Resonant-phonon Terahertz Quantum Cascade Lasers Research Articles

Analyzing the effect of doping concentration in split-well resonant-phonon terahertz quantum cascade lasers.

The effect of doping concentration on the temperature performance of the novel split-well resonant-phonon (SWRP) terahertz quantum-cascade laser (THz QCL) scheme supporting a clean 4-level system design was analyzed using non-equilibrium Green's functions (NEGF) calculations. Experimental research showed that increasing the doping concentration in these designs led to better results compared to the split-well direct-phonon (SWDP) design, which has a larger overlap between its active laser states and the doping profile. However, further improvement in the temperature performance was expected, which led us to assume there was an increased gain and line broadening when increasing the doping concentration despite the reduced overlap between the doped region and the active laser states. Through simulations based on NEGF calculations we were able to study the contribution of the different scattering mechanisms on the performance of these devices. We concluded that the main mechanism affecting the lasers' temperature performance is electron-electron (e-e) scattering, which largely contributes to gain and line broadening. Interestingly, this scattering mechanism is independent of the doping location, making efforts to reduce overlap between the doped region and the active laser states less effective. Optimization of the e-e scattering thus could be reached only by fine tuning of the doping density in the devices. By uncovering the subtle relationship between doping density and e-e scattering strength, our study not only provides a comprehensive understanding of the underlying physics but also offers a strategic pathway for overcoming current limitations. This work is significant not only for its implications on specific devices but also for its potential to drive advancements in the entire THz QCL field, demonstrating the crucial role of e-e scattering in limiting temperature performance and providing essential knowledge for pushing THz QCLs to new temperature heights.

Doping study of two-well resonant-phonon terahertz quantum cascade lasers part I: doping profile dependence

The influence of impurity doping on GaAs-based two-well resonant-phonon terahertz quantum cascade lasers is investigated theoretically, and efficient doping schemes are discussed. By using the rate equation model, the impacts of dopant amount, position, and distribution on the performance of a high-performance device is simulated focusing on a single module. The calculated optical gain is found to have a peak over the range of sheet doping density from 1.0 × 1010 to 1.0 × 1012 cm−2 in all eight doping conditions examined in this work. Among these patterns, the devices with the undoped condition and homogeneous-doping in phonon-wells mark high optical gain, and the latter is also resistant against the detuning of subband alignment due to band-bending under the high doping conditions. Furthermore, based on the simulation results, a modulation doping scheme whose active cores include both doped and undoped modules is suggested and discussed.

Barrier height study of two-well resonant-phonon terahertz quantum cascade lasers. I. The third-order tunneling current theory

The third-order tunneling current model, reflecting the effect of carrier leakage from bound to continuum states, is modeled based on the second-order tunneling current and semi-classical leakage current theories and is incorporated in a simulation based on the rate equation. GaAs/AlGaAs-based two-well resonant-phonon terahertz quantum cascade lasers, published in the past, are theoretically examined by this method. Based on the latest recorded device, the influence of barrier height is investigated by changing the Al-composition from 12.5% to 45% with a fixed-layer thickness. As a result, there are carrier dynamics that diminish the carrier density of lasing subbands in both high and low barrier conditions. On the one hand, carrier leakage into continuum states increases drastically below 25% Al-composition. On the other hand, in high barrier conditions, substantial carriers accumulate in the ground state due to a strong confinement effect in addition to oscillator strength lowering.

Contrasting influence of charged impurities on transport and gain in terahertz quantum cascade lasers

Transport and gain properties of a resonant-phonon terahertz quantum cascade laser are calculated using nonequilibrium Green's functions. Impurity scattering is shown to be responsible for contrasting nonlinear effects in the transport and the gain properties. For typical doping concentrations, the current density is found to be weakly sensitive to the impurity scattering strength. In contrast, the calculated gain is found to be very sensitive to the impurity scattering strength. This difference is attributed to the strong momentum dependence of the long-range coupling to charged impurities. Small-momentum impurity scattering is shown to be responsible for an incoherent regime of resonant tunneling processes. These new insights into the crucial role of impurity scattering open a new route of improvement of terahertz quantum cascade lasers by engineering of the doping profile.

Non-equilibrium Green’s function calculation for GaN-based terahertz-quantum cascade laser structures

We theoretically investigated GaN-based resonant phonon terahertz-quantum cascade laser (QCL) structures for possible high-temperature operation by using the non-equilibrium Green’s function method. It was found that the GaN-based THz-QCL structures do not necessarily have a gain sufficient for lasing, even though the thermal backfilling and the thermally activated phonon scattering are effectively suppressed. The main reason for this is the broadening of the subband levels caused by a very strong interaction between electrons and longitudinal optical (LO) phonons in GaN.

Temperature profile modelling and experimental investigation of thermal resistance of terahertz quantum-cascade lasers

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.

The effect of phonon extraction level separation on the performance of three-well resonant-phonon terahertz quantum-cascade lasers

The authors present a simulation and experimental study on the effect of phonon extraction level separation on the performance of GaAs-based three-well resonant-phonon terahertz quantum-cascade lasers (QCLs). The phonon extraction level separation is varied from 30 to 42 meV. Because of the efficient longitudinal-optical phonon scattering, the 36 meV QCL shows the largest gain, the best temperature performance and the highest output power. As for the lower (30 meV) or higher (42 meV) energy separation QCLs, the electron–longitudinal-optical phonon interaction still works by involving a transfer of in-plane momentum. The measured lasing characteristics are in qualitative agreement with simulation.

186 K operation of terahertz quantum-cascade lasers based on a diagonal design

Resonant-phonon terahertz quantum-cascade lasers operating up to a heat-sink temperature of 186 K are demonstrated. This record temperature performance is achieved based on a diagonal design, with the objective to increase the upper-state lifetime and therefore the gain at elevated temperatures. The increased diagonality also lowers the operating current densities by limiting the flow of parasitic leakage current. Quantitatively, the diagonality is characterized by a radiative oscillator strength that is smaller by a factor of two from the least of any previously published designs. At the lasing frequency of 3.9 THz, 63 mW of peak optical power was measured at 5 K, and approximately 5 mW could still be detected at 180 K.

Monte Carlo simulation of extraction barrier width effects on terahertz quantum cascade lasers

The effect of extraction barrier width on the performance of a resonant-phonon terahertz quantum cascade laser is studied using the ensemble Monte Carlo method. The width of extraction barrier is varied from 48to33A with a stepsize of −3A. The calculated threshold current density increases monotonously from 436to1054A∕cm2 with decreasing extraction barrier width, while the peak gain shows nontrivial behavior. These findings agree well with an experimental study. The simulation results suggest an optimal extraction barrier width of 36A for peak gain and for the best temperature performance.

Temperature dependence of the gain profile for terahertz quantum cascade lasers

We study the rapid decrease of peak gain in resonant-phonon terahertz quantum cascade lasers with increasing temperature. The effect of various microscopic scattering processes on the gain profile as a function of temperature is discussed. We argue that increased broadening, primarily due to increased impurity scattering, and not diminishing population inversion, is the main reason for the reduction of peak gain.

Effects of extraction barrier width on performance of terahertz quantum-cascade lasers

Experimental results are presented on the effects of extraction barrier on the performance of resonant phonon terahertz quantum-cascade lasers. The extraction barrier is varied from 48 to 39 A in an otherwise identical 3.4 THz laser structure. It is found that, when the extraction barrier width decreases from 48 to 39 A, the threshold current density at 10K increases monotonously from 900 A/cm2 to 1.4 kA/cm2, and the maximum lasing temperature increases from 95 to 110K. No optimum extraction barrier width was found for maximum lasing temperature.