Inki Kim

Nanophotonic modal dichroism: mode-multiplexed modulators.

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.

Tungsten-based Ultrathin Absorber for Visible Regime

Utilizing solar energy requires perfect absorption of light by the photovoltaic cells, particularly solar thermophotovoltaics (STPVs), which can be eventually converted into useful electrical energy. Ultrathin nanostructures, named metasurfaces, provide an intriguing platform to develop the miniaturized solar energy absorbers that can find potential applications in integrated photonics, optical sensing, color imaging, thermal imaging and electromagnetic shielding. Therefore, the quest of novel materials and designs to develop highly efficient absorbers at minuscule scale is an open topic. In this paper, novel absorbers using tungsten-metasurface are developed which give ultrahigh absorbance over a wide frequency spectrum. The proposed designs are two-dimensional, polarization insensitive, broadband and are predicted to give better response under high temperatures ascribed to high melting point of tungsten i.e. 3422 °C. Amongst these designs, cross alignment is found optimum for tungsten, because it is impedance matched with the free space for visible spectrum. This cross arrangement is further tweaked by changing width, height and length resulting in 7 different optimized solutions giving an average absorbance greater than 98%. One, amongst these solutions, gave a maximum average absorbance of 99.3%.

Photonic spin Hall effect by the spin-orbit interaction in a metasurface with elliptical nano-structures

The metasurface with elliptical nano-structures containing doubly degenerate geometrical charge is designed to investigate the enhanced spin-orbit optical interactions, numerically as well as experimentally. It is found that localized surface plasmon (LSP) resonance with orbital angular momentum (i.e., rotating SP vortex mode carrying extrinsic orbital angular momentum) can be induced under linearly polarized illumination. On the contrary, the LSP resonance without orbital angular momentum is formed under circularly polarized illumination. Moreover, based on the different LSP modes as results of spin-orbit interaction with alternative geometrical charge, directional propagations of surface plasmon polariton in two orthogonal trajectories depending on spin states of the incident light are presented with experimental demonstration, a phenomenon called photonic spin Hall effect.

Plasmonic metasurface cavity for simultaneous enhancement of optical electric and magnetic fields in deep subwavelength volume

It has been hard to achieve simultaneous plasmonic enhancement of nanoscale light-matter interactions in terms of both electric and magnetic manners with easily reproducible fabrication method and systematic theoretical design rule. In this paper, a novel concept of a flat nanofocusing device is proposed for simultaneously squeezing both electric and magnetic fields in deep-subwavelength volume (~λ3/538) in a large area. Based on the funneled unit cell structures and surface plasmon-assisted coherent interactions between them, the array of rectangular nanocavity connected to a tapered nanoantenna, plasmonic metasurface cavity, is constructed by periodic arrangement of the unit cell. The average enhancement factors of electric and magnetic field intensities reach about 60 and 22 in nanocavities, respectively. The proposed outstanding performance of the device is verified numerically and experimentally. We expect that this work would expand methodologies involving optical near-field manipulations in large areas and related potential applications including nanophotonic sensors, nonlinear responses, and quantum interactions.

Fabrication of three-dimensional suspended, interlayered and hierarchical nanostructures by accuracy-improved electron beam lithography overlay

Nanofabrication techniques are essential for exploring nanoscience and many closely related research fields such as materials, electronics, optics and photonics. Recently, three-dimensional (3D) nanofabrication techniques have been actively investigated through many different ways, however, it is still challenging to make elaborate and complex 3D nanostructures that many researchers want to realize for further interesting physics studies and device applications. Electron beam lithography, one of the two-dimensional (2D) nanofabrication techniques, is also feasible to realize elaborate 3D nanostructures by stacking each 2D nanostructures. However, alignment errors among the individual 2D nanostructures have been difficult to control due to some practical issues. In this work, we introduce a straightforward approach to drastically increase the overlay accuracy of sub-20 nm based on carefully designed alignmarks and calibrators. Three different types of 3D nanostructures whose designs are motivated from metamaterials and plasmonic structures have been demonstrated to verify the feasibility of the method, and the desired result has been achieved. We believe our work can provide a useful approach for building more advanced and complex 3D nanostructures.

The role of current loop in harmonic generation from magnetic metamaterials in two polarizations

Abstract In this paper, we investigate the role of current loop in the generation of second and third harmonic signals from magnetic metamaterials and we are clarifying why two polarized harmonics are generated from magnetic metamaterials. We show that the current loop formed in the magnetic resonant frequency acts as a source for nonlinear effects. The current loop that has a circular shape can be divided into two orthogonal parts, where each of these parts acts as a source for generating a harmonic signal parallel to itself. The type of harmonic signal is determined by the metamaterial’s inversion symmetry in that direction. This claim is also supported by the experimental results of another group.

Thermally robust ring-shaped chromium perfect absorber of visible light

Abstract A number of light-absorbing devices based on plasmonic materials have been reported, and their device efficiencies (or absorption) are high enough to be used in real-life applications. Many light-absorbing applications such as thermophotovoltaics and energy-harvesting and energy-sensing devices usually require high-temperature durability; unfortunately, noble metals used for plasmonics are vulnerable to heat. As an alternative, refractory plasmonics has been introduced using refractory metals such as tungsten (3422°C) and transition metal nitrides such as titanium nitride (2930°C). However, some of these materials are not easy to handle for device fabrications owing to their ultra-high melting point. Here, we propose a light absorber based on chromium (Cr), which is heat tolerant due to its high melting temperature (1907°C) and is compatible with fabrication using conventional semiconductor manufacturing processes. The fabricated device has >95% average absorption of visible light (500–800 nm) independent of polarization states. To verify its tolerance of heat, the absorber was also characterized after annealing at 600°C. Because of its compactness, broadband operational wavelength, and heat tolerance, this Cr perfect absorber will have applications in high-temperature photonic devices such as solar thermophotovoltaics.

Micron-scale light structuring via flat nanodevices

Miniaturized devices with multiple functionalities are exceedingly required in integrated optical systems. Flat nanostructures, named metasurfaces, provide fascinating boulevard for complex structuring and manipulation of light such as optical vortex generation, lensing, imaging, harmonic generation etc. at micron scale. Since, the performance of metal-based plasmonic metasurfaces is significantly limited by their optical absorption and losses, lossless dielectric materials (in the operational spectrum) provide decent alternative to attain higher efficiency. Here, a novel, polarization insensitive and highly efficient method for light structuring is demonstrated based on amorphous silicon (with subwavelength thickness of 400 nm) at an operational wavelength of 633 nm. The proposed phase gradient metasurface is based on circular cylindrical nanopillars of amorphous silicon exhibits two optical properties, the lensing and orbital angular momentum generation. The cylindrical nature of the pillar plays a pivotal role to make the overall structure as polarization insensitive. The proposed innovative methodology will provide an interesting road towards the development and realization of multi-functional ultrathin nanodevices which will find numerous applications in integrated photonics.

Highly Efficient Visible Hologram through Dielectric Metasurface

To achieve applied aspect of metasurfaces in the visible regime, dielectric materials with low absorption are indispensable. This work presents highly efficient generation of hologram via processed amorphous silicon, which exhibits significantly low absorption in the region of interest. The phase and the polarization of transmitted light are tailored by varying the orientation of dielectric nanorods whereas their conversion efficiency is optimized by adjusting their structural parameters. Better image fidelity and higher conversion efficiency (up-to 75%) are achieved as compared to previously reported work. The proposed design methodology paves a way toward on-chip realization of various novel phenomena with substantially enhanced performance.

Demonstration of a Hyperlens-integrated Microscope and Super-resolution Imaging

The use of super-resolution imaging to overcome the diffraction limit of conventional microscopy has attracted the interest of researchers in biology and nanotechnology. Although near-field scanning microscopy and superlenses have improved the resolution in the near-field region, far-field imaging in real-time remains a significant challenge. Recently, the hyperlens, which magnifies and converts evanescent waves into propagating waves, has emerged as a novel approach to far-field imaging. Here, we report the fabrication of a spherical hyperlens composed of alternating silver (Ag) and titanium oxide (TiO2) thin layers. Unlike a conventional cylindrical hyperlens, the spherical hyperlens allows for two-dimensional magnification. Thus, incorporation into conventional microscopy is straightforward. A new optical system integrated with the hyperlens is proposed, allowing for a sub-wavelength image to be obtained in the far-field region in real time. In this study, the fabrication and imaging setup methods are explained in detail. This work also describes the accessibility and possibility of the hyperlens, as well as practical applications of real-time imaging in living cells, which can lead to a revolution in biology and nanotechnology.