Namkyoo Park

A terahertz metamaterial with unnaturally high refractive index

Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial ‘atoms’ has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.

Active terahertz nanoantennas based on VO2 phase transition.

Unusual performances of metamaterials such as negative index of refraction, memory effect, and cloaking originate from the resonance features of the metallic composite atom(1-6). Indeed, control of metamaterial properties by changing dielectric environments of thin films below the metallic resonators has been demonstrated(7-11). However, the dynamic control ranges are still limited to less than a factor of 10,(7-11) with the applicable bandwidth defined by the sharp resonance features. Here, we present ultra-broad-band metamaterial thin film with colossal dynamic control range, fulfilling present day research demands. Hybridized with thin VO(2) (vanadium dioxide) (12-18) films, nanoresonator supercell arrays designed for one decade of spectral width in terahertz frequency region show an unprecedented extinction ratio of over 10000 when the underlying thin film experiences a phase transition. Our nanoresonator approach realizes the full potential of the thin film technology for long wavelength applications.

Actively gain-flattened erbium-doped fiber amplifier over 35 nm by using all-fiber acoustooptic tunable filters

We demonstrate an actively gain-flattened erbium-doped fiber amplifier (EDFA) using an all-fiber gain-flattening filter with electronically controllable spectral profiles. A good gain flatness ( 35 nm) is achieved for a wide range of operational gain levels as well as input signal and pump powers.

Atomic layer lithography of wafer-scale nanogap arrays for extreme confinement of electromagnetic waves

Squeezing light through nanometre-wide gaps in metals can lead to extreme field enhancements, nonlocal electromagnetic effects and light-induced electron tunnelling. This intriguing regime, however, has not been readily accessible to experimentalists because of the lack of reliable technology to fabricate uniform nanogaps with atomic-scale resolution and high throughput. Here we introduce a new patterning technology based on atomic layer deposition and simple adhesive-tape-based planarization. Using this method, we create vertically oriented gaps in opaque metal films along the entire contour of a millimetre-sized pattern, with gap widths as narrow as 9.9 Å, and pack 150,000 such devices on a 4-inch wafer. Electromagnetic waves pass exclusively through the nanogaps, enabling background-free transmission measurements. We observe resonant transmission of near-infrared waves through 1.1-nm-wide gaps (λ/1,295) and measure an effective refractive index of 17.8. We also observe resonant transmission of millimetre waves through 1.1-nm-wide gaps (λ/4,000,000) and infer an unprecedented field enhancement factor of 25,000.

Flexible, Angle-Independent, Structural Color Reflectors Inspired by Morpho Butterfly Wings

Thin-film color reflectors inspired by Morpho butterflies are fabricated. Using a combination of directional deposition, silica microspheres with a wide size distribution, and a PDMS (polydimethylsiloxane) encasing, a large, flexible reflector is created that actually provides better angle-independent color characteristics than Morpho butterflies and which can even be bent and folded freely without losing its Morpho-mimetic photonic properties.

Optofluidic maskless lithography system for real-time synthesis of photopolymerized microstructures in microfluidic channels

The authors propose an optofluidic maskless lithography technique that can dynamically synthesize free-floating polymeric microstructures inside microfluidic channels by selectively polymerizing photocurable resin with high-speed two-dimensional spatial light modulators. The combination of programable optical projection and microfluidic devices allows one to precisely control the timing and location of the photopolymerization process for microstructure fabrication. Real-time generation of microparticles with various shapes, sizes, ordering, and material contents are experimentally demonstrated. Long polymeric structures of which size is not limited by the exposure field of view can also be fabricated.

Efficiency of broadband four-wave mixing wavelength conversion using semiconductor traveling-wave amplifiers

We present a theoretical analysis and experimental measurements of broadband optical wavelength conversion by four-wave mixing in semiconductor traveling-wave amplifiers. In the theoretical analysis, we obtain an analytical expression for the conversion efficiency. In the experiments, both up and down-conversion efficiencies are measured as a function of wavelength shift for shifts up to 27 nm. The experimental data are well explained by the theoretical calculation. The observed higher conversion efficiency for wavelength down-conversion is believed to be caused by phase interferences that exist between various mechanisms contributing to the four-wave mixing process.<<ETX>>

Efficient formulation of Raman amplifier propagation equations with average power analysis

For the first time, we derive efficient modeling equations for the average power analysis of Raman amplifiers (RAs) from the standard propagation equations. Applications of these equations to the numerical analysis of practical RA-based systems show a reduction in computation time of over two orders of magnitude compared with the direct integration approach based on ordinary coupled differential equations, while reproducing all the essential system performances precisely. In addition to enhanced computational efficiency, the derived equations also give deeper insights into the detailed dynamics of RAs.

Four-wave mixing wavelength conversion efficiency in semiconductor traveling-wave amplifiers measured to 65 nm of wavelength shift

The efficiency of broadband optical wavelength conversion by four-wave mixing in semiconductor traveling-wave amplifiers is measured for wavelength shifts up to 65 nm using a tandem amplifier geometry. A quantity we call the relative conversion efficiency function, which determines the strength of the four-wave mixing nonlinearity, was extracted from the data. Using this quantity, gain requirements for lossless four-wave mixing wavelength conversion are calculated and discussed. Signal to background noise ratio is also measured and discussed in this study.<<ETX>>

All fiber, low threshold, widely tunable single‐frequency, erbium‐doped fiber ring laser with a tandem fiber Fabry–Perot filter

An all fiber, widely tunable, single‐frequency, erbium‐doped fiber ring laser was constructed with a threshold pump power as low as 10 mW. Tuning over more than 30 nm was obtained by applying 0 to 17 dc V to an intracavity fiber Fabry–Perot filter. Threshold pump power versus wavelength data showed low variation over the tuning range. Mode hopping suppression with a tandem fiber Fabry–Perot filter is proposed and demonstrated. Stable single‐frequency operation was demonstrated with side mode suppression higher than 35 dB.