Weixin Lu

Optical characteristics of gold nanoparticle-doped multilayer thin film

A variety of surface plasmon resonance (SPR) peaks of gold nanoparticles was detected in the wavelength range between the SPR peak of Au/SiO2(∼535 nm) and Au/TiO2(∼655 nm) with the parameter x in the triple-component composite films, (SiO2)1−x/Au/(TiO2)x. Based on the spectrum of the dielectric constant of Au and the location of the SPR peaks, the effective refractive index neff of these composite films was found to be almost equal to nSiO2∙(1−x)+n_TiO2∙x. These composite films can be used as optical filters owing to their high damage threshold value (>6.5 MW/cm2). Their potential applications in nonlinear optical devices are due to their larger third-order nonlinear susceptibilities, chi(3), which were measured by the degenerate four-wave-mixing method at a laser wavelength of 532 nm. The chi(3) value reached 2.6×10^−6 esu.

Broadband perfect absorption of ultrathin conductive films with coherent illumination:Superabsorption of microwave radiation

Absorption of microwave by metallic conductors is exclusively inefficient, though being natively broadband, due to the huge impedance mismatch between metal and free space. Reducing the thickness to ultrathin conductive film may improve the absorbing efficiency, but is still bounded by a maximal 50% limit induced by the field continuity. Here, we show that broadband perfect (100%) absorption of microwave can be realized on a single layer of ultrathin conductive film when it is illuminated coherently by two oppositely incident beams. Such an effect of breaking the 50% limit maintains the intrinsic broadband feature from the free carrier dissipation, and is frequency-independent in an ultrawide spectrum, ranging typically from kilohertz to gigahertz and exhibiting an unprecedented bandwidth close to 200%. In particular, it occurs on extremely subwavelength scales, ~{\lambda}/10000 or even thinner, which is the film thickness. Our work proposes a way to achieve total electromagnetic wave absorption in a broadband spectrum of radio waves and microwaves with a simple conductive film.

An equivalent realization of coherent perfect absorption under single beam illumination

We have experimentally and numerically demonstrated that the coherent perfect absorption (CPA) can equivalently be accomplished under single beam illumination. Instead of using the counter-propagating coherent dual beams, we introduce a perfect magnetic conductor (PMC) surface as a mirror boundary to the CPA configuration. Such a PMC surface can practically be embodied, utilizing high impedance surfaces, i.e., mushroom structures. By covering them with an ultrathin conductive film of sheet resistance 377 Ω, the perfect (100%) microwave absorption is achieved when the film is illuminated by a single beam from one side. Employing the PMC boundary reduces the coherence requirement in the original CPA setup, though the present implementation is limited to the single frequency or narrow band operation. Our work proposes an equivalent way to realize the CPA under the single beam illumination, and might have applications in engineering absorbent materials.

Lasing from dye-doped photonic crystals with graded layers in dichromate gelatin emulsions

We report on optically pumped lasing from dye-doped, graded-spacing layer structures of dichromate gelatin emulsions fabricated using two-beam holographic interference. The graded layers exhibited deep and wide photonic band gaps. Multimode lasing with both a low threshold and a high quality factor was observed at the band edge of the photonic band gap. We modeled the emissions from the dye-doped graded layer system using a finite difference time domain technique and achieved good agreement with experimental results.

Experimental realization of a broadband conformal mapping lens for directional emission

We experimentally realize a transformation lens for directional emission at microwave frequencies. The refractive index of the lens, based on the design of conformal mapping, has variation from 1 to 4 inside a semi-circular geometry. By drilling subwavelength holes in dielectric plates inhomogeneously, we design the lens sample. The scanning measurements of the electric field of the lens demonstrate the high directivity of emission, within a broadband working frequency region of 7-11 GHz.

Simultaneous perfect phase matching for second and third harmonic generations in ZnS/YF 3 photonic crystal for visible emissions

Theoretically designed and experimentally realized simultaneous perfect phase matching of second and third harmonic generations were demonstrated in a one-dimensional ZnS/YF(3) photonic crystal (PC) structure. Dramatic enhancement of second harmonic generation (SHG) and third harmonic generation (THG) in forward and backward directions near the photonic band edge were observed. This enhancement came from a combination of large ZnS nonlinear susceptibility coefficients, high density of optical modes and perfect phase matching of the fundamental and the harmonic waves near the photonic band edge due to modification of the dispersion curve by the PC structure. Total SHG and THG conversion efficiency over 4% is measured in only six micrometers length of photonic crystal. Theoretical calculations show good agreement with experimental measurements.

Perfect electromagnetic absorption at one-atom-thick scale

We experimentally demonstrate that perfect electromagnetic absorption can be realized in the one-atom thick graphene. Employing coherent illumination in the waveguide system, the absorbance of the unpatterned graphene monolayer is observed to be greater than 94% over the microwave X-band, 7–13 GHz, and to achieve a full absorption, >99% in experiment, at ∼8.3 GHz. In addition, the absorption characteristic manifests equivalently a wide range of incident angle. The experimental results agree very well with the theoretical calculations. Our work accomplishes the broadband, wide-angle, high-performance absorption in the thinnest material with simple configuration.

Applying Effective Medium Theory in Characterizing Dielectric Constant of Solids

We present a simple approach to measure the dielectric constant of solid materials. In this approach, the powder for the solid under investigation is mixed with the oil at a specific volume fraction. By measuring the oil and the mixture, the permittivity of the inclusion, i.e. the solid, can be accurately derived from the Maxwell-Garnett effective medium theory. With this method, the strict requirements for the solid shape and surface flatness in the conventional measuring configurations can be waved off, and meanwhile the broadband permittivity can be obtained. The demonstrations on alumina and glucose show this approach is valid and robust.

A broadband polarization-insensitive cloak based on mode conversion

In this work, we demonstrate an one-dimensional cloak consisting of parallel-plated waveguide with two slabs of gradient index metamaterials attached to its metallic walls. In it objects are hidden without limitation of polarizations, and good performance is observed for a broadband of frequencies. The experiments at microwave frequencies are carried out, supporting the theoretical results very well. The essential principle behind the proposed cloaking device is based on mode conversion, which provides a new strategy to manipulate wave propagation.

Oblique total transmissions through epsilon-near-zero metamaterials with hyperbolic dispersions

Zero-index metamaterials with near-zero permittivity and/or permeability usually reflect oblique incident waves due to total reflection that occurs at the air-metamaterial interface. In this work, we show that if one component of the near-zero–permittivity tensor of metamaterial turns from positive to negative due to a small disturbance, the dispersion surface changes dramatically from a tiny circle to a hyperbola, which enables oblique transmissions. A series of high-order total-transmission peaks at large incident angles is predicted and observed. These peaks are induced by Fabry-Perot effects. Our work may have potential applications for filters, sensors and switches.