Polarizer-Free Acousto-Optic Monochromators

Monochromators based on a noncollinear acousto-optic filter without external polarizers have been studied. As an element to select an operating diffracted light beam, (1) a spatial filter-telescope and (2) an output plane of a filter crystal rotated in a diffraction plane have been used. In the latter case, dispersion of the acousto-optic filter crystal is compensated using a correcting prism made from the same crystal and placed behind an output filter plane along a path of a filtered light beam. As a result, the transmission coefficient of the monochromator is increased upon an effective compensation of the angular drift of the filtered light beam, the monochromator design is simplified, and its sizes are decreased.

     

Spectral phase shift and residual angular dispersion of an acousto-optic programmable dispersive filter

Spectral phase shift and both types of wavelength dependent angular deviation introduced by an acousto-optic programmable dispersive filter (AOPDF) into a femtosecond laser beam at 800 nm have been measured with high precision. With the use of spectrally and spatially resolved interferometry, we have proved experimentally that the AOPDF controls the group delay dispersion (GDD) and third-order dispersion (TOD) with an accuracy of better than 1% and 3% of the preset values within the range of 10–1000 fs² and 1000–40,000 fs³, respectively. The values of angular deviation and angular dispersion are primarily dependent on the set absolute value of GDD of the AOPDF when it is operating in continuous mode. Besides of a small offset value, there is no added angular dispersion at 0 fs², while it can increase up to 0.2 μrad/nm at 10,000 fs².     

A dedicated small‐angle X‐ray scattering beamline with a superconducting wiggler source at the NSRRC

At the National Synchrotron Radiation Research Center (NSRRC), which operates a 1.5 GeV storage ring, a dedicated small‐angle X‐ray scattering (SAXS) beamline has been installed with an in‐achromat superconducting wiggler insertion device of peak magnetic field 3.1 T. The vertical beam divergence from the X‐ray source is reduced significantly by a collimating mirror. Subsequently the beam is selectively monochromated by a double Si(111) crystal monochromator with high energy resolution (ΔE/E? 2 × 10^?4) in the energy range 5–23 keV, or by a double Mo/B₄C multilayer monochromator for 10–30 times higher flux (～10¹¹ photons s^?1) in the 6–15 keV range. These two monochromators are incorporated into one rotating cradle for fast exchange. The monochromated beam is focused by a toroidal mirror with 1:1 focusing for a small beam divergence and a beam size of ～0.9 mm × 0.3 mm (horizontal × vertical) at the focus point located 26.5 m from the radiation source. A plane mirror installed after the toroidal mirror is selectively used to deflect the beam downwards for grazing‐incidence SAXS (GISAXS) from liquid surfaces. Two online beam‐position monitors separated by 8 m provide an efficient feedback control for an overall beam‐position stability in the 10 µm range. The beam features measured, including the flux density, energy resolution, size and divergence, are consistent with those calculated using the ray‐tracing program SHADOW. With the deflectable beam of relatively high energy resolution and high flux, the new beamline meets the requirements for a wide range of SAXS applications, including anomalous SAXS for multiphase nanoparticles (e.g. semiconductor core‐shell quantum dots) and GISAXS from liquid surfaces.     

Acousto-optic filter of nonpolarized electromagnetic radiation

An acousto-optic tunable filter that is insensitive to the polarization of incident light is studied both theoretically and experimentally. The fundamental possibility of designing an acousto-optic filter of nonpolarized light is demonstrated. The filter operates in the visible range of electromagnetic waves with λ=480–750 nm and provides a high spatial and temporal resolution. The number of resolvable spots of the filtered image reaches a value of 400×400 with the spectral bandwidth of the device being Δλ=1.7 nm. The results of the processing of optical images formed by arbitrarily polarized light beams are discussed. An acousto-optic filter based on a TeO₂ crystal can be used in optics and spectroscopy in the processing of light beams with an arbitrary polarization, as well as in analyzing optical images formed by light beams whose polarization varies with time.     

Transverse grating-assisted narrow-bandwidth acousto-optic tunable filter

An experimental acousto-optic tunable filter that has a narrow spectral bandwidth (0.2 nm at 1550 nm) and a fast (10-micros) tuning capability with a continuous tuning range of approximately 50 nm is described. The tunable filter consists of an acousto-optic beam deflector with a diffraction grating whose grating vector is transverse to the direction of light propagation.     

Quasi-monochromatic radiation source with the wavelength tunable in the entire visible range

A quasi-monochromatic radiation source whose wavelength is continuously tuned in the visible spectral range is presented. The source is based on a short-arc high-pressure xenon lamp and an acousto-optic filter. The tuning range is 420–670 nm, the characteristic time of the wavelength tuning is 18 μs, and the radiation spectral width is 10 nm.     

声光可调谐滤波器(AOTF)消色散设计

介绍了非共线声光可调谐滤波器的构成及基本原理,计算了非共线声光可调谐滤波器衍射出的±1级衍射偏振光的偏转角,采用在光路中添加光楔的方法补偿非共线声光可调谐滤波器晶体色散引起的图像漂移.分析结果表明使用TeO_2材料设计的光楔在楔角为7.6°的时候可以控制图像漂移在一个像元以内,并在400～900 nm的工作波段范围内满足稳定成像的要求.     

窄带集成声光可调谐滤波器的研制

利用模式耦合理论分析了声光模式转换,并对声光可调谐滤波器的滤波带宽、调谐范围、驱动功率等性能参数进行分析计算。设计并制作出窄带集成声光可调谐滤波器(IAOTF)样品,搭建相应的IAOTF测试平台,得到中心波长在1.55μm,滤波带宽达到0.85 nm,波长调谐范围达到80 nm,驱动功率低于40mW的IAOTF。并对进一步压缩滤波带宽和旁瓣效应提出了解决途径。     

AOTF成像光谱仪光学系统的最优方案选择

基于声光可调谐滤波器（AOTF）的成像光谱仪是一种新型的成像光谱仪,它除了具有一般成像光谱仪的二维空间信息与一维光谱信息外,还能获得目标的偏振信息。在实际应用中AOTF可接收的光束孔径角一般不大于5～6,因而受到所采用声光晶体可接收角度的限制,AOTF光谱成像仪的光学系统不能同时兼顾大孔径与大视场。要求AOTF成像光谱仪视场满足128128像元,TeO2晶体尺寸为10 mm10 mm,根据声光可调谐滤波器成像光谱仪光学系统的特点,分析比较了2种光学系统总体方案,最终将系统的孔径光栏放置在晶体上,TeO2晶体尺寸限制了孔径光栏的尺寸,晶体可接收的角度决定了系统的视场角,提高了能量利用率,获得较高的图像信噪比。     

基于一维耦合腔光子晶体的声光可调谐平顶滤波器的研究

本文利用一维耦合腔光子晶体,提出了一种声光可调谐平顶滤波器.该滤波器利用声光效应,通过改变超声波频率使一维耦合腔光子晶体透射谱的平顶滤波器的中心波长产生漂移,从而实现可调谐的滤波功能.基于传输矩阵法和声光效应理论,建立了这种平顶滤波器的理论模型;利用COMSOL软件,对平顶滤波器的矩形度、通带带宽、插入损耗、可调谐特性、加工精度进行仿真研究.研究结果表明,通过施加频率为6-11 MHz的超声波,可实现通带带宽为5-6 nm及中心波长在1514-1562 nm范围内可调谐的平顶滤波器;在可调谐范围内通带带宽内插入损耗不超过2.23 dB,最低仅为0.78 dB,矩形度最低可达1.4;加工误差在±10 nm内平顶滤波器的中心波长、矩形度、插入损耗、通带带宽出现的偏差很小.该平顶滤波器具有易于设计和集成、通带平坦、可调谐范围宽、通带带宽稳定、插入损耗低、品质因素高的特点,在光开关、可调谐光纤激光器、光纤传感等光通信领域有重要应用.     

Resonance Excitation of Photoluminescence in Crystalline Uranyl Acetate Dihydrate

A method for rapid identification of uranyl compounds based on resonance fiber-optic photoluminescence (PL) excitation by ultraviolet-laser or LED radiation is proposed. This method was used to measure the PL spectra of an extremely small volume (10^–9 cm³) of crystalline uranyl acetate dehydrate UO₂(CH₃COO)₂ ? 2H₂O with an exposure of 10^–3 s. Semiconductor LEDs with wavelengths of 369, 385, 410, and 466 nm and a repetitively pulsed nitrogen laser with a lasing wavelength of 337 nm served as sources of excitation radiation. The operating range of a small-sized minispectrometer used in these experiments was 200–1000 nm.     

一种高通量宽带近红外声光可调滤光器的设计

声光可调滤光器(AOTF)是20世纪90年代以来近红外光谱仪最突出的进展,传统的单换能器结构使滤光器存在工作带宽较窄的问题。本文采用双换能器结构设计了一种高通量、宽带宽的近红外声光可调滤光器,基于“切面平行条件”波矢量布局方案,通过仿真计算确定声光晶体最优切向切角及换能器长度等最佳参数。两段换能器分别工作于两个不同的波段范围,使得滤光器的工作波段范围在光谱分辨率、衍射效率等参数满足要求的前提下大为改善。测试结果表明,设计的滤光器在900～2 400 nm波段范围内分辨率优于15 nm,衍射效率最高可到41%。     

MOCVD grown MWIR HgCdTe detectors for high operation temperature conditions

The paper reports on photoelectrical performance of the mid-wave infrared HgCdTe detector for high operating temperature condition. Detector structure was simulated with APSYS numerical platform by Crosslight Inc. The comprehensive analysis of the detector performance such as dark current, detectivity, time response vs. device architecture and applied bias has been performed. The N⁺pP⁺n⁺ HgCdTe heterostructure photodiode operating in room temperature at a wavelength range of 2.6–3.6 μm enabled to reach: detectivity ～8.7×10¹⁰ cmHz^1/2/W, responsivity ～1.72 A/W and time response ～ 145 ps (V = 200 mV).     

A pure rotational Raman lidar using double-grating monochromator for temperature profile detection

A pure rotational Raman lidar is constructed for measurements of vertical temperature profiles of atmosphere. A fiber-coupled, double-grating monochromator is used as the filter system so that a high spectral resolution of <0.23 nm and an out-of-band rejection rate of 10⁷ are achieved. Comparison with in-situ measurements indicates that the accuracy of atmospheric temperature measurements using this lidar system is better than 1.1 K up to an altitude of 10 km. Different effects on signal accuracy resulting from various noise sources are analyzed and uncertainties in temperature due to detection noises are estimated.     

Selective cleaning of ions of heavy metals from water solutions using the H-form of todorokite synthesized by the hydrothermal method

A phase of Mg-todorokite with a size of structural tunnels of ～10 Å has been synthesized by the hydrothermal method in the form of lamellar crystalline aggregates 50–100 nm in size. Topotactic interaction with concentrated nitric acid results in the formation of the H-form of todorokite, having a high ability to adsorb Cs⁺, Pb²⁺, and Ba²⁺ ions from water solutions. H-todorokite can be used as an ion filter for cleaning ions of heavy metals from water solutions.     

Type-II superlattice detectors for free space optics applications and higher operating temperature conditions

The utmost limit performance of interband cascade detectors optimized for the longwave range of infrared radiation is investigated in this work. Currently, materials from the III–V group are characterized by short carrier lifetimes limited by Shockley-Read-Hall generation and recombination processes. The maximum carrier lifetime values reported at 77 K for the type-II superlattices InAs/GaSb and InAs/InAsSb in a longwave range correspond to ～200 and ～400 ns. We estimated theoretical detectivity of interband cascade detectors assuming above carrier lifetimes and a value of ～1–50 μs reported for a well-known HgCdTe material. It has been shown that for room temperature the limit value of detctivity is of ～3–4×10¹⁰ cmHz^1/2/W for the optimized detector operating at the wavelength range ～10 μm could be reached.     

Atmospheric ozone concentration measurement by UV light-emitting diode radiation absorption

The concentration of ozone produced in the atmosphere under mercury lamp light is measured using an UV light-emitting diode by the absorption spectrum in the range of 260–270 nm. The absorbance and ozone concentration measurement sensitivity is k = 8 · 10^?7 cm^?1 and ～3 ppbv, respectively.     

Phase size effect in thin Ge-Se polycrystalline films

The Raman spectra of thin (d = 60–170 nm) Ge-Se polycrystalline films obtained by vacuum thermal evaporation of Ge₁₀Se₉₀ glass are investigated in the spectral range 110–310 cm^?1. The coexistence of the glasslike and crystalline phases α-Se, β-Se, and β-GeSe₂ is established using the X-ray diffraction method. Analysis of diffraction patterns and the Raman spectra of polycrystalline samples of various thicknesses demonstrates a phase size effect in the transition of Se from the α-monoclinic to the β monoclinic modification (d ～ 120 nm). It is found that the crystalline phase of Se is of the nanodisperse type with an average grain size of ～30–50 nm. Crystallites of β-GeSe₂ have an average size of ～100–130 nm.     

Tunable large angular aperture TeO2 acousto-optic filters for use in the 1.0–1.6 μm region

The design of tunable large angular aperture TeO₂ noncollinear acousto-optic filters for use in the 1.0–1.6m region is discussed. It is shown that it is possible to maximize the angular aperture of such a filter by the correct choice of the direction for the acoustic wave vector and that this required direction is independent of the operating wavelength in the range 1.0–1.6m. The theoretical model adopted is verified by comparison with experimentally measured filter characteristics and the design of filters with narrow bandwidths and low power requirements is discussed.     

Surface‐enhanced Raman scattering sensing on black silicon

Reactive ion etching was used to fabricate black‐Si over the entire surface area of 4‐inch Si wafers. After 20 min of the plasma treatment, surface reflection well below 2% was achieved over the 300–1000 nm spectral range. The spikes of the black‐Si substrates were coated by gold, resulting in an island film for surface‐enhanced Raman scattering (SERS) sensing. A detection limit of 1 × 10^?6 M (at count rate > 10² s^?1 . mW^?1) was achieved for rhodamine 6G in aqueous solution when drop cast onto a ～ 100‐nm‐thick Au coating. The sensitivity increases for thicker coatings. A mixed mobile‐on‐immobile platform for SERS sensing is introduced by using dog‐bone Au nanoparticles on the Au/black‐Si substrate. The SERS intensity shows a non‐linear dependence on the solid angle (numerical aperture of excitation/collection optics) for a thick gold coating that exhibits a 10 times higher enhancement. This shows promise for augmented sensitivity in SERS applications.