Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers

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.

Ultrafast terahertz saturable absorbers using tailored intersubband polaritons

“…A further increase of the pump intensity saturates the ISB transition, which reduces the strength of light-matter coupling and leads to a reduction of the nonlinear signal. In comparison, the nonlinearities of the bare ISB transition 28 show qualitatively similar behaviour (Fig. 4a, b, blue curves), yet for a tenfold increase of the intensity.…”

“…ISB transitions are promising candidates, enabling straightforward tunability and monolithic integration into the QCL cavity. Still, they suffer from relatively high saturation intensities 28 . A much more powerful solution is provided by ultrastrong coupling of such ISB transitions to the near field of metallic resonators.…”

Two-dimensional spectroscopy on a THz quantum cascade structure

“…In contrast to previous studies on similar, but simpler, quantum well samples, our system shows a population decay as well as non-trivial oscillation (see Figure 4). As already mentioned, Raab et al [26] investigated a multiquantum-well system which contains only two levels. Such a system will possess only two PP signals (A pump B probe and B pump A probe ).…”

“…Since both mechanisms rely on χ (3) processes, it is essential to investigate the fundamental nonlinear material properties. In addition, devices such as quantum cascade detectors [25], saturable absorbers [26], and nonlinear conversion media [27] with giant nonlinearities rely on the same operation principle of intersubband transitions in multi-quantum-well structures.…”

“…Since this multi-quantum-well system is far from a trivial system and involves many coupled electron states, an advanced modeling is required to fully understand and interpret the experimental results. Therefore, we go beyond the commonly employed models based on classical [33] or fewlevel Maxwell-Bloch equations with phenomenological scattering and dephasing rates [26,30,34,35]. We model our experiment using a time-resolved density matrix model formalism which takes into account all the relevant intersubband states, as well as the relevant scattering mechanisms (except electron-electron interactions) treated with a master equation in Lindblad form [36].…”