Yun-Song Zhou

Phase Diagram of Ising Nano‐Particles with Cubic Structures

The method of variational cumulant expansion (VCE) on the Ising model is applied to investigate phase transitions in magnetic alloy nano-particles, in which a random site distribution of component atoms is assumed. By introducing the effect exchange constant, it is shown numerically that nano-particles can be treated the same way as bulk materials. The Curie temperature and the Neel temperature of binary magnetic nano-particles with cubic lattices are calculated. It is found that both the Curie temperature and the Neel temperature decrease with decreasing size of the nano-particle. Phase diagrams of binary alloy nano-particles are investigated for variable doping concentration and the results are discussed. As the nano-particle size grows larger, the critical point approaches that of a bulk. On the other hand, the tri-critical point is independent of the nano-particle size and is always the same as that of the bulk.

Ab initio study of oxygen adsorption on the Ti(0001) surface

The adsorption of oxygen atoms on the Ti(0001) surface is investigated by the use of ab initio total-energy density-functional methods within the generalized gradient approximation. Two kinds of supercell model (a single-layer and a multiple-layer adsorption model) are designed to study the preference of occupation sites for oxygen atoms. In the single-layer adsorption model, the oxygen adatoms are restricted in the same adsorption layer. In the multiple-layer adsorption model, the adsorbed oxygen atoms occupy sites in different adsorption layers at the same time. Our calculated results show that the surface face-centred cubic (SFCC) sites are the most favourable sites for oxygen atoms to occupy when the oxygen coverage is 0.25 monolayer (ML); however, the system with one oxygen adatom occupying the SFCC sites and the other one occupying the octahedral sites between second and third Ti layers (Octa(2, 3)) is the most stable configuration when the oxygen coverage is 0.50 ML. As the oxygen coverage is increased to 1.0 ML, each of the oxygen atoms prefers occupying the SFCC and octahedral sites such as Octa(2, 3), Octa(4, 5) and Octa(6, 7) in the alternate layers, respectively. It can be found that there are strong repulsive interactions between the adsorbed oxygen atoms when they occupy adjacent interstitial sites. Our calculated results of the work functions and the total density of states with the multiple-layer adsorption model are in good agreement with experimental results of work function change and ultraviolet photoelectron spectroscopy. It should be noticed that the multiple-layer adsorption model must be taken into account for oxygen adsorption on the Ti(0001) surface.

Multi-reflection process of extraordinary optical transmission in a single subwavelength metal slit

The process of the light passing through a single subwavelength metal slit is analyzed by the finite-difference time-domain method. The simulation results show that the slit can be considered as an effective dielectric region sandwiched by two scattering regions at the two slit ends. Thus, the slit is analogous to a Fabry-Perot (FP) etalon consisting of a dielectric slab with high-reflection coatings on its two surfaces. A formula is presented to evaluate the phase shift caused by the amplitude transmissivity of the coatings.

Coupled third harmonic generations and multiple mode effects in aperiodic optical superlattices with a finite lateral width

We study the coupled third harmonic generations (CTHGs) from quasi-one-dimensional aperiodic optical superlattices (AOSs) with a finite lateral width. We design the special AOS that can implement multiple wavelength CTHG with identical effective nonlinear coefficient at the preassigned wavelengths of incident light with the use of simulated annealing method. We reveal the effects of mode-mode coupling and the lateral width of the AOSs on the CTHG for the two cases: One case corresponds to the incident light beam of a plane wave profile and other case to the incident light beam of Gaussian profile. The results show that the characteristics of the CTHG from the AOSs are closely related to the profiles of the incident fundamental frequency light beam. For the incidental plane wave beam, the dependence of the effective nonlinear coefficient on the width of the AOS is quite weak. On the contrary, for the incident Gaussian profile beam, this dependence exhibits a rapid decrease at the beginning and then tends t...

Wavelength squeeze of surface plasmon polariton in a subwavelength metal slit

One of the keys to understanding the extraordinary optical transmission in a subwavelength metal slit is in what field mode is excited. We find that, compared to the usual surface plasmon polariton (SPP), the wavelength of the excited mode is squeezed and will become smaller as the slit width does. This mode is called an in-slit SPP. We show how the wavelength of this mode can be solved by a formula and clearly analyze the physical reason for the wavelength squeeze. Furthermore, the similarities and differences of the wavelength squeeze between normal incidence and oblique incidence are pointed out.

Guide modes in photonic crystal heterostructures composed of rotating non-circular air cylinders in two-dimensional lattices

We investigate the properties of guide modes localized at the interfaces of photonic crystal (PC) heterostructures which are composed of two semi-infinite two-dimensional PCs consisting of non-circular air cylinders with different rotating angles embedded in a homogeneous host dielectric. Photonic band gap structures are calculated with the use of the plane-wave expansion method in combination with a supercell technique. We consider various configurations, for instance, rectangular (square) lattice–rectangular (square) air cylinders, and different rotating angles of the cylinders in the lattices on either side of the interface of a heterostructure. We find that the absolute gap width and the number of guide modes strongly depend on geometric and physical parameters of the heterostructures. It is anticipated that the guide modes in such heterostructures can be engineered by adjusting parameters.

Zero refraction in natural materials and the mechanism of metal superlens

We found that a single negative material has the characteristic of zero refraction in near field, and point out that the mechanism of a metal superlens to image with a resolution exceeding the diffraction limitation is different from that of a perfect lens made of a double negative material. The principle of metal superlens is disclosed. Our numerical results based on the zero-refraction characteristic is well in agreement with the experimental results. This work brings new understanding about the single negative materials and will lead to the applications of the superlens in the right direction.

Second-harmonic generation and multiple mode effects in aperiodic optical superlattices with finite lateral width

We have studied second-harmonic generation (SHG) from quasi-one-dimensional aperiodic optical superlattices (AOSs) of finite lateral width by inverting poled ferroelectric domains. The search for optimal AOS structures corresponds with solving a difficult inverse source problem. We describe the design principle in real-space representation and undertake model designs. The numerical simulations show that the constructed AOSs can implement multiple-wavelength SHG with identical effective nonlinear coefficients at the pre-assigned wavelengths of the incident light. We investigate the effects of mode–mode coupling and the lateral width of the superlattice on the SHG for two cases: incident light beams of plane-wave and Gaussian profiles. When the number of modes increases, the effective nonlinear coefficient decreases in an oscillatory fashion at the beginning and then tends to a constant. For an incident plane-wave beam, the dependence of the effective nonlinear coefficient on the width of the sample is quite weak, while for an incident Gaussian beam this dependence exhibits a rapid decrease at the beginning and then tends to a constant. We display the variation in the effective nonlinear coefficients with the distance of propagation of the optical wave from where the incident light beam impinges on the sample surface and find that this variation exhibits monotonically increasing behaviour. This clearly infers that the contribution of every block to the optical SHG process takes the form of constructive addition. It is expected that this new design method may provide an effective and useful technique for constructing nonlinear optical material to match various practical applications.

External electric field control of THz pulse generation in ambient air

A theoretical model has been proposed to describe the dependence of the THz wave generated in a laser-induced air plasma on the external electric field. Using this model we predict the following, (i) previously observed results show that the THz pulse enhances linearly with the increase of the external field; (ii) the THz pulse varies as a cosine function with the angle between the direction of the external electric field and the polarization of the incident exciting beam; (iii) and the amplitude is proportional to the square of the intensity of the incident pulse in a low energy region. These predictions are validated by our experiment.

Effect of pump depletion on second harmonic generation in multiple quasi-phase-matching gratings.

Second harmonic generation (SHG) from the aperiodic optical superlattice (AOS) in considering the pump depletion is investigated. It is found the domain configuration designed in undepleted pump approximation (UPA) can also be used to achieve multiple wavelength SHGs with high enough conversion efficiency for an exact solution. The applicable scope of UPA was estimated by a relative tolerance based on the related SHG conversion efficiency calculated in UPA and an exact solution. Results reveal that the relative tolerance is solely determined by the conversion efficiency, and unrelated to the sample configuration, pump intensity, incidental wavelength and nonlinear media. A model to evaluate an exact solution is proposed, and it is suggested that the SHG conversion efficiency can be easily assessed by the developed model. These results can be used to provide direct guidance for practical application, and can also make the estimation of practical samples more convenient.