Gain recovery time in a terahertz quantum cascade laser

Abstract

The gain recovery time of a bound-to-continuum terahertz frequency quantum cascade laser, operating at 1.98 THz, has been measured using broadband terahertz-pump-terahertz-probe spectroscopy. The recovery time is found to reduce as a function of current density, attaining a value of 18 ps as the laser is brought close to threshold. We attribute this reduction to improved coupling efficiency between the injector state and the upper lasing level as the active region aligns.

Gain dynamics in a heterogeneous terahertz quantum cascade laser

“…The gain recovery time between 34 ps and 50 ps that we determined by studying BTC QCHs with resonantphonon extraction above threshold is generally longer than the GRT of 15 -26 ps found in other similar studies of THz BTC QCHs. 17,18,22,29 The theoretical calculation of a bound-to-continuum quantum cascade at 3.74 THz also indicated a quite short upper state lifetime of ∼ 12 ps. 30 However, all those results have been obtained for the classical BTC design of THz QCH.…”

“…All present studies of the gain dynamics have been performed on QCLs with the bound-to-continuum (BTC) design that features a relatively narrow gain spectrum of ∼ 200 GHz and have reported a GRT in the range of 15 -26 ps without the phonon extraction scheme. 17,18,21,22 Following the recent successful demonstration of a THz QCL with octave spanning gain, 23 heterogeneous QCL active regions composed from several different quantum cascades structures become increasingly important. 24 While for standard active regions of THz QCLs, a single cascade design (typically 3 -8 quantum wells) is repeated hundreds of times to form a quantum cascade heterostructure (QCH), in a heterogeneous active region several heterostructures with different design details are stacked to form a broadband QCL.…”

“…[13][14][15] This flexible spectroscopic tool helped to identify individual gain degradation mechanisms, 16 and provided direct access to the gain recovery dynamics. 17,18 Fast gain dynamics, which is described by a short characteristic time -the gain recovery time (GRT) -is important for the high speed modulation of THz QCLs and for the formation and the sustainability of THz pulses in QCLs. Thereby the gain modulation and the pulse dispersion are key issues for the active/passive mode-locking 19,20 of lasers.…”

Passive and hybrid mode locking in multi-section terahertz quantum cascade lasers

“…In fact, to the our best knowledge, to present date there have been only two publications experimentally investigating the gain recovery time in bound-to-continuum (BTC) THz devices, and none in resonant-phonon QCLs. Interestingly, these experimental results indicate sub-threshold lifetimes on the order of several tens of picoseconds [20,21]. These measurement techniques are based on a pump-probe experimental method where a perturbing resonant pump pulse is injected into the gain medium followed by a temporally detuned probe pulse interacting with the saturated gain.…”

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

“…However, it is difficult to form stable modelocked pulses from this type of device owing to the interplay of the inherent characteristic lifetimes, most notably the ratio of the gain recovery time (GRT) and round-trip time of the laser cavity. In this work [3], we measure the GRT of a bound-to-continuum THz QCL directly using a THz-pump-THz-probe technique, based on a THz time-domain spectroscopy (TDS) [4,5 ], arrangement. The GRT of the THz-QCL is dependent on the applied bias and is found to reduce as the device approaches threshold, at which point a value of 18 ps is obtained.…”