Shaojie Ma

Tailor the functionalities of metasurfaces based on a complete phase diagram

Metal/insulator/metal metasurfaces have widely applications ranging from perfect absorption to phase modulation, but the mechanism of these functionalities are not yet fully understood. Here, based on a coupled-mode analysis, we establish a complete phase diagram through two simple parameters, which lays a solid basis for realizing functional and tunable photonic devices with such structures.Metasurfaces in metal/insulator/metal configuration have recently been widely used in photonics research, with applications ranging from perfect absorption to phase modulation, but why and when such structures can realize what kind of functionalities are not yet fully understood. Here, based on a coupled-mode theory analysis, we establish a complete phase diagram in which the optical properties of such systems are fully controlled by two simple parameters (i.e., the intrinsic and radiation losses), which are in turn dictated by the geometrical/material parameters of the underlying structures. Such a phase diagram can greatly facilitate the design of appropriate metasurfaces with tailored functionalities (e.g., perfect absorption, phase modulator, electric/magnetic reflector, etc.), demonstrated by our experiments and simulations in the Terahertz regime. In particular, our experiments show that, through appropriate structural/material tuning, the device can be switched across the functionality phase boundaries yielding dramatic changes in optical responses. Our discoveries lay a solid basis for realizing functional and tunable photonic devices with such structures.

Dynamical control on helicity of electromagnetic waves by tunable metasurfaces

Manipulating the polarization states of electromagnetic (EM) waves, a fundamental issue in optics, attracted intensive attention recently. However, most of the devices realized so far are either too bulky in size, and/or are passive with only specific functionalities. Here we combine theory and experiment to demonstrate that, a tunable metasurface incorporating diodes as active elements can dynamically control the reflection phase of EM waves, and thus exhibits unprecedented capabilities to manipulate the helicity of incident circular-polarized (CP) EM wave. By controlling the bias voltages imparted on the embedded diodes, we demonstrate that the device can work in two distinct states. Whereas in the “On” state, the metasurface functions as a helicity convertor and a helicity hybridizer within two separate frequency bands, it behaves as a helicity keeper within an ultra-wide frequency band in the “Off” state. Our findings pave the way to realize functionality-switchable devices related to phase control, such as frequency-tunable subwavelength cavities, anomalous reflectors and even holograms.

Tunable microwave metasurfaces for high-performance operations: dispersion compensation and dynamical switch

Controlling the phase distributions on metasurfaces leads to fascinating effects such as anomalous light refraction/reflection, flat-lens focusing, and optics-vortex generation. However, metasurfaces realized so far largely reply on passive resonant meta-atoms, whose intrinsic dispersions limit such passive meta-devices’ performances at frequencies other than the target one. Here, based on tunable meta-atoms with varactor diodes involved, we establish a scheme to resolve these issues for microwave metasurfaces, in which the dispersive response of each meta-atom is precisely controlled by an external voltage imparted on the diode. We experimentally demonstrate two effects utilizing our scheme. First, we show that a tunable gradient metasurface exhibits single-mode high-efficiency operation within a wide frequency band, while its passive counterpart only works at a single frequency but exhibits deteriorated performances at other frequencies. Second, we demonstrate that the functionality of our metasurface can be dynamically switched from a specular reflector to a surface-wave convertor. Our approach paves the road to achieve dispersion-corrected and switchable manipulations of electromagnetic waves.

A bi-layered quad-band metamaterial absorber at terahertz frequencies

In this work, a quad-band metamaterial absorber is proposed and experimentally demonstrated at terahertz frequency using a bi-layer isotropic structure. The absorber is designed by distributing four metallic square loops with different lengths on two polyimide layers (PI). The backside of PI is covered by an opaque gold film to eliminate the transmission of terahertz waves. The proposed absorber exhibits almost unity absorptions at 0.51, 0.69, 1.05, and 1.34 THz under the normal illuminance. As the incident angle increases from 0° to 50°, the absorption intensities remain above 95%, and the absorption frequencies almost keep stable for both transverse-electric and transverse-magnetic polarizations. The absorption mechanism is interpreted by giving the electromagnetic field distributions at each absorption peak. Experimental results show good agreement with numerical simulations both in the frequency and strength of the four absorption peaks.

Aberration-free and functionality-switchable meta-lenses based on tunable metasurfaces

Constructing a meta-lens with tunable meta-atoms with varactor diodes incorporated, we can precisely control the phase profile of the meta-lens by varying the external voltages imparted on the diodes, such that the dispersion-induced phase distortions at off-working frequencies can be rectified and the functionality of the meta-lens can be dynamically changed. As an illustration, we design and fabricate a tunable meta-lens in the microwave regime and employ both experiments and numerical simulations to demonstrate the aberration-free and dynamically switchable focusing performances of the meta-lens. Our approach paves the road to achieve dispersion-corrected and switchable manipulations of electromagnetic waves in the microwave regime.

Tailor the surface-wave properties of a plasmonic metal by a metamaterial capping

We show that putting an ultra-thin anisotropic metamaterial layer on a plasmonic surface significantly enriches the surface wave (SW) characteristics of the system, which now supports SWs with transverse-magnetic (TM) and transverse-electric (TE) polarizations simultaneously. In addition, the generated SWs exhibit hybridized polarization characteristics in certain cases, and a SW band gap opens within a particular propagation direction range. We designed and fabricated a realistic structure based on the proposed model, and combined microwave experiments with full-wave simulations to verify the fascinating theoretical predictions. Several potential applications of the proposed system are discussed in the end.

Theoretical analysis and simulation of pulsed laser heating at interface

Quantitative yet simple analytical solutions of surface temperature under pulsed laser illumination are presented for a quick estimation in optical spectroscopy studies. Dependence of steady state surface temperature as well as its temporal evolution on laser parameters, such as repetition rate and beam radius, together with medium properties is thoroughly investigated using the analytical solution, which is supported by numerical simulation. It is found that when the pulse number is larger than 100 within the heat diffusion time, the steady-state temperature rise reaches more than 85% of the temperature rise induced by CW laser heating of the same power. We provide a summary of the results to allow their use for a quick estimate of surface temperature evolution from pulse laser heating if laser parameters and medium properties are known.