Using a drop of liquid hydrogen, Kohzo Hakuta and colleagues at the University of Electro-Communications (Tokyo, Japan) have carried out stimulated Raman scattering (SRS) experiments. The droplet itself forms an optical cavity that supports whispering gallery modes (with a Q of more than 109) for wavelengths from the ultraviolet (UV) spectral region through the visible region. When pumped at UV wavelengths, the cavity emitted a series of multi-order SRS sidebands that cover the spectral region from 200 to 900 nm, offering promise of an extremely wide band optical comb generator. Hakuta reported on his work at SPIE's Photonics West meeting (2126 January; San Jose, CA) in paper #4270-03.
"Professor Hakuta's work on the optical properties of the liquid-hydrogen droplet has attracted broad attention in the quantum-optics and microcavity research community," explains Laser Resonators conference chair Vladimir Ilchenko of the Jet Propulsion Lab (Pasadena, CA). "It combines the simple elegance of spherical geometry (supporting whispering-gallery modes with extremely high quality factor) with unique properties of liquid hydrogen as the simplest impurity-free molecular quantum liquid." why liquid hydrogen?
Most work on high-Q optical cavities has been performed using solid materials or normal liquids. Unlike those materials, however, liquid hydrogen is a quantum liquid with narrow spectral widths for the relevant transitions. "Even in the liquid phase, hydrogen molecules possess nearly the same vibrational and rotational spectra as in the gas," Ilchenko explains. The rotational and vibrational Raman widths for liquid hydrogen (with half-width half-maximum values of 1.4 and 5 X 10-2 cm-1 respectively) are extremely narrow compared with conventional condensed media (which have widths of more than 10 cm-1). This suggests that liquid hydrogen will have a large Raman gaina useful property for nonlinear optics.
Also, liquid hydrogen is transparent to vacuum UV wavelengths. Because hydrogen boils at 20 K, and the material must be cooled below this point, it freezes out impurities that might otherwise absorb or scatter light.
The researchers experimented on a 400-µm diameter droplet of liquid hydrogen hanging from a capillary, held in an optical cell at temperatures of 14 to 15 K. They pumped the droplet with pulsed single-longitudinal-mode lasers operating at either 202 nm or 532 nm. They also experimented with CW pumping at 410 nm. The emission pattern from the droplet was detected using a pump-wavelength-filtered CCD camera along the pump axis. The temporal and spectral emission properties were measured perpendicular to the pump axis using two grating spectrometers.
Among many other experimental results, the researchers showed that stimulated Raman scattering in the droplet results in rich multi-order sidebands (see figure below).
"Professor Hakuta's work spotlights the broad capabilities of microcavities," says Ilchenko, "and it triggers new ideas toward novel principles of broad-band sources, wide-frequency comb generation, and others."