Slightly larger than a dime, a newly devised frequency comb (whose spectrum contains equally spaced elements) provides a powerful light source, spanning frequencies of over 550 GHz with a total power of 5 mW. At the heart of the frequency combs are terahertz (THz) quantum cascade lasers (QCLs), which have the advantage of having both high power (in the form of THz radiation) and broadband capabilities (since the QCLs have gain over a wider frequency range).
While extremely useful, terahertz spectrum has traditionally remained unexploited despite its usefulness in fields ranging from physics to biology because of the large size and cost of the systems. Frequency combs are powerful tools for high-precision measurements and spectroscopy. Combining these two elements to make a compact frequency comb generating long-wavelength light in the terahertz range can produce a useful source of radiation for a variety of applications in imaging, diagnostics, remote sensing, and identifying molecular "fingerprints" of extremely complex molecules.
Researchers at the Massachusetts Institute of Technology and Sandia National Laboratories have fabricated high-performance QCLs and integrated them into a device to demonstrate new, high-power broadband terahertz frequency combs. By canceling out intracavity wavelength dispersion (through corrugated etching of the laser), a robust comb can be formed since long-wavelength waves travel to the end of the cavity before reflecting back, while short-wavelength waves reflect back earlier.
This produces frequency combs covering a frequency range of almost 500 GHz with more than 70 lines at 3.5 terahertz. The comb's frequency bandwidth covers 14% of its center frequency. This is the highest fractional bandwidth of integrated semiconductor frequency combs to date, suggesting that similar techniques can be used to improve frequency combs at wavelengths other than terahertz, such as the mid-infrared. A derivative of Fourier-transform spectroscopy, Shifted wave interference Fourier-transform (SWIFT) spectroscopy, was used to quantitatively measure the performance of the lasers frequency combs and to measure the effectiveness of comb formation. By utilizing intracavity mixing, these lasers can compactly measure the frequency of single-mode lasers without the need for a high-speed terahertz detector or an external solid-state laser.
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