Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers

TL;DR

This study uses advanced 2D THz spectroscopy to observe and control ultrafast saturation dynamics in semiconductor quantum wells, revealing carrier-wave Rabi flopping and guiding the design of optimized THz saturable absorbers.

Contribution

It introduces amplitude and phase-resolved 2D THz spectroscopy to directly observe saturation and coherent control of ISB transitions in MQWs, supported by a Maxwell-Bloch model.

Findings

Nonlinear signals peak at 11 kV/cm electric field.

Carrier-wave Rabi flopping observed at high fields.

Numerical model matches experimental results.

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.