Active directional switching of surface plasmon polaritons using a phase transition material

Plasmonic switches based on VO2as the phase change material

In this paper, a comprehensive review of the recent advancements in the design and development of plasmonic switches based on vanadium dioxide (VO2) is presented. Plasmonic switches are employed in applications such as integrated photonics, plasmonic logic circuits and computing networks for light routing and switching, and are based on the switching of the plasmonic properties under the effect of an external stimulus. In the last few decades, plasmonic switches have seen a significant growth because of their ultra-fast switching speed, wide spectral tunability, ultra-compact size, and low losses. In this review, first, the mechanism of the semiconductor to metal phase transition in VO2is discussed and the reasons for employing VO2over other phase change materials for plasmonic switching are described. Subsequently, an exhaustive review and comparison of the current state-of-the-art plasmonic switches based on VO2proposed in the last decade is carried out. As the phase transition in VO2can be activated by application of temperature, voltage or optical light pulses, this review paper has been categorized into thermally-activated, electrically-activated, and optically-activated plasmonic switches based on VO2operating in the visible, near-infrared, infrared and terahertz frequency regions.

Femtosecond photoemission electron microscopy of surface plasmon polariton beam steering via nanohole arrays

Directional control over surface plasmon polariton (SPP) waves is a prerequisite for the development of miniaturized optical circuitry. Here, the efficacy of single and dual component SPP steering elements is explored through photoemission electron microscopy. Our imaging scheme relies on two-color photoemission and counter-propagating SPP generation, which collectively allow SPPs to be visualized in real space. Wave-vector difference mixing between the two-dimensional nanohole array and photon momenta enables SPP steering with directionality governed by the array lattice constant and input photon direction. In our dual component configuration, separate SPP generation and Bragg diffraction based steering optics are employed. We find that array Bragg planes principally influence the SPP angles through the array band structure, which allows us to visualize both positive and negative refractory waves.

Reconfigurable all-dielectric Fano metasurfaces for strong full-space intensity modulation of visible light

Dynamically reconfigurable nanoscale tuning of visible light properties is one of the ultimate goals both in the academic field of nanophotonics and the optics industry demanding compact and high-resolution display devices. Among various efforts incorporating actively reconfigurable optical materials into metamaterial structures, phase-change materials have been in the spotlight owing to their optical tunability in wide spectral regions including the visible spectrum. However, reconfigurable modulation of visible light intensity has been limited with small modulation depth, reflective schemes, and a lack of profound theoretical background for universal design rules. Here, all-dielectric phase-change Fano metasurface gratings are demonstrated for strong dynamic full-space (reflection and transmission) modulation of visible intensities based on Fano resonances. By judicious periodic couplings between densely arranged meta-atoms containing VO₂, phase-change induced thermo-optic modulation of full-space intensities is highly enhanced in the visible spectrum. By providing intuitive design rules, we envision that the proposed study would contribute to nanophotonics-enabled optoelectronics technologies for imaging and sensing.

Large-angle beaming from asymmetric nanoslit-corrugation structures

The beaming effect in single apertures surrounded by periodic corrugations and the manipulation of beaming directions from such structures has gained considerable attention since discovery. Different materials and structural profiles have been studied in this context but directional beaming at angles larger than 45° has not been achieved. We design and demonstrate nanoslits in a gold film flanked by corrugations, which give rise to beaming angles ranging from 45° to 60°. While the previous designs are based on achieving constructive interference at the aimed beaming angle, our approach complements such constructive interference with destructive interference at 0° and, as a result, enhances the directional beaming effect at angles larger than 45°. The structures are fabricated by electron beam lithography with two consecutive lift-off processes. The experimental far-field intensity distributions agree well with the designs.

Ultracompact meta-pixels for high colour depth generation using a bi-layered hybrid metasurface

Nano-structural colour pixels have attracted much attention as promising solutions for compact display devices. However, it is difficult to miniaturize and integrate conventional transmissive colour filtering components for high resolution pixels within subwavelength scale without sacrificing colour depth. Here, we propose a novel colour pixel structure using bi-layered hybrid metasurfaces that are composed of aluminum nanograting and amorphous silicon nanorod layers. The independent high-contrast control of resonance intensity and spectral position is achieved by anisotropic Mie resonances and cavity effect between stacked two metasurfaces. Moreover, the proposed structures permit the wide colour gamut through the novel physical principles. In addition, a meta-pixel which can provide gradual tuning of colour is demonstrated to obtain high colour depth. The proposed structures are expected to be fruitful for the development of next generation display and imaging systems.

Broadband efficient modulation of light transmission with high contrast using reconfigurable VO2 diffraction grating

Ultra-compact dynamically reconfigurable modulation of optical transmission has been widely studied by using subwavelength-spaced resonant metasurface structures containing reconfigurable optical materials. However, it has been difficult to achieve high transmissivity, large modulation depth, and broad bandwidth simultaneously with the conventional resonance-based metasurface schemes. Here, we propose a reconfigurable phase-transition diffractive grating, made of thick VO₂ ridge waveguides, for achieving the above-mentioned three goals simultaneously in the near-infrared range. Based on the large dielectric-to-plasmonic transition characteristic of VO₂ in the near-infrared range, diffraction directivity of dual-VO₂ ridge waveguide is designed to be tuned by thermally driven phase transition of VO₂ for transverse electrically polarized illumination. Then, the diffractive VO₂ ridge waveguide grating composed of the periodically arranged dual VO₂ ridge waveguides is designed with on-state efficiency around 0.3 and minimum modulation depth about 0.35 over a broad bandwidth of 550 nm (1100-1650 nm). The working principle and excellent modulation performance are thoroughly verified through numerical and experimental studies.

Electrically tunable multifunctional metasurface for integrating phase and amplitude modulation based on hyperbolic metamaterial substrate

Active metasurfaces, which are tunable and reconfigurable nanophotonic structures with active materials, have been in spotlight as a versatile platform to control the profiles of scattered light. These nanoscale structures show surpassing functionalities compared to the conventional metasurfaces. They also play an important role in a wide range of applications for imaging, sensing, and data storage. Hence, the expansion of functionalities has been highly desired, in order to overcome the limited space constraints and realize the integration of several optical devices on a single compact platform. In this context, an electrically tunable metasurface that enables respective modulation of the phase and amplitude of reflected light, depending on the angle of incidence at the targeted wavelength, is proposed. This resonance-based device with hyperbolic metamaterial substrate excites different kinds of highly confined modes, according to the incident angle. Indium tin oxide is employed to offer electrically tunable optical properties in the near-infrared regime. At the wavelength of 1450 nm, the proposed device modulates the phase of reflected light with ~207 degrees of modulation depth for normal incidence, whereas it shows ~86% of relative reflectance change for oblique incidence of 60 degrees. In principle, the proposed scheme might provide a path to applications for the next-generation ultracompact integrated systems.

Switching of the direction of reflectionless light propagation at exceptional points in non-PT-symmetric structures using phase-change materials

We introduce a non-parity-time-symmetric three-layer structure, consisting of a gain medium layer sandwiched between two phase-change medium layers for switching of the direction of reflectionless light propagation. We show that for this structure unidirectional reflectionlessness in the forward direction can be switched to unidirectional reflectionlessness in the backward direction at the optical communication wavelength by switching the phase-change material Ge₂Sb₂Te₅ (GST) from its amorphous to its crystalline phase. We also show that it is the existence of exceptional points for this structure with GST in both its amorphous and crystalline phases which leads to unidirectional reflectionless propagation in the forward direction for GST in its amorphous phase, and in the backward direction for GST in its crystalline phase. Our results could be potentially important for developing a new generation of compact active free-space optical devices.

Metasurfaces Based on Phase-Change Material as a Reconfigurable Platform for Multifunctional Devices

Integration of phase-change materials (PCMs) into electrical/optical circuits has initiated extensive innovation for applications of metamaterials (MMs) including rewritable optical data storage, metasurfaces, and optoelectronic devices. PCMs have been studied deeply due to their reversible phase transition, high endurance, switching speed, and data retention. Germanium-antimony-tellurium (GST) is a PCM that has amorphous and crystalline phases with distinct properties, is bistable and nonvolatile, and undergoes a reliable and reproducible phase transition in response to an optical or electrical stimulus; GST may therefore have applications in tunable photonic devices and optoelectronic circuits. In this progress article, we outline recent studies of GST and discuss its advantages and possible applications in reconfigurable metadevices. We also discuss outlooks for integration of GST in active nanophotonic metadevices.