Nanofabrication in transparent materials with a femtosecond pulse laser

Femtosecond lasers have been applied for materials processing when high accuracy and small structure size are required. Various induced structures have been observed inside glasses after the femtosecond laser irradiation. We report the femtosecond laser induced refractive-index change, space-selective valence state change of active ions, formation of nanograting, and precipitation and distribution of nanoparticles. We systematically studied the morphology of structures that are induced in the bulk of transparent materials by the tightly focused femtosecond laser radiation. Rugby-ball-like asymmetric induced structures were observed inside Ag⁺-doped silicate glass. These structures are due to the aggregation of Ag nanoparticles at the depth of the focal point. The size of the induced structure depended on the time interval between successive femtosecond laser pulses. In the case of zinc-tellurite glass, TeO₂ rich parts were formed in the center of the focal spot, while zinc migrated to the outside. The mechanisms of the observed phenomena are discussed.     

The structural and refractive index changes in the waveguides written by femtosecond laser in Er-doped silicate glasses

Waveguides with low propagation losses were written in erbium-doped silicate glasses by tightly focused femtosecond laser pulses. Raman spectra measurements and 2-D refractive index mapping in the micro-areas near the focus were carried out. The results supported that the refractive index change induced by fs laser pulses comes from the increase of the low rank ring structures in the silicate network.     

Preparation and optical properties of silicate glasses containing Pd nanoparticles

We report the space selective precipitation of Pd nanoparticles in Pd²⁺-doped silicate glass by ultrashort laser pulses irradiation and further annealing. Absorption spectra, transmission electron microscopy, refractive index measurement and Z-scan technique demonstrated that metallic Pd nanoparticles were precipitated in the glass sample after irradiation by an 800-nm femtosecond laser and subsequent annealing at 600 °C. We discuss a refractive index change and nonlinear absorption that combines the precipitation of Pd nanoparticles.     

Ultrafast modification of elements distribution and local luminescence properties in glass

The success in construction of three-dimensional micro optical components or devices inside transparent materials is highly dependent on the ability to modify materials’ local structure. Especially, the realization of space-selective manipulation of element distribution is highly desirable since most of optical parameters such as refractive index and luminescence are closely related to element distribution. Up to present, the only way to control selective element distribution is local melting of glass. Here, we reported, for the first time to our knowledge, the success in realization of space-selective manipulation of element distribution in glassy state region (i.e., un-melted region) inside glass with the irradiation of high repetition rate femtosecond laser. Confocal fluorescence spectra and micro-Raman spectra show that the luminescence distribution of Cu⁺ ions and the glass network structure can be controlled with femtosecond laser irradiation, revealing the potential applications of this technique in the fabrication of functional waveguides and integrated optical devices.     

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystal

Focused infrared femtosecond laser pulses (wavelength ∼800 nm, emission pulse duration 100 fs) were employed to fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F₂ color centers of about 2 × 10¹⁸ cm⁻³ and refractive index change of about 1% at 633 nm were induced by the fs-laser irradiation. This technique was applied to fabricate a distributed-feedback (DFB) F₂ color center laser structure inside LiF single crystal. The LiF DFB laser exhibited laser oscillation at 707 nm at room temperature. The slope efficiency of ∼10% and beam divergence of ∼20 mrad were achieved.     

Femtosecond laser-induced microfeatures in glasses and their applications

In the past decades, femtosecond laser has been widely used to study the interaction between light and matter, and dynamic processes of various reactions. In this article, we review femtosecond laser-induced microfeatures in glasses and their promising applications in micro-optics and optoelectronics. Various highly localized microstructures, e.g. color center defects, refractive index change, micro-void and micro-crack, have been observed in the glasses after the femtosecond laser irradiation. The mechanisms of the observed phenomena are discussed. We also demonstrate the fabrication of various micro-optical components, e.g. three-dimensional optical memory, optical waveguide, micro-grating, micro-lens and fiber attenuator, by using the femtosecond laser-induced structures.     

High refractive index contrast in fused silica waveguides by tightly focused,high-repetition rate femtosecond laser

A new domain of optical waveguide writing with record high refractive index contrast (0.022) is reported in fused silica by strong focusing of a 522 nm wavelength, 500 kHz repetition rate femtosecond laser with oil-immersion optics. The strongly confining waveguide supports a mode of only 7 μm mode field diameter at 1550 nm wavelength, opening the door for higher density integration in photonic circuits formed by femtosecond lasers. It is found that green and fundamental wavelengths have similar absorption in femtosecond laser waveguide writing in fused silica and that the advantage of the second harmonic is simply from an increased fluence through a smaller focal volume.     

Influence of deposition parameters on composition and refractive index of femtosecond and nanosecond pulsed laser deposited gallium lanthanum oxysulphide

The influence of the deposition parameters on femtosecond and nanosecond pulsed laser deposited gallium lanthanum oxysulphide (GLSO) glass films has been investigated. A comparison between films deposited by femtosecond and nanosecond pulsed laser deposition (PLD) shows that the compositional range of each ablation regime varies significantly; in particular, femtosecond PLD shows a unique potential for selective fabrication of films with a high lanthanum content well outside the conventional glass-melting region. We demonstrate how manipulation of the PLD growth parameters can influence the stoichiometric transfer of the PLD process, leading to films with compositions that differ significantly from the GLSO target material. We also reveal how the refractive index of as-deposited films is dependent upon the composition and briefly discuss the thermal properties of bulk GLSO material of various compositions which indicate the potential for films grown by PLD to be used in optical data-storage device applications.     

Fabrication of buried waveguides in planar silica films using a direct CW laser writing technique

A CW CO₂ laser ablation technique is used to form buried waveguides in planar silica films. It is shown that the refractive index of a silica thin film is reduced sufficiently adjacent to the laser processed region to allow the fabrication of low loss waveguides. The refractive index distribution of these structures is measured using the reflectance of a focussed spot from the surface of the films. The change in refractive index is measured to be of the order of the core cladding refractive index difference of typical single mode waveguides. The spatial resolution of the reflectance technique is 1.3 μm with a refractive index resolution of ±5 × 10^?4. Devices such as 1 × 2 and 1 × 4 multi-mode interference (MMI) splitters have also been demonstrated and shown to exhibit low transmission losses.     

Thermal lens and Z-scan measurements: Thermal and optical properties of laser glasses – A review

In this paper, the application of thermal lens and Z-scan techniques to the study of the thermo-optical and spectroscopic properties of solid-state laser glasses is described. The theoretical basis for quantitative measurements of thermal diffusivity and conductivity, temperature coefficient of the optical path length change, heat efficiency, fluorescence quantum efficiency, losses mechanisms (Auger upconversion and concentration quenching) and the line shape of the nonlinear refractive index are presented and discussed. The electronic contribution to the nonlinearity was investigated using the Z-scan technique in the time-resolved mode. The measurements were performed spectroscopically, allowing the determination of the line shapes of real and imaginary parts of the nonlinear refractive index, n₂, in resonance with laser transitions. The results were interpreted by considering resonant and nonresonant contribution to n₂. The magnitude of electronic and thermal contributions to the refractive index changes in solid-state laser glasses were also compared, and the thermal properties as a function of the temperature in the range of 20 up to 600 K are presented.     

Optical properties and valence change of samarium ions in a sol–gel Al2O3–B2O3–SiO2 glass by femtosecond laser irradiation

10Al₂O₃–5B₂O₃–85SiO₂–xSm₂O₃ glasses were prepared by the sol–gel method. The emission spectra of the glasses indicate that the quench concentration of the Sm³⁺ ions is about 0.2 mol%. The emission spectra of the glasses after high-temperature treatment with H₂ gas exhibit the coexistence of the Sm³⁺ and Sm²⁺ ions. We observed the strong emission line of the Sm²⁺ ions and the emission band of the non-bridging oxygen hole center when the glasses were exposed to a femtosecond laser. It indicates that some Sm³⁺ ions were reduced to Sm²⁺ ions by femtosecond laser pulses and non-bridging oxygen hole centers were formed. The ⁵D₀–⁷F₀ emission line of the Sm²⁺ ions by femtosecond laser irradiation shows a red shift, compared with the emission of the Sm²⁺ ions by reduction with H₂ gas. The strong absorption band and weak, sharp absorption lines in the range from the UV to IR come from charge transfer and the transition from the ⁶H_5/2 state to the various excited states of the Sm³⁺ ions. The reduction mechanism of Sm³⁺ ions is discussed.     

Influence of γ-Irradiation on Some Properties of NaCl: Eu++ Crystals (I). Optical and Dielectrical Characteristics of the Dopant Ions

The present work opens a series of papers describing a variety of properties of the same material, and deals with the effect of Eu⁺⁺ ions, and y–irradiation on the optical and dielectrical characteristics of NaCl crystals. For nonirradiated crystals four types of Eu⁺⁺-related absorption spectra have been identified and correlated with I.V. dipoles and three types of precipitates stable in the well aged crystals. For the solution—treated samples the effect of irradiation was to diminish the concentration of I.V. dipoles and to change the absorption spectrum in a remarkable way. These changes are explained in terms of the formation of some dopant—related clusters different of those obtainable thermally. No spectroscopic evidences have been obtained for the radiation—induced changes of the valence state of the dopant irrespective of its dispersion degree.     

Photosensitive erbium doped tin-silicate glass

Erbium doped tin-silicate samples were prepared by sol-gel method. Optical absorption, infrared photoluminescence at 1.5 μm from ⁴I_13/2-⁴I_15/2 Er³⁺ electronic transition and data of refractive index change after exposure to 266 nm pulsed radiation were collected. The results show that Er-doped tin-silicate can be produced by the proposed sol-gel method with photosensitivity comparable to that of only tin-doped silica, showing refractive index changes up to 4×10⁻⁴.     

Synthesis,characterization and local changes induced by a cw argon laser in silica glass xerogel doped with transition metal ions

We report on the synthesis, characterization and local changes induced by a cw argon laser in SiO₂ xerogel doped with low metal ion concentrations, and obtained by the sol–gel process. The V, Mn and Cr metal ions were introduced into the SiO₂ matrix as inorganic salts and as an oxide in the case of Mo. The characterization of these materials showed that an amorphous system with high porosity was obtained; the metals were incorporated as ions with several different oxidation states, and the thermal diffusivity was around 10⁻⁹ m²/s. In these xerogel monoliths it is possible to induce local changes in the refractive index with the incidence of a focused laser beam at 488 nm, about 80 μm in diameter, with power intensities from 1 to 48 mW and an incidence time from 0.5 to 5 min. After this process, a high contrast in the transmitted light between the processed area and the homogenous matrix is obtained, resulting in an optical memory effect.     

Periodic modulation on the refractive index in a Bi12TiO20 crystal

It is, well know that some, optical materials to change his refractive index when these exposed to light, depending on particular conditions, they can generate several distributions of refractive index. In this work, an experimental periodic arrangement of refractive index generated on the Bi₁₂TiO₂₀ photorrefractive crystal is presented, the initial condition used was generated by superposing two He‐Ne laser linearly polarized beams in a perpendicular direction to the external field applied to the crystal at 632.8 nm. The diagrams of bands generated with this arrangement is studied, based on the similarities presented by the periodic refractive index with the photonic bandgap (PBG) diagrams one‐dimensional superlattices composed of alternating layers of two distinct materials. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vacuum ultraviolet absorption spectrum of photorefractive Sn-doped silica fiber preforms

Vacuum ultraviolet (VUV) absorption data have been obtained up to 8.3 eV on Sn-doped silica fibre preforms. Measurements have been carried out before and after exposure to 248 nm radiation from KrF excimer laser. The absorption spectrum is composed of three structures peaking at about 4.9, 5.8 and 7 eV, with the absorption edge at about 8.2 eV. The main effect of 5 eV irradiation is the decrease of the spectral components at 4.9 and 7 eV, whereas a small increase of absorption intensity is only observed just below the band at 4.9 eV. According to the observed negative absorption changes in the whole region of point-defect bands one would expect a negative refractive index change, contrary to the positive change previously reported in optical fibres of the same composition. Structural modifications accompanying the defect photoconversion process are suggested to be responsible for positive refractive index changes.     

The leaching behavior of PbOSiO2 glasses

The leached layer of PbOSiO₂ glasses formed by diluted nitric acid solution has been investigated by ellipsometry and Auger electron spectroscopy (AES). The leaching behavior of PbOSiO₂ glasses in 10^?4 N aqueous solution of NHO₃ at 30°C was measured in real time using a Nikon auto-ellipsometer.The results were applied by curve fitting to the two-layer model from the concentration profile obtained by AES, and the refractive index profile against the film thickness was determined.The leached layer is inhomogeneous and consists of a low refractive index region and a transition region. The gradient of the refractive index in the former region is extremely small and the refractive index becomes nearly constant between 1.42 and 1.44. The shape of latter region becomes stable with its thickness at 100–310 Å, and moves in the direction of depth without changing the shape as the leaching proceeds.     

Investigation of optical parameters of Ag-In-Se thin films deposited by e-beam technique

The optical properties of the Ag-In-Se (AIS) thin films deposited by e-beam technique were investigated by means of the optical transmittance measurements. The optical absorption coefficients of the films were found to vary from 10³ to 10⁵ cm⁻¹ over the wavelength range of 300-1100 nm. The real and imaginary parts of the refractive index and dielectric constant for the as-grown and annealed films in between 100 and 400 °C were evaluated by means of both envelope method (EM) and continuous wavelet transform (CWT) method and the results were in quite good agreement with each other. The refractive index, n, dispersion over the measured wavelength range was explained by applying single-oscillator model (SOM) and the related parameters were calculated. The optical absorption process for the AIS thin films was characterized by three direct transitions from three closely spaced valence bands to a single conduction band due to the splitting of the valence band under the influence of the tetragonal crystalline field and spin-orbit interaction. The direct optical band gap energy decreases as the annealing temperature increases because of the increase in the width of band tail states near valence-band edge caused by the Se segregation. The two distinct parameters of the quasicubic model; crystal-field splitting, Δ_CF, and spin-orbit splitting, Δ_SO, were calculated for as-grown and annealed AIS thin films.     

Optical Field Induced Scattering in Polymer Dispersed Liquid Crystal Films

Abstract

Polymer dispersed liquid crystals are composite materials consisting of inclusions of liquid crystalline materials dispersed in a polymer binder. If the refractive indices of the constituent liquid crystal and polymer are appropriately matched, then films of these materials may be switched between an optically scattering state and a non–scattering transparent state^1?2 by the application of electric fields which reorient the liquid crystal in the inclusions. In this paper we discuss the response of these materials to intense laser radiation, and examine the mechanisms associated with optical field induced reorientation.