Tunable light-matter interaction and the role of hyperbolicity in graphene-hBN system

Hexagonal boron nitride (hBN) is a natural hyperbolic material which can also accommodate highly dispersive surface phonon-polariton modes. In this paper, we examine theoretically the mid-infrared optical properties of graphene-hBN heterostructures derived from their coupled plasmon-phonon modes. We found that the graphene plasmon couples differently with the phonons of the two Reststrahlen bands, owing to their different hyperbolicity. This also leads to distinctively different interaction between an external quantum emitter and the plasmon-phonon modes in the two bands, leading to substantial modification of its spectrum. The coupling to graphene plasmons allows for additional gate tunability in the Purcell factor, and narrow dips in its emission spectra

Investigation of gold nanorods as a sensing material in plasmonic sensor for triclopyr butotyl detection

Gold nanorods (GNRs) have a unique optical property of metal nanoparticles (MNPs) due to the localised surface plasmon resonance (LSPR) effect, which depends on the size, shape and dielectric property of the surrounding medium. LSPR, or commonly known as the plasmonic effect, refers to the optical phenomena resulting from the interaction of free electrons on a nano-sized metal surface with incident light at specific wavelengths. The plasmonic effect of rod-shaped nanoparticles shows dual absorption bands corresponding to transverse surface plasmon resonance (t-SPR) and longitudinal surface plasmon resonance (l-SPR). These two bands are sensitive to size changes and the surrounding medium’s refractive index. In GNR formation, particles size, homogeneity and shape are crucial elements to be investigated during the synthesis process. Therefore, three parameters are studied in this research, which are centrifugation speed, seed solution concentration and growth solution ageing period. Through the variation of parameters during the synthesis procedure, the optimum GNRs with a surface density of 74.81 %, an average length of 59.80 ± 0.53 nm and an average width of 14.14 ± 0.19 nm produce an aspect ratio of 4.23 ± 0.36 via the seed�mediated growth method (SMGM). The optimum GNR sample is prepared by adding 10 µl of a seed solution into a raw growth solution and left undisturbed for 20 hours and then centrifuged at a rotational speed of 5000 rpm. The optical spectrum from that sample exhibits two plasmon bands at the transverse axis of 535.02 nm and the longitudinal axis of 782.65 nm. For sensing application, the GNRs are used as a sensing material to detect the targeted analyte, namely triclopyr butotyl (Cଵଷ Hଵ଺ClଷNOସ). The sensitivity, stability and repeatability of GNRs in deionized water and triclopyr butotyl medium is studied by observing the changes in the absorption intensity and the peak position of the plasmon resonance. The optical response of 10 % triclopyr butotyl without GNRs shows no significant peaks and proves that GNRs are able to increase the ability of detection through the plasmonic effect. In sensitivity testing, it is found that the presence of triclopyr butotyl changes the absorption intensity and shifts the resonance peak position of the GNRs. The vi detection limit of GNRs is as low as 3 %. Furthermore, the GNRs depict good response during 600 seconds of the stability test. Moreover, the fast response and recovery time in the change of medium observed in five cycles show good repeatability of GNRs

Geometry-diversified Coulomb excitations in trilayer AAB stacking graphene

The lower-symmetry trilayer AAB-stacked graphene exhibits rich electronic properties and thus diverse Coulomb excitations. Three pairs of unusual valence and conduction bands create nine available interband excitations for the undoped case, in which the imaginary (real) part of the polarizability shows 1D square root asymmetric peaks and 2D shoulder structures (pairs of antisymmetric peaks and logarithm type symmetric peaks). Moreover, the low frequency acoustic plasmon, being revealed as a prominent peak in the energy loss spectrum, can survive in a narrow gap system with the large-density-of-states from the valence band. This type of plasmon mode is similar to that in a narrow gap carbon nanotube. However, the decisive mechanism governing this plasmon is the intraband conduction state excitations. Its frequency, intensity and critical momentum exhibit a non-monotonic dependence on the Fermi energy. The well-defined electron-hole excitation boundaries and the higher frequency optical plasmons are transformed by varying the Fermi energy. There remain substantial differences between the electronic properties of trilayer AAB, ABC, AAA and ABA graphene stackings.Comment: 20 pages, 8 figures. arXiv admin note: text overlap with arXiv:1601.00223 by other author

Emergent scale invariance of non-classical plasmons in graphene nanoribbons

Using a nearest-neighbor tight-binding model we investigate quantum effects of plasmons on few-nanometer wide graphene nanoribbons, both for zigzag and armchair edge terminations. With insight from the Dirac description we find an emerging scale-invariant behavior that deviates from the classical model both for zigzag and armchair structures. The onset of the deviation can be related to the position of the lowest parabolic band in the band structure. Dirac theory is only valid in the parameter subspace where the scale invariance holds that relates narrow ribbons with high doping to wide ribbons with low doping. We also find that the edge states present in zigzag ribbons give rise to a blueshift of the plasmon, in contrast to earlier findings for graphene nanodisks and nanotriangles

Tuning localized plasmons in nanostructured substrates for surface-enhanced Raman scattering

Comprehensive reflectivity mapping of the angular dispersion of nanostructured arrays comprising of inverted pyramidal pits is demonstrated. By comparing equivalently structured dielectric and metallic arrays, diffraction and plasmonic features are readily distinguished. While the diffraction features match expected theory, localised plasmons are also observed with severely flattened energy dispersions. Using pit arrays with identical pitch, but graded pit dimensions, energy scaling of the localised plasmon is observed. These localised plasmons are found to match a simple model which confines surface plasmons onto the pit sidewalls thus allowing an intuitive picture of the plasmons to be developed. This model agrees well with a 2D finite-difference time-domain simulation which shows the same dependence on pit dimensions. We believe these tuneable plasmons are responsible for the surface-enhancement of the Raman scattering (SERS) of an attached layer of benzenethiol molecules. Such SERS substrates have a wide range of applications both in security, chemical identification, environmental monitoring and healthcare

Surface Plasmon Dispersion Relations in Chains of Metallic Nanoparticles: Exact Quasistatic Calculation

We calculate the surface plasmon dispersion relations for a periodic chain of spherical metallic nanoparticles in an isotropic host, including all multipole modes in a generalized tight-binding approach. For sufficiently small particles ($kd \ll 1$, where $k$ is the wave vector and $d$ is the interparticle separation), the calculation is exact. The lowest bands differ only slightly from previous point-dipole calculations provided the particle radius $a \lesssim d/3$, but differ substantially at smaller separation. We also calculate the dispersion relations for many higher bands, and estimate the group velocity $v_g$ and the exponential decay length $\xi_D$ for energy propagation for the lowest two bands due to single-grain damping. For $a/d=0.33$, the result for $\xi_D$ is in qualitative agreement with experiments on gold nanoparticle chains, while for larger $a/d$, such as $a/d=0.45$, $v_g$ and $\xi_D$ are expected to be strongly $k$-dependent because of the multipole corrections. When $a/d \sim 1/2$, we predict novel percolation effects in the spectrum, and find surprising symmetry in the plasmon band structure. Finally, we reformulate the band structure equations for a Drude metal in the time domain, and suggest how to include localized driving electric fields in the equations of motion.Comment: 19 pages 3 figures To be published in Phy. Rev.