Gain recovery dynamics of a terahertz quantum cascade laser

Abstract

The gain recovery time of a bound-to-continuum terahertz quantum cascade laser is measured using a stimulated emission photocurrent technique. At low temperatures a value ϳ50 ps is found. This tends to decrease with rising temperature, and suggests that class B laser dynamics should be expected from these devices. Both the value and the trend in the gain recovery time are found to be consistent with hopping transport through the miniband. Power-dependent measurements of the photocurrent clearly show the effects of inhomogeneous broadening of the gain transition.

Gain recovery time 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. The values that we find are also significantly lower than those obtained by Green et al, 13 where the photo-current through the QCL structure was measured; this may represent a recovery time for the device (rather than the laser transition) and hence be an upper limit for the GRT. 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.…”

“…13 This work did not, however, measure gain directly, but rather the recovery of photocurrent as a function of time delay between two high-power THz pulses, generated by a free-electron laser. A value of 50 ps for the GRT was found, which is significantly longer than the 2-3 ps measured for mid-infrared (mid-IR) frequency QCLs 14,15 using an IR pump-probe technique.…”

“…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. The GRT of the THz QCL is a function of bias and is found to reduce as the device approaches threshold, at which point a value of 18 ps is obtained.…”

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

“…The role of the laser dynamics, particularly the gain recovery time, T 1 , is a critical parameter for stable mode-locking and should also strongly affect the temporal output of seeded QCLs. Despite the importance of this parameter, there has been only one attempt to measure this parameter in THz QCLs directly [14] due to the difficulty of the measurement.…”

Investigation of Time-resolved Gain Dynamics in an Injection Seeded Terahertz Quantum Cascade Laser

“…The lifetime of the intersubband transitions mainly determines QCL gain dynamics which are expected to be on the order of picoseconds. Several previous studies have been performed on the gain dynamics of mid-infrared QCLs [3][4], and terahertz (THz) QCLs [5]. Typically these studies require a very strong initial pump pulse in order to saturate the gain.…”