Vertex correction effects on nonlinear optical response

The effects of vertex corrections due to frequency modulation are exactly evaluated on the emission spectra from the two-level electronic system pumped by the laser of arbitrary intensity and the absorption spectra of the weak probe field of this system. Here the frequency modulation is assumed to obey the Gaussian-Markoffian process. The two-time correlation function of the polarizations which describe the emission and absorption spectra is represented as the product of two propagators and a vertex correction. This correction is described in the form of the summation of the products of the matrix continued fractions. This correction is numerically demonstrated to be quite large for the emission spectrum in the case of the slow or strong frequency modulation and weak laser field, and for the absorption spectrum in the case of intermediate modulation and laser intensity.     

Local-field correction of the charged Bose condensate for two and three dimensions

The local-field correction for the dielectric function of the two-dimensional and of the three-dimensional Bose condensate is calculated within a sum-rule version of the Singwi et al. (Phys. Rev.176, 589 (1968)) approach. We derive analytical expressions for small and large wave numbers and give analytical expressions for the density dependence. We compare the results of the groundstate energy for the three-dimensional system with Monte-Carlo computations. In two dimensions a roton structure in the plasmon dispersion is found at low boson density. The plasmon density of states is calculated. A correlation induced charge-density-wave instability in layered structures of two-dimensional Bose gases is discussed.     

Local-field effects on the optical properties of non-cubic crystals

A method is developed that allows effective calculation of local-field effects on optical properties of non-cubic crystals in the Random Phase Approximation, expanding electron wave-functions on localized orbitals. Adler and Wiser's formula of the macroscopic dielectric constant is generalized to the case of non-cubic crystals, where a macroscopic dielectric tensor is defined in terms of Wannier functions.     

Instability of layered quantum liquids: 1. Local-field correction in superlattices

For a superlattice with periodd the Singwi et al. (Phys. Rev.176, 589 (1968)) approach for the local-field correction and the static structure factor is formulated. With two approximations we reduce the resulting three-dimensional integral-equation into a one-dimensional integral-equation. For the local-field correction we present analytical results for small wave numbers and large wave numbers. An expression of Hubbard-typ is derived for the local-field correction. Explicit results for boson superlattices and electron superlattice are given. A charge-density-wave instability in a layered Bose condensate withr _s>r_scd^3/4 is discovered.r _s is the small parameter of the random-phase-approximation. The charge-density-wave instability is due to a many-body anomaly (short-range correlations) in layered structures and is a general property of layered quantum liquids. We find the charge-density-wave instability in a layered electron gas forr _s>r_scd. Double-quantum-well structures are also considered. The effects of a finite well width is calculated. The general implications of the charge-density-wave instability for microscopically layered quantum liquids are pointed out.     

Determination of optical constants and nonlinear optical coefficients of Violet 1-doped polyvinyl alcohol thin film

The optical properties of Violet 1-doped polyvinyl alcohol (PVA) have been investigated using Wemble and Didomenico (WD) method. The optical constants such as refractive index n, the dispersion energy E _d, the oscillation energy E ₀, the lattice dielectric constant \(\varepsilon _{\infty } \), light frequency dielectric constant ε ₀ and the ratio of carrier concentration to the effective mass N/m* have been determined using reflection spectra in the wavelength range 300–900 nm. The single- beam Z-scan technique was used to determine the nonlinear optical properties of Violet 1:polyvinylalcohol (PVA) thin film. The experiments were performed using continuous wave (cw) laser with a wavelength of 635 nm. The calculated nonlinear refractive index of the film, n ₂ = ?2.79×10^?7 cm²/W and nonlinear absorption coefficient, β = 6.31×10^?3 cm /W. Optical limiting characteristics of the dye-doped polymer film was studied. The result reveals that Violet 1 can be a promising material for optical limiting applications.     

极化子效应对量子盘中线性和非线性光吸收系数的影响

利用密度矩阵的方法,得出了考虑极化子效应的量子盘的线性和非线性光吸收系数的解析表达式,并以GaAs为例讨论了光吸收系数与不同的入射光子能量和量子盘的厚度之间的关系.结果表明,极化子效应对吸收系数有相当的影响 关键词： 量子盘 光学吸收系数 极化子效应     

Linear and nonlinear optical absorption coefficients in an asymmetric graded ridge quantum wire

In this article, the optical absorption coefficients in an asymmetric ridge quantum wire within the framework of the density matrix formalism are studied. The energy spectrum and wave functions of a quantum wire with graded confinement potential using the effective mass approximation are analytically calculated. The results show that parameters such as the asymmetry and width of the potential well change the position and magnitude of the absorption peak and saturation intensity. The incident optical intensity also has a great effect on the total absorption.     

Study of intensity-dependent nonlinear optical coefficients of GaP optical crystal at 800 nm by femtosecond pump-probe experiment

The intensity-dependent two-photon absorption and nonlinear refraction coefficients of GaP optical crystal at 800 nm were measured with time-resolved femtosecond pump-probe technique.A nonlinear refraction coefficient of 1.7 × 10-17 m2/W and a two-photon absorption coefficient of 1.5 × 10-12 m/W of GaP crystal were obtained at a pump intensity of 3.5 × 1012 W/m2.The nonlinear refraction coefficient saturates at 3.5 × 1012 W/m2,while the two-photon absorption coefficient keeps linear increase at 6 × 1012 W/m2.Furthermore,fifth-order nonlinear refraction of the GaP optical crystal was revealed to occur above pump intensity of 3.5 × 1012 W/m2.     

Linear and nonlinear optical absorption coefficients and binding energy of a spherical quantum dot

The binding energy and wavefunctions of the 1s, 1p, 1d and 1f energy states of a spherical quantum dot (QD) with parabolic potential were calculated by using a method which is a combination of the quantum genetic algorithm (QGA) and the Hartree–Fock–Roothaan (HFR) approach. In addition, the linear and the third-order nonlinear optical absorption coefficients based on optical transitions in QDs with and without impurity were calculated. The results show that the parabolic potential has a great effect not only on the binding energies and but also on the optical absorption coefficients. Moreover, the calculated results also reveal that the linear and nonlinear optical absorption coefficients are strongly affected by the existence of impurity and the incident optical intensity.     

粉末倍频系数的模拟研究

根据Kurtz粉末倍频理论,使用光纤光谱仪自行设计搭建一个粉末倍频测试系统,该系统可在基频光波入射方向不变的情况下,通过转动旋转臂在不同方向探测二次谐波,并且具有较高的精确度和灵敏度。使具有位相匹配特性的颗粒尺寸范围扩大为46μm～250μm 。使用波长为1.064μm的基频光,对KIO3晶体粉末的倍频效应进行了测试。测试结果表明,KIO3晶体粉末透射倍频光具有位相匹配特性。这一测试结果与已报道结果相一致。通过数值模拟实验测试曲线,获得了KIO3晶体粉末的倍频系数d2ω的值为9.7×10-12m/V。这一模拟值与已报道的KIO3晶体对该基频光波产生的倍频系数d32的值相吻合。由此可见,该方法切实可行。并且可推广应用到其他晶体材料的非线性特性测试研究。     

Designing novel chalcone single crystals with ultrafast nonlinear optical responses and large multi-photon absorption coefficients

A chalcone single crystal, 1-(4-chlorophenyl)-3-(4-methoxyphenyl)prop-2-en-1-one that is transparent over the visible to infrared region is introduced as a new potential material to third-order nonlinear optical applications. The crystal exhibits ultrafast optical response (≤90 fs) and large optical nonlinearity in the wavelength range 800–1200 nm. A very large effective two-photon absorption coefficient β_eff exceeding 120 cm/GW can be obtained with this chalcone crystal, at a low intensity threshold of 41 MW/cm². The mechanism of nonlinear absorption at different levels of intensity has been discussed. The crystal shows no damage against the laser pulse intensity as high as 8 GW/cm². We discuss the molecular and crystal designing of chalcones with large and ultrafast optical nonlinearity combined with low optical cut-off (<450 nm).     

Local-field fluctuations in paramagnets

Equations are derived for the local-field fluctuations in a paramagnet. A comparison is made with the second moment and with the Lorentz local field.     

Redetermination of the nonlinear optical coefficients of proustite by comparison with pyrargyrite and gallium arsenide

A redetermination of the values of the nonlinear coefficients of proustite has been performed in order to remove the present inconsistency between the values appearing in the literature. It has been found that

$\text{|d}{}_{\mathrm{+}}\text{/d\_| = 1.8 ± 0.1, |d\_(Ag}{}_3\text{AsS}{}_3\text{)| = (3.2 ± 0.5)|d\_(Ag}{}_3\text{SbS}{}_3\text{)|, |d}{}_{22}\text{/d}{}_{31}\text{| = 1.6 ± 0.1, |d}{}_{22}\text{(Ag}{}_3\text{AsS}{}_3\text{)| = (0.2 ± 0.03)|d}{}_{14}\text{(GaAs)|}$

.     

Dielectric superlattices for nonlinear optical effects

Dielectric superlattices show a lot of new phenomena that are promising in applications, such as nonlinear optical frequency conversion and optical bistability. This paper reviews the main achievements in this field. Much has been focused on the work done in our Lab. Since 1980, we have extended the concept of superlattice from semiconductor to dielectrics. When quasi-phase-matching technique is introduced into a quasiperiodic dielectric superlattice, more interesting phenomena can occur. For example, multiwavelength second-harmonic generation and coupled parametric processes such as direct third-harmonic generation have been realized with high efficiency. A new mechanism for optical bistability in a 2-dimensional dielectric superlattice, i.e. a 2-dimensional nonlinear photonic crystal, has been proposed theoretically had proved experimentally.     

On optical diffusion coefficients for step-index multimode guides

Optical diffusion coefficients, computed from ray-tracing in step-index slab guides with small continuous width perturbations, show an angular dependence which does not in general correspond well with the mechanical deformation spectrum. This contrasts with assumptions of a one-to-one correspondence in earlier reports. The results indicate that, in a fibre having a steep cut-off in the perturbation spectrum at high mechanical frequencies, the corresponding diffusion coefficient will have a significantly less pronounced cut-off. It is also shown that, in a cylindrical guide with diameter perturbations, the effect of skew rays is to further reduce the cut-off rate in the diffusion coefficient.An immediate consequence of these findings is that, in a perturbed fibre without appreciable cladding loss, the loss penalty (due to radiation) may be much higher than would be predicted by the mechanical spectrum alone.As a prelude to the above ray-tracing studies, a derivation of the optical diffusion equation is given from the analogy with Brownian motion.     

Measurement of the third order nonlinear optical coefficients of GaP and chirp parameter of laser pulse

The spectral resolved femtosecond pump-probe experiment is applied to measure the third order nonlinear refractive index coefficient, two-photon absorption coefficient of GaP crystal, and the chirp parameter of the input laser pulses. The results show that nonlinear refractive index coefficient is 2× 10-18 m2/W, twophoton absorption coefficient is 2× 10-11 m/W at wavelength of 783.5 nm, and the chirp parameter of laser pulse is l× 1025 s-2. Furthermore, the mechanism of nonlinear refraction due to two-photon absorption in GaP crystal and experimental results are discussed.