Permanent Refractive Index Change Research Articles

Multiphoton process investigation in silica by UV femtosecond laser

We investigated the interaction processes between high intensity femtosecond ultraviolet laser pulses and amorphous silica, leading to permanent refractive-index changes that are at the basis of advanced manufacturing for photonics devices. The experiment, carried out as a function of the laser power, improves our understanding on the strong-field ionization process by the monitoring of the 1.9 eV and 2.65 eV emissions, related to nonbridging oxygen hole centers and self-trapped exciton, respectively, induced in the exposed glass region. Our results clearly proved that the UV laser light band-to-band absorption is allowed in the multiphoton ionization limit, whose consecutive relaxation leads to the generation of these photoluminescence (PL) signatures. Furthermore, we coupled the online PL investigation with post mortem analysis of the irradiated volume through phase contrast microscopy, Raman and steady state PL, providing a complete view of the silica-femtosecond laser interaction.

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.

Study of femtosecond laser writing in the bulk of Nd3+, Y3+ co-doped CaF2 crystals

We reported on the femtosecond (fs) laser induced permanent refractive index changes, stress-induced birefringence, and micro-photoluminescence properties of Nd3+, Y3+ codoped CaF2 crystals. Permanent changes in the linear optical properties were studied for various pulse energies (0.01 - 5 µJ) at two different repetition rates (10 kHz and 500 kHz). Optimum quantitative phase changes are close to - 1.7π radians within an irradiated area, surrounded by positive phase changes due to the stress field that can reach + 0.7π. Provided that, we avoid heat-accumulation at 500 kHz, written laser lines offer higher linear birefringence on the order of λ/4 at 200 nm. From the spectroscopic point of view, we observed the Raman changes and micro-photoluminescence spectra of irradiated fs-laser Nd3+, Y3+ doped CaF2.

Self-inscribed antisymmetric long-period grating in a dual-core As2Se3-PMMA fiber.

We report for the first time that transmission of optical pulses centered at a wavelength of 1550 nm through a tapered dual-core As2Se3-PMMA fiber inscribes an antisymmetric long-period grating. The pulse power is equally divided between even and odd modes that superpose along the dual-core fiber to form an antisymmetric intensity distribution. A permanent refractive-index change that matches the antisymmetric intensity distribution is inscribed due to photosensitivity at the pulse central wavelength. The evolution of the transmission spectrum of the dual-core fiber is experimentally measured as the accumulated time that the fiber is exposed to the pulse is increased. A theoretical model of an antisymmetric long-period grating in a dual-core fiber computationally reproduces the experimentally observed evolution of the transmission spectrum. Experimental results indicate that antisymmetric long-period gratings induce effective group-velocity matching between the even and odd modes of the dual-core fiber, and reveal for the first time that long-period gratings can lead to slow light propagation velocities.

Fabrication of long-period fiber gratings using end-coupled ultrafast laser pulses.

We propose a novel technique for the fabrication of long-period fiber gratings (LPFGs) using ultrafast laser pulses coupled into opposite ends of an optical fiber. The pulses counter-propagate in the fiber and induce a permanent refractive index change where they overlap. Based on absorption rate measurements, it was confirmed that the pulses were partially absorbed at the overlapping position, with a maximum absorption rate of 6%. Furthermore, a LPFG with a smooth transmission spectrum and low background loss was successfully fabricated using this technique.

A review of the photo-thermal mechanism and crystallization of photo-thermo-refractive (PTR) glass

ABSTRACTPhoto-thermo-refractive (PTR) glass is an optically transparent photosensitive multicomponent silicate glass. UV-exposure produces silver nanoclusters, and NaF nanocrystals upon heating, giving rise to permanent refractive index changes. This is the basis for the production of Bragg gratings. In this article, we present a comprehensive overview of the photo-thermo-induced effects in PTR glass. We first give a general introduction of PTR glasses and their applications. Then, we perform a thorough analysis of the mechanisms of refractive index changes starting with the photo-induced process followed by the crystal nucleation mechanism and NaF crystallisation kinetics. Finally, we concentrate on the PTR glass nanostructure and its effect on the crystallisation mechanisms and properties, finishing with the kinetics and origin of the refractive index change. Despite the fact that much is known about this septuagenarian glass and that it is already being commercially produced, a number of open problems still exist that warrant further investigation.

UV–vis spectroscopic studies of CaF2 photo-thermo-refractive glass

A photo-thermo-refractive glass based on the system Na2O/K2O/CaO/CaF2/Al2O3/ZnO/SiO2 doped with Ag2O, CeO2, SnO2, Sb2O3 and KBr was investigated. This glass undergoes a permanent refractive index change after UV irradiation and subsequent two step heat treatment at temperatures above Tg. This is due to the formation of Ag metal clusters which act as nucleation centers for CaF2 crystallization. Oxidation of Ce3+ by UV light is the initial reaction and acts as photosensitizer in the glass. The UV–vis absorption spectra during this photo-induced crystallization process were measured. The spectral components that form the absorption spectra of cerium were studied in detail by a band separation with Gaussian functions. Deconvolution of the cerium absorption bands shows an envelope of five spectral components for the trivalent cerium due to the 4f-5d transitions and two spectral components for the tetravalent cerium caused by charge transfer transitions. The effect of different dopants and melting conditions on the photo-thermal process were studied to investigate the influence of glass technology on the photoprocess.

Electron Paramagnetic Resonance (EPR) studies on the photo-thermo ionization process of photo-thermo-refractive glasses

Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive sodium alumino silicate glass, containing NaF and KBr additives, along with cerium, silver, tin and antimony oxide dopants. UV-exposed regions of this glass produce NaF nanocrystals upon heating, giving rise to a permanent localized refractive index change. In this article we examine the initial stages of this crystallization process by continuous-wave and pulsed X-band electron paramagnetic resonance (EPR) spectroscopy. UV exposure of PTR glass produces unpaired electrons whose EPR spectrum is characterized by pronounced peak splitting arising from nuclear magnetic hyperfine interactions with spin-5/2 and spin-7/2 nuclei suggesting close proximity of the unpaired electrons with 121Sb and 123Sb nuclei. These results indicate that the Sb2O3 dopant plays a key role in the initial stages of the crystallization mechanism. Upon thermal annealing, leading to the crystallization of NaF, these species disappear, indicating their transient nature. A number of other unpaired electron species identified in the dopant free matrix appear to be unrelated to the crystallization process. These results clearly challenge the classical mechanism proposed decades ago to explain the complex crystallization process of PTR glass. Together with more recent results from optical spectroscopy they support the Nikonorov model involving (1) photoionization of Ce3+, (2) transfer of this electron to Sb5+ species to create a Sb4+ species, (3) upon annealing electron transfer from Sb4+ to Ag+ ions, producing silver atoms, (4) coalescence of these species into Ag clusters, which (5) serve as nucleation catalysts for NaF nanocrystals.

Threshold for permanent refractive index change in crystalline silicon by femtosecond laser irradiation

An optical damage threshold for crystalline silicon from single femtosecond laser pulses was determined by detecting a permanent change in the refractive index of the material. This index change could be detected with unprecedented sensitivity by measuring the resonant wavelength shift of silicon integrated optics microring resonators irradiated with femtosecond laser pulses at 400 nm and 800 nm wavelengths. The threshold for permanent index change at 400 nm wavelength was determined to be 0.053 ± 0.007 J/cm2, which agrees with previously reported threshold values for femtosecond laser modification of crystalline silicon. However, the threshold for index change at 800 nm wavelength was found to be 0.044 ± 0.005 J/cm2, which is five times lower than the previously reported threshold values for visual change on the silicon surface. The discrepancy is attributed to possible modification of the crystallinity of silicon below the melting temperature that has not been detected before.

Requirements on glasses for femtosecond-laser based micro-structuring

In this work, glasses with systematically varied compositions were manufactured and irradiated by single Ti:sapphire fs-laser pulses (800 nm, 120 fs), focused at the surface and into the bulk of the glass materials. The samples were tested for their ablation threshold fluence as well as for structural changes using μ-Raman-spectroscopy. Correlations between the glass composition, the material-ablation on the glass surface and the permanent changes of the refractive index inside the glass volume after the irradiation by fs-laser pulses were obtained. The results show, that the structural modifications found at the surface of the glasses and inside its volume are closely related. However, while the ablation threshold fluence of the glass surface primarily depends on the glass dissociation energy, the permanent refractive index change inside the volume is rather determined by its ability for absorbing the fs-laser pulses and the subsequent relaxation processes. The results of this work provide some guidance on how the glass composition can be varied in order to optimize the fs-laser induced modification of dielectrics.

Femtosecond laser writing in the monoclinic RbPb2Cl5:Dy3+ crystal

Monoclinic RbPb2Cl5:Dy single crystal was tested for femtosecond laser writing at wavelength of 800nm. Dependence of permanent refractive index change upon input pulse energy was investigated. Non-linear coefficients of multiphoton absorption and self-focusing were measured. Kerr non-linear coefficient was found to be as high as 4.0∗10−6cm2/GW.

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.

Er 3+ doped waveguide amplifiers written with femtosecond laser in germanate glasses

The authors report the fabrication and characterization of active waveguides in GeO 2–PbO–Ga 2O 3 glass samples doped with Er 3+, written with a femtosecond laser delivering pulses of 150 fs duration at 1 kHz repetition rate. Permanent refractive index change was obtained and waveguides were formed under different laser pulse energies and scan velocities. The passive and active optical properties of the waveguides were investigated. The minimum value of propagation loss was of 4.8 dB/cm. Optical amplification at 1.5 μm under 980 nm excitation was observed showing a maximal internal gain of 2.7 dB/cm.

Time-resolved interferometry of femtosecond-laser-induced processes under tight focusing and close-to-optical breakdown inside borosilicate glass

We use an interferometric time-resolved observation of a femtosecond-laser pulse (800 nm/45 fs) interaction with glass from 100 fs to 10 ns at spatial lateral resolution down to the wavelength of the pulse. The phase and amplitude images reveal sequence of events after the irradiation of a single ultra-short laser pulse at close-to-threshold intensity when permanent refractive index changes occur. The proposed method is applicable to characterization of the processes induced by tightly focused fs-laser pulses during three-dimensional structuring of glasses and crystals for fundamental studies and optical applications. Generation of carriers, thermal expansion, generation and propagation of shockwaves, and formation of refractive index changes are experimentally observed and resolved in time and space with the highest resolution. Quantitative estimations of the threshold energies of different processes are achieved. The threshold energy of carrier generation is found the same as that of shockwave generation while the threshold energy of refractive index changes was by 40% higher. Application potential of the method is discussed.

Photosensitivity in GeO 2–SiO 2 glasses and optical waveguides

Permanent refractive index changes caused by a KrF excimer laser operating at 248 nm in GeO 2–SiO 2 glasses deposited on Si (100) substrates using flame hydrolysis deposition (FHD) are presented. The sample was amorphouslike investigated by X-ray diffraction (XRD), and the ratio of Ge:Si is 14:86 from X-ray photoelectron spectroscopy (XPS) analysis. The 10-min irradiation with a KrF excimer laser (10 Hz, 187 mJ/cm 2) induced a positive refractive index change of 0.341% at 1550 nm, which has achieved an international level of this field. An innovative photomask with a Cr-loaded structure coated on a UV quartz glass, was used to fabricate a 50 μm gap with a period of 100 μm waveguide grating under 1460 mJ/cm 2/pulse at 6 Hz, and the diffraction patterns were observed clearly. The extinction coefficients of the samples are also measured over the range 250–1600 nm.

Photoinduced Changes in Sn-Doped Sol-Gel Silica Glass Films

Sn-doped SiO2 thin films were fabricated on Si substrates by the Sol-Gel method. Permanent refractive index change was found using ultraviolet (UV) laser radiation at 248 nm. The photoinduced refractive index change increased after radiation and variety in the contents of Tin. The optical and surface properties of the films were characterized using X-ray photoelectron spectroscopy (XPS) and variable angle spectroscopic ellipsometry (VASE).

Trimming phase and birefringence errors in planar lightwave circuits with deep ultraviolet femtosecond laser

A deep ultraviolet femtosecond laser was employed to trim phase and birefringence errors in silica planar lightwave circuits. A permanent refractive index change of ∼3.8×10−4 and a birefringence change of 1.0×10−4 were induced in hydrogen-free Mach-Zehnder planar waveguide circuits. The ultrafast laser enhances the ultraviolet photosensitivity response in silica waveguides by two orders of magnitude greater than that of a nanosecond 248 nm KrF excimer laser.

Femtosecond Micromachining Applications for Optical Devices

This paper investigates applications of femtosecond lasers for the micromachining of transparent materials and fabrication of optical devices. We show commercial micromachining examples of transparent materials which have been fabricated for various applications. Near infrared femtosecond laser processing is an attractive method to fabricate three-dimensional optical waveguides into various transparent materials. Focused femtosecond laser pulses induce a permanent refractive-index change only near the focal point. We also demonstrate a Y coupler with the splitting ratio of 1:1 written by femtosecond laser pulses into a fused silica glass. The minimum propagation loss of 0.8 ㏈/㎝ awl the refractive-index change of 0.006-0.01 at the wavelength of 1550 ㎚ were achieved by optimization of the laser fluence.

Photon sensitive high index metal oxide films

In this work, we review the electrical and optical properties ofInOx and ZnO films, and their use as dynamic optical materials. The transport properties ofthese films are analysed and correlated with the growth parameters. We havestudied the permanent light-induced refractive index changes in our dc-sputteredInOx thin films using pulsed ultraviolet laser radiation at 193 nm.Non-permanent holography recording of information has been achieved inInOx films upon illumination with UV radiation (325 nm). The recording, which appears to beindependent of the electrical state of the material, disappears in the absence of UV lightwith decay times that depend on the material’s specific properties, as these are influencedby the film fabrication conditions. We also review our work on waveguide reflectionsubmicron relief gratings on multilayer waveguides operating near 1550 nm, fabricated byexcimer laser ablation at 248 nm.

Waveguide formation in niobium tellurite glasses by pico- and femtosecond laser pulses

We obtained a temporal and permanent refractive index change in niobium tellurite glasses by using ps- and fs-pulses, respectively. A He–Ne laser beam was guided by a line waveguide structure drawn by the fs-pulses, indicating positive refractive index change. It is suggested that the refractive index change by the ps-pulses is due to an increase in the absorption coefficient while that caused by the fs-pulses results from the rearrangement of the glass structure.