. The reason lies in the change in the composition and number of silver nuclei under different photo-thermo-nucleation treatment systems. According to the obtained results, the photo-thermo-induced nucleation processes of PTR glass will be helpful for improving the properties of volume Bragg gratings (VBGs) and other optical elements based on PTR glass.

A novel photo-thermo-refractive glass with chlorine instead of bromine for holographic application

The well-known standard photo-thermo-refractive (PTR) glass has some limitations in holographic applications in the visible region due to light scattering on the large NaF nanocrystals formed during the photo-thermo-induced (PTI) crystallization process. The large size of the NaF nanocrystals can be reduced by forming more nucleation centers (silver nanoparticles). We suppose this can be achieved by introducing more silver into the batch composition of the glass. However, bromine does not permit high silver concentrations due to the spontaneous crystallization and high Mie-scattering. Therefore, we introduce a novel PTR glass by replacing bromine with chlorine. The novel glass enables us to increase the silver concentration and consequently increase the concentration of the formed nanoparticles. The optical properties, differential scanning calorimetry, and X-ray diffraction analyses were performed on the novel glasses. An experimental and theoretical investigation of the PTI crystallization process was carried out by monitoring the evolution of the surface plasmon resonance (SPR) absorption band. It was found that the UV exposure and heat treatment lead to precipitation of the NaF crystalline phase on a chlorine-containing dielectric shell that surrounds the silver nanoparticle. It was also found that using chlorine instead of bromine leads to a blue shift in the SPR absorption peak location.Volume Bragg gratings were successfully recorded in the novel PTR glasses. The maximum value of the refractive index modulation amplitude obtained for the novel glass is comparable with that for the standard PTR glass.

Versatile approach to laser beam shaping and analyzing by holographic phase masks

A new technology that includes a flexible holographic recording system using photo-thermo-refractive (PTR) glass is demonstrated for creating so-called ‘holographic phase masks’ or HPMs. These diffractive optical elements are permanently recorded in the PTR glass, can be multiplexed as with normal holograms, do not require electric power to operate, and can perform near arbitrary beam phase transformations. The holographic setup includes a spatial light modulator that enables recording transmitting volume Bragg gratings (VBGs) with arbitrary phase patterns encoded into them. As a result, the desirable phase pattern is introduced in a beam that is diffracted by the VBG. These HPMs are tunable within the visible and near IR spectral regions and can be made achromatic, with spectral widths of up to 50 nm. Furthermore, they tolerate laser power densities on the order of several kW cm−2. Applications of HPMs include high-power, broadband laser beam shaping and modal analysis of complex beams profiles, both of which are demonstrated here.

High efficiency multiplexed volume Bragg gratings for angle deflection at 1064nm

The volume Bragg grating (VBG) recorded in the photo-thermo-refractive (PTR) glass has high diffraction efficiency (DE), excellent angle selectivity, multiplexed and flexible design, which is an ideal device to achieve the angle magnification of beam scanning. In this study, a 4-channel multiplexed VBGs with the average relative diffraction efficiency (RDE) greater than 96% at 1064 nm and the maximum discrete angle deflection of 12° was designed and fabricated. And the angle deviation of experiments and design schemes were controlled less than 0.3°. The laser damage of PTR glass and multiplexed VBGs were also tested at 1064 nm. Laser damage tests were performed with a beam diameter of 0.3 mm by 1-on-1 mode. It showed that the laser damage threshold of PTR glass and multiplexed VBGs were 44.33J/cm2 and 30.15 J/cm2 respectively.

Effects of CeO2 and Sb2O3 on the Nonlinear Photochemical Process in Ultrashort Laser Gaussian-Bessel Beams Irradiated Photo-Thermo-Refractive Glass.

Microfluidic chips and optical elements can be fabricated based on the nonlinear photosensitivity in photo–thermo–refractive (PTR) glass by controlling the growth of nanocrystals in the femtosecond (fs) laser–irradiated region. Here, we focus on CeO2 and Sb2O3 that play important roles in UV irradiation, experimentally investigate the effects of the dopants on the nonlinear photochemical process in PTR glass triggered by fs Gaussian–Bessel beams. The results show that the generation of Ag0 atoms and the Ag nanoparticles can be improved by CeO2 and Sb2O3 co–doping. Besides, each multivalent ion in PTR glass possibly participates in the electron transfer processes and contributes to the generation of Ag0 atoms. Finally, X–ray diffraction analysis reveals the precipitation of NaF nanocrystals with an average size of 10 to 12 nm after laser irradiation and thermal treatment, which is unrelated to the dopants.

Photochemical response triggered by ultrashort laser Gaussian-Bessel beams in photo-thermo-refractive glass

Photosensitivity in photo-thermo-refractive (PTR) glass can be triggered by UV and near-infrared fs laser irradiation. Here we focus on the nonlinear photochemical process triggered by ultrashort laser Gaussian-Bessel beams. The transmission and absorption spectra show that the primary difference between UV and fs laser exposure is the formation of color centers and kinetic process of silver nanoparticles growth. It is contributed to the nonlinear ionization of PTR glass matrix and thermal effects during interaction of glass matrix and ultrashort laser pulses. Transmission electron microscopy verifies the generation of nanoscale crystals in the irradiated region, and X-ray diffraction shows the existence of quartz crystal and NaF after laser irradiation and thermal treatment. Moreover, the dependence of photochemical reaction on laser parameters is investigated, as well as the tailoring of silver nanoparticles. On this basis, volume Bragg gratings with ultrashort laser Gaussian-Bessel beams are inscribed as an application which possess good diffraction characteristics.

Tunable photoluminescence of silver molecular clusters formed in Na+-Ag+ ion-exchanged antimony-doped photo-thermo-refractive glass matrix

Influence of Na+-Ag+ low-temperature ion exchange and subsequent heat treatment durations on the spectral-luminescent properties of silver molecular clusters formed in antimony-doped photo-thermo-refractive (PTR) glass matrix was studied. The ion exchange of PTR glass leads to long-wavelength shift of the ultra-violet (UV) edge of strong absorption. The shift magnitude is shown to linearly depend on the square root of the ion exchange duration. Increase in the ion exchange duration is shown to result in changing photoluminescent properties of silver molecular clusters, which allows the tuning of the emission color from cool and warm white to yellow and orange. Model for growth of silver molecular clusters in Na+-Ag+ ion-exchanged layers of the antimony-doped PTR glass matrix was proposed based on the observed changes in spectral-luminescent properties of the samples. The studied PTR glass samples doped with silver molecular clusters are characterized by photoluminescence quantum yield up to 57%, which paves the wave for the development of solid-state light-emitting devices and spectral converters with tunable emission spectrum.

Spectroscopic and laser properties of erbium and ytterbium co-doped photo-thermo-refractive glass

Spectral-luminescent characteristics of erbium ions in photo-thermo-refractive (PTR) glasses doped with different concentrations of ytterbium and erbium oxides were investigated in this work. X-ray diffraction pattern of studied glasses was carried out. Judd-Ofelt parameters of intensity Ωt were obtained and used for deriving emission cross-sections and radiative lifetimes. It was found that the peak emission cross-sections values at a wavelength of 1.53 μm obtained from F-L and McCumber methods are 0.59 × 10−20 cm2 and 0.60 × 10−20 cm2, respectively, which are very similar and compared with those of silicate glasses. A quite high value of quantum yield at the wavelength of 1.53 μm for the sample with 0.1 mol% Er2O3 co-doped with ytterbium oxide has been investigated. Energy transfer efficiency from ytterbium ions to erbium ones was found to be 80% and 86% for erbium- and 1 mol% and 2 mol% Yb2O3-co-doped PTR glass, respectively. Laser properties of 2Yb-01Er PTR glass were carried out. The gain spectrum starts to be positive when the population inversion parameter gets to 0.5. Laser action was demonstrated at room temperature. It was found that the round-trip losses of the Er/Yb-PTR element are 0.28%.

Preparation and optical properties of boron-doped Si-Na-Al-Zn photo-thermal-refractive glass

Photo-thermo-refractive (PTR) glass, which has the advantages of high refractive index modulation value, large working bandwidth and wide environmental adaptability, is an excellent material for preparing volume Bragg grating (VBG). However, the application of PTR glass is limited because of its high melt processing temperature and high optical loss. The introduction of boron into PTR glass can significantly reduce the melting temperature. In this study, boron-doped Si-Al-Na-Zn PTR glass was prepared at a low temperature by the high-temperature melting method, and its temperature–viscosity relationship, heat treatment procedure and optical properties were investigated. The results show that the introduction of boron into PTR glass can significantly reduce the high-temperature viscosity of PTR glass. The melting temperature is lower than 1,400 °C, and the optimum nucleation and crystallisation temperatures are 490 °C and 590 °C, respectively. The optical transmittance of UV-exposed and heat-treated glass is more than 90%, which indicates its excellent optical properties and its applicability for use in preparing VBG with high diffraction efficiency and low optical loss.

The effect of UV irradiation and thermal treatments on structural properties of silver-containing photo-thermo-refractive glasses: Studies by Raman spectroscopy

Structural changes in silver-containing photo-thermo-refractive (PTR) silicate glasses during photo-thermo-crystallization process were studied by methods of Raman spectroscopy. Raman spectra were measured in frequency ranges 300-700 cm−1 and 900-1300 cm−1 after UV irradiation and subsequent thermal treatments at T = 360 OC and 510 OC. Raman spectra of silver-containing PTR glass were compared with Raman spectra of glasses without Ag, and glasses without Ag, Ce and Sb. It was shown that mentioned external treatments result in the sequence of structural changes, which may be interpreted as breaks and restoration of siloxane rings, appearance of new rings, appearance and disappearance of non-bridging oxygen centers. These processes can influence on the features of photo-thermo-crystallization processes in PTR glasses.

Optical, spectroscopic properties and Judd–Ofelt analysis of Nd3+-doped photo-thermo-refractive glass

A comparative study of spectral-luminescent and Judd–Ofelt analysis of photo-thermo-refractive (PTR) glasses doped with a variable content of neodymium (0.5–2.11 mol%) has been carried out for the first time. The change of Judd-Ofelt parameters (Ω2, Ω4, Ω6), radiation lifetime, experimental lifetime, branching ratio and emission cross sections were demonstrated as a function of Nd2O3 concentration and compared to the equivalent parameters in other works. It was found that the 4F3/2 → 4I11/2 transition is enhanced in Nd-doped PTR glass. The quantum efficiency of 0.5 mol% Nd-PTR for the same transition is 90.3% and the induced emission cross section is 1.743 × 10−20 cm2. The saturation intensity for 4F3/2 → 4I11/2 laser transition in the same sample is 20.62 kW/cm2, it is lower than that in some commercially produced glasses of same Nd3+-concentration. Laser action of a 0.5 mol% Nd-PTR was demonstrated, it was found that the round-trip loss of this gain medium is 0.34%, this value is quite low, and it is comparable to that in some commercially fabricated Nd-YAG crystals.

Short- and Medium-Range Order in Photothermal Refractive Glass Revealed by Solid-State NMR Techniques

Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive Na2O–ZnO–Al2O3–SiO2 glass, containing NaF and KBr additives, along with cerium, silver, tin, and antimony oxide dopants. After heating above 500 °C, UV-exposed regions of this glass produce permanent refractive index changes, resulting from precipitation of NaF nanocrystals. Short- and medium-range order of this glass system is studied via multinuclear single- and double-resonance solid-state nuclear magnetic resonance (NMR) spectroscopy in regular PTR glass and in model glasses with simplified compositions. The results, when combined with data from energy-dispersive X-ray spectroscopy, indicate that the NaF component modifies the standard aluminosilicate framework, producing small amounts of F-bonded five- and six-coordinated aluminum species. The fluoride speciation is obtained from 19F magic-angle spinning NMR spectra, supported by 19F{27Al} and 19F{23Na} dipolar recoupling experiments. The majority of fluoride within the PTR glass is found within Na-dominated local environments, which also interact strongly with the aluminum. 23Na{19F} rotational echo double resonance reveals that about 1/3 of the Na+ ions have fluoride ions in their first coordination spheres.