Linhua Liu

Parallel LC circuit model for multi-band absorption and preliminary design of radiative cooling

We perform a comprehensive analysis of multi-band absorption by exciting magnetic polaritons in the infrared region. According to the independent properties of the magnetic polaritons, we propose a parallel inductance and capacitance(PLC) circuit model to explain and predict the multi-band resonant absorption peaks, which is fully validated by using the multi-sized structure with identical dielectric spacing layer and the multilayer structure with the same strip width. More importantly, we present the application of the PLC circuit model to preliminarily design a radiative cooling structure realized by merging several close peaks together. This omnidirectional and polarization insensitive structure is a good candidate for radiative cooling application.

Wide-angle and polarization independent perfect absorber based on one-dimensional fabrication-tolerant stacked array.

We propose a wide-angle, polarization independent and fabrication-tolerant perfect absorber, which is based on a one-dimensional stacked array consisted of vertically cascaded two pairs of metal-dielectric bilayers. The results show that the absorption peaks are over 99% at the wavelength of 5.25 μm for different polarization angles, and remain very high within wide ranges of incident and azimuthal angles. We attribute those excellent performances to the excitation of the magnetic resonance (MR) and the guided mode resonance (GMR) for the TM and TE polarization, respectively, and are further expounded by the inductor-capacitor (LC) circuit model and the eigen equation of the GMR, respectively. More importantly, this one-dimensional absorber is very robust to the spacing distance between the neighboring stacks and the metallic strip thickness, which releases degrees of freedom in design and makes the absorber extremely flexible and simple in fabrication, thus it can be a good candidate for many fascinating applications.

Temperature-dependent dielectric functions in atomically thin graphene, silicene, and arsenene

The dielectric functions of atomically thin graphene, silicene, and arsenene have been investigated as a function of temperature. With zero energy gap, more carriers in graphene and silicene are thermally excited as temperature increases and intraband transition strengthens, resulting in the strengthened absorption peak. Yet with large energy gap, interband transition dominates optical absorption of arsenene but it reduces as lattice vibration enhances, inducing the redshift and decreased absorption peak. To validate the theoretical method, the calculated optical constants of isolated graphene are compared with ellipsometry results and demonstrate good agreement.

Omnidirectional and polarization insensitive nearly perfect absorber in one dimensional meta-structure

We design and numerically investigate an omnidirectional and polarization insensitive nearly perfect absorber in the mid-infrared regime, which is just a one dimensional metallic grating with a pair of metal–dielectric bilayer on the grating ridge. Absorption peaks of over 99% are simultaneously achieved around the wavelength of 5.73 μm for both TM and TE polarizations, and they also remain very high over a wide range of incident angle for both polarizations. According to the analysis of the electromagnetic field distributions, we attribute the nearly perfect absorption to the magnetic resonances and the cavity modes for the TM and TE polarizations, respectively, which are further confirmed by inductor-capacitor (LC) circuit model and cavity resonance condition, respectively. This simple and flexible one dimensional nearly perfect absorber is particularly desirable for various potential applications including micro-bolometers, band-stop filters and selective thermal emitters.

Radiative heat transfer in many-body systems: Coupled electric and magnetic dipole approach

The many-body radiative heat transfer theory [P. Ben-Abdallah, S.-A. Biehs, and K. Joulain, Phys. Rev. Lett. 107, 114301 (2011)] only considered the contribution from the electric dipole moment. For metal particles, however, the magnetic dipole moment due to eddy current plays an important role, which can further couple with the electric dipole moment to introduce crossed terms. In this work, we develop coupled electric and magnetic dipole (CEMD) approach for the radiative heat transfer in a collection of objects in mutual interaction. Due to the coupled electric and magnetic interactions, four terms, namely the electric-electric, the electric-magnetic, the magnetic-electric and the magnetic-magnetic terms, contribute to the radiative heat flux and the local energy density. The CEMD is applied to study the radiative heat transfer between various dimers of nanoparticles. It is found that each of the four terms can dominate the radiative heat transfer depending on the position and composition of particles. Moreover, near-field many-body interactions are studied by CEMD considering both dielectric and metallic nanoparticles. The near-field radiative heat flux and local energy density can be greatly increased when the particles are in coupled resonances. Surface plasmon polariton and surface phonon polariton can be coupled to enhance the radiative heat flux.

Temperature-dependent infrared dielectric functions of MgO crystal: an ellipsometry and first-principles molecular dynamics study.

In this work, the state-of-the-art infrared variable angle spectroscopic ellipsometry (IR-VASE) and first-principles molecular dynamics (FPMD) method were combined to obtain the infrared dielectric functions of MgO crystal in the spectral range 300-1000 cm(-1) and for temperatures up to 1950 K. The IR-VASE can measure the infrared dielectric functions of MgO crystal at temperatures ranging from 300 to 573 K and reproduce previous infrared-reflectivity experiments. As temperature increases, it demonstrates that the amplitude of dominant absorption peak centered around 400 cm(-1) reduces, the width broadens, and the position shifts to longer wavelength. Besides ellipsometry study, the FPMD method was implemented, seeking to theoretically predict the infrared spectra of MgO crystal at elevated temperatures. Comparing with experimental measurements, the FPMD method can reproduce the essential feature of ellipsometry and previous infrared-reflectivity experiments even at elevated temperatures, though with some deviations in predicting the exact position and amplitude of dominant absorption peak. On the other hand, the FPMD method can predict the temperature effect on the infrared dielectric functions of MgO crystal, e.g., redshift and broadened absorption peak with increasing temperature.

General design method of ultra-broadband perfect absorbers based on magnetic polaritons

Starting from one-dimensional gratings and the theory of magnetic polaritons (MPs), we propose a general design method of ultra-broadband perfect absorbers. Based on the proposed design method, the obtained absorber can keep the spectrum-average absorptance over 99% at normal incidence in a wide range of wavelengths; this work simultaneously reveals the robustness of the absorber to incident angles and polarization angles of incident light. Furthermore, this work shows that the spectral band of perfect absorption can be flexibly extended to near the infrared regime by adjusting the structure dimension. The findings of this work may facilitate the active design of ultra-broadband absorbers based on plasmonic nanostructures.

Implementation of Outstanding Electronic Transport in Polar Covalent Boron Nitride Atomic Chains: another Extraordinary Odd-Even Behaviour.

A theoretical investigation of the unique electronic transport properties of the junctions composed of boron nitride atomic chains bridging symmetric graphene electrodes with point-contacts is executed through non-equilibrium Green’s function technique in combination with density functional theory. Compared with carbon atomic chains, the boron nitride atomic chains have an alternative arrangement of polar covalent B-N bonds and different contacts coupling electrodes, showing some unusual properties in functional atomic electronic devices. Remarkably, they have an extraordinary odd-even behavior of conductivity with the length increase. The rectification character and negative differential resistance of nonlinear current-voltage characteristics can be achieved by manipulating the type of contacts between boron nitride atomic chains bridges and electrodes. The junctions with asymmetric contacts have an intrinsic rectification, caused by stronger coupling in the C-N contact than the C-B contact. On the other hand, for symmetric contact junctions, it is confirmed that the transport properties of the junctions primarily depend on the nature of contacts. The junctions with symmetric C-N contacts have higher conductivity than their C-B contacts counterparts. Furthermore, the negative differential resistances of the junctions with only C-N contacts is very conspicuous and can be achieved at lower bias.

Accurate Geometry Design of Magnetic Polariton With Specified Resonance Wavelength: A Combined LC Circuit Model and Inverse Technique

For enhancement of absorption and transmission at a specified wavelength using magnetic polariton (MP) resonance, it is necessary to determine the accurate geometry parameters at the corresponding resonance condition. In this work, the feature of the geometry design problem is analyzed and a method is presented for accurately determining the geometry parameters for specified MP resonance mode, which combines the LC circuit model for MP and inverse technique. The LC circuit model is used to give an initial rough design of geometric parameters and parameters range for the inverse algorithm. The particle swarm optimization (PSO) algorithm is used to minimize the objective function and determine the optimized geometric parameters. The forward problem to evaluate the objective function is solved using rigorous method. The presented method is demonstrated to have good performance in the geometry design of MP resonance structure using several example cases.Copyright

COMPUTATIONAL EFFICIENCY OF THE FINITE ELEMENT METHOD BASED ON THE SECOND-ORDER RADIATIVE TRANSFER EQUATION

The second-order radiative transfer equation (SORTE) [Numerical Heat Transfer B, Vol. 51, pp. 391-409, 2007] is in a form like diffusion equation, hence no additional artificial diffusion or upwinding treatment is needed in the numerical discretization for stabilization. The computational efficiency of the finite element method based on SORTE is investigated by comparison with that of the finite element methods based on original first order radiative transfer equation (FORTE). The FORTE based finite element methods considered are the finite element method with Galerkin approach (Galerkin-FORTE) and the finite element method with least-square approach (LS-FORTE). By comparison, the accuracy of the finite element method based on the SORTE is generally better than those based on the FORTE under the same discretization scheme, spatial grid and angular grid. The finite element method based on the SORTE shows the best computational efficiency among the three finite element methods, i.e., to obtain the same target accuracy, the least computational time is required.