Effective Three-photon Absorption Research Articles

Metalated porphyrin-napthalimide based donor-acceptor systems with long-lived triplet states and effective three-photon absorption

• Excited state electron dynamics of novel Metalated porphyrins-napthalimide molecules was investigated by femtosecond transient absorption spectroscopy (fs-TAS). • The rate constants of excited states are estimated using the global analysis of the obtained fs-TAS data. • Long-lived triplet state (lifetime ∼10 μ s for PN-Zn) formed via intersystem crossing. • Open aperture z-scan at non-resonant excitation of 800 nm revealed strong three-photon absorption by these molecules. • The experimental second hyperpolarizability was estimated to be ∼ 10 -31 esu and verified by DFT calculations. Porphyrin applications are primarily related to electronic transitions across a broad spectrum. In this context, understanding the excited state electron dynamics is necessary. Herein, we report the ultrafast photophysical characterization of one free base and three transition metallated porphyrin - napthalimide based donor - acceptor systems ( PN-Fb , PN-Ni , PN-Cu , and PN-Zn ) in dichloromethane (DCM) solution using transient absorption spectroscopy (TAS) in the spectral range of 430-780 nm by the Soret band excitation at 400 nm. The different rate constants were extracted using a target model analysis of the collected transient absorption spectra and established a consistent photophysical model of molecular excited state population relaxations. The photophysical model consisted of the following various processes: (a) internal conversion in the range of 215-400 fs, (b) vibrational relaxation in the range of 1.35-62 ps, and (c) singlet state relaxation times in the range of 0.17-2.06 ns, and finally (d) 0.03-10 μs was attributed to the triplet state lifetimes. We have also measured the nonlinear optical properties of these molecules by the open and closed aperture Z-scan studies, which was carried out by 800 nm, 70 fs photoexcitation. The theoretical fittings to the experimental data resulted in an effective three-photon absorption coefficient and positive nonlinear refractive index. Further, from the Z-scan data, the second-order hyperpolarizability of the molecules was calculated, and the values were 10 - 31 esu. Additionally, complete optimization of individual structures and construction of these PN systems was achieved using density functional theory (DFT) calculations and the obtained results were found to be consistent with the experimental observations.

The role of defects in the nonlinear optical absorption behavior of pristine and Co-doped V2O5 layered 2D nanostructures

V2O5 nanostructures doped with 1% Co have been successfully synthesized by simple hydrothermal method. Structural and morphological aspects, vibrational spectra and optical properties (linear and nonlinear) of pristine as well as Co-doped V2O5 (V2O5:Co) nanoparticles are investigated. X-ray diffraction patterns indicate that both V2O5 and Co doped V2O5 are stabilized in an orthorhombic structure. Energy band gaps measured using the Tauc plot give 2.24 eV for V2O5 and 2.16 eV for Co doped V2O5. When excited using 5 ns laser pulses at 532 nm in the open aperture Z-scan configuration, pristine as well as Co-doped V2O5 nanoparticles are found to show excellent nonlinear absorption, which arises from strong reverse saturable absorption occurring in the sample at this wavelength. The corresponding effective two-photon absorption coefficients are in the range of 10−11 to 10−12 m/W, and effective three-photon absorption coefficients are in the range of 10−22 to 10−23 m3/W2. When excited at the same wavelength using a continuous wave laser beam, the effective two-photon absorption coefficient gets enhanced to 10−6 m/W. These results confirm the suitability of V2O5 and Co-doped V2O5 nanoparticles for the fabrication of optical limiter devices for the protection of human eyes and sensitive optical detectors from hazardous laser radiation.

Third- and fifth-order optical nonlinearities of norbixin

In this work, we investigated the nonlinear response of norbixin (C24H28O4) in acetone. The third- and fifth-order optical nonlinearities were measured using the conventional Z-scan technique, with 357 fs laser pulses at 1040 nm and low-repetition rate (1 kHz). It is shown that, at intensities lower than 99GW/cm2, the nonlinear refraction process was determined by the only third-order process. However, at higher intensities, the third- and fifth-order refractive nonlinearities coexist with opposite signs: n2 = (4.43 ± 0.44) × 10−16 cm2/W and n4 = (−4.65 ± 0.46) × 10−27 cm4/W2. On the other hand, the open-aperture Z-scan measurements show an effective three-photon absorption (3PA) with 9.19±0.92×10-23 cm4/W2. The physical mechanisms of the observed nonlinearity are also discussed.

Nonlinear optical characteristics of non-covalently functionalised graphene-pyrene-conjugated phthalocyanine hybrids

Improving the non-linear absorption properties of graphene materials is valuable for optical limiting applications. We report the non-linear optical characteristics of new hybrid materials formed by the non-covalent functionalisation of graphene nanoplatelets (GNs) with pyrene-conjugated Zn(II) phthalocyanines and pyrene-conjugated Zn(II) azaphthalocyanines. The GN–pyrene-conjugated Zn(II) phthalocyanine hybrid (GPc) shows a promising change from the two-photon absorption behaviour of GNs to three-photon absorption, the first such example in a graphene based hybrid system under nanosecond pulse excitation. Fluorescence and transient absorption spectroscopies are employed to describe the non-linear optical phenomena of the prepared hybrid materials. The GPc hybrid exhibits a low optical limiting threshold induced by three-photon absorption and a favourable linear transmittance (above 90%). The GN–pyrene-conjugated azaphthalocyanine hybrid (GAzaPc) also shows promising non-linear optical coefficients. A unique optical limiting performance (80% reduction in intensity) is achieved throughout the experimental range of excitation intensities. Our research offers a way to design hybrid material structures with a large effective three-photon absorption coefficient and nearly constant optical limitation across a wide intensity range with 92% linear transmittance, which would be highly useful in non-linear photonic applications.

Two-photon and three-photon absorption in ZnO nanocrystals embedded in Al2O3 matrix influenced by defect states.

The broadband nonlinear absorption in ZnO nanocrystals embedded in Al2O3 matrix was investigated by Z-scan and pump-probe techniques from 400nm to 800nm. The effective two-photon absorption and three-photon absorption coefficients were determined to be ∼1.1×103 cm/GW at 400nm and ∼1.1×10-1 cm3/GW2 at 800nm, respectively, which are at least two orders of magnitude greater than that in ZnO bulk crystal. It may be attributed to the defect-states-mediated multiphoton absorption process, which was proofed by comparison experiments with different densities of interfacial defect states. The corresponding lifetimes for the intraband relaxation, defect-states trapping, and interband recombination processes were measured by femtosecond transient absorption measurements as τ1 ∼ 1 ps, τ2 ∼ 13 ps, and τ3 ∼ 350 ps, respectively.

Solvent effects on the fluorescence and effective three-photon absorption of a Zn(II)-[meso-tetrakis(4-octyloxyphenyl)porphyrin

The fluorescence and effective three-photon absorption (3PA) properties of Zn(II)-[meso-tetrakis(4-octyloxyphenyl)porphyrin] (labeled Zn(II)-porphyrin) dissolved in three different polar solvents were systematically investigated. The electrochemical and photophysical properties of Zn(II)-porphyrin were investigated by 1H NMR spectra, IR spectra, mass spectroscopy, and electronic absorption spectra. The fluorescence emission of Zn(II)-porphyrin in three different solvents excited at the wavelengths of 420 nm (Soret band) and 550 nm (Q-band) were analyzed. By performing Z-scan experiments with femtosecond laser pulses at a wavelength of 800 nm, the effective 3PA process of Zn(II)-porphyrin in three different solvents was observed and the underlying mechanism was discussed in detail. It is found that the fluorescence spectra slightly depend on the polarity of the solvent. Interestingly, the effective 3PA properties of Zn(II)-porphyrin strongly depend on the solvent polarity. The lower the solvent polarity is, the larger effective 3PA cross-section is. Low polar solvents are beneficial to applications of Zn(II)-porphyrin in optical limiting, photodynamic therapy, etc.

Nonresonant and Resonant Nonlinear Absorption of CdSe-Based Nanoplatelets**Supported by the National Natural Science Foundation of China under Grant Nos 61234005 and 11304197.

We present a comprehensive understanding of the nonlinear absorption characteristics of CdSe-based nanoplatelets (NPLs) synthesized by the solution-phase method and the colloidal atomic layer deposition approach through Z-scan techniques at 532 nm with picosecond pulses. The CdSe NPLs exhibit strong two-photon induced free carrier absorption (effective three-photon absorption) upon the nonresonant excitation, resulting in a remarkable optical limiting behavior with the limiting threshold of approximately 75 GW/cm2. A nonlinear optical switching from saturable absorption (SA) to reverse saturable absorption (RSA) with increasing the laser intensity is observed when coating CdSe NPLs with a monolayer of CdS shell to realize the resonant absorption. The SA behavior originates from the ground state bleaching and the RSA behavior is attributed to the free carrier absorption. These findings explicitly demonstrate the potential applications of CdSe-based NPLs in nonlinear optoelectronics such as optical limiting devices, optical pulse compressors and optical switching devices.

D-A-π-D Synthetic approach for thienyl chalcones – NLO – a structure activity study

With the growing interest for organic molecules in Nonlinear Optical (NLO) applications, we have synthesized nine novel thienyl chalcones based on the D-A-π-D design. In order to establish the identity, these have been characterized in detail. Having followed the design based synthetic route, we have focused on two prime criteria for comparison; namely second harmonic generation (SHG) and non-linear absorption. In this work the role of the electron withdrawing groups, electron donating groups and extended conjugation, have been extensively studied vis-à-vis the NLO properties. The change in these properties by virtue of the molecular structure has been elucidated in this work as the structure activity relationship. Optical nonlinearity is studied using ultrafast (100 fs) laser pulses at 800 nm, employing the open aperture Z-scan technique. The compounds exhibit large effective three-photon absorption (3PA) coefficients, in the order of 10−28 m3/W2. These observations show that these compounds possess potential for application in all-optical limiting and switching devices.

Excited state assisted three-photon absorption based optical limiting in nanocrystalline Cu2Se and FeSe2

Transition metal selenides (FeSe2 and Cu2Se) are synthesized by the hydrothermal co-reduction method. XRD results revealed the crystalline nature of their single phase and the elemental compositions are obtained using EDS. TEM images of the as-prepared samples show the formation of nanorods of 10–20nm diameter in case of iron selenide and nanoparticles of 10–35nm diameter in case of copper selenide. The energy bandgap values are calculated using tauc plots obtained from UV–Visible absorption spectra. The open aperture Z-scan measurements carried out using 5ns pulses at 532nm revealed that the samples showed excellent optical limiting behavior owing to strong nonlinear absorption (NLA). Through numerical simulations, the mechanism of NLA is found to be effective three-photon absorption which has significant contribution from excited state absorption.

Three-photon absorption and vibrational spectroscopic study of 2-methylamino-5-chlorobenzophenone

In this paper, the vibrational spectral analysis and three-photon absorption properties of an organic material of 2-methylamino-5-chlorobenzophenone have been reported. The geometry and harmonic vibrational wavenumbers are calculated with the help of B3LYP density functional theory method. The detailed interpretation of the vibrational spectra has been carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology (SQM). SQM force fields have also been used to calculate potential energy distribution (PED) in order to make conspicuous vibrational assignments. The nonlinear absorption properties have been investigated in ethanol solution at 532nm using the Z-scan technique employing laser pulses of 5ns duration. Open aperture Z-scan data reveal the presence of effective three-photon absorption for ns pulses at 532nm resulting in a strong optical limiting behavior, indicating possible photonic applications.

Picosecond and femtosecond optical nonlinearities of novel corroles

We present our results from the experimental and modeling studies of picosecond (ps) and femtosecond (fs) nonlinearities of two novel corroles (a) tritolyl corrole (TTC) (b) triphenyl corrole (TPC) using the Z-scan technique. Both open and closed aperture Z-scan curves were recorded with ~2 ps/~40 fs laser pulses at a wavelength of 800 nm and nonlinear optical coefficients were extracted for both studies. Both the molecules possessed negative nonlinear refractive index (n2) as revealed by signature of the closed aperture data in both (ps and fs) time domains. Picosecond nonlinear absorption data of TPC obtained at a concentration of 5 × 10-4 M demonstrated complex behavior with switching from reverse saturable absorption (RSA) within saturable absorption (SA) at lower peak intensities to RSA at higher peak intensities. TTC data recorded at the similar concentration exhibited saturable absorption (SA) type of behavior at lower peak intensities to switching from RSA with in SA at higher peak intensities. At a concentration of 2.5 × 10-4 M, the ps open aperture data at higher peak intensities illustrated effective three-photon absorption (3PA) for both the molecules. We also report the picosecond spectral dependent Z-scan studies performed at 680 nm, 700 nm, and 740 nm. Nonlinear absorption and refraction of both the samples at these three wavelengths were studied in detail. Femtosecond nonlinear absorption data of TPC and TTC demonstrated the behavior of saturable absorption (SA) at a concentration of 1 × 10-3 M. Solvent contribution to the nonlinearity was also identified. We have also evaluated the sign and magnitude of third order nonlinearity. We discuss the nonlinear optical performance of these organic molecules.

Synthesis and optical limiting studies of HAP and HAP@SiO2 core–shell nanoparticles

We report the spectral and optical limiting properties of two kinds of HAP nanoparticles (pure HAP and HAP@SiO2 core–shell nanoparticles) that were wet-chemically prepared and coated with silica by means of Stober method. The size, framework substitution and silica coating formation of the resultant particles were confirmed by Transmission Electron Microscopy, X-ray diffraction, Fourier Transform Infrared Spectroscopy and Thermogravimetric analysis. The nonlinear optical limiting properties of HAP and HAP@SiO2 nanoparticles suspended in ethylene glycol is studied at 532 nm using 5 ns Nd:YAG laser pulses. Observations indicate that effective three-photon absorption is found to be responsible for strong optical limiting characteristics in these nanoparticles and it is seen that the optical nonlinearity is enhanced in core–shell structures as compared with single counterparts. The nonlinear optical parameters calculated from the data show that these materials are efficient optical limiters.

Two-photon and three-photon absorption in dinaphthoporphycenes

We present our results from the detailed investigations of both metal free-base and metal substituted dinaphthoporphycenes using the Z-scan technique with picosecond pulses. The compounds exhibited singular nonlinear absorption properties because of their molecular structure allowing us to access the two-photon and three-photon absorption states with different peak intensities when excited at the wavelength of 800nm. This type of behaviour is explained using the unique energy level configuration of porphycene molecules. The orders of magnitude of both two-photon absorption and effective three-photon absorption coefficients and cross-sections were estimated along with sign and magnitude of the third order nonlinearity.

Synthesis and optical limiting studies of Au-doped TiO2 nanoparticles

A series of Au-doped TiO 2 nanoparticles with different Au concentrations were prepared by the sol–gel method using titanium (IV) isopropoxide and tetrachloroaurate (III) trihydrate as precursors. The framework substitution of Au in TiO 2 nanoparticles was established by x-ray diffraction (XRD) and Fourier transform infrared (FT-IR) techniques. X-ray diffraction (XRD) and transmission electron microscopy (TEM) image results confirmed the anatase phase and nanocrystalline nature of TiO 2–Au. The optical properties revealed an extended tailing of the absorption edge toward the visible region upon Au doping. The incorporation of Au in TiO 2 was also confirmed by fluorescence quenching. The increase in Au doping enhanced the ‘red-shift’ in the UV-Vis absorption spectra. The spectral and nonlinear optical properties were studied using fluence-dependent transmittance measurements in order to reveal the optical limiting mechanism. The nonlinear optical response and optical limiting effects of TiO 2–Au nanocomposites dispersed in ethylene glycol were studied at 532 nm using 5 ns Nd:YAG laser pulses. Effective three-photon absorption mechanisms play a major role for good optical limiting characteristics in these nanoparticles and it is seen that the optical nonlinearity enhances with lower threshold as the volume fraction of the Au concentration increased.

Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films

We report a dramatic change in effective three-photon absorption coefficient of amorphous Ge16As29Se55 thin films, when its optical band gap decreases by 10 meV with 532 nm light illumination. This large change provides valuable information on the higher excited states, which are otherwise inaccessible via normal optical absorption. The results also indicate that photodarkening in chalcogenide glasses can serve as an effective tool to tune the multiphoton absorption in a rather simple way.

OPTICAL LIMITING CHARACTERISTICS OF CORE-SHELL NANOPARTICLES

TiO2 nanoparticles were synthesized by hydrothermal method at 180°C from TiOSO4 aqueous solution with 1 m/l concentration. The obtained products were coated with silica by means of a seeded polymerization technique for a coating time of 1440 minutes to obtain well-defined TiO2@SiO2 core-shell structure. The uncoated and coated nanoparticles were characterized by using X-Ray diffraction technique (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) to study their physico-chemical properties. Evidence from XRD and FTIR results show that SiO2 is homogenously coated on the surface of titania particles. FTIR spectra show that there exists an interaction between TiO2 and SiO2 and results in the formation of Ti-O-Si chemical bonds at the interface of TiO2 particles and SiO2 coating layer. The open aperture z-scan was carried out in both pulsed and CW laser. The RSA observed using CW laser and effective three-photon type absorption seen using pulsed laser at this wavelength, is of potential application in fabricating optical limiting devices. The non linear optical limiting properties of TiO2 and TiO2@SiO2 nanoparticles dispersed in ethylene glycol were studied at 532 nm using 5 ns Nd:YAG laser pulses and diode pumped Nd:YAG laser. Results indicate that these compounds are a potential candidate for low-power optical limiting applications and it is seen that the optical nonlinearity is enhanced in core-shell structures when compared with single counterparts.

Nonlinear Optical Studies on a New Poly{2-(biphenyl-4-yl)-5-[3,4-didecyloxy-5-(1,3,4-oxadiazol-2-yl)thiophen-2-yl]-1,3,4-oxadiazole}

Measurements of nonlinear optical properties of a newly synthesized thiophene based polymer are reported. The nonlinear transmission measurements were performed on the polymer dissolved in N,N-dimethylformamide by employing the single beam Z-scan technique using a Q-switched laser output at 532-nm. The polymer shows strong optical limiting behavior, where the transmittance decreases when the pump fluence is increased. It was found that an effective three-photon absorption (3PA) model at the excitation wavelength gives the best fit to the obtained experimental data. The value of the 3PA coefficient has been numerically calculated. This study clearly reveals the potential of new polymeric material has, for optical limiting applications.

Optical limiting properties of CdS nanowires

Intensity-dependent nonlinear optical transmission studies of cadmium sulfide (CdS) nanowires (∼ 50–100 nm diameter) suspended in dimethylformamide have been carried out in the visible region using the Z-scan technique with 7 ns pulses from the second harmonic of an Nd:YAG laser. The optical limiting threshold of CdS nanowires suspension was determined to be 1.3 J cm − 2 , with normalized transmittance of 0.47, which is relatively lower when compared with those of many popular metal nanowire suspensions reported in the literature. Based on an effective three-photon absorption model, nonlinear absorption and nonlinear scattering were identified as the dominant processes for the measured reduced transmittance.

Dynamics of two-photon-induced three-photon absorption in nanosecond, picosecond, and femtosecond regimes

Based on five-level rate-equation theory, we develop the laser-pulse-duration dependence of two-photon-absorption-induced singlet and triplet excited-state absorptions (ESAs). We present analytical expressions for the effective three-photon absorption coefficients caused by both singlet and triplet ESAs under the pulsed excitation on time scales from femtoseconds to microseconds. We demonstrate that the triplet ESA is predominant with longer laser pulses (microseconds to tens of nanoseconds) and that the resultant nonlinear absorption (NLA) can be adequately interpreted by a simplified four-level model. Under the excitation of picosecond laser pulses, generally speaking, the competition between singlet and triplet ESAs is observable. In this instance, the photodynamics of the system can be understood by a five-level model. In the femtosecond regime, however, a three-level model is validated in the prediction of NLA, because the triplet ESA becomes negligible.

Study of optical properties and effective three-photon absorption in Bi-doped ZnO nanoparticles

In this paper we report the linear and nonlinear optical properties of Bi-doped ZnO nanoparticles. Bi-doped ZnO nanoparticles were prepared through the wet chemical method at room temperature. Optical absorption measurements show that the exciton peaks are situated at 272 and 368 nm, which are attributed to the n=2 and n=1 exciton states, respectively. Transmission electron microscopy measurements reveal the size and shape of the particles and energy dispersive X-ray measurements confirm the doping of Bi in ZnO. Steady state photoluminescence measurements show that the emission is composed of five peaks. Open aperture z-scan measurements done at 532 nm using 5 ns, 300 μJ laser pulses reveal nonlinear absorption which arises from an effective three-photon absorption process.