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Register a new userA concentric plasmonic platform for the efficient excitation of surface plasmon-polaritons
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Added by
Christophe Couteau Asst Prof
Publication date
2016
fields
Physics
and research's language is
English
Authors
N. Rahbany
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We propose a plasmonic device consisting of a concentric ring grating acting as an efficient tool for directional launching and detection of surface plasmon-polaritons (SPPs). Numerical simulations and optical characterizations are used to study the fabricated structured gold surface. We demonstrate that this circularly symmetrical plasmonic device provides an efficient interface between free space radiation and SPPs. This structure offers an excellent platform for the study of hybrid plasmonics in general and of plasmon-emitter couplings in particular, such as those occurring when exciting dye molecules placed inside the ring. As illustrated in this work, an interesting property of the device is that the position of excitation determines the direction of propagation of the SPPs, providing a flexible mean of studying their interactions with molecules or dipole-like emitters placed on the surface.
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We study a surface plasmon polariton mode that is strongly confined in the transverse direction and propagates along a periodically nanostructured metal-dielectric interface. We show that the wavelength of this mode is determined by the period of the structure, and may therefore, be orders of magnitude smaller than the wavelength of a plasmon-polariton propagating along a flat surface. This plasmon polariton exists in the frequency region in which the sum of the real parts of the permittivities of the metal and dielectric is positive, a frequency region in which surface plasmon polaritons do not exist on a flat surface. The propagation length of the new mode can reach a several dozen wavelengths. This mode can be observed in materials that are uncommon in plasmonics, such as aluminum or sodium.
Surface plasmon polaritons in a strained slab of a Weyl semimetal with broken time-reversal symmetry are investigated. It is found that the strain-induced axial gauge field reduces frequencies of these collective modes for intermediate values of the wave vector. Depending on the relative orientation of the separation of Weyl nodes in momentum space, the surface normal, and the direction of propagation, the dispersion relation of surface plasmon polaritons could be nonreciprocal even in a thin slab. In addition, strain-induced axial gauge fields can significantly affect the localization properties of the collective modes. These effects allow for an in situ control of the propagation of surface plasmon polaritons in Weyl semimetals and might be useful for creating nonreciprocal devices.
The electrodynamics of Weyl semimetals (WSMs) is an extension of Maxwells theory where in addition to field strength tensor $F_{mu u}$, an axion field enters the theory which is parameterized by a four-vector $b^mu=(b_0,bf b)$. In the tilted Weyl matter (TWM) an additional set of parameters ${bfzeta}=(zeta_x,zeta_y,zeta_z)$ enter the theory that can be encoded into the metric of the spacetime felt by electrons in TWM. This allows an extension of Maxwells electrodynamics that describes electric and magnetic fields in TWMs and tilted Dirac material (TDM) when $b^mu=0$. The tilt parameter $bfzeta$ appearing as off-diagonal metric entries mixing time and space components mingles $bf E$ and $bf B$ fields whereby modifies the inhomogeneous Maxwells equations. Surface plasmon polariton (SPP) in these systems describes the propagation of electromagnetic waves at the {it interface of two different spacetime geometries}. In the case of TDM, we find a characteristic dependence of SPP spectrum on the tilt parameter $zeta$ which can be used map $zeta$ from SPP measurements. In the case of TWM, depending on whether the interface with vacuum supports a Fermi arc or not, and whether the propagation direction is along the Fermi arc or transverse to it, we find many unusual spectral features for SPP modes. Our detailed study of the dependence of SPP spectra on the arrangements of three vectors $(bf b, q,zeta)$, the first two of which are at our control, can be utilized to map the tilt characteristics and Fermi arc characteristics from SPP measurements.
We use mono-crystalline gold platelets with ultra-smooth surfaces and superior plasmonic properties to investigate the formation of interference patterns caused by surface plasmon polaritons (SPPs) with scattering-type scanning near-field microscopy (s-SNOM) at 521~nm and 633~nm. By applying a Fourier analysis approach, we can identify and separate several signal channels related to SPPs launched and scattered by the AFM tip and the edges of the platelet. Especially at the excitation wavelength of 633~nm, we can isolate a region in the center of the platelets where we find only contributions of SPPs which are launched by the tip and reflected at the edges. These signatures are used to determine the SPP wavelength of $lambda_{SPP}=606$ nm in good agreement with theoretical predictions. Furthermore, we were still able to measure SPP signals after 20~$upmu$m propagation, which demonstrates impressively the superior plasmonic quality of these mono-crystalline gold platelets.
We theoretically investigate surface plasmon polaritons propagating in the thin-film Weyl semimetals. We show how the properties of surface plasmon polaritons are affected by hybridization between plasmons localized at the two metal-dielectric interfaces. Generally, this hybridization results in new mixed plasmon modes, which are called short-range surface plasmons and long-range surface plasmons, respectively. We calculate dispersion curves of these mixed modes for three principle configurations of the axion vector describing axial anomaly in Weyl semimetals. We show that the partial lack of the dispersion and the non-reciprocity can be controlled by fine-tuning of the thickness of the Weyl semimetals, the dielectric constants of the outer insulators, and the direction of the axion vector.
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