Effects of mode mixing and avoided crossings on the transverse spin in a metal-dielectic-metal sphere

Abstract

We study transverse spin in a sub-wavelength metal-dielectric-metal (MDM) sphere when the MDM sphere exhibits avoided crossing due to hybridization of the surface plasmon with the Mie localized plasmon. We show that the change in the absorptive and dipersive character near the crossing can have significant effect on the transverse spin. An enhancement in the transverse spin is shown to be possible associated with the transparency (suppression of extinction) of the MDM sphere. The effect is attributed to the highly structured field emerging as a consequence of competition of the electric and magnetic modes.Keywords: Scattering, Mie theory, Avoided crossing, Surface plasmons, Transverse spin Light-matter interaction in the strong coupling regime resulting in vacuum-field Rabi splittings has been one of the central themes in cavity quantum electrodynamics [1][2][3][4][5]. The coupling occurs when the dispersion branches of the uncoupled systems cross and results in the avoided crossing phenomenon and normal mode splitting. The resulting physics can have interesting implications for various applications ranging from fast and slow light [6,7] and optical sensing [8] to counting and sizing of nanoparticles [9]. The avoided crossing phenomena have been observed in a variety of optical as well as condensed matter systems. These include planar or spherical plasmonic or guided wave structures, metamaterial cavities, in photonic crystal fibers [8,[10][11][12], etc. Contrary to the belief that the split modes can be resolved only with high-finesse optical modes, recent focus on systems with substantial losses (eg. plasmonic nanocavities, and leaky cavities) demonstrated the avoided crossing phenomena [13][14][15] in such systems. Our interest in the strongly coupled systems is motivated by the fact that there is a significant change in the dispersive and absorptive properties near the avoided crossing caused by mode mixing and exchange [16][17][18]. This could result in a highly structured field which is essential for observing yet another important recent discovery, namely, the transverse spin [19] in optical systems [20][21][22].It is well known that the linear momentum (P) carried by light waves can be decomposed into orbital (P o ) and spin (P s ) parts [23,24]. The orbital momentum (P o ) is the so-called canonical momentum of light which is responsible for the energy transport and radiation pressure. The spin momentum, on the other hand, has long been known as a 'virtual' entity which does not transport energy or exerts pressure on the dipolar particle and is only responsible for generating the spin angular momentum (SAM) (P s ≈ ∇ × S) [19]. However, recent studies have shown that

Spectral singularity enhances transverse spin

“…A comparison of these panels clearly reveal the rotation of the electric field vector in the plane of incidence (x − z plane). In fact, this circulation leads to the transverse spin along the y direction in full conformity with the earlier results [4,24].…”

“…Recall that for the typical parameters used in our system one of the spectral singularities occurs for a 0 = 0.062045958076 µm, d 0 = 1.004358293913 µm and ω/ω 0 = 1. Rotation of the field vector has been identified as the source of transverse spin and spin momentum [4,24]. Analogous circulation is shown in Fig.…”

“…One of the major directions that emerge is how to enhance the tiny Belinfante momentum. Strong coupling mediated dispersion management was proposed to be one of the possible ways [24] to enhance the transverse spin. Coherent perfect absorption [25,26] was shown to further enhance the effects [27].…”

Manipulating the transverse spin angular momentum and Belinfante momentum of spin-polarized light by a tilted stratified medium in optical tweezers

“…This novel type of SOI dealing with the transverse angular momentum (both SAM and OAM) and spin-dependent transverse BSM of light has led to observation of remarkable effects in spin-controlled directional coupling between circularly polarized incident light beams and transversely propagating surface modes in nano-fibers, metal surfaces, waveguides etc. [13][14][15][16]. This has opened up exciting new opportunities for the development of spin orbit photonic nano devices for control and manipulation of light at nanometer length scales.…”

Towards the development of new generation spin-orbit photonic techniques

“…From the fundamental point of view, coupling and interconversion between the spin and OAM degrees of freedom of light are thus expected under certain circumstances. A number of non-trivial spin-orbit optical phenomena like spin and orbital Hall effect of light, optical Rashba effect, photonic Aharonov-Bohm effect, rotational Doppler effect, transverse spin, Belinfante's spin-momentum and spin-momentum locking in optical fields etc have recently been observed in diverse micro, and nanoscale optical system [15][16][17][18][19]. These have created a new area in photonics, namely, the spinorbit photonics that not only deals fundamentally with lightmatter interaction effects but has also opened up the possibility of the development of a new generation of miniaturized and on-chip integrable multifunctional photonic devices based on the angular momentum and geometrical phase of light.…”