E. R. Youngdale

Auger lifetime enhancement in InAs–Ga1−xInxSb superlattices

We have experimentally and theoretically investigated the Auger recombination lifetime in InAs–Ga1−xInxSb superlattices. Data were obtained by analyzing the steady‐state photoconductive response to frequency‐doubled CO2 radiation, at intensities varying by over four orders of magnitude. Theoretical Auger rates were derived, based on a k⋅p calculation of the superlattice band structure in a model which employs no adjustable parameters. At 77 K, both experiment and theory yield Auger lifetimes which are approximately two orders of magnitude longer than those in Hg1−xCdxTe with the same energy gap. This finding has highly favorable implications for the application of InAs–Ga1−xInxSb superlattices to infrared detector and nonlinear optical devices.

Interface roughness scattering in semiconducting and semimetallic InAs‐Ga1−xInxSb superlattices

An analysis of magnetotransport results for InAs‐Ga1−xInxSb superlattices with a range of layer thicknesses demonstrates that interface roughness scattering dominates the electron mobility under most conditions of interest for infrared detector applications. However, the dependence on well thickness is much weaker than the d16 relation observed in other systems with thicker barriers, which is consistent with predictions based on the sensitivity of the energy levels to roughness fluctuations. Theory also correctly predicts an abrupt mobility decrease at the semiconductor‐to‐semimetal transition point, as well as the coexistence of two electron species in semimetallic samples.

Electron transport in InAs/Ga1-xInxSb superlattices

Abstract We have experimentally investigated the electron transport properties of a series of n -typeInAs/Ga 1- x In x Sb superlattices with InAs thicknesses ( d 1 ) between 25 and 86 A and a fixed Ga 1- x In x Sb thickness ( d 2 ) of 25 A. Interface roughness scattering is found to dominate the electron mobility ( μ n ), but with a much weaker dependence on layer thickness than the conventional d 1 6 . We observe an abrupt decrease in μ n ( d 1 ) at the semiconductor-to-semimetal transition point and the coexistence of two electron species in semimetallic samples. All of these findings can be understood by considering the theoretical band structures for InAs/Ga 1- x In x Sb superlattices with thin d 2 , which differ considerably from those in InAs/GaSb structures with comparable energy gaps and thick d 2 , but are strikingly similar to results for HgTe/CdTe. Since the system is strongly 3D rather than 2D in character, it is surprising that in some samples the quantum oscillations in the Hall conductivity are much larger than those in the diagonal conductivity.

Nonlinear optical properties of molecular beam epitaxy grown Bi1-xSbx

We discuss the first investigation of Bi1−xSbx as an infrared nonlinear optical material. Nondegenerate four‐wave mixing experiments at CO2 laser wavelengths yield a large nonlinearity (χ(3)≊3×10−4 esu) which does not saturate at power densities up to 0.5 MW/cm2. Both the ambient and substrate interfaces of the film are highly reflective and the etalon they form is found to have a large effect on the transmission and reflectivity spectra of the as‐grown films. This suggests the possibility that constructive interference of the film’s internal optical fields could be used to considerably enhance the nonlinear signal.

Momentum-space reservoir for enhancement of intersubband second-harmonic generation

We analyze fundamental limits to the second-harmonic conversion efficiency attainable from semiconductor intersubband devices employing asymmetric stepped and double quantum wells. The coupled propagation equations have been solved numerically, accounting for saturation, absorption, and optical heating. It is found that the key figure of merit is the conversion efficiency at the onset of saturation, which has a remarkably simple form depending only on the ratio of broadening time to intersubband relaxation time and on another ratio involving the optical matrix elements. We show that since there are fundamental limits to the values of these ratios, it is unlikely that conversion efficiencies exceeding /spl ap/10% can be attained in devices of the type considered in the previous literature, and for surface incidence even efficiencies approaching that value will require impractically-thick active regions. While detuning from the double resonance condition is often advantageous, net improvements to the optimum performance are relatively modest. However, these limitations ran be transcended by placing the subband system in contact with an optically-inactive momentum-space reservoir, which shunts the intersubband relaxation and delays saturation by refilling the depleted subband states with electrons from the reservoir. We propose a specific device based on /spl Gamma/-valley active states and L-valley reservoir states in InAs-GaSb-AlSb asymmetric double quantum wells, whose energy levels and optical matrix elements are modeled using an 8-band finite-element calculation. It is predicted that a conversion efficiency of 20% can be achieved in an active-layer thickness of less than 10 /spl mu/m. >

Recombination lifetime in InAs–Ga1−xInxSb superlattices

We report an experimental investigation of Shockley–Read and Auger lifetimes in InAs–Ga1−xInxSb and InAs–GaSb superlattices, based on measurements of the photoconductive response to excitation by a frequency‐doubled CO2 laser (4.63 μm) at intensities up to 100 kW/cm2. Results at 77 K for low excitation levels yield Shockley–Read lifetimes between 0.13 and 6 ns. The scaling of the lifetime with carrier concentration also provides the first determinations of Auger coefficients in narrow‐gap type II superlattices: γ3≊8×10−25 cm6/s at 300 K and γ3≊1.3×10−27 cm6/s at 77 K. The observed Auger lifetime at 77 K is two orders of magnitude longer than that in Hg1−xCdxTe with the same energy gap, which has highly favorable implications for InAs–Ga1−xInxSb superlattices in both IR detector and nonlinear optical applications.

Effects of energy gap and band structure on free‐carrier nonlinear susceptibilities in semiconductors

Using dispersion relations from the Kane band model, we obtain limiting forms for the third‐order nonlinear susceptibilities due to nonparabolicity, thermal‐carrier generation, and nonequilibrium optical‐carrier generation. We show that whereas χ(3)’s for all three processes increase with decreasing energy gap, there is no further benefit once Eg becomes smaller than either the Fermi or thermal energies. In fact, simultaneous optimization of both the magnitude of χ(3) and the saturation properties favors materials with a large direct gap and large effective mass, coupled with a smaller thermal gap which may be indirect in either real or momentum space.

Novel FIR nonlinear optical materials

The authors report the first experimental and theoretical study of nonlinear optical mechanisms in alpha -Sn and alpha -Sn1-xGex grown by MBE. Non-degenerate four-wave mixing has been employed to measured third-order nonlinear susceptibilities at 10.6 mu m as a function of temperature, laser intensity and difference frequency. There is good agreement between experiment and theoretical calculations based on free carrier contributions to the susceptibility. Comparison is made with optical nonlinearities in Hg-based materials grown by MBE.

LARGE, WEAKLY SATURATING THIRD-ORDER NONLINEAR SUSCEPTIBILITIES IN SEMIMETALS AND NARROW-GAP SEMICONDUCTORS

We review recent experimental and theoretical work on optical nonlinearities in semimetals and narrow-gap semiconductors. A strong nonlinear response due to optical modulation of the free carrier susceptibility is seen to be a common feature of these materials, with large third-order nonlinear susceptibilities being reported for Hg1−xCdxTe, Hg1−xMnxTe, Hg-based superlattices, α-Sn1−xGex, Pb1−xSnxSe, and Bi1−xSbx. Effects of differences in the various band structures are discussed for nonlinearities due to nonparabolicity, thermal carrier generation, and nonequilibrium carrier generation. Mechanisms leading to either partial or severe saturation are also considered. Optimized nonlinearities for the various mechanisms and materials are critically compared, and promising future directions for the field are suggested.

Étalon enhancement of nonlinear optical response in Bi1−xSbx

Measurements of the nondegenerate four‐wave mixing of CO2 laser beams in a Bi1−xSbx film have yielded the largest high‐power third‐order nonlinear susceptibilities ever reported at that wavelength (χ(3)≳6×10−4 esu at P0≳2×105 W/cm2). Furthermore, an etalon effect resulting from the high reflectivity of the Bi1−xSbx films at both the air and substrate interfaces leads to an additional enhancement of the four‐wave signal by as much as a factor of 30. A theoretical model based on optical modulation of the free‐carrier susceptibility gives results which are in excellent agreement with the data.Measurements of the nondegenerate four‐wave mixing of CO2 laser beams in a Bi1−xSbx film have yielded the largest high‐power third‐order nonlinear susceptibilities ever reported at that wavelength (χ(3)≳6×10−4 esu at P0≳2×105 W/cm2). Furthermore, an etalon effect resulting from the high reflectivity of the Bi1−xSbx films at both the air and substrate interfaces leads to an additional enhancement of the four‐wave signal by as much as a factor of 30. A theoretical model based on optical modulation of the free‐carrier susceptibility gives results which are in excellent agreement with the data.