Nonlinear optical properties media

When metallic nanoparticles are added to a biological medium, its linear and nonlinear optical properties are changed. In particular, the analysis of the linear optical absorption is one of the methods most commonly used to study colloids... 

Most of the studies devoted to the nonlinear optical properties of metal nanoparticles use the notation x (<>>) to refer to the susceptibility for the optical Kerr effect. Unless otherwise specified, we will also adopt this simplified designation in the following. Let us just recall that it corresponds, in fact, to an experimental situation where a unique plane wave, linearly polarized (or three plane waves with same polarization and frequency), generates the third-order nonlinear optical phenomenon in an isotropic medium at the same frequency, and that the susceptibility is a priori a complex quantity. 

As for the linear optical response, different approaches have been proposed to describe the nonlinear optical properties of nanocomposite media. Nevertheless, a few general principles can be identified. First, each component of such a medium possesses its own susceptibility however, as the typical structure size is much smaller than the wavelength, the observable result of light interaction with the medium is different from a simple combination of the individual responses of the separated constituents (again, we do not treat the case of spatially-resolved studies of die optical response). One is then again led to introduce the concept of effective medium, extended to the case of nonlinear optical properties. 

Whatever the degree of approximation used in evaluating the effective nonlinear susceptibility of a composite medium, it can be seen in Eqs. (22), (23) or (27) that the result depends on the product of two complex quantities One linked with the medium morphology and composition (the local field factor), the other linked with the nonlinear optical properties of the metal inclusions themselves (the intrinsic third-order susceptibility, Xm ) - inasmuch as the own contribution of the host matrix to the whole nonlinear response still remains negligible. We will focus here on the second factor. It is noteworthy that very few theoretical work has been accomplished regarding the value of Xm for noble metal nanoparticles after the pioneering smdies of Flytzanis and coworkers [79, 80, 89, 90]. Moreover, as will be underlined below, their results may not be used in every experimental situation as they are. 

One of the main consequences of dielectric confinement for the third-order nonlinear optical properties is the fact that the response of a composite medium can be very different in both sign and magnitude from the one of its constituents [73, 89, 94]. 

Thermal lensing contribution to the measured nonlinear optical properties. If the pulse duration is longer than the characteristic time of the heat diffusion in the medium, or if this time is itself longer than the delay between successive pulses, material heating may lead to an observable transient thermal lens phenomenon [120, 165, 212, 218, 219], This can show itself, in experiments, with characteristics similar to those of a pure (electronic) Kerr effect. There have been some attempts to extract the respective values of the thermal and electronic contributions to y from z-scan measurements [136, 160, 165, 166, 175, 220], However, de Nalda et al. proved later that this method was not reliable enough to get quantitative results [219],... 

Laser-induced molecular reorientation is a common cause of optical nonlinearity in a fluid medium. In this respect, liquid crystals are often strongly nonlinear because of their large molecular anisotropy and strong correlation between molecules. The nonlinear optical properties of liquid crystals in the isotropic phase have already been studied quite extensively by a number of researchers in the past decade, This is, however, not true for liquid crystals in the mesophases. 

The first optical effect pointed out by Wang [13], and studied by computational simulations, is so-called dielectric confinement. Dielectric confinement is caused by the difference in refractive indices of a polymer medium (which has lower refractive index) and a semiconductor or metal particle (which usually has hi er refractive index). When illuminated by light, the field intensity near, at and inside the particle surface can be enhanced considerably compared to the inddent intensity becau of tte boundary established by the different refractive indices. This local ld enhancement eflect can have important con quences on photophysical and nonlinear optical properties of such polyn r-nancqmrtide systems. 

For isotropic media < 2> and < 4> are zero. Hence X333 reduces to the first term of the Langevin function. If the first-order parameters approach unity, the value of is five times larger than for an isotropic medium. This large difference has created interest in the use of liquid crystals, both low-molar-mass and polymeric, for nonlinear optics [24]. The nonlinear optical properties of poled polymers and poled pre-oriented polymers have been analysed thoroughly by the group at AT T Bell Laboratories [25, 26]. 

Nonlinear second order optical properties such as second harmonic generation and the linear electrooptic effect arise from the first non-linear term in the constitutive relation for the polarization P(t) of a medium in an applied electric field E(t) = E cos ot. 

A novel second-order nonlinear optical medium which should offer considerable fabrication flexibility has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation of complex dynamical and thermal behavior indicates that an important role is played by the polymer liquid crystalline host. Additional properties of modified members of this family of lc polymers were consequently investigated. The explanations of guest alignment stabilization and thermal dependence of the alignability remain unresolved issues. 

When a strong static electric field is applied across a medium, its dielectric and optical properties become anisotropic. When a low frequency analyzing electric field is used to probe the anisotropy, it is called the nonlinear dielectric effect (NLDE) or dielectric saturation (17). It is the low frequency analogue of the Kerr effect. The interactions which cause the NLDE are similar to those of EFLS. For a single flexible polar molecule, the external field will influence the molecule in two ways firstly, it will interact with the total dipole moment and orient it, secondly, it will perturb the equilibrium conformation of the molecule to favor the conformations with the larger dipole moment. Thus, the orientation by the field will cause a decrease while the polarization of the molecule will cause an... 

We examine the optical properties of a nonlinear medium in which nonlinearities of order higher than the second are negligible, so that... 

Many of the different susceptibilities in Equations (2.165)-(2.167) correspond to important experiments in linear and nonlinear optics. x<(>> describes a possible zero-order (permanent) polarization of the medium j(1)(0 0) is the first-order static susceptibility which is related to the permittivity at zero frequency, e(0), while ft> o>) is the linear optical susceptibility related to the refractive index n" at frequency to. Turning to nonlinear effects, the Pockels susceptibility j(2)(- to, 0) and the Kerr susceptibility X(3 —to to, 0,0) describe the change of the refractive index induced by an externally applied static field. The susceptibility j(2)(—2to to, to) describes frequency doubling usually called second harmonic generation (SHG) and j(3)(-2 to, to, 0) describes the influence of an external field on the SHG process which is of great importance for the characterization of second-order NLO properties in solution in electric field second harmonic generation (EFISHG). 

---