Gain dynamics in a heterogeneous terahertz quantum cascade laser

Abstract

The gain recovery time of a heterogeneous active region terahertz quantum cascade laser is studied by terahertz-pump terahertz-probe spectroscopy. The investigated active region, which is based on a boundto-continuum optical transition with an optical phonon assisted extraction, exhibits a gain recovery time in the range of 34 -50 ps dependent on the operation condition of the laser. The recovery time gets shorter for stronger pumping of the laser while the recovery dynamics slows down with increasing operation temperature. These results indicate the important role of the intracavity light intensity for the fast gain recovery.Semiconductor terahertz lasers based on the quantum cascade design, so called THz QCLs, have become an established THz technology. 1 Besides the generation of spectrally bright THz light, todays THz QCL can be used as frequency comb source, 2 as transceiver at THz frequencies for sensing and communication, 3 or as a source of tailored coherent pulses generated on demand. 4,5 Although the physics of QCLs is well understood 6 and can be modeled to a satisfactory level, 7 their performance is still not under full control. In experimental studies, discrepancies between the theoretical and actual performance of THz QCLs were observed that were attributed to deviations of the fabricated devices from the design model. To address these issues and to identify their origin THz time-domain spectroscopy (THz-TDS) has been applied to access the internal processes in QCLs, 8,9 which allowed to study the spectral gain curve at all operation points of a QCL (hence even above the lasing threshold), 8,10-12 the gain clamping dynamics, 4 and the gain induced dispersion. [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. 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. As a consequence, the different heterostructures can fea...

RF Injection of THz QCL Combs at 80 K Emitting over 700 GHz Spectral Bandwidth

“…The fact that we often observe the high-phase noise instead of the harmonic state indicates that there must be additional parameters influencing the locking mechanism. Other factors to be taken into account are the relatively long upper state life time of the laser (surely longer than 40 ps [30] measured in our previous, less diagonal design), which would result in an intermediate behaviour between AM and FM comb regimes, and the spatial hole burning [31]. The latter is probably reduced for lower temperatures due the particular photon-driven transport in these devices, which leads to a low differential conductance even though the low-loss subwavelength waveguides leads to higher fields.…”

Mode-locked short pulses from an 8 μm wavelength semiconductor laser

“…Thus, MIR QCLs favor a quasi-continuous intensity waveform rather than short pulses 19 . Note that in THz QCLs, where gain recovery times between 5 ps and 50 ps have been reported 20 , actively mode-locked pulses were observed in several works [21][22][23][24] with durations as short as 4 ps 25 .…”

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.…”