In this work, the results are presented of a nanorod LED array. If the lateral size of the nanorods is small enough, it is
possible to achieve a degree of lateral confinement. If the nanorods are ordered into a suitable photonic lattice, then this
will reduce the lateral spontaneous emission and enhance emission along the vertical axis via the Purcell effect.
Additionally there is a degree of dislocation filtering that can occur [1]. However, one potential drawback of this device
is the large free surface that borders the multi-quantum well active region. Nevertheless, it has been shown that the
surface recombination in the nitride materials is the lowest of all III-V semiconductors.
Results of SEM, PL, EL, and far field pattern are presented to compare the progressive effect of using photo-assisted
electroless and wet etching [2]. It can be seen that over time the photo-assisted electroless method clearly delineates the
active MQW region, possibly as a result of the different etch rate of InGaN. Alternatively, a purely chemical etching
method was used. With a narrowing of the nanorods, there is a progressive blue shift of the photoluminescence peak. The
optical image of the emission shows that there are well-defined lines of enhanced light propagation that match the
symmetry of the nanorod array, thus showing there is a photonic crystal effect.
A resonance tunnelling LED structure having a high efficiency, low droop and negligible wavelength shift with current
is reported in this study. The LED structure contains a thick InGaN bottom spacer between an n-GaN contact layer and a
multiple quantum well (MQW) active region, and a thin InGaN top spacer between the MQW and an AlGaN electron
blocking layer (EBL). The observed high efficiency and negligible wavelength shift with applied current are attributed
to the thick InGaN bottom spacer that nucleates V-pits and acts as a strain control layer for the MQW. The thick InGaN
layer also provides an electron reservoir for efficient electron tunnelling injection into the MQW and reduces the electropotential
difference between the n-emitter and the p-emitter, to suppress current leakage at high driving current and
reduce droop. The top InGaN spacer was designed to act as a magnesium back-diffusion barrier and strain relief layer
from EBL so as to obtain high efficiency.
The characteristics of surface plasmon wave (SPW) modes supported by multiple layer configurations are analysed in this paper. The transmission-line analysis based on the transverse resonance condition is implemented to describe SPW excitation via a prism coupler. This accurate yet convenient modeling technique is also applied to investigate the characteristics of SPWs in multilayer media with optical gain as a means of reducing/overcoming the typical, undesirably large loss associated with SPWs in conventional structures. The paper will also present experimental and theoretical results to quantify the operational characteristics to be expected from prism coupled SPW sensors that use semiconductor optical sources.
The development of new medical tools for minimal-invasive surgery is essential for the reduction of pain, side-effects and hospitalisation costs. The benefits of using optical methods in some procedures are now well recognised. In this context semiconductor optical sources offer the advantages of high efficiency, compactness, low cost, long lifetime, low power consumption and high reliability. The optical source presented in this paper comprises a phase-locked, high-power laser diode array and beam shaping optics designed to optimise the coupling to small-diameter optical fibres. The high-power, index-guided laser array used in this work was developed to achieve high-brightness by adopting a specially designed optical cavity based on the parabolic taper for each individual array element. With this design the individual elements are phase-coherent and the fundamental mode of the array is dominant, thus achieving quasi-diffraction-limited operation without the use of external lenses. Compared to other high-brightness laser arrays, the parabolic bow-tie laser array used here offers the advantages of simple device fabrication, reduced costs and compactness. In this work, specially designed lenses are used to circularise the beam and, therefore, to focus the beam to a small spot-size to improve coupling to single mode fibres. Details of the device characteristics, with emphasis on beam quality and phase front, and of the design of anamorphic optics for beam shaping and focusing will be presented in the context of the integration of the high-brightness laser array with the specially design optics to achieve optical power delivery exclusively where necessary via small-diameter optical fibres.
With the widespread use of laser diodes in modern industry there has been an increasing demand for high optical output power devices with good beam quality and, ideally, low production and packaging costs. Reliability and long lifetime are essential requirements since they determine the extent to which such sources will be utilised. The devices of interest here are arrays of parabolic bow-tie lasers which have been specially designed to achieve high power with high brightness without the need for re-growth or sophisticated device fabrication. This paper presents a comparative study on laser diode arrays to investigate the effects of scaling and device geometry on device operation, including degradation and ageing. Temperature profiles at the array facets have been obtained using a thermal imaging system. The HgCdTe-based detector operates in the 1-5.5μm wavelength range. The results obtained indicate a smaller increase in temperature (2-5°C) in uncoupled arrays with respect to phase-coherent arrays and a considerable increase in temperature with increasing number of elements in the array. Such considerations are essential to properly manage thermal dissipation and improve the operational characteristics of such devices.
Superluminescent Diodes are high power semiconductor optical sources with relatively broad spectral linewidth used for a variety of applications. The basic, crucial device feature needed to achieve high-power superluminescent operation is low facet reflectivity to prevent lasing and optical gain saturation. Theoretical calculations show that by appropriately designing the device length and output facet reflectivity a significant increase in optical output power and wall-plug efficiency can be obtained. This paper presents theoretical and preliminary experimental results indicating that it is possible to further improve the operational characteristics of superluminescent diodes and achieve high optical output power with high conversion efficiency. The control of the facet reflectivity and the overall device optical geometry are obtained by using Focused Ion Beam post-fabrication processing. Results obtained from the characterisation of superluminescent diodes before and after facet reflectivity alteration achieved by creating patterns on the subwavelength scale will be discussed.
A wealth of data point to Delayed Luminescence (DL) as a good candidate for early and reliable detection technique in neoplastic cells and tissues sorting. Aiming at a DL experimental set up for such a kind of information, a testing technique for morphological analysis should be provided. This could certify the early identification of pathologies and abnormalities in cells and tissues by DL. DL technique may be coupled with FIB (Focused Ion Beam) imaging
analysis to give a correlated, both spectroscopic and morphological investigation, at the submicron scale.
A strong link among others has been reported to exist between DL signal characteristics and cytoskeleton structure and dynamics: FIB (Focused Ion Beam) imaging is for the moment being the best non invasive check at all and it can detect morphological alterations as early as possible since its resolution can go down to 2-5 nm.
The cells, that can be highlighted by the fast DL and slow and efficient FIB, can be in parallel analysed by a metabolic manometric technique that uses differential pressure sensors: the different cellular activity of normal and abnormal cells can be recorded and this allows fast and non-invasive investigations, although requiring a minimal number of cells.
In addition it’s possible to study, by the confocal microscopy spectroscopic analysis, DNA fragments, exploiting the optical characteristics of a dye, like ethidium bromide, to detect dynamic and conformational changes in DNA chains. These changes can be artificially induced in cells (e.g. by irradiation) or found in neoplastic cells. The acquired experience allows an independent check of spectroscopic, morphologic and metabolic testing by a control on nucleic acid defects.
These four techniques may be used together creating a "protocol" in order to permit an early and reliable alterations diagnosis of cells and tissues, guaranteeing an high accuracy standard.
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