Sequential Two-photon Absorption Research Articles

Localized surface plasmon resonance induced nonlinear absorption and optical limiting activity of gold decorated graphene/MoS2 hybrid

The synergistic supremacy of a hybrid nanocomposite made of reduced graphene oxide (rGO), molybdenum disulfide (MoS2) and gold nanoparticles (Au) for Q-switching and optical limiting applications is investigated. Hydrothermally synthesized Au-rGO-MoS2 hybrid with varying concentrations of Au (2.5, 5, 7.5 and 10 wt%) was subjected to interact with Q-switched Nd:YAG nanosecond green laser pulses. The intensity dependent nonlinear absorption studies revealed a switch over in the nonlinear behaviour from saturable absorption (SA) to reverse saturable absorption (RSA) behaviour. SA occurs due to ground state bleaching at comparatively lower laser irradiances serving the need for Q-switching and optical communications. The RSA trait at higher laser irradiance is attributed to sequential two-photon absorption which serves as the platform for optical limiting behaviour prompting laser safety and effective photothermal therapy applications. The strong mid-visible and UV absorption promotes the availability of multiple energy bands for energy transitions of excited state absorption (ESA). Enhanced local fields due to localized surface plasmon resonance effect, structural defects, hierarchical morphology, increased absorption cross-section and plasmon induced energy transfer promotes robust ESA process. These tunable nonlinear optical properties make Au-rGO-MoS2 hybrid promising futuristic candidates for applications in nonlinear photonic devices, ultrafast optical switches and all-optical signal processing systems.

Strong Electron-Electron-Nuclei Correlations in Two-Photon Double Ionization of H_{2}.

Two-photon double ionization is a paradigmatic example of how electron correlation manifests. In molecular targets, its coupling with the slower nuclear motion introduces an additional complication and induces electron-electron-nuclei correlations. Experimentally, momentum-coincident measurements can provide a complete kinematical image of the molecular full Coulomb breakup. Previous theoretical studies have described this process by ignoring nuclear motion and the subsequent Coulomb explosion of the dication. Here we show, by means of a full-dimensional treatment of two-photon double ionization of the H_{2} molecule, that nuclear motion plays a decisive role even for pulses as short as 1.5fs, a time during which the nuclei are not expected to move significantly. We find strong correlations between nuclear and electronic degrees of freedom, giving access to different electronic processes as a function of nuclear kinetic energy. In particular, we observe unexpectedly strong back-to-back asymmetry in the photoelectron angular distributions, as well as novel interferences resulting from the coherent contributions from two-photon sequential absorption paths via different molecular cationic states.

Growth, DFT, spectroscopic analysis, third-order NLO properties and optical limiting behavior of Bis(p-aminobenzoic acid-N) dichloro cadmium (II) (BPANDC) crystal

A new organic nonlinear optical crystal, Bis(p-aminobenzoic acid-N) dichloro cadmium (II) (BPANDC) has been grown by Slow Evaporation Solution Technique (SEST). The experiments were carried out to study the structural, optical, DFT and nonlinear optical properties of the grown crystals. The BPANDC crystal belongs to triclinic system with space group Pī with lattice parameters a = 6.359(4)Å, b = 7.509(11)Å, c = 17.263(7)Å, α= 94.50(2)˚, β=104.56(2)˚, γ=96.79(4)˚, and volume V = 787.20Å3. Using the TD-DFT method, the electronic absorption spectra of BPANDC was calculated in gas phase. The calculated FT-IR, FT-Raman spectrum, Molecular geometrical analysis, TD-DFT, HOMO–LUMO energy gap and Molecular electrostatic potential surface of BPANDC were calculated by DFT/ LANL2DZ basis set. The Z-scan analysis evidenced that the BPANDC exhibits sequential two-photon absorption induced reverse saturable absorption.

Visible-light optical limiting of vanadia–polyvinylpyrrolidone nanofibers

In this work, vanadium pentoxide (V2O5) nanoparticles-filled electrospun polyvinylpyrrolidone (PVP) nanofibers were investigated systematically at various nanofiller weight percentages (8 and 10 wt%) and input intensities to reveal the effective optical limiting feature in the visible spectrum. XRD analysis demonstrated the purity of the produced V2O5 nanoparticles. According to SEM findings, V2O5 nanoparticles were effectively integrated into the PVP nanofibers. Two distinct absorption bands were observed at around 400 and 217 nm. These bands were related to PVP and V2O5 nanoparticles in linear absorption measurements, respectively. Moreover, an increased Urbach energy value was obtained with an increase in V2O5 nanofiller content within PVP. Open-aperture Z-scan measurements were taken at 532 nm considering the band gap energy of the V2O5 nanofillers in PVP composite nanofibers. In 8 wt% V2O5 nanofilled PVP nanofibers, one-photon absorption (OPA) was the main nonlinear absorption (NA) mechanism, and the defect states of the V2O5 nanoparticles had no contribution to NA. On the other hand, sequential two-photon absorption was the main NA mechanism, and the defect states of the nanoparticles caused more efficient NA behavior in 10 wt% V2O5 nanofilled PVP nanofibers. The effective optical limiting behavior was obtained for this composite nanofiber with lower limiting threshold as 1.49 × 10–5 J/cm2. The V2O5 nanofilled PVP nanofibers presented strong potential optical limiters in the visible wavelength region. This was attributed to their high linear transmittance at low input intensities and their robust NA behavior at higher input intensities.

Second harmonic generation and two photon absorption assisted power limiting mechanism of l-Phenylalanine (LPA) - poly-ethylene-oxide (PEO) electrospun nanofibers

Nanofibers consisting of organic nonlinear optical l-Phenylalanine (LPA) nanocrystals embedded in poly-ethylene-oxide (PEO) polymer were synthesized by the electrospinning technique. The preferred crystallographic orientation of the organic molecules that crystallized inside the nanofibers was demonstrated through X-ray diffraction and calculations show that the average crystallite size is around 28 nm. SEM images clearly show the formation of smooth fibers with average diameter of 246 nm. The emission of green light from the nanofiber powder sample approved the second harmonic generation (SHG) under the excitation of Nd: YAG laser (1064 nm). Further the generation of second harmonic from the fibers was confirmed by nonlinear microscope imaging and power dependent SHG studies, performed using femtosecond fiber laser (1040 nm). The nonlinear absorption and its power limiting characteristics of LPA-PEO electro-spun nanofibers were studied utilizing the Z-scan method employing Q switched Nd: YAG laser as an excitation source (532 nm, 5 ns, 10 Hz). Open aperture Z-scan measurement (with intensity 1.23 × 1012 W/m2 and 2.4623 × 1012 W/m2) revealed the reverse saturable absorption property of LPA-PEO nanofibers. The existence of a sequential two-photon absorption process involving real resonant intermediate states has been demonstrated by the intensity dependent two-photon absorption coefficient. The usefulness of LPA optical fibers as power limiters is ensured by their lower optical limiting threshold than other optical limiting materials.

Nonlinear optical studies of phenothiazine Schiff bases for optical limiting application

Synthesized Schiff-based organic molecules consisting of phenothiazine backbone (Phe-DNP, Phe-Ma, Phe-3) to investigate its optical limiting applications. The structural confirmation of compounds is carried out by FTIR and NMR spectroscopy. The quantum mechanical calculations of compounds are measured using Density functional theory (DFT) to investigate the electronic orientation in molecular orbitals and the theoretical bandgap. All the molecules show a broad absorption in the visible domain and the conjugated molecule absorption shifts towards the higher wavelength region. The effective intramolecular charge transfer of molecules in the order Phe-DNP> Phe-Ma>Phe-3 is understood from the ground state absorption spectra results. All the organic moieties exhibited sequential two-photon absorption upon studying the open aperture Z-scan technique. Phe-Ma molecule shows the highest nonlinear absorption (1.212× 10−10 m/W) and lowest optical limiting threshold (3.19 × 1012 W/m2) compared to other molecules suggesting that Phe-Ma could be a potential nonlinear optical limiter.

Two-dimensional Kβ-Kα fluorescence spectrum by nonlinear resonant inelastic X-ray scattering

High sensitivity of the Kβ fluorescence spectrum to electronic state is widely used to investigate spin and oxidation state of first-row transition-metal compounds. However, the complex electronic structure results in overlapping spectral features, and the interpretation may be hampered by ambiguity in resolving the spectrum into components representing different electronic states. Here, we tackle this difficulty with a nonlinear resonant inelastic X-ray scattering (RIXS) scheme, where we leverage sequential two-photon absorption to realize an inverse process of the Kβ emission, and measure the successive Kα emission. The nonlinear RIXS reveals two-dimensional (2D) Kβ-Kα fluorescence spectrum of copper metal, leading to better understanding of the spectral feature. We isolate 3d-related satellite peaks in the 2D spectrum, and find good agreement with our multiplet ligand field calculation. Our work not only advances the fluorescence spectroscopy, but opens the door to extend RIXS into the nonlinear regime.

Exploring the nonlinear optical limiting activity of para-aminobenzoic acid by experimental and DFT approach

The present study accounts for the nonlinear optical behavior of para-aminobenzoic acid. The intramolecular charge transfer was established using electrostatic potential and Hirshfeld’s surfaces. The fingerprint plots identified that the bonds associated with the functional groups of para-aminobenzoic acid (amino and carboxyl) have major contributions to the total intramolecular charge transfer. It was observed that the para-aminobenzoic acid gave a broad absorption spectrum between 240 and 310 nm wavelength experimentally as well as computationally. From Tauc’s plot, the obtained direct band gap of para-aminobenzoic acid is 3.66 eV. The vibrational peaks of experimental FT-IR spectra were also found similar to the peaks observed in computational FT-IR. Moreover, high-frequency peaks were observed for the associated functional groups. The first-order hyperpolarizability of para-aminobenzoic acid was computed around 29.99 × 10–30 esu. We derived from the open-aperture Z-scan data that the para-aminobenzoic acid exhibits strong reverse saturable absorption. The positive value of the nonlinear absorption coefficient established the potential application of para-aminobenzoic acid in strong multi-photon absorption. Theoretical fit and intensity dependent Z-scan experiment confirm the occurrence of excited absorption (or) sequential two-photon absorption. The present study promises para-aminobenzoic acid for optical limiting applications.

Tunable nonlinear absorption and optical limiting behavior of NaBi(MoxW1−xO4)2 single crystals with ratio of Molybdenum/Tungsten

The compositional effect of Mo/W ratio on linear, nonlinear absorption and optical limiting behavior of the NaBi(MoxW1−xO4)2 single crystals grown by Czochralski technique was investigated. X-ray diffraction patterns of the studied crystals presented well-defined peaks associated with the tetragonal crystalline structure. The nonlinear absorption performance and optical limiting threshold were determined using an open-aperture Z-scan technique. A theoretical model including one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption was used to determine the nonlinear absorption parameters. All of the results showed that defect states, which strongly affect nonlinear absorption (NA) and optical limiting behaviors, can be tuned with the Mo/W ratio, enabling NaBi(MoxW1−xO4)2 single crystals to be used in desired optoelectronic applications. Linear optical absorption analysis revealed that bandgap energy and defect states can be tuned by changing the Mo/W ratio in the crystal structure. The obtained results showed that all the studied crystals had NA behavior and the nonlinear absorption coefficient decreased with increasing Mo/W ratio. Sequential TPA is the main NA mechanism for these crystals due to the fact that the incident light energy is lower than the bandgap energies and the existence of the real intermediate state around 2.32 eV.

Excitation wavelength dependent nonlinear absorption mechanisms and optical limiting properties of Bi12SiO20 single crystal

Nonlinear absorption mechanisms (NA), excitation wavelength dependence, and defect states of Bi12SiO20 (BSO) single crystal were investigated. The band gap and Urbach energies were found to be 2.51 and 0.4 eV from the absorption spectra. To evaluate the effect of excitation energy on the NA mechanism of the BSO single crystal, open aperture Z-scan experiment with 4 ns laser pulse at 532 and 1064 nm wavelengths with different intensities was performed. Obtained data were analyzed with a theoretical model considering the contributions of one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption (FCA) to NA. The results indicated that the NA behavior decreased with increasing of the pump intensity as the defect states at around 2.32 eV by OPA at 532 nm, and TPA at 1064 nm excitations. The dominant NA mechanisms are OPA and sequential TPA at 532 nm as compared to the 1064 nm. A higher NA coefficient was obtained at 532 nm as compared to 1064 nm excitation. This observation was attributed to higher contribution of OPA at 532 nm even at lower input intensities compared to TPA contribution at 1064 nm. Onset optical limiting thresholds were found as 0.34 and 0.68 mJ/cm2 for 532 and 1064 nm input beams, respectively. In the light of the results, the BSO single crystal may be used as a saturable absorber or an optical limiter at convenient input intensity by effectively adjusting defect states and excitation wavelength.

Nonlinear absorption-based analysis of energy deposition in melanosomes for 532-nm short-pulsed laser skin treatment.

The clinical use of 532-nm short-pulsed lasers has provided effective treatment of epidermal pigmented lesions. However, the detection of significant differences in treatment effects between picosecond and nanosecond lasers has still varied among clinical studies. For robust evaluation of the differences based on the treatment mechanism, this study presents a nonlinear absorption-based analysis of energy deposition in melanosomes for 532-nm short-pulsed laser treatment. Nonlinear absorption by melanin is modeled based on sequential two-photon absorption. Absorption cross-sections and nonradiative lifetimes of melanin, which are necessary for the nonlinear absorption-based analysis, are determined from transmittance measurement. Using the model and parameters, energy deposition in melanosomes was calculated with varying fluence and pulse width settings, including actual clinical parameters. The energy deposition in melanosomes increased with shorter laser pulses, and subnanosecond laser pulses were found to be most efficient. The comparison of energy deposition calculated using clinical parameters demonstrated the differences in treatment effects between picosecond and nanosecond lasers reported in clinical studies. The nonlinear absorption-based analysis provides quantitative evidence for the safety and efficacy evaluation of short-pulsed laser treatments, which may lead to the establishment of numerical indices for determining treatment conditions. Future studies considering the effects of the surrounding tissue on energy deposition in melanosomes will be needed.

Controlled plasmon-induced nonlinear absorption and optical limiting in Al/PVP composite nanofibers.

The effect of aluminum (Al) concentration on the surface plasmon resonance (SPR) band position of aluminum/polyvinylpyrrolidone (Al/PVP) composite nanofibers was investigated to strengthen nonlinear absorption (NLA) and widen its spectral range. With increasing Al content in PVP nanofibers, the SPR band was shifted towards excitation wavelength and an improved NLA response was achieved. The NLA response was examined both experimentally, by conducting Z-scan experiments, and theoretically, using two models. In the first model, the contributions of one-photon absorption (OPA), two-photon absorption (TPA), excited state absorption (ESA) and saturated absorption (SA) are considered. The second model, on the other hand, is a model that is widely used in the literature, and while taking into account the contributions of OPA and TPA, it neglects the ESA. The first model provides more accurate results due to the high concentration of free carriers in the samples examined. In order to reveal the contribution of Al to the nonlinear absorption, a laser excitation wavelength of 532 nm was chosen to minimize both the defect-assisted sequential and genuine two-photon absorption contributions of PVP. While the nonlinear absorption of pure PVP is quite weak, the NLA performance of Al/PVP nanofibers significantly improved as the Al content increased. As the amount of Al increased, the aggregation effect increased and a broadening and red shift in the SPR band were observed in the plasmonic behavior. This indicates a decreasing interparticle distance in Al particles. The sample with the highest amount of Al is anticipated as a potential candidate for optical limiting (OL) applications due to its superior NLA performance and SPR band furthest towards the near infrared (NIR) region, allowing a wider range of wavelength set to be used in OL applications.

Wavelength dependence of the nonlinear absorption performance and optical limiting in Bi12TiO20 single crystal

In this study, the influence of excitation wavelength and input intensity on the nonlinear absorption (NA) mechanism and optical limiting behavior of the Bi12TiO20 (BTO) single crystal were reported. The energy band gap of the BTO single crystal was obtained to be 2.38 eV. Urbach energy revealed that the single crystal has a highly defective structure. Open aperture (OA) Z-scan experiments were conducted at 532 and 1064 nm excitation wavelengths at various input intensities. Obtained experimental data were analyzed with a theoretical model considering one photon, two-photon and free carrier absorption contributions to NA. The obtained results revealed that the BTO single crystal possesses NA. The NA coefficient increased with increasing input intensity at 532 nm excitation wavelength, while it decreased with increasing input intensity at 1064 nm excitation wavelength. Due to the intense localized defect states distribution at the energy of 532 nm excitation wavelength within the band gap, increasing contribution to NA came from one photon absorption (OPA), sequential two-photon absorption (TPA) and free carrier absorption (FCA) with increasing input intensity. The filling of the defect states at 1064 excitation wavelength caused a reduction in NA due to increasing saturable absorption with increasing input intensity. TPA coefficients were also found from the fitting ignoring the defect states. As expected, the values of the nonlinear absorption coefficient βeff are higher than that of the TPA coefficients for both excitation wavelengths. The optical limiting threshold of the BTO single crystal was obtained to be 6.62 mJ/cm2. The results of the present works indicated that BTO single crystal can be used as a potential optical limiter.

Third-order nonlinear optical responses of Cr3+ ions on La2(WO4)3 nanoparticles for optical limiting applications

In this article, we discuss the nonlinear optical limiting characteristics of pure and Cr3+ doped La2(WO4)3nanoparticles using soft chemical method. We studied the third-order nonlinear optical properties of nanoparticle using a continuous-wave (CW) laser system pumped by a diode laser at 785 nm. The results reveal that the materials possess reverse saturable absorption (RSA) and self-defocusing in nature. Due to the presence of near-resonant excitation state, the material exhibits sequential two-photon absorption (excited state induced thermally) process. We observe a higher non-linear absorption coefficient (β) and nonlinear refractive index (n2) for 0.7 % Cr3+ doped La2(WO4)3 systems which is higher than that of pure La2(WO4)3 nanoparticles. The transmittance decreases with increased input fluence for both pure and Cr3+ doped La2(WO4)3 samples exposing the optical limiting behavior of the prepared sample. The presence of larger concentrations of more polarizable Cr3+ ions, increased density of states, and higher oxygen defects resulted in enhanced OL behaviour for 0.7 % Cr3+ doped La2(WO4)3 sample. It is found that two photon absorption (β) is increased from 0.41 to 1.16 × 10-3 and third order nonlinear susceptibility (χ(3)) enhances from 3.909 x10-6 to 6.696 × 10-6 esu on Cr doping. The optical limiting threshold of pure and CrxLa1-xWO4 nanoparticles was also studied and found to be in the range of 91.2 to 86.2 W/cm2. As a consequence of the obtained results Cr3+ doped La2(WO4)3 is likely to be an appropriate candidate for CW low-power optical limiting (OL) applications.

A comparative analysis on electrical and nonlinear optical properties of pure and Co–Ni co-doped SnO2 nanoparticles

Compared to bulk materials, nanostructured materials possess ultrafast optical response and strong two-photon absorption which leads to the utility of materials in various laser related applications. Hence, Cobalt (Co)-Nickel (Ni) co-doped SnO2 nanoparticles were synthesized via co-precipitation technique and its optical limiting behavior was studied. PXRD confirms the formation of tetragonal rutile SnO2with space group P42/mnm with decrease in crystallite size due to co-doping. Shift in PXRD peak position towards higher angle and suppression of Sn–O–Sn vibrational peaks confirm the intervention of Co2+ and Ni2+ ions into the SnO2 lattice. Upon co-doping, the band gap decreases (3.35, 3.28 and 3.20 eV) and the PL intensity of blue-green emission increases due to the increase in density of oxygen vacancies on the surface of SnO2. Low dielectric constant, dielectric loss and ac conductivity indicates the prepared materials are having lesser defects. Nonlinear optical (NLO) absorption and optical limiting behavior of developed NPs were recorded by Z-scan method (open aperture) using nano pulsed green laser excitation. Open aperture Z-scan curves with a valley-like pattern indicate the presence of reverse saturable nonlinear absorption due to the sequential two-photon absorption (2PA) process. The 2PA coefficient increases (0.27–0.56 ×10−10W/m) and onset optical limiting threshold decreases (4.5 –1.7 ×10−12W/m2) with increasing dopant concentration of Co–Ni (2 & 4 wt%) in SnO2 NPs. These results demonstrate that Co–Ni (4 wt%) NPs have virtuous dielectric and efficient NLO absorption and limiting properties which suggest that it could be an eminent candidate for the potential applications such as high-performance dielectric material and optical limiting components in optical devices.

Wavelength-dependent nonlinear absorption of gold nanobipyramids with large saturable modulation depth

The light–matter interaction in noble metal nanoparticles is intrinsically determined by the surface plasmon resonance (SPR) and bandgap. To determine the process of electronic transition and the limitation of energy bands on nonlinearity at different excitation wavelengths, we investigate the broadband nanosecond nonlinear absorption of gold nanobipyramids from 532 nm to 850 nm. The experimental results indicate that with the increase of the incident light intensity, gold nanobipyramids have the characteristic of transforming from saturable absorption (SA) to reverse saturable absorption (RSA) at 532 nm to 690 nm, and exhibit a single SA after 690 nm until 850 nm. Based on the wavelength-dependent response, the SA is attributed to the intraband transition of sp-band, whereas sequential two-photon absorption leads to RSA. Unlike other metal nanoparticles, gold nanobipyramids exhibited a significantly higher modulation depth up to 47%. Lastly, building on the excellent SA characteristic, we achieve pulse width compression of nanosecond laser pulses at multi-wavelengths.

Gradual Carrier Filling Effect in "Green" InGaN/GaN Quantum Dots: Femtosecond Carrier Kinetics with Sequential Two-Photon Absorption.

Quantum dots (QDs) allow for a significant amount of strain relaxation, which is helpful in GaN systems where a large lattice mismatch needs to be accommodated. InGaN QDs with a large indium composition are intensively investigated for light emitters requiring longer wavelengths. These are especially important for developing high-efficiency white light sources. Understanding the carrier dynamics in this large lattice-mismatched system is essential to improving the radiative efficiency while circumventing high defect density. This work investigates femtosecond carrier and photon dynamics in self-organized In0.27Ga0.73N/GaN QDs grown by molecular beam epitaxy using transient differential absorption spectroscopy, which measures the differential absorption coefficient (Δα) with and without an optical pump. Due to 3D quantum confinement and the small effective mass of InGaN, the low density of states in the conduction band is easily filled with electrons. In contrast, the GaN barrier region is replete with a high density of electrons due to a large effective mass. This contrast in carrier density creates a unique phenomenon in the dynamics, showing a change in the differential absorption coefficient (Δα) sign from negative to positive with time. The ultrafast microscopic processes indicate that right after the optical pump and first photon absorption, the valence (conduction) band states are depleted (replete) of electrons. This ground-state bleaching process makes Δα negative, and the probe beam is not absorbed. The electrons are then gradually transferred from the GaN barrier into InGaN QDs, which absorb the second photon from the probe beam (excited-state absorption), making Δα positive. The presence of excited-state carriers with a long lifetime is indicative of the enhanced availability of carriers for radiative recombination. This effect also promotes stimulated emission and amplified spontaneous emission, which can be used to develop lasers and superluminescent LEDs, respectively. Measurements with multiple pump powers and temperatures further confirm that the efficacy of InGaN QDs is enhanced by this effective mass contrast and 3D reservoir of carriers from the GaN barrier. This effect can be used to improve the internal quantum efficiency of GaN-based light emitters.

Synthesis and characterization of the novel 4-(1-(pyridin-4-yl) ethoxyl) substituted bis(phthalocyaninato) rare earth complexes and investigation of their two-photon absorption-based third-order non-linear optical properties

Novel 4-(1-(pyridin-4-yl) ethoxyl) substituted double-decker rare earth phthalocyanine complexes were synthesized starting from (R)-4-(1-(pyridin-4-yl) ethoxy) phthalonitrile with proper rare earth metal acetates in octanol catalyzed by 1,8-diazabicyclo[5.4.0]undec‑7-ene at 300 °C. The new starting phthalonitrile compound was obtained from (R)-1-(pyridin-4-yl) ethan-1-ol and 4-nitrophthalonitrile in acetonitrile at reflux temperature in the presence of potassium carbonate as a catalyst. The characterization of the synthesized and isolated compounds was performed by elemental analysis, infrared, ultraviolet-visible, proton and correlation spectroscopy nuclear magnetic resonance, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopic methods. Nonlinear optical responses of all complexes are investigated using the Z-scan method. The nonlinear absorption coefficient, two-photon absorption cross-section, and the imaginary part of the nonlinear susceptibility of complexes are calculated for each discrete concentration. All these nonlinear parameters of our rare-earth coordinated double-decker phthalocyanines present a decrement due to the aggregation under high optic powers, which result in sequential two-photon absorption for smaller ionic radii.

A novel organic-inorganic ionic cocrystal - piperazine-1,4-diium tetrachloridocuprate(II) dihydrate delivering efficient optical limiting

The aqueous reaction of piperazine and copper chloride dihydrate yielded piperazine-1,4-diium tetrachloridocuprate(II) hydrate (PCu) cocrystal. Acid hydrolysis during pH optimization produced cationic piperazine-1,4-diium, which was neutralized by anionic tetrachloridocuprate(II) to form a stable ionic cocrystal. The PCu single crystal belongs to the space group C2/c of the monoclinic crystal system. High linear absorption with indirect bandgap and least possible crystal defects are coherently supported to the two-photon absorption assisted optical limiting phenomena in PCu cocrystal. The third order nonlinear refractive index (n2), absorption coefficient (β) and susceptibility (χ(3)) of title material are found to be (0.635±0.0058)×10-8 cm2W−1, (0.232±0.001)×10-4 cmW−1, (2.63±0.01)×10-6 esu respectively under continuous-wave laser (100 mW, 532 nm) excitation shows its suitability for nonlinear optical device applications. The variation in β measured at different nanosecond pulsed laser intensities confirmed the presence of sequential two-photon absorption. The noncovalent intermolecular charge transfer responsible for sequential two-photon absorption in PCu cocrystal is theoretically proved possible using DFT-UB3LYP/LANL2DZ+6-311G** level of quantum mechanical calculation. The onset optical limiting threshold values were measured to be (0.394±0.02)×1003 Wcm−2 and (1.62±0.02)×1012 Wm−2 for 100 mW continuous wave laser and 100 μJ nanosecond pulsed green laser (532 nm) irradiance respectively. Hence the PCu cocrystal can be a promising candidate for optofunctional applications.

A novel chlorocadmate hybrid cocrystal delivering intermolecular charge transfer enhanced nonlinear optical properties and optical limiting

Employing the donor-acceptor configuration of cocrystals, the intermolecular charge transfer enhanced third order nonlinear optical properties and the optical limiting behaviour of a novel chlorocadmate cocrystal Cd 2 Cl 6 OH 2 2 C 4 H 12 N 2 n ( 1 ) are reported. Optimization of crystal growth parameters is discussed to achieve bulk single crystal with reduced crystal defects by slow solvent evaporation technique. The acid hydrolysis of the piperazine molecule during the pH optimization converted it into piperazine-1,4-diium, which is neutralized by Cd 2 Cl 6 OH 2 2 2 - via hydrogen bond interactions. The intermolecular charge transfer responsible for optical nonlinearity in 1 is proved possible by calculating the intermolecular interaction energies at HF/3-21G level of quantum mechanical calculations. The two-photon absorption coefficient measured at different nanosecond pulsed laser intensities is found to vary, which explicitly confirmed sequential 2 PA in 1 . Lower optical limiting threshold of 1 measured under CW and nanosecond pulsed green laser regime ( ( 1.027 ± 0.02 ) × 10 3 Wcm -2 and ( 1.87 ± 0.02 ) × 10 12 Wm -2 respectively) with coherent dielectric, thermal, mechanical, linear absorption and emission properties showed its suitability for an efficient optical limiting device application. • A Novel intermolecular charge transfer cocrystal for NLO applications is reported. • Sequential two-photon absorption was conclusively proved from intensity-dependent β. • Intermolecular charge transfer interactions assisted optical nonlinearity is proved possible in the title material. • Third order nonlinear optical susceptibility (χ (3) ) of title cocrystal is found to be (0.527 ± 0.008) × 10 −6 esu. • Optical limiting threshold of title cocrystal for nano pulsed (100 mJ, 532 nm) and CW (100 mW, 532 nm) laser irradiance are calculated.