Nonlinear Absorption and Refraction of Highly Monodisperse and Luminescent ZnTe Quantum Dots and Their Self-Assembled Nanostructures: Implications for Optoelectronic Devices.

Zinc telluride (ZnTe) quantum dots (QDs) were synthesized by a unique supersaturation-controlled aqueous route. For a given pH, increasing the degree of initial supersaturation led to a decrease in the average diameter (d avg) of the QDs and increased monodispersity. Three samples of ZnTe QDs having average sizes of 0.8, 1.7, and 2.2 nm were synthesized (hence named ZnTe_0.8, ZnTe_1.7, and ZnTe_2.2). Nonlinear absorption (NLA) and nonlinear refraction (NLR) of these colloidal ZnTe QDs of different sizes were investigated by the Z-scan technique using a continuous He-Ne laser (632.8 nm, 15 mW). Isotropic assembly of ZnTe_2.2 leads to the formation of nanoballs (hence named ZnTe_NB). The NLA profile of smaller QDs, ZnTe_1.7 and ZnTe_0.8, was found to follow a three-photon absorption (3PA) model, while relatively bigger QDs, ZnTe_2.2, followed a two-photon absorption (2PA) model. On moving from ZnTe_0.8 to ZnTe_1.7, the three-photon absorption coefficient (γ) decreases by 26% (3.00 × 10-4 → 2.21 × 10-4 cm3/MW2). The two-photon absorption coefficient (ß) for ZnTe_2.2 is 0.3 cm/MW. For a 63% decrease in average diameter (2.2 → 0.8 nm), the refractive index (n 2) increases by 45% (2.48 × 10-2 → 3.6 × 10-2 cm2/MW). Overall, the NLR coefficient shows a decreasing trend with size. Upon isotropic self-assembly, ZnTe_NB, there is a significant increase in the NLR coefficient by 40% (2.48 × 10-2 → 3.48 × 10-2 cm2/MW) and a simultaneous decrease in the NLA coefficient by 45% (0.3 → 0.166 cm/MW). The figure of merit was also determined for all of the samples, and it was found that ZnTe_2.2 and ZnTe_0.8 were best suited for all-optical device applications. Further, the self-assembled nanostructures are promising for making optical waveguides for supercontinuum generation (SCG).

Shape and size dependent nonlinear refraction and absorption in citrate-stabilized, near-IR plasmonic silver nanopyramids.

Using a combination of a mild stabilizer and a mild reductant, sodium citrate and hydrazine hydrate, anisotropic silver nanocrystals (NCs) were synthesized with tunable plasmon peaks at 550 nm, 700 nm, 800 nm, 900 nm and 1010 nm (the samples are named Ag-550, Ag-700, Ag-800, Ag-900 and Ag-1010, respectively). TEM investigations revealed that Ag-550 NCs were pentagonal nanoplates while the other four samples were nanopyramids with a pentagonal base with the edge length varying between 15 and 30 nm. The non-linear optical (NLO) properties of these NCs were studied by the Z-scan technique using the CW He-Ne laser (632.8 nm, 15 mW). The shape change from 2D nanoplates (Ag-550) to 3D nanopyramids (Ag-700) resulted in sign reversal of the non-linear refractive index, n2, from a negative (-3.164 × 10-8 cm2 W-1) to a positive one (1.195 × 10-8 cm2 W-1). This corresponds to a change from a self-defocussing effect to a self-focussing one. Besides shape, the size effect is also prominently observed. Amongst nanopyramids, as the edge length increases, n2 increases linearly and reaches a maximum of 3.124 × 10-8 cm2 W-1. Doubling the edge length from 15 nm to 30 nm resulted in 162% increase in n2. On moving from Ag-550 to Ag-900 NCs, with the increasing plasmon wavelength, the non-linear absorption (NLA) coefficient increased exponentially to a high value of 8.52 × 10-4 cm W-1. However, Ag-1010 showed 29% decrease in NLA which is attributed to twinning present in the crystal structure as seen in the HR-TEM images. Due to the tunable NLO properties, these anisotropic Ag NCs hold great potential for applications in optical limiting, switching and data storage.

Optical Properties of Mg, Fe, Co-Doped Near-Stoichiometric LiTaO3 Single Crystals.

Mg, Fe co-doped near-stoichiometric lithium tantalite (SLT) single crystals were grown by employing the zone-leveling Czochralski (ZLCz) technique. The optical properties, holographic parameters, as well as the composition of the grown crystals were measured. It was found that the Li/Ta ratio decreased with the doping of Mg and Fe ions. A red shift was observed in absorption spectrum for the Mg, Fe co-doped crystals compared to the undoped and Mg-doped ones. The effect of the iron ions (Fe2+ and Fe3+) was further discussed based on the specified absorption bands. Moreover, the occupation mechanism for the defects was discussed by using the IR absorption spectrum, which was attributed to the FeTa3- defects in the highly Fe-doped crystal. In addition, the holographic parameters were also found to be improved with a higher Fe/Ta ratio in the crystals.