Optical limiters nonlinear absorption mechanism

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]. 

Nonlinear absorption provides the most useful mechanism for optical limiting, so it is desirable to use the induced reflectivity in conjunction with nonlinear absorption. The target material is then one in which alternate layers contain a dye that possesses both nonlinear absorption and nonlinear refraction. This material has a modulation in both the real and imaginary (absorptive) parts of the index. This paper describes the progress towards such a material. 

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

It is important to note that since the thermal/density and order parameter changes could be induced in microseconds or tens of nanoseconds (cf preceding chapters), these director-axis-reorientation or order-parameter-change mediated limiting actions will work well for sensor protection in these time scales. For shorter laser pulses, for example, nanosecond and picosecond or subpicosecond laser pulses, the response will not be able to build up sufficiently in time to provide the necessary attenuation effect. In those time regimes, electronic optical nonlinear mechanisms, in particular, nonlinear photonic absorptions, have to be employed. This is discussed in... 

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