Soo Bong Choi

Nanopattern enabled terahertz all-optical switching on vanadium dioxide thin film

We demonstrate ultrafast all-optical control of terahertz (THz) radiation through nanoresonators, slot antennas with a hundred micron length but submicron width in thin gold layers, fabricated on vanadium dioxide (VO2) thin films. Our THz nanoresonators show almost perfect transmission at resonance. By virtue of phase transition of VO2 from insulating to metallic state, induced in subpicosecond time scale by moderate optical pump, ultrafast control of THz transmission is enabled. This is compared to bare VO2 films where no switching dynamics are observed under similar conditions.

Directional control of surface plasmon polariton waves propagating through an asymmetric Bragg resonator

We have demonstrated the directional control of surface plasmon polariton (SPP) waves propagating through an asymmetric plasmonic Bragg resonator using femtosecond temporal-phase control via the resonant coupling of SPPs and the interference of SPPs. The near-field images display significant temporal-phase dependence, switching between left and right propagation after the Bragg resonator. Our results would be a key step toward the control of surface plasmon propagation direction in nanoscaled plasmonic applications

Terahertz near-field enhancement in narrow rectangular apertures on metal film

We report huge field accumulations in rectangular aperture arrays on thin metal film by using shape resonance in THz frequency region. A huge far-field transmission enhancement is observed in samples of various widths ranging from 10 mum to 1.8 mum which correspond to only an order of lambda/100. Theoretical calculations based on vector diffraction theory indicates 230 times near-field enhancement in case of the 1.8 mum wide rectangular aperture. Transmission measurement through the single rectangular aperture shows that the shape resonance, not the periodicity, is mainly responsible for the transmission enhancement and the corresponding field enhancement.

Emission properties of hydrothermal Yb3 + , Er3 + and Yb3 + , Tm3 + -codoped Lu2O3 nanorods: upconversion, cathodoluminescence and assessment of waveguide behavior

Yb(3+) and Ln(3+) (Ln(3+) = Er(3+) or Tm(3+)) codoped Lu(2)O(3) nanorods with cubic Ia3 symmetry have been prepared by low temperature hydrothermal procedures, and their luminescence properties and waveguide behavior analyzed by means of scanning near-field optical microscopy (SNOM). Room temperature upconversion (UC) under excitation at 980 nm and cathodoluminescence (CL) spectra were studied as a function of the Yb(+) concentration in the prepared nanorods. UC spectra revealed the strong development of Er(3+) (4)F(9/2) → (4)I(15/2) (red) and Tm(3+) (1)G(4) → (3)H(6) (blue) bands, which became the pre-eminent and even unique emissions for corresponding nanorods with the higher Yb(3+) concentration. Favored by the presence of large phonons in current nanorods, UC mechanisms that privilege the population of (4)F(9/2) and (1)G(4) emitting levels through phonon-assisted energy transfer and non-radiative relaxations account for these observed UC luminescence features. CL spectra show much more moderate development of the intensity ratio between the Er(3+) (4)F(9/2) → (4)I(15/2) (red) and (2)H(11/2), (4)S(3/2) → (4)I(15/2) (green) emissions with the increase in the Yb(3+) content, while for Yb(3+), Tm(3+)-codoped Lu(2)O(3) nanorods the dominant CL emission is Tm(3+) (1)D(2) → (3)F(4) (deep-blue). Uniform light emission along Yb(3+), Er(3+)-codoped Lu(2)O(3) rods has been observed by using SNOM photoluminescence images; however, the rods seem to be too thin for propagation of light.

Terahertz Wave Applications of Single-Walled Carbon Nanotube Films with High Shielding Effectiveness

We demonstrate that a filtration method is efficient for the fabrication of thick single-walled nanotube films and is capable of shielding terahertz waves. Shielding effectiveness can be engineered by controlling the film thickness and we achieved 38 dB for a 950-nm-thick film. In addition, we found that the films exhibit a dispersion of dielectric constant obeying the Drude free-electron model, whereas the plasma frequency decreases with increasing film thickness. Based on the nanotube films with a thickness greater than the skin depth, we fabricated grid polarizers by laser-machining process, which enable us to achieve a large polarization extinction ratio.

Optical field enhancement of nanometer-sized gaps at near-infrared frequencies

We report near-field and far-field measurements of transmission through nanometer-sized gaps at near-infrared frequencies with varying the gap size from 1 nm to 10 nm. In the far-field measurements, we excluded direct transmission on the metal film surface via interferometric method. Kirchhoff integral formalism was used to relate the far-field intensity to the electric field at the nanogaps. In near-field measurements, field enhancement factors of the nanogaps were quantified by measuring transmission of the nanogaps using near-field scanning optical microscopy. All the measurements produce similar field enhancements of about ten, which we put in the context of comparing with the giant field enhancements in the terahertz regime.

Epsilon-Near-Zero meta-lens for high resolution wide-field imaging

Herein, we will propose a new application possibility of epsilon-near-zero (ENZ) materials: high resolution wide-field imaging. We show that the resolution can be dramatically enhanced by simply inserting a thin epsilon-near-zero (ENZ) material between the sample and substrate. By performing metal half-plane imaging, we experimentally demonstrate that the resolution could be enhanced by about 47% with a 300-nm-thick SiO2 interlayer, an ENZ material at 8-μm-wavelength (1250 cm-1). The physical origin of the resolution enhancement is the strong conversion of diffracted near fields to quasi-zeroth order far fields enabled by the directive emission of ENZ materials.

Pyroprotein‐Based Electronic Textiles with High Stability

Thermally reducible pyroprotein-based electronic textiles (e-textiles) are fabricated using graphene oxide and a pyroprotein such as cocoon silk and spider web without any chemical agents. The electrical conductivity of the e-textile is 11.63 S cm-1 , which is maintained even in bending, washing, and temperature variation.

Contactless photoconductive terahertz generation

We describe a pulsed terahertz (THz) emitter that uses a rapidly oscillating, high-voltage bias across electrodes insulated from a photoconductor. Because no carriers are injected from the electrodes, trap-enhanced electric fields do not form. The resulting uniform field allows excitation with a large laser spot, lowering the carrier density for a given pulse energy and increasing the efficiency of THz generation. Compared to a dc bias, less susceptibility to damage is observed.

Femtosecond phase control of spatial localization of the optical near-field in a metal nanoslit array

We demonstrate spatial control of optical near-fields by femtosecond phase shaping in one-dimensional plasmonic structures. The near-field images display striking temporal-phase dependence, switching between double- and single-peak images within one lattice constant. The change of the near-field distribution is studied in the time and spectral domain. The spectral composition change observed by varying the time delay between two phase-locked femtosecond pulses explains the spatial control of the near-field images. Modal expansion calculations of linear light transmission using the surface impedance boundary condition are in excellent agreement with experiments.