Plasmonic nanostructure design for enhanced IR modulation: wavelength-scale optical buffer and amplitude modulator

Abstract

Nowadays, infrared technology is attracting enormous attention in various industries and research fields with the advent of industry 4.0. In particular, infrared communication, stealth, LiDAR, and holograms are closely related to the development of military technology as well as the industry 4.0 technologies. For the advancement of infrared applications, the ability to control and modulate infrared light with high resolution must first be built. However, unlike electronic devices, optical devices have difficulties in miniaturization due to the diffraction limit of light, and have obvious limitations in terms of resolution and also practicality. To control and modulate the infrared light with high resolution that overcomes the diffraction limit, research and development on plasmonic devices with wavelength or sub-wavelength scale is necessary.In this paper, we first design a plasmonic waveguide that forms a non-Hermitian exceptional point without material gain or loss. Furthermore, we demonstrate the formation of a higher-order exceptional point by coalescence of two separate exceptional points in the parameter space of the plasmonic waveguide. The presented results show the alleviated conditions for the complex permittivity (or complex refractive index) which has been an obstacle to the applications in non-Hermitian optics, suggesting the possibility of using an optical buffer with wavelength or subwavelength scale based on the light trapping effect.Next, we suggest an active amplitude modulator in the near-infrared range using twisted bilayer graphene. The optical conductivity of the twisted bilayer graphene is designed to be suitable for near-infrared modulation, which is calculated from the periodic moiré superlattice depending on its twist angle. In this case, an active near-infrared modulation with high potential was confirmed at specific twist angles, while the modulation conditions can be eased by exciton-plasmon coupling. Furthermore, we design a twisted bilayer graphene-based meta-surface with a Salisbury screen that the significant near-infrared modulation is achieved with the gate voltage-dependent perfect absorption. The presented results reveal the possibility of an active near-infrared modulation in subwavelength scale by twist angle engineering in various stacked van der Waals structures, which can be widely expanded from visible to infrared wavelength range.Finally, we propose an active mid-infrared amplitude modulator using an ionic liquid. In the ionic liquid, water (H$_2$O) molecules are bound via hydrogen bonding with anions. The formed anion-H$_2$O clusters can be bound with (or dissociated from) the surface-bound H$_2$O layer on a hydrophilic surface by applying a voltage, while the frequency of OH stretching mode changes depending on whether they are bound or not. As a result, the electro-optic modulation in the mid-infrared range is achieved by a change in the frequency of OH stretching mode, and the modulation was further amplified through the design of a plasmonic pattern. The presented results show that an active amplitude modulation in a mid-infrared range can be simply achieved without complicated fabrication processes, and point out that ionic liquid can modulate mid-infrared light even above 3300 cm$^{-1}$ by itself on a hydrophilic surface, which is frequently used in the research of electronic and optical device.