Nonlinear optical properties of free standing films of PbS quantum dots in the nonresonant femtosecond regime

Abstract

Devices based on optical technology for high speed communication networks require materials with large nonlinear optical response in the ultrafast regime. Nonlinear optical materials have also attracted wide attention as potential candidates for the protection of optical sensors and eyes while handling lasers. Optical limiters have a constant transmittance at low input influence and a decrease in transmittance at higher fluences and are based on a variety of mechanisms such as nonlinear refraction, nonlinear scattering, multiphoton absorption and free carrier absorption. As we go from bulk to nanosized materials especially in the strong quantum confinement regime where radius of the nanoparticle is less than the bulk exciton Bohr radius, the optical nonlinearity is enhanced due to quantum confinement effect. This paper is on the ultrafast nonresonant nonlinearity in free standing films of PbS quantum dots stabilized in a synthetic glue matrix by a simple chemical route which provides flexibility of processing in a variety of physical forms. Optical absorption spectrum shows significant blue shift from the bulk absorption onset indicating strong quantum confinement. PbS quantumdots of mean size 3.3nm are characterized by X-ray diffraction and transmission electron microscopy. The mechanism of nonlinear absorption giving rise to optical limiting is probed using open z-scan technique with laser pulses of 150 fs pulse duration at 780 nm and the results are presented in the nonresonant femtosecond regime. Irradiance dependence on nonlinear absorption are discussed. KeywordsPbS quantum dots; femtosecond z-scan; non resonant optical nonlinearity; optical limiting 1.INTRODUCTIONBoth the linear and nonlinear optical properties of semiconductor nanocrystals are known to differ appreciably from those of the bulk semiconductor 1,2 , particularly in the regime of strong quantum confinement, when the cluster sizes are below a critical size called bulk exciton Bohr radius. In this regime, the bulk semiconductor energy bands are split into discrete energy levels with an increase in the effective bandgap. Materials with large optical nonlinearities have immense potential in various photonics device applications like optical switching and optical limiting. This has led to a lot of interest in exploring the linear as well as nonlinear optical processes and mechanisms in these materials. PbS is a direct band gap material with bulk bandgap of 0.41eV and exciton bohr radius of 9nm.

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“…Shashkov et al demonstrated PA signals acquired from core-shell CdSe/ZnS Qds. PbS nanocrystals, however, which demonstrate very weak PA signals, are generally not used for photoacoustic imaging [ 95 ].…”