Micromachining of polyurea aerogel using femtosecond laser pulses

We successfully sliced cylindrical polyurea aerogel samples of 10-15 mm in diameter into 1-3 mm disks using femtosecond laser. The experiments were performed using a Ti:sapphire laser with 800 nm wavelength in ambient air with a pulse duration of ~ 40 fs. We found that the laser fluence to breakdown this material is 1.3 J/cm². The ablation rate at different energy levels was evaluated. The factors influencing the ablation surface quality were investigated. The proper fluence to slice the porous polyurea is 6.4- 8.9 J/cm² with the beam linearly scanning the sample at a speed of 0.1 mm/s, or 5.1-7.6 J/cm² with the beam circularly scanning the sample at a speed of 3.5-4°/s, and high quality machining surface was obtained under these conditions. The material removal mechanisms are proposed. Structural details of the machined area were characterized using a number of techniques such as optical microscopy and scanning electron microscopy. This work provides insights for micromachining nanostructured porous polymers using femtosecond lasers.     

Laser-induced thermomigration of Te precipitates in CdZnTe crystals

The thermomigration of tellurium precipitates in a cadmium zinc telluride (CdZnTe) crystal was observed using an infrared (IR) CO₂ laser beam. Te precipitates present in CdZnTe have been shown to thermally migrate along a temperature gradient. Selective energy deposition from an IR laser beam in Te precipitates was investigated as a potentially advantageous approach to speed the annealing of Te precipitates in CdZnTe. Initial results indicate Te precipitates do thermally migrate under IR laser heating.     

Laser-assisted chemical beam epitaxy for selective growth

A recent exciting development in the growth of compound semiconductors is the use of light to modify the film growth rate in the irradiation area. We report Ar⁺-laser-assisted CBE of GaAs, InP, GaP and InGaAs to generate various patterned films without lithography. A linked-circle pattern is formed by laser beam scanning and a 0.85 μm pitch corrugation pattern formed by a holographic interference technique. Relationships between the growth rate and substrate temperature for the materials are compared. The mechanism of the growth rate enhancement is revealed to be photolytic decomposition of metalorganic molecules. In the case of InGaAs, laser irradiation above 500°C results in new phenomena of growth rate suppression and composition variation.     

Determination of light-scattering properties of glass surfaces

Glass surfaces were damaged in a defined manner by sandblasting with an adjustable sand blasting machine and subsequent etching with HF. Investigations with the scanning electron microscope (SEM) showed surface defects with an ellipsoidal shape. A quantitative evaluation of the exact profile of these defects was made using mathematical evaluation of stereographic pairs. A new method is described for characterizing surfaces with optical light scattering. In contrast to the usual method of a fixed specimen and a photocell moving on a circle around it, here the specimen revolves on an axis perpendicular to a laser beam and rotates on an axis parallel to it in order to average the scattering over a large area of the surface, thus preventing interferences of the coherent laser beam with surface defects. A theory is described which enables a numerical estimation of roughness parameters using a distribution function of angles of small mirrors to the average surface. In this special case the theory was extended for the special type of defects having an ellipsoidal shape. The results are discussed with respect to creation of surface defects by sandblasting, which are accompanied by subsurface cracks.     

Study of phosphoric acid crystallization using a focused beam reflectance measurement method

A way for restoring the crystal size distributions (CSD) from measured chord length distributions (CLD) was reported in this paper. The kinetics of phosphoric acid crystallization process was investigated in cooling mode using focused beam reflectance measurement (FBRM) and digital photo technique. In order to restore the CSD from measured CLD and verify the reliability of FBRM data, digital photo technique in real time and optical microscope were applied in large crystal size and small range, respectively. Results indicated a converting constant A existed between CLD and CSD when crystal growth follows size‐independent growth (Mcabe's ΔL law) law. It was verified by Malvern particles size analysis method. The converting constant A varied with crystal morphology. The crystal growth order increased with the stirring increasing speed during phosphoric acid crystallization process. The trend was especially notable at higher speed situations. It can illustrate that the state of phosphoric acid hemihydrate crystal growth was controlled by both diffusion and surface‐integration with the increasing stirring speed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation mechanism of nanostructures in soda-lime glass using femtosecond laser

This paper reports on the formation of nanostructures produced in soda-lime glass by controlling the irradiation conditions of a single-beam femtosecond laser. Periodic nanoholes are fabricated in the sample with the lowest diameter of 200 nm (approximately one fourth of the incident light wavelength). Self-organized nanogratings with a period of 120 nm are fabricated for the first time on soda-lime glass surface by applying many pulses at the same spot. We discover that the nanogratings’ period decreases with the increase of the applied pulses. We investigate that the direction of the nanogratings is perpendicular to the direction of laser polarization. Further, it is discovered that the microholes, due to the illumination by many pulses, are elliptical in shape with the major axis perpendicular to the direction of laser polarization. Finally, long distance horizontal and vertical gratings are fabricated by scanning a femtosecond laser beam only in the horizontal direction.     

Emission properties of cathodes based on B-N-C as a result of laser processing

Emission properties of cathodes produced under the action of laser radiation are presented. Cathodes were fabricated in nitrogen flow as a result of surface modification induced by laser treatment of the compacted samples of fine-grained graphite-like h-BN powders and 25 wt.% lamp black composite. Optical and scanning electron microscopes revealed new structures and morphologies on the surface of samples formed due to the interaction of plume of ionized B, C and N with nitrogen ambient. X-ray diffraction (XRD) study demonstrated complicated phase composition of the precipitates formed by B-N-C composite after heating by laser beam.     

Cr:KNSBN晶体两波耦合及其图像存储

本文以He-Ne 632.8nm激光为写入光,在非同时读出条件下,实验研究了e偏振光写入Cr:KNSBN晶体两波耦合过程中信号光和泵浦光的透射光强随时间的变化,以及单束泵浦光的透射光强随时间的变化,实验结果表明,泵浦光损失的能量几乎全部转移到了信号光方向,基本不存在散射光;并以二值化图像作为物在晶体内进行了图像存储实验,其再现图像清晰,信噪比高,没有观察到扇形光的影响.     

Cw argon laser crystallization of silicon films: Structural properties

This paper deals with the structural characterization of amorphous silicon films deposited on glass in the amorphous state and then post-crystallized using a continuous wave argon laser. In opposite to the excimer laser crystallization method, the processing window is wider. Due to the low cooling rate induced by the continuous irradiation, very large grains are obtained. With an epitaxial growth induced by an adequate overlapping of the laser traces, grains as large as 100 μm can be reached. Electron back-scattered diffraction analysis highlights the single crystalline character of the large size grains crystallized with this kind of laser. The technique is able to produce large area single crystalline regions, suitable to fabricate high speed circuits.     

Photo-quenched of the luminiscence signal in Co(II)-doped alumina prepared by the sol-gel method

Co(II)-doped alumina monoliths prepared by the sol-gel method were laser irradiated producing fluorescence. The intensity of this defect-induced fluorescence was exponentially reduced with the irradiation time to practically disappear. The rate the fluorescence intensity decays was modeled as a double exponential function of the irradiation time; the characteristic times associated with the decays are in the range of seconds. The suppression of the luminescence was associated with the local heating produced by the highly focused laser beam in a small area (≈ 2 μm in diameter) on the sample. This heating process reduces physical (grain boundaries and surface states) and chemical (oxygen vacancies) defects present in the sol-gel samples. Some residual fluorescence still remains after long periods of illumination. The characteristic times for alumina samples are compared with those obtained for other metal oxides prepared also by the sol-gel method.     

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al_xGa_1−xAs layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.     

Quantum Size Effect and very localized random laser in ZnO@mesoporous silica nanocomposite following a two-photon absorption process

ZnO@mesoporous silica nanocomposite was prepared by the impregnation of a CMI-1 material in a Zn(NO₃)₂ solution followed by calcination under O₂. Intensive characterization was carried out by N₂ adsorption–desorption measurements, scanning and transmission electron microscopy. The optical properties of the ZnO@mesoporous silica nanocomposite were studied by photoluminescence spectroscopy. Quantum Size Effect was firstly demonstrated by subjecting the sample to a 254 nm excitation light, and was further confirmed by using a 680 nm excitation laser beam, which implies a two-photon absorption process. By focusing the 680 nm laser beam on different places in the sample, a very localized random laser effect, also induced by a two-photon absorption process, was detected.     

Reorientation of Liquid Crystals by Superposed Optical and Quasistatic Electric Fields

The electric field of a CW laser beam can reorient a nematic liquid crystal. Experiments on the influence of a superposed quasistatic electric field are reported showing that this can reinforce or weaken the laser induced reorientation depending on the substance and geometry.     

光固化树脂基陶瓷单层成型工艺研究

在陶瓷光固化三维成型工艺中,单层成型效果对坯体堆积成型起着决定性作用.文章主要研究在单层坯体打印成型中,光源波长、扫描速度、光照时间和光源的照射距离对成型件质量和精度的影响.实验结果表明,光源波长越接近引发剂吸收波长范围,固化效果越好;光源的曝光量达到临界曝光量时,浆料才能发生完全固化;随着扫描速度的不断增大,陶瓷坯体的硬度不断降低,固化厚度逐渐减小,当扫描速度很大时,坯体表面致密度极低,组织样貌变得疏松.此外,紫外光源的照射距离对坯体的成型质量没有影响.     

Laser Induced Reorientation of Nematic Liquid Crystals

A significant increase of the divergence and appearance of a fringe system has been observed by illuminating of MBBA (p-n-methoxybenzilidene-p-butylaniline) and OCB (octyl-cyano-biphenyl) nematic liquid crystalline samples with a collimated beam of an argon ion laser. The dependence of this effect on laser power, beam polarization, and angle of incidence has been studied in homeopolar and planar sandwich-like cells of 50-150 μm thickness, in the nematic phase. At a homeopolar cell of MBBA (150 μm thickness), at normal incidence a threshold laser power of 45 m W was found. The phenomena can be explained as deformation of the orientation by the Fredericks effect due to light fields. The observations are in good agreement with the theoretical predictions including an estimation of the laser power threshold.     

Features of controlled laser thermal cleavage of crystal quartz

Controlled laser thermal cleavage of crystalline quartz has been simulated. The thermoelastic fields formed in a square single-crystal quartz plate as a result of successive laser heating and exposure to coolant have been calculated for five different versions specified by the crystal cut orientation and direction of laser beam displacement. The results have been verified experimentally using a CO₂ laser. The simulation results can be used in the electronics industry to optimize laser cutting of quartz crystals.     

The influence of a ferroelectric domain wall on the fabrication of a pyroelectric laser energymeter

A region across a ferroelectric domain wall has been scanned, using an Nd:YAG laser beam (beam size 440 μm), in a thin crystal of TGS (triglycine sulphate), cut normal to the polar axis and the pyroelectric response of the crystal has been recorded. If a laser pulse of fixed beam size falls on the domain completely, then the output pyroelectric signal is maximum. But when the laser beam (diameter of beam greater than width of domain wall) falls on the domain wall, the pyroelectric signals from two opposite domains tries to cancel each other and net signal depends on the position of laser beam across the domain wall. When the domain wall lies in the middle of the laser beam the output pyroelctric signal will be zero. In our experiment when beam of size 440 μm falls on positive domain the pyroelectric voltage of ‐2.76 mV recorded and when beam starts crossing the domain wall (i.e from positive domain to negative domain), the pyoelectric signal changes its sign from negative to positive through zero. After 220 μm the signal decreased to minimum and again increases to maximum (+2.46 mV) in opposite direction after 440 μm i.e when beam falls on the negative domain completely. This shows that the polarization in the domain wall region (separating two domains) is zero i.e the paraelectric nature of domain wall. So before detector fabrication the sample must be poled sufficiently to avoid the non uniformity of pyroelectric signal due to the domain wall. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamical aspects of Czochralski growth of alkali halides

The variation of the crystal diameter in KC1, KBr, and KI after a sudden small temperature step was measured by means of an optical diameter controller using a laser beam. The diameter reaches exponentially a new equilibrium with time constants of about 30 to 60 min in the given Czochralski apparatus. The time constants increase with the length of the grown crystal. The same exponential behaviour and time constants were obtained from photographs of the solid-liquid interface after a temperature step. The maximum diameter changes for a given temperature step also increase with the length of the grown crystal. The results can be used for a construction of an automatic Czochralski growth apparatus.     

3D Damage Analysis of Laser-irradiated Materials inside a Scanning Electron Microscope Using Ion Beam Slope Cutting Technique

In practical use of high temperature materials extreme thermal loads at materials surfaces often occur which can effect a non-reversible material damage or material failure consequently. These generated damages can be studied using an SEM which is coupled with a pulsed Nd:YAG high power laser. The ion beam slope cutting technique is a very suitable method to produce a highly localized cross section in laser irradiated material. By this the three-dimensional electron microscopic analysis of the damaged bulk material was possible. For the experiments a combination of ion beam slope cutting and conventional ion etching was used. It enables the high quality of microstructure characterization.     

Characterization of etched facets for GaN-based lasers

Dry-etching of laser facets is commonly used for (InAl)GaN/sapphire-based structures since the epitaxial planes of the nitride layers are rotated with respect to the substrate planes making cleaving impractical. To achieve steep and smooth facets by chemically assisted ion beam etching, a 3-layer resist system is developed for patterning. Characterization by scanning electron microscopy and atomic force microscopy shows facets with root-mean-square roughnesses of 7 nm and inclination angles of 2–4°. Optically pumped lasers yield low threshold excitation densities for fully doped separate confinement heterostructure lasers.