Ultrafast response of harmonic mode-locked THz lasers

Sukhdeep S Dhillon

doi:10.1117/12.2578208

Abstract

The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. Here, using time-resolved THz techniques, we show the first of demonstration harmonic injection and mode- locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. This generates multiple THz pulses per round trip in both active and self-starting harmonic regimes. This behavior is supported by time-resolved Maxwell-Bloch simulations of induced gain and loss in the system. This work exploits the inherent ultrafast dynamics of QCLs and opens up new avenues in THz pulse generation.

Highlights

Harmonic modelocking is routinely used in the visible and near-infrared range and consists in the generation of multiple light pulses within the photon round-trip time of a laser cavity
Harmonic active modelocking is inherently adapted to quantum cascade lasers (QCLs), as the unique fast dynamics with picosecond relaxation dynamics permit an ultrafast modulation of the gain and loss
We show the first demonstrations of harmonic active modelocking of QCLs, as well as selfharmonic emission at multiple (15th) harmonics with 15 pulses per round-trip

Summary

Introduction

Harmonic modelocking is routinely used in the visible and near-infrared range and consists in the generation of multiple light pulses within the photon round-trip time of a laser cavity. This offers the possibility of high-repetitionrate laser systems, reaching GHz and tens of GHz rates, beyond rates that are limited by the laser cavity length. This is of particular interest in high-bit-rate optical communication[1], photonic analogue-to-digital conversion[2], multi-photon imaging[3], and astronomical frequency comb generation[4].

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