Terahertz source frequency comb based on difference frequency generation from a mid-IR quantum cascade laser

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Compact, Chip-based Terahertz Frequency CombAbstractNon-techniical: A frequency comb is a light source whose spectrum consists of a series of discrete, equally spaced elements. Since their discovery in the 1990s, frequency combs have revolutionized metrology, spectroscopy, and frequency synthesis with extraordinary precision and speed. However, spectroscopic applications are limited by the wavelength range that is accessible by the comb. The terahertz (THz) spectral range, where abundant molecules have strong spectral fingerprints or feature broad absorption bands, is not easily accessible with current comb technology. The main challenge is the need for multiple optical components or cryogenic cooling, which leads to a complicated and expensive source. A novel approach is proposed to develop a chip-based THz frequency comb source operating at room temperature. This simple to use and compact source will be an enabling technology which will allow easy access to THz spectroscopy/ imaging for the broader scientific community. Technical: The objective of the proposed research is to demonstrate a room temperature, continuous wave, monolithic THz frequency comb source with sub-milliwatt level output power. The approach is to use a mid-IR quantum cascade laser (QCL) engineered with a flat-top gain and large 2nd and 3rd order optical nonlinearities for the intracavity difference frequency generation (DFG) of a THz frequency comb. A dual-section cavity design is proposed to render the mid-infrared pump source with an output exhibiting a unique combination of single- and multi-mode operation. The multi-mode output can be locked to equally spaced comb modes via four wave mixing and an engineered near-zero group velocity dispersion. Such a cavity scheme provides the perfect conditions for creating a THz comb output via the DFG process and a broadband Èerenkov phase matching scheme. In contrast to a mode-locked laser comb source, which is bulky and expensive, the proposed THz comb source offers a monolithic solution that, once developed, has potential for cost-effective mass production using the existing semiconductor laser fabrication infrastructure. This project is an excellent example of a multidisciplinary approach used to circumvent existing technological limitations. Solid state physics, material science, nonlinear optics, and laser physics are all major components of the research plan, which are all supported generally by NSF, and are used, in this case, together to combine multiple functional elements into a single, compact, high power device.