Abstract
Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. Here, we exploit amplitude and phase-resolved two-dimensional (2D) THz spectroscopy on the sub-cycle time scale to observe directly the saturation dynamics and coherent control of ISB transitions in a metal-insulator MQW structure. Clear signatures of incoherent pump-probe and coherent four-wave mixing signals are recorded as a function of the peak electric field of the single-cycle THz pulses. All nonlinear signals reach a pronounced maximum for a THz electric field amplitude of 11 kV/cm and decrease for higher fields. We demonstrate that this behavior is a fingerprint of THz-driven carrier-wave Rabi flopping. A numerical solution of the Maxwell-Bloch equations reproduces our experimental findings quantitatively and traces the trajectory of the Bloch vector. This microscopic model allows us to design tailored MQW structures with optimized dynamical properties for saturable absorbers that could be used in future compact semiconductor-based single-cycle THz sources.
Highlights
Terahertz photonics has emerged as an exciting field of research owing to its enormous promise in a fast-growing variety of applications
We use field-sensitive 2D spectroscopy to investigate the ultrafast nonlinear response, as well as the saturation dynamics, of an ISB transition in a multi-quantum well (MQW) system, which is coupled to a THz electromagnetic field via a metallic grating
Note that all dynamics occur in a regime where the Rabi frequency, the intersubband transition frequency and the carrier frequency of the THz field, all are of comparable size
Summary
Terahertz photonics has emerged as an exciting field of research owing to its enormous promise in a fast-growing variety of applications These range from ultrafast spectroscopy and sub-cycle control of condensed matter to non-invasive imaging, chemical fingerprinting and security screening [1,2]. Electrically-pumped quantum cascade lasers (QCLs) offer a promising alternative They exploit intersubband transitions in semiconductor heterostructures, enabling a wide operational frequency range through bandstructure engineering. In the visible to mid-infrared spectral region, semiconductor-based saturable absorbers have been routinely used to achieve passive mode-locking [22] In these schemes, the high peak intensity of pulsed light suffers substantially lower losses within the saturable absorber than the cw background, which allows for a robust formation of ultrashort pulses in the cavity. While the concept of ISB transitions is well known and the dynamics of their interaction with light have been studied in great detail [25,26,27,28,29,30], their coherent and incoherent nonlinear dynamics in the THz frequency range have yet to be evaluated
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