Two-dimensional coherent spectroscopy of a THz quantum cascade laser: observation of multiple harmonics

Abstract

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ω = 0, ω = ω), (ω = ω, ω = ω), and (ω = 2ω, ω = ω) where ω denotes the lasing frequency. The peak at ω = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery.

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.…”

“…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 a) Electronic mail: christian.g.derntl@tuwien.ac.at b) Now with Crystalline Mirror Solutions GmbH, Austria. c) Electronic mail: juraj.darmo@tuwien.ac.at become increasingly important.…”

Two-dimensional spectroscopy on a THz quantum cascade structure

“…observe nontrivial system dynamics attributed to fieldinduced coherences, the so-called coherent population exchange. We find incoherent lifetimes to be several times smaller than the typical gain recovery times in THz QCLs [44][45][46], as well as saturation times to be shorter than the QCL gain saturation time. Under these circumstances, one can consider such a structure as a monolithically integrated saturable absorber into a QCL cavity in order to achieve a passive mode locking.…”

Two-dimensional spectroscopy on a THz quantum cascade structure

“…Moreover, we observe nontrivial system dynamics attributed to fieldinduced coherences, the so-called coherent population exchange. We find incoherent lifetimes to be several times smaller than the typical gain recovery times in THz QCLs [44][45][46], as well as saturation times to be shorter than the QCL gain saturation time. Under these circumstances, one can consider such a structure as a monolithically integrated saturable absorber into a QCL cavity in order to achieve a passive mode locking.…”