Seyoon Kim

Hybrid Surface-Phonon-Plasmon Polariton Modes in Graphene/Monolayer h-BN Heterostructures

Infrared transmission measurements reveal the hybridization of graphene plasmons and the phonons in a monolayer hexagonal boron nitride (h-BN) sheet. Frequency-wavevector dispersion relations of the electromagnetically coupled graphene plasmon/h-BN phonon modes are derived from measurement of nanoresonators with widths varying from 30 to 300 nm. It is shown that the graphene plasmon mode is split into two distinct optical modes that display an anticrossing behavior near the energy of the h-BN optical phonon at 1370 cm(-1). We explain this behavior as a classical electromagnetic strong-coupling with the highly confined near fields of the graphene plasmons allowing for hybridization with the phonons of the atomically thin h-BN layer to create two clearly separated new surface-phonon-plasmon-polariton (SPPP) modes.

Tunable large resonant absorption in a midinfrared graphene Salisbury screen

The optical absorption properties of periodically patterned graphene plasmonic resonators are studied experimentally as the graphene sheet is placed near a metallic reflector. By varying the size and carrier density of the graphene, the parameters for achieving a surface impedance closely matched to free-space (Z_0 = 377Ω) are determined and shown to result in 24.5% total optical absorption in the graphene sheet. Theoretical analysis shows that complete absorption is achievable with higher doping or lower loss. This geometry, known as a Salisbury screen, provides an efficient means of light coupling to the highly confined graphene plasmonic modes for future optoelectronic applications.

Electronic modulation of infrared radiation in graphene plasmonic resonators

All matter at finite temperatures emits electromagnetic radiation due to the thermally induced motion of particles and quasiparticles. Dynamic control of this radiation could enable the design of novel infrared sources; however, the spectral characteristics of the radiated power are dictated by the electromagnetic energy density and emissivity, which are ordinarily fixed properties of the material and temperature. Here we experimentally demonstrate tunable electronic control of blackbody emission from graphene plasmonic resonators on a silicon nitride substrate. It is shown that the graphene resonators produce antenna-coupled blackbody radiation, which manifests as narrow spectral emission peaks in the mid-infrared. By continuously varying the nanoresonator carrier density, the frequency and intensity of these spectral features can be modulated via an electrostatic gate. This work opens the door for future devices that may control blackbody radiation at timescales beyond the limits of conventional thermo-optic modulation.

Electronically tunable extraordinary optical transmission in graphene plasmonic ribbons coupled to subwavelength metallic slit arrays

Subwavelength metallic slit arrays have been shown to exhibit extraordinary optical transmission, whereby tunnelling surface plasmonic waves constructively interfere to create large forward light propagation. The intricate balancing needed for this interference to occur allows for resonant transmission to be highly sensitive to changes in the environment. Here we demonstrate that extraordinary optical transmission resonance can be coupled to electrostatically tunable graphene plasmonic ribbons to create electrostatic modulation of mid-infrared light. Absorption in graphene plasmonic ribbons situated inside metallic slits can efficiently block the coupling channel for resonant transmission, leading to a suppression of transmission. Full-wave simulations predict a transmission modulation of 95.7% via this mechanism. Experimental measurements reveal a modulation efficiency of 28.6% in transmission at 1,397 cm−1, corresponding to a 2.67-fold improvement over transmission without a metallic slit array. This work paves the way for enhancing light modulation in graphene plasmonics by employing noble metal plasmonic structures.

Influence of thermochemical properties on ion mixing of markers in Cu and β-Zr at 77 K with Kr

Ion mixing of Nb, Ru, Ag, In, Sb, Hf, Pt, Au, and Bi markers in a Cu matrix and of Ti, Cr, Fe, Co, Ni, Cu, Hf, W, and Au markers in a β-Zr matrix has been studied by irradiation with 750 keV Kr+ ions of doses from 5 × 1015 to 2 × 1016/cm2= at 77 K. Cu and β-Zr have quite different atomic properties and impurities in them also behave quite differently. Thus, through a systematic investigation, the influence of parameters and mechanisms on ion mixing is clarified. The mixing was analyzed in situ, using 1.8 and 1.9 MeV He ion backscattering spectrometry. The overall mixing efficiency, Dt/φFD, is significantly higher in Cu than in β-Zr. This difference is explained in terms of the thermal spike mechanism in these matrices. In Cu, the mixing efficiencies correlate with impurity tracer diffusivities and impurity-vacancy binding energies for the marker atoms in Cu. Vacancies apparently play a major role during thermal spike mixing in Cu. In β-Zr, the markers that are likely to dissolve substitutionally in the matrix, have slightly higher mixing efficiencies than the markers that are likely to dissolve interstitially. The results are interpreted with the diffusion properties of these impurities in β-Zr.

Ion mixing and thermochemical properties of markers in Cu

Markers of Nb, Ru, Ag, In, Sb, Hf, Pt, Au, and Bi in Cu were mixed by irradiation with 750 keV Kr at 77 K and analyzed in situ by backscattering of 1.9 MeV He+. Cu with Pt and Au markers were also irradiated and analyzed at 7 K. The results were identical to those obtained at 77 K results. The measured mixing efficienciesDt/øFD, for the various markers correlate with their respective impurity tracer diffusivities and impurity-vacancy binding energies in Cu. The correlation suggests that diffusion by a vacancy mechanism during a thermal spike as an important process in ion mixing of marker atoms in Cu.

Electronically Tunable Perfect Absorption in Graphene

The demand for dynamically tunable light modulation in flat optics applications has grown in recent years. Graphene nanostructures have been extensively studied as means of creating large effective index tunability, motivated by theoretical predictions of the potential for unity absorption in resonantly excited graphene nanostructures. However, the poor radiative coupling to graphene plasmonic nanoresonators and low graphene carrier mobilities from imperfections in processed graphene samples have led to low modulation depths in experimental attempts at creating tunable absorption in graphene devices. Here we demonstrate electronically tunable perfect absorption in graphene, covering less than 10% of the surface area, by incorporating multiscale nanophotonic structures composed of a low-permittivity substrate and subwavelength noble metal plasmonic antennas to enhance the radiative coupling to deep subwavelength graphene nanoresonators. To design the structures, we devised a graphical method based on effective surface admittance, elucidating the origin of perfect absorption arising from critical coupling between radiation and graphene plasmonic modes. Experimental measurements reveal 96.9% absorption in the graphene plasmonic nanostructure at 1389 cm-1, with an on/off modulation efficiency of 95.9% in reflection.

Resonant thermoelectric nanophotonics

Photodetectors are typically based either on photocurrent generation from electron-hole pairs in semiconductor structures or on bolometry for wavelengths that are below bandgap absorption. In both cases, resonant plasmonic and nanophotonic structures have been successfully used to enhance performance. Here, we show subwavelength thermoelectric nanostructures designed for resonant spectrally selective absorption, which creates large localized temperature gradients even with unfocused, spatially uniform illumination to generate a thermoelectric voltage. We show that such structures are tunable and are capable of wavelength-specific detection, with an input power responsivity of up to 38 V W-1, referenced to incident illumination, and bandwidth of nearly 3 kHz. This is obtained by combining resonant absorption and thermoelectric junctions within a single suspended membrane nanostructure, yielding a bandgap-independent photodetection mechanism. We report results for both bismuth telluride/antimony telluride and chromel/alumel structures as examples of a potentially broader class of resonant nanophotonic thermoelectric materials for optoelectronic applications such as non-bandgap-limited hyperspectral and broadband photodetectors.

Ion mixing and thermochemical properties of tracers in Ag

Very thin films of Ni, Ta, W, Pb, and Bi in a Ag matrix were irradiated at 77 K with 330 keV Kr ions at doses from 3 to 7×10^15 ions/cm^2 and analyzed at room temperature by backscattering of 1.9 MeV He + . The measured mixing efficiencies, Dt/phiFD, for the various tracers correlate with their respective tracer impurity diffusion coefficients and impurity-vacancy binding energies in Ag. The results concur with previous ones with a Cu matrix and further support the idea that the parameters that are important for thermal diffusion are also important for ion mixing in a thermal spike.

Controlling and creating plasmonic absorption processes in graphene nanostructures (presentation video)

In this presentation, it will be shown that the plasmonic absorption of a graphene sheet can be enhanced and perturbed in controllable ways by controlling the thickness and permittivity of the supporting substrate. We will show the results of recent experiments where 25% absorption is achieved in the plasmonic modes of a graphene sheet by carefully selecting the properties of an underlying silicon nitride substrate. We also demonstrate how additional absorption pathways can be created by modifying the surrounding dielectric environment to have optical resonances that can couple to the graphene plasmons.