Xiang Zhai

Theoretical Investigation of Broadband and Wide-Angle Terahertz Metamaterial Absorber

Broadband absorbers have attracted considerable attention due to their great prospect for practical applications. The mechanism is usually a superposition of several sets of structures with different geometrical dimensions. Herein, we numerically investigate an unconventional to existing metamaterial-based broadband terahertz absorber based on the multilayer same-sized square plate structure. Greater than 99% absorption across a frequency range of 300 GHz with the central frequency ~ 1.96 THz can be obtained. The FWHM of this device can be up to 42% (with respect to the central frequency), which is 2.6 times greater than that of the single layer structure. Such a property is retained well at a very wide range of incident angles. The mechanism of the broadband absorber is attributed to longitudinal coupling between layers. The results of the designed metamaterial absorber appear to be very promising for solar cell, detection, and imaging applications.

A novel dual-band terahertz metamaterial absorber for a sensor application

We present a new type of dual-band terahertz metamaterial absorber formed by a patterned metallic strip and a dielectric layer on top of a metallic ground plane. It is found that besides a strong absorption in the fundamental resonance, a prominent high-order resonance with near-unity absorption is also unveiled. The origin of the induced dual-band absorption was elucidated. Importantly, the quality factor (Q) and the figure of merit (FOM) of the high-order resonance are 8.4 and 22.7 times larger than that of the fundamental resonance, respectively, which makes the proposed absorber to have significant potential in biological monitoring and sensing. Moreover, we demonstrate a dual-band and insensitive for two orthogonal polarizations terahertz absorber based on a metallic cross and a metallic ground plane separated by a dielectric layer. The Q and FOM of the high-order resonance are still larger than that of the fundamental resonance. The proposed absorbers appear to be very promising for solar cells, detection, and imaging applications.

Investigation of the graphene based planar plasmonic filters

We investigate numerically the edge modes supported by graphene ribbons and the planar band-stop filter consisting of a graphene ribbon lateral coupled a graphene ring resonator by using the finite-difference time-domain method. Simulation results reveal that the edge modes can enhance the electromagnetic coupling between objects indeed and this structure realizes perfect, tunable filtering effect. Successively, the channel-drop filter is constructed. Especially, the proposed structures can be designed and the size of the ring is changed by creating non-uniform conductivity patterns on monolayer graphene. Our studies will benefit the fabrication of the planar, ultra-compact devices in the mid-infrared region.

Frequency Continuous Tunable Terahertz Metamaterial Absorber

Metamaterial-based perfect absorbers utilize the intrinsic loss, with the aid of appropriate structural design (completely suppress transmission and reflection), to achieve near unity absorption at a certain frequency. The frequency of the reported absorbers is usually fixed and operates over a limited bandwidth, which greatly hampers their practical applications. Active or dynamic control over their resonance frequency is urgently necessary. Herein, we propose a novel approach for efficient tuning of the frequency of the absorber by shifting the movable part of the composite structure composed of the fixed and movable parts. The concept is rather general and applicable to various absorbers as long as the sandwich structure design is valid. The demonstrated continuous tuning of metamaterial absorber can find practical applications in detection, imaging, spectroscopy and selective thermal emitters.

Frequency tunable metamaterial absorber at deep-subwavelength scale

Metamaterial-based absorbers utilize the intrinsic loss, with the aid of appropriate structure design, to achieve near unity absorption at a certain frequency. The frequency of the reported absorbers is usually fixed and operates over a limited bandwidth, which greatly hampers their practical applications. Active or dynamic control over their resonance frequency is urgently necessary. Herein, we theoretically present a novel frequency tunable terahertz metamaterial absorber formed by a square metallic patch and a ground plane separated by a strontium titanate dielectric layer. Up to 80.2% frequency tuning is obtained by changing the temperature of the absorber, and there is very little variation in the strength of the absorption. The frequency shift is attributed to the temperature-dependent refractive index of the dielectric layer. Furthermore, the ratio between the lattice period and the resonance wavelength is close to 1/36 at 0.111 THz, which is smaller than the previously reported results. The proposed absorber has potential applications in detection, sensors, and selective thermal emitters.

Design of a Four-Band and Polarization-Insensitive Terahertz Metamaterial Absorber

We present a four-band and polarization-insensitive terahertz metamaterial absorber formed by four square metallic rings and a metallic ground plane separated by a dielectric layer. It is found that the structure has four distinctive absorption bands whose peaks are over 97% on average. The mechanism of the four-band absorber is attributed to the overlapping of four resonance frequencies, and the mechanism of the absorption is investigated by the distributions of the electric field. In particular, the frequency of each absorption peak can be flexibly controlled by varying the size of the corresponding metallic ring. The proposed concept is applicable to other types of absorber structures and can be readily scaled up to the structures that are working in the microwave frequency range. Moreover, the characteristic of the design can be used to design a five-band metamaterial absorber by adding one more metallic ring. The proposed absorber has potential applications in detection, imaging, and stealth technology.

Dynamically tunable plasmonically induced transparency in sinusoidally curved and planar graphene layers.

To achieve plasmonically induced transparency (PIT), general near-field plasmonic systems based on couplings between localized plasmon resonances of nanostructures rely heavily on the well-designed interantenna separations. However, the implementation of such devices and techniques encounters great difficulties mainly to due to very small sized dimensions of the nanostructures and gaps between them. Here, we propose and numerically demonstrate that PIT can be achieved by using two graphene layers that are composed of a upper sinusoidally curved layer and a lower planar layer, avoiding any pattern of the graphene sheets. Both the analytical fitting and the Akaike Information Criterion (AIC) method are employed efficiently to distinguish the induced window, which is found to be more likely caused by Autler-Townes splitting (ATS) instead of electromagnetically induced transparency (EIT). Besides, our results show that the resonant modes cannot only be tuned dramatically by geometrically changing the grating amplitude and the interlayer spacing, but also by dynamically varying the Fermi energy of the graphene sheets. Potential applications of the proposed system could be expected on various photonic functional devices, including optical switches, plasmonic sensors.

Combined theoretical analysis for plasmon-induced transparency in integrated graphene waveguides with direct and indirect couplings

By taking a graphene nanoribbon as a resonator, we have numerically and analytically investigated the spectral characteristics of plasmon-induced transparency in integrated graphene waveguides. For the indirect coupling, the formation and evolution of the transparency window are determined by the excitation of the super resonances, as well as by the destructive interference and the coupling strength between the two resonators, respectively, while for the indirect coupling, the peak transmission and corresponding quality factor can be dynamically tuned by adjusting the Fermi energy of graphene nanoribbons and the transparency peak shifts periodicity with the round-trip phase accumulated in the graphene waveguide region. Analytical results based on temporal coupled mode theory (CMT) show good consistence with the numerical calculations. Our findings may support the design of ultra-compact plasmonic devices for optical modulating.

Metamaterial-Based Low-Conductivity Alloy Perfect Absorber

We propose a simple way to enormously broaden the bandwidth of the metamaterial absorber formed by a low-conductivity square alloy patch and a dielectric layer on top of an alloy ground plane. The FWHM of the device can be up to 38.8%, which is 3.6 times larger than that of the high-conductivity (Au) absorber. Moreover, we demonstrate an ultra-broadband and polarization insensitive absorber by simply stacking three different-sized square alloy patches. Greater than 90% absorption is obtained across a frequency range of 1.34 THz with the central frequency around 1.90 THz. The relative absorption bandwidth of the device is greatly improved to 70.4%, which is much larger than previous results. The mechanism for the ultra-broadband absorption is attributed to the overlapping of four different but closely positioned resonance frequencies. The results of the proposed alloy metamaterial absorber appear to be very promising for solar cells, detection, and imaging applications.

Tailoring optical transmission via the arrangement of compound subwavelength hole arrays

The transmission properties of light through metal films with compound periodic subwavelength hole arrays is numerically investigated by using the finite-difference time-domain (FDTD) method. The sharp dips in the transmission bands, together with the suppression of surface Plasmon resonance (SPR) (0, 1) peak, are found when two square holes in every unit cell are arranged asymmetrically along the polarization direction of the incident light. However, the shape of transmission spectra is not sensitive to the symmetry if the holes are arranged perpendicular to the propagation direction of surface plasmon polaritons (SPPs). The physics origin of these phenomena is explained qualitatively by the phase resonance of SPPs.