A. R. Adams

Studies on egg albumen and whey protein interactions by FT-Raman spectroscopy and rheology

Large deformation rheological studies of either egg albumen or whey protein isolate (15% protein w/w) gels made by heating at 90 °C for 30 min indicated a higher gel strength and Youngs modulus values for egg albumen compared to whey protein isolate. The proteins mixed in the ratio 10:5 whey/egg albumen indicated synergistic interactions producing greater than expected gel strength values. Small deformation rheological studies of egg albumen 15% (w/w) and whey protein isolate 15% (w/w) showed very low values of the storage modulus (G′) at 20 °C suggesting that both proteins do not easily aggregate at room temperature. Whey proteins gelled at 80 °C, whereas egg albumen proteins were already cross-linked at 20 °C (G′>G″). A mixture of 7.5% whey/7.5% egg albumen showed the highest G′ value at the end of both heating and cooling cycles but this value was lower than that of the individual proteins. Changes in the conformation, molecular structure and protein–protein interactions in whey and egg albumen (15% w/w in D2O pD7.2) and their mixtures (7.5:7.5 protein w/w) were investigated by FT-Raman spectroscopy. Upon heating to 90 °C for 30 min, individual whey and egg albumen proteins showed changes in the CH (1350 cm−1), CH2 (1450 cm−1) bending vibrations and Trp residues at 760 cm−1, reflecting involvement of hydrophobic interactions. Changes in the disulphide stretching vibrations were also observed. Peak intensity increased for β-sheet structures in the Amide 111′ region 980–990 cm−1 and decreased for helix structures (900–960 cm−1) for both egg albumen and whey proteins. Differences in the experimental and theoretical average spectra indicated changes in β-sheet structures, CH2 bending, carboxyl and hydrophobic groups in heated binary mixtures of egg albumen and whey proteins, providing evidence of protein–protein interactions, observed by large deformation rheology.

Determination of the band structure of disordered AlGaInP and its influence on visible-laser characteristics

Using hydrostatic pressure techniques, we have obtained new energies for the X-minima, L-minima and band offsets in GaInP-AlGaInP. Theoretical calculations of the threshold current density in bulk and strained quantum-well visible lasers are shown to be in good agreement with experimental results, obtained as a function of both temperature and hydrostatic pressure. Our results show that heterobarrier leakage current is a dominant limiting factor in the performance at shorter wavelength (/spl sim/635 nm) operation, but is of less significance for longer wavelength (/spl sim/675 nm) operation. >

Optical gain in wide bandgap GaN quantum well lasers

We investigate the gain characteristics of III - V nitride lasers operating at wavelengths around 400 nm, comparing the transparency and threshold characteristics with both GaAs and GaInP bulk and quantum well laser devices. We find that the calculated threshold carrier density and radiative current density have a stronger dependence on well width, , in GaN-based single quantum well lasers than is the case for either GaAs- or GaInP-based devices, with a relatively sharp minimum predicted in both carrier and current density for of about 3.5 nm. Although the threshold carrier densities are found to be much larger than in typical GaAs lasers, our calculations indicate that nitride-based lasers may compare favourably with current GaInP visible laser devices.

High pressure determination of AlGaInP band structure

Abstract Low temperature photoluminescence measurements have been carried out at hydrostatic pressures up to 80 kbar to determine the band structure of the disordered ( Al x Ga 1 − x ) 0.5 In 0.5 P alloy system. Using a series of specifically designed samples we have measured how the Γ, L and X conduction band energies vary with strain and aluminium content. The direct band gap varies linearly from that of GaInP at a rate of 0.61 x eV. The position of the X minimum in GaInP is found to be 280 meV above the Γ, increasing linearly with aluminium content at a rate of 0.085 x eV. Measurements of unstrained and 1% compressively strained GaInP put lower limits on the Γ c -L c separation of 125 meV and 175 meV, respectively.

Quantifying the Effect of Indirect Carrier Leakage on Visible Al(GaInP) Lasers Using High Pressures and Low Temperatures

The extreme temperature sensitivity of the threshold current, I th , above room temperature in visible edge-emitting lasers (EELS) is investigated. From measurements as a function of temperature we find that I th is dominated by the radiative current, I Rad up to ∼230 K so I th I Rad as expected for an ideal device. However, above 230 K, I th increases sharply due to the onset of indirect carrier leakage which we calculate to be ∼20% of I th at room temperature at an emission wavelength of 672 nm. By 350 K it accounts for >70% of I th . The operating wavelength of the device can be reduced by applying pressure and then we observe that the leakage rapidly increases and at 655 nm it forms ∼70% of I th at room temperature. This gives rise to a significant deterioration in the light output characteristics and has important implications for producing high power EELS and VCSEL structures based upon Al(GaInP) at these shorter operating wavelengths.

The light-hole mass in a strained InGaAs/GaAs single quantum well and its pressure dependence

Abstract The pressure dependence of the light-hole mass in a compressively strained In0.20Ga0.80As/GaAs single quantum well has been deduced from the temperature dependence of Shubnikov-De Haas oscillations over a pressure range from 0 to 15 kbar. The sample was grown by MBE on a GaAs substrate. Low temperature hole densities in the well were (3.64 ± 0.03) × 1011 cm−2 independent of pressure. The mass of (0.156 ± 0.005)m0 measured at atmospheric pressure agrees well with cyclotron resonance measurements reported elsewhere. The effective mass did not increase with pressure as expected but appeared to decrease between 0 and 8 kbar. It then remains constant with increasing pressure up to 15 kbar. Details of the experimental results are described and compared with theoretical calculations using a second-order 8-band k·p model. This predicts a mass which increases with pressure at a rate of about 0.5% kbar−1 in disagreement with our experimental results. The importance of the warping of the valence band and the limitations of the model are described.

Unusual increase of the Auger recombination current in 1.3 μm GaInNAs quantum-well lasers under high pressure

The pressure dependence of the total threshold current and its respective recombination components in 1.3 μm GaInNAs single-quantum-well lasers using spontaneous emission measurements up to 13 kbar is presented. We observed an unusual increase of the nonradiative Auger recombination current with increasing pressure in this material, which is opposite to those in 1.3 μm InP-based InGaAsP and AlGaInAs devices where the Auger current decreases with pressure. It is shown that the high-pressure-induced increase of the threshold current in GaInNAs is associated with the increase of the Auger current, while the defect-related monomolecular nonradiative current remains nearly unchanged in the pressure range studied. Theoretical calculations show that the unusual increase of the Auger current with pressure in GaInNAs is due to a large increase in the threshold carrier density.

Semiconductor lasers take the strain

Compact infrared lasers made from gallium arsenide and indium phosphide have opened up many significant opportunities over the past decade – optical communications, compact discs and related optical data storage applications, in particular. The recent commercialisation of visible semiconductor lasers by Toshiba and others opens up potentially huge markets in displays and lighting. Until now most of these lasers have been based on lattice-matched structures with the materials in each layer of the semiconductor structure having the same lattice constant. But there are only a few suitable combinations of lattice-matched materials for laser applications and these cover only very limited wavelength ranges. They also suffer from serious intrinsic loss mechanisms. Therefore a lot of effort is currently being directed towards increasing the range of available wavelengths and reducing these losses, and strained-layer lasers are proving most successful in this quest.

Experimental determination of the band gap dependence of Auger recombination in InGaAs∕InP multiple quantum well lasers at room temperature

The band gap dependencies of the threshold current and its radiative component are measured using high pressure techniques. Detailed theoretical calculations show that the band gap dependence of the internal losses plays a significant role in the band gap dependence of the radiative current. Temperature dependence measurements show that the radiative current accounts for 20% of the total threshold current at room temperature. This allows us to determine the pressure dependence of the non-radiative Auger recombination current, and hence to experimentally obtain the variation of the Auger coefficient C with band gap.

Direct measurement of band offsets in GaInP/AlGaInP using high pressure

High pressure photoluminescence experiments carried out at 2 K in the diamond anvil cell have been used to determine band offsets in GaInP/AlGaInP as functions of aluminium content and strain. We have used accurate measurements of the positions of the conduction band Γ and X minima to design samples specifically for high pressure experiments in which we have measured the band offsets. In the case of unstrained GaInP/(Al x Ga 1-x ) 0.5 In 0.5 P we obtain a value for the conduction b-and offset (ΔE c ) of ΔE c =0.73 ΔE g independent of x. We have also measured samples in which the erects of aluminium content and strain are combined (typical of many laser devices). A 1% compressively strained quantum well in (Al 0.3 Ga 0.7 ) 0.5 In 0.5 P barriers gives ΔE c =0.7±0.07ΔE g