Changing the length of the cavity is perhaps the simplest way to tune the wavelength of a laser, but is almost never used for continuous tuning over a large fractional range. This is because, to avoid multi-mode lasing and mode hopping, the cavity must be kept optically short to ensure a large free-spectral-range compared to the gain bandwidth of the amplifying material. The metasurface VECSEL architecture is shown to be an effective approach for widely tunable lasers based upon cavities that operate on low-order longitudinal modes. Since the gain resides in the amplifying reflectarray metasurface, and not a bulk medium, there is no gain/loss penalty to making the cavity length wavelength scale. Fractional tuning of a THz quantum-cascade laser up to 25% is observed in a multi-mode regime, and up to 19% in a single-mode regime with high quality beam pattern. We discuss the fundamental limits to broadband single-mode tuning using this approach.
The terahertz quantum-cascade (QC) VECSEL is a recently demonstrated approach to designing single-mode terahertz lasers based on the coupling of an amplifying reflect-array metasurface with an external optical cavity. The QC-VECSEL has demonstrated single-mode terahertz lasing with high output power and near-diffraction limited beam quality. The QC-VECSEL is also a natural candidate for demonstrating broadband, continuous, single-mode frequency tuning as the VECSEL’s lasing frequency is determined by the length of the its external cavity, which can be mechanically tuned. In this work, we use a piezoelectric translational stage to actively adjust the length of the QC-VECSEL’s external cavity and demonstrate >500 GHz of single-mode tuning around a center frequency of 3.5 THz (>20% fractional tuning). High-quality, circular beam patterns are observed with a divergence angle of ~15° throughout the tuning range, and tens of milliwatts of peak terahertz output power are observed. In order to maintain single mode behavior, the external cavity is made to be extremely short, increasing the spacing between the external cavity’s neighboring longitudinal resonances. Cavity lengths as short as 250 µm have been studied, but the free-spectral range of the external cavity could not be made larger than the gain bandwidth of the metasurface, providing testament to the bandwidth of both the metasurface and the QC-gain material.
The VECSEL architecture is shown to be an effective approach for building THz quantum-cascade lasers with scalable watt-level output power in a high quality beam pattern. The enabling component is a “metasurface” made up of sub-wavelength arrays of antenna-coupled sub-cavities loaded with quantum-cascade active material. By using a sub-cavity antenna based upon a third-order resonance (rather than a first order resonance), metasurfaces with higher fill factors are demonstrated which are suitable for large output powers. Watt-level pulsed output powers have been demonstrated in a single mode, with tunability achieved by intra-cryostat tuning of the cavity length.
Terahertz quantum-cascade lasers based upon active reflectarray metasurfaces are shown to be a viable platform for scalable power with high-quality beams. The “metasurface” is made up of sub-wavelength arrays of antenna-coupled sub-cavities loaded with quantum-cascade active material. One has the ability to spatially engineer the amplitude, phase, spectral, and polarization response of the metasurface. We present several recent results. By placing the metasurface as part of external cavity, focusing QC-VECSELs have been demonstrated with high slope efficiency, high cw power, and near diffraction-limited beam quality. Additionally, VECSELs with electrically switchable polarization have been demonstrated.
Achieving high power in combination with high quality beam pattern is a ubiquitous challenge for semiconductor lasers. The demonstration of vertical-external-cavity surface-emitting lasers (VECSELs) in 1997 for visible and near-infrared semiconductor lasers has been a very successful approach. Terahertz (THz) quantum-cascade (QC) lasers, also have the challenge of combining high power and good beam pattern into one device – even more so because they typically use sub-wavelength metallic waveguides. The concept of VECSEL has been impossible to implement for QC lasers, since the optical gain is based on intersubband transitions of electrons, which only interact with the electric field polarized perpendicular to the quantum wells plane according to the "intersubband selection rule". To address this issue, we have developed an amplifying metasurface reflector that can couple the incident THz wave with the QC gain medium via metal-metal micro-cavity antenna reflectarray. Pairing the active metasurface with an output coupler, we demonstrated the first VECSEL in the THz regime in 2015. Based upon the prototype design, we have achieved a number of improvements to the QC-VECSEL including designing an inhomogeneous focusing metasurface to achieve a near-diffraction limited beam pattern with M2 = 1.3 and high brightness of 1.86×106 Wsr-1m-2, designing compact cavities and optimizing metasurface bias area to achieve continuous-wave operation above 77 K, achieving record high slope efficiency of 745 mW/A, as well as extending the VECSEL concept to cover a broad frequency range from 2.5 - 4.4 THz.
Terahertz quantum-cascade vertical external cavity surface emitting laser (VECSELs) are made possible through the development of amplifying reflectarray metasurfaces. The metasurface is made up of sub-wavelength arrays of antenna coupled sub-cavities loaded with quantum-cascade active material. The QC-VECSEL approach allows scaling of laser power while maintaining a high quality, near diffraction limited beam - something which has been a long standing challenge for THz quantum-cascade lasers with sub-wavelength metallic waveguides. The latest results of cavity and metasurface engineering are presented, including the demonstration of a focusing reflectarray metasurface that enables a "flat-optics" hemispherical VECSEL cavity, with improved geometric stability and a Gaussian profile beam with beam quality parameter of M2=1.3.
Vertical-external-cavity surface-emitting lasers (VECSELs) have been successfully used in the visible and near-infrared to achieve high output power with excellent Gaussian beam quality. However, the concept of VECSEL has been impossible to implement for quantum-cascade (QC) lasers due to the "intersubband selection rule". We have recently demonstrated the first VECSEL in the terahertz range. The enabling component for the QC-VECSEL is an amplifying metasurface reflector composed of a sparse array of metallic sub-cavities, which allows the normally incident radiation to interact with the electrically pumped QC gain medium. In this work, we presented multiple design variations based on the first demonstrated THz QC-VECSEL, regarding the lasing frequencies, the output coupler and the intra-cavity aperture. Our work on THz QC-VECSEL initiates a new approach towards achieving scalable output power in combination with a diffraction-limited beam pattern for THz QC-lasers. The design variations presented in this work further demonstrate the practicality and potential of VECSEL approach to make ideal terahertz QC-laser sources.