Exciton-induced Fano resonance in metallic nanocavity with tungsten disulfide atomic layer.

Photon-exciton coupling behaviors in optical nanocavities attract broad attention due to their crucial applications in light manipulation and emission. Herein, we experimentally observed a Fano-like resonance with asymmetrical spectral response in an ultrathin metal-dielectric-metal (MDM) cavity integrated with an atomic-layer tungsten disulfide (WS2). The resonance wavelength of an MDM nanocavity can be flexibly controlled by adjusting dielectric layer thickness. The results measured by the home-made microscopic spectrometer agree well with the numerical simulations. A temporal coupled-mode theoretical model was established to analyze the formation mechanism of Fano resonance in the ultrathin cavity. The theoretical analysis reveals that the Fano resonance is attributed to a weak coupling between the resonance photons in the nanocavity and excitons in the WS2 atomic layer. The results will pave a new way for exciton-induced generation of Fano resonance and light spectral manipulation at the nanoscale.

Asymmetric nanocavities with wide reflection color gamut for color printing.

Symmetric metal-dielectric-metal (MDM) nanocavities based on Fabry-Perot resonance play a crucial role in transmission colors. However, their reflection color gamuts are generally limited owing to the narrow dip of resonance spectrum. In this work, we propose and fabricate symmetric titanium-indium tin oxide-silver (Ti/ITO/Ag) nanocavities to realize the reflection colors. The experimental and simulation results show that reflection color gamut of the asymmetric nanocavity is wider than that of symmetric MDM nanocavity due to the generation of broader resonance spectral dip. Moreover, a grayscale focused ion beam (FIB) etching method is employed to fabricate the thickness-controlled microstructures, and the etching depth satisfies a linear relationship with the gray value. The reflection color image can be observed by fabricating the ITO layer in the asymmetric MDM nanocavity with grayscale FIB etching method, which is more vivid than the image from fabricated symmetric MDM nanocavities. This work will provide a new way for color printing, color display, and ultra-small anti-counterfeiting technology.

Magnetic plasmon resonances in nanostructured topological insulators for strongly enhanced light-MoS2 interactions.

Magnetic resonances not only play crucial roles in artificial magnetic materials but also offer a promising way for light control and interaction with matter. Recently, magnetic resonance effects have attracted special attention in plasmonic systems for overcoming magnetic response saturation at high frequencies and realizing high-performance optical functionalities. As novel states of matter, topological insulators (TIs) present topologically protected conducting surfaces and insulating bulks in a broad optical range, providing new building blocks for plasmonics. However, until now, high-frequency (e.g. visible range) magnetic resonances and related applications have not been demonstrated in TI systems. Herein, we report for the first time, to our knowledge, a kind of visible range magnetic plasmon resonances (MPRs) in TI structures composed of nanofabricated Sb2Te3 nanogrooves. The experimental results show that the MPR response can be tailored by adjusting the nanogroove height, width, and pitch, which agrees well with the simulations and theoretical calculations. Moreover, we innovatively integrated monolayer MoS2 onto a TI nanostructure and observed strongly reinforced light-MoS2 interactions induced by a significant MPR-induced electric field enhancement, remarkable compared with TI-based electric plasmon resonances (EPRs). The MoS2 photoluminescence can be flexibly tuned by controlling the incident light polarization. These results enrich TI optical physics and applications in highly efficient optical functionalities as well as artificial magnetic materials at high frequencies.

Graphene-tuned EIT-like effect in photonic multilayers for actively controlled light absorption of topological insulators.

As newly emerging nanomaterials, topological insulators with unique conducting surface states that are protected by time-reversal symmetry present excellent prospects in electronics and photonics. The active control of light absorption in topological insulators are essential for the achievement of novel optoelectronic devices. Herein, we investigate the controllable light absorption of topological insulators in Tamm plasmon multilayer systems composed of a Bi1.5Sb0.5Te1.8Se1.2 (BSTS) film and a dielectric Bragg mirror with a graphene-involved defect layer. The results show that an ultranarrow electromagnetically induced transparency (EIT)-like window can be generated in the broad absorption spectrum. Based on the EIT-like effect, the Tamm plasmon enhanced light absorption of topological insulators can be dynamically tuned by adjusting the gate voltage on graphene in the defect layer. These results will pave a new avenue for the realization of topological insulator-based active optoelectronic functionalities, for instance light modulation and switching.

High-performance optical sensing based on electromagnetically induced transparency-like effect in Tamm plasmon multilayer structures.

We present a novel kind of optical sensor based on the electromagnetically induced transparency (EIT)-like effect in a Tamm plasmon multilayer structure, which consists of a metal film on a dielectric Bragg grating with alternatively stacked TiO2 and SiO2 layers and a defect layer. The defect layer can induce a refractive-index-sensitive ultranarrow peak in the broad Tamm plasmon reflection dip. This nonintuitive phenomenon in analogy to the EIT effect in atomic systems originates from the coupling and destructive interference between the defect and Tamm plasmon modes in the multilayer structure. Taking advantage of this EIT-like effect, we achieve an ultrahigh sensing performance with a sensitivity of 416 nm/RIU and a figure of merit (FOM) of 682 RIU-1. The numerical simulations agree well with the theoretical calculations. Additionally, the spectral line shape can be effectively tailored by changing the defect layer thickness, significantly promoting the dimensionless FOM from 0.76×104 to more than 2.4×104. Our findings will facilitate the achievement of ultrasensitive optical sensors in multilayer structures and open up perspectives for practical applications, especially in gas, biochemical, and optofluidic sensing.

Induced reflection in Tamm plasmon systems.

We present an induced reflection response analogue to electromagnetically induced transparency (EIT) in a novel Tamm plasmon system, consisting of a thin metal film and a Bragg grating with a defect layer. The results show that an induced narrow peak can be generated in the original broad reflection dip, which is attributed to the coupling and interference between the Tamm plasmon and defect modes in the grating structure. It is found that the EIT-like induced reflection is strongly dependent on the thickness of defect layer, grating period number between the metal and defect layers, thickness of Bragg grating layer, refractive index of defect layer, and thickness of metal film. Additionally, the induced reflection can be dynamically tuned by adjusting the angle of incident light. The numerical simulations agree extremely well with theoretical calculations. The coupling strength between the Tamm plasmon and defect modes is determined by the above parameters. These results will provide a new avenue for light field control and devices in multilayer photonic systems.