Taeyoon Hong

Reflection terahertz time-domain spectroscopy of RDX and HMX explosives

We report on our study of RDX and HMX, two of the most commonly used explosive materials, in bulk pellets with reflection terahertz time-domain spectroscopy in the frequency range of 0.3–3 THz. The maximum entropy method was utilized to correct our raw reflection data against the phase error due to the relative displacement between the sample and the reference. Both the refractive index n and the extinction coefficient k in the terahertz region were acquired for these two explosives without a Kramers-Kronig analysis. Both RDX and HMX exhibit a series of distinct peaks not quite detectable in the more conventional transmission-type measurements due to their high terahertz absorptivity. Our results are compared with the literature data on powder samples.

Terahertz and optical study of monolayer graphene processed by plasma oxidation

We report on our terahertz and optical study of monolayer graphene grown by chemical vapor deposition and processed by plasma oxidation. The plasma oxidation induces oxygen-related defects, and the resulting disorder increases the sheet resistance of graphene as measured via terahertz spectroscopy. The excitonic absorption peak weakens considerably and blue shifts upon plasma oxidation, resulting in higher transmittance in both the visible and ultraviolet regions. Our oxygen plasma-treated graphene also exhibits a free-carrier doping effect as confirmed by the blue shift of the Raman G band.

Double Fano resonances in a composite metamaterial possessing tripod plasmonic resonances

Double Fano resonances are observed in a planar composite metamaterial possessing tripod plasmonic resonances, where a common subradiant driven oscillator is coupled with two superradiant oscillators. As a classical analogue of four-level tripod atomic system, the extinction spectrum of the composite metamaterial exhibits a coherent effect based on double Fano resonances. It is shown that a transfer of the absorbed power between two orthogonal superradiant oscillators is mediated by a common subradiant oscillator.

Terahertz electrodynamics and superconducting energy gap of NbTiN

Terahertz electrodynamics of superconducting NbTiN has been studied in the spectral range of 8-70 cm–1 above and below the critical temperature of Tc = 14.1 K. Our transmission terahertz time-domain spectroscopy technique allows for independent and accurate determination of both the real part σ1 and the imaginary part σ2 of the optical conductivity σ as a function of frequency ω and temperature T without a Kramers-Kronig analysis. A clear signature of the superconducting energy gap Δ(T) is observed in the real part σ1 of the optical conductivity below Tc, with 2Δ(0) = 36.5 cm−1 corresponding to 2Δ(0)/kBTc = 3.72. No indication of strong-coupling effects was observed in the imaginary part σ2 toward zero frequency. Our results, including the temperature dependence of the penetration depth λ(T), are in excellent quantitative agreement with weak coupling BCS theory and Mattis-Bardeen formula.

Double Fano resonances in a composite metamaterial possessing tripod plasmonic resonances (Presentation Recording)

We experimentally demonstrate a classical analogue of double Fano resonances in a planar composite metamaterial possessing tripod plasmonic resonances, where a common subradiant dipole corresponding to the dark mode is coupled with two superradient dipoles corresponding to two bright modes. The composite metamaterial is structured such that four-rod resonators (FRR) are embedded inside double-split ring resonators (DSRR) superlattice. Two dipole resonances of DSRR are superradiant modes and one dipole resonance of FRR is subradiant mode. Proximity of the inner diameter of DSRR and the rod-length of FRR permits near field coherent couplings, and the composite metamaterial is a tripod metamaterial system possessing two superradiant dipole resonances of DSRR coupled coherently with one common subradiant dipole resonance of FRR in near field, similar to a tripod atomic system with four atomic levels coupled coherently by coherent optical fields. Important finding is that double Fano resonances in the composite metamaterial are correlated, showing up as a transfer of the absorbed power from one superradiant dipole to the other superradient dipole in DSRR. The general feature of plasmonic Fano resonance is examined where both superradiant and subradiant oscillators are externally driven. Analysis based on two coupled oscillators model leads to the fact that the characteristic asymmetric Fano resonance formula of plasmonic structure is kept the same with a modication in the asymmetry parameter q.

Time-domain terahertz spectroscopy of LaSrAlO 4

Single crystal lanthanum strontium aluminate, LaSrAlO4 (001), was investigated by means of time-domain terahertz transmission spectroscopy. We extracted the optical constants at frequencies from 3 to 100 cm−1 and at temperatures from 4 to 300 K by inverting the measured complex transmission coefficient without resorting to a Kramers-Kronig analysis. The refractive index was almost independent of temperature in the terahertz range.