Hongru Ma

Highly sensitive response of anisotropic terahertz metamaterials to electromagnetic waves has attracted considerable attention due to its potential applications in terahertz modulators and biosensing devices. We designed three microstructure samples with split ring arrays. The terahertz transmission spectra are experimentally measured at different rotation angle. It is found that as the rotation angle increases, the resonance mode has gradually evolved from a single dipole oscillation mode to the double oscillation modes accompanied with LC and dipole oscillations, we further simulated the polarization conversion characteristics of the single splitting rings. It shows that the highest conversion efficiency appears at 45 degrees. But it doesn’t have polarization conversion effect at 0° and 90°. Subsequently, we have utilized ultrashort pulse laser to optically control the electromagnetic response by exciting the photogenerated carriers in the metamaterial samples. The experimental data show that the transmittance change obviously with the pump light. Additionally, when the angle is 90 degrees, it can be observed that the LC resonance annihilates earlier than the resonance of the dipole resonance with the increase of pump power, implying the former is particularly sensitive to the variation of the dielectric environment. To further explore the influence of dielectric environment on the resonance characteristics of the terahertz metamaterials, we have further performed the simulations with the applied surface analyte of different refractive index. The simulated data show that with the increase of the refractive index of the surface analyte, the resonance frequency of the dipole oscillation has a more significant blue shift than the resonance frequency of the LC oscillation. Our obtained results could provide the idea for designing terahertz modulators and sensitive biosensing devices.

We modulate the electromagnetic response of terahertz metamaterials by exciting photogenerated carriers with ultra-short pulsed laser, and realize the active control of terahertz transmission and resonances in a wide band. The experimental results show that when the terahertz electric vector is perpendicular and parallel to the bar direction with the open gap in the circular split ring resonators, respectively, the transmission of the non-resonant region and resonance absorption are significantly tuned by the irradiated pump pulse. With the increase of the pump power, the resonance absorption peak decreases and the frequency of resonance dip has a remarkable blue-shift. Meanwhile, the transmission of the non-resonant region decreases correspondingly. It is also found that the blue-shift of the resonance is mainly determined by the change of conductivity and dielectric constant in the photoexcited layer of the substrate. Through the numerical simulation, we have further proved the introduction of pump light has a significant modulation effect on the electromagnetic properties of terahertz metamaterials. Additionally, it can be observed that the LC resonance annihilates earlier than the resonance of the dipole resonance with the increase of pump power, implying the former is particularly sensitive to the variation of the active control. In the theoretical analysis, based on the Lorenz oscillator model, we have derived and calculated the resonance response intensity of the coupled oscillators under an external field. The calculated results indicate the coupling coefficient and damping rate have an impact on the spectra evolution, showing that the resonant response peak has a remarkable blue-shift with the increase of coupling coefficient and the resonance response intensity decreases with the increase of damping rate.

Colloidal suspension is composed of particles with sizes between 1 nm and 1 m, suspended in liquid phase. The interaction between the particles consists of a hard core repulsive interaction and other kinds of repulsive and attractive interacions. Hard interaction forbids the particles from occupying the same places, resulting in a depletion effect. When big colloid particles are immersed in a colloid of small particles, each big particle has a depletion layer where the small particles cannot enter due to the hard interaction. The depletion layers of two big particles overlap when they are close enough so that extra free volume of the small particles increases and therefore the entropy of the small particles increase, thus an effective interaction between big particles is induced. This effective interaction is the so-called depletion interaction. In this review the concepts and an intuitive explanation of depletion interaction of colloidal suspensions are presented. The numerical calculation methods, including the acceptance ratio method, Wang-Landau-type method, and density functional theory method, are briefly reviewed. Several useful analytic approximations are presented. Stating from the depletion interaction between two flat plates, the Derjaguin approximation is introduced through the Asakura- Oosawa model. With this approximation, the approximate formulas of depletion interaction between two hard spheres, between a hard sphere and a hard wall, and between a hard sphere and curved hard walls in a small hard sphere colloid are derived. The depletion interaction between two hard spheres in a thin rod colloid and a thin disk colloid are also derived in the Derjaguin approximation.

We investigate the structure of the nematic director field on a toroidal surface, on the basis of two different free-energy models. The two models treat the variation of the nematic field differently; one in full-derivative form and the other in covariant-derivative form. Through solving the Euler–Lagrange equation used to minimize the energy model and conducting a simulated annealing Monte Carlo simulation, we confirm that both models produce a trivial solution as a defect-free state. In the first model, however, there exists a second-order phase transition, beyond which the energy bifurcates into a non-trivial solution as the ground state, as the torus ring radius grows. Using the simulated annealing technique on both models, we also trap the system in excited, metastable states that display nematic-field disclinations.

The depletion interactions between two ellipsoids in three configurations were studied by both Monte Carlo simulation with the Wang-Landau algorithm and the density functional theory in the curvature expansion approximation. Common features of the depletion interactions were found and the results were as expected. By comparing the results of the two methods, it is concluded that density functional theory under the curvature expansion approximation gave very good results to the depletion forces.

We model the orientational and positional order of tetratically shaped molecules, each having four-fold structural symmetry, confined on a spherical surface. Our Monte Carlo simulation shows that at a high molecular density, a tetratic orientational order develops in the system, accompanied by eight disclinations arranged in an anticube configuration form on the hard spherical surface. We also consider an elastic-energy model, which consists of both Helfrich and Frank energies for a soft surface; the solution confirms the Monte Carlo study and further predicts the tetratic morphology that can be realized on the surface of a soft vesicle. Assuming that the induced interaction between these disclinations is repulsive, we demonstrate that the anticube structure has a lower free energy than, for example, the cubic structure.