Non-linear optical effect

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

Spontaneous polarization and non-linear optical effect in niobium and tantalum fluoride compounds... 

Flash Photolysis and Pulse Radiolysis Studies Non-linear Optical Effects 142... 

The fact that the absorption spectrum of CdS changes when electrons are produced on it may be regarded as a non-linear optical effect, i.e. a dependence of the absorption coefficient on the light intensity. Other non-linear effects, which are caused... 

One of the more interesting applications of non-linear optical effects is the generation of the second harmonic. This phenomenon results when a laser beam passes through a material having second-order NLO properties (hence, composed by non-centrosymmetric molecules) the light emitted has a frequency double that of the incident radiation (or the wavelength has been halved). 

With a non-linear susceptibility, the polarization is composed of non-linear terms such as eox E, eoX E as well as the linear term cqx E. It is clear that these elements in the polarization maybe oscillating at 2co and 3co respectively, giving rise to harmonics of the original frequency co. Because the higher order terms in the susceptibility are small compared with the first term, non-linear optical effects were not observed until after the invention of the laser in 1960. 

Non-linear optical effects were observed for the 551 polymer-encased gold particles... 

A single crystal is a homogeneous solid, which means that all parts within it have identical properties. However, it is not in general isotropic, so that physical properties such as thermal and electrical conductivity, refractive index, and non-linear optical effect generally vary in different directions. 

Liquid crystals are interesting supramolecular systems which can show second harmonic generation when they are aligned appropriately. Ferroelectric LCs [250] as well as bent-core molecules have been used to this purpose, and show reasonable second harmonic generation [251]. These materials combine non-linear optical effects with simple processing procedures on account of their liquid crystalline flow characteristics and the possibility of organising them with electric and magnetic fields. 

Prasad PN, Williams DJ (1991) Introduction to Non-linear Optical Effects in Molecules and Polymers. Wiley, New York... 

For most materials the higher-order non-linear optical effects are small and extremely difficult to detect. So the NLO effects investigated most are the second-order effects. 

Non-linear optical effect An effect brought about by electromagnetic radiation the magnitude of which is not proportional to the irradiance. Non-hnear optical effects of importance to photochemists are harmonic frequency generation, lasers, Raman shifting, upconversion, and others. 

In the last few decades, metal clusters and nanomaterials have attracted increasing attention due to their unique properties which differ from those of the bulkd Metal nano-objects are of great interest in the field of nanosciences. They are the ideal structures for fundamental research and applications. Indeed, from the confinement of the charge carriers in such limited objects, one expects a shift of the plasmon resonance absorption, non-linear optical effects, non-metallic conductivity, and nanocatalytic effects. Nanomaterials can have important applications in several fields such as catalysis, electrocatalysis, electronics, optical limitation, biology, etc. 

Non-linear Optical Effects.—Usual light (thus, a weak analysing li t beam), when traversing a transparent body, does not affect the latter s optical proporties because the electric and magnetic polarizations induced are linear functions [equations (7) and (8)] of the vectors E and HJ, ... 

Generation of harmonic optical frequencies. In 1961, Franken et al. performed an experiment of Mgh aesthetic value in wMch the red beam of a ruby laser (A = 694.3 nm), focused on to the surface of a quartz plate, gave rise to an emerging beam which contained a weak component of doubled frequency with respect to the inddent beam (u.v., A = 347.15 run). Since the detection of this earliest non-linear optical effect, second and third harmonics have been obso ed in various materials in both transmitted and reflected light. ". 

Figure 8 Non-linear optical effects. Scheme for the formation of two types of scattering centres at increasing fluences ofa laser pulse used to excite gold clusters. High fluence vaporized and expanded metal clusters limit light transmission Immediately after the pulse and are replaced, after energy transfer to the solvent, by solvent bubbles acting as secondary scattering centres. Medium fluence only solvent bubblesare formed. Low fluence. light transmission depends linearly on the Intensity.

X and x are the non-linear contributions to the susceptibility. Non-linear optical effects become possible with high-powered lasers. SHG and VSFS depend on x the second-order non-linear susceptibility of the medium at an interface. More details about these two methods are given below. 

Fig. 36.25. Relative importance of electronic and vibrational contributions for various third-order non-linear optical effects.

The 38-metal-atom cluster (Ph3P)j2Auj8Ag2oCli4 is reported to be a highly efficient optical limiter in a polymer matrix or in solution. It was proposed that the optical limiting behavior is related to the third-order non-linear optical effect. 

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