Abstract
Mode-locked comb sources operating at optical frequencies underpin applications ranging from spectroscopy and ultrafast physics, through to absolute frequency measurements and atomic clocks. Extending their operation into the terahertz frequency range would greatly benefit from the availability of compact semiconductor-based sources. However, the development of any compact mode-locked THz laser, which itself is inherently a frequency comb, has yet to be achieved without the use of an external stimulus. High-power, electrically pumped quantum cascade lasers (QCLs) have recently emerged as a promising solution, owing to their octave spanning bandwidths, the ability to achieve group-velocity dispersion compensation and the possibility of obtaining active mode-locking. Here, we propose an unprecedented compact architecture to induce both frequency and amplitude self-modulation in a THz QCL. By engineering a microwave avalanche oscillator into the laser cavity, which provides a 10 GHz self-modulation of the bias current and output power, we demonstrate multimode laser emission centered around 3 THz, with distinct multiple sidebands. The resulting microwave amplitude and frequency self-modulation of THz QCLs opens up intriguing perspectives, for engineering integrated self-mode-locked THz lasers, with impact in fields such as nano- and ultrafast photonics and optical metrology.
Highlights
quantum cascade lasers (QCLs)-based frequency combs have been demonstrated in the THz frequency range, without the need for external seeding, by engineering the refractive index dispersion of the QCL active region[17], or through self-seeding devices that exploit an octave-spanning emission[18]
By engineering a THz frequency QCL in a double-metal waveguide with an integrated GaAs-based tunable avalanche oscillator, we report observation of the emission of frequency and amplitude modulated THz radiation
The reduction of the contact layer thickness is expected to change the electrostatics of the active region in proximity to the metal-semiconductor junction, effectively forming a diode that can be driven into the avalanche regime
Summary
QCL-based frequency combs have been demonstrated in the THz frequency range, without the need for external seeding, by engineering the refractive index dispersion of the QCL active region[17], or through self-seeding devices that exploit an octave-spanning emission[18]. We propose an alternative technique to achieve self-mode-locking in THz QCLs, based on the generation of a microwave seed within the QCL cavity itself. By engineering a THz frequency QCL in a double-metal waveguide with an integrated GaAs-based tunable avalanche oscillator, we report observation of the emission of frequency and amplitude modulated THz radiation. This provides an integrated solution to achieve gain modulation at the cavity round-trip frequency and, at the same time, phase locking through the stabilization of the microwave oscillator
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