Tapashree Roy

A super-oscillatory lens optical microscope for subwavelength imaging

The past decade has seen an intensive effort to achieve optical imaging resolution beyond the diffraction limit. Apart from the Pendry-Veselago negative index superlens, implementation of which in optics faces challenges of losses and as yet unattainable fabrication finesse, other super-resolution approaches necessitate the lens either to be in the near proximity of the object or manufactured on it, or work only for a narrow class of samples, such as intensely luminescent or sparse objects. Here we report a new super-resolution microscope for optical imaging that beats the diffraction limit of conventional instruments and the recently demonstrated near-field optical superlens and hyperlens. This non-invasive subwavelength imaging paradigm uses a binary amplitude mask for direct focusing of laser light into a subwavelength spot in the post-evanescent field by precisely tailoring the interference of a large number of beams diffracted from a nanostructured mask. The new technology, which--in principle--has no physical limits on resolution, could be universally used for imaging at any wavelength and does not depend on the luminescence of the object, which can be tens of micrometres away from the mask. It has been implemented as a straightforward modification of a conventional microscope showing resolution better than λ/6.

Super-oscillatory optical needle

Super-oscillatory optical lenses have recently been shown to achieve subwavelength focusing and have been used for super-resolution imaging. However, the subwavelength hotspots created by these lenses are always accompanied by sidebands containing a significant fraction of the optical energy and are highly localised in the axial direction. Here, we report a class of super-oscillatory lenses that form extended subwavelength optical needles on a 15λ field of view.

Sub-wavelength focusing meta-lens

We show that a planar plasmonic metamaterial with spatially variable meta-atom parameters can focus transmitted light into sub-wavelength hot-spots located beyond the near-field of the metamaterial. By nano-structuring a gold film we created an array of meta-lenses generating foci of 160 nm (0.2λ) in diameter when illuminated by a wavelength of 800 nm. We attribute the occurrence of sub-wavelength hotspots beyond the near field to the phenomenon of superoscillation.

1.7 Gbit/in.2 gray-scale continuous-phase-change femtosecond image storage

We demonstrate high-density, multi-level crystallization of a Ge2Sb2Te5 thin film using tightly focused femtosecond laser pulses. The submicron spots with 8 distinct data storage states are written on a 1.08 μm square grid. The significant change in reflectivity of every specific state of crystallized spot allows easy optical reading and identification. As a demonstration, two gray-scale images are written into the storage medium. Our results open up potential applications in ultra-fast two-dimensional parallel cognitive computing and holography.

Flat super-oscillatory lens for heat-assisted magnetic recording with sub-50nm resolution

Heat-assisted magnetic recording (HAMR) is a future roadmap technology to overcome the superparamagnetic limit in high density magnetic recording. Existing HAMR schemes depend on a simultaneous magnetic stimulation and light-induced local heating of the information carrier. To achieve high-density recorded data, near-field plasmonic transducers have been proposed as light concentrators. Here we suggest and investigate in detail an alternative approach exploiting a far-field focusing device that can focus light into sub-50 nm hot-spots in the magnetic recording layer using a laser source operating at 473 nm. It is based on a recently introduced super-oscillatory flat lens improved with the use of solid immersion, giving an effective numerical aperture as high as 4.17. The proposed solution is robust and easy to integrate with the magnetic recording head thus offering a competitive advantage over plasmonic technology.

Point spread function of the optical needle super-oscillatory lens

Super-oscillatory optical lenses are known to achieve sub-wavelength focusing. In this paper, we analyse the imaging capabilities of a super-oscillatory lens by studying its point spread function. We experimentally demonstrate that a super-oscillatory lens can generate a point spread function 24% smaller than that dictated by the diffraction limit and has an effective numerical aperture of 1.31 in air. The object-image linear displacement property of these lenses is also investigated.

A meta-diffraction-grating for visible light

We report an experimental realization of a visible range planar diffraction grating, formed by sub-wavelength elements, with periodically variable parameters. At normal incidence the grating exhibits asymmetric diffraction into the positive and negative first diffraction orders and operates at visible wavelengths with peak efficiency at 736 nm wavelength.

Solid-immersion super-oscillatory lens for heat assisted magnetic recording technology and nanoscale imaging

We demonstrate a solid-immersion super-oscillatory planar lens for super-resolution applications with a 102 nm full-width at half maximum focal spot in a low-loss immersion medium.

Transformation optics with planar metamaterials: Diffraction grating and lens

We use the resonance properties of elemental building blocks of metamaterial array with spatially variable parameter to control the phase and intensity of light beam. Dispersive and focusing devices are reported for the first time.