Photonic lab-on-a-chip portable platforms have proved to be very sensitive, rapid in analysis and easy-to-use. However, they still rely on a bulk light source to operate, thus hindering the actual portability and potential for commercial realization. In the present paper we have proposed a design for a light emitting structure that could be easily implemented on chip. The design consists of a Si3N4 strip waveguide on SiO2 substrate, with an active material that emits light as top and lateral cladding. The cross-section of the waveguide was optimised to support both excitation and emission as guided modes, with a high mutual overlap and high confinement to the cladding. This ensures an efficient light emission activation from the cladding and a stable propagation along the waveguide. The proposed structure shows to be operative along the visible range; demonstrated from 400nm to 633nm. The procedure we have followed along this report can be virtually used for designing the cross-section geometry of any strip waveguide system so that the performance is optimised for a given cladding refractive index and emission and excitation wavelengths. In addition we have proposed the use of polymeric quantum dots as the gain material to be used as active cladding. The ease of on-chip integration of this gain material via spin-coating, together with the simplicity of our light emitting waveguide, makes our light source design suitable for large-scale integration on Si chip. Specially, for lab-on-chip applications where multiplexed operation is essential.
We study four compressively strained GaInSb/AlGaInSb type I multi quantum-well (QW) laser
structures grown on GaAs, with increasingly strained QWs, aimed at emitting at ~4μm. This
wavelength region is highly important for applications such a free space communication,
biomedical imaging and trace gas sensing. The structures are analysed using photoluminescence,
photo-modulated reflectance and, at room temperature, using our novel, recently developed Fourier
transform infrared surface photo-voltage spectroscopy technique (FTIR-SPS). Neither
photoluminescence nor photo-modulated reflectance managed to give any characterisation
information at room temperature or such detailed information even at low temperatures. However,
FTIR-SPS clearly yielded a full set of transitions for all four samples including not only the barrier
bandgap, but also the QW ground state transition, from which the device operating wavelengths can
be inferred, and up to five excited state QW transitions. The full set of measured transition energies
are then compared closely with those predicted by an 8-band k.p model which takes account of the
band anisotropy and strain. There is generally a good agreement between the QW transitions
predicted by the model and those measured experimentally, but there is also a strong indication that
the current literature values for the AlGaInSb bandgap seem to be in considerable error for the
present alloy compositions. The FTIR-SPS technique gives information of great importance when
designing future devices to emit in this wavelength region.
Negative luminescent (NL) devices, which to an IR observer can appear colder than they actually are, have a wide range of possible applications, including use as modulated IR sources in gas sensing systems and as thermal radiation shields in IR cameras. A further important use would be a calibration source for IR focal plane arrays where there are many potential advantages over conventional sources, including high speed operation (for multi-point correction) and lower power consumption. Such applications present considerable technological challenges as they require large area uniform devices (>1cm2) with a large apparent temperature range.
In this paper we report on recent progress in fabricating large area (1.5cm × 1.5cm) negative luminescence devices from Hg1-xCdxTe grown on silicon substrates using a segmented device architecture.
Negative luminescent devices, which absorb more light than they emit when reverse biased, have a large number of applications including, reference planes for thermal cameras, infrared (IR) sources and IR scene projection. This paper describes devices made from mercury cadmium telluride grown on silicon substrates, focusing on large area arrays with reduced operating powers. Novel growth structures and device designs have been investigated in order to reduce the series resistance. Results from the first dry etched, LW MCT on Si, 1 cm2 device with optical concentrators are presented.
Infrared LEDs and negative luminescent devices, where less light is emitted than in equilibrium, have been attracting an increasing amount of interest recently. They have a variety of applications, including as a 'source’ of IR radiation for gas sensing; radiation shielding for and non-uniformity correction of high sensitivity starring infrared detectors; and dynamic infrared scene projection. Similarly, IR detectors are used in arrays for thermal imaging and, discretely, in applications such as gas sensing. Multi-layer heterostructure epitaxy enables the growth of both types of device using designs in which the electronic processes can be precisely controlled and techniques such as carrier exclusion and extraction can be implemented. This enables detectors to be made which offer good performance at higher than normal operating temperatures, and efficient negative luminescent devices to be made which simulate a range of effective temperatures whilst operating uncooled.
In both cases, however, additional performance benefits can be achieved by integrating optical concentrators around the diodes to reduce the volume of semiconductor material, and so minimise the thermally activated generation-recombination processes which compete with radiative mechanisms. The integrated concentrators are in the form of Winston cones, which can be formed using an iterative dry etch process involving methane/hydrogen and oxygen. We will present results on negative luminescence in the mid and long IR wavebands, from devices made from indium antimonide and mercury cadmium telluride, where the aim is sizes greater than 1cm x 1cm. We will also discuss progress on, and the potential for, operating temperature and/or sensitivity improvement of detectors, where very higher performance imaging is anticipated from systems which require no mechanical cooling.
A dynamic infrared scene projector based on IR luminescent devices has many potential advantages compared with existing systems based on micro-resistor arrays. These include very fast response times, as individual devices can be driven at frequencies greater than 1 MHz, and no need for cryogenic cooling. Additionally, luminescent sources can not only appear hot to an IR observer when in forward bias, but also appear cold in reverse bias (commonly referred to as negative luminescence), so that a large apparent temperature range around ambient can be simulated. For a scene projector a large array of photodiodes is required, where each photodiode can be biased individually. As a precursor to the manufacture of a scene projector, we have already fabricated large area MW devices, consisting of arrays of photodiodes, suitable for use as calibration sources in IR cameras. To reduce the currents needed to achieve maximum dynamic temperature range, we have used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb devices. We present here recent results from a large area (~0.86cm2) medium wavelength (MW) device, consisting of an array of photodiodes each with an integrated optical concentrator. The reverse saturation current of the device was measured to be ~2.3A/cm2, which is significantly smaller than the value of ~9A/cm2 reported previously for similar devices without optical concentrators. The device also displays a large apparent temperature range in line with device modelling. Finally, we will discuss the perspectives on using similar devices for dynamic infrared scene projection.
Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as IR sources in gas sensing systems and as thermal radiation shields in IR cameras. Additionally these devices can be used as calibration sources for very large IR focal plane arrays and have many potential advantages over conventional calibration sources, including high speed operation and low power consumption. For many of these applications a large area device which displays as large area 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 dynamic range. We have therefore used a novel micromachining techniqe to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption and the required currents are thus reduced. To fabricate the concentrators, spherical resist masks are first produced by resist reflow. Inductively coupled plasma etchign is then used to alternatley etch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Recent results from large area medium wavelength devices with integrated optical concentrators are presented, together with a description of the continuing optimization of the process and progress towards the fabrication of large area long wavelength devices.
Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as thermal radiation shields in IR cameras, and as IR sources in gas sensing systems. For many of these applications a large area (>1cm2) device is required, together with as large as possible apparent temperature range. However, under reverse bias significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. These may lead to current heating of the device, which in turn reduces the apparent temperature range. 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 parabolic resist masks are first produced, which are approximately 10 μm high and approximately 53 μm wide, by resist reflow at 120 degrees C. Inductively coupled plasma (ICP) etching is then used to alternately etch 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 approximately 15 μm. Continuing optimization of the process to reach the theoretical limits of optical gain is described.
One of the most important factors limiting the optical efficiency of LEDs is total internal reflection of generated light, where photons incident to the surface at angles greater than the critical angle are reflected back into the semiconductor and absorbed. Most semiconductors have a large refractive index and hence a small critical angle. Narrow gap semiconductors, such as InSb, have particularly large refractive indexes and corresponding smaller critical angles. Additionally, strong absorption of light in the 3-5(mu) m range means that epoxy immersion lenses, which are used for GaAs Ir LEDs, cannot be used in InSb based IR LEDs. We have therefore used a novel micromachining technique to fabricate optical concentrators inInSb and HgCdTe layers. Inductively coupled plasma (ICP) etching is used to alternatively eatch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with parabolic profiles. Continuing optimization of the process to reach the theoretical limits of optical gain is described together with some of the main issues associated with the fabrication process.
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