J. C. Cao

Terahertz quantum-cascade lasers based on a three-well active module

The authors report on a design of terahertz quantum-cascade lasers based on three-well active modules. Each module consists of two tunnel-coupled wells for the two lasing states and another well for both resonant-phonon depopulation and carrier injection. This design is the simplest so far among the various published working devices. The test device has a lasing frequency of 3.4THz and maximum operating temperature of 142K.

Monte Carlo simulation of hot phonon effects in resonant-phonon-assisted terahertz quantum-cascade lasers

We study the influence of nonequilibrium optical phonons on the electron transport in resonant-phonon-assisted terahertz (THz) quantum-cascade lasers (QCLs). The hot phonon effect is included in the Monte Carlo simulation by introducing a time constant accounting for the decay of polar optical phonons into other phonon modes. We find that nonequilibrium polar optical phonons may modify the electron-phonon interaction, affect the electron distribution in different subbands, and consequently change the device current. We conclude that the hot phonon effect should be taken into account in the study of the transport properties of resonant-phonon-assisted THz QCLs.

Coulomb scattering in the Monte Carlo simulation of terahertz quantum-cascade lasers

The authors compare different Coulomb screening models in the Monte Carlo simulation of terahertz quantum-cascade lasers: the static multisubband screening model and two single subband models. In quantum-cascade structures, electrons are located in many different subbands. Coulomb screening from all these subbands influences the intra- and intersubband scattering processes. The simulation results show that one of the two single subband models overestimates the screening effect, while the other underestimates it. The authors show the reasons for this and propose simple modification to the single subband models.

Simulation of negative-effective-mass terahertz oscillators

We present a model calculation of hole current oscillations in doped p+pp+ ballistic diodes using the nonparabolic balance-equation theory and a time-dependent drift-diffusion model. Such an oscillation originates from a negative effective mass (NEM) region in the hole dispersion relation. In the present balance-equation calculation, we consider the scatterings by hole-impurity, hole-acoustic phonon, hole-polar-phonon, and hole-nonpolar-phonon–hole interactions, and yield a “N-shape” velocity-field relation, which are quite different from the two-valley results for electrons in bulk GaAs. We provide a detailed analysis of the resulting oscillations as a function of the applied voltage, base length, base doping, and the dispersion relation. Typical frequencies for a 0.2 μm structure NEM oscillator are in the terahertz range. Qualitative agreement is obtained between the present calculations and the existing Boltzmann results.

Monte Carlo simulation of carrier transport and output characteristics of terahertz quantum cascade lasers

An ensemble Monte Carlo method, including electron-phonon, electron-electron, electron-impurity scatterings and the hot phonon effect, is used to simulate the carrier transport and output characteristics of a lasing terahertz quantum cascade laser. A simulated I-V curve fits well with the measurement at the lasing domain. Extracted output characteristics, e.g., the gain, threshold current density, and threshold bias, are in good agreement with the experimental results. All the above indicate that the proposed Monte Carlo model is a useful tool for investigating the physical characteristics of terahertz quantum cascade lasers, as well as for analyzing and optimizing device performances.

A study of terahertz quantum cascade lasers: Experiment versus simulation

The authors present the testing method and electrical and optical characteristics of a 4.1 THz quantum cascade laser with a four-well resonant-phonon design. The mode evolution in pulsed and continuous wave modes as a function of drive current has been investigated. An ensemble Monte Carlo model is used to analyze the carrier transport and output characteristics of the device. The calculated current density, lasing domain, and lasing frequency as a function of electric field are consistent with the measured results.

Interface and confined optical-phonon modes in wurtzite multi-interface heterostructures

Interface and confined optical-phonon modes in wurtzite multi-interface heterostructures are studied numerically based on the dielectric continuum model and Loudon’s uniaxial crystal model. The phonon dispersion relation and electrostatic potential are calculated using the transfer matrix method. The anisotropic characteristics of wurtzite phonons are demonstrated. The electron-optical-phonon Frohlich interaction is studied in a fully numerical manner. The dispersion relation is taken into account in the derivation of Fermi’s golden rule. The method presented here is general and can be easily applied to the design and simulation of optoelectronic devices based on wurtzite multi-interface heterostructures.

Current self-oscillation and driving-frequency dependence of negative-effective-mass diodes

We have analyzed spatio-temporal current patterns and current–voltage characteristics of negative-effective-mass (NEM) p+pp+ diodes driven by dc bias and terahertz (THz) electromagnetic radiation. Interesting nonlinear dynamics are presented, including current synchronization, frequency doubling, and transition to chaos. Discussions of suppressing possible chaos in NEM semiconductor devices are included.

Optical absorption in terahertz-driven quantum wells

The optical absorption spectra in a quantum well driven both by an intense terahertz (THz) and by an optical pulse are theoretically investigated within the theory of density matrix. We found that the optical absorption spectra and the splitting of the excitonic peaks splitting can be controlled by changing the THz field intensity and/or frequency. The Autler–Towns splitting is a result of the THz nonlinear dynamics of confined excitons, which is in agreement with the experiments. In addition, the dependence of the optical absorption on the quantum well width and the carrier density is also discussed.

Spectrum dynamics of negative-effective-mass oscillators under terahertz radiation

We report on a theoretical investigation of power spectrum dynamics in negative-effective-mass (NEM) p+pp+ oscillators under the influence of terahertz (THz) electromagnetic radiation. Possible types of transport states (periodic or chaotic) and transitions between them are examined with the intensity and frequency of the radiation as controlling parameters. When the driving frequency is fixed to the self-oscillating frequency times the inverse Golden ratio, the resulting power spectrum pattern displays a very complex mosaic scenario with a self-similar emergence of high-order mixing frequencies.