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Terahertz quantum-well photodetectors (THz QWPs) have advantages in constructing THz high-speed communication and fast imaging systems. In this paper, GaAs/(Al,Ga)As THz QWPs with metal random-grating couples are designed, fabricated, and characterized. Both one-dimensional metal random grating devices and a contrast device with 45°-incident facet are fabricated, and their peak responsivities and photocurrent spectra are characterized with the samples soaked in the liquid helium. Compared with 45°-incident coupling, it is found that normal incidence with the random metal grating is an efficient method to improve the responsivity. The random metal grating THz QWP is ∼400% improved in the peak responsivity. [ABSTRACT FROM AUTHOR]

Due to intersubband transitions, the quantum cascade laser (QCL) is free of relaxations and able to work under fast modulations. In this work, the authors investigate the fast modulation properties of a continuous wave (cw) terahertz QCL emitting around 3 THz (∼100 μm). Both simulation and experimental results show that the 3 dB modulation bandwidth for the device can reach 11.5 GHz and the modulation response curve is relatively flat upto ∼16 GHz. The radio frequency (RF) injection measurements verify that around the laser threshold the inter-mode beat note interacts strongly with the RF signal and the laser can be modulated at the round trip frequency of 15.5 GHz. [ABSTRACT FROM AUTHOR]

We investigate the temperature effects on electron transport in a bound-to-continuum terahertz quantum cascade laser using Monte Carlo simulation which includes electron-electron and electron-phonon scattering. The effects of carrier transport paths and mechanisms, especially for those related to laser levels, on the device's temperature performance are evaluated. The simulation shows that the parasitic leakage of carriers from the upper laser level and the thermal backfilling to the lower laser level is the main limiting factor for high-temperature operation. The calculations are in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

The authors perform a temperature profile modelling of a four-well resonant-phonon terahertz quantum-cascade laser (QCL) with a double-metal waveguide configuration by solving a steady-state heat transfer equation. The effects of ridge width and substrate thickness on the active region temperature are investigated. In addition, we also present two methods, i.e. light power-duty cycle measurement (method I) and threshold current-heat sink temperature measurement (method II), to investigate the thermal characteristics of the device. The two methods are briefly described. The extracted values of (normalized) thermal resistance from method I and method II are 11.7 K W[?]1 (4.9 K cm W[?]1) and 8.1 K W[?]1 (3.4 K cm W[?]1), respectively. The results show that method I is a valid and simple measurement especially for the thermal investigation of terahertz QCLs with relatively low continuous-wave (cw) operating temperatures. The measured results of method I are close to those from microprobe band-to-band photoluminescence measurements. Because of the lack of sampling points due to the relatively low cw operating temperature, method II shows a wide deviation from method I. [ABSTRACT FROM AUTHOR]

We report the electrical and the optical characteristics of 2 THz quantum-cascade lasers with a similar four-well resonant-phonon design. A Monte Carlo simulation, employed to evaluate the temperature performance of the device, indicates that the degradation of material gain with increasing temperature is attributed to the rapid decrease in the lifetime of the upper lasing level and the relatively stable lifetime of the lower lasing level. Because of the broadening effect of the temperature dependent gain profile, our calculations overestimate the peak gain and subsequently overrate the maximum operating temperature. Under a linear approximation condition, the deduced maximum operating temperature is in good agreement with experiment. Both experiment and simulation show that the lasing frequency is insensitive to temperature. [ABSTRACT FROM AUTHOR]