Sanghoon Han

Colossal Absorption of Molecules Inside Single Terahertz Nanoantennas

Molecules have extremely small absorption cross sections in the terahertz range even under resonant conditions, which severely limit their detectability, often requiring tens of milligrams. We demonstrate that nanoantennas tailored for the terahertz range resolves the small molecular cross section problem. The extremely asymmetric electromagnetic environment inside the slot antenna, which finds the electric field being enhanced by thousand times with the magnetic field changed little, forces the molecular cross section to be enhanced by >10(3) accompanied by a colossal absorption coefficient of ~170,000 cm(-1). Tens of nanograms of small molecules such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and lactose drop-cast over an area of 10 mm(2), with only tens of femtograms of molecules inside the single nanoslot, can readily be detected. Our work enables terahertz sensing of chemical and biological molecules in ultrasmall quantities.

Electromagnetic Saturation of Angstrom-Sized Quantum Barriers at Terahertz Frequencies.

Metal-graphene-metal hybrid structures allow angstrom-scale van der Waals gaps, across which electron tunneling occurs. We squeeze terahertz electromagnetic waves through these λ/10 000 000 gaps, accompanied by giant field enhancements. Unprecedented transmission reduction of 97% is achieved with the transient voltage across the gap saturating at 5 V. Electron tunneling facilitated by the transient electric field strongly modifies the gap index, starting a self-limiting process related to the barrier height. Our work enables greater interplay between classical optics and quantum tunneling, and provides optical indices to the van der Waals gaps.

Superfocusing of electric or magnetic fields using conical metal tips: effect of mode symmetry on the plasmon excitation method.

We compare single- and double-sided excitation methods of adiabatic surface plasmon polariton (SPP) wave superfocusing for scattering-type metallic near-field scanning optical microscopy (s-NSOM). Using the results of full 3D finite difference time domain analyses, the differences in field enhancement factors are explained and reveal the mode selectivity of a conical NSOM tip for adiabatic SPP superfocusing. Exploiting the mode-symmetric nature of the tip further, we also show that it is possible to selectively confine either the electric or magnetic field at the NSOM tip apex, by simply adjusting the relative phase between the SPP waves in the double-sided excitation approach.

Accurate and Robust Pressure Weight Advection Upstream Splitting Method for Magnetohydrodynamics Equations

Two variants of the advection upstream splitting method are modified to solve the equations of ideal magnetohydrodynamics. Discontinuity-sensing functions used in AUSMPW+ and M-AUSMPW+ for the magnetohydrodynamics equations are newly defined in consideration of a magnetic field. The new pressure-based weight functions are shown to effectively remove the oscillations behind a strong shock wave as well as provide a highly accurate solution for a stationary contact discontinuity. To satisfy the divergence-free constraint of a magnetic field, the hyperbolic divergence cleaning method is chosen and applied for all the test cases. Ryu and Jones one-dimensional magnetohydrodynamics shock-tube problem and Orszag and Tangs two-dimensional magnetohydrodynamics vortex system are evaluated to validate and show the advantages of the newly developed schemes.

Terahertz pinch harmonics enabled by single nano rods

A pinch harmonic (or guitar harmonic) is a musical note produced by lightly pressing the thumb of the picking hand upon the string immediately after it is picked [J. Chem. Educ. 84, 1287 (2007)]. This technique turns off the fundamental and all overtones except those with a node at that location. Here we present a terahertz analogue of pinch harmonics, whereby a metallic nano rod placed at a harmonic node on a terahertz nanoresonator suppresses the fundamental mode, making the higher harmonics dominant. Strikingly, a skin depth-wide nano rod placed at the mid-point turns off all resonances. Our work demonstrates that terahertz electromagnetic waves can be tailored by nanoparticles strategically positioned, paving important path towards terahertz switching and detection applications.

Analysis of Segmented Arc-Heater Flows with High Argon Concentration

The existing computer code to solve the airflow in a segmented constrictor-type arc-heated wind tunnel named ARCFLO4 is improved to accept an air-argon mixture as the working gas. The new version of the code is used to calculate the flows in the Aerodynamic Heating Facility of NASA Ames Research Center where argon concentration is relatively high. The calculation shows that argon tends to increase the diameter of the arc column, increase ionization fraction, decrease thermal efficiency of the arc heater, and push the ratio of the centerline-to-average enthalpy toward unity. The calculated operating characteristics of the arc heater agree well with the experimental data and the results of the calculations made by Sakai using a similar code developed earlier, ARCFLO3.

Terahertz field enhancement in asymmetric and tapered nano-gaps.

We investigate field enhancement inside metal-insulator-metal gaps with asymmetric thicknesses and tapered shapes in the terahertz regime. Finite-difference time-domain simulations were conducted for calculation of field enhancement factor. The calculation indicates that for asymmetric sample, field enhancement increases proportionally with the decrease of the thinner of the two metal film thicknesses surrounding the gap. Concomitantly, angle variation has little effect on the field enhancement if the thickness of the narrowest gap region is fixed. A model based on the capacitor concept is proposed for intuitive understanding of the phenomena.

Quantum dots-nanogap metamaterials fabrication by self-assembly lithography and photoluminescence studies.

We present a new and versatile technique of self-assembly lithography to fabricate a large scale Cadmium selenide quantum dots-silver nanogap metamaterials. After optical and electron microscopic characterizations of the metamaterials, we performed spatially resolved photoluminescence transmission measurements. We obtained highly quenched photoluminescence spectra compared to those from bare quantum dots film. We then quantified the quenching in terms of an average photoluminescence enhancement factor. A finite difference time domain simulation was performed to understand the role of an electric field enhancement in the nanogap over this quenching. Finally, we interpreted the mechanism of the photoluminescence quenching and proposed fabrication method of new metamaterials using our technique.

Terahertz transmission through rings of quantum dots-nanogap

We report resonant funneling of terahertz (THz) waves through (9 ± 1) nm wide quantum dots-nanogap of cadmium selenide quantum dots silver nanogap metamaterials. We observed a giant THz intensity enhancement (~104) through the quantum dots-nanogap at the resonant frequency. We, further report the experimentally measured effective mode indices for these metamaterials. A finite difference time domain simulation of the nanogap enabled by the quantum dots supports the experimentally measured THz intensity enhancement across the nanogap. We propose that these low effective mode index terahertz resonators will be useful as bio/chemical sensors, gain-enhanced antennas, and wave guides.

High Resolution Numerical Study on the Coaxial Supersonic Turbulent Flame Structures

A diagnostic investigation regarding the turbulent combustion is analyzed for supersonic co-axial flame. The Mathematical formulation is based on the hybrid RANS/LES formulation for compressible reacting flows. A validation case study is performed for an experimental case with three levels of different grid system with three different high resolution schemes namely 3-order MUSCL scheme, a 5th-order WENO scheme and multi-dimensional 5th-order oMLP scheme. The time-averaged results of the experimental case shows the importance of fine grid resolution and high-order accurate numerical scheme for reliable prediction, capturing the fine scale instabilities of the supersonic turbulent combustion. The oMLP scheme exhibits a remarkable performance without a big expense. An analysis of the flow field is carried out to investigate the supersonic turbulent flame structure. The comparison of the combustion parameters including OH mass fraction, scalar dissipation rate and flame index reveals that the supersonic combustion of the validation case has a characteristics of turbulent lifted flame where the combustion is held mostly at premixed mode following turbulence mixing at the shear layer.