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We present z-scan measurements of magnetic nanoparticles made from magnetite in both thin film form and colloidal solutions. In order to avoid heating and, thus, spurious effects that could lead to misinterpretation of the z-scan results, an electro-mechanical shutter was added along the beam path in order to guarantee samples thermal relaxation. Two photon absorption coefficient β and nonlinear refractive index n2 were measured as a function of concentration N of absorbing units (Fe3O4). Our magnetite samples presented n2≈−1.5×10−14 cm2/W, similar to that of the liquid carrier, for concentrations below 2×1020 cm−3. n2 increases, in absolute value, to about −10×10−14 cm2/W for a sample three times more concentrated and then decreases with N until about −7×10−14 cm2/W for the most concentrated sample. β presented a linear dependence with N and the two-photon absorption cross section σ2PA was calculated, resulting in σ2PA=50(2) GM for magnetite nanoparticles.

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The nonlinear index of refraction (n_{2}) and the two-photon absorption coefficient (β) of water-based ferrofluids made of magnetite nanocrystals of different sizes and with different coatings have been measured through the Z-scan technique, with ultrashort (femtoseconds) laser pulses. Their third-order susceptibility is calculated from the values of n_{2} and β. The influence of different particles' coatings and sizes on these nonlinear optical properties are investigated. The values of n_{2} and β depend more significantly on the nanoparticles' size than on the particular coating. We observe a decrease of β as the nanoparticles' diameters decrease, although the optical gap is found to be the same for all samples. The results are interpreted considering modifications in the electronic orbital shape due to the particles' nanosize effect.

Practical applications implementing integrated photonic circuits can benefit from nonlinear optical functionalities such as wavelength conversion, all-optical signal processing, and frequency-comb generation, among others. Numerous nonlinear waveguide platforms have been explored for these roles; the group of materials capable of combining both passive and active functionalities monolithically on the same chip is III–V semiconductors. AlGaAs is the most studied III–V nonlinear waveguide platform to date; it exhibits both second- and third-order optical nonlinearity and can be used for a wide range of integrated nonlinear photonic devices. In this review, we conduct an extensive overview of various AlGaAs nonlinear waveguide platforms and geometries, their nonlinear optical performances, as well as the measured values and wavelength dependencies of their effective nonlinear coefficients. Furthermore, we highlight the state-of-the-art achievements in the field, among which are efficient tunable wavelength converters, on-chip frequency-comb generation, and ultra-broadband on-chip supercontinuum generation. Moreover, we overview the applications in development where AlGaAs nonlinear functional devices aspire to be the game-changers. Among such applications, there is all-optical signal processing in optical communication networks and integrated quantum photonic circuits.

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