Passive Broadband Reflector: Elaboration and Spectral Properties

Abstract

We present passive broadening obtained by assembling two layers of cholesteric photo-polymerisable monomers having different pitches at the same temperature. A more or less large diffusion occurs between the two layers, which is dependent of various parameters such as degree of cross-linking of the two layers, temperature dependence, time evolution at a given temperature, thickness of the layers… The presented work consider the spectral properties of the sandwich structure and its correlation with the diffusion profile at the interface between the two layers. The band shape of the reflected and transmitted light were studied. Then, the reflected intensity shows a widening of the reflection associated with the existence of a pitch gradient inside the sample, which can be blocked by photo-polymerisation. At short time, the two peaks corresponding to each liquid crystal polymer are still present but the diffusion occurs at the interface. After a longer time, the broadening band still decreases and a single large peak is observed.     

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.     

Fabrication of moth-eye structure on photoresist film by laser control of reaction time constant

We have proposed the fabrication of high-contrast-moth-eye structure using continuous wave (CW) laser. Nonlinear-optical reaction time constant of the photoresist can be controlled by manipulating the exposure intensity and scanning speed of the laser. As a result, shortening the time constant in the intense beam area and lengthening that in the weak beam area compared with the scanning speed, we can limit the exposed area.     

Antisolvent crystallization of aqueous ammonium dihydrogen phosphate solutions by addition of acetone at different rates

Results of an investigation of in situ measurements of laser‐beam intensity I_s and I_m transmitted through aqueous ammonium dihydrogen phosphate (ADP) solutions saturated at 30 °C and water, respectively, and temperature T_s of solution and T_m of water during feeding of antisolvent acetone at different rates R_A, using an indigenously designed experimental setup, are presented and discussed. It was found that: (1) for the measurement of MSZW, defined as the maximum volume fraction of acetone content Δx_max in the solution, obtained from temperature measurements are more reliable than transmitted laser‐beam intensity measurements for solutions, (2) two minima ΔT_min1 and ΔT_min2 associated with endothermic reactions, separated by a maximum ΔT_max due to exothermic reactions appear in the plots of temperature difference ΔT = T_s−T_m against acetone feeding time t, and (3) in the ΔT(t) plots there are time intervals Δt of constant rates R_T of increase in ΔT of aqueous ADP solutions, and these values of R_T increase linearly with acetone feeding time rate R_A. The experimental data on the observed dependence of MSZW on antisolvent feeding rate R_A, the appearance of minima ΔT_min1 and ΔT_min2 and maximum ΔT_max and their dependence on R_A, and the relationship between R_T and R_A are discussed from consideration of processes of nucleation and growth of crystallites.     

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)     

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.     

The influence of the growth rate and V/III ratio on the crystal quality of InGaAs/GaAs QW structures grown by MBE and MOCVD methods

The influence of the growth rate and V/III ratio on the crystal quality of In_0.2GaAs/GaAs quantum well structures was examined. The investigated heterostructures were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD). Reflection high energy electron diffraction (RHEED), photoluminescence measurements (PL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) were applied for evaluation of the interfaces smoothness and the overall layer quality. Comprehensive characterisation of InGaAs/GaAs structures allowed us to establish optimal values of analysed technological parameters. Moreover, the comparison between the results obtained for samples grown by two different epitaxial techniques allowed us to find, which of the analysed growth parameters has the strongest influence on the quality of MBE and MOCVD grown structures. In contrast with the growth temperature and the interruption time, which in different manner impact on the crystal quality of QWs obtained by different method, the growth rate and the V/III ratio play similar role in both epitaxial techniques.     

Evolution of Optical Transmittance in Precipitants Solutions. A Computer Simulation

We present a computer simulation model of the evolution of the transmitted light intensity traversing a reservoir that contains a supersaturated solution. From the analysis of the noise associated with the main signal, it seems to be possible to differentiate whether mass precipitation occurs by homogeneous nucleation or by pure crystal growth.     

MOVPE-grown (AlGa)As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence

We present a (AlGa)As laser diode based on a MOVPE-grown heterostructure modified toward reduction of the vertical (perpendicular to junction plane) light beam divergence. Insertion of thin, wide-gap barrier layers at the interfaces between the MQW active region and the cladding layers allows for separate controlling the carrier and optical confinements. According to theoretical modeling, the antiguiding influence of the barriers on the primary waveguiding properties of a MQW structure causes a weakening of the optical confinement and thereby a reduction of the vertical beam divergence. This occurs however without weakening of the carrier confinement and an excessive increase of threshold current density. As a result, the beam divergence of 17-13° has been experimentally achieved (depending on construction details), for devices manufactured from the heterostructure theoretically expected to give 12°. MOVPE growth conditions of this modified “double-barrier multiquantum well” (DBMQW) heterostructure are described. The light-current characteristics presented show threshold current densities of 0.92-2.1 kA cm⁻² for laser cavity lengths of 0.75-0.25 mm, respectively and good quantum efficiencies.     

Re-examination of effects of nucleation temperature and time on glass crystallization

The effects of nucleation temperature and time on the kinetics of non-isothermal glass crystallization have been re-examined to demonstrate the limitations of some approximate solutions used to extract kinetic parameters from differential thermal analysis (DTA) experiments. Those features were analyzed by numerical solutions of equations describing the dependence of fraction crystallized on the rates of nucleation and growth, and the corresponding transient time, reported for lithium disilicate. It was shown that the temperature of maximum nucleation rate varies on changing the nucleation time. Some guidelines were established to assist the selection of suitable conditions to perform crystallization studies by DTA, and to extract the values of activation energy and dimensionality of growth from the dependences of crystallization peak temperature on heating rate, and nucleation time. The main limitations of these methods were identified and discussed.     

声学超表面的非对称声分束特性研究

非对称声分束超表面是由人工微单元结构按照特定序列构建的二维平面结构,可将垂直入射的声波分成两束传播方向和分束比自由调控的透射波,在声功能器件设计及声通信领域具有广泛的应用前景。本文系统研究了一种实现非对称声分束的设计理论和实现方法,基于局域声功率守恒条件研究了声分束器的设计理论、阻抗矩阵分布、法向声强分布、声压场分布等。利用遗传算法对四串联共振腔结构进行参数优化实现了声分束器所需的阻抗矩阵分布,声压场分布表明声波入射到声分束器后在入射侧激发出两列传播方向相反且幅值和衰减系数均相同的表面波,实现了入射侧与透射侧的局域声功率相互匹配。声波经过声分束器后被分为两束透射波,两束透射波的折射角和透射系数与理论值十分吻合,证明了设计理论及实现方法的正确性和可行性。本文的研究工作可以为新型非对称声分束结构设计提供理论参考、设计方法和技术支持,并促进其在工程领域的实际应用。     

Influence of non-uniformity of laser beam intensity on the surface layer structure of implanted silicon crystals

The influence of the non-uniformity of intensity distribution in a laser beam on the surface layer structure of As-implanted silicon crystals as well as a method for homogenizing a pulsed laser beam is described.     

Calculation of the parameters of the X-ray diffraction station with adaptive segmented optics on the side beam from the wiggler of the Sibir’-2 storage ring

The mounting of an X-ray diffraction station on the side beam of a 19-pole superconducting wiggler makes it possible not only to use the central synchrotron radiation beam with a wavelength of 0.5 ?, but also to solve problems requiring softer X rays at a synchrotron radiation (SR) intensity exceeding that for the beams from the bending magnet. A numerical simulation of the formation of photon beams from a source and their transmission through the elements of the station (and through the station as a whole) allows one to calculate the parameters of the station, compare it with the existing analogs, determine its potential and actual efficiency of its elements, and estimate the adjustment quality. A numerical simulation of the SR source on the side beam from the wiggler and the focusing channel (segmented condenser mirror, monochromator with sagittal focusing by the segmented second crystal, and segmented focusing mirror) has been performed. The sizes of the focus and the divergence of rays in it are determined with allowance for the finite sizes of segments. The intensity of radiation with a wavelength ?? = 1.0 ? in the focus is determined taking into account the loss in the SR extraction channel and in the focusing channel. The values of the critical wavelength for the side beam from the wiggler and the wavelength resolution are calculated. The intensities in the X-ray diffraction pattern and its angular resolution are found.     

X-ray phase contrast of laser-heated air space

The image of a thin cylindrical air space heated by the beam of a continuous CO₂ laser has been obtained by the method of X-ray phase contrast in the scheme with a slit (cut) crystal-analyzer. It is shown that the method is extremely sensitive to superlow density values and relative X-ray refractive index of an object. The air temperature in the laser beam is evaluated for several values of the laser power.     

Surface Deformation and Walls in the Conduction Regime of Nematic Liquid Crystals

Reflection of a laser beam from the free surface of a nematic liquid crystal shows that the surface is periodically deformed when a high electric field (conduction regime) is continuously applied. Walls which appear to be of the alignment-inversion type have also been observed for these fields. These results are associated with flow cells that were discussed earlier.     

Photoluminescence characterization of GaxIn1−xAs (0 ≤ x ≤ 0.32) strained quantum wells grown on InP by chemical beam epitaxy

We have investigated some characteristics of Ga_xIn_1−xAs/InP (x = 0, 0.2, 0.32) strained materials by growing multi-quantum wells with various well widths from 3 to 60 Å by chemical beam epitaxy. Growth conditions such as valve sequences and growth interruptions were optimised to have single-peaked photoluminescence spectra for all strained samples. Measured room temperature emission wavelengths were compared with theoretically estimated values. Photoluminescence linewidths were around 16 meV for most samples at 77 K, but broadened for well widths of less than 20 Å. Photoluminescence intensity also peaked at well width of 20 Å for all compositions. Performances of optical devices may be closely related to this “critical thickness” determined probably by the growth system limitation.     

Investigations on nucleation kinetics,growth and characterization of sulphanilic acid single crystals

Nucleation parameters such as radius of critical nucleus, critical free energy change and interfacial tension were evaluated for Sulphanilic acid (SAA) single crystals. Metastable zone width and induction period values were determined to optimize the growth parameters. The interfacial tension values estimated using the experimentally determined induction period is found to be comparable with theoretical values. SAA crystals were grown with the optimized growth parameters. The formation of SAA crystals was confirmed by powder X‐ray diffraction and FT‐IR studies. The density measurements were carried out by both theoretical and experimental methods. The NLO behaviour of SAA crystals was tested by Kurtz‐ Perry technique. The mechanical hardness was studied by Vickers Microhardness tester. The UV‐Vis spectral analysis was carried out on the SAA crystals to study the optical properties. The laser damage threshold values of the SAA crystals are found to be 7.6 and 6.6 GW/cm² for single and multiple shots, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compression of Acoustic Beams at Conversion Reflection in Hexagonal Crystals

It is shown how the entire energy of an acoustic beam incident on a boundary of a hexagonal crystal can be directed into a narrow reflected beam propagating at a small angle to the surface. This process is accompanied by mode conversion: the incident and reflected beams belong to different acoustic branches. The conversion near the total internal reflection is implemented by matching the orientations of the sagittal plane and surface and thereby eliminating the loss for parasitic wave reflection. The found conversion conditions and resonant reflection parameters are expressed in terms of the moduli of elasticity of crystal.     

The kinetics of parasitic growth in GaAs MOVPE

Gallium arsenide (GaAs) deposition was carried out in a horizontal quartz reactor tube with trimethylgallium (TMGa) and arsine (AsH₃) as precursors, using a hydrogen (H₂) carrier gas. Temperatures were in the range 400–500 °C, where surface reactions limit deposition rate. Nucleation time and deposition rate were monitored using laser interferometry, optimum reflectance was gained by aligning a quartz wafer to back reflect the incident beam. The 980 nm infrared laser beam was sufficiently long in wavelength to be able to penetrate the wall deposit. Results showing the effect of temperature and V/III ratio on the nucleation time and deposition rate are presented, where with temperature the nucleation delay was observed to reduce and the growth rate to increase. The nucleation delay is consistent with a thermally activated surface nucleation for the parasitic GaAs. A theoretical growth rate model, based on a restricted set of reaction steps was used to compare with the experimental growth rates. Without any free parameters, the growth rates from theoretical calculation and experiment agreed within a factor of two and showed the same trends with V/III ratio and temperature. The non-linearity of the theoretical growth rates on an Arrhenius plot indicates that there is more than one dominant reaction step over the temperature range investigated. The range of experimental activation energies, calculated from Arrhenius plots, was 17.56–23.59 kJ mol⁻¹. A comparison of these activation energies and minimum deposition temperature with the literature indicates that the wall temperature measurement on an Aixtron reactor is over 100 °C higher than previously reported.     

Second harmonic generation in selenium-metal structures

Results of second harmonic (SH) generation in amorphous and crystalline selenium films and selenium-metal (Ga, Zn, In, Sb, Bi) sandwich structures induced by titan-sapphire femtosecond laser (wavelength λ – 690–1040 nm) are presented. It is found that the highest intensity of SH is provided by fundamental wave at wavelength 1000 nm and it reaches maximum in approximately 100 s. The intensity of transmitted SH depends on film thickness while that of reflected does not. The highest SH intensities in selenium-metal structures are provided by Se–Ga and Se–Zn films. A possible mechanism for central symmetry breaking in amorphous films also is proposed.