Ultrafast terahertz saturable absorbers using tailored intersubband polaritons

Jürgen Raab,Laura K Diebel,Edmund H Linfield,Christoph Lange,Nils Dessmann,Leonardo Viti,Lianhe Li,A Giles Davies,Rupert Huber,Francesco P Mezzapesa,Miriam S Vitiello

doi:10.1038/s41467-020-18004-8

Abstract

Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. While nonlinearities play a central role for cutting-edge functionality, they require strong field amplitudes owing to the weak light-matter coupling of electronic resonances of naturally occurring materials. Here, we ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of light-matter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation. We complement our data by a quantitative theory. Our results highlight a path towards passively mode-locked QCLs based on polaritonic saturable absorbers in a monolithic single-chip design.

Highlights

Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices
We prepare a multiquantum well (MQW) structure consisting of GaAs quantum wells (QWs) of a thickness of nm and a nominal doping concentration of 5.0 × 1010 cm−2 charge carriers, per QW
The QWs are separated by Al0.15Ga0.85As barriers of a thickness of 20 nm

Summary

Introduction

Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. Energy-efficient and cost-effective devices with extraordinary design flexibility, which has led to countless applications ranging from data storage to ultrafast solid-state lasers[1,2,3] While their lasing process exploits interband transitions of electrons, quantum cascade lasers[4,5] (QCLs) are based on intersubband (ISB) transitions in electrically biased semiconductor multiquantum well (MQW) structures and extend the emission spectrum towards the mid-infrared and THz spectral regions. Whereas commercial QCLs available today operate in continuous-wave (cw) mode, there has been a strong demand for QCLs generating few or even single-cycle pulses Such inexpensive, compact, and highly stable devices could replace complex and expensive table-top laser sources and boost exciting applications ranging from rapid gas sensing via ultrafast communications[6] to fundamental research investigating low-energy elementary excitations in condensed matter, including superconducting currents[14], magnons[15] or Dirac currents[16], on subcycle time scales. Generating single-cycle pulses, requires passive mode locking facilitated by saturable absorber (SA) structures with short response times and low saturation thresholds

Methods

Results

Conclusion