Optical limiting applications

Most promising applications of C60 fullerene are in the field of optoelectronics and nonlinear optics (NLO). Photovoltaic cells and infrared detectors are among actively pursued optoelectronic applications while, in the field of NLO, after some initial emphasis on third-order responses, such as intensity dependent refractive index, the focus is now mainly on optical limiting based on the mechanisms of reverse saturable absorption (RSA) and possibly other multiphoton processes. In the first section we define the ideal behavior of an optical limiter and summarize the physical mechanisms that may lead a real material to approach such an ideal behavior. The reverse saturable absorption (RSA) mechanism is emphasized and the nonlinear optical techniques for determining optical limiting properties are described. In section 3 the linear and nonlinear optical properties of fullerenes are illustrated. The third order NLO responses indicate that fullerene is a good optical material, while photophysical properties and absorption spectra of the ground the and excited states suggest the presence of a strong RSA in a large visible range. The experimental OL properties of fullerenes are described in section 4. The OL behavior of C60 in solution has been measured in a large wavelength range and compared with the calculated data, based on the RSA model. The presence of a prevailing RSA contribution is demonstrated but nonlinear scattering and nonlinear refractive changes are shown to contribute. In section 5 the optical properties of fullerene derivatives in solution are characterized, and compared with those of C60 and of a benchmark OL material like Sn-phthalocyanine. The issues related with preparing a solid OL material by inclusion of fullerene derivatives in a host matrix are discussed in section 6 and it is shown that with the use of suitably designed hybrid sol-gel glasses the OL performances of solution samples can be preserved. In section 7 it is shown that fullerenes, thanks to their electro-active properties, offer opportunities to implement different mechanisms of OL with the aim of extending the wavelength range of operation. Finally, section 8 addresses some current issues pertaining to the design and manufacture of practical OL devices and section 9 concludes this chapter with a summary and a few final remarks.