By converting low-energy photons to optical frequencies, researchers can capture images at ultrahigh speeds.
For imaging through opaque materials, x rays are an obvious choice. But their high energies can ionize atoms. At frequencies between the far-IR and microwave regions of the electromagnetic spectrum, terahertz radiation can penetrate materials such as paper, cloth, and plastics without damaging them. That property makes the frequency band useful for such applications as nondestructive testing, security screening, and biomedicine. But the frequency band is an elusive part of the spectrum: Terahertz photons lie below the reach of most optical technologies and just above the reach of electronics.
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| Credit: Adapted from L. A. Downes et al., Phys. Rev. X 10, 011027 (2020) |
The upshot is that terahertz sources tend to have low power and their detectors, low sensitivity. One promising approach to circumvent those limitations is to use an atomic gas that converts terahertz-frequency photons into easily detectable optical-frequency ones. Rydberg atoms—atoms excited to a high principal quantum number n—turn out to be excellent sensors for the job. Lucy Downes, a doctoral student at Durham University, her advisers Kevin Weatherill and Charles Adams, and their colleagues have now demonstrated a terahertz imaging system using cesium vapor to make the conversion. The researchers produced optical photons at frame rates two orders of magnitude higher than the current state of the art in terahertz imaging.
The transitions between Rydberg states of alkali atoms have large dipole moments. And with a high vapor pressure at room temperature and easily accessible transition wavelengths, Cs is an advantageous choice for the demonstration. In their experiment, illustrated in panels a and b of the first figure, Downes and her colleagues place Cs gas in a quartz cell. IR radiation (yellow) focused on the cell produces a two-dimensional sheet of excited Cs atoms. Onto that sheet the researchers project a terahertz field (red) that has passed through an object of interest. The subsequent interactions of terahertz photons with Cs atoms excite the atoms to the n = 13 D5/2 level and give rise to fluorescence (green), whose emission is captured with an optical camera.
Credit: Adapted from L. A. Downes et al., Phys. Rev. X 10, 011027 (2020)
For their demonstration of high-speed terahertz imaging, the researchers captured subsequent video of dynamical processes—including a rotating optical chopper—at frame rates up to 3 kHz, as shown in the second figure. The white arrow highlights the movement of one spoke of the chopper between frames. (L. A. Downes et al., Phys. Rev. X 10, 011027, 2020.)
