Gain Measurements of THz Quantum Cascade Lasers using THz Time-Domain Spectroscopy

Abstract

Terahertz (THz) time domain spectroscopy is used to investigate the gain and losses of a THz quantum cascade laser (QCL) operating at 2.86THz. This measurement technique allows access to the amplitude and phase spectra, allowing the direct determination of the gain. At the emission frequency of the QCL, a value of 6.5cm-1 is found. The gain can also be studied as a function of different operating conditions, even when no laser action is present. Effects such as gain clamping and spectral narrowing are also observed. Furthermore, temperature measurements illustrate the reduction of the gain as the temperature is increased.

Laser-seeding dynamics with few-cycle pulses: Maxwell-Bloch finite-difference time-domain simulations of terahertz quantum cascade lasers

“…1. This leads to a reflection of around 30% at the boundary, in good agreement with other studies and experiments [23][24][25]. The region outside the cavity is made highly lossy to 063817-2 ensure that reflections from the fixed boundary conditions are suppressed.…”

Terahertz time domain spectroscopy of phonon-depopulation based quantum cascade lasers

“…For current densities greater than 792A/cm 2 (when the bandstructure starts to align up to the design field) oscillations of the field are observed owing to the amplification of the input pulse and last for approximately 3ps when the device is above threshold. The short time period of the oscillations, compared to bound-to-continuum designs, 7,8 are attributed to both the small cavity length 13 and the broad gain spectra. The latter can be observed in Fig.…”

“…1(b) shows the difference transmitted electric fields (i.e. that measured relative to the modulation frequency of the QCL) 7 of the THz pulses for various current densities. For current densities greater than 792A/cm 2 (when the bandstructure starts to align up to the design field) oscillations of the field are observed owing to the amplification of the input pulse and last for approximately 3ps when the device is above threshold.…”

Spectral gain profile of a multi-stack terahertz quantum cascade laser

“…Because additional requirements, such as identical threshold currents of the different stacks, make the overall design very crucial, THz-TDS gain measurements provide valuable information for bandstructure design and consequently enable further improvement of heterogeneous active regions. 14,15 Gain measurements of a two-stack THz QCL, processed in a single plasmon waveguide, were presented in Ref. 16.…”

“…14,15 The emitter section was square wave modulated at a repetition rate of 20 kHz and a duty cycle of 20%, whereas the QCL gain section was independently modulated at 10 kHz and a duty cycle of 10%. Due to the missing top contact layer of the active region, the emitter section could be reverse-biased with amplitudes up to À30 V, without driving a significant dark current through the structure.…”