Volume 16, Issue 4

In this paper we summarize the recent results on nonlinear dynamics of vertical-cavity surface-emitting lasers (VCSELs) which are bearing the fingerprints of the particular polarization properties of VCSELs. Due to the surface emission and cylindrical symmetry, VCSELs lack strong polarization anisotropy and may undergo polarization switching and polarization mode-hopping. This provides new specificities to the rich nonlinear dynamics induced in VCSELs by an external perturbation. We unveil both experimentally and theoretically these new specificities for the case of optical feedback, optical injection, and unidirectional synchronization between VCSELs.

External cavity tunable lasers have been around for many years and now constitute a large group of semiconductor lasers featuring very unique properties. The present review has been restricted to the systems based on the edge emitting diode lasers set-up in a hybrid configuration. The aim was to make the paper as concise as possible without sacrificing, however, most important details. We start with short description of the fundamentals essential for operation of the external cavity lasers to set the stage for explanation of their properties and some typical designs. Then, semiconductor optical amplifiers used in the external cavity lasers are highlighted more in detail as well as diffraction gratings and other types of wavelength-selective reflectors used to provide optical feedback in these lasers. This is followed by a survey of designs and properties of various external cavity lasers both with mobile bulk gratings and with fixed wavelength selective mirrors. The paper closes with description of some recent developments in the field to show prospects for further progress directed towards miniaturization and integration of the external cavity laser components used so far to set-up hybrid systems.

The spectral and orientational properties of some dichroic fluorescent dyes (naphthalene derivatives of bicarboxylic acid) in some isotropic solvents and in an anisotropic matrix have been studied. The order parameter of the dyes in the nematogenic liquid crystal 7CB has been determined from measurements of polarized electronic absorption and fluorescence. The influence of the dye addition on the nematic-isotropic phase transition temperature of 7CB has been investigated. It was found that some of the dyes studied could be successfully utilized in guest-host liquid crystal display devices.

A statistical theory of dipole flexoelectric (FE) polarization in liquid crystals is used to calculate temperature dependence of order parameters, elastic constants and FE coefficients. Two systems with polar wedge-shaped and banana-shaped molecules are investigated. In both cases, the FE coefficients are proportional to the dipole moment component parallel to the molecule symmetry axis. It results from the symmetries of interactions and of the Mayer function. The origin of the FE effect and microscopic pictures of the distorted phases are discussed.

Compounds with moderate and large negative dielectric anisotropy (Δɛ) are very attractive liquid crystal (LC) for vertical alignment mode (VA). Materials with such properties can be achieved by lateral substitution of a polar group into a mesogenic molecule. We synthesized some new LC materials with a negative value of Δɛ, a moderately high birefringence (Δn), and a low viscosity. The mesomorphic and physical behaviour of the novel biphenyls, terphenyls and quaterphenyls fluorosubstituted in the rigid core and also with fluorinated alkyl and alkoxy chains are investigated. The prepared series of four LC compounds are promising for new LC mixtures for various applications. Examples of nematic mixtures with Δɛ∼−3.25 will be presented.

Static Kerr effect and the pre-transition temperature were investigated for the 6CHBT, 6BOBT and the nematic mixtures, M1 (40% 6CHBT: 60% 6BOBT), M2 (30% 6CHBT: 70% 6BOBT) and M3 (20% 6CHBT: 80% 6BOBT) at temperatures above the nematic-isotropic transition temperature. The linear dependence of (T-T*)−1 on the Kerr constant is found to be in good agreement with the predications of the Landau-De Gennes model. The hypothetical second order phase transition temperatures and susceptibility values were determined for these compounds.

Possible influence of a nematics splay-bend elastic constant and corresponding free energy functional term on adequacy of description of nematics states is studied by comparing results of theoretical considerations and computer simulations with experimental data. Planar deformation states enforced in flat nematics cells by symmetric aligning cover coatings and external electric or magnetic fields are studied in one-dimensional approximation. Nematics material parameters and nematics-substrate coupling characteristics are determined from experimental data by immediate measurement or by solving inverse problems. Two series of cells with thicknesses between twenty and forty-four micrometers (in fact two wedge cells) with two different orienting substrates are investigated by measuring a cell optical response in a birefringence experimental system. Good agreement between optical retardation measured and simulated is observed. Posterior comparison of free energy contributions from substrate interactions and a hypothetical surface-like term leads to a conclusion that the last one together with a splay-bend elastic constant should be very small and can be neglected.

The paper presents a review of recent results on static dielectric behaviour in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices. Temperature behaviour of static permittivity and its derivative are interpreted in terms of a basic thermodynamic quantities increment (internal energy, entropy and Helmholtz free energy) induced by a probing electric field.

Electron beam irradiation studies on liquid crystal material 5CB have been carried out at a temperature where the compound exists in the isotropic liquid phase. In situ time-resolved spectroscopic characterization was carried out during the irradiation. Three different transients were observed during the 2-μs electron pulse. After about 50 μs, only one transient species was found to be present, which has an absorption peak at 360 nm. Radiolysed sample exhibits a broad absorption at ∼400 nm. The dielectric measurements show that even a low level of irradiation results in a dramatic increase in the component of dielectric permittivity normal to the long axes of the molecules ε⊥′, and a corresponding decrease in the dielectric anisotropy (Δε′=ε∥′−ε⊥′ ). These studies show that 5CB is prone to substantial radiation damage on exposure to the beam of high-energy electrons.

The crosstalk between concentration changes in oxygenated haemoglobin and deoxygenated haemoglobin calculated by the modified Lambert-Beer law in near-infrared topography is theoretically investigated. The changes in intensity detected with probe pairs on the scalp caused by the concentration change in either oxygenated or deoxygenated haemoglobin induced by the focal brain activation is predicted by Monte Carlo simulation. The topographic images of the changes in oxygenated and deoxygenated haemoglobin are obtained from the changes in the intensity of light at two wavelengths detected by probe pairs to evaluate the crosstalk. The crosstalk slightly depends on the positional relationship between the probe arrangement and the focal brain activation and is minimised when the focal brain activation is located below a measurement point that is the midpoint between a probe pair. The 690-/830-nm wavelength pair is practically effective for reducing the crosstalk, especially the crosstalk from oxygenated haemoglobin to deoxygenated haemoglobin, in the NIR topography.

This paper gives an overview of principles, technologies, and applications using near infrared spectrometry and imaging (NIRS and NIRI) to study brain function. The physical background is reviewed and technologies and their properties are discussed. Advantages and limitations of NIRI are described. The basic functional signals obtained by NIRI, the neuronal and the hemodynamic signal are described and in particular publications about the former are reviewed. Applications in adults and neonates are reviewed, too.

The paper presents a signal processing system used for nitrogen dioxide detection employing cavity enhanced absorption spectroscopy. In this system, the absorbing gas concentration is determined by the measurement of a decay time of a light pulse trapped in a cavity. The setup includes a resonance optical cavity, which was equipped with spherical and high reflectance mirrors, the pulsed diode laser (414 nm) and electronic signal processing system. In order to ensure registration of low-level signals and accurate decay time measurements, special preamplifier and digital signal processing circuit were developed. Theoretical analyses of main parameters of optical cavity and signal processing system were presented and especially signal-to-noise ratio was taken into consideration. Furthermore, investigation of S/N signal processing system and influence of preamplifier feedback resistance on the useful signal distortion were described. The aim of the experiment was to study potential application of cavity enhanced absorption spectroscopy for construction of fully optoelectronic NO2 sensor which could replace, e.g., commonly used chemical detectors. Thanks to the developed signal processing system, detection limit of NO2 sensor reaches the value of 0.2 ppb (absorption coefficient equivalent = 2.8 × 10−9 cm−1).

Wider dissemination of medical digital video libraries is affected by two correlated factors, resource effective content compression that directly influences its diagnostic credibility. It has been proved that it is possible to meet these contradictory requirements halfway for long-lasting and low motion surgery recordings at compression ratios close to 100 (bronchoscopic procedures were a case study investigated). As the main supporting assumption, it has been accepted that the content can be compressed as far as clinicians are not able to sense a loss of video diagnostic fidelity (a visually lossless compression). Different market codecs were inspected by means of the combined subjective and objective tests toward their usability in medical video libraries. Subjective tests involved a panel of clinicians who had to classify compressed bronchoscopic video content according to its quality under the bubble sort algorithm. For objective tests, two metrics (hybrid vector measure and hosaka Plots) were calculated frame by frame and averaged over a whole sequence.

We revealed that the phase function of a thin-film structure measured by a white-light spectral interferometric technique depends on the path length difference adjusted in a Michelson interferometer. This phenomenon is due to a dispersion error of a beam splitter cube, the effective thickness of which varies with the adjusted path length difference. A technique for eliminating the effect in measurement of the phase function is described. In a first step, the Michelson interferometer with same metallic mirrors is used to measure the effective thickness of the beam splitter cube as a function of the path length difference. In a second step, one of the mirrors of the interferometer is replaced by a thin-film structure and its phase function is measured for the same path length differences as those adjusted in the first step. In both steps, the phase is retrieved from the recorded spectral interferograms by using a windowed Fourier transform applied in the wavelength domain.

The recent development of table top extreme ultraviolet (EUV) lasers have enabled new applications that so far were restricted to the use of large facilities. These compact sources bring now to the laboratory environment the capabilities that will allow a broader application of techniques related to nanotechnology and nanofabrication. In this paper we review the advances in the utilization of EUV lasers in nanopatterning. In particular we show results of the nanopatterning using a table-top capillary discharge laser producing 0.12-mJ laser pulses with 1.2-ns time duration at a wavelength λ = 46.9 nm. The nanopatterning was realized by interferometric lithography using a Lloyd’s mirror interferometer. Two standard photoresists were used in this work, polymethyl methacrylate (PMMA) and hydrogen silsesquioxane (HSQ). Pillars with a full width half maximum (FWHM) diameter of 60 nm and holes with FWHM diameter of 130 nm were obtained over areas in excess of 500×500 μm2.

In a previous paper we proposed a modification of metal-coated tapered-fibre aperture probes for scanning near-field optical microscopes (SNOMs). The modification consists in radial corrugations of the metal-dielectric interface oriented inward the core. Their purpose is to facilitate the excitation of surface plasmons, which increase the transport of energy beyond the cut-off diameter and radiate a quasi-dipolar field from the probe output rim. An increase in energy output allows for reduction of the apex diameter, which is the main factor determining the resolution of the microscope. In two-dimensional finite-difference time-domain (FDTD) simulations we analyse the performance of the new type of SNOM probe. We admit, however, that the two-dimensional approximation gives better results than expected from exact three-dimensional ones. Nevertheless, optimisation of enhanced energy throughput in corrugated probes should lead to at least twice better resolution with the same sensitivity of detectors available nowadays.

Third-generation infrared (IR) systems are being developed nowadays. In the common understanding, these systems provide enhanced capabilities-like larger numbers of pixels, higher frame rates, and better thermal resolution as well as multicolour functionality and other on-chip functions. In this class of detectors, two main competitors, HgCdTe photodiodes and quantum-well photoconductors, have being developed. Recently, two new material systems have been emerged as the candidates for third generation IR detectors, type II InAs/GaInSb strain layer superlattices (SLSs) and quantum dot IR photodetectors (QDIPs). In the paper, issue associated with the development and exploitation of multispectral photodetectors from these new materials is discussed. Discussions is focused on most recently on-going detector technology efforts in fabrication both photodetectors and focal plane arrays (FPAs). The challenges facing multicolour devices concerning complicated device structures, multilayer material growth, and device fabrication are described.