Li-Ming Zhao

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...

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.

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.

An optical one-way device constructed with an epsilon-near-zero prism inserted in a metal slit

We demonstrate theoretically that an optical one-way transmission device can be constructed by inserting an epsilon-near-zero (ENZ) prism into a metal slit. Similarly to the junction in the electron diode, the oblique surface of the ENZ prism is a key factor that can block the transmission of the light striking on it and allows the transmission of the light leaving from it. The performance of the device is shown by numerical simulation based on the finite-difference time-domain technique.

A PHENOMENOLOGICAL CONSTITUTIVE MODEL OF ALUMINUM ALLOY FOAMS AT VARIOUS STRAIN RATES

A nonlinear elasto-plastic phenomenological constitutive model for aluminum alloy foams subjected to quasi-static and dynamic compression is proposed. The six-parameter model can fully capture the three typical features of stress-strain response, i.e., linearity, plasticity-like stress plateau, and densification phases. Moreover, the parameters of the model can be systematically varied to describe the effect of initial density of foams that may be responsible for changes in yield stress and hardening-like or softening-like behavior at various strain rates. The experimental results at various loading rates are provided to validate the model. It is shown that the proposed model can be used in the selection of the optimal-density and energy absorption foam for a specific application based on certain design criteria.

Optimal design of aperiodic optical superlattices for achieving parametric amplification or second harmonic generation with consideration of the depletion of pumping light power

The parametric amplification (PA) and second harmonic generation (SHG) from quasi-one-dimensional aperiodic optical superlattices (AOSs) are investigated in the cases of large, intermediate, and small signals of the pumping light wave. The optimal design of the AOSs is carried out with the use of the simulated annealing (SA) method. The numerical simulations show that the constructed AOSs can achieve multiple wavelength PA (or SHG) with identical amplification (or identical conversion efficiency) at the pre-assigned wavelengths. The variations of the normalized intensities of the PA signal (or the pumping fundamental wave (PFW)) for the PA process with the distance of propagation of the light wave from the interface of the AOSs at the pre-assigned wavelengths exhibit monotonically increasing (or decreasing) behavior. So do the variations of the normalized intensities of the second harmonic wave (SHW) (or the fundamental wave (FW)) for the SHG. This strongly supports the fact that the contribution of each individual unit domain in the constructed samples to the PA (or SHG) is constructive accumulation, favorable for the PA (or SHG) process. The saturation phenomenon of the PA (or SHG) signal is observed.

Multiple wavelength second-harmonic generation in one-dimensional nonlinear photonic crystals

We investigate one-dimensional nonlinear photonic crystals consisting of ferroelectric domains with the modulated polarization direction. The optimal arrangement of the polarization directions of the ferroelectric domains can be achieved with the use of the simulated annealing method. We find that the second-harmonic generation (SHG) is increased significantly when the frequencies of the fundamental wave aim at the photonic band edges and the special arrangement of the polarization directions of the ferroelectric domains is satisfied. The optimal structure can achieve simultaneously multiple wavelength SHGs with nearly identical conversion efficiencies for both the “forward” and “backward” SHGs.

CHARACTERISTICS OF DEFECT STATES IN ONE-DIMENSIONAL PHOTONIC CRYSTALS CONTAINING MULTIPLE PHOTONIC QUANTUM-WELLS

We calculate the omnidirectional dispersion spectrum for one-dimensional (1D) photonic crystals (PCs) inserting multiple photonic quantum-wells (PQWs) with the use of the plane-wave expansion method in combination with a supercell technique. We find that the localized defect modes significantly depend on the number of the PQWs and the configuration. This may provide a new way to engineer the dispersions of the composite PCs to meet the requirements of optical or microwave devices.

EVOLUTION OF THE BAND GAP STRUCTURES IN ONE-DIMENSIONAL PHOTONIC CRYSTALS WITH THE MODIFICATIONS OF THE STRUCTURES FROM PHOTONIC QUANTUM WELLS TO HETEROSTRUCTURES

We investigate the evolution of the band gap in one-dimensional (1D) photonic crystals (PC) with the gradual modifications of structural configurations from photonic quantum wells to PC heterostructures. The transmissive spectra are calculated with the use of scattering matrix method. We find that the transmissive spectra and confined states strongly depend on the structural configurations of samples.

A promising nonlinear conjugate-gradient method proposed to design nonlinear domains with a disordered distribution

A new method, namely the nonlinear conjugate-gradient (NCG) method, is proposed to design nonlinear domains with a disordered distribution, in which an efficient broadband second harmonic generation can be achieved simultaneously with high conversion efficiency. It is demonstrated by numerical simulation that the NCG method has obvious advantages in realizing the optimal quasi-phase-matching, in comparison with the traditional simulated annealing method.