Yanxia Cui

Ultrabroadband Light Absorption by a Sawtooth Anisotropic Metamaterial Slab

We present an ultrabroadband thin-film infrared absorber made of sawtoothed anisotropic metamaterial. Absorptivity of higher than 95% at normal incidence is supported in a wide range of frequencies, where the full absorption width at half-maximum is about 86%. Such property is retained well at a very wide range of incident angles too. Light of shorter wavelengths are harvested at upper parts of the sawteeth of smaller widths, while light of longer wavelengths are trapped at lower parts of larger tooth widths. This phenomenon is explained by the slowlight modes in anisotropic metamaterial waveguide. Our study can be applied in the field of designing photovoltaic devices and thermal emitters.

Ultra-broadband microwave metamaterial absorber

A microwave ultra-broadband polarization-independent metamaterial absorber is demonstrated. It is composed of a periodic array of metal-dielectric multilayered quadrangular frustum pyramids. These pyramids possess resonant absorption modes at multi-frequencies, of which the overlapping leads to the total absorption of the incident wave over an ultra-wide spectral band. The experimental absorption at normal incidence is above 90% in the frequency range of 7.8–14.7 GHz, and the absorption is kept large when the incident angle is smaller than 60°. The experimental results agree well with the numerical simulation.

Self-Alignment of Plasmonic Gold Nanorods in Reconfigurable Anisotropic Fluids for Tunable Bulk Metamaterial Applications

We demonstrate the bulk self-alignment of dispersed gold nanorods imposed by the intrinsic cylindrical micelle self-assembly in nematic and hexagonal liquid crystalline phases of anisotropic fluids. External magnetic field and shearing allow for alignment and realignment of the liquid crystal matrix with the ensuing long-range orientational order of well-dispersed plasmonic nanorods. This results in a switchable polarization-sensitive plasmon resonance exhibiting stark differences from that of the same nanorods in isotropic fluids. The device-scale bulk nanoparticle alignment may enable optical metamaterial mass production and control of properties arising from combining the switchable nanoscale structure of anisotropic fluids with the surface plasmon resonance properties of the plasmonic nanorods.

A thin film broadband absorber based on multi-sized nanoantennas

We experimentally demonstrate an infrared broadband absorber based on an array of nanostrip antennas of several different sizes. The broadband property is due to the collective effect of magnetic responses excited by these nanoantennas at distinct wavelengths. By manipulating the differences of the nanostrip widths, the measured spectra clearly validate our design for the purpose of broadening the absorption band.

Enhancing extraordinary transmission of light through a metallic nanoslit with a nanocavity antenna

The extraordinary transmission of light through a vertical nanoslit in a metal film is enhanced by introducing a nanocavity antenna formed by a nearby metallic nanostrip over the slit opening. For a fixed wavelength, the width of the metallic nanostrip should be chosen to make the horizontal metal-insulator-metal waveguide of finite length resonant as a Fabry-Perot cavity. When such a cavity antenna is used to enhance the transmission through a nonresonant nanoslit, the slit should be opened at a position with a maximal magnetic field in the horizontal resonant cavity. It is shown that an optimized cavity antenna can enhance greatly the transmission of light through a nonresonant nanoslit (by about 20 times) or a resonant nanoslit (by 124%). Such a transmission enhancement with a nanocavity antenna is studied for the first time, and the physical mechanism is explained.

Improved performance of organic solar cells by incorporating silica-coated silver nanoparticles in the buffer layer

It is demonstrated that the use of silica-coated silver nanoparticles (AgNPs) in the buffer layer improves the performance of organic solar cells (OSCs). It is found that only large sized AgNPs are advantageous for increasing the electric field distribution in the active layer, and therefore, increasing light absorption, caused by the localized surface plasmonic resonance and far-field scattering. Furthermore, the scattering of silica-coated AgNPs is more important to the light harvesting because of the existence of the silica coating. It is also demonstrated that the silica coating is favorable for enhancing the exciton dissociation because of the reduction of the exciton quenching that occurred at the interface between the bare AgNPs and the active layer. Furthermore, silica-coated AgNPs also promote hole transport and extraction, which is presumably explained by the introduction of “dopant” levels within the band gap of the poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) and reduction of hole trapping of a bare silver surface. The combination of all these benefits results in a 25.4% improvement in photocurrent density and an increase of 19.2% in power conversion efficiency. This work indicates that using silica-coated AgNPs as light trapping elements is more efficient than using bare AgNPs in plasmonic organic solar cells. The systematic exploration of the optical and electrical effects of silica-coated AgNPs contributes to a more comprehensive understanding of the mechanism of performance improvement of the plasmonic OSCs.

Efficient multiband absorber based on one-dimensional periodic metal–dielectric photonic crystal with a reflective substrate

We propose an efficient multiband absorber comprised of a truncated, one-dimensional periodic metal-dielectric photonic crystal and a reflective substrate. The reflective substrate is essentially an optically thick metallic film. Such a planar device is easier to fabricate compared to absorbers with complicated shapes. For a four-unit cell device, all four of the absorption peaks can be optimized with efficiencies higher than 95 percent. Moreover, those absorption peaks are insensitive to the polarization and incident angle. The influences of the geometrical parameters and the refractive index of the dielectric on the device performance also are discussed. Furthermore, we found that the number of absorption peaks within each photonic band precisely corresponds to the number of unit cells because the truncated photonic crystal lattices select resonant modes. We also show that the total absorption efficiency gradually increases when there are more periods of the metal-dielectric composite layer placed on top of the metallic substrate. We expect this work to have potential applications in solar energy harvesting and thermal emission tailoring.

Magnetodielectric effect in Z-type hexaferrite

The authors report on the magnetodielectric (MD) effect of Z-type hexaferrite Sr3Co2Fe24O41 at various temperatures and frequencies. A fairly large negative MD effect was observed with a peak near room temperature and a maximum at low frequencies. Analysis suggests that the MD effect shows a quadratic dependence on magnetization. The results were discussed by considering the magnetic field induced change of transverse conical spin structure and spin-phonon coupling. This work is helpful for understanding the MD effect in materials with complicated spin structures.

High-efficiency, broad-band and wide-angle optical absorption in ultra-thin organic photovoltaic devices

Metal nanogratings as one of the promising architectures for effective light trapping in organic photovoltaics (OPVs) have been actively studied over the past decade. Here we designed a novel metal nanowall grating with ultra-small period and ultra-high aspect-ratio as the back electrode of the OPV device. Such grating results in the strong hot spot effect in-between the neighboring nanowalls and the localized surface plasmon effect at the corners of nanowalls. These combined effects make the integrated absorption efficiency of light over the wavelength range from 400 to 650 nm in the active layer for the proposed structure, with respect to the equivalent planar structure, increases by 102% at TM polarization and by 36.5% at the TM/TE hybrid polarization, respectively. Moreover, it is noted that the hot spot effect in the proposed structure is more effective for ultra-thin active layers, which is very favorable for the exciton dissociation and charge collection. Therefore such a nanowall grating is expected to improve the overall performance of OPV devices.

A theoretical re-examination of giant transmission of light through a metallic nano-slit surrounded with periodic grooves

We show that the slit-to-groove distance for a maximal transmission through the nano-slit surrounded with periodic grooves cannot be predicted by the theory of constructive interference between the groove-generated surface plasmon wave (SPW) and the incident wave. A clear physical explanation is given for the dependence of the transmission on the slit-to-groove distance. It is shown that the influence to the transmission comes from three parts: the groove-generated SPW, the incident wave and the nano-slit-generated SPW. The groove-generated SPW is the main factor determining the local field distribution around the nano-slit opening. The influence of the incident wave is very weak when strong SPW is generated on the input surface by many periods of deep grooves. The nano-slit-generated SPW can also be considered as a disturbance to the light distribution on the input surface.