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

We report the third order nonlinear optical properties of Er3+-doped BaMoO4 nanostructures, and its dependence on Er-dopant concentration. BaMoO4 nanostructures with different concentration of Er were synthesized by chemical precipitation method and were characterized by UV–Vis absorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and fluorescence measurements. The incorporation of Er ions shifted the absorption band of BaMoO4 towards higher wavelength and enhanced the light absorption in the visible region. XRD patterns showed that the powders crystallize in scheelite-type tetragonal structure. The nonlinear optical behavior of the nanostructures was investigated by a Z-scan technique at 532 nm using continuous wave Nd:YAG laser. Experimental results suggested that the addition of Er can considerably enhance the nonlinear absorption coefficient and refractive index of BaMoO4 which could be used as a potential for nonlinear optical device applications.

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Third-order nonlinear optical characteristics of Er3+-doped BaMoO4 nanostructures

Crystal structure, physicochemical and intermolecular charge-transfer-enhanced third order nonlinear optical properties of a new metal-organic cocrystal $$\{[\hbox {NiCl}_2(\hbox {C}_4\hbox {H}_{11}\hbox {N}_2)_2(\hbox {OH}_2)_2](\hbox {Cl})_2 (\hbox {OH}_2)_2\hbox {n}\}$$
 (PNi) is reported. An intermediate piperazinium chloride created during the pH optimization reacts with the metal centre (Ni), forming mono-substituted piperazine metal-organic cocrystal. The PNi adopts octahedral coordination geometry with triclinic (
 $$P\overline{1}$$
 ) crystal system at its single crystalline phase. The crystal packing stabilization via $$\hbox {O}-\hbox {H}\cdots \hbox {Cl}$$
 , $$\hbox {C}-\hbox {H}\cdots \hbox {Cl}$$
 and $$\hbox {O}-\hbox {H}\cdots \hbox {O}$$
 noncovalent charge-transfer interactions is observed from single-crystal X-ray diffraction analysis. The PNi single crystal is thermally stable up to 87 $$^\circ$$
 C. The $$\hbox {H}_2\hbox {O}$$
 molecules tend to evaporate at lower temperatures bringing moderate, rather sufficient thermal stability for using the title material in lasing applications. Vickers microhardness study revealed that the title crystal belongs to the soft material category. Investigation on dielectric properties showed polar dielectric nature of the material with consistent electronic polarizability (
 $$\alpha =1.11\times 10^{22}\ \hbox {cm}^3$$
 ). The optical bandgap of 5.17 eV from UV-DRS spectroscopic analysis shows PNi belongs to the wide bandgap material category. The third order nonlinear optical absorption (
 $$\beta$$
 ), nonlinear refractive index (
 $$n_2$$
 ) and third order nonlinear optical susceptibility (
 $$\chi ^{(3)}$$
 ) of PNi cocrystal using 532 nm CW DPSS laser are measured to be $$(0.067\pm 0.001)\times 10^{-4}\ \hbox {cmW}^{-1}$$
 , $$(0.211\pm 0.0057)\times 10^{-8}\ \hbox {cm}^2\hbox {W}^{-1}$$
 and $$(0.511\pm 0.09)\times 10^{-6}$$
 esu, respectively. Intermolecular charge-transfer interactions and electronic polarizability well influence nonlinear optical properties and are proved possible by calculating the intermolecular charge-transfer interactions at the B3LYP/6-31G(d,p) level of calculations. Further, the nonlinear absorption coefficient (
 $$\beta$$
 ) was found to vary with the laser intensities, explicitly indicating the presence of excited state absorption assisted sequential 2PA in PNi cocrystal. Low onset limiting threshold for nanosecond pulsed and continuous-wave laser irradiance (
 $$(1.66\pm 0.02)\times 10^{12}\ \hbox {Wm}^{-2}$$
 and $$(0.804\pm 0.02)\times 10^{03}$$
 $$\hbox {Wcm}^{-2}$$
 , respectively) promotes the title material as an efficient candidate for optical limiting application.

Crystal structure and physicochemical properties of a new optofunctional metal-organic cocrystal delivering intermolecular charge-transfer-enhanced nonlinear optical and optical limiting properties

Structural, spectral, thermal and nonlinear optical analysis of anhydrous citric acid crystal

Graphene has generated enormous research interest during the last decade due to its significant unique properties and wide applications in the field of optoelectronics and photonics This research studied the structural and nonlinear absorption properties of reduced graphene oxide (rGO) synthesized by Modified Hummer’s method Structural and physiochemical properties of the rGO were explored with the help of Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman) Nonlinear absorption property in rGO, was investigated by open aperture Z-scan technique by using a continuous wave (CW) laser The Z-scan results demonstrate saturable absorption property of rGO with a nonlinear absorption coefficient, β, of −262 × 10−4 cm/W, making it suitable for applications in Q switching, generation of ultra-fast high energy pulses in laser cavity and mode lockers

Study of nonlinear absorption properties of reduced graphene oxide by Z-scan technique

A series of photo addressable polymers bearing azo-naphthol moiety in the polymer side chain containing polymethacrylate backbone were synthesized for NLO applications. Various halogen atoms were substituted in the para position of the side chain azobenzene, and the effects of the substituents were investigated. The polymers were characterized by UV, FT-IR, 1H-NMR and 13C-NMR spectroscopy. The photoisomerization property of all the polymers shows a clear dependence on the size of the halogen, indicating the rate of the switching time in the increasing order of F > Cl > Br > I. The glass transition temperature and thermal stability of the polymers were analyzed by DSC and TGA respectively. The third order nonlinear optical properties of the polymer film were measured by the Z-scan technique using Ar-ion laser and exhibited negative optical nonlinearity. The results revealed that these polymers possess potential applications in nonlinear optics and the value of nonlinear susceptibility for the iodine substituted polymer is higher than the other halogen substituted polymers chosen in the study.

G. Vinitha

Papers

Third-order nonlinear optical characteristics of Er3+-doped BaMoO4 nanostructures

Crystal structure and physicochemical properties of a new optofunctional metal-organic cocrystal delivering intermolecular charge-transfer-enhanced nonlinear optical and optical limiting properties

Structural, spectral, thermal and nonlinear optical analysis of anhydrous citric acid crystal

Study of nonlinear absorption properties of reduced graphene oxide by Z-scan technique

Effect of halogen substitution on azo-naphthol dye based poly(alkyloxymethacrylate)s and their Z-scan measurements