Many Applications In Nanophotonics Research Articles

Twisted Nanotubes of Transition Metal Dichalcogenides with Split Optical Modes for Tunable Radiated Light Resonators

AbstractSynthesized micro‐ and nanotubes composed of transition metal dichalcogenides (TMDCs) such as MoS2 are promising for many applications in nanophotonics because they combine the abilities to emit strong exciton luminescence and to act as whispering gallery microcavities even at room temperature. In addition to tubes in the form of hollow cylinders, there is an insufficiently studied class of twisted tubes, the flattened cross‐section of which rotates along the tube axis. As shown by theoretical analysis, in such nanotubes the interaction of electromagnetic waves excited at the opposite sides of the cross‐section can cause splitting of the whispering gallery modes. By studying micro‐photoluminescence spectra measured along individual MoS2 tubes, it has been established that the splitting value, which controls the energies of the split modes, depends exponentially on the aspect ratio of the cross‐section, which varies in “breathing” tubes, while the relative intensity of the modes in a pair is determined by the angle of rotation of the cross‐section. These results open up the possibility of creating multifunctional tubular TMDC nanodevices that provide resonant amplification of self‐emitting light at adjustable frequencies.

Resonance coupling between chiral quasi-BICs and achiral molecular excitons in dielectric metasurface J-aggregate heterostructures.

The realization of flexible tuning and enhanced chiral responses is vital for many applications in nanophotonics. This study proposes to manipulate the collective optical responses with heterostructures consisting of chiral dielectric metasurfaces and achiral J-aggregates. Owing to the resonance coupling between the chiral quasi-bound states in the continuum (QBICs) and the achiral exciton mode, large mode splitting and anticrossing are observed in both the transmission and circular dichroism (CD) spectra, which indicates the formation of hybrid chiral eigenmodes and the realization of the strong coupling regime. Considering that the radiative and dissipative damping of the hybrid eigenmodes depends on the coherent energy exchange, the chiral resonances can be flexibly tuned by adjusting the geometry and optical constants for the heterostructure, and the CD of the three hybrid eigenmodes approach the maximum (∼1) simultaneously when the critical coupling conditions are satisfied, which can be promising for enhanced chiral light-matter interactions.

Directional excitation of surface plasmon using multi-mode interference in an aperture

Plasmonics is a promising technology that can find many applications in nanophotonics and biosensing. Local excitation of surface plasmons with high directionality is required for many of these applications. We demonstrate that by controlling the interference of light in a metal slot with the adjustment of the angle of incidence, it is possible to achieve highly directional surface plasmon excitation. Our numerical analysis of the structure showing a strong directionality of excited surface plasmon is confirmed by near field scanning measurements. The proposed structure can be useful for many applications including excitation of plasmonic waveguides, nanolithography, and optical sensing. To illustrate its usefulness, we experimentally demonstrate that it can be used for highly directional excitation of a dielectric loaded plasmonic waveguide. We also propose a simple structure for surface plasmon interference lithography capable of providing high image contrast using this scheme.

Study of electric field vector, angular momentum conservation and Poynting vector of nonparaxial beams

The angular momentum (AM) of light, comprising spin and orbital AMs, is conserved and produces a spin-Hall shift in this process for paraxial beams. For nonparaxial beams, the spin and orbital AMs are non-separable and produce many changes in the beams’ spatial profile contrary to paraxial beams. These changes can be manifested as polarization modulation in the transverse plane, and conversion to orbital angular momentum (OAM) structured beams in the transverse and longitudinal planes, which can be estimated by studying the electric field vector in detail. We have calculated theoretically and simulated numerically the electric field vector components in the focal plane, to study the polarization modulation and AM conservation for OAM and Gaussian light beams of circular and linear polarizations and compared the results. Further, we have calculated and simulated the Poynting vector components for the corresponding fields to study the energy flow. We have considered the focusing of light beams using a high Numerical Aperture objective lens to obtain the nonparaxial beam, and presented a detailed theoretical analysis therein. The interpretation studies presented here are new, which may have many applications in nanophotonics and help in understanding the spin–orbit interaction at the fundamental level.

Hyperbolic surface waves on anisotropic materials without hyperbolic dispersion.

We theoretically analyze directional surface electromagnetic waves supported at an interface between an isotropic medium and anisotropic metal with effective uniaxial negative permittivity. We identify two types of surface wave solutions, resulting in unique hyperbolic dispersion in the wavevector space. Such anisotropic metal can be realized by alternating dielectric and metallic layers with deep subwavelength thicknesses or metallic nanowires in dielectric host. Such systems serve as a platform for many applications in nanophotonics.

Long optical coherence times of shallow-implanted, negatively charged silicon vacancy centers in diamond

The creation of single, negatively charged silicon vacancy (SiV−) centers in well-defined diamond layers close to the host surface is a crucial step for the development of diamond-based quantum optic devices with many applications in nanophotonics, quantum sensing, or quantum information science. Here, we report on the creation of shallow (10 nm below the surface), single SiV− centers in diamond using low energy Si+ ion implantation with subsequent high temperature annealing at 1500 °C. We show transition linewidths down to 99 MHz and narrow inhomogeneous distributions. Furthermore, we achieved a reduction of homogeneous linewidths by a factor of 2 after removing subsurface damage using oxygen plasma processing. These results not only give insights into the formation process of SiV− centers but also indicate a favorable processing method to fabricate shallow single quantum emitters in diamond perfectly suited for coupling to nanostructures on the diamond surface.

Polymer dispersed liquid crystal-mediated active plasmonic mode with microsecond response time

Active plasmonics combined with liquid crystal (LC) has found many applications in nanophotonics. In this Letter, we propose a fast response active plasmonic device based on the interplay of the plasmonic spectrum and Fabry-Perot (FP) modes. The plasmonic spectrum and FP modes are excited in a layer of gold nanoparticle (NP) islands and an LC microcavity, respectively. The FP mode splits the extinction spectrum of the NP to narrow bands, which are named hybrid modes (HMs). Due to multiple reflections of photons inside the cavity, the extinction coefficient is enhanced compared to a bare NP layer. An external electric field shifts the HM leading to a significant increase in the figure of merit (FoM) related to the activation ability by up to a factor of 45. Additionally, we could reduce the response time of active plasmonics. This decrease in response time is achieved through polymer-dispersed LC (PDLC) in the microcavity. Utilizing a mesogenic monomer in PDLC reduces the response time of the HM into the microsecond range, while the sample remains transparent.

Asymmetric Metasurfaces with High-Q Resonances Governed by Bound States in the Continuum.

We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-Q resonances arising from a distortion of symmetry-protected bound states in the continuum. We develop a rigorous theory of such asymmetric periodic structures and demonstrate a link between the bound states in the continuum and Fano resonances. Our results suggest the way for smart engineering of resonances in metasurfaces for many applications in nanophotonics and metaoptics.

High Q-factor with the excitation of anapole modes in dielectric split nanodisk arrays.

The simultaneous realization of high Q-factor resonances and strong near-field enhancements around and inside of dielectric nanostructures is important for many applications in nanophotonics. However, the incident fields are often confined within dielectric nanoparticles, which results in poor optical interactions with external environment. Near-field enhancements can be extended outside of dielectric nanostructures with proper design, but the Q-factor is often reduced caused by additional radiation losses. This paper shows that the obstacles to achieve high Q-factor, that is, the radiative losses can be effectively suppressed by using dielectric nanodisk arrays, where the Q-factor is about one order larger than that of the single disks associated with the nonradiating anapole modes and the collective oscillations of the arrays. When the resonance energies of the electric dipole mode and the subradiant mode are degenerate with each other, the destructive interference produces an effect analogous to electromagnetically induced transparency. Furthermore, the Q-factor can be extremely enlarged with dielectric split nanodisk arrays, where the present of the split gap does not induce additional losses. Instead, the coupling between the two interfering modes is modified by adjusting the gap width, which makes it possible to achieve high Q-factor and strong near-field enhancements around and inside of the split disks simultaneously. It is shown that the Q-factor is approaching to 106 when the gap width is about 110 nm, and the near-field enhancements around and inside of the split disks are about two orders stronger than that of the single disk.

Tunable graphene-based hybrid plasmonic modulators for subwavelength confinement

Electro-optical modulators which work at the near-infrared range are significant for a variety of applications such as communication and sensing. However, currently available approaches result in rather bulky devices which suffer from low integration and can hardly operate at low power consumption levels. Graphene, an emerging advanced material, has been widely utilized due to its tunability by gating which allows one to realize active optical devices. Plasmonic waveguides, one of the most promising candidates for subwavelength optical confinement, provide a way to manipulate light on scales much smaller than the wavelength. In this paper, we combine the advantages of graphene and plasmonic waveguides and propose a tunable graphene-based hybrid plasmonic modulator (GHPM). Considering several parameters of the GHPM, the modulation depth can reach approximately 0.3 dB·μm−1 at low gating voltages. Moreover, we combine GHPM with metal-insulator-metal (MIM) structure to propose another symmetrical GHPM with a modulation depth of 0.6 dB·μm−1. Our modulators which utilize the light-matter interaction tuned by electro-doped graphene are of great potential for many applications in nanophotonics.

Tailoring the energy distribution and loss of 2D plasmons

The ability to tailor the energy distribution of plasmons at the nanoscale has many applications in nanophotonics, such as designing plasmon lasers, spasers, and quantum emitters. To this end, we analytically study the energy distribution and the proper field quantization of 2D plasmons with specific examples for graphene plasmons. We find that the portion of the plasmon energy contained inside graphene (energy confinement factor) can exceed 50%, despite graphene being infinitely thin. In fact, this very high energy confinement can make it challenging to tailor the energy distribution of graphene plasmons just by modifying the surrounding dielectric environment or the geometry, such as changing the separation distance between two coupled graphene layers. However, by adopting concepts of parity-time symmetry breaking, we show that tuning the loss in one of the two coupled graphene layers can simultaneously tailor the energy confinement factor and propagation characteristics, causing the phenomenon of loss-induced plasmonic transparency.