Theoretical analysis of spectral gain in a THz quantum cascade laser: prospects for gain at 1 THz

In a recent Letter [Appl. Phys. Lett. 82, 1015 (2003)], Williams et al. reported the development of a terahertz quantum cascade laser operating at 3.4 THz or 14.2 meV. We have calculated and analyzed the gain spectra of the quantum cascade structure described in their work, and in addition to gain at the reported lasing energy of ~= 14 meV, we have discovered substantial gain at a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue for the development of a terahertz laser at this lower energy, or of a two-color terahertz laser.Comment: in press APL, tentative publication date 29 Sep 200

Self-Consistent Theory of the Gain Linewidth for Quantum Cascade Lasers

The linewidth in intersubband transitions can be significantly reduced below the sum of the lifetime broadening for the involved states, if the scattering environment is similar for both states. This is studied within a nonequilibrium Green function approach here. We find that the effect is of particular relevance for a recent, relatively low doped, THz quantum cascade laser.Comment: 3 pages, figures include

Double-heterostructure GaAs-GaAIAs injection lasers on semi-insulating substrates using carrier crowding

GaAs‐GaAlAs double‐heterostructure lasers were fabricated on semi‐insulating substrates. Laser action based on carrier confinement via the crowding effect has been demonstrated. Laser action takes place in a narrow (10–20 μm) region near the edge of the mesa where the current is injected. The threshold current is low and is comparable to that of stripe‐geometry lasers

Roughening of ion-eroded surfaces

Recent experimental studies focusing on the morphological properties of surfaces eroded by ion-bombardment report the observation of self-affine fractal surfaces, while others provide evidence about the development of a periodic ripple structure. To explain these discrepancies we derive a stochastic growth equation that describes the evolution of surfaces eroded by ion bombardment. The coefficients appearing in the equation can be calculated explicitly in terms of the physical parameters characterizing the sputtering process. Exploring the connection between the ion-sputtering problem and the Kardar-Parisi-Zhang and Kuramoto-Sivashinsky equations, we find that morphological transitions may take place when experimental parameters, such as the angle of incidence of the incoming ions or their average penetration depth, are varied. Furthermore, the discussed methods allow us to calculate analytically the ion-induced surface diffusion coefficient, that can be compared with experiments. Finally, we use numerical simulations of a one dimensional sputtering model to investigate certain aspects of the ripple formation and roughening.Comment: 20 pages, LaTeX, 5 ps figures, contribution to the 4th CTP Workshop on Statistical Physics "Dynamics of Fluctuating Interfaces and Related Phenomena", Seoul National University, Seoul, Korea, January 27-31, 199

A MEMS electrostatic particle transportation system

We demonstrate here an electrostatic MEMS system capable of transporting particles 5-10μm in diameter in air. This system consists of 3-phase electrode arrays covered by insulators (Figs. 1, 2). Extensive testing of this system has been done using a variety of insulation materials (silicon nitride, photoresist, and Teflon), thickness (0- 12μm), particle sizes (1-10μm), particle materials (metal, glass, polystyrene, spores, etc), waveforms, frequencies, and voltages. Although previous literature [1-2] claimed it impractical to electrostatically transport particles with sizes 5-10μm due to complex surface forces, this effort actually shows it feasible (as high as 90% efficiency) with the optimal combination of insulation thickness, electrode geometry, and insulation material. Moreover, we suggest a qualitative theory for our particle transportation system which is consistent with our data and finite-element electrostatic simulations

GaAs-GaAIAs injection lasers on semi-insulating substrates using laterally diffused junctions

Low‐threshold GaAs‐GaAlAs lasers operating in a stable single mode have been fabricated using laterally diffused junctions. The lasers are fabricated on semi‐insulating substrates and can be integrated with other components

Integration of an injection laser with a Gunn oscillator on a semi-insulating GaAs substrates

The integration of an injection semiconductor laser with an active electronic device (Gunn oscillator) in a single epitaxial crystal device is demonstrated

Simulation of Transport and Gain in Quantum Cascade Lasers

Quantum cascade lasers can be modeled within a hierarchy of different approaches: Standard rate equations for the electron densities in the levels, semiclassical Boltzmann equation for the microscopic distribution functions, and quantum kinetics including the coherent evolution between the states. Here we present a quantum transport approach based on nonequilibrium Green functions. This allows for quantitative simulations of the transport and optical gain of the device. The division of the current density in two terms shows that semiclassical transitions are likely to dominate the transport for the prototype device of Sirtori et al. but not for a recent THz-laser with only a few layers per period. The many particle effects are extremely dependent on the design of the heterostructure, and for the case considered here, inclusion of electron-electron interaction at the Hartree Fock level, provides a sizable change in absorption but imparts only a minor shift of the gain peak.Comment: 12 pages, 5 figures included, to appear in in "Advances in Solid State Physics", ed. by B. Kramer (Springer 2003