Huixu Deng

Broadband perfect absorber based on one ultrathin layer of refractory metal

Broadband perfect absorber based on one ultrathin layer of the refractory metal chromium without structure patterning is proposed and demonstrated. The ideal permittivity of the metal layer for achieving broadband perfect absorption is derived based on the impedance transformation method. Since the permittivity of the refractory metal chromium matches this ideal permittivity well in the visible and near-infrared range, a silica-chromium-silica three-layer absorber is fabricated to demonstrate the broadband perfect absorption. The experimental results under normal incidence show that the absorption is above 90% over the wavelength range of 0.4-1.4 μm, and the measurements under angled incidence within 400-800 nm prove that the absorber is angle-insensitive and polarization-independent.

Experimental realization of epsilon-near-zero metamaterial slabs with metal-dielectric multilayers

Epsilon-near-zero (ENZ) metamaterial slabs at visible frequencies based on metal-dielectric multilayers are experimentally realized. Transmission, reflection, and absorption spectra are measured and used to determine the complex refractive indices and the effective permittivities of the ENZ slabs, which agree with the results obtained from both the numerical simulations and the optical nonlocalities analysis. Furthermore, light propagation in ENZ slabs and directional emission from ENZ prisms are also analyzed. The accurate determination of the ENZ wavelength for metal-dielectric multilayer metamaterial slabs is important for realizing many unique applications, such as phase front manipulation and enhancement of photonic density of states.

Infrared perfect absorber based on nanowire metamaterial cavities

An infrared perfect absorber based on a gold nanowire metamaterial cavities array on a gold ground plane is designed. The metamaterial made of gold nanowires embedded in an alumina host exhibits an effective permittivity with strong anisotropy, which supports cavity resonant modes of both electric dipole and magnetic dipole. The impedance of the cavity modes matches the incident plane wave in free space, leading to nearly perfect light absorption. The incident optical energy is efficiently converted into heat so that the local temperature of the absorber will increase. Results show that the designed absorber is polarization-insensitive and nearly omnidirectional for the incident angle.

Broadband infrared absorbers with stacked double chromium ring resonators

A broadband absorber in the infrared wavelength range from 1 μm up to 5 μm is designed and demonstrated with stacked double chromium ring resonators on a reflective chromium mirror. The near-perfect broadband absorption is realized by combining the multilayer impedance match in the short wavelength range and the double plasmonic resonances in the long wavelength range, which is illustrated with an equivalent circuit model for the impedance analysis. The broadband absorber is proved to be angle-insensitive and polarization-independent due to the geometrical symmetry. The thermal analysis for heat generation and temperature distributions inside the absorber structure is also investigated.

Transmission Spectrum of Asymmetric Nanostructures for Plasmonic Space Propulsion

NANOSATELLITES are defined as spacecraft with a mass of 1– 50 kg. The demand for and use of these and other small satellites iswidespread and is projected to continue to increase according to the 2015 SpaceWorks report on the nano/microsatellite market [1]. The SpaceWorks report also makes note that most nano/microsatellites are launched in large clusters and that certain companies such as SpaceX and OneWeb plan to use large constellations of small satellites for communications purposes. Satellites within sizeable constellationswill need the ability tomaneuver and orient themselves precisely in relation to the other satellites of the cluster. Coupling these requirements with the constraints of mass and volume demand the use of novel propulsion systems that are optimized for use on smallsats (a nano/micro type satellite defined in the SpaceWorks report). This propulsion system must also be adaptable for satellites that continue to decrease in size. Plasmonics is a subfield of optics that addresses the nanoscale interactions of light and metallic nanostructures. If a beam of light is allowed to strike the surface of a metal, then the oscillating electric field component of the light will cause the electrons in the metal to oscillate. A group of oscillating electrons is known as a plasmon. If the metal is restricted in dimensions to the nanoscale regime, the plasmons are able to resonate with the incident light, which creates a strong electromagnetic field about the location of the nanostructure. This electromagnetic field can be tuned by changing the size and shape of themetallic nanostructures and used to control themotion of particles within the vicinity of the nanostructure. The control of nanoparticles in the vicinity of a plasmon interaction is well known as “plasmon nano-optical tweezers” [2]. This technique demonstrates optical particle trapping coupled with the plasmon interaction to trap nanoparticles beyond the diffraction limit. Previous research [3] extended this idea of plasmonic nanoparticle manipulation from trapping to acceleration andwas specifically aimed at propulsion for smallsats. This work developed a thruster design that made use of the plasmonic force concept. Results predicted that, with plasmonic force propulsion, the relative position (distance) and angular orientation (degrees) between two spacecraft was able to be controlled to a resolution that was one to two orders of magnitude smaller than state of the art. Results for a conceptual design of a plasmonic thruster that had 35 layers, 86 array columns, a multistage length of 5 mm, a 5-cm-diam light focusing lens, and used 100 nm polystyrene nanoparticles expelled at a rate of 1 × 10 per second would have a thrust of 250 nN, a specific impulse of 10 s, and a minimum impulse bit of 50 pN · s. The thrustermass andvolumewere estimated at 100 g and 50 cm, respectively [3]. Previous numerical simulations have shown that asymmetric nanostructures can resonate strongly within the visible spectrum, but this has never been demonstrated in experiments until now. Here, we report the first time these types of nanostructures have been manufactured and optically characterized.We report on the first experiment that demonstrates this resonance, with a nanostructure designed to resonate at λ 770 nm, for use as a plasmonic space propulsion thruster.

Fabrication of asymmetric nanostructures for plasmonic force propulsion

The objective of this research is to manufacture and investigate the characteristics and use of asymmetric, metallic, nanostructures for plasmonic force propulsion, a developing method of nano-/picosatellite thrust generation. Visible to near-infrared light is focused onto sub-wavelength nanostructures to generate polarized oscillations of electrons on the surface of the metallic nanostructures (surface plasmon polaritons). The surface plasmon polaritons accelerate nanoparticle propellant away from the nanostructure, creating thrust. Previous numerical simulations have shown that asymmetric nanostructures can resonate strongly within the visible spectrum. This is the first experiment ever attempted and first to successfully demonstrate this resonance where the resonance peak is λ = 830 nm. The resonance peak of the experimental optical characterization agrees well with our computed model, showing an 11.2% difference. However, the off resonance behavior exhibits peak broadening where the variation of intensity with wavelength, off resonance, has an experimental slope that is 3.7 times less steep than the computed model. Furthermore, the optical transmittance of the sample is 2.1 times higher than computationally modeled. It is shown that the nanostructures are thermodynamically stable in the projected environmental conditions and have an equilibrium temperature of 746.4 K. Upon review of the experimental optical setup, we conclude that thrust generation is not possible with continuous irradiation of light and propose a method of synchronous dynamic acceleration of nanoparticle propellant by use of a pulsed light beam.

Experimental Demonstration of Near-Infrared Epsilon-Near-Zero Multilayer Metamaterial Slabs

Near-infrared epsilon-near-zero metamaterial slabs based on Ag-Ge multilayers are experimentally demonstrated and numerically analyzed. A post-annealing process and multilayer grating structures are introduced to reduce the optical loss and also tune the epsilon-near-zero wavelength.