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

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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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