Jiawei Cong

Broadband visible-light absorber via hybridization of propagating surface plasmon.

We demonstrate a broadband visible-light absorber based on excitation of multiple propagating surface plasmon (PSP) resonances. The simple structure is constructed of continuous gold/silica multi-layers covered by a one-dimensional gold grating. The broadening of bandwidth arises from the inter-layer hybridization and spectral superposition of PSPs, which is predicted with the analytical coupled oscillator model and validated using the RCWA simulation. The average absorption increases with the number of gold/silica pairs and exceeds 95% over the whole visible spectrum when only five pairs are included. Moreover, results show that the absorption can be further enhanced by grading the thickness of silica layers. The presented design might enable promising applications in the fields of photovoltaic cells and thermal emitters, owing to its advantages of wideband, near-unity absorption and simple fabrication simultaneously.

Angle-insensitive and narrow band grating filter with a gradient-index layer.

We demonstrate the design of an efficient angle-insensitive guided mode resonance filter (GMRF), with narrow bandwidth and low sideband reflection, for TE-polarized waves. The reflection properties of the multilayer structure have been studied, and the results verify that the thin film design of the gradient-index layer is important for the realization of an angle-insensitive filter. Various gradient coefficients of the thin film have distinct effects on the reflection spectrum. For an increasing incident angle, although the line-shape symmetry becomes less perfect, the positions of the resonant peak remain the same. The GMRF proposed here has many desirable attributes that lends itself to being an excellent platform, for devices such as lasers, detectors, filters, and sensors.

Angularly dense comb-like enhanced absorption of graphene monolayer with attenuated-total-reflection configuration

A multiline absorber based on the excitation of guided-mode resonance of one-dimensional photonic crystals (1D-PhCs), including a surface graphene monolayer under the attenuated-total-reflection configuration, is proposed and demonstrated. By carefully designing the structure parameters of the 1D-PhCs, the guided mode can be modulated by the periodic distribution of the refractive index. Our results reveal that the critical coupling of the guided resonance in periodical PhCs to graphene produces the perfect absorption. The number of absorption peaks within the photonic band corresponds to the number of unit cells. An ultrahigh Q-factor value of 4.75×106 is obtained at resonance with unity absorption, which could serve as a promising replacement of metallic thin film as a sensor probe for future biosensing applications.

Polarization-insensitive and wide-incident-angle optical absorber with periodically patterned graphene-dielectric arrays

A polarization-insensitive and angle-independent graphene absorber (GA) with periodically patterned grating is demonstrated. A periodic nanocavity composed of multilayer subwavelength grating and metal substrate supports a strongly localized mode inside the cavity, where the mode helps to absorb more electromagnetic waves. The proposed GA exhibits polarization-insensitive behavior and maintains the high absorption above 90% within a wide range of incident angle (more than 80°). We attribute the high absorption to the excitation of the cavity mode resonance and magnetic resonance for the transverse electric and transverse magnetic polarizations, respectively. The proposed GA has potential applications in the design of various devices, such as optical modulators or tunable absorption filters because of its remarkable angle-insensitive absorption performance.

High-resolution surface plasmon resonance sensor with Fano resonance in waveguide-coupled multilayer structures

An ultra-high resolution refractive-index sensor with the Kretschmann configuration was proposed and experimentally demonstrated. The Fano resonance (FR) in the attenuated total reflection curve arose from the interactions between the surface plasmon polariton and planar waveguide modes. It was shown to depend strongly on the structural parameters that governed the position of the FR and to be in good agreement with the results of electromagnetic calculations. The sensitivity by intensity was estimated to be 3.56 × 102-fold higher than that of conventional surface plasmon resonance sensors.

Reducing the pump power of optically controlled terahertz metamaterial via tailoring the resistance of the silicon gap region

Optically tunable metamaterials provide an ultrafast and active manipulation of terahertz wave. We demonstrate a strategy to alleviate the tradeoff between the requirement of low optical pump power and the achievement of significant resonance modulation in the photoexcited metamaterials. We have shown that the resonance strength of split-ring resonator metamaterial is determined by the resistance of its silicon gaps. By reducing the resistance through geometry adjustment of the gap region, the needed photoconductivity and hence the pump power can be substantially reduced without deteriorating the resonance tuning effect. The presented design rule may offer an avenue to realize low-power optically tunable terahertz devices.

Rod-shaped nanomotor powered by magnetic field gradients and its application to surface-enhanced Raman-scattering-based detection

Magnetically powered nanomotors have received increasing attention for decades because of their diverse potential applications. Here, we demonstrate a rod-shaped Au–Ni bimetallic nanomotor that can be powered by the gradients of applied magnetic fields. Efficient and steady control of locomotion over the Brownian motion is achieved by modulating the magnetic field and tailoring the length of Ni and Au segments. The nanomotors are ideal for use as a dynamic surface-enhanced Raman scattering (SERS) sensor platform, in that the average enhancement is up to almost one order of magnitude higher than that in a conventional nanostructured Au substrate, thus providing a promising alternative for real-time biochemical sensing.

Broadening of absorption band by coupled gap plasmon resonances in a near-infrared metamaterial absorber

We propose a strategy to broaden the absorption band of the conventional metamaterial absorber by incorporating alternating metal/dielectric films. Up to 7-fold increase in bandwidth and ~95% average absorption are achieved arising from the coupling of induced multiple gap plasmon resonances. The resonance coupling is analytically demonstrated using the coupled oscillator model, which reveals that both the optimal coupling strength and the resonance wavelength matching are required for the enhancement of absorption bandwidth. The presented multilayer design is easily fabricated and readily implanted to other absorber configurations, offering a practical avenue for applications in photovoltaic cells and thermal emitters.

Magnetically Powered Annelid-Worm-Like Microswimmers

A bioinspired magnetically powered microswimmer is designed and experimentally demonstrated by mimicking the morphology of annelid worms. The structural parameters of the microswimmer, such as the surface wrinkling, can be controlled by applying prestrain on substrate for the precise fabrication and consistent performance of the microswimmers. The resulting annelid-worm-like microswimmers display efficient propulsion under an oscillating magnetic field, reaching a peak speed of ≈100 µm s-1 . The speed and directionality of the microswimmer can be readily controlled by changing the parameters of the field inputs. Additionally, it is demonstrated that the microswimmers are able to transport microparticles toward a predefined destination, although the translation velocity is inevitably reduced due to the additional hydrodynamic resistance of the microparticles. These annelid-worm-like microswimmers have excellent mobility, good maneuverability, and strong transport capacity, and they hold considerable promise for diverse biomedical, chemical sensing, and environmental applications.