This paper discusses key issues related to the quantum dot infrared photodetector (QDIP). These are the normal incidence response, the dark current, and the responsivity and detectivity. We attempt to address the following questions of what is QDIP’s potential, what is lacking, and what is needed to make the device interesting for practical applications. It is argued that so far the present QDIP devices have not fully demonstrated the potential advantages. Representative experimental results are compared with characteristics of quantum well infrared photodetectors. Areas that need improvements are pointed out.
Keywords: quantum dot, infrared, photodetector, normal incidence, dark current, responsivity.
A review is given of efforts to prepare thick gallium nitride films on lattice-matched g-LiAlO2 substrates. Much progress in the design of new high performance nitride device structures is presently impeded by the lack of GaN substrates, leading to large defect concentrations in layers grown on foreign materials. These problems could be alleviated if a true GaN substrate were to become available, allowing homoepitaxial growth. The preparation of 50 mm diameter boules of g -LiAlO2 from the melt will be discussed, including wafer preparation. Growth of thick (300–400 µm) GaN layers on the g -LiAlO2 wafers will be presented. The GaN is deposited by the halide vapour phase epitaxy (HVPE) method. Characteristics of these 50 mm diameter wafers are explained in detail. Much progress has been made, but several problems remain to be overcome.
Keywords: nitrides, g -LiAlO2 wafers, GaN substrates, HVPE method
A microstructural mechanism of photoinduced structural transformations in amorphous As2Se3 thin films was studied by IR Fourier-transform spectroscopy in the range of 300–100 cm–1. It was shown that the stage of irreversible photostructural changes in the freshly-deposited films was connected with coordination of topological defects formation accompanied by homopolar chemical bonds switchings in heteropolar ones. By contrast, the reversible stage of photoinduced effects were caused by two different types of transformations – heteropolar bonds switching in homopolar ones, as well as heteropolar bridge bonds switching in short layer ones. Both processes were associated with the formation of anomalously coordinated topological defect pairs and atomic displacements at the medium-range ordering level.
Keywords: amorphous semiconductors, photoinduced changes, IR Fourier spectroscopy.
In the paper, an analysis of the influence of low frequency characteristics of digital optical links on bit error rate performance is undertaken. Effects caused by both baseline wander and flicker noise are investigated. Links with one and two poles in the low frequency range are taken into account. Also some practical conclusions emerging from the analysis are pointed out.
Keywords: baseline wander, bit error rate, optical links.
Along with the conventional optogalvanic (OG) signal an anomalous light-induced response has been detected in hollow cathode discharge. This response manifests itself both in the amplitude and time-resolved OG reactions. The anomalous OG signal is found to be less dependent on the absorbing optical transition and yet, more informative on the parameters of the plasma medium, that is the OG detector. This circumstance is proposed for usage in four applications based on the behaviour of the anomalous OG signal. The latter is taken as a spectral marker, as an amplified quasi-amplitude signal as well as a sensitive tool in monitoring procedure.
Keywords: hollow cathode discharge, optogalvanic effect, dynamic optogalvanic signal, inflection operating point, spectral marker.
A sensor was developed to measure the gloss of curved metal objects. The crucial part of the sensor is a diffractive optical element (DOE). The advantages of the present sensor are non-contact measurement mode, normal incidence of the probe light, and possibility to measure gloss of curved surfaces. The sensor yields information also on the surface texture, such as finishing marks, of curved metal surfaces. The operation of the sensor was verified by measuring draw-off pipes of water cranes, which were obtained from metal industry.
Keywords: gloss, curved metallic products, surface texture, diffractive optical element (DOE).
Although excellent research tools, the ultrafast laser systems are poorly suited for routine laboratory instrumentation, simply due to their high price. Our research group has shown that application of two-photon excitation need not to be limited to those laboratories that can afford the price and maintenance of ultrafast laser systems. With certain compromises and well-designed experiments, low-cost lasers can be used in utilizing the positive properties of two-photon excitation. The purpose of this publication is to show examples of bioanalytical applications that are possible using low-cost lasers. A bioaffinity assay using microparticles as bioactive carriers and an assay for characterization of cell surface antigens are presented. By direct comparison of the example assays in two instrumental set-ups, a quantity is given to the ‘compromises’ that are made by replacing an ultrafast system with low cost equipment.
Keywords: two-photon excitation, bioanalytics, bioaffinity assays, in-vitro diagnostics.
A new type of passive infrared human body detecting photoreceiver with popcorn noise reduction systems is presented. When the range of operating temperatures and temperature gradients of a pyroelectric detector are spread, the popcorn noise occurs. To avoid error signal at the system output we designed two-signal processing circuits. We observed a difference of frequency components at the photoreceiver output. The output current resulting from popcorn noise holds high frequency components compared with frequency components of a moving person. So, we can distinguish popcorn noise by analysing frequency components of the output current from the pyroelectric detector.
Keywords: pyroelectric detectors, low-noise preamplifier, thermal imaging systems.
Based on the excitation space anisotropy a partial polarisation of the spontaneous emission is found in hollow cathode discharge. This polarisation is ascribed to existing spontaneous coherence, i.e., self-alignment of the excited states. The aligning factors are the beam-like fast electrons from the cathode dark space. An expression for observed signal of the spontaneous emission magnetic depolarisation is obtained. Within the frames of the ordinary opto-galvanic effect a separate coherent conductivity is analysed. A poor coherent conductivity due the self-aligned states is detected.
Keywords: hollow cathode discharge, excitation space anisotropy, coherence, self-alignment, opto-galvanic effect and coherent conductivity.
Two-channel acousto-optic modulator (AOM) makes simultaneous assignment of the frequency and phase of radio-signals possible. AOM is a fundamental element of a spectrum analyser based on acousto-optic receiver. In the paper, an analysis of influence of structural elements on characteristics of a two-channel modulator was shown and the procedure of optimisation of its construction described. Basing on this procedure the modulator for spectrum analysis at GSM band was designed, performed and investigated. The two-channel acousto-optic modulators give the possibility of detection and defining direction of radiation with accuracy of 0.50.
Keywords: acousto-optics, modulator, phase detection.
A comprehensive, fully self-consistent, optical-electrical-thermal-gain, three-dimensional model of edge-emitting (EE) nitride diode lasers has been developed and used to simulate a room-temperature (RT) continuous-wave (CW) threshold operation of typical EE designs of a nitride diode laser. With the aid of the model, some RT CW performance characteristics of the laser have been anticipated taking into consideration important features of nitride materials. These features have proved already to be often essentially different from those of other AIIIBV materials, therefore expected properties of nitride devices cannot be estimated on the basis of known properties of analogous devices manufactured from other materials.
Keywords: nitride diode lasers, modelling of a diode-laser operation, edge-emitting diode lasers.