Sangin Kim

Highly efficient photonic crystal-based multichannel drop filters of three-port system with reflection feedback

We have derived the general condition to achieve 100% drop efficiency in the resonant tunneling-based channel drop filters of a threeport system with reflection feedback. According to our theoretical modeling based on the coupled mode theory in time, the condition is that the Q-factor due to coupling to a bus port should be twice as large as the Q-factor due to coupling to a drop port and the phase retardation occurring in the round trip between a resonator and a reflector should be a multiple of 2pi. The theoretical modeling also shows that the reflection feedback in the threeport channel drop filters brings about relaxed sensitivity to the design parameters, such as the ratio between those two Q-factors and the phase retardation in the reflection path. Based on the theoretical modeling, a fivechannel drop filter has been designed in a two-dimensional photonic crystal, in which only a single reflector is placed at the end of the bus waveguide. The performance of the designed filter has been numerically calculated using the finite-difference time domain method. In the designed filter, drop efficiencies larger than 96% in all channels have been achieved.

Higher order optical resonant filters based on coupled defect resonators in photonic crystals

In this paper, a general methodology for the design of higher order coupled resonator filters in photonic crystals (PCs) is presented. In the proposed approach, the coupling between resonators is treated as though it occurs through a waveguide with an arbitrary phase shift. The coupling through the waveguide is analyzed theoretically, based on the coupled-mode theory in time. The derived theoretical model suggests a way to extend an equivalent circuit approach, previously demonstrated with a certain value of a phase shift, to the higher order filter design with an arbitrary phase shift. The validity of the proposed approach is confirmed by the design of a third-order Chebyshev filter having a center frequency of 193.55 THz, a flat bandwidth of 50 GHz, and ripples of 0.3 dB in the passband. The characteristics of the designed filter are suitable for wavelength-division-multiplexed (WDM) optical communication systems with a 100-GHz channel spacing. The performance of the designed filter is numerically calculated using the two-dimensional (2-D) finite-difference time-domain (FDTD) method.

Tunable guided-mode resonances in coupled gratings

We present a rigorous numerical analysis on tunable characteristics of guided-mode resonances (GMRs) in coupled gratings. Two schemes of strong and negligible evanescent coupling of guided modes are treated. Both show wide range tunability. In the case of strong evanescent coupling, independent control of the center wavelength and the linwidth of the resonance is obtained via variations of a gap size between the gratings and lateral alignment conditions. We believe that this characteristic will provide a useful means to realize a tunable filter in conjunction with micro/nano-electromechanical system technologies. We also present a generalized theoretical analysis on the tunable characteristics of the GMRs in coupled gratings, which is qualitatively in good agreement with the numerical analysis.

Optical bistable devices based on guided-mode resonance in slab waveguide gratings

We investigate properties of nonlinear resonant gratings with emphasis on optical bistability. Slab waveguide gratings with various quality factors are designed and their characteristics analyzed with a finite-difference time-domain method. Considerable field enhancements are observed in the gratings and the performance compares favorably with metallic bistable devices. Bistability based on coupled gratings is also treated. Mechanically controllable switching intensity realized by varying a gap distance between two gratings is demonstrated. Resonant nonlinear elements in this work may find applications in all-optical information processing and optical switching, and our investigation on the dependence of the normalized switching intensity and the response time on quality factor will provide a general guide line for grating-based bistable device design.

Long-range channel plasmon polaritons in thin metal film V-grooves

We numerically investigate the propagation characteristics of guided modes in a thin metal film V-groove embedded in a dielectric medium, with a particular emphasis on long-ranging channel plasmon polaritons (LR-CPPs). The LR-CPP shows several orders of magnitude larger propagation length than the previously studied short-range channel plasmon polariton (SR-CPP). Moreover, the LR-CPP possesses a peculiar mode cutoff mechanism when surrounding dielectric media are asymmetric and this makes its propagation characteristics very sensitive to index change of the surrounding dielectric media.

Leaky-mode resonance photonics: Technology for biosensors, optical components, MEMS, and plasmonics

Resonant leaky modes can be induced on dielectric, semiconductor, and metallic periodic layers patterned in one or two dimensions. Potential applications include bandpass and bandstop filters, laser mirrors, ultrasensitive biosensors, absorption enhancement in solar cells, security devices, tunable filters, nanoelectromechanical display pixels, dispersion/slow-light elements, and others. As there is now a growing realization worldwide of the utility of these devices, it is of interest to summarize their physical basis and present their applicability in photonic devices and systems. In particular, we have invented and implemented highly accurate, label-free, guided-mode resonance (GMR) biosensors that are being commercialized. The sensor is based on the high parametric sensitivity inherent in the fundamental resonance effect. As an attaching biomolecular layer changes the parameters of the resonance element, the resonance frequency (wavelength) changes. A target analyte interacting with a bio-selective layer on the sensor can thus be identified without additional processing or use of foreign tags. Another promising pursuit in this field is development of optical components including wideband mirrors, filters, and polarizers. We have experimentally realized such devices that exhibit a minimal layer count relative to their classical multilayer thin-film counterparts. Theoretical modeling has shown that wideband tuning of these filters is achievable by perturbing the structural symmetry using nano/microelectromechanical (MEMS) methods. MEMS-tuned resonance elements may be useful as pixels in spatial light modulators, tunable lasers, and multispectral imaging applications. Finally, mixed metallic/dielectric resonance elements exhibit simultaneous plasmonic and leaky-mode resonance effects. Their design and chief characteristics is described.

Angle- and position-insensitive electrically tunable absorption in graphene by epsilon-near-zero effect

We propose an electrically tunable absorber based on epsilon-near-zero (ENZ) effect of graphene embedded in a nanocavity, which is composed of metal grating and substrate. Due to strong surface-normal electric field confined in ENZ graphene in the proposed structure, greatly enhanced light absorption (~80%) is achieved in an ultrathin graphene monolayer. By electrically controlling the Fermi-level of graphene, a sharp peak absorption wavelength is tuned over a wide range. The proposed device can work as an optical modulator or a tunable absorption filter, which has a unique feature of incident angle insensitiveness owing to the ENZ effect and magnetic dipole resonance. Moreover, existence of a significantly dominant electric field and its uniformity make the device performance independent of the position of the graphene layer in the nanocavity, which provides great fabrication tolerance.

Low bending loss characteristics of hybrid plasmonic waveguide for flexible optical interconnect

The bending loss characteristics of the hybrid plasmonic waveguide are investigated theoretically and experimentally. Simulation results showed that the guided mode is confined mainly into outer high index slab as the bending radius decreases. Thus, the radiation loss due to bending is greatly suppressed. We fabricate flexible hybrid plasmonic waveguide consisted of 5 nm-thick Au stripe and flexible multiple polymer cladding layers. The measured bending loss is lower than 1 dB/180° at a wavelength of 1310 nm for the bending radii down to 2 mm.

Circular hybrid plasmonic waveguide with ultra-long propagation distance

We propose a novel plasmonic waveguide structure, which is referred to as a circular hybrid plasmonic waveguide (HPW) and consists of a metal wire covered with low- and high-index dielectric layers. The circular HPW exhibits two distinctly different modes, namely, the strongly localized mode and the extremely low-loss mode. Our numerical calculation demonstrates that the strongly localized mode exhibits 10-4 order scale in normalized mode area and can be performed even in tens of nanometer sizes of waveguide geometry. In the extremely low-loss mode, the HPW exhibits ultra-long propagation distance of more than 103μm that can be achieved by forming the dipole-like hybrid mode and properly adjusting the radius of the metal wire. It is also shown that, even with this long-range propagation, the mode area of the dipole-like hybrid mode can be maintained at subwavelength scale. The simultaneous achievement of a small mode area and ultra-long propagation distance contributes to the ultra-high propagation distance to mode size ratio of the waveguide. The HPW results are very helpful for plasmonic device applications in the fields of low-threshold nanolasers, ultrafast modulators, and optical switches.

Impact of varying the DC bias stripline connection angle on terahertz coplanar stripline dipole antenna characteristics

We investigated the dependence of the input impedance and radiation characteristics on the connection angle between the DC bias stripline and the center dipole of a terahertz coplanar stripline dipole antenna in a semi-infinite substrate. Three connection angles, namely 0° (typical horizontal connection), 45° (diagonal connection), and 90° (vertical connection), were examined and an overall performance comparison was provided. The antenna with a sufficiently long bias stripline exhibited a traveling-wave behavior with stable impedance variation, regardless of the connection angle between the DC bias stripline and the center dipole. The antenna with diagonal and vertical bias connections performed much better than the typical H-shaped electrode with a horizontal bias connection in terms of radiation characteristics exhibiting flat gain and low frequency cut-off of the radiation efficiency. In addition, the antenna with a diagonal bias connection exhibited a better radiation pattern and more efficient use of a given substrate area than that with a vertical bias connection. The result indicates that in terms of the overall aspect of the terahertz stripline dipole antenna designs on a high-dielectric-constant substrate, the antenna with a diagonal bias connection is the best configuration.