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Researchers developed an optical cavity that improves the sensitivity of terahertz spectroscopy. [Image: Francis Hindle, Université du Littoral-Côte d'Opale]

High-sensitivity cavity-based spectroscopic techniques work well at microwave and infrared frequencies, but the lack of a high-finesse Fabry-Pérot cavity for terahertz waves has hindered the extension of those techniques to crucial applications, such as gas-phase chemical analysis.

Researchers at a French university have devised a high-finesse cavity that boosts a spectrometer’s sensitivity in the terahertz region (Optica, doi: 10.1364/OPTICA.6.001449).

Confining terahertz waves

Molecular rotations and vibrations at frequencies between 300 GHz and 10 THz already give radio astronomers, with their expensive and dedicated electronics, a view of the interstellar medium and developing planetary systems. But the lack of high-finesse cavities for terahertz radiation has hindered the transfer of cavity-based infrared techniques to practical, Earth-bound applications of terahertz spectroscopy, such as breath analysis, environmental monitoring and detection of food spoilage. In particular, researchers have not been able to develop the right kind of dielectric spherical mirrors to confine terahertz waves in a cavity.

Scientists at the Université du Littoral Côte d'Opale (ULCO) in France built their Fabry-Pérot cavity out of a 48-cm-long corrugated waveguide with highly reflective, low-loss photonic mirrors at each end. The pitch of the corrugation was 166 μm, with a groove depth of about 125 μm. The researchers built the mirrors as a sandwich of thin silicon layers separated by a vacuum; the mirrors were mounted on piezo actuators for fine-tuning of the cavity to within about 250 μm in length. According to the researchers, silicon exhibits some of the lowest losses in the terahertz region, compared with other materials.

Maintaining finesse

The ULCO team incorporated their bespoke cavity into a Fabry-Pérot terahertz absorption spectrometer with an interaction length of 1 km. With that system, the researchers measured line intensities in the 620-GHz range of up to 10–27 cm–1 per molecule per cm2 with a signal-to-noise ratio of 3 and a finesse above 3000. To test the instrument, the researchers used it to measure the occurrence of a rare isotopic version of carbonyl sulfide, which naturally occurs in the atmosphere at 21 ppm.

The spectrometer works at room temperature, which is an important consideration for a practical device. The cavity mirrors, however, remain the limiting factor for the spectrometer, which must operate between 600 and 650 GHz to maintain their finesse. Adding wire grid polarizers might extend the operating range of the device, the researchers suggest.