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Bayesian Optimization of Terahertz Quantum Cascade Lasers
摘要： We use Bayesian optimization algorithms in combination with a nonequilibrium Green’s function transport model to increase the maximum operating temperature of terahertz quantum cascade lasers. This procedure lead to the recent temperature record of 210 K in terahertz quantum cascade lasers, and here we provide even-further-improved structures. The Bayesian optimization algorithm, which takes into account all the available history of the optimization, converges much faster and more securely than the commonly used genetic algorithm. Designs based on two and three wells per period are considered, and using the large amount of data generated, we systematically evaluate the studied schemes in terms of optimal extraction energy and relevance of electron-electron correlations. This analysis shows that the two-well scheme is superior for reaching high operating temperatures, while the three-well scheme is more robust to variations in layer thicknesses. Furthermore, we study the sensitivity to ?ux-rate ?uctuations during growth and simulation-model inaccuracies, showing the period thickness needs to be controlled to within a few percent, which is challenging but achievable with present-day molecular-beam epitaxy. These limits to the growth accuracy can be a guiding principle for experimentalists, along with the suggestion to fabricate devices across the wafer radius so as to ?nd the optimal period thickness.
关键词： Bayesian optimization,electron-electron correlations,nonequilibrium Green’s function,terahertz quantum cascade lasers,molecular-beam epitaxy
更新于2025-09-23 15:19:57
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An antipodal Vivaldi antenna for improved far-field properties and polarization manipulation of broadband terahertz quantum cascade lasers
摘要： We present an antipodal Vivaldi antenna for broadband double metal waveguide terahertz quantum cascade lasers and frequency combs. Its exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing an improved far-field with a single-lobed beam of (23° × 19°) full width half maximum with an octave-spanning bandwidth. The antenna also acts as a wave retarder, rotating the polarization from vertical toward horizontal polarization by a frequency-dependent angle. The laser’s emission spectrum and current–voltage characteristics are not affected, as well as frequency comb operation. Measurements agree well with numerical simulations, and the proposed antenna covers a broad spectral range (1.5–4.5 THz).
关键词： antipodal Vivaldi antenna,broadband,polarization manipulation,far-field properties,terahertz quantum cascade lasers
更新于2025-09-23 15:19:57
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Electrical tuning of a terahertz quantum cascade laser based on detuned intersubband absorption
摘要： A mechanism to electrically tune the frequency of terahertz quantum cascade lasers (QCLs) is developed that allows for tuning, while the QCL is operated close to its peak bias and temperature. Two optically coupled but electrically isolated cavities are used in which the bias of a control cavity tunes the resonant-mode of the coupled QCL cavity independent of the QCL’s operating bias. Approximately 4 GHz electrical tuning is realized for a 3:6 THz distributed-feedback QCL operating in pulsed mode at 58 K in a Stirling cooler. The single-mode QCL emits near-constant peak-power in the range of 5 (cid:2) 5:3 mW through the tuning range and radiates in a narrow single-lobed beam with a far-?eld divergence of (cid:3)4(cid:4) (cid:5) 11(cid:4). The superlattice structure of the QCL is designed to implement a low-voltage intersubband absorption transition that is detuned from that of its gain transition, the strength of which could be controlled sensitively with applied voltage utilizing resonant-tunneling injection of electrons in the absorption subband. The tuning is realized by the application of small bias voltages ((cid:3)6 (cid:2) 7 V) and requires a narrow bias range ((cid:3)1 V, (cid:3)40 A=cm2) to traverse across the entire tuning range, and the method should be generally applicable to all intersubband lasers including mid-infrared QCLs.
关键词： detuned absorption,cavity pulling,terahertz quantum cascade lasers,electrical tuning,intersubband absorption
更新于2025-09-16 10:30:52
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Temporal stability and absolute composition issues in molecular beam epitaxy of AlGaAs/GaAs THz QCL
摘要： The operation of the terahertz quantum cascade lasers (THz QCL) is strongly dependent on the repeatable fabrication of the well and barrier layers with the certain thicknesses throughout the whole active region stack epitaxial growth. This emphasizes the importance of the strict control of the growth rates and the stability of Si and group III effusion cell fluxes during growth [1]. It was reported [2] that two THz QCLs based on nominally identical multilayer heterostructures Al0.15Ga0.85As/GaAs emit at the different frequencies of 2.59 and 2.75 THz because of unintentional small deviations in the GaAs and AlAs growth rates (4 and 1.6 % respectively). Authors [3] determined the thickness tolerance for working lasing heterostructures to be minimally above 2% while the structures with thickness deviation 4.3 and 6.5 % are not lasing. In [4] the Ga cell temperature has to be increased to maintain a GaAs constant growth rate while the Al cell temperature remains nearly constant because variation in AlAs growth rate can be neglected. By using such growth rate compensation technique the two nominally identical structures approximately 10 mkm overall thick were found to show thickness difference of ~ 1 %. Thus calibration procedures and accurate analysis techniques become crucial to provide enhanced metrology possibilities.
关键词： molecular beam epitaxy,metrology,AlGaAs/GaAs,terahertz quantum cascade lasers,growth rates
更新于2025-09-10 09:29:36