Xiaolong Weng

Dual-band absorption of mid-infrared metamaterial absorber based on distinct dielectric spacing layers

We present the simulation, fabrication, and characterization of a dual-band metamaterial absorber in the mid-infrared regime. Two pairs of circular-patterned metal-dielectric stacks are employed to excite the dual-band absorption peaks. Dielectric characteristics of the dielectric spacing layer determine energy dissipation in each resonant stack, i.e., dielectric or ohmic loss. By controlling material parameters, both two mechanisms are introduced into our structure. Up to 98% absorption is obtained at 9.03 and 13.32 μm in the simulation, which is in reasonable agreement with experimental results. The proposed structure holds promise for various applications, e.g., thermal radiation modulators and multicolor infrared focal plane arrays.

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.

Structural and Visible-Near Infrared Optical Properties of Cr-Doped TiO 2 for Colored Cool Pigments

Chromium-doped TiO2 pigments were synthesized via a solid-state reaction method and studied with X-ray diffraction, SEM, XPS, and UV-VIS-NIR reflectance spectroscopy. The incorporation of Cr3+ accelerates the transition from the anatase phase to the rutile phase and compresses the crystal lattice. Moreover, the particle morphology, energy gap, and reflectance spectrum of Cr-doped TiO2 pigments is affected by the crystal structure and doping concentration. For the rutile samples, some of the Cr3+ ions are oxidized to Cr4+ after sintering at a high temperature, which leads to a strong near-infrared absorption band due to the 3A2 → 3 T1 electric dipole-allowed transitions of Cr4+. And the decrease of the band gap causes an obvious redshift of the optical absorption edges as the doping concentration increases. Thus, the VIS and near-infrared average reflectance of the rutile Ti1 − xCrxO2 sample decrease by 60.2 and 58%, respectively, when the Cr content increases to x = 0.0375. Meanwhile, the color changes to black brown. However, for the anatase Ti1 − xCrxO2 pigments, only the VIS reflection spectrum is inhibited by forming some characteristic visible light absorption peaks of Cr3+. The morphology, band gap, and NIR reflectance are not significantly affected. Finally, a Cr-doped anatase TiO2 pigment with a brownish-yellow color and 90% near-infrared reflectance can be obtained.

A novel self-healing electrochromic film based on a triphenylamine cross-linked polymer

The cracking of electrochromic materials due to aging or reiterative bending is a major problem which noticeably degrades the performance of electrochromic devices. In the present research, we successfully induced a triphenylamine derivative into furfuryl glycidyl ether, and further by a DA cross-linking polymerization reaction with maleimide (MA) to yield polymer DATPFMA. The color of the polymer film could be switched from faint yellow to green grey to dark blue. The observed coloration efficiency values of the polymer for the electrochromic process were 82.2 and 175.3 cm2 C−1 at 580 and 1060 nm, and the coloring and bleaching response times were 4.1 and 11.0 s, respectively. The self-healing process of the polymer film at 110 °C was observed by optical microscopy and the self-healing rate was about 80%. The results indicate that the polymer DATPFMA is a novel electrochromic material with noteworthy self-healing properties.

ZrO2-TiO2 Thin Films and Resonators for Mid-Infrared Integrated Photonics

Mid-infrared (MIR, 2-6 μm wavelength) transparent metal oxides are attractive materials for planar integrated photonic devices for sensing applications. In this study, we present reactive sputtering deposited ZrO2-TiO2 (ZTO) thin films as a new material candidate for integrated MIR photonics. We demonstrate that amorphous ZTO thin films can be achieved with Ti concentration of 40 at.%. With increasing Ti concentration, the optical band gap decreases monotonically from 4.34 eV to 4.11 eV, while the index of refraction increases from 2.14 to 2.24 at 1 μm wavelength. MIR micro-disk resonators on MgO substrates are demonstrated using Ge23/Sb7S70/Zr0.6Ti0.4O2 strip-loaded waveguides with a loaded quality factor of ~11,000 at 5.2 μm wavelength. By comparing with a reference device of Ge23Sb7S70 resonator on MgO and simulating the optical confinement factors, the ZTO thin film loss is estimated to be below 10 dB/cm. Single mode shallow ridge waveguides with a ridge height of 400 nm and a slab height of 1.7 μm are also demonstrated using ZrO2 thin films on MgO substrates. The low loss, relatively high index of refraction, superior stability and proven CMOS compatibility of ZTO thin films make them highly attractive for MIR integrated photonics.

Design and implementation of electromagnetic soft surfaces for decoupling in between two microstrip patch antennas

Abstract In this paper, the design of the electromagnetic soft surfaces realized by metal strips loaded with C-shaped slots is demonstrated having good suppression of electromagnetic coupling in between two patch antennas. Electromagnetic bandgap properties of the as-prepared electromagnetic soft surfaces with C-shaped slots loading is proposed for normal incidence, and compared with that from the one of the same size but none C-shaped slots on the patches. The proposed structure possesses excellent miniaturization properties with working frequency of bandgap toward lower frequency but keeping the filtering level unchangeable. Then, the proposed structure is loaded in two patch antennas for decoupling in between them. Experimental results are presented and compared to numerical simulations, and they demonstrated that the proposed electromagnetic soft surfaces loading advances excellent suppression of electromagnetic coupling and the coupling decrease about 17 dB near the working frequency band.

Broadband thermal tunable infrared absorber based on the coupling between standing wave and magnetic resonance

In this paper, a simple thermal tunable metamaterial absorber with broadband absorption in the mid-infrared regime is designed and fabricated. The TiN/VO2/Al2O3/Al four-layer structure (with top Al circular patch arrays) introduces a specific dual-dielectric method to manipulate the coupling between standing wave and magnetic resonance for the metal-insulator-metal sandwich absorber. After structural optimization, the 80% absorption bandwidth reaches 2.9 μm in both simulation and experimental results. By virtue of the insulator-to-metal transition (IMT) of VO2, the reflectance increases from 5% to 42% by heating up the absorber from room temperature to 358 K leading to a relative stable radiation temperature at the crucial IMT stage (323~348 K). This design method is quite useful for active IR absorption (or camouflage) as well as thermal tuning.

TM electromagnetic scattering analysis of a finite conducting cone loaded with a slot

The aperture integral equation (AIE) and method of moment (MoM) technique is proposed for analyzing the electromagnetic scattering properties of a finite perfectly conducting cone loaded with a slot. The solution allows us to deal accurately with slots of any shape. Two kinds of slots representing for electrical large and electrical small structure respectively, are considered in this paper. Bi-static radar cross sections (RCS) of the slotted cone and near field scattering at slot aperture are discussed for the incidence plane wave located near the axial direction. Numerical results are presented and demonstrated the slot apertures whether electrical large size or electrical small size can be significantly improved to increase electromagnetic scattering of the considered finite cone in certain direction. Moreover, grooved conducting cone are also treated as a special case, and its electromagnetic scattering properties are compared to the slotted ones.

Passive cooling effect of infrared metamaterial absorber

Over the past decade, infrared metamaterial absorbers have been extensively studied. Great efforts have been dedicated to design broadband, omi-directional, and high-efficient infrared absorbing structures with the potential sensing, detecting, and imaging applications. As a matter of fact, infrared absorber is also infrared emitter. Due to Kirchhoffs law, the frequency selective absorption equals to the frequency selective emitting which can be applied for passive cooling. In this work, infrared metamaterial absorbers at three crucial bands are realized by carefully designed sandwich structure of metallic pattern - dielectric spacer - metallic ground plane. The relationship between electromagnetic wave energy dissipation for absorption design and thermal radiation for heat transfer is discussed to explore the physical mechanism of passive cooling. Effective method associated with equivalent circuit approach reveals that the localized magnetic resonance determines the energy transfer. According to heat transfer theory, thermal cooling model of metamaterial covered heat object is proposed to analyze the effect of selective emitting. The integration form of Wiens law shows that the passive cooling effect is determined by both the emitting spectrum and heat source. After optimization, the proposed absorber can effectively improve the cooling rate of heat object experimentally. This technique can be used for the thermal control of electronic device.

Effects of hydrogen bonds and volatilization on absorption properties for a microwave absorbing coating

In this work, we demonstrated the effects on reflection loss (RL) for a microwave absorbing coating. The mixture of an absorbing coating before curing process including flake carbonyl iron particles (FCIPs), polymer matrix, and moderate diluting agent butanone were distributed uniformly. Microwave absorption properties RL of the coating were measured during curing process at the frequency range of 2-18 GHz by the arch method, which showed RL changing clearly. Volatilization of diluting agent was investigated, resulting in the increasing of the FCIPs volume fraction, which could elucidate the changing of RL. Polymer matrix for the absorbing coating was generated by reacting polyurethane to epoxy resin, emerging the hydrogen bonds between secondary amides N-H and carbonyl groups C = O. Effect of hydrogen bonds was discussed, revealing it played an significant role in the enhancing of interfacial polarization, resulting in changing of the microwave absorbing coating RL.