Abstract
Terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation are currently the only room-temperature mass-producible diode-laser-like emitters of coherent 1–6 THz radiation. Device performance has improved dramatically over the past few years to reach milliwatt-level power output and broad tuning from 1.2 to 5.9 THz, all at room-temperature. Terahertz output in these sources originates from intersubband optical nonlinearity in the laser active region. Here we report the first comprehensive spectroscopic study of the optical nonlinearity and investigate its dependence on the mid-infrared pump frequencies. Our work shows that the terahertz generation efficiency can vary by a factor of 2 or greater depending on the spectral position of the mid-infrared pumps for a fixed THz difference-frequency. We have also measured for the first time the linewidth for transitions between the lower quantum cascade laser states, which is critical for determining terahertz nonlinearity and predicting optical loss in quantum cascade laser waveguides.
Highlights
The terahertz (THz) spectral range (0.3–10 THz) has a proven potential for a variety of applications, such as spectroscopy, communications, and security screening[1,2,3,4]
Terahertz generation in THz difference-frequency generation (DFG)-quantum cascade lasers (QCL) is produced via the nonlinear mixing of the two mid-IR pump fields generated in the QCL active region
The ability to accurately predict and maximize χ(2) in the active region is critical for further advancement of THz DFG-QCL technology
Summary
The terahertz (THz) spectral range (0.3–10 THz) has a proven potential for a variety of applications, such as spectroscopy, communications, and security screening[1,2,3,4]. Room-temperature THz sources analogous to diode lasers in the near-infrared/visible or quantum cascade lasers (QCL) in the mid-infrared (mid-IR), i.e. electrically-pumped, compact, widely-tunable, and suitable for low-cost production, are highly desired for feasible and inexpensive THz systems. THz sources based on intra-cavity difference-frequency generation (DFG) in dual-wavelength mid-IR QCLs11–20 are currently the only electrically-pumped mass-producible monolithic semiconductor laser sources that can operate at room temperature in 1–6 THz region. A variety of monolithic electronically-tunable THz DFG-QCL sources were demonstrated at room temperature[17,18,19]. Terahertz generation in THz DFG-QCLs is produced via the nonlinear mixing of the two mid-IR pump fields generated in the QCL active region.
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