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THz (QCLs) are promising large output power semiconductor‐based THz sources with narrow bandwidths and wide operating frequency ranges. However, the operating temperature of THz QCLs is still limited by the design of its active region. Matyas et al. (2012) theoretically predicted that AlGaAs/GaAs THz QCLs with higher optical gain could be obtained by introducing a variable barrier height structure (Matyas et al., J. Appl. Phys. 2012, 111, 103106). Based on the proposed variable barrier QC structure, we fabricate AlGaAs/GaAs three‐well type THz QCLs with an additional very thin second extraction barrier and a low barrier height emission barrier. With this device, we obtain lasing operation at 3.7 THz at a maximum operating temperature of 145 K. [ABSTRACT FROM AUTHOR]

Terahertz quantum cascade lasers (THz-QCLs) are compact THz laser source with narrow emission linewidth and high output power. Currently, a maximum operation temperature ( Tmax) of 199.5 K is reported by using a state-of-the-art longitudinal optical (LO) phonon depopulation scheme AlGaAs/GaAs structure. We propose an effective method to obtain higher operating temperature for AlGaAs/GaAs THz-QCL by increasing LO-phonon scattering energy by utilizing high Al composition AlGaAs barrier. We have successfully fabricated an LO phonon depopulation scheme AlGaAs/GaAs THz-QCL with double metal waveguide, and obtained 3.7 THz lasing. We obtained the Tmax of 150 K. We observed an increase of Tmax by about 7 K by increasing the Al composition of AlGaAs barrier from 15% to 30%. The characteristic temperature ( T0) was also improved from 44.7 K to 69.9 K by changing the Al composition of AlGaAs barriers. The increase of Al composition in AlGaAs-based THz-QCL was shown to be promising way for obtaining higher operating temperature of THz-QCL. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. This enabled us to avoid the subband misalignment induced by the strong band bending effect, to provide efficient carrier transport even at high doping concentrations, and to improve the output power. The maximum output power of a GaAs/Al0.16Ga0.84As 3.5 THz-band QCL at 10 K was enhanced from 390 to 600 mW by increasing the doping concentration at the depopulation layers from 6 × 1016 to 1.2 × 1017 cm−3. The average power of 21 mW was achieved by implementing the appropriate pulse drive conditions. [ABSTRACT FROM AUTHOR]

Non-polar m-plane GaN terahertz quantum cascade laser (THz-QCL) structures have been studied. One is traditional three-well resonant-phonon (RP) design scheme. The other is two-well phonon scattering injection (PSI) design scheme. The peak gains of 41.8 and 44.2 cm−1 have been obtained at 8.2 and 7.7 THz respectively at 300 K according to the self-consistent non-equilibrium Green's function calculation. Different from the usual GaAs two-well design, the upper and lower lasing levels are both ground states in the GaN quantum wells for the PSI scheme, mitigating the severe broadening effect for the excited states in GaN. To guide the fabrication of such devices, the doping effect on the peak gain has been analyzed. The two designs have demonstrated distinct doping density dependence and it is mainly attributed to the very different doping dependent broadening behaviors. The results reveal the possibility of GaN based THz-QCL lasing at room temperature. [ABSTRACT FROM AUTHOR]

For terahertz quantum cascade lasers employing the nonalignment injection scheme via direct-phonon resonance, this study presents the crucial effect of parasitic absorption in the narrow module architecture based on only two quantum wells. The optical gain can be restricted to small (or negative) values even at the low temperature. To address this issue, a suppression strategy by increasing the injection energy is used to isolate the relevant levels. The feasibility of this strategy is experimentally demonstrated by observing lasing at 194 K. [ABSTRACT FROM AUTHOR]

The recent designs of terahertz quantum cascade lasers usually employ the short periodic length and also the tall barriers for high-temperature operation. In this work, the effect of high-energy lying non-relevant subbands is studied based on nonequilibrium Green's function formalisms model, demonstrating those subbands are probable to play a minor role on the population inversion, but play a major role on the optical gain at high temperatures. The phenomenon can be ascribed to the appearance of leakages crossing neighboring periods via sequential resonant tunneling, and those leakages are inherently created by the specific features of the two-well configuration in this design that the phonon well should be wide enough for performing the phonon scattering to depopulate the lower-laser subband. The narrower periodic length design can strengthen this inter-period leakage. A parasitic absorption between the first high-lying nonrelevant subbands from two laser wells can closely overlap the gain shape and thus significantly reduce the peak gain. [ABSTRACT FROM AUTHOR]

Recent renewed operating temperatures in terahertz quantum cascade lasers emphasize on narrowing the periodic length in a 2-well resonant-phonon design for a clean quantum level structure, in which the depopulation energy is significantly higher than one longitudinal phonon. In this study, various depopulation energies (small and large) are engineered in a 2-well design; the effect of the high-lying nonrelevant levels on the currents are systematically studied by using the non-equilibrium Green's function method. The engineering of the depopulation energy is unable to avoid the formation of leakage channels, which are activated within at least three neighboring periods via sequential close tunneling. However, a large depopulation energy relaxes the thermal backfilling process; as a result, the net leakages at high temperatures can be significantly suppressed. In addition, pre-alignment remains a critical issue in the design when using a large depopulation energy, which requires improved engineering for the barriers to obtain better laser dynamics. [ABSTRACT FROM AUTHOR]

A GaAs/AlGaAs‐based terahertz quantum cascade laser with a peak power of 1.31 W and average power of 52 mW is demonstrated. To realize a high output power, an effective way is to increase the electron density at the active region by using a high doping level. This work aims to optimize a design enabling to use a heavy doping level, by realigning the resonant tunneling which can be seriously degraded due to a band bending from this heavy local doping. With variable quantum well and barrier height, the design owns an improved parameter tuning freedoms, and a saturation of peak gain can be achieved at a relatively high doping concentration of 1.0 × 1017 cm−3, which is twice higher than the recently reported device. Nonequilibrium Green's function method including full carriers scattering effect is used to carry out the design optimization, mainly including the realignment of resonant tunneling and suppression of leakages via high‐energy confined levels. In the experiment, a 14 μm thick active layer for this design is grown and also large mesa size is developed. To ensure a quasi‐continuous wave (quasi‐CW) operation, the heat dissipation is also managed. [ABSTRACT FROM AUTHOR]

The terahertz quantum cascade laser (THz-QCL) is a promising compact THz laser source with high output power and narrow emission line width, and could be put to use in a variety of applications. However, the operating temperature of lower frequency (< 2 THz) QCLs is particularly low, and is limited by the kBT line. This paper demonstrates a 1.9 THz GaAs/Al0.175Ga0.825As QCL operating at temperatures much higher than the kBT line. A selective injection scheme (using a four-level design) with high-Al composition barrier was used to build the low-frequency QCL with high operating temperature. We succeeded in surpassing the kBT line limit for a 1.9 THz QCL that operates up to 160 K (∼1.8 ħw/kB). The selective injection 1.9 THz QCL shows one-frequency lasing operation over the entire injection bias voltage range. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The key challenge for terahertz quantum cascade lasers (THz-QCLs) is to make it operating at room-temperature. The suppression of thermally activated leakages via high lying quantum levels is emphasized recently. In this study, we employ the advanced self-consistent method of non-equilibrium Green's function, aiming to reveal those kinds of leakages in the commonly used THz-QCL designs based on 2-, 3- and 4-quantum well. At the high temperature of 300 K, if all the confined high lying quantum levels and also the continuums are included within three neighboring periods, leakages indeed possess high fraction of the total current (21%, 30%, 50% for 2-, 3- and 4-quantum well designs, respectively). Ministep concept is introduced to weaken those leakage channels by isolating the desired levels from high lying ones, thus the leakages are well suppressed, with corresponding fractions less than 5% for all three designs. [ABSTRACT FROM AUTHOR]

Carrier transport in GaN terahertz (THz) quantum cascade laser (QCL) structures is theoretically investigated using a non-equilibrium Green's function method. Although scattering due to polar optical phonons in GaN is greatly enhanced with respect to GaAs/AlGaAs THz QCLs, the phonon-induced broadening of the laser levels is found to remain much smaller than other sources of broadening arising from impurity and electron-electron scattering. The gain is calculated self-consistently accounting for the correlation effects in level broadening. Three-well based design with resonant-phonon scheme shows a peak gain of 88/cm at 10 K, and 34/cm at 280 K, which remains above the calculated loss of a double metal waveguide. The results suggest that lasing at 6.6 THz, which is beyond the traditional GaAs THz QCLs, is possible up to 280 K. [ABSTRACT FROM AUTHOR]

Highlights • GaN-based THz-QCL active region was grown by MOCVD on Si substrate. • A GaN/AlGaN QC active layer consisting of two-QW for one-period was grown. • Samples were evaluated by HR-XRD and cross-sectional TEM. • FWHM of the XRD satellite peaks became narrower when growth temperature decreased. Abstract GaN/AlGaN THz quantum-cascade laser (QCL) structures were grown on AlGaN/Si templates by a metal-organic chemical vapor deposition (MOCVD). We aim to fabricate a double metal waveguide (DMW) based GaN THz QCL through removing the Si substrate by wet etching. The DMW is essential for realizing low cavity loss and a very high optical confinement factor (99%). We grew 100 periods of GaN/Al 0.21 Ga 0.79 N two quantum wells (QW) type active region on Al 0.08 Ga 0.92 N/Si templates. Samples were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). We found that the full width at half maximum (FWHM) of the XRD satellite peaks became narrower when the growth temperature decreased, and the steepness of the QC layer hetero-interfaces has been improved. [ABSTRACT FROM AUTHOR]