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1. Continuous operation of a monolithic semiconductor terahertz source at room temperature

   https://doi.org/10.1063/1.4881182  

   Lu, Q Y; Bandyopadhyay, N; Slivken, S; Bai, Y; Razeghi, M (June 2014, Applied Physics Letters)

   We demonstrate room temperature continuous wave THz sources based on intracavity difference-frequency generation from mid-infrared quantum cascade lasers. Buried ridge, buried composite distributed-feedback waveguide with Čerenkov phase-matching scheme is used to reduce the waveguide loss and enhance the heat dissipation for continuous wave operation. Continuous emission at 3.6 THz with a side-mode suppression ratio of 20 dB and output power up to 3 μW are achieved, respectively. THz peak power is further scaled up to 1.4 mW in pulsed mode by increasing the mid-infrared power through increasing the active region doping and device area. 

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2. Room temperature terahertz quantum cascade laser sources with 215  μ W output power through epilayer-down mounting

   https://doi.org/10.1063/1.4812814  

   Lu, Q Y; Bandyopadhyay, N; Slivken, S; Bai, Y; Razeghi, M (July 2013, Applied Physics Letters)

   We report room temperature terahertz (THz) quantum cascade laser sources with high power based on difference frequency generation. The device is Čerenkov phase matched and spectrally purified with an integrated dual-period distributed-feedback grating. Symmetric current injection and epilayer-down mounting of the device onto a patterned submount are used to improve the electrical uniformity and heat removal, respectively. The epilayer-down mounting also allows for THz anti-reflective coating to enhance the THz outcoupling efficiency. Single mode emission at 3.5 THz with a side-mode suppression ratio and output power up to 30 dB and 215 μW are obtained, respectively. 

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3. Widely tunable room temperature semiconductor terahertz source

   https://doi.org/10.1063/1.4902245  

   Lu, Q Y; Slivken, S; Bandyopadhyay, N; Bai, Y; Razeghi, M (November 2014, Applied Physics Letters)

   We present a widely tunable, monolithic terahertz source based on intracavity difference frequency generation within a mid-infrared quantum cascade laser at room temperature. A three-section ridge waveguide laser design with two sampled grating sections and a distributed-Bragg section is used to achieve the terahertz (THz) frequency tuning. Room temperature single mode THz emission with a wide tunable frequency range of 2.6–4.2 THz (∼47% of the central frequency) and THz power up to 0.1 mW is demonstrated, making such device an ideal candidate for THz spectroscopy and sensing. 

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4. Room temperature terahertz semiconductor frequency comb

   https://doi.org/10.1038/s41467-019-10395-7  

   Lu, Quanyong; Wang, Feihu; Wu, Donghai; Slivken, Steven; Razeghi, Manijeh (June 2019, Nature Communications)

   Abstract A terahertz (THz) frequency comb capable of high-resolution measurement will significantly advance THz technology application in spectroscopy, metrology and sensing. The recently developed cryogenic-cooled THz quantum cascade laser (QCL) comb has exhibited great potentials with high power and broadband spectrum. Here, we report a room temperature THz harmonic frequency comb in 2.2 to 3.3 THz based on difference-frequency generation from a mid-IR QCL. The THz comb is intracavity generated via down-converting a mid-IR comb with an integrated mid-IR single mode based on distributed-feedback grating without using external optical elements. The grating Bragg wavelength is largely detuned from the gain peak to suppress the grating dispersion and support the comb operation in the high gain spectral range. Multiheterodyne spectroscopy with multiple equally spaced lines by beating it with a reference Fabry-Pérot comb confirms the THz comb operation. This type of THz comb will find applications to room temperature chip-based THz spectroscopy. 

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5. Recent Advances in Room Temperature, High-Power Terahertz Quantum Cascade Laser Sources Based on Difference-Frequency Generation

   https://doi.org/10.3390/photonics3030042  

   Lu, Quanyong; Razeghi, Manijeh (September 2016, Photonics)

   We present the current status of high-performance, compact, THz sources based on intracavity nonlinear frequency generation in mid-infrared quantum cascade lasers. Significant performance improvements of our THz sources in the power and wall plug efficiency are achieved by systematic optimizing the device’s active region, waveguide, and chip bonding strategy. High THz power up to 1.9 mW and 0.014 mW for pulsed mode and continuous wave operations at room temperature are demonstrated, respectively. Even higher power and efficiency are envisioned based on enhancements in outcoupling efficiency and mid-IR performance. Our compact THz device with high power and wide tuning range is highly suitable for imaging, sensing, spectroscopy, medical diagnosis, and many other applications. 

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