Optical super­oscillation refers to a wave packet that can oscillate locally in a frequency exceeding its highest Fourier component. This intriguing pheno­menon enables production of extremely localized waves that can break the optical diffraction barrier. Indeed, super­oscillation has proven to be an effective technique for overcoming the diffraction barrier in optical super­resolution imaging. The trouble is that strong side lobes accompany the main lobes of super­oscillatory waves, which limits the field of view and hinders appli­cation.

There also are tradeoffs between the main lobes and the side lobes of super­oscillatory wave packets: reducing the super­oscillatory feature size of the main lobe comes at the cost of enlarging the side lobes. This happens mainly because super­oscillation is a local phenomenon, yet the overall width of the wave packet is wider than the optical diffraction limit. Precise engineering of the inter­ference of diffracted light fields emitted from complex nano­structures can produce structural masks that enable significant optical super­oscillation. But structural masks require optimization and complex fabri­cation, and the resulting light field is still limited by high-intensity side lobes. Producing super­oscillatory waves with appreciable feature size while maintaining a larger field of view has remained challenging until now.

Researchers from Jinan University, Guangzhou, China, recently developed a way to eliminate, to some extent, the tradeoffs involved in super­oscillatory wave packets. They demonstrate, both experi­mentally and theo­retically, generation of super­oscillatory light spots without side lobes. A central microdisc with cylindrical diffraction gives rise to a super­oscillatory light spot of a size within the optical diffraction limit. A pair of sharp-edged apertures ensures constructive interference with the high-spatial-frequency waves. That inter­ference effectively eliminates side lobes along a symmetric cut that can be adjusted in the transverse plane by rotating the moonlike apertures.

According to Yanwen Hu, a doctoral student working under the supervision of Zhenqiang Chen in the Department of Opto­electronic Engineering at Jinan University, “Due to its easy design, based on clear physics, the sharp-edged aperture is a promising candidate for realization of super­oscillatory waves.” Hu explains further that the cylindrical diffraction of the central microdisc produces super­oscillatory waves with Bessel-like forms. These forms enable the delicate structures of the super­oscillatory waves propagating in free space to travel much farther than the evanescent light waves. According to Hu, this intriguing propa­gation effect of super­oscillation holds promise for potential application in nanoparticle mani­pulation, as well as super­resolution imaging. (Source: SPIE)

Reference: Y. Hu et al.: Optical superoscillatory waves without side lobes along a symmetric cut, Adv. Phot. 3, 045002 (2021); DOI: 10.1117/1.AP.3.4.045002

Link: Dept. of Optoelectronic Engineering, Jinan University, Guangzhou, China