Showing papers by "Junsuk Rho published in 2016"

Deep sub-wavelength nanofocusing of UV-visible light by hyperbolic metamaterials

Abstract: Confining light into a sub-wavelength area has been challenging due to the natural phenomenon of diffraction. In this paper, we report deep sub-wavelength focusing via dispersion engineering based on hyperbolic metamaterials. Hyperbolic metamaterials, which can be realized by alternating layers of metal and dielectric, are materials showing opposite signs of effective permittivity along the radial and the tangential direction. They can be designed to exhibit a nearly-flat open isofrequency curve originated from the large-negative permittivity in the radial direction and small-positive one in the tangential direction. Thanks to the ultraflat dispersion relation and curved geometry of the multilayer stack, hyperlens can magnify or demagnify an incident beam without diffraction depending on the incident direction. We numerically show that hyperlens-based nanofocusing device can compress a Gaussian beam down to tens-of-nanometers of spot size in the ultraviolet (UV) and visible frequency range. We also report four types of hyperlenses using different material combinations to span the entire range of visible frequencies. The nanofocusing device based on the hyperlens, unlike conventional lithography, works under ordinary light source without complex optics system, giving rise to practical applications including truly nanoscale lithography and deep sub-wavelength scale confinement.

Sensitive method for measuring third order nonlinearities in compact dielectric and hybrid plasmonic waveguides.

Abstract: We demonstrate a sensitive method for the nonlinear optical characterization of micrometer long waveguides, and apply it to typical silicon-on-insulator nanowires and to hybrid plasmonic waveguides. We demonstrate that our method can detect extremely small nonlinear phase shifts, as low as 7.5·10<(-4) rad. The high sensitivity achieved imparts an advantage when investigating the nonlinear behavior of metallic structures as their short propagation distances complicates the task for conventional methods. Our results constitute the first experimental observation of χ((3)) nonlinearities in the hybrid plasmonic platform and is important to test claims of hybrid plasmonic structures as candidates for efficient nonlinear optical devices.

Nanophotonic modal dichroism: mode-multiplexed modulators.

Abstract: As the diffraction limit is approached, device miniaturization to integrate more functionality per area becomes more and more challenging. Here we propose a strategy to increase the functionality-per-area by exploiting the modal properties of a waveguide system. With such an approach the design of a mode-multiplexed nanophotonic modulator relying on the mode-selective absorption of a patterned indium-tin-oxide (ITO) is proposed. Full-wave simulations of a device operating at the telecom wavelength of 1550 nm show that two modes can be independently modulated, while maintaining performances in line with conventional single-mode ITO modulators reported in the recent literature. The proposed design principles can pave the way to a class of mode-multiplexed compact photonic devices able to effectively multiply the functionality-per-area in integrated photonic systems.

High-throughput super-resolution hyperlens imaging

Abstract: Super-resolution imaging is of prime importance for nanoscience. However, the maximum resolution of a conventional optical system is determined by Abbe’s diffraction limit,1 i.e., at half the wavelength of the light employed. To overcome this limitation, it is necessary to transfer both evanescent waves—which decay exponentially in naturally occurring materials—and propagating waves. Various super-resolution imaging methods take this approach, including near-field scanning optical microscopy,2, 3 stochastic optical reconstruction microscopy,4 and stimulated emission depletion microscopy.5 However, these methods can be difficult to control and have serious noise problems because they obtain evanescent waves directly from the near field using a detector. More recently, metamaterials—artificially structured materials whose optical properties are determined by the design of subwavelength building blocks—have also been suggested. Whereas conventional materials have properties originating from a particular combination of elements and their arrangement, metamaterials have desired optical properties to support evanescent waves as a result of the specific design of their unit cells. One interesting branch of this technology is hyperbolic metamaterials, which behave like both metal and insulator simultaneously. Hyperbolic metamaterials offer new possibilities for far-field super-resolution imaging, while eliminating the need for complex optical systems or algorithms. Hyperbolic metamaterials used for super-resolution have a multilayer structure, in which metals and dielectrics with subwavelength thickness are stacked alternately in a curved geometry. Incident electromagnetic waves cause the electron density to oscillate collectively along the metal-dielectric interfaces as a result of the subwaveFigure 1. (a) A scanning electron microscope (SEM) image of the crosssectional view of a spherical hyperlens. (b) Right: SEM images of objects. Left: Images obtained using the spherical hyperlens.8

Towards 3D metamaterials at optical frequencies

Abstract: Recent development of hierarchical fabrication techniques for three-dimensional metamaterial and plasmonic structures based on ultra-accurate and ultra-precise electron-beam lithography overlay will be discussed in this article.