Fused Silica Plates Research Articles

Production of submicrometre fused silica gratings using laser-induced backside dry etching technique

Laser micromachining of transparent materials is a promising technique for producing micro-optical elements. Several types of both direct (e.g. ablation) and indirect (e.g. laser-induced backside wet etching: LIBWE) procedures have already been developed and presented in the last two decades. Here we present a new method (laser-induced backside dry etching (LIBDE)) in the analogy of LIBWE for the micro and nanoprocessing of transparent materials.In our experiments 1 mm thick fused silica plates were used as transparent work pieces. The plates were covered with 100 nm thick silver layers. The metal absorbing films were irradiated through the fused silica by a KrF excimer laser beam (λ = 248 nm, FWHM = 30 ns). The illuminated area was 1.05 mm2 and the fluence on the silver–quartz interface varied in the range 0–1800 mJ cm−2. We have provided evidence that LIBDE is more effective and simple than LIBWE, its etch rate being much higher at a given laser fluence. Our interference experiments proved that the LIBDE etching technique is suitable to fabricate gratings displaying submicrometre periods in transparent materials. On the basis of all these, it is suggested that this method may be useful to produce other nano and microoptical elements, too.

Pulsed laser deposition of polyhydroxybutyrate biodegradable polymer thin films using ArF excimer laser

We demonstrated the pulsed laser deposition (PLD) of high quality films of a biodegradable polymer, the polyhydroxybutyrate (PHB). Thin films of PHB were deposited on KBr substrates and fused silica plates using an ArF ( λ = 193 nm, FWHM = 30 ns) excimer laser with fluences between 0.05 and 1.5 J cm −2. FTIR spectroscopic measurements proved that at the appropriate fluence (0.05, 0.09 and 0.12 J cm −2), the films exhibited similar functional groups with no significant laser-produced modifications present. Optical microscopic images showed that the layers were contiguous with embedded micrometer-sized grains. Ellipsometric results determined the wavelength dependence ( λ ∼ 245–1000 nm) of the refractive index and absorption coefficient which were new information about the material and were not published in the scientific literature. We believe that our deposited PHB thin films would have more possible applications. For example to our supposal the thin layers would be applicable in laser induced forward transfer (LIFT) of biological materials using them as absorbing thin films.

Fabrication of 150 nm period grating in fused silica by two-beam interferometric laser induced backside wet etching method

Fused silica gratings with periods of 154 nm, 266 nm, and 550 nm have been fabricated by the method of two-beam interferometric laser induced backside wet etching (TWIN-LIBWE). The spatially filtered pulses at 266nm were splitted into two parts and interfered at an incident angle of 60(o), 30(o), and 14(o), respectively, on the backside surface of a fused silica plate contacting with the liquid absorber. The morphology of the etched gratings was characterized by atomic force microscope. According to our knowledge, the produced 154 nm period is the smallest grating constant generated by laser techniques directly in fused silica at present.

Adaptive shaping system for both spatial and temporal profiles of a highly stabilized UV laser light source for a photocathode RF gun

We have been developing a stable and highly qualified ultraviolet (UV) laser pulse as a light source of an RF gun for an injector candidate of future light sources. Our gun cavity is a single-cell pillbox, and the copper inner wall is used as a photocathode. The chirped pulse amplification (CPA) Ti:sapphire laser system is operated at a repetition rate of 10 Hz. At the third harmonic generation (central wavelength—263 nm), the laser pulse energy after a 45 cm silica rod is up to 850 μJ/pulse. In its present status, the laser's pulse energy stability has been improved down to 0.2∼0.3% at the fundamental, and 0.7–1.4% (rms; 10 pps; 33,818 shots) at the third harmonic generation, respectively. This stability has been held for 1 month continuously, 24 h a day. The improvements we had passively implemented were to stabilize the laser system as well as the environmental conditions. We introduced a humidity-control system kept at 50–60% in a clean room to reduce damage to the optics. In addition, we prepared a deformable mirror for spatial shaping and a spatial light modulator based on fused-silica plates for temporal shaping. We are applying both the adaptive optics to automatic optimization of the electron beam bunch to produce lower emittance with the feedback routine. Before the improvements, the electron beam produced from a cathode suffered inhomogeneous distribution caused by the quantum efficiency effect, and some pulse distortions caused by its response time. However, we can now freely form any arbitrary electron beam distribution on the surface of the cathode.

Comparing study of subpicosecond and nanosecond wet etching of fused silica

Abstract The effectiveness of the laser induced backside wet etching (LIBWE) of fused silica produced by subpicosecond (600 fs) and nanosecond (30 ns) KrF excimer laser pulses (248 nm) was studied. Fused silica plates were the transparent targets, and naphthalene–methyl-methacrylate ( c = 0.85, 1.71 M) and pyrene–acetone ( c = 0.4 M) solutions were used as liquid absorbents. We did not observe etching using 600 fs laser pulses, in contrast with the experiments at 30 ns, where etched holes were found. The threshold fluences of the LIBWE at nanosecond pulses were found to be in the range of 360–450 mJ cm −2 depending on the liquid absorbers and their concentrations. On the basis of the earlier results the LIBWE procedure can be explain by the thermal heating of the quartz target and the high-pressure bubble formation in the liquid. According to the theories, these bubbles hit and damage the fused silica surface. The pressure on the irradiated quartz can be derived from the snapshots of the originating and expanding bubbles recorded by fast photographic setup. We found that the bubble pressure at 460 mJ cm −2 fluence value was independent of the pulse duration (600 fs and 30 ns) using pyrene–acetone solution, while using naphthalene–methyl-methacrylate solutions this pressure was 4, 5 times higher at 30 ns pulses than it was at 600 fs pulses. According to the earlier studies, this result refers to that the pressure should be sufficiently high to remove a thin layer from the quartz surface using pyrene–acetone solution. These facts show that the thermal and chemical phenomena in addition to the mechanical effects also play important role in the LIBWE procedure.

Diffraction characteristics of 10 fs laser pulses passing through an aperture

Results are presented on experimental and theoretical work performed to compare diffraction phenomena for ultrashort 10 fs pulses and continuous-wave propagation modes illuminating different-sized pinholes and slits. Results demonstrate that 10 fs pulses do not produce high-frequency diffraction like that produced with continuous-wave illumination. The diffraction through a 1 mm pinhole of temporally stretched pulses obtained by using fused silica plates whose frequency spectrum remains the same is compared with those of 10 fs pulses. The overall diffraction intensity profiles are, however, nearly identical in this case. The simulations of diffraction patterns for 100 fs, 10 fs, and 1 fs incident pulse were compared theoretically for different aperture sizes and frequencies. Calculations indicate that the lack of high-frequency diffraction for the mode-locked case is due to the broadband nature of the ultrashort laser pulses; i.e., the distribution of the frequency contained in the pulse ends up washing out when objects are illuminated with pulses of broad frequency content. The results of this work have important application in biomedical imaging and remote imaging applications, to name only a few.

Effects of Degassing and Ionic Strength on AFM Force Measurements in Octadecyltrimethylammonium Chloride Solutions

Sakamoto et al. (Langmuir 2002, 18, 5713) conducted AFM force measurements between silica sphere and fused-silica plate in aqueous octadecyltrimethylammonium chloride (C18TACl) solutions and concluded that long-range attractive force is not observed in carefully degassed solutions. In the present work, AFM force measurements were conducted by following the procedures described by Sakamoto et al. The results showed the presence of an attractive force that was much stronger than the van der Waals force both in air-saturated and degassed solutions. The force was most attractive at 5 x 10(-6) M C18TACl, where contact angle was maximum. At this concentration, which is close to the charge compensation point (ccp) of the glass sphere, the long-range decay lengths (D) were 34 and 38 nm in air-saturated and degassed solutions, respectively. At 10(-5) M, the decay length decreased from 30 to 4 nm upon degassing. This decrease in decay length can be explained by a pH increase (from 5.7 to 6.6), which in turn causes additional surfactant molecules to adsorb on the surface with inverse orientation. The attractive force was screened by an added electrolyte (NaCl), indicating that the attractive force may be of electrostatic origin. Therefore, the very long decay lengths observed in the absence of electrolyte may be ascribed to the fact that the ccp occurs at a very low surfactant concentration.

Creation of second-order nonlinearity and quasi-phase-matched second-harmonic generation in Ge-implanted fused silica planar waveguide

A planar waveguide was formed in fused silica plate by implantation of 5-MeV Ge2+ ions to a dose of 1015ions∕cm2. Thermal poling created larger second-order nonlinearity in the waveguide than in blank fused silica plate. The nonlinearity could be erased by 266nm ultraviolet exposure. Periodical nonlinearity distribution in the waveguide was implemented by periodical UV exposure. The first-order quasi-phase-matched second-harmonic generation from 1064nm fundamental mode to 532nm fundamental mode was achieved in the waveguide. The d33:d31 ratio was found to be 3.2:1, in favor of the built-in electric field model for Ge-implanted silica waveguide.

Visualization of second-order nonlinear layer in thermally poled fused silica glass

Second-harmonic microscopy has been applied to characterize the second-order nonlinear layer in fused silica plates thermally poled at 280 °C and 3.5 kV for different time intervals. The nonlinear layer is found only under the anode surface and to be ∼5μm deep under the anode for a poling time of 30 min. Progression of this layer into the bulk glass with poling time is also characterized.

Fabrication of Microarrays on Fused Silica Plates Using the Laser-Induced Backside Wet Etching Method

A novel approach in the fabrication of microarrays of dye and protein on fused silica plates using the laser-induced backside wet etching (LIBWE) technique is described. The surface of fused silica plates was initially precoated using trimethoxysilane self-assembled monolayers (SAMs) and then etched using the LIBWE method to obtain the desired microstructures on the plate surface. Using this technique, the SAMs on the nonirradiated areas were able to survive the LIBWE process and were used as templates for the subsequent deposition of dye molecules or proteins via chemical bonding or physical adsorption. In the case of fused silica plates precoated with fluorinated SAMs, the LIBWE method is used to remove the SAMs to expose the etched silica surfaces, on which a thin layer of pyranine molecules can be site-selectively deposited using an aqueous solution of pyranine. In another application, an ethanol solution of rhodamine 6G was preferentially deposited onto the nonirradiated areas. In yet another application, bovine serum albumin was preferentially deposited onto the laser-irradiated areas; in this case, the fused silica plates were precoated with poly(ethylene oxide) SAMs. Interestingly, when an aqueous suspension of polystyrene (PS) microbeads was cast onto the fused silica precoated with the fluorinated SAMs, hexagonally close-packed PS microbeads were deposited into the etched cavities. Depositions of the dye, protein, and microbeads were confirmed by visualization using a fluorescence microscope and scanning electron microscope.

Wet etching of fused silica: a multiplex study

The machining process of transparent materials using the laser induced backside wet etching (LIBWE) procedure was studied. In the course of this, experimental investigations and numerical calculations were carried out. Fused silica plates were irradiated by an ArF excimer laser, using a naphthalene–methyl methacrylate solution as an absorbing liquid (concentration 0.85 mol dm−3, absorption coefficient at 193 nm 52 200 cm−1). The etch rate dependence on the applied laser fluence (varied from 110 to 860 mJ cm−2) was derived from the etch depths, measured using an atomic force microscope (AFM). The etch rate was found to be 4.7–49.5 nm/pulse, depending on the laser fluence. The surface morphology of the etched edges was also investigated by AFM. A fast photographic arrangement was used for time resolved observation of bubble development in the liquid absorbent, which is an important phenomenon of LIBWE. The internal pressure of the expanding bubbles was calculated using recorded snapshots. It was found to be 22–120 MPa 17.2 ns after the excimer pulse peak. The one-dimensional heat flow equation, including the melting of the treated fused silica layer and the vaporization of the absorbing solution, was solved using the finite difference method. The surface temperature of the fused silica was found to be a maximal 17.2 ns after the excimer pulse peak. Based on our results, we present a possible interpretation of the LIBWE procedure of fused silica.

Experiments and numerical calculations for the interpretation of the backside wet etching of fused silica

Experimental investigations and thermal calculations of laser induced backside wet etching (LIBWE) of fused silica are presented. Fused silica plates having 1 mm thickness were irradiated by an ArF excimer laser in the presence of liquid absorber. The absorbing liquids were naphthalene–methyl-methacrylate solutions with different concentrations (0, 0.21, 0.43, 0.85 and 1.71 mol/dm3). The absorption coefficients (α) of these solutions at the applied wavelength were determined using a plano–concave microcuvette. It was found that the value of α could be varied in the range of 39400–62300 cm−1 by the change of the naphthalene content. The dependence of the etch rate on both the absorption coefficient of the liquid absorbers and the applied fluence was studied. The etch depths were measured by an atomic force microscope. At 450 mJ/cm2 laser fluence, the etch rates were found to be between 13.3 and 22.2 nm/pulse depending on the actual absorption coefficient. In the next experiment the laser fluence was varied from 110 to 860 mJ/cm2 and the etch rate was found to vary between 4.7 and 49.5 nm/pulse when the concentration of the applied solution was constant 0.85 mol/dm3. The mechanism of the LIBWE can be explained in the first approximation by thermal effects and their consequences. We calculated the maximal depth of melted fused silica layer by numerical solution of the one-dimension heat flow equation taking into account the phase transformations of the liquid reagents and the fused silica target. These calculations proved that the thickness of the melted quartz layer depends linearly on both of the absorption coefficient of the liquid and the laser fluence. Our calculations give an upper estimation of the etch rate. On the basis of our results a possible interpretation of the LIBWE was suggested.

Thermal poling and ultraviolet erasure characteristics of type-III ultraviolet-grade fused silica and application to periodic poling on planar substrates

A periodically poled structure on type-III ultraviolet (UV)-grade planar fused silica was realized by periodic UV erasure of second-order nonlinearity of the thermally poled fused silica plate. Poling and UV-erasure characteristics are given. The second-order nonlinearity profile of thermally poled fused silica was found to correlate with a buried Gaussian function. The nonlinear depth, calculated by curve fitting Maker fringes assuming a step-like nonlinear profile, corresponded to the location of the 1/e peak in the buried Gaussian function. The etching rate in hydrofluoric acid of the UV-bleached thermally poled fused silica was found to be the same as that of unpoled fused silica along the direction of the poling field but was larger along the direction perpendicular to the poling field.

Quasi-phase-matched second-harmonic generation in ultraviolet-assisted periodically poled planar fused silica.

First-order quasi-phase-matched second-harmonic generation of 1064 to 532 nm in a thermally poled planar fused-silica plate with periodic UV erasure of the second-order nonlinearity was successfully implemented. We obtained a 1:2.9 ratio of d31:d33 for UV-grade fused silica in support of the proposed mechanism for electric-field-induced second-order nonlinearity in this material.

Site-selective dye deposition on microstructures of fused silica fabricated using the LIBWE method.

Using laser-induced backside wet etching (LIBWE) technique, microstructures were fabricated onto the surface of fused silica plates, which were pre-coated with self-assembled monolayers (SAMs). Dye molecules and proteins were alternately deposited onto the laser-irradiated or non-irradiated areas by either chemical bonding or physical adsorption.

Slightly dispersive white-light spectral interferometry to measure distances and displacements

Summary A new spectral-domain interferometric technique of measuring distances and displacements is realized when the effect of low dispersion in a Michelson interferometer, which comprises two coated plates of a beam splitter and a compensator, is known and the spectral interference fringes are resolved over a wide wavelength range. First, processing the recorded spectral interferograms by an adequate method, the unmodulated spectrum, the spectral fringe visibility function and the unwrapped phase function are obtained. Then, knowing the dispersion relation for the fused-silica plates, the ambiguity of the unwrapped phase function is removed and the thickness of fused silica and the nonlinear phase function due to the effect of the coatings are determined by using a new procedure. It is based on the linear dependence of the overall optical path difference between interferometer beams on the refractive index of fused silica. Once the thickness and the nonlinear phase function are known, the positions of the interferometer mirror are determined precisely by a least-squares fitting of the theoretical spectral interferograms to the recorded ones.

SHG and AFM study of PECVD a-Si:H films

In the present paper, results are presented on the dependence of the second harmonic generation (SHG) in a-Si:H on the substrate material. The investigated structures were a-Si:H films deposited onto three different substrates, namely, fused silica plates, Corning glass 7059 and soda-lime glass. It was established that the SHG depended substantially on the substrate material. The effect of the intrinsic stress due to the film–substrate mismatch strain has been considered. Investigation of the roughness of the substrate surface as estimated from atomic force microscopy measurements has shown no significant differences. Formation of microvoids in the a-Si:H films is observed with a possible implication as an SHG source.

Laser-induced high-quality etching of fused silica using a novel aqueous medium

Laser-induced backside wet etching of fused-silica plates using an aqueous solution of naphthalene-1,3,6-trisulfonic acid trisodium salt (Np(SO3Na)3) is reported. A KrF excimer laser was employed as a light source. The etch rate varied greatly with the concentration of the solution and the laser fluence. For lower concentration solutions, the etch rate increased linearly with laser fluence. For highly concentrated solutions, however, the etch rate increased abruptly at higher fluence. Well-defined line-and-space and grid micropatterns were fabricated using a low etch rate. The etched surface was as flat as the surface of the virgin plates and the etched pattern was free of debris and microcracks. The formation and propagation of shockwaves and bubbles in the solution during the etch process were monitored. High pressure, as well as the high temperature generated by the photothermal process, plays a key role in the etching process.

Relief of the internal stress in ternary boron carbonitride thin films

Ternary boron carbonitride (BCN) thin films were deposited by radio frequency reactive sputtering. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) absorption measurements suggested that the films are atomic-level hybrids composed of B, C and N atoms. The structure of the samples is graphitic BC2N. High compressive stresses were observed in the films deposited on silicon and fused silica substrates. When the films were peeled off the substrates, the absorption bands in the FT-IR spectra shift to lower wavenumbers due to the stress relief. The compressive stresses caused buckling of the films, and the buckling patterns were related to the used substrates. For the samples deposited on Si wafer, a branching type buckling pattern was observed, while a telephone-cord morphology was noted for the samples deposited on fused silica plate. The telephone-cord morphology can be explained based on the buckling-driven delamination theory proposed by Oritiz and Gioia [J. Mech. Phys. Solids 42 (1994) 531].

Formation of Ytterbium Silicide Thin Films by Solid-Phase Crystallization

Ytterbium silicide (YbSi1.7) thin films were prepared on single-crystal Si (111) wafers and fused silica plates by solid-phase crystallization. The structure and phase transformations of the films were analysed by X-ray diffraction and Rutherford backscattering spectroscopy (RBS). The electrical properties of these silicides were studied by Hall mobility measurements and four-point probe measurements of sheet resistance. Solid-phase crystallization was carried out by irradiating the samples with infrared radiation in a vacuum, the process being monitored by in situ sheet resistance measurements to obtain the phase transformation temperature. The crystallization of YbSi1.7 phase occurs at a temperature as low as 260 � C on Si substrate, and at about 400 � C on fused silica. The results of 2.5 MeV þ Li 7 RBS showed that the atomic ratio of Si to Yb in the irradiated films is close to 1.7, and the Yb atomic density is almost constant through the depth of the films.