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

Abstract

We implement a Maxwell-Bloch simulation for a two-level system within the finite-difference time-domain method to simulate the seeding of lasers by broadband pulse injection. The model does not make the slowly varying envelope approximation, and the full electromagnetic field is simulated so that we are able to obtain time-resolved seeding by few-cycle pulses. The model is compared to recent results on seeding of THz quantum cascade lasers to aid in the interpretation of their complex signals. The simulations are found to be in good agreement with the data when gain recovery times of 15 ps are used. Furthermore, we find that the emission from the laser depends only weakly on the seed used to initiate laser action. The model is readily applicable to any seeded laser system where few-cycle seed pulses are used.

Pump-probe measurements of gain in a terahertz quantum cascade laser

“…This value is significantly higher than has been measured for QCLs operating in the mid-IR range (2-3 ps) where the active region transport is based on rapid phonon-depopulation designs. We note that the values of GRT agree well with the estimate of 15 ps obtained by Maxwell-Bloch simulations of pulsed seeding of a bound-to-continuum THz QCL [7]. The fast gain recovery measured in this work explains why conventional methods for modelocking THz QCLs are problematic.…”

Gain recovery time in a terahertz quantum cascade laser

“…We also note that the values we measure here agree well with the estimate of 15 ps obtained by Maxwell-Bloch simulations of pulsed seeding of a bound-to-continuum THz QCL. 16 The fast gain recovery measured in this work explains why conventional methods for modelocking THz QCLs are problematic. Given that the cavity round-trip time is generally 50-100 ps for most devices, THz QCLs will tend to support multiple pulses in the same cavity.…”

“…The GRT has also been inferred indirectly from simulations of pulse-seeding in THz QCLs; this enabled an estimate of the GRT to be made of around 15 ps. 16 In our work, we measure the GRT of a bound-to-continuum THz QCL, similar to the device used in Ref. 13, directly, using a THz-pump-THzprobe technique.…”

Injection seeding and modelocking of metal-metal Terahertz quantum cascade lasers

“…This is an important application for QCLs as it allows access to both the amplitude and phase of the emission on ultrafast timescales. It has permitted, for example, the demonstration of modelocking [2] as well as bringing insights into the QCL's ultrafast dynamics [3]. It also allows QCLs to be integrated with THz time-domain spectroscopy (TDS) systems, which could allow the combination of a powerful, narrow-band QCL pump and a broadband THz probe.…”