Jiaming Hao

High performance optical absorber based on a plasmonic metamaterial

High absorption efficiency is particularly desirable at present for various microtechnological applications including microbolometers, photodectors, coherent thermal emitters, and solar cells. Here we report the design, characterization, and experimental demonstration of an ultrathin, wide-angle, subwavelength high performance metamaterial absorber for optical frequencies. Experimental results show that an absorption peak of 88% is achieved at the wavelength of ∼1.58 μm, though theoretical results give near perfect absorption.

Super-reflection and cloaking based on zero index metamaterial

A zero index metamaterial (ZIM) can be utilized to block wave (super-reflection) or conceal objects completely (cloaking). The “super-reflection” device can be realized by a Mu zero (Epsilon zero) metamaterial with a perfect electric (magnetic) conductor inclusion of arbitrary shape and size for a transverse electric (magnetic) incident wave. In contrast, a Mu zero (Epsilon zero) metamaterial with a perfect magnetic (electric) conductor inclusion for a transverse electric (magnetic) incident wave can be used to conceal objects of arbitrary shape. The underlying physics here is determined by the intrinsic properties of the ZIM.

A transparent metamaterial to manipulate electromagnetic wave polarizations

We design an anisotropic ultrathin metamaterial to allow perfect transmissions of electromagnetic (EM) waves for two incident polarizations within a common frequency interval. The transparencies are governed by different mechanisms, resulting in significant differences in transmission phase changes for two polarizations. The system can thus manipulate EM wave polarizations efficiently in transmission geometry, including polarization conversion and rotation. Microwave experiments performed on realistic samples are in excellent agreement with numerical simulations.

Photothermal reshaping of gold nanoparticles in a plasmonic absorber

We experimentally demonstrate that a metamaterial nanostructure can have a localized heating response owing to plasmonic resonances in the near-infrared wavelength range (from 1.5 to 2 µm). With a broadband nanosecond-pulse light, the temperature of composing gold particles in the nanostructure can be easily increased to over 900K within only several nanoseconds, resulting in re-shaping of the particles. The photothermal effect is elaborated with finite-element based numerical simulations. The absorption resonance can in principle be tailored with a great freedom by choosing appropriate metamaterial parameters. The light-induced heating in an artificial metamaterial can be potentially used for all-optical acute temperature tuning in a micro-environment, which may open new frontiers especially in nanotechnology and biotechnology.

Shape-dependent absorption characteristics of three-layered metamaterial absorbers at near-infrared

We experimentally demonstrate the absorption properties of designed metamaterial absorbers in the near-infrared wavelength regime. For the rectangular-shaped case, we demonstrate its polarization dependent absorbance at various incident angles. For each polarization, the absorbance is insensitive to the incident angle (up to 60°) and a maximum absorbance of 0.95 is obtained. Of particular interest we experimentally observe an absorption peak corresponding to a high-order resonance at 60° with an absorbance of 0.68 excited by the TM polarization. For the square-shaped case, we show its polarization-independent absorption property. A maximum absorbance around 0.65 is achieved at normal incidence and it remains high for incidence angles up to 50°.

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.

Theoretical realization of robust broadband transparency in ultrathin seamless nanostructures by dual blackbodies for near infrared light

We propose a counter-intuitive mechanism of constructing an ultrathin broadband transparent device with two perfect blackbodies. By introducing hybridization of plasmon modes, resonant modes with different symmetries coexist in this system. A broadband transmission spectrum in the near infrared regime is achieved through controlling their coupling strengths, which is governed by the thickness of high refractive index layer. Meanwhile, the transparency bandwidth is found to be tunable in a large range by varying the geometric dimension. More significantly, from the point view of applications, the proposed method of achieving broadband transparency can perfectly tolerate the misalignment and asymmetry of periodic nanoparticles on the top and bottom, which is empowered by the unique dual of coupling-in and coupling-out processes within the pair of blackbodies. Moreover, roughness has little influence on its transmission performance. According to the coupled mode theory, the distinguished transmittance performance is physically interpreted by the radiative decay rate of the entire system. In addition to the feature of uniquely robust broadband transparency, such a ultrathin seamless nanostructure (in the presence of a uniform silver layer) also provides polarization-independent and angle-independent operations. Therefore, it may power up a wide spectrum of exciting applications in thin film protection, touch screen techniques, absorber-emitter transformation, etc.

Design of an ultrathin broadband transparent and high-conductive screen using plasmonic nanostructures

In this Letter, we present a new type of ultrathin antireflection transparent and high-conductive screen based on plasmonic nanostructures that does not suffer from high loss and thickness coating and also can be used as good conductive material due to super electrical conductivity of the component (noble metal). Low reflection and greatly enhanced transmissions over a broad spectral range are observed at optical telecommunication frequencies in arbitrary polarizations. The performance is almost insensitive of the angle of incidence.

All-dimensional subwavelength cavities made with metamaterials

By exploiting the reflection phase properties of metamaterial reflectors, the authors propose a method to break the size restrictions imposed strictly on conventional cavities. They design the all-dimensional subwavelength cavities and perform experiments and simulations to demonstrate their subwavelength functionalities. For the smallest cavity that they fabricated, each dimension is only a quarter of the resonance wavelength.

Effective-medium models and experiments for extraordinary transmission in metamaterial-loaded waveguides

We show that a metallic waveguide behaves as an electric (magnetic) plasma for transverse-electric (transverse-magnetic) polarized electromagnetic (EM) waves at frequencies below cutoff value. Inserting anisotropic resonance structures of either electric or magnetic type into a waveguide, we find extraordinary transmissions of EM waves with different polarizations through the waveguide at frequencies well below the waveguide’s cutoff value, following two different mechanisms. Microwave experiments, in excellent agreements with finite-different-time-domain simulations, are performed to demonstrate all theoretical predictions.