Haipeng Lu

Ultra-Thin Reflective Metamaterial Polarization Rotator Based on Multiple Plasmon Resonances

We propose the design, simulation, and measurement of a broadband reflective metamaterial polarization rotator in the microwave region based on multiple plasmon resonances. An utra-thin wideband polarization conversion composing of arrays of oval ring pattern, a dielectric layer and a continuous metallic layer is further demonstrated both numerically and experimentally. Two plasmon resonances are generated by magnetic and electric resonances, leading to bandwidth expansion of cross-polarization reflection. The simulated results show that the polarization conversion ratio (PCR) is greater than 68.6% in 8.0-18.0 GHz with incidence angle up to 30° for both y-and x-polarized waves and the maximum conversion efficiency is nearly 100% at the two plasmon resonance frequencies. Experimental results under normal and oblique incidence agree well with simulated ones. The proposed rotator has applications in the area of polarization control.

Resistance Selection of High Impedance Surface Absorbers for Perfect and Broadband Absorption

High impedance surface absorbers comprising of lossless dielectric substrate and resistive pattern were studied based on equivalent circuit method. Resonant conditions in terms of potential resonant frequencies and corresponding equivalent resistance were deduced. Theoretical calculation, numerical simulation and experiment showed that expected absorption was obtained by matching the capacitance, inductance and the resistance. We demonstrated that the optimal resistance of frequency-dispersive was between 0 and 377 Ω. For square, dipole and square ring pattern, a mean resistance value was required to achieve broadband absorption. It offered a way to create broadband and perfect absorbers by controlling the surface resistance of sub-wavelength structures.

Achieving Ultrafast Hole Transfer at the Monolayer MoS2 and CH3NH3PbI3 Perovskite Interface by Defect Engineering

The performance of a photovoltaic device is strongly dependent on the light harvesting properties of the absorber layer as well as the charge separation at the donor/acceptor interfaces. Atomically thin two-dimensional transition metal dichalcogenides (2-D TMDCs) exhibit strong light-matter interaction, large optical conductivity, and high electron mobility; thus they can be highly promising materials for next-generation ultrathin solar cells and optoelectronics. However, the short optical absorption path inherent in such atomically thin layers limits practical applications. A heterostructure geometry comprising 2-D TMDCs (e.g., MoS2) and a strongly absorbing material with long electron-hole diffusion lengths such as methylammonium lead halide perovskites (CH3NH3PbI3) may overcome this constraint to some extent, provided the charge transfer at the heterostructure interface is not hampered by their band offsets. Herein, we demonstrate that the intrinsic band offset at the CH3NH3PbI3/MoS2 interface can be overcome by creating sulfur vacancies in MoS2 using a mild plasma treatment; ultrafast hole transfer from CH3NH3PbI3 to MoS2 occurs within 320 fs with 83% efficiency following photoexcitation. Importantly, our work highlights the feasibility of applying defect-engineered 2-D TMDCs as charge-extraction layers in perovskite-based optoelectronic devices.

A Broadband Radar Absorber Based on Perforated Magnetic Polymer Composites Embedded With FSS

This paper presents a design method of broadband radar absorber (RA) combining together the features of frequency selective surfaces (FSSs), subwavelength hole array, and magnetic absorbing sheets. The absorber is constructed of a periodic array of square conducting patches embedded into a magnetic absorbing substrate perforated with circular hole arrays and backed by a metal ground. The absorption characteristics of the magnetic absorbing substrate are tuned and improved by means of perforating holes and embedding FSSs. After optimizing the dimensions of the holes and FSSs, the RA with a thickness of 2.4 mm achieves a reflection coefficient less than -10 dB from 6.3 to 17.3 GHz, which is nearly 3.6 times the bandwidth of the magnetic absorbing substrate. Meanwhile, the weight of the RA decreases by 15% due to the hole perforation, and the reflection coefficient is insensitive to incident angle from 0° to 30° for both TE and TM polarizations.

Realization of broadband reflective polarization converter using asymmetric cross-shaped resonator

Cross-shaped resonator (CSR) in metamaterial usually corresponds to adjust resonant characteristics. In this report, a simple and broadband reflective polarization converter is realized at GHz frequencies by breaking the symmetry of the CSR. The experimental results demonstrate that the polarization conversion ratio (PCR) over 0.8 is achieved from 8.3 GHz to 14.3 GHz for linearly polarized (LP) incident waves under normal incidence. The high polarization conversion efficiency can be sustained when the incident angle increases to 60 degrees. By taking advantage of surface current distribution and the interference theory, the physical mechanisms are elucidated in detail. Finally, the broadband polarization conversion can also be achieved by decreasing the geometrical parameters of the converter in the mid-infrared frequency range.

A Study on the Effective Permittivity of Carbon/PI Honeycomb Composites for Radar Absorbing Design

Radar absorbing honeycomb composites with different coating thicknesses are prepared by impregnation of aramid paper frame with solutions containing conductive carbon blacks (non-magnetic) and PI (polyimide). Expressions for the effective permittivity of the composites are studied and validated both in theory and experiment. It is found that a theoretical equivalent panel with given permittivity can be obtained to represent the honeycomb structure in the quasistatic approximation, which provides a feasible way to optimize the design of radar absorbing honeycomb structure by connecting the effective electromagnetic parameters with the unit cell dimensions. The effective permittivity is measured by a network analyzer system in the frequency range of 8-12 GHz and compared with the theoretical result.

Adjustable wideband reflective converter based on cut-wire metasurface

We present the design, analysis, and measurement of a broadband reflective converter using a cut-wire (CW) metasurface. Based on the characteristics of LC resonances, the proposed reflective converter can rotate a linearly polarized (LP) wave into its cross-polarized wave at three resonance frequencies, or convert the LP wave to a circularly polarized (CP) wave at two other resonance frequencies. Furthermore, the broad-band properties of the polarization conversion can be sustained when the incident wave is a CP wave. The polarization states can be adjusted easily by changing the length and width of the CW. The measured results show that a polarization conversion ratio (PCR) over 85% can be achieved from 6.16 GHz to 16.56 GHz for both LP and CP incident waves. The origin of the polarization conversion is interpreted by the theory of microwave antennas, with equivalent impedance and electromagnetic (EM) field distributions. With its simple geometry and multiple broad frequency bands, the proposed converter has potential applications in the area of selective polarization control.

ZrO 2 -TiO 2 thin films: a new material system for mid-infrared integrated photonics

Mid-infrared (MIR, 2 - 6 μm wavelength) transparent metal oxides are attractive materials for planar integrated MIR photonic devices and sensing applications. In this report, we present reactive sputtering deposited ZrO2-TiO2 (ZTO) thin films as a new material candidate for integrated MIR photonics. The material structure and optical properties were systematically studied as a function of Ti concentration. The thin film index of refraction monotonically increases with Ti concentration, while the film crystallinity decreases. Fully amorphous ZTO films were achieved with 40 at.% Ti doping on various substrates. MIR micro-disk resonators on MgO substrates were demonstrated using Zr0.6Ti0.4O2 strip-loaded waveguides with a loaded quality factor of ~11,000 at 5.2 μm wavelength.

Ultrabroadband Design for Linear Polarization Conversion and Asymmetric Transmission Crossing X- and K- Band

In this work, a high-efficiency and broadband reflective converter using ultrathin planar metamaterial (MM) composed of single-layered SRR is firstly realized. Numerical and experimental results demonstrate that the cross-polarization conversion reflectance above 0.84 is achieved from 8.6 to 18.6 GHz for linearly polarized (LP) incident waves under normal incidence. Subsequently, a multi-layered MM based on SRR enables a dramatic improvement of the recently demonstrated asymmetric transmission (AT) effect. Theoretical and measured results present that strong one-way transmission of two orthogonally polarized waves crossing C- and K- band has been observed. These two separated AT pass-bands have a function of selective polarization filter, which can be switched on/off by changing the polarization state of incident waves. The physical mechanisms are elucidated by taking advantage of electric fields and current distributions. Considering the broad bandwidth and the dual band, we believe that these two structures will be beneficial for designing polarization-controlled and selective transmission converter.

Influence of Interface Structure on Magnetic Proximity Effect in Pt/Y3Fe5O12 Heterostructures.

The influence of interface structure on the magnetic proximity effect (MPE) in Pt/Y3Fe5O12 (YIG) bilayered heterostructures is studied by first-principles calculations based on the density functional theory (DFT). When Pt atoms are in close proximity with Y or Fe ions at the interface, Pt-Y and Pt-Fe bonds are observed. The crystalline orientations and interface termination layers of the YIG strongly modify both the strength and the length of the Pt-Fe bonding and thereby influence the magnetic properties of the Pt. Point defects including tetrahedral Fe, octahedral Fe, and Y vacancies are introduced at the Pt/YIG interface to quantitatively evaluate the influence on the MPE from individual atoms. For the Pt(100)/YIG(100) structure, the interface tetrahedral Fe vacancies can significantly reduce or even completely diminish the magnetic moments in the Pt. In a stark contrast, the octahedral Fe vacancies slightly enhance the Pt magnetism, and the nonmagnetic Y vacancies cause little influences to the Pt magnetism. These results indicate that the strength of the MPE at the Pt/YIG interface strongly depends on the interface structure. This dependence originated from the direct exchange interaction between the Fe 3d and Pt 5d electrons via electronic state hybridization as well as the electron exchange coupling between the Pt atoms.