Study of non-diffracting light beams from broad-stripe edge-emitting semiconductor lasers

Broad-stripe edge-emitting semiconductor lasers have been used to obtain propagation-invariant (nondiffracting) light beams with powers and diameters of the central ray acceptable for optical manipulation and tweezing. The results of investigations of the propagation of Bessel beams generated from broad-stripe lasers with spectrally selective resonator show that the spatial homogeneity of emission plays a much greater role than the temporal coherence in the formation of Bessel beams. The main factors limiting the length of non-diffracting beam propagation (without distortion of the central ray) are the astigmatism and multimode character of laser radiation.     

Micromachined optical concentrators for IR negative luminescent devices

Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including use as thermal radiation shields in IR cameras, and as IR sources in gas-sensing systems. For many of these applications a large area (>1 cm²) device which displays as large as possible apparent temperature range is required. However, under reverse bias, significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. We have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. To fabricate the concentrators, spherical resist masks are first produced, which are ～10 μm high and ～53 μm wide, by resist reflow at 120°C. Inductively coupled plasma (ICP) etching is then used to etch alternately the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Currently, the concentrators are typically 30 μm high, with a top diameter of ～15 μm. Continuing optimization of the process to reach the theoretical limits of optical gain is described.     

High-power 1.8-μm InGaAsP/InP lasers

Separately bounded InGaAsP/InP laser heterostructures with two stressed quantum wells emitting at a wavelength of 1.8 μm were obtained by metalorganic vapor-phase epitaxy. The laser diodes with a strip width of 100 μm provide for an output radiation power of 1.2 W in the continuous operation mode at a temperature of 20°C. A minimum threshold current density was 320 A/cm² and a differential quantum efficiency was η_d=28% for a Fabry-Perot resonator length of 1.4 mm. The internal optical losses in the laser heterostructure studied amounted to 5.6 cm^?1.     

Ultrafast beam self-switching by using coupled vertical-cavity surface-emitting lasers

Dynamic beam switching of vertical-cavity surface-emitting lasers (VCSELs) has important applications for switching and routeing in optical interconnect networks. VCSEL arrays of various kinds have been quite extensively researched for tailoring and engineering near- and far-field patterns. In this paper, a new method of directional beam switching is proposed that uses two coupled VCSELs. When two VCSELs are coupled by a small separation and biased at the same steady current near threshold, then the resulting light output is dynamic at an extremely high frequency. The model equations are based on an approximation to the semiconductor Maxwell-Bloch equations. The simulations are for coupled VCSELs operating at 980 nm with circular current apertures of 5.6 μm diameter. Figures of the results will show far-field beam intensity patterns during a cycle of oscillation. The simulation results show directional switching at a speed of about 40 μm GHz and between directions about 8° apart. Results of addition simulations are also presented. Simulations that use two square VCSELs show that the frequency of oscillation increases to 50 GHz and that the far-field pattern remains similar. Finally, for four round VCSELs in a square pattern, two far-field circularly shaped beams moved left and right at a frequency of 50 GHz.     

Sum-frequency mixing of radiation from two extended-cavity laser diodes using a doubly resonant external cavity for laser cooling of trapped ytterbium ions

Over 100 μW of continuous-wave tunable ultraviolet radiation at 370 nm is generated by the sum-frequency mixing of radiation from two extended-cavity laser diodes having powers of 60 mW at 822 nm and 8 mW at 671 nm in a lithium iodate crystal. The crystal is placed in an external cavity that enhances the powers of two fundamental beams simultaneously by approximately 40. This light source is successfully applied to the laser cooling of trapped ytterbium ions.     

Synthesis of large single crystals of CoAPO-5 molecular sieves

Large single crystals of CoAPO-5 have been synthesized using 2-diethylaminoethanol (DAE) as a template reagent, at the mole ratio of Al₂O₃:P₂O₅:(0.1–0.4)DAE:18H₂O:0.045CoCl₂, after aging at various temperatures (25, 50, 100, 150°C) for 1 day and heating hydrothermally at 200°C for 72 h. The optimum synthetic conditions for large single crystals of CoAPO-5 are a DAE ratio of 0.2–0.25 and an aging temperature of around 50°C. The average size of the crystals is 230 μm length and 75 μm width. The maximum size is 400 μm length and 150 μm width.     

High-power semiconductor radiation sources based on 100-W diode laser bars for pumping solid-state lasers

High-power semiconductor radiation sources with a total optical output power of up to 5 kW and a power density of 400 W/cm² have been developed, representing a vertical stack of quasi-continuous pulsed (500 μs) 100-W diode laser bars. Based on such sources, a solid-state laser with an output radiation energy of up to 150 mJ has been created, which is intended for information systems, laser radars, spectrum analyzers, etc.     

基于色敏二极管的可见光辐射测量

为了用半导体光电二极管探测可见光波段辐射能量,采用多传感器融合技术,首先分别测定红绿蓝3种色敏二极管的光谱响应,再根据求解超定方程组的方法求得各曲线权数,最后通过加权融合的方式获得接近平坦的系统光谱响应曲线。实验结果表明:用3种色敏二极管构成的测试装置具有较宽的均匀光谱响应范围,响应速度较快,能够降低可见光辐射测量不确定度,获得1种新的可见光辐射测量方法。在420~720nm波长范围内,光谱响应不均匀相对误差小于3.74%。     

High-power external-cavity AlGaAs/GaAs/InGaAs quantum-dimensional heterolasers (λ = 1.06 μm)

Use of an external cavity with a grating ensures effective narrowing of the linewidth (～0.35 nm) of a high-power multimode semiconductor laser with a broad (100 μm) stripe contact. An output power of up to 550 mW has been reached with experimental external-grating-cavity laser diodes. It is demonstrated that a 3-mm-long multimode laser diode based on a quantum-dimensional AlGaAs/GaAs/InGaAs heterostructure (λ = 1.06 μm) can be used with a directly pumped PPLN crystal waveguide to obtain second-harmonic radiation with λ = 0.532 μm.     

Laser pumped light emitting diodes as broad area sources of coherent radiation

This paper describes the use of large area light emitting diodes, pumped with various laser sources, as extended area emitters of coherent radiation. The photon recycling takes place through the intermediary of electron–hole pair formation and subsequent stimulated recombination. It is possible to generate both spontaneous and stimulated emission together and the two channels are then independent of each other. This allows the generation of a mixture of coherent and non-coherent radiation in any desired proportion. The technique described is a broad-band resonant process with diffusive feedback and can be used for generating non-collimated laser radiation for a variety of applications.     

Signal,recombination effects and noise in amorphous silicon detectors

Some properties of hydrogenated amorphous silicon diodes are described. Back biased diodes of the Schottky, p-i-n type, in thicknesses ranging from 5–15 μm, have been tested with 6 MeV alpha particles and with 1 and 2 MeV protons. Large signal saturation, due to electron-hole recombination, occurs for high LET particles. Diodes have been exposed to fast neutron fluences up to 10¹³ cm⁻² and shown to have better radiation resistance than similarly exposed crystalline silicon detectors. From our measurements we extrapolate that minimum ionizing particles can be detected with stacked layers 100–120 μm thick, with adequate signal/noise levels.     

Superfocusing of mutimode semiconductor lasers and light-emitting diodes

The problem of focusing multimode radiation of high-power semiconductor lasers and light-emitting diodes (LEDs) has been studied. In these sources, low spatial quality of the output beam determines theoretical limit of the focal spot size (one to two orders of magnitude exceeding the diffraction limit), thus restricting the possibility of increasing power density and creating optical field gradients that are necessary in many practical applications. In order to overcome this limitation, we have developed a method of superfocusing of multimode radiation with the aid of interference. It is shown that, using this method, the focal spot size of high-power semiconductor lasers and LEDs can be reduced to a level unachievable by means of traditional focusing. An approach to exceed the theoretical limit of power density for focusing of radiation with high propagation parameter M ₂ is proposed.     

Surface-plasmon-enhanced light emitters based on InGaN quantum wells

Since 1993, InGaN light-emitting diodes (LEDs) have been improved and commercialized, but these devices have not fulfilled their original promise as solid-state replacements for light bulbs as their light-emission efficiencies have been limited. Here we describe a method to enhance this efficiency through the energy transfer between quantum wells (QWs) and surface plasmons (SPs). SPs can increase the density of states and the spontaneous emission rate in the semiconductor, and lead to the enhancement of light emission by SP-QW coupling. Large enhancements of the internal quantum efficiencies (eta(int)) were measured when silver or aluminium layers were deposited 10 nm above an InGaN light-emitting layer, whereas no such enhancements were obtained from gold-coated samples. Our results indicate that the use of SPs would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.     

Increasing output power of LEDs (λ = 1.7–2.4 μm) by changing directions of reflected light fluxes in GaSb/GaInAsSb/GaAlAsSb heterostructures

It is demonstrated, using the example of light-emitting diodes (LEDs) based n-GaSb/n-GaIn-AsSb/p-GaAlAsSb heterostructures, that the formation of a curvilinear reflecting surface consisting of hemispherical etch pits on the rear side of an LED chip leads to an increase in the output radiation power by a factor of 1.9–2 in the entire wavelength interval studied (λ = 1.7–2.4 μm) as compared to the LED chip design with a continuous absorbing ohmic contact. This increase in the LED efficiency is related to a change in the directions of reflected light fluxes upon reflection from the hemispherical etch pits.     

Radiation Thermometry and Emissivity Measurements Under Vacuum at the PTB

A new experimental facility was realized at the PTB for reduced-background radiation thermometry under vacuum. This facility serves three purposes: (i) providing traceable calibration of space-based infrared remote-sensing experiments in terms of radiation temperature from  −173 °C to 430 °C and spectral radiance; (ii) meeting the demand of industry to perform radiation thermometric measurements under vacuum conditions; and (iii) performing spectral emissivity measurements in the range from 0 °C to 430 °C without atmospheric interferences. The general concept of the reduced background calibration facility is to connect a source chamber with a detector chamber via a liquid nitrogen-cooled beamline. Translation and alignment units in the source and detector chambers enable the facility to compare and calibrate different sources and detectors under vacuum. In addition to the source chamber, a liquid nitrogen-cooled reference blackbody and an indium fixed-point blackbody radiator are connected to the cooled beamline on the radiation side. The radiation from the various sources is measured with a vacuum infrared standard radiation thermometer (VIRST) and is also imaged on a vacuum Fourier-transform infrared spectrometer (FTIR) to allow for spectrally resolved measurements of blackbodies and emissivity samples. Determination of the directional spectral emissivity will be performed in the temperature range from 0 °C to 430 °C for angles from 0° to ±70° with respect to normal incidence in the wavelength range from 1 μm to 1,000 μm. References to commercial products are provided for identification purposes only and constitute neither endorsement nor representation that the item identified is the best available for the stated purpose.     

A multi-source portable light emitting diode spectrofluorometer

A portable luminescence spectrofluorometer weighing only 1.5 kg that uses multiple light emitting diodes (LEDs) as excitation sources was developed and evaluated. Excitation using a sequence of seven individual broad-band LED emission sources enabled the generation of excitation-emission spectra using a light weight (<1.5 kg) spectrometer. Limits of detection for rhodamine 6G, rhodamine B, and fluorescein were 2.9, 3.2, and 11.0 nM, respectively. Generation of excitation-emission matrices (EEMs) enabled the analysis of samples containing mixtures of rhodamine B and fluorescein. Buffered saline plant and animal feed extracts were also analyzed using this instrument. These samples included the woody plants Pistacia lentiscus (Evergreen pistache or Mastic) and Philyria latifolia, and the herbaceous species Medicago sativa (alfalfa), Trifolium spp. (clover), and a feed concentrate. Application of multi-way principal component analysis (MPCA) to the resulting three-dimensional data sets enabled discernment among these various diet constituents.     

Laser diodes (λ=0.98 μm) with a narrow radiation pattern and low internal optical losses

Quantum-confined InGaAs/AlGaAs/GaAs heterostructures for laser diodes emitting at λ=0.98 μm, optimized to provide for reduced radiation divergence in the vertical plane and decreased internal optical losses, have been synthesized by metalorganic-hydride vapor-phase epitaxy. For the obtained laser diodes the far field pattern full width at half-maximum in the vertical plane falls within 16°–19°, the internal optical losses are about 0.7 cm⁻¹, the internal quantum yield amounts to 97%, and the maximum continuous emission power reaches 8.6 W.     

Resonance Excitation of Photoluminescence in an Aqueous Uranyl Chloride Solution

We have measured photoluminescence spectra of an aqueous uranyl chloride solution under excitation by various light sources: semiconductor light-emitting diodes and cw laser. The excitation wavelength lay within a resonance absorption band of uranyl chloride, which ensured photoluminescence detection at exposure times of 10^–3 s using an extremely small volume of the substance (10^–9 cm³). The photoluminescence spectra were measured using a small minispectrometer, which allowed us to analyze the spectra in the range 200–1000 nm.     

An apparatus for applying tensile,compressive and cyclic stresses on foil specimens during light ion irradiation

An apparatus is described which allows irradiation of thin folded foils under tensile, compressive or cyclic load by light ions at well defined temperatures. The foils are stabilized against buckling under compressive stresses by folding them at angles of 50° or 70°. By this means, more than 100 MPa compressive stress can be safely loaded on 50 μm foils of 20% cold worked stainless steel. Details are also given of the helium-gas cooling circuit, the temperature control device and the irradiation system.     

Semiconductor strained layers

During the past 25 years (after the growth of the first pseudomorphic GeSi strained layers on Si) we have seen a steady accumulation of new materials and devices with enhanced performance made possible by strain. The past year has been good for strained layers. Until recently, short wavelength (violet to green) and mid-infrared (2–5 μm) regions of the spectrum were not accessible to photonic devices. Short wavelength light emitting diodes and laser diodes have been developed using III—Nitride and II–VI strained layers. Improved mid-infrared lasers, using Sb-based III–V semiconductor strained layers, have also been fabricated. These advances have been possible due to improvements in growth techniques as well as in the modelling and computer simulations of layers compositions and device structures. Full band Monte Carlo simulations of electron transport in strained Si predict very high electron mobilities. Computer simulations show that complementary metal oxide silicon circuits, fabricated with strained layer GeSi transistors will have considerably improved performance. Electronic devices based on GeSi strained layers have shown further improvements in high frequency and high power performance. Many devices based on strained layers are being used in commercial equipment. Advances have been made both in modelling and determining the nonuniform strain, using a Raman technique.