Hyunwoong Kim

High-harmonic generation by field enhanced femtosecond pulses in metal-sapphire nanostructure

Plasmonic high-harmonic generation (HHG) drew attention as a means of producing coherent extreme ultraviolet (EUV) radiation by taking advantage of field enhancement occurring in metallic nanostructures. Here a metal-sapphire nanostructure is devised to provide a solid tip as the HHG emitter, replacing commonly used gaseous atoms. The fabricated solid tip is made of monocrystalline sapphire surrounded by a gold thin-film layer, and intended to produce EUV harmonics by the inter- and intra-band oscillations of electrons driven by the incident laser. The metal-sapphire nanostructure enhances the incident laser field by means of surface plasmon polaritons, triggering HHG directly from moderate femtosecond pulses of ∼0.1 TW cm−2 intensities. The measured EUV spectra exhibit odd-order harmonics up to ∼60 nm wavelengths without the plasma atomic lines typically seen when using gaseous atoms as the HHG emitter. This experimental outcome confirms that the plasmonic HHG approach is a promising way to realize coherent EUV sources for nano-scale near-field applications in spectroscopy, microscopy, lithography and atto-second physics.

Terahertz Modulator based on Metamaterials integrated with Metal-Semiconductor-Metal Varactors

The terahertz (THz) band of the electromagnetic spectrum, with frequencies ranging from 300 GHz to 3 THz, has attracted wide interest in recent years owing to its potential applications in numerous areas. Significant progress has been made toward the development of devices capable of actively controlling terahertz waves; nonetheless, further advances in device functionality are necessary for employment of these devices in practical terahertz systems. Here, we demonstrate a low voltage, sharp switching terahertz modulator device based on metamaterials integrated with metal semiconductor metal (MSM) varactors, fabricated on an AlGaAs/InGaAs based heterostructure. By varying the applied voltage to the MSM-varactor located at the center of split ring resonator (SRR), the resonance frequency of the SRR-based metamaterial is altered. Upon varying the bias voltage from 0 V to 3 V, the resonance frequency exhibits a transition from 0.52 THz to 0.56 THz, resulting in a modulation depth of 45 percent with an insertion loss of 4.3 dB at 0.58 THz. This work demonstrates a new approach for realizing active terahertz devices with improved functionalities.

High-brightness laser imaging with tunable speckle reduction enabled by electroactive micro-optic diffusers

High coherence of lasers is desirable in high-speed, high-resolution, and wide-field imaging. However, it also causes unavoidable background speckle noise thus degrades the image quality in traditional microscopy and more significantly in interferometric quantitative phase imaging (QPI). QPI utilizes optical interference for high-precision measurement of the optical properties where the speckle can severely distort the information. To overcome this, we demonstrated a light source system having a wide tunability in the spatial coherence over 43% by controlling the illumination angle, scatterer’s size, and the rotational speed of an electroactive-polymer rotational micro-optic diffuser. Spatially random phase modulation was implemented for the lower speckle imaging with over a 50% speckle reduction without a significant degradation in the temporal coherence. Our coherence control technique will provide a unique solution for a low-speckle, full-field, and coherent imaging in optically scattering media in the fields of healthcare sciences, material sciences and high-precision engineering.

Self-optimization of plasmonic nanoantennas in strong femtosecond fields

Plasmonic dimer nanoantennas can significantly boost the electric field strength in the gap region, allowing for a modification of the feed gap geometry by femtosecond laser illumination. Using resonant bowtie antennas to enhance the electric field of a low-fluence femtosecond oscillator, here we experimentally demonstrate highly localized reshaping of the antennas, resulting in a self-optimization of the antenna shape. From high-resolution scanning electron micrographs and two-dimensional energy dispersive x-ray maps, we analyze the near-field enhanced subwavelength ablation at the nanotips and the resulting deposition of ablated materials in the feed gap. The dominant ablation mechanism is attributed to the nonthermal transient unbonding of atoms and electrostatic acceleration of ions. This process is driven by surface plasmon enhanced electron emission, with subsequent acceleration in the vacuum. This ablation is impeded in the presence of an ambient gas. A maximum of sixfold enhancement of the third-harmonic yield is observed during the reshaping process.

Quad-band THz frequency selective surface based on a split ring resonator loaded with multiple slots

A quad-band terahertz (THz) frequency selective surface with bandstop response is designed, fabricated, and characterized. The unit cell of the filter consists of a split ring resonator (SRR) and surrounding four groups of slots. Each group of slots has three slots with different lengths. The lowest frequency band is determined by the resonance of the SRR and the other three upper bands are selected by the resonance of the slots which exhibit bandstop characteristics rather than bandpass characteristics. The filter transformation characteristic originates from the slits placed between the unit cells which act as series-connected capacitive components. The fabricated quad-band THz filters exhibited bandstop characteristics with attenuation higher than 20 dB in all the operating frequency bands.

3DIV: A 3D-genome Interaction Viewer and database

Abstract Three-dimensional (3D) chromatin structure is an emerging paradigm for understanding gene regulation mechanisms. Hi-C (high-throughput chromatin conformation capture), a method to detect long-range chromatin interactions, allows extensive genome-wide investigation of 3D chromatin structure. However, broad application of Hi-C data have been hindered by the level of complexity in processing Hi-C data and the large size of raw sequencing data. In order to overcome these limitations, we constructed a database named 3DIV (a 3D-genome Interaction Viewer and database) that provides a list of long-range chromatin interaction partners for the queried locus with genomic and epigenomic annotations. 3DIV is the first of its kind to collect all publicly available human Hi-C data to provide 66 billion uniformly processed raw Hi-C read pairs obtained from 80 different human cell/tissue types. In contrast to other databases, 3DIV uniquely provides normalized chromatin interaction frequencies against genomic distance dependent background signals and a dynamic browsing visualization tool for the listed interactions, which could greatly advance the interpretation of chromatin interactions. ‘3DIV’ is available at http://kobic.kr/3div.

Investigating the origin of third harmonic generation from diabolo optical antennas

We propose to use diabolo nanoantennas for experimentally investigating the origin of the enhanced third harmonic generation by localized surface plasmon polaritons. In such a geometry, the opposing apexes of bowties are electrically connected by a thin gold nanorod, which has two important functions in discriminating the point of harmonic generation. First, the inserted gold nanorod shifts the field enhancement area to be far away from the dielectric substrate material. Next, the accumulation of free charges at the adjacent bowtie tips produces a strong electric field inside the gold nanorod. The diabolo nanoantennas allow us to examine the contribution of the bare gold susceptibility to the third harmonic conversion. Our results reveal that the bare gold does not significantly enhance the harmonic generation at high pump intensity. From this, we deduce that in regular bowtie antennas, the enhanced harmonic photons mainly arise from the substrate sapphire that is located in the feedgap of the bowtie, whe...

Enhancing the modulation depth of a dynamic terahertz metasurface by integrating into an asymmetric Fabry-Pérot cavity

The modulation capability of a terahertz dynamic metasurface was enhanced by integrating it with an asymmetric Fabry-Perot cavity. The dynamic metasurface consists of split ring resonators combined with high frequency metal semiconductor metal varactors. Integrating the above dynamic metasurface into an asymmetric Fabry-Perot cavity results in a modulation performance improvement of up to 3.3 times. An analytical model based on transmission line theory illustrates the modulation enhancement mechanism, and provide guidelines for designing dynamic metamaterial devices based on cavity structures.The modulation capability of a terahertz dynamic metasurface was enhanced by integrating it with an asymmetric Fabry-Perot cavity. The dynamic metasurface consists of split ring resonators combined with high frequency metal semiconductor metal varactors. Integrating the above dynamic metasurface into an asymmetric Fabry-Perot cavity results in a modulation performance improvement of up to 3.3 times. An analytical model based on transmission line theory illustrates the modulation enhancement mechanism, and provide guidelines for designing dynamic metamaterial devices based on cavity structures.

Nonlinear third harmonic generation at crystalline sapphires

Third harmonic generation (THG) is a nonlinear optical phenomenon which can be applied in diverse research areas including interfacial studies, sub-wavelength light manipulation, and high sensitivity bio-molecular detection. Most precedent studies on THG have focused on dielectric and metallic materials, including silicon, gold, and germanium, due to their high nonlinear susceptibility. Sapphire, a widely-used optical substrate, has not been studied in depth for its third harmonic characteristics, despite its excellent optical transmission in the UV-visible range, high thermal conductance, and superior physical and chemical stability. In this research, we comprehensively studied THG at thin air-dielectric interfaces of sapphire wafers by controlling the wafer cutting planes, focusing depth, incidence angle, laser intensity, and input polarization of the input laser beam. These findings can lead to broader use of third harmonics for high-precision sapphire characterization, such as surface quality inspection, crystallinity determination, interfacial studies, delamination check, and real-time monitoring of crack propagation.

Monolithic integration of microlaser with passive waveguide via selective quantum well etching

Monolithic integration of a photonic crystal laser and a passive waveguides via selective quantum well etching are demonstrated. Coupling efficiency between photonic crystal cavity and waveguide is investigated numerically.