Liu Xiao-Jun

Enormous enhancement of electric field in active gold nanoshells

The electric field enhancement properties of an active gold nanoshell with gain material inside have been investigated by using Mie theory. As the gain coefficient of the inner core increases to a critical value, a super-resonance appears in the active gold nanoshell, and enormous enhancements of the electric fields can be found near the surface of the particle. With increasing shell thickness, the critical value of the gain coefficient for the super-resonance of the active gold nanoshell first decreases and then increases, and the corresponding surface enhanced Raman scattering (SERS) enhancement factor (G factor) also first increases and then decreases. The optimized active gold nanoshell can be obtained with an extremely high SERS G factor of the order of 1019–1020. Such an optimized active gold nanoshell possesses a high-efficiency SERS effect and may be useful for single-molecule detection.

Extraordinary Resonant Scattering in Imperfect Acoustic Cloak

We present a systematic study on the extraordinary resonant scattering in imperfect acoustic cloak by means of acoustic scattering theory. Analysis results demonstrate that the resonances are inevitable due to the perturbation to the ideal cloak, and specific resonance modes are excited by specific order waves. The strength of resonance is determined by the magnitude of perturbation and each order waves sensitivity to the perturbation. Further studies reveal the unique scattering characters of different resonance modes.

Manipulation of extraordinary acoustic transmission by a tunable bull's eye structure

Extraordinary acoustic transmission (EAT) has been investigated in a tunable bulls eye structure. We demonstrate that the transmission coefficient of acoustic waves can be modulated by a grating structure. When the grating is located at a distance of 0.5 mm from the base plate, the acoustic transmission shows an 8.77-fold enhancement compared to that by using a traditional bulls eye structure. When the distance increases to 1.5 mm, the transmission approaches zero, indicating a total reflection. Thus, we can make an efficient modulation of acoustic transmission from 0 to 877%. The EAT effects have been ascribed to the coupling of structure-induced resonance with the diffractive wave and the waveguide modes, as well as the Fabry-Perot resonances. As a potential application, the modulation of far-field collimation is illustrated in the proposed bulls eye structure.

Insulator–Metal Transition due to La Doping in Double Perovskite Sr2MnMoO6

Resistivity, thermal diffusivity, lattice and magnetic properties of double perovskite Sr2−xLaxMnMoO6 are investigated with systematic change of La doping concentration x from 0.0 to 0.4. The insulator to metal phase transition is observed with increasing x above 0.3, suggesting that the extra electrons via substitution of La3+ for Sr2+ ions occupy mainly the conduction Mo-4d band. According to the insulator to metal phase transition, the thermal diffusivity of Sr2−xLaxMnMoO6 enhances from 0.33 cm2/s at x = 0.0 to 0.49 cm2/s at x = 0.4. We further investigate the La doping effects on the lattice and magnetic properties.

Manipulated localized surface plasmon resonances in silver nanoshells coated with a spherical anisotropic layer

The influences of the anisotropy of the outer spherically anisotropic (SA) layer on the far-field spectra and near-field enhancements of the silver nanoshells are investigated by using a modified Mie scattering theory. It is found that with the increase of the anisotropic value of the SA layer, the dipole resonance wavelength of the silver nanoshell first increases and then decreases, while the local field factor (LFF) reduces. With the decrease of SA layer thickness, the dipole wavelength of the silver nanoshell shows a distinct blue-shift. When the SA layer becomes very thin, the modulations of the anisotropy of the SA layer on the plasmon resonance energy and the near-field enhancement are weakened. We further find that the smaller anisotropic value of the SA layer is helpful for obtaining the larger near-field enhancement in the Ag nanoshell. The geometric average of the dielectric components of the SA layer has a stronger effect on the plasmon resonance energy of the silver nanoshell than on the near-field enhancement.

Influence of polarization direction, incidence angle, and geometry on near-field enhancement in two-layered gold nanowires

The influences of polarization direction, incidence angle, and geometry on near-field enhancements in two-layered gold nanowires (TGNWs) have been investigated by using the vector wave function method. When the polarization direction is perpendicular to the incidence plane, the local field factor (LFF) in TGNW decreases first and then increases with the increase in the incidence angle. The minimum LFF is observed at an incidence angle of 41°. It is found that the increase in the dielectric constant of the inner core leads to a decrease in the LFF. With the increase in the inner core radius, the LFF in TGNW increases first and then decreases, and the maximum LFF is observed at an inner core radius of 27 nm. On the other hand, when the polarization direction is parallel to the incidence plane, the collective motions of the induced electrons are enhanced gradually with the decrease in the incidence angle, and hence the near-field enhancement is increased.

Optical Properties and Photocatalytic Activity of Marokite-Type CaMn2O4

The optical properties and electronic structure of marokite-type CaMn2O4 are investigated by using UV-vis spectroscopy and the local-spin-density approximation plus the Hubbard-U (LSDA+U) method. Four absorption bands are observed at 638 nm (1.94 eV), 512 nm (2.42 eV), 377 nm (3.29 eV) and 248nm (5.00eV), which are ascribed to the charge transfer transitions O2p↑→Mn3d eg↑, O2p↓→Mn3d eg↑, Mn3d eg↑→Mn3d t2g↓ and O2p↑→Mn3d t2g↓, respectively. We further use CaMn2O4 as a photocatalyst to decompose an azo-dye acid orange 7 (AO7) under irradiation of visible light and find that the decomposition ratio of AO7 reaches 15.9% under the irradiation of visible light for two hours.

Photoinduced Spin Disorder in Half-Metal CrO2 films

We investigate the photoinduced effects on the spin state for half-metallic ferromagnet CrO2 (TC~390 K), in which the conducting electrons are totally polarized, by means of the time-resolved pump–probe method at the temperature range from 300 K to 470 K. A significant negative change ΔT/T for the transmittance spectrum at 1.55 eV under photo-excitation is found. The ΔT/T value monotonically decreases on approaching to TC from the low temperature side, suggesting a photoinduced spin disorder state. Furthermore, we calculate the saturation magnetization MS of CrO2 in both the ground and photo-excited states by using the local-spin-density approximation plus U (LSDA+U) method, and find a decrease of the MS-value in the photo-excited state. The suppressed MS-value in the photo-excited state is consistent with the experimental data.

Thermal Diffusivity of Ordered Double Perovskite A2FeMoO6 (A = Ca, Sr and Ba)

Thermal diffusivity has been investigated in ordered double perovskite Sr2FeMoO6 by means of transient surface grating technique in the temperature range of 300–450 K. The thermal diffusivity shows an appreciable decrease from 39 mm2/s at 300 K to 37 mm2/s at 360 K in the ferromagnetic phase, and then steeply drops to 10 mm2/s with further increasing temperature above the critical temperature Tc~380 K. Such an abrupt decrease of the thermal diffusivity has been ascribed to the structural phase transition at Tc. We further investigate the lattice effect on the thermal diffusivity in A2FeMoO6 (A = Ca, Sr and Ba) by substitution of Ca2+ or Ba2+ ions for Sr2+ ions at 300 K. We find that the thermal diffusivity increases from 35 mm2/s for A = Ca to 41 mm2/s for A = Ba. Considering the change of the Fe–O–Mo bond angle from 152.4 degrees for A = Ca to 180 degrees for A = Ba, the increased thermal diffusivity for Ba compound has been ascribed to the enhanced hybridization between transition-metal d and oxygen p states due to the larger Fe–O–Mo bond angle and hence the wider one-electron bandwidth W.

Pulse decomposition-based analysis of PAT/TAT error caused by negative lobes in limited-view conditions*

Pulse decomposition has been proven to be efficient to analyze complicated signals and it is introduced into the photo-acoustic and thermo-acoustic tomography to eliminate reconstruction distortions caused by negative lobes. During image reconstruction, negative lobes bring errors in the estimation of acoustic pulse amplitude, which is closely related to the distribution of absorption coefficient. The negative lobe error degrades imaging quality seriously in limited-view conditions because it cannot be offset so well as in full-view conditions. Therefore, a pulse decomposition formula is provided with detailed deduction to eliminate the negative lobe error and is incorporated into the popular delay-and-sum method for better reconstructing the image without additional complicated computation. Numerical experiments show that the pulse decomposition improves the image quality obviously in the limited-view conditions, such as separating adjacent absorbers, discovering a small absorber despite disturbance from a big absorber nearby, etc.