Nonlinear scattering

Fig. 11. Nonlinear scattering spectra for chloroform solution of nitro-azobenzene dendrimer (curve a) and neat chloroform solution (curve b). Inset is absorption spectrum of sample...

SF radiation may be accompanied with inelastic nonlinear scattering, although the scattering cross section is extremely small, typically in the order of 10 cm" per molecule. Such an inelastic scattering effect in the second-order nonlinear optical... 

There is a close relation between NLO and optical limiting (OL) properties. The main mechanisms to achieve OL are nonlinear absorption (NLA) and nonlinear refraction (NLR), but other effects such as nonlinear scattering can also contribute to OL. Materials with a positive NLA coefficient exhibit reverse saturable absorption (RSA), causing a decrease in transmittance at high intensity levels, and so operate as optical limiters [14]. 

Elimination of intense scattering contributions requires high-quality optical components. The finite contrast factor of polarization optics (106-107) and possible higher-order processes (e.g., six-wave mixing) to the nonlinear scattering limit the suppression potential (39). 

Torres-Verdin, C., and T. M. Habashy, 1994, Rapid 2.5-dimensional forward modeling and inversion via a new nonlinear scattering approximation Radio Sci., 29, 1051-1079. 

The case (3 > 0, opposite to the previous one, can stem in nanocomposite media from reverse saturation of absorption (RSA), multi-photon absorption, or nonlinear scattering. It is of course of high interest for optical limiting applications. 

Let us now examine the case of a nonlinear scattering process. Ispasoiu et al. deduced from considerations regarding the excited-state lifetime that the optical limiting observed using nanosecond laser pulses at X = 532 nm in their silver-dendrimer nanocomposite aqueous solution was due to absorption-induced nonlinear scattering [142]. They suggested that the scattering centres were micro-bubbles... 

Simple analytical results can be obtained for asymmetric configuration when the nonlinear scattering takes place only in the first waveguide and damping is neglected. One eigenvalue is zero, A5 = 0, the others are roots of biquadratic equation... 

The propogation of an intense, nearly resonant, laser pulse, through sodium vapor results in a number of nonlinear scattering processes. These processes have been previously observed in the spontaneous fluorescence Spectra emitted by Na vapor interacting with a laser tuned near the D lines.( ) Here we report the observation of stimulated emission due to the same processes which we obtain whenever the appropriate population differences exist between the states involved in the scattering.(2)... 

The RSA mechanism is typical of moderate input fluences, while at large input fluences (larger than some J cm ) other mechanisms, such as nonlinear scattering, are activated, particularly in solution. 

The use of tuned lasers increases possibilities of the RLS method, however, the radiation intensity still remains much lower than the intensity of the exciting light. This linear spectroscopic method possesses a very low sensitivity and, therefore, it is rarely used for studies in the gas phase. The sensitivity of nonlinear scattering methods is much higher. 

Nonlinear scattering is exemplified by hyper-Raman scattering, induced Raman scattering, coherent anti-Stokes Raman scattering (CARS). 

This temperature independence is also noted in nonlinear scattering involving the mixing of two coherent laser beams in nematic hquid crystals. The coherently scattered wave intensity is also shown to be proportional to Az lK and is thus independent ofT - T. 

Figure 5.9. Typical laser-nematic director axis interaction geometry in a wave mixing experiment to measure the nonlinear refractive index change induced in the nematic liquid crystal. Note that an external dc or ac voltage can be applied in conjunction with the optical field. Also the liquid crystal could be doped with photosensitive molecules or nanoparticulates to enhance their nonlinear scattering efficiency.

---