The invention of optical frequency combs generated by mode-locked lasers revolutionized time and frequency metrology in the late 1990s. This concept has been explored in several laser systems; the quantum cascade laser (QCL) is one such system that operates in the terahertz (THz) frequency range.THz QCL was first invented in 2001 as a reliable semiconductor source for compact, high-power THz radiation. The inherently strong third-order nonlinearity in its QC-gain medium allows for spontaneous frequency comb formation as a result of spatial hole burning induced by Fabry-Perot cavities and four-wave mixing, which synchronizes the dispersed cavity modes.
It was noticed that the self-generated combs are naturally frequency-modulated with quasi-continuous power output, whereas amplitude-modulated combs, i.e., mode-locking, are considered challenging in THz QCLs because of the inherent fast gain recovery time.
One effective method to trigger active mode-locking is RF injection locking. It involves injecting RF current modulation into the QC-device at a frequency that is close to the cavity round-trip frequency. This locks the spacing between adjacent lasing modes, and pulses with a duration of 4-5 ps have been reported.
In recent years, the study of frequency comb/mode-locking in THz QCLs has raised increasing interest because of its potential for a number of applications, including astronomy, biomedicine, fast spectroscopy, non-invasive imaging, and non-destructive evaluation.
So far, research has concentrated on ridge-waveguide and ring QCLs.
On the other hand, THz quantum-cascade vertical-external-cavity surface-emitting-laser (QC-VECSEL) was introduced in 2015 as a novel external cavity configuration of THz QCLs.The key concept of THz QC-VECSEL is to engineer its gain chip into a millimeter-scale reflectarray metasurface for free-space THz radiation and further incorporation into a resonant laser cavity as an active reflector.
This enables watt-level output power with near-Gaussian distributed beam quality; versatile functionality may be incorporated into the amplifying metasurface; and broadband frequency tunability is provided by the VECSEL architecture.
Despite the fact that VECSELs are widely used for mode-locking at near-infrared and optical frequencies, THz QC-VECSELs have not yet been exploited in frequency comb and mode-locking applications.
In this thesis, we report for the first time the techniques utilized to achieve frequency comb/mode-locking operations in THz QC-VECSELs. Both the metasurface design and VECSEL cavity geometry are optimized for this purpose. The double-patch metasurface design is considered optimal for broadband frequency response and low dispersion, and a well-designed RF package is needed for efficient RF signal injection and extraction. On the other hand, an off-axis parabolic (OAP) mirror is introduced to build a V-shaped intra-cryostat focusing VECSEL cavity. This OAP-focusing cavity design eliminates most of the intra-cavity diffraction losses and, therefore, enables lasing in an ultra-long external cavity using a small-sized metasurface that supports continuous wave (CW) biasing. It is highly suited for frequency comb/mode-locking applications as the cavity round-trip frequency is lowered to a typical value of 3-5 GHz.
In contrast to ridge-waveguide or ring QCLs, self-generated frequency combs have not been observed in THz QC-VECSELs --- in fact, they prefer to lase in a single-mode regime primarily due to a lack of spatial hole burning.To promote multimode operation in THz QC-VECSELs, we present a technique based on a specific combination of output coupler thickness and external cavity length. Through Vernier selection and reflectance compensation in a cascaded Fabry-Perot cavity, we are able to perform simultaneous nine modes lasing with a free-spectral range (FSR) of ~21 GHz. The number of lasing modes that can be generated using this method is limited by the maximum available output coupler thickness.
A more effective way to promote multimoding, as well as possible frequency comb or even mode-locking operations, is through RF injection locking.The successful demonstration of RF injection locking in THz QC-VECSELs for the first time is the main focus of this thesis. Lasing spectral broadening has been observed under strong RF modulation, with a maximum bandwidth of around 100–300 GHz.
An intermodal beat-note is produced as a result of beating between each of the two lasing modes. It is locked to the RF injection signal as the injection frequency is tuned around the cavity round-trip frequency.
This suggests that the lasing modes are equally spaced, which is a prerequisite of frequency comb/mode-locking.
Several impacting factors, including metasurface design, external cavity length, and optical feedback, are experimentally investigated in the RF-injection locked QC-VECSELs, which may help control and tune the laser states.
THz QC-VECSEL is consequently considered to be a superior platform that enables a more thorough investigation of the fundamental physics of mode-locking/frequency comb operation in QCL systems.
Our research on mode-locked THz QC-VECSELs opens the way for future development of semiconductor lasers operating in the 2–5 THz region that produce picosecond-scale pulses.
