Cha-Hwan Oh

Resonant coupling of surface plasmons to radiation modes by use of dielectric gratings

Efficient outcoupling of surface-plasma waves to radiation modes by use of dielectric diffraction gratings on a flat metallic surface is discussed. The dielectric gratings, which have a surface-relief structure with only several tens of nanometers in peak-to-trough height on a flat metal surface, can efficiently extract radiation modes propagating in free space from the surface-plasmon modes. An outcoupling efficiency of 50% is estimated with the rigorous coupled-wave diffraction theory, and it is confirmed by the experiment.

Vertical coupling of long-range surface plasmon polaritons

We evaluated and demonstrated strong vertical-coupling characteristics of vertical directional couplers based on long-range surface plasmon polaritons (LRSPPs) at 1.55μm wavelength. Fundamental even and odd modes supported by LRSPP metal-stripe waveguides compete more strongly on vertical coupling structures than on lateral coupling structures, possibly leading to less power consumption for switching and to compactness in device length and width. LRSPP-based vertically coupled routing of signals can also be a powerful means of developing three-dimensional photonic integrated circuits and optical printed circuit boards.

Multidimensional manipulation of carbon nanotube bundles with optical tweezers

Optical manipulation of carbon nanotubes (CNTs) in aqueous solution was performed using a linearly polarized infrared tweezers system. Vertical and horizontal manipulation of single-walled and multiwalled carbon nanotubes (SWNTs, MWNTs) was carried out by changing the size of the CNTs and the trapping position. Rotation of MWNT bundles was confirmed using a circular polarized infrared optical tweezers system. Patterning of dots and letters with CNTs was successfully carried out on glass substrates.

Surface-plasmon photonic band gaps in dielectric gratings on a flat metal surface

Photonic band gaps created by Bragg scattering of the surface plasmon polaritons are observed from dielectric grating structures on a flat metal surface. Observation results that directly image the band gaps are confirmed by the well-known numerical calculation method of diffraction, the rigorous coupled-wave analysis method. A numerical model based on the plane wave expansion method is also developed for estimation of the surface-plasmon band-gap characteristics in our dielectric-on-metal system. Consistency among the results of the band structures obtained from the experiment and the two numerical methods is achieved.

Determination of the space-charge field in polymeric photorefractive material

We proposed a method for measuring the magnitude of the space-charge field of the polymeric photorefractive materials. In the case of polymeric photorefractive material with low glass transition temperature, optically anisotropic chromophores are known to be reoriented under space-charge field. Simply by adding a pair of crossed polarizer units to a conventional degenerated four wave mixing setup, we could measure the birefringence of the photorefractive materials induced by a newly formed space-charge field. Since the birefringence of a given material is governed by the applied electric field, the space-charge field can be determined from the variation of birefringence using the oriented gas model.

Arbitrary structuring of two-dimensional photonic crystals by use of phase-only Fourier gratings

The computer-generated holography technique is applied to the structuring of two-dimensional (2D) photonic crystals with inherently embedded arbitrary defects. The technique uses phase-only Fourier gratings as a generator of spot arrays in the focal plane, such that a single exposure produces a 2D array of focused spots with desired defects or modifications in the lattice structure. We demonstrate several types of large-area 2D lattice structures with square, hexagonal, or hybrid lattices embedded with point and (or) line defects. Scanning the Fourier plane in the depth direction throughout multiphoton polymerization media allows 3D lattices with stacked defect layers to be formed.

Gradient-index planar optics for optical interconnections

Use of a gradient-index (GRIN) substrate is proposed as a novel signal-propagation medium for planar optics. The GRIN substrate provides planar optics designers not only a three-dimensional light propagation space but also smart optical diffraction-limited imaging. Experimental results on the imaging of an input signal to multiple destinations by the GRIN planar optics are presented.

Observation of exceptional points in reconfigurable non-Hermitian vector-field holographic lattices

Recently, synthetic optical materials represented via non-Hermitian Hamiltonians have attracted significant attention because of their nonorthogonal eigensystems, enabling unidirectionality, nonreciprocity and unconventional beam dynamics. Such systems demand carefully configured complex optical potentials to create skewed vector spaces with a desired metric distortion. In this paper, we report optically generated non-Hermitian photonic lattices with versatile control of real and imaginary sub-lattices. In the proposed method, such lattices are generated by vector-field holographic interference of two elliptically polarized pump beams on azobenzene-doped polymer thin films. We experimentally observe violation of Friedels law of diffraction, indicating the onset of complex lattice formation. We further create an exact parity-time symmetric lattice to demonstrate totally asymmetric diffraction at the spontaneous symmetry-breaking threshold, referred to as an exceptional point. On this basis, we provide the experimental demonstration of reconfigurable non-Hermitian photonic lattices in the optical domain and observe the purest exceptional point ever reported to date.

Photonic Band Gaps for Surface Plasmon Modes in Dielectric Gratings on a Flat Metal Surface

For dielectric gratings on a flat metal surface, photonic band gaps created by Brags scattering of surface plasmon polaritons are observed. The observation result that directly images this gap is compared with that predicted by a numerical model based on a plane wave expansion. Consistency between the experimental and numerical results is also confirmed by comparison with the well-known calculation method of diffraction, the rigorous coupled wave analysis method.

Plasmonic gain in long-range surface plasmon polariton waveguides bounded symmetrically by dye-doped polymer

The plasmonic gain of a top-pumped active symmetric metal slab waveguide is investigated theoretically and experimentally. The structure consists of a thin Ag film cladded above and below by gain media (IR140-doped poly (methyl methacrylate)), and operating with long-range surface plasmon polaritons (LRSPPs) at near-infrared wavelengths. We consider the spatial distribution of the pump intensity and the position dependence of the dipole lifetime within the claddings when computing the LRSPP gain. We find that the bottom cladding provides significant gain to the LRSPP, despite the low pump transmittance through the Ag film, as long as the pump intensity is strong enough to saturate the gain material (∼4 MW/cm2). In this situation, the LRSPP gain is doubled compared to the case where the top cladding only is active. The LRSPP gain was measured in a fabricated structure using the variable stripe length method, yielding gmod = 16.7 cm−1 at a pump intensity of ∼4 MW/cm2. The measured LRSPP gain agrees very wel...