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1. Tunable quantum-cascade VECSEL operating at 1.9 THz

   https://doi.org/10.1364/OE.438636  

   Wu, Yu; Shen, Yue; Addamane, Sadhvikas; Reno, John_L; Williams, Benjamin_S (October 2021, Optics Express)

   We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) emitting around 1.9 THz with up to 10% continuous fractional frequency tuning of a single laser mode. The device shows lasing operation in pulsed mode up to 102 K in a high-quality beam, with the maximum output power of 37 mW and slope efficiency of 295 mW/A at 77 K. Challenges for up-scaling the operating wavelength in QC metasurface VECSELs are identified. 

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2. Long wavelength interband cascade lasers

   https://doi.org/10.1063/5.0084565  

   Massengale, J. A.; Shen, Yixuan; Yang, Rui Q.; Hawkins, S. D.; Klem, J. F. (February 2022, Applied Physics Letters)

   InAs-based interband cascade lasers (ICLs) can be more easily adapted toward long wavelength operation than their GaSb counterparts. Devices made from two recent ICL wafers with an advanced waveguide structure are reported, which demonstrate improved device performance in terms of reduced threshold current densities for ICLs near 11  μm or extended operating wavelength beyond 13  μm. The ICLs near 11  μm yielded a significantly reduced continuous wave (cw) lasing threshold of 23 A/cm²at 80 K with substantially increased cw output power, compared with previously reported ICLs at similar wavelengths. ICLs made from the second wafer incorporated an innovative quantum well active region, comprised of InAsP layers, and lased in the pulsed-mode up to 120 K at 13.2  μm, which is the longest wavelength achieved for III–V interband lasers. 

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3. RF Injection Locking of THz Metasurface Quantum‐Cascade VECSEL

   https://doi.org/10.1002/lpor.202300007  

   Wu, Yu; Curwen, Christopher A.; Shahili, Mohammad; Reno, John L.; Williams, Benjamin S. (March 2023, Laser & Photonics Reviews)

   Abstract Radiofrequency (RF) injection locking and spectral broadening of a terahertz (THz) quantum‐cascade vertical‐external‐cavity surface‐emitting laser (QC‐VECSEL) is demonstrated. An intracryostat VECSEL focusing cavity design is used to enable continuous‐wave lasing with a cavity length over 30 mm, which corresponds to a round‐trip frequency near 5 GHz. Strong RF current modulation is injected to the QC‐metasurface electrical bias to pull and lock the round‐trip frequency. The injection locking range at various RF injection powers is recorded and compared with the injection locking theory. Moreover, the lasing spectrum broadens from 14 GHz in free‐running mode to a maximum spectral width around 110 GHz with 20 dBm of injected RF power. This experimental setup is suitable for further exploration of active mode‐locking and picosecond pulse generation in THz QC‐VECSELs. 

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4. Broadband metasurface design for terahertz quantum‐cascade VECSEL

   https://doi.org/10.1049/el.2020.1963  

   Curwen, CA; Reno, JL; Williams, BS (October 2020, Electronics Letters)

   A broadband active metasurface is designed around quantum‐cascade (QC) gain material for use in a broadband tunable QC vertical external‐cavity surface‐emitting laser (VECSEL). The metasurface is based on a unit cell containing two resonant waveguide elements that couple with the periodicity of the metasurface to produce a multi‐resonant reflection spectrum. Simulated reflectance spectra exhibit full‐width half‐maximum bandwidths up to 50% of the centre frequency. The tested QC‐VECSEL demonstrates up to 15 mW of peak pulse power at 77 K and supports multimode lasing from 2.85 to 3.9 THz (>30% fractional bandwidth around 3.37 THz). The frequency coverage is limited by how short the cavity can be made, rather than the metasurface bandwidth. Consistent multimoding results from non‐uniform distribution of spectral energy across the surface. 

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5. Continuous recoil-driven lasing and cavity frequency pinning with laser-cooled atoms

   https://doi.org/10.1038/s41567-025-02854-4  

   Schäfer, Vera_M; Niu, Zhijing; Cline, Julia_R_K; Young, Dylan_J; Song, Eric_Yilun; Ritsch, Helmut; Thompson, James_K (April 2025, Nature Physics)

   Abstract Laser-cooled gases of atoms interacting with the field of an optical cavity are a versatile tool for quantum sensing and the simulation of quantum systems. These systems can exhibit phenomena such as self-organization phase transitions, lasing mechanisms, squeezed states and protection of quantum coherence. However, investigations of these phenomena typically occur in a discontinuous manner due to the need to reload atomic ensembles. Here we demonstrate hours-long continuous lasing from laser-cooled⁸⁸Sr atoms loaded into a ring cavity. The required inversion to produce lasing arises from inversion in the atomic-momentum degrees of freedom, which is linked to the self-organization phase transitions and collective atomic recoil lasing observed previously only in a cyclic fashion. We find that over a broad parameter range, the sensitivity of the lasing frequency to changes in cavity frequency is significantly reduced due to an atomic loss mechanism, suggesting a potential approach for mitigating low-frequency cavity noise. Our findings open opportunities for continuous cavity quantum electrodynamics experiments and robust and continuous super-radiant lasers. 

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