Fluctuation-induced dispersion forces on thin DNA films.

In this work, the calculation of Casimir forces across thin DNA films is carried out based on the Lifshitz theory. The variations of Casimir forces due to the DNA thicknesses, volume fractions of containing water, covering media, and substrates are investigated. For a DNA film suspended in air or water, the Casimir force is attractive, and its magnitude increases with decreasing thickness of DNA films and the water volume fraction. For DNA films deposited on a dielectric (silica) substrate, the Casimir force is attractive for the air environment. However, the Casimir force shows unusual features in a water environment. Under specific conditions, switching sign of the Casimir force from attractive to repulsive can be achieved by increasing the DNA-film thickness. Finally, the Casimir force for DNA films deposited on a metallic substrate is investigated. The Casimir force is dominated by the repulsive interactions at a small DNA-film thickness for both the air and water environments. In a water environment, the Casimir force turns out to be attractive for a large DNA-film thickness, and a stable Casimir equilibrium can be found. The influences of electrolyte screening on the Casimir pressure of DNA films are also discussed at the end. In addition to the adhesion stability, our finding could be applicable to the problems of condensation and decondensation of DNA, due to fluctuation-induced dispersion forces.

Casimir interaction with black phosphorus sheets.

We calculate the Casimir interaction between isotropic plates (gold or graphene) and black phosphorus (BP) sheets with Lifshitz theory. It is found that the Casimir force with BP sheets is of the order of α times the perfect metal limit, and α is the fine structure constant. Strong anisotropy of the BP conductivity gives rise to a difference in the Casimir force contribution between the two principal axis. Furthermore, increasing the doping concentration both in BP sheets and graphene sheets can enhance the Casimir force. Moreover, introducing substrate and increased temperature can also enhance the Casimir force, by this way we reveal that the Casimir interaction can be doubled. The controllable Casimir force opens a new avenue for designing next generation devices in micro- and nano-electromechanical systems.

Toroidal dipole resonances by a sub-wavelength all-dielectric torus.

Electromagnetic toroidal excitations open up a new avenue for strong light-matter interactions. Although toroidal dipole resonances (TDRs) based on artificial meta-molecules were reported intensely, the TDRs supported in a single dielectric particle remain largely unknown. In this work, we show that an all-dielectric sub-wavelength torus can support a dominant TDR. The magnetic field can be enhanced greatly, and it shows a "vortex-like" configuration in the torus, confirming the toroidal excitation. The evolutions of the TDRs due to the geometrical parameters, dielectric permittivity, and polarization are discussed. It is found that the toroidal excitation is achieved mainly for TM polarization, while the anapole state is uncovered for TE polarization. This work suggests a new strategy for toroidal excitations based on a simple dielectric resonator.

Near-field radiative heat transfer between topological insulators via surface plasmon polaritons.

Recently, thanks to its excellent opto-electronic properties, two-dimension topological insulator not only has attracted broad interest in fields such as tunable detectors and nano-electronics but also shall yield more interesting prospect in thermal management, energy conversion, and so on. In this work, the excellent near-filed radiative heat transfer (NFRHT) resulting from monolayer topological insulator (Bi2Se3) is demonstrated. The NFRHT of this system is mainly dominated by the strong coupling effect of the surface plasmon polaritons (SPPs) between two Bi2Se3 sheets. Moreover, the non-monotonic dependence of the Fermi energy of Bi2Se3 on NFRHT is then discovered. It is indicated that the system can provide great thermal adjustability by controlling the Fermi energy, achieving a modulation factor of heat flux as high as 98.94%. Finally, the effect of substrate on the NFRHT is also explored. This work provides a promising pathway for the highly efficient thermal management.

Optical metasurface composed of multiple antennas with anti-Hermitian coupling in a single layer.

Metasurfaces consisting of different shapes of resonant units are used to manipulate light beams at subwavelength scales. In many cases, interactions among the resonant units are suppressed or avoided because of mode splitting in metasurfaces. Here we theoretically and numerically investigate metasurfaces composed of multiple antennas with anti-Hermitian coupling in a single layer. By utilizing the anti-Hermitian coupling, the results show that antennas with similar resonance frequencies at a subwavelength distance can individually absorb their corresponding frequency photons. The antennas whose reflection phase can be tailored by changing the number of antennas have the same resonance frequencies. This Letter paves the way for various potential applications in broadband absorption, photon sorting, image sensors, and phase modulation.

Angle-dependent optical response of the plasmonic nanoparticle clusters with rotational symmetry.

Plasmonic nanoparticle clusters are widely considered experimentally and numerically. In the clusters consisting of one central particle and N satellite particles, not only the magnetic modes but also the toroidal modes can exist. Here, the eigenmodes of such clusters and the corresponding excitation efficiency under the illumination of a plane wave are studied analytically by using the eigen-decomposition method. The angular dependence of the optical response of these clusters is clearly demonstrated. The behavior of excitation efficiency is dependent on both the value and the parity of N, the number of satellite particles. Our results may provide a guide for the selective excitation of plasmonic modes in the plasmonic nanoparticle clusters.

Giant near-field radiative heat transfer between ultrathin metallic films.

Understanding energy transfer via near-field thermal radiation is essential for applications such as near-field imaging, thermophotovoltaics and thermal circuit devices. Evanescent waves and photon tunneling are responsible for the near-field energy transfer. In bulk noble metals, however, surface plasmons do not contribute efficiently to the near-field energy transfer because of the mismatch of wavelength. In this paper, a giant near-field radiative heat transfer rate that is orders-of-magnitude greater than the blackbody limit between two ultrathin metallic films is demonstrated at nanoscale separations. Moreover, different physical origins for near-field thermal radiation transfer for thick and thin metallic films are clarified, and the radiative heat transfer enhancement in ultrathin metallic films is proved to come from the excitation of surface plasmons. Meanwhile, because of the inevitable high sheet resistance of ultrathin metal films, the heat transfer coefficient is 4600 times greater than the Planckian limit for the separation of 10 nm in ultrathin metallic films, which is the same order or even greater than that in other 2D materials with low carrier density. Our work shows that ultrathin metallic films are excellent materials for radiative heat transfer, which may find promising applications in thermal nano-devices and thermal engineering.

Modulation of near-field radiative heat transfer between graphene sheets by strain engineering.

In this article, we study the near-field radiative heat transfer (NFRHT) between two graphene sheets under mechanical strain. The modulation of NFRHT due to the strain modulus and stretching direction is investigated under two types of strain configurations. For the first type, one graphene sheet is strained whereas the other one is unstrained. It is found that the spectra of NFRHT undergo a redshift and the magnitudes drop remarkably as the strain modulus increases. For the second type of configuration, two graphene sheets have the same strain modulus while the stretching direction could be arbitrary. It is found that the differences in stretching directions lead to the mismatch of anisotropic plasmonic modes. Under proper choices of stretching directions, a large modulation with the reduction of heat transfer coefficient over 60% is possible for strain modulus 0.2. Our findings may have promising applications in thermal management for micro/nano-electromechanical devices.

Unidirectional scattering induced by the toroidal dipolar excitation in the system of plasmonic nanoparticles.

The interference between conventional multipoles (e.g., electric and magnetic dipole, electric quadrupole, etc.) is known as the cause of unidirectional backward and forward scattering of nanoparticles. However, an unconventional multipole moment, toroidal dipole moment is generally overlooked in the unidirectional scattering. In this work, we systematically investigate the unidirectional scattering in the system of plasmonic nanoparticles. It is found that the toroidal dipole moment can play a significant role in the unidirectional backward scattering. The structural tunability of the unidirectional scattering is also demonstrated. Our results can find applications in the design of nanoantennas.

Broadband light absorption in graphene ribbons by canceling strong coupling at subwavelength scale.

We theoretically investigate the broadband light absorption in the THz range by canceling the strong coupling in an array of graphene ribbons at subwavelength scale. A series of resonators with different absorption frequencies can achieve a broadband absorber, however, the suppression of absorption always accompanies since the mutual coupling between resonators cause the mode splitting. By adjusting the near- and far-field coupling between the plasmon resonances of the graphene ribbon array to the critical point, the absorption linewidth is broadened for almost one magnitude larger than that of individual graphene ribbon, to be ~1 THz. Our study provides not only insight understanding but also new approaches towards the broadband graphene absorber.

Topological edge modes in multilayer graphene systems.

Plasmons can be supported on graphene sheets as the Dirac electrons oscillate collectively. A tight-binding model for graphene plasmons is a good description as the field confinement in the normal direction is strong. With this model, the topological properties of plasmonic bands in multilayer graphene systems are investigated. The Zak phases of periodic graphene sheet arrays are obtained for different configurations. Analogous to Su-Schrieffer-Heeger (SSH) model in electronic systems, topological edge plasmon modes emerge when two periodic graphene sheet arrays with different Zak phases are connected. Interestingly, the dispersion of these topological edge modes is the same as that in the monolayer graphene and is invariant as the geometric parameters of the structure such as the separation and period change. These plasmonic edge states in multilayer graphene systems can be further tuned by electrical gating or chemical doping.

Determination of the quantized topological magneto-electric effect in topological insulators from Rayleigh scattering.

Topological insulators (TIs) exhibit many exotic properties. In particular, a topological magneto-electric (TME) effect, quantized in units of the fine structure constant, exists in TIs. Here, we theoretically study the scattering properties of electromagnetic waves by TI circular cylinders particularly in the Rayleigh scattering limit. Compared with ordinary dielectric cylinders, the scattering by TI cylinders shows many unusual features due to the TME effect. Two proposals are suggested to determine the TME effect of TIs simply by measuring the electric-field components of scattered waves in the far field at one or two scattering angles. Our results could also offer a way to measure the fine structure constant.

Unusual electromagnetic scattering by cylinders of topological insulator.

Topological insulators (TIs) show unusual optical responses resulting from a topological magnetoelectric (TME) effect. In this paper, we study theoretically the scattering of electromagnetic waves by circular TI cylinders. In certain configurations, the bulk scattering can be suppressed, leading to strong scattering in the backward direction in both Rayleigh and Mie scattering regimes due to the TME effect. At antiresonances, an interesting filed trapping phenomenon is found which is absent in conventional dielectric cylinders.

[Study on the fluorescence characteristic and mechanism of ether-water solution].

The fluorescence spectra of low concentration diethyl ether-water solution excited by different UV light, and the variation rule of the fluorescence characteristic along with the incidence light wavelength and the solution concentration were studied. Also the emission mechanism together with the spectral characteristic were analyzed. The result shows that the ether solution exhibits an obvious fluorescence peak nearby 306 nm, for which the best excitation wavelength is 245 nm, moreover, there is an inferior peak at 292 nm. The shape of the fluorescence spectrum is invariable ultimately along with the change in excitation light wavelength, and the peak value assumes the Gaussian distribution with the stimulation light wavelength. Meanwhile the emission intensity of the inferior peak and that of the prominent peak are in competition with each other. With increasing the concentration, the fluorescence intensity at 306 nm strengthens linearly, and after increasing the concentration to 7%, concentration-quenching occurs, so the intensity linearly weakens. The findings can provide the reference for detecting concentration and purity of the virulent and anesthetic matter-diethyl ether and so on.