Alex M. H. Wong

Spatially shifted beam approach to subwavelength focusing.

Although negative-refractive-index metamaterials have successfully achieved subwavelength focusing, image resolution is limited by the presence of losses. In this Letter, a metal transmission screen with subwavelength spaced slots is proposed that focuses the near-field beyond the diffraction limit and, furthermore, is easily scaled from microwave frequencies to the optical regime. An analytical model based on the superposition of shifted-beam patterns is developed that agrees very well with full-wave simulations and is corroborated by experimental results at microwave frequencies.

Holography-Inspired Screens for Sub-Wavelength Focusing in the Near Field

Inspired by the principle of holography we present a concept for making simple transmission screens that can focus an incident wave into a sub-wavelength spot in the near field. The screen is made out of closely spaced, unequal slits cut on a metallic sheet. Fullwave simulations are presented for an example screen that focuses an incident plane wave down to a spot having a peak-to-null beamwidth equal to lambda/10 .

An Optical Super-Microscope for Far-field, Real-time Imaging Beyond the Diffraction Limit

Optical microscopy suffers from a fundamental resolution limitation arising from the diffractive nature of light. While current solutions to sub-diffraction optical microscopy involve combinations of near-field, non-linear and fine scanning operations, we hereby propose and demonstrate the optical super-microscope (OSM) - a superoscillation-based linear imaging system with far-field working and observation distances - which can image an object in real-time and with sub-diffraction resolution. With our proof-of-principle prototype we report a point spread function with a spot size clearly reduced from the diffraction limit, and demonstrate corresponding improvements in two-point resolution experiments. Harnessing a new understanding of superoscillations, based on antenna array theory, our OSM achieves far-field, sub-diffraction optical imaging of an object without the need for fine scanning, data post-processing or object pre-treatment. Hence the OSM can be used in a wide variety of imaging applications beyond the diffraction limit, including real-time imaging of moving objects.

Adaptation of Schelkunoff's Superdirective Antenna Theory for the Realization of Superoscillatory Antenna Arrays

We have adapted Schelkunoffs method of antenna pattern synthesis to the design of superoscillatory waveforms. Our method stems from the observation that superdirectivity and superoscillation are dual phenomena in the space and spectral domains; it can be applied to construct any periodic superoscillatory wave functions. Using this method, we designed two subwavelength focusing schemesin free space and within a waveguideat an image distance of five wavelengths. We first describe how we extend the concept of superdirectivity toward constructing superoscillatory waveforms, then provide in-depth formulations as they apply to each design example. We also report full-wave simulation results verifying subwavelength focusing to 0.6 times that of the diffraction limit at a distance five wavelengths away from the source.

Sub-Wavelength Focusing at the Multi-Wavelength Range Using Superoscillations: An Experimental Demonstration

We experimentally demonstrate the formation of a superoscillatory sub-wavelength focus at a multi-wavelength working distance. We first discuss and distinguish superlensing, superdirectivity and superoscillation as different methods which, in their respective ways, achieve sub-diffraction resolution. After establishing superoscillation as a potential way towards sub-wavelength focusing at the multi-wavelength range, we proceed to design, synthesize and demonstrate a superoscillatory sub-wavelength focus in a waveguide environment. Our measurements confirm the formation of a focus at 75% the spatial width of the diffraction limited sinc pulse, 4.8 wavelengths away from the source distributions. This working distance is an order of magnitude extended from those of superlenses and related evanescent-wave-based devices, and should pave way to various applications in high-resolution imaging.

Temporal Pulse Compression Beyond the Fourier Transform Limit

It is a generally known that the Fourier transform limit forbids a function and its Fourier transform to both be sharply localized. Thus, this limit sets a lower bound to the degree to which a band-limited pulse can be temporally compressed. However, seemingly counterintuitive waveforms have been theoretically discovered, which, across finite time intervals, vary faster than their highest frequency components. While these so-called superoscillatory waveforms are very difficult to synthesize due to their high amplitude sidebands and high sensitivity, they open up the possibility toward arbitrarily compressing a temporal pulse, without hindrances from bandwidth limitations. In this paper, we report the design and realization of a class of superoscillatory electromagnetic waveforms for which the sideband amplitudes, and hence, the sensitivity can be regulated. We adapt Schelkunoffs method for superdirectivity to design such temporally compressed superoscillatory pulses, which we ultimately realize in an experiment, achieving pulse compression 47% improved beyond the Fourier transform limit.

Superoscillatory Radar Imaging: Improving Radar Range Resolution Beyond Fundamental Bandwidth Limitations

In this work, we propose to improve the range resolution in a conventional radar system by employing a superoscillatory pulse as the radar pulse. A superoscillatory waveform is a waveform which contains, across a finite time interval, faster variations than its highest constituent frequency component. As such, radar imaging using a superoscillatory pulse allows one to detect an object with a range resolution improved beyond a fundamental bandwidth limitation. In this work, we experimentally compare the radar resolution performance of a 500 MHz superoscillatory pulse with that of a sinc pulse of the same bandwidth, and demonstrate that the superoscillatory pulse reduces distance uncertainty by 36%. We also suggest future directions of development to our proposed radar system.

Superoscillations without Sidebands: Power-Efficient Sub-Diffraction Imaging with Propagating Waves

A superoscillation wave is a special superposition of propagating electromagnetic (EM) waves which varies with sub-diffraction resolution inside a fixed region. This special property allows superoscillation waves to carry sub-diffraction details of an object into the far-field, and makes it an attractive candidate technology for super-resolution devices. However, the Shannon limit seemingly requires that superoscillations must exist alongside high-energy sidebands, which can impede its widespread application. In this work we show that, contrary to prior understanding, one can selectively synthesize a portion of a superoscillation wave and thereby remove its high-energy region. Moreover, we show that by removing the high-energy region of a superoscillation wave-based imaging device, one can increase its power efficiency by two orders of magnitude. We describe the concept behind this development, elucidate conditions under which this phenomenon occurs, then report fullwave simulations which demonstrate the successful, power-efficient generation of sub-wavelength focal spots from propagating waves.

Advances in Imaging Beyond the Diffraction Limit

Considerable research has been devoted to the development of imaging apparatus capable of high-resolution imaging beyond the diffraction limit. Innovative subdiffraction imaging systems have been proposed and successfully demonstrated. These systems either focus the electromagnetic near-field more tightly than a Hertzian dipole or resolve the electromagnetic far-field beyond Abbes diffraction limit and, hence, can be termed subdiffraction imaging systems. This paper reviews major advances in the field of subdiffraction imaging in the year 2011, along the research fronts of superlenses, hyperlenses, meta- screens, and superoscillations.

Superoscillatory antenna arrays for sub-diffraction focusing at the multi-wavelength range in a waveguide environment

In the past decade there has been tremendous interest in focusing electromagnetic waves to subwavelength levels. While most works achieve subwavelength focusing using evanescent waves, [1, 2] recently proposed to achieve subwavelength focusing using propagating waves, through the concept of superoscillation. Superoscillation is a phenomenon whereby the delicate interference of propagating electromagnetic waves results in an overall waveform which, within a limited stretch of space, contains variations faster than the highest spatial frequency component of the involved electromagnetic wave. Since only propagating waves are involved, i) the focusing is certainly of a sub-diffraction type; and ii) the subwavelength focusing capability can be extended to much longer imaging distances – to several wavelengths and beyond.