Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the functi...

Nonlinear Optical Materials for the Smart Filtering of Optical Radiation.

g-C3N4/carbon dot-based nanocomposites serve as efficacious photocatalysts for environmental purification and energy generation: A review

In this review, we survey the recent advances in nonlinear optics and the applications of two-dimensional (2D) materials. We briefly cover the key developments pertaining to research in the nonlinear optics of graphene, the quintessential 2D material. Subsequently, we discuss the linear and nonlinear optical properties of several other 2D layered materials, including transition metal chalcogenides, black phosphorus, hexagonal boron nitride, perovskites, and topological insulators, as well as the recent progress in hybrid nanostructures containing 2D materials, such as composites with dyes, plasmonic particles, 2D crystals, and silicon integrated structures. Finally, we highlight a few representative current applications of 2D materials to photonic and optoelectronic devices.

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Nonlinear optical properties and applications of 2D materials: theoretical and experimental aspects

Metal nanoparticles (MNPs) hold great technological promise because of the possibility of engineering their electronic and optical properties through material design. One of the effective methods to fabricate MNPs is ion implantation. In this review, recent results on the nonlinear optical properties of nanoparticles (including copper, silver, gold, and bismuth nanoparticles) doped in various bases have been discussed. Some specific optical nonlinear properties, such as nonlinear refraction, two-photon absorption, and optical limiting, for femtosecond, picosecond, and nanosecond laser pulses have also been covered. In addition to ion implantation, we have summarized several other methods for the preparation of composite materials, and Z-scan has been used to study the nonlinear optical properties of these materials.

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Nonlinear optical properties of metal nanoparticles: a review

The structure–property relation of a novel metal–organic cocrystal piperazine-1,4-diium tetrachloridocobaltate(II) monohydrate (abbreviated as PCo) for third-order nonlinear optical applications is reported. The acid hydrolysis during pH optimization yielded anionic $$[\hbox {CoCl}_4]^{2-}$$
 and cationic $$(\hbox {C}_{4}\hbox {H}_{12}\hbox {N}_{2})^{2+}$$
 which together formed a stable chemical structure. The bulk single crystals were grown by slow solvent evaporation method with optimized solution pH of 3.5. Its response to the single-crystal X-ray diffraction confirmed PCo belongs to the monoclinic (
 $$P2_1/c$$
 ) crystal system. Investigated thermal, mechanical, linear absorption and emission properties show the suitability of PCo for optoelectronic device applications. The calculated molecular interaction energy at B3LYP/6-31G(p,d) level reveals the possibility of noncovalent intermolecular charge transfer via $${\hbox {N}- \hbox {H}\cdots \hbox {Cl}}$$
 , $${\hbox {N}-\hbox {H}\cdots \hbox {O}}$$
 and $${\hbox {O}-\hbox {H}\cdots \hbox {Cl}}$$
 types of interactions. Rich availability of polarizable electronegative interactions between radicals upon laser stimuli enhanced nonlinear optical property in PCo cocrystal. The magnitude of third-order nonlinear optical susceptibility (
 $$\chi ^{(3)}$$
 ), nonlinear refractive index (
 $$n_2$$
 ) and nonlinear absorption coefficient (
 $$\beta $$
 ) of PCo cocrystal under continuous-wave laser excitation were found to be $$(9.32\pm 0.01)\times 10^{-6}$$
 esu, $$(1.22\pm 0.006)\times 10^{-8}\,\mathrm{cm}^2\mathrm{W}^{-1}$$
 and $$(1.43\pm 0.001)\times 10^{-4}\,\mathrm{cmW}^{-1}$$
 , respectively. The excited state-assisted sequential two-photon absorption responsible for optical limiting is demonstrated by measuring the $$\beta $$
 at different intensities of nanosecond pulsed laser excitations. Superior physicochemical properties with a low optical limiting threshold for both nanosecond pulsed and CW laser irradiance (
 $$(1.44\pm 0.02)\times 10^{12} \hbox { Wm}^{-2}$$
 and $$(0.365\pm 0.02)\times 10^{3} \hbox { Wcm}^{-2}$$
 , respectively) promote the PCo cocrystal as a promising candidate for optical limiting application.

Chloridocobaltate(II) metal–organic cocrystal delivering intermolecular-charge transfer-enhanced passive optical limiting: A comprehensive study on structure–property relation

Imidazolinium l-tartrate (IMLT) crystals and l-histidine-doped IMLT crystals were grown by slow evaporation technique at room temperature. The powder x-ray diffraction technique confirms the lattice parameters and shifts in the peak positions attributed to the dopant l-histidine. The Fourier transform infrared (FTIR) spectrum reveals the assignments of characteristic bondings present in the grown crystals. The frequency-dependent dielectric constant and dielectric loss of pure and l-histidine-doped IMLT crystals have been investigated by the dielectric measurements. Doping has improved the optical parameters and was studied using UV–Vis-near infrared (NIR) spectral studies. The cut-off wavelengths of pure and 1 mol.% l-histidine-doped IMLT crystals were observed at 234 nm and 229 nm, respectively. The etching study examines the growth mechanism and surface morphology of the pure and l-histidine-doped IMLT crystals. The carbon-hydrogen-nitrogen (CHN) analysis conveys the percentage of carbon, hydrogen and nitrogen elements present in pure and l-histidine-doped IMLT crystals. The consequences of doping l-histidine in IMLT single crystal and their dominance in various properties of the crystal grown in aqueous solution by slow evaporation technique have been explored.

Growth and Characterization of a Nonlinear Optical Material: l-Histidine-Doped Imidazolinium l-Tartrate

Growth, Spectral and quantum chemical investigation on hexamethylenetetramine 4-nitrophenol monohydrate single crystals for second harmonic generation and optical limiting applications

Synthesis, spectral characterisation, third-order nonlinear optical properties and quantum chemical studies on (E)-4-bromo-N'-(2,4-dimethoxybenzylidene)benzohydrazide crystal for optoelectronics applications

G. Vinitha

Papers

Chloridocobaltate(II) metal–organic cocrystal delivering intermolecular-charge transfer-enhanced passive optical limiting: A comprehensive study on structure–property relation

Growth and Characterization of a Nonlinear Optical Material: l-Histidine-Doped Imidazolinium l-Tartrate

Growth, Spectral and quantum chemical investigation on hexamethylenetetramine 4-nitrophenol monohydrate single crystals for second harmonic generation and optical limiting applications

Synthesis, spectral characterisation, third-order nonlinear optical properties and quantum chemical studies on (E)-4-bromo-N'-(2,4-dimethoxybenzylidene)benzohydrazide crystal for optoelectronics applications

Self-defocusing, Kerr nonlinearity of green light emitting phosphors, Calcium–Strontium bimetallic tungstate nanoparticles decorated rGO