Search

The characteristics of four-layer chiral metamaterials, optically active in the important mid-infrared region, have been investigated using simulations and experiments. Results show that the spectral response of the materials can be tailored, while preserving the magnitude of the circular dichroism, relative to standard double layer metamaterials. An analysis of the coupling in these four-layer structures shows that they offer greater design freedom than might be expected from a simple consideration of double layer structures. [ABSTRACT FROM AUTHOR]

The ability to confine light to subwavelength scales has led to exciting developments in fields ranging from sensing to single molecule chemistry. In this paper, we demonstrate how arrays of annular hole resonators can be used to confine surface acoustic waves (SAWs) to regions of the propagating surface that are much smaller than the wavelength of the SAWs. These microscopic elastic waves are used in devices for signal processing and sensing and, increasingly, in areas such as quantum information and microfluidics. This work potentially transforms the ability to enhance interactions with SAWs and could pave the way for advances that mirror those in nano-photonics. [ABSTRACT FROM AUTHOR]

Emission characteristics, spectral properties, and quantum efficiencies of AlxIn1-xSb light-emitting diodes, with aluminum compositions between 0% and 8.75%, have been investigated as a function of temperature from 25 to 300 K, and a function of current from 1 to 100 mA. As both current and temperature are varied a change in the dominant recombination mechanism is observed as indicated by changes in the measured emission. An analysis of the light-current characteristics shows that Auger processes become important in all devices at temperatures above 100 K, implying an activation energy of approximately 7–13 meV depending on the aluminum composition. [ABSTRACT FROM AUTHOR]

The spatial and spectral characteristics of mid-infrared thermal emission from devices containing a large area multilayer graphene layer, encapsulated using hexagonal boron nitride, have been investigated. The devices were run continuously in air for over 1000 h, with the emission spectrum covering the absorption bands of many important gases. An approximate solution to the heat equation was used to simulate the measured emission profile across the devices yielding an estimated value of the characteristic length, which defines the exponential rise/fall of the temperature profile across the device, of 40 μm. This is much larger than values obtained in smaller exfoliated graphene devices and reflects the device geometry, and the increase in lateral heat conduction within the devices due to the multilayer graphene and boron nitride layers. [ABSTRACT FROM AUTHOR]

The acoustoelectric current in graphene has been investigated as a function of illumination, using blue (450 nm) and red (735 nm) light-emitting diodes (LEDs), and surface acoustic wave (SAW) intensity and frequency. The measured acoustoelectric current increases with illumination, more than the measured change in the conductivity of the graphene, whilst retaining a linear dependence on the SAW intensity. The latter is consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model suggesting that the change in the acoustoelectric current is caused by the effect of the illumination on the electronic properties of the graphene. The increase in the acoustoelectric current is greatest under illumination with the blue LED, consistent with the creation of a hot electron distribution. [ABSTRACT FROM AUTHOR]

The acoustoelectric current in graphene has been investigated as a function of temperature, surface acoustic wave (SAW) intensity, and frequency. At high SAW frequencies, the measured acoustoelectric current decreases with decreasing temperature, but remains positive, which corresponds to the transport of holes, over the whole temperature range studied. The current also exhibits a linear dependence on the SAW intensity, consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model. At low temperatures and SAW frequencies, the measured acoustoelectric current no longer exhibits a simple linear dependence on the SAW intensity, and the direction of the acoustoelectric current is also observed to reverse under certain experimental conditions. [ABSTRACT FROM AUTHOR]

We have investigated the spatial and spectral characteristics of mid-infrared thermal emission from large area Chemical Vapor Deposition (CVD) graphene, transferred onto SiO2/Si, and show that the emission is broadly that of a grey-body emitter, with emissivity values of approximately 2% and 6% for mono- and multilayer graphene. For the currents used, which could be sustained for over one hundred hours, the emission peaked at a wavelength of around 4 μmand covered the characteristic absorption of many important gases. A measurable modulation of thermal emission was obtained even when the drive current was modulated at frequencies up to 100 kHz. [ABSTRACT FROM AUTHOR]

Graphene was transferred to the propagation path of quartz surface acoustic wave devices and the attenuation due to gas loading of air and argon measured at 70 MHz and 210 MHz and compared to devices with no graphene. Under argon loading, there was no significant difference between the graphene and non-graphene device and the values of measured attenuation agree well with those calculated theoretically. Under air loading, at 210 MHz, there was a significant difference between the non-graphene and graphene devices, with the average value of attenuation obtained with the graphene devices being approximately twice that obtained from the bare quartz devices. [ABSTRACT FROM AUTHOR]

The characteristics of mid-infrared laser diodes have been investigated before and after the patterning of wavelength-scale metallic apertures on the emitting facet. Before modification of the facet, the emitted spectrum consisted of a large number of peaks associated with different spatial modes, whereas afterwards, the spectrum was dominated by a single peak. Simulations showed that the patterning of the facet caused the effective reflectivity to be different for each lateral mode, suggesting that the peak in the measured spectra is associated with the single lateral mode which is most strongly reflected from the modified facet. [ABSTRACT FROM AUTHOR]

A technique for the reduction in Shockley–Read–Hall recombination in semiconductor diodes, in which areas of the depletion region are removed, is described and preliminary results obtained using AlInSb light-emitting-diodes (LEDs) are presented. Both the electrical and optical characteristics of the devices were significantly improved by removing parts of the depletion region using dry etching, with an average increase in the zero bias differential resistance and LED emission at low currents by a factor of 2. [ABSTRACT FROM AUTHOR]

Electroluminescence from GaInSb/AlGaInSb type I quantum well diode lasers, grown on GaAs, has been investigated as a function of strain in the quantum wells. Lasing was observed, in pulsed operation, up to temperatures of 161, 208, 219, and 202 K for structures containing 0.55%, 0.62%, 0.78%, and 1.1% strain, respectively, with lasing occurring at ∼3.3 μm at 200 K for the 1.1% structure. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) from GaInSb/AlInSb type I multi-quantum-wells, grown on GaAs, has been investigated as a function of strain in the quantum wells. Luminescence, between 3 and 4 μm, was observed for all samples, with good agreement between the measured and calculated peak emission energies. Analysis of the temperature dependence of the luminescence suggests that population of excited quantum well hole subbands occurs at high temperature, leading to a reduction in the PL signal. Room temperature luminescence was obtained from a sample with ∼0.8% strain in the quantum wells. Preliminary results from laser diodes fabricated from companion wafers indicate lasing up to 220 K. [ABSTRACT FROM AUTHOR]

The realization of midinfrared GaInSb/AlGaInSb type I quantum well diode lasers grown on GaAs is reported. Lasing was observed up to 95 K, at an emission wavelength of ∼3.5 μm, threshold current density of 115 A/cm2, and with a characteristic temperature T0∼51 K. [ABSTRACT FROM AUTHOR]

The properties of AlxIn1-xSb light-emitting diodes (LEDs) have been investigated as a function of aluminum concentrations between 0% and 8.8%. By varying the aluminum concentration it is possible to tailor the peak emission wavelength to match the characteristic absorption of CO2, CO, CH4, NO, and NO2, making these diodes suitable for use in infrared gas sensing applications. The total emitted power and internal quantum efficiency were found to have maxima of 27 mW/cm2 and 4.2%, respectively, at a composition of 2.5%, where the peak emission was found to be 5.3 μm, making LEDs of this composition particularly suited to the detection of NO. [ABSTRACT FROM AUTHOR]

The properties of InSb/AlxIn1-xSb quantum-well light-emitting diodes have been investigated as a function of temperature from 300 to 15 K. Over the whole range, the peak emission occurred at significantly higher energies than the band gap of InSb but below the band gap of the AlxIn1-xSb barriers, confirming that emission is from the quantum wells. Maximum internal quantum efficiencies of 65% and 85% were measured at 15 K for diodes containing 40 and 20 nm quantum wells, respectively. [ABSTRACT FROM AUTHOR]

Lateral light emitting diodes have been fabricated in InSb/AlxIn1-xSb quantum wells using a simple bevel etching technique. The peak in emission was found to be in the range of 4-5 μm, confirming that the emission was from the quantum well. [ABSTRACT FROM AUTHOR]

The luminescent properties of single InSb quantum wells have been investigated as a function of temperature and incident laser power. A large peak was observed in the photoluminescence, corresponding to emission from the quantum well, which moves to lower energy with increasing temperature and tracks the bulk InSb band gap with a constant energy up-shift of ∼50 meV. The integrated photoluminescence intensity was observed to fall with increasing temperature, with a well defined activation energy of 17 meV at high temperatures, and to be proportional to the square of the excitation intensity. [ABSTRACT FROM AUTHOR]

We have investigated the room-temperature electroluminescent properties of InSb/AlxIn1-xSb quantum-well light-emitting diodes. The maximum emission from diodes containing quantum wells occurred at significantly higher energies than the band gap of InSb. Close agreement between experimental and theoretical data confirms that recombination occurs within the quantum well. [ABSTRACT FROM AUTHOR]

We have investigated the negative luminescent properties of a device fabricated from metalorganic vapor phase epitaxy grown HgCdTe on a Si substrate. The peak emission was at 7.2μm, and the intrinsic Auger processes were found to be very well suppressed. The low currents (minimum current density, Jmin, of 0.84 A/cm2 at 295K) needed to drive these devices makes them suitable for a range of device applications. [ABSTRACT FROM AUTHOR]

We have investigated the negative luminescent properties of a HgCdTe device, fabricated from a 1 mm diameter array of photodiodes having peak emission at a wavelength of 8.5 μm. This long-wavelength luminescence is of sufficient efficiency and area to be useful in device applications. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the activation energy for fluorine transport in amorphous silicon. Deduction of the diffusion coefficient; Activation energy for F transport; Influence of implantation-induced defects.

The spatial variation of thermal emission from large area graphene grown by chemical vapor deposition, transferred onto SiO2/Si substrates and fabricated into field effect transistor structures, has been investigated using infra-red microscopy. A peak in thermal emission occurs, the position of which can be altered by reversal of the current direction. The experimental results are compared with a one dimensional finite element model, which accounts for Joule heating and electrostatic effects, and it is found that the thermal emission is governed by the charge distribution in the graphene and maximum Joule heating occurs at the point of minimum charge density. [ABSTRACT FROM AUTHOR]

We demonstrate macroscopic acoustoelectric transport in graphene, transferred onto piezoelectric lithium niobate substrates, between electrodes up to 500 μm apart. Using double finger interdigital transducers we have characterised the acoustoelectric current as a function of both surface acoustic wave intensity and frequency. The results are consistent with a relatively simple classical relaxation model, in which the acoustoelectric current is proportional to both the surface acoustic wave intensity and the attenuation of the wave caused by the charge transport. [ABSTRACT FROM AUTHOR]

Efficiency droop in InSb/AlxIn1-xSb quantum-well light-emitting diodes has been investigated as a function of temperature for devices containing 20 nm, 40 nm, and 100 nm wide quantum well active regions. The amount of droop is greatest at low temperatures in device with the widest wells. [ABSTRACT FROM AUTHOR]