Silver-based surface plasmon waveguide for terahertz quantum cascade lasers

Abstract

Terahertz-frequency quantum cascade lasers (THz QCLs) based on ridge waveguides incorporating silver waveguide layers have been investigated theoretically and experimentally, and compared with traditional gold-based devices. The threshold gain associated with silver-, gold- and copper-based devices, and the effects of titanium adhesion layers and top contact layers, in both surface-plasmon and double-metal waveguide geometries, have been analysed. Our simulations show that silver-based waveguides yield lower losses for THz QCLs across all practical operating temperatures and frequencies. Experimentally, QCLs with silver-based surface-plasmon waveguides were found to exhibit higher operating temperatures and higher output powers compared to those with identical but gold-based waveguides. Specifically, for a three-well resonant phonon active region with a scaled oscillator strength of 0.43 and doping density of 6.83 × 10 cm, an increase of 5 K in the maximum operating temperature and 40% increase in the output power were demonstrated. These effects were found to be dependent on the active region design, and greater improvements were observed for QCLs with a larger radiative diagonality. Our results indicate that silver-based waveguide structures could potentially enable THz QCLs to operate at high temperatures.

Bayesian Optimization of Terahertz Quantum Cascade Lasers

“…Recently, our group increased the operating temperature of terahertz QCLs using a design optimized using the IAPT described above and varying the four layer widths to maximize the gain at 300 K. The details of this optimization are given in Supplemental Material [37]. The optimized design had a simulated gain of 25 cm −1 , which predicts a much-higher operating temperature than observed experimentally [7], considering total optical losses of about 20 cm −1 at 300 K for Cu-Cu double-metal waveguides [39]. However, these simulations did not consider the effect of electron-electron (e-e) correlations, which act to redistribute the carriers (thermalization) and greatly reduce the peak gain due to level broadening [40].…”

Design and simulation of losses in Ge/SiGe terahertz quantum cascade laser waveguides

“…For GaAs/AlGaAs THz QCLs, most single and double metal plasmon waveguides use Au as the metal waveguide. More recently both Cu [37] and Ag [38] have been used for double metal waveguides due to their higher electrical and thermal conductivity compared to Au. A comparison of Ag, Au and Cu on GaAs/AlGaAs THz QCLs has demonstrated that Ag provides the lowest waveguide losses and lowest lasing thresholds [38].…”

“…More recently both Cu [37] and Ag [38] have been used for double metal waveguides due to their higher electrical and thermal conductivity compared to Au. A comparison of Ag, Au and Cu on GaAs/AlGaAs THz QCLs has demonstrated that Ag provides the lowest waveguide losses and lowest lasing thresholds [38]. Au is a fast diffuser and a deep level impurity in Si, Ge and SiO 2 [39][40][41] and therefore it is incompatible with all silicon foundries that might microfabricate Ge/Si 1−x Ge x QCL devices.…”

Температурная Зависимость Порогового Тока И Выходной Мощности Квантово-Каскадного Лазера С Частотой Генерации 3.3 ТГц

“…Со стороны " высоких" частот расстояние между рабочими уровнями (около 20 мэВ для частоты 5 ТГц) становится сопоставимо с энергией LO-фонона в GaAs (E LO = 36 мэВ), что приводит к испусканию LO-фононов " горячими" электронами на верхнем рабочем уровне и уменьшению инверсной населенности. Следует отметить, что в области низких частот 1−2 ТГц наблюдается рост потерь, обусловленных поглощением излучения свободными носителями заряда, а в области частот свыше 6 ТГц -резонансным поглощением на TO-фононах [7,8], что также ограничивает частотный диапазон работы ТГц ККЛ.…”