Opt. Express 19, 22350–22357 (2011)
The resolving capabilities of far-field optical imaging schemes are limited by their inability to collect rapidly decaying evanescent waves, which contain detailed spatial information. To overcome this restriction, Leonid Alekseyev, Evgenii Narimanov and Jacob Khurgin have now proposed a time-multiplexed approach for recovering evanescent waves in the far-field. In their scheme, the object to be imaged is placed in the near-field of an acousto-optic modulator and illuminated with a plane wave from a light source. Waves scattered from the object strike a phonon grating formed by the acousto-optic modulator, thereby causing the evanescent components to be shifted in both frequency and transverse wave vector. The researchers discovered that for a sufficiently large phonon wave vector, the evanescent components with high spatial frequency can be converted into propagating waves, which can then be collected and imaged. Super-resolved fingerprinting and digital holography are suggested as two example applications of this scheme. Simulations show that mixing the frequency-shifted fields with a reference wave can create a high-spatial-frequency beat note photocurrent at the detector, and that true super-resolved imaging can be achieved when the reference signal is Bragg-shifted. Although this scheme is particularly suitable for super-resolution imaging at infrared and terahertz wavelengths, the researchers say that it might also be able to work at optical frequencies by replacing the acousto-optic medium with a moving nanostructured grating.
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Won, R. Acoustic help. Nature Photon 6, 5 (2012). https://doi.org/10.1038/nphoton.2011.343