Abstract
The exploitation of ultrafast electron dynamics in quantum cascade lasers (QCLs) holds enormous potential for intense, compact mode-locked terahertz (THz) sources, squeezed THz light, frequency mixers, and comb-based metrology systems. Yet the important sub-cycle dynamics have been notoriously difficult to access in operational THz QCLs. Here, we employ high-field THz pulses to perform the first ultrafast two-dimensional spectroscopy of a free-running THz QCL. Strong incoherent and coherent nonlinearities up to eight-wave mixing are detected below and above the laser threshold. These data not only reveal extremely short gain recovery times of 2 ps at the laser threshold, they also reflect the nonlinear polarization dynamics of the QCL laser transition for the first time, where we quantify the corresponding dephasing times between 0.9 and 1.5 ps with increasing bias currents. A density-matrix approach reproducing the emergence of all nonlinearities and their ultrafast evolution, simultaneously, allows us to map the coherently induced trajectory of the Bloch vector. The observed high-order multi-wave mixing nonlinearities benefit from resonant enhancement in the absence of absorption losses and bear potential for a number of future applications, ranging from efficient intracavity frequency conversion, mode proliferation to passive mode locking.
Highlights
The terahertz (THz) window of the electromagnetic spectrum is at the heart of an ongoing revolution in optical sciences and technology
Our quantum cascade lasers (QCLs) is based on an Al0.1Ga0.9As/GaAs heterostructure that shows laser action at 2.2 THz above a threshold bias current of Ith = 910 mA
Single-cycle THz pulses with a tunable peak field strength of up to 3 kV cm−1 and a repetition rate of 1 MHz generated by tilted-pulse-front optical rectification of intense near-infrared (NIR) laser pulses in lithium niobate are polarized parallel to the growth direction of the heterostructure and coupled into the QCL waveguide through the front facet (Fig. 1a)
Summary
The terahertz (THz) window of the electromagnetic spectrum is at the heart of an ongoing revolution in optical sciences and technology. Strong THz fields have paved the way to extreme nonlinearities[2,8,9,10,11] such as multi-wave mixing[12,13,14,15], high-harmonic generation[9,16], lightwave electronics[8], and ultrafast nanoscopy[10,11,17,18]. A wide range of novel photonics applications has employed THz radiation including high-resolution spectroscopy[1,19], near-field microscopy[20], and telecommunication concepts[21]. In order to meet the growing demands of nonlinear optics and ultrashort pulse generation and to explore novel application fields, a detailed understanding and control of coherent and incoherent electron dynamics in operational THz QCLs is indispensable. In THz pump-probe measurements using weak pulses, incoherent gain dynamics of THz QCLs38–40
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