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2Stimulated emission in HgCdTe-based quantum wells: toward continuous wave lasing in THz range
S.V. Morozov
Institute for Physics of Microstructures of RAS, Nizhny Novgorod, Russia, 603950
more@ipmras. ru
Hg(Cd)Te/CdHgTe heterostructures with quantum wells (QW) are an attractive a material for mid infrared (IR) lasers. Due to importance of HgCdTe-based heterostructures for the industry of IR detectors their quality is reaching the quality of A3B5 heterostructures. Hg(Cd)Te/CdHgTe QWs also provide a unique opportunity to change the bandgap from 0 to over 1200 meV, while maintaining the ability to tailor the energy spectrum of the carriers by changing the content of solid solution of the barriers and the QW. In the long-wavelength part of mid Hg(Cd)Te/CdHgTe QWs offer the quasi-relativistic dispersion law of the carriers which suppresses Auger recombination, enabling stimulated emission (SE) up to 31 ^m, and laser generation up to 24 ^m in the temperature range from 10 to 80 K [1,2]. The wavelength of emission, which is demonstrated in our experiments, is six times larger than the previous results for HgCdTe lasers [3,4]. The record wavelength of 31 ^m (inaccessible for existing cascade lasers) was achieved by a peculiar design of the structure utilizing the reflection of the waveguide mode from the substrate near the Reststrahlen band of GaAs. Quasi-relativistic dispersion law of the carriers in HgTe/CdHgTe QWs is useful at even longer wavelengths, in terahertz range, where we have recently managed to experimentally demonstrate "Landau emission" (optical transitions between non-equidistant Landau levels formed when quasi-relativistic electron system is placed in magnetic field) between 1 and 3 THz with the frequency adjustable by magnetic field and carrier concentration [5]. These results open up an avenue for a new type of terahertz Landau lasers controlled by magnetic field and gate voltage.
In this work, by carefully optimizing the waveguides and mitigating carrier heating, we achieve stimulated emission at 14-24 ^m in HgCdTe QWs under optical pumping in quasi-continuous wave regime (pulse duration 20-500 ^s). The intensity is as low as 1.5-2 W/cm2 [6]. The impact of AR happening right after the excitation on the carrier temperature is investigated both theoretically (using the balance equations and calculated AR rates) and experimentally (via PL spectrum analysis). When such 'hot' AR is eliminated due to long-wavelength pumping carrier lifetimes are shown to be only slightly limited by Shockley-Read-Hall recombination. Its contribution is also directly investigated via time resolved measurements of the photoconductivity decay and PL spectroscopy of trap states in the bandgap. Finally, we estimate that implementing microdisc cavities would allow continuous-wave operation of HgCdTe lasers in the very long-wavelength infrared range (14-30 ^m) and beyond when pumped by last generation quantum cascade lasers.
Considering the short-wavelength pat of the IR spectrum, we focus our attention to the atmospheric transparency window of 3-5 ^m. In this range the competition between different types of lasers is very stacked because of its importance for chemical analysis. In this range the advantage of Hg(Cd)Te/CdHgTe QWs is that in addition to the suppression of threshold Auger process, we are able to mitigate QW-specific non-threshold processes associated with non-radiative transitions into barriers. Our results show that by optimizing the parameters of the QE and barriers and increasing the band offset in the valence band it is possible to reach the maximum temperature of SE up to 270 K and optically pumped laser action utilizing whispering gallery modes up to 230 K in the wavelength range of 3-4 ^m [7].
This work was supported by Russian Science Foundation project # 22-12-00310.
[1] S.V. Morozov, et al, ACS Photonics, Vol. 8, No. 12, 3526-3535 (2021).
[2] V.V. Rumyantsev, et al, Appl. Phys. Lett., Vol. 121, No. 18, 182103 (2022).
[3] J.M. Arias, et al, Semicond. Sci. Technol., Vol. 8, S255-5260 (1993).
[4] E. Hadji, et al, Appl. Phys. Lett., Vol. 67, 2591 (1995).
[5] S. Gebert, et al, Nat. Photon., Vol. 17, 244-249 (2023).
[6] V.V. Rumyantsev, et al, Appl. Phys. Lett. 124, 161111 (2024).
[7] A.A. Razova, et al, Appl. Phys. Lett. 123, 161105 (2023).
