Hanjo Lim

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.

Fabrication and characterization of ultrafast carbon nanotube saturable absorbers for solid-state laser mode locking near 1μm

Transmitting and reflecting ultrafast saturable absorbers based on single-walled carbon nanotubes are developed that are applicable for stable mode locking of bulk solid-state lasers operating near 1μm. For fabrication of these saturable absorbers, relatively simple spin coating and spray methods are employed. Parameters important for stable mode locking, such as transient nonlinear absorption, saturation fluence, and recovery time, are investigated by nonlinear transmission and time-resolved pump-probe measurements near 1μm. Typical modulation depths and recovery times amount to ∼0.21%–0.25% and <1ps, respectively.

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.

Four-leaf-clover-shaped antenna for a THz photomixer

To improve the output power of a photomixer used as a THz source, we propose a four-leaf-clover-shaped antenna structure composed of a highly resonant radiation element and a stable DC feed element. The resonance characteristics of the proposed structure were first investigated on a half-infinite substrate as a simplified radiation environment to reduce the computation time. Based on the antenna characteristics on that half-infinite substrate, the antenna structure was designed to have a maximum total efficiency and a maximum directivity on an extended hemispherical lens. The input resistance of this structure was six times that of a full-wavelength dipole, significantly improving the mismatch efficiency between a photomixer and the antenna. The terahertz output power from this structure is expected to be 2.7 times that of a full-wavelength dipole.

Enhanced ultrafast optical nonlinearity of porous anodized aluminum oxide nanostructures

Enhanced ultrafast optical nonlinearities of porous anodized aluminum oxide (AAO) nanostructures, well-known templates for quantum dots fabrication, have been investigated using the differential optical Kerr gate technique at 800 nm. The optical nonlinearity is strongly influenced by the pore number density, the pore size and the shape. Large values of the third-order nonlinear optical susceptibility (chi((3))) of the order of 10(-10)esu are measured. The nonlinear response time is faster than or comparable to the laser pulse width (90 fs) used. The origin and variation of such remarkable optical nonlinearities has been discussed by considering the nanoporous AAO as an effective medium and utilizing the extended Maxwell Garnet theory, and by considering the additional influence from pore diameter, pore shape and surface states.

Particle size-dependent giant nonlinear absorption in nanostructured Ni-Ti alloys

The nonlinear absorption in nanostructured Ni-Ti alloys, fabricated by electrochemical deposition, was investigated at 532 and 1064 nm. The type of nonlinear absorption (saturable or reverse saturable absorption) was observed to depend on the laser intensity as well as on the nanoparticle size. The nanostructured Ni-Ti alloys comprising particle mean diameters of 20 and 30 nm exhibited large three-photon absorption (3PA coefficient approximately 10(6) cm(3)/GW(2)) and large two-photon absorption (2PA coefficient approximately 10(5) cm/GW) at 532 nm, respectively. The observed change over from reverse saturable absorption to saturable absorption at high peak intensities has qualitatively been analyzed by the excited-state theory of conduction electrons.

Signal processing using Fourier & wavelet transform for pulse oximetry

Current pulse oximeters use a weighted moving average technique to compute oxygen saturation (SpO/sub 2/) values. This method has many limitations including susceptibility to motion artifact, background light, and low perfusion errors. The goal for developing an alternate method for pulse oximetry was to overcome these limitations. The hypothesis was that frequency domain analysis could more easily extract the cardiac rate and amplitude of interest from time domain signal. The focus was on the digital signal processing algorithms that had potential to improve pulse oximetry readings, and then test those algorithms. This was accomplished using the fast Fourier transform (FFT) to analysis in spectral domain, and the discrete wavelet transform (DWT) to estimate oxygen saturation. The result indicate that the FFT and DWT computation of oxygen saturation were accurate and erroneous without weighted moving average (WMA) algorithms currently being used.

Quantitative analysis of water distribution in human articular cartilage using terahertz time-domain spectroscopy

The water distribution in human osteoarthritic articular cartilage has been quantitatively characterized using terahertz time-domain spectroscopy (THz TDS). We measured the refractive index and absorption coefficient of cartilage tissue in the THz frequency range. Based on our measurements, the estimated water content was observed to decrease with increasing depth cartilage tissue, showing good agreement with a previous report based on destructive biochemical methods.

Improved bending loss characteristics of asymmetric surface plasmonic waveguides for flexible optical wiring

We present improved characteristics of the curved plasmonic waveguide which consists of a thin metal stripe with asymmetric cladding layers. It is shown that in the proposed curved asymmetric plasmonic waveguides, a balance between a radiation due to bending and a radiation due to the asymmetric claddings allows a bending with a smaller radius curvature and a lower loss compared to the waveguide with symmetric claddings. At the same time, a symmetric metal stripe waveguides typical trade-off between the bending characteristics and the propagation loss of a straight waveguide is relaxed with proper amount of asymmetry. With the proposed structure, a plasmonic waveguide bending whose radius is as small as 2 mm with a total loss of 1.8 dB/90 degrees is designed. Enhanced sensitivity to the surrounding medium and its application are discussed.

Computational design of one-dimensional nonlinear photonic crystals with material dispersion for efficient second-harmonic generation

A computational study of the second-harmonic generation in one-dimensional photonic crystals made of GaAs and AlAs with quadratic optical nonlinearity and material dispersion is presented. The computational approach uses a shooting method to solve nonlinear wave equations for coupled fundamental and second-harmonic fields and the invariant imbedding method to obtain the linear transmittance and group index spectra. The photonic crystal is built with an elementary cell consisting of four sublayers whose thicknesses are systematically varied. Doubly-resonant second harmonic generation with high conversion efficiency is achieved by choosing the geometrical parameters of the elementary cell optimally and controlling the band structure.