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This paper reviews the vibrant field of nonlinear optical (NLO) properties of metal oxide nanoparticles and nanocomposites. It includes a historic introduction which discusses the birth and successive developments of NLO. An attempt is made to give a summary in a nutshell about the basic concepts of nonlinear optics and different NLO phenomena with special attention to third-order phenomenon and $Z$-scan technique. The review also touches on the topic of science and technology of metal oxide nanoparticles and nanocomposites, various synthesis processes that include co-precipitation, micro-emulsion, sol–gel route, etc. have been discussed. Metal oxide nanoparticles and nanocomposites are introduced with a deep focus on the origin of various physical mechanisms and optical properties. The origin of NLO properties from these important structures has also been discussed. Some recently studied metal oxide nanocomposites along with their NLO coefficients are also given.

The optical limiting properties of three meso-pyridyl-distyrylBODIPY dyes with differing alkyl substitution patterns at the 2,6- and 1,7-positions on the pyrrole moieties are investigated. Styryl groups are introduced at the 3,5-positions to form dyes with strong donor-$\pi$-acceptor (D-$\pi$-A) properties. Favorable optical limiting properties are obtained for all three dyes, with the highest second-order hyperpolarizability obtained for a dye that is unsubstituted at both the 2,6- and 1,7-positions suggesting that the optical limiting properties are enhanced when the meso-aryl ring can freely rotate into the plane of the BODIPY core.

The lack of suitable optical limiters that cover the entire UV-visible-NIR region has motivated research to identify novel efficient and stable optical limiting materials. There has been a strong focus on attenuating the second harmonic of Nd:YAG pulsed lasers at 532 nm on the nanosecond timescale. The photophysicochemical processes of a series of mono- and di-(4-dimethylamino)styryl-meso-pyrenylBODIPY and mono- and di-(4-dimethylamino)buta-1,3-dienyl-meso-pyrenylBODIPY dyes were studied by UV-visible absorption and fluorescence emission spectroscopy, and by determining the fluorescence and triplet state lifetimes. Z-scan measurements were carried out at a fixed concentration of 10 $\mu$M so that the effective nonlinear coefficients, third-order susceptibilities, second-order hyperpolarizabilities and limiting fluences of the dyes can be compared and trends in the structure-property relationships for these optical limiting parameters can be identified.

The optical limiting properties of a 1,7-dithien-3-yl-3,5-di-4-hydroxyphenyl-azaBODIPY dye and its silver nanoparticle (AgNP) conjugate on the nanosecond timescale at 532 nm are reported. The photophysicochemical properties are compared to those of the parent 1,3,5,7-tetraphenylazaBODIPY complex. The thien-3-yl rings at the distal positions facilitate conjugation to AgNPs through the Ag-S interaction and a significant red shift of the main azaBODIPY spectral band. Favorable optical limiting properties are obtained that suggest 1,3,5,7-tetraarylazaBODIPYs and their nanoparticle conjugates merit further in-depth study in this context. This is further enhanced by conjugation to AgNPs.