Abstract-Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers

Jürgen Raab, Christoph Lange, Jessica L. Boland, Ignaz Laepple, Martin Furthmeier, Enrico Dardanis, Nils Dessmann, Lianhe Li, Edmund Linfield, A. Giles Davies, Miriam S. Vitiello, and Rupert Huber

(a) Schematic diagram of the THz saturable absorber structure showing the grating and the MQW stack. 𝛿$\delta$-Si: Silicon delta-doping layers. (b) Electron envelope functions of the first (𝛹1${\Psi }_1$, red) and second (𝛹2${\Psi }_2$, blue) subbands, and the conduction band edge (CB, black), in the MQW structure. (c) Cross section of the sample, showing the simulated field enhancement of the z-component ℰ𝑧${\mathcal{E}}_z$at 2.7 THz underneath one period of the gold grating, normalized to the incident electric field. Dashed horizontal lines indicate a GaAs layer, separating the MQW section from the metal grating. Lower panel: magnified view of the marked part of the upper panel. (d) Electric field waveform of the THz pulses used to excite the ISB system. (e) Amplitude spectrum of the THz transient shown in (d) along with the measured field transmission of the sample. The blue arrow indicates the expected ISB transition frequency. (f) Experimental principle showing the two identical THz pulses with fieldsℰA${\mathcal{E}}_{\text{A}}$andℰB${\mathcal{E}}_{\text{B}}$ delayed by a time 𝜏$\tau$, which prepare and interrogate the structure’s nonlinear response.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-3-2248

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.

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