Effective Two-photon Absorption Research Articles

Multifunctional Photoelectroactive Materials for Optoelectronic Applications Based on Thieno[3,4-b]pyrazines and Thieno[1,2,5]thiadiazoles.

In this study, we introduce a novel family of symmetrical thiophene-based small molecules with a Donor-Acceptor-Donor structure. These compounds feature three different acceptor units: benzo[c][1,2,5]thiadiazole (Bz), thieno[3,4-b]pyrazine (Pz), and thieno[1,2,5]thiadiazole (Tz), coupled with electron donor units based on a carbazole-thiophene derivative. Using Density Functional Theory (DFT), we investigate how the molecular geometry and strength of the central acceptor unit impact the redox and spectroscopic properties. Notably, the incorporation of Pz and Tz moieties induces a significant redshift in the absorption and emission spectra, which extend into the near-infrared (NIR) region, simultaneously reducing their energy gaps (~1.4-1.6 eV). This shift is attributed to the increased coplanarity of the oligomeric inner core, both in the ground (S0 ) and excited (S1 ) states, due to the enhanced quinoidal character as supported by bond-length alternation (BLA) analysis. These structural changes promote better π-electron delocalization and facilitate photoinduced charge transfer processes in optoelectronic devices. Notably, we show that Pz- and Tz-containing molecules exhibit NIR electrochromic behavior and present ambivalent character in bulk heterojunction (BHJ) solar cells. Finally, theoretical calculations suggest that these molecules could serve as effective two-photon absorption (2PA) probes, further expanding their potential in optoelectronic applications.

Tailoring the optical limiting response of methyl orange via protonation

In order to realize effective optical limiting (OL), it is necessary to utilize materials that exhibit distinct traits, such as a large and rapid nonlinear optical response, high photothermal stability, and cost-effectiveness. In the present study, the nonlinear optical (NLO) response of methyl orange (MO) is modified by inducing conformational and structural changes through appropriate modification of pH conditions. The NLO properties of MO under two different pH conditions were investigated via the z-scan technique under 7 nanoseconds excitation in the visible region (532 nm). From z-scan results, the nonlinear absorption coefficient of pristine MO and protonated-MO (P-MO) was estimated. Compared to MO, the nonlinear absorption coefficient of P-MO increased by 3.6 times, and it can be attributed to the redistribution of energy levels of MO, which favored resonant absorption in the system. Molecular energy shift and the corresponding apparent red shift in the absorption spectrum of MO are confirmed from both experimental and theoretical analysis. The mechanism behind the nonlinear absorption (NLA) and OL activity of MO is found to be effective two-photon absorption. The closed aperture (CA) z-scan data reveal a negative nonlinear refraction response of MO and P-MO. These findings suggest an efficient way to tune the NLO response of MO through protonation, and this method readily applies to numerous nonlinear optical applications, including but not limited to OL.

A study on novel semi-organic material L-phenylalanine mixed with cadmium nitrate single crystals

Slow evaporation solutions turned out to be a good way to get single crystals of semi-organic NLO from an aqueous solution. During this experiment, the ambient temperature was used. Through X-ray diffraction analysis, a single crystal of L-phenylalanine (C9H11NO2) cadmium nitrate (Cd (NO3)2)was characterised. As a result of this construction, the P21 space group has the following cell parameters: a = 12.47 Å, b = 5.43 Å, c = 15.00 Å, α = γ = 90°, β = 107.61°. It was found that a 257 nm cut-off wavelength was found from UV–Vis-NIR spectroscopy. Aside from linear optical characteristics such as the extinction coefficient (K) and reflectance (R), this study examined a number of geometrical features. In addition, the crystal was shown to have a high level of optical transparency. Fluorescence (FL) spectrum analysis has been used in the process of investigating how the luminescence of LPCN behaves. Evidence that functional groups are present may be gleaned from the FT-IR vibrational patterns seen in the LPCN. We used a Vickers microhardness hardness tester to figure out the brittleness index (Bi), as well as the hardness (Hv), Meyer's index (n), yield strength (σy), fracture toughness (Kc), and elastic stiffness constant (C11). The evaluation of the produced crystals' dielectric properties was accomplished by adjusting the frequency. In contrast to KDP and other organic materials, the threshold value for laser-induced damage was found to be much higher. The crystal's surface morphology was analysed using SEM to get a better idea of what it looked like. EDAX analysis of the LPCN crystal was conducted to determine what elements are present in it. The non-linear optical quality of the compound that was discovered with Kurtz powder's second harmonic production in the case of Nd: YAG laser light shows that it is a potential candidate for frequency conversion. The nonlinearity coefficient (β) was measured using the open aperture Z-scan technique (Nd: YAG, 532 nm, 9 ns), and the results showed that it was 0.36 × 10–11m/W. This coefficient is the consequence of a phenomenon known as effective genuine two-photon absorption. The crystal that was created now has a fluency threshold of 3.52 × 1012 W/m2, which makes it a possible candidate for use in optical limiting devices because of its consequent optical limiting behaviour.

Red fluorescent protein dimer for singlet oxygen (1O2)/ superoxide radical (O2•−) simultaneously mediated photodynamic therapy and two-photon fluorescence imaging

In this work, two lysosome-targeting red fluorescent protein analogues (APFP-lyso and DAPFP-lyso) were synthesized by phenolthiazine and anthracene anchoring strategies for singlet oxygen (1O2)/superoxide radical (O2•−) simultaneously mediated photodynamic therapy (PDT) in A-549 cells and two-photon fluorescence imaging in zebrafish. Compared with APFP-lyso of single-branch structure, fluorescent protein dimer DAPFP-lyso shows larger Stokes shift (Δλ = 5431 cm−1) with near-infrared emission at 664 nm and achieves 1.2- and 10.2-fold enhancement in 1O2 and O2•− production efficiency, respectively. In particular, effective two-photon absorption property of fluorescent protein dimer DAPFP-lyso (δ2PA = 81 GM) imparts the vivid two-photon fluorescence imaging in zebrafish under 800 nm excitation. Theoretical calculation indicated that fluorescent protein dimer DAPFP-lyso shows small singlet-triplet (ΔEST = 0.32 eV) to boost reactive oxygen species production efficiency. Intracellular PDT in A-549 cells, it is worth mentioning that DAPFP-lyso showed good biocompatibility, negligible cell dark toxicity (MTT assay >90%) and high phototoxicity (IC50 = 4.52 μM). The fluorescence imaging in A-549 cells and zebrafish manifests dimer DAPFP-lyso could generate 1O2 and O2•− under irradiation (460 nm, 23 mW cm−2). AO/EB dual staining assays and cell migration experiment in A-549 cells demonstrated that DAPFP-lyso could promote cancer cell apoptosis and migration under irradiation. The fluorescent protein dimer photosensitizer DAPFP-lyso had a precise lysosome-targeting ability (R = 0.95) could recognize lysosome specifically. These experimental results show that the fluorescent protein dimer design is of great significance for the development of new photosensitizers in the field of PDT.

Photophysical properties of Pt(II) and Pd(II) complexes for two-photon photodynamic therapy: A computational investigation

Two-photon photodynamic therapy (TP-PDT) plays a critical role of in cancer treatment due to its deep penetration into tissue and low photobleaching effect. In this paper, density functional theory (DFT) and time-dependent density functional theory (TDDFT) are used to calculate the one and two-photon absorption properties, spin-orbit coupling constants, triplet excited state lifetimes and type I/II mechanisms of Pt(II) complexes and two designed Pd(II) complexes. The results reveal that all complexes meet the standard of photosensitizer. More importantly, the designed Pd(II) complex 3 possesses both large two-photon absorption cross section up to 134.5 GM and long triplet excited state lifetime of 85.5 μs. It is expected that the inexpensive Pd(II) complex will replace the reported Pt(II) complexes as an effective two-photon absorption photosensitizer, which will provide some guidance for the design and synthesis of novel photosensitizers experimentally.

Surface plasmon resonance induced impressive absorptive nonlinearity from C-2-phenylethenilcalix [4]resorcinarene silver hybrid system

We experimentally demonstrate the enhanced absorptive nonlinear response from a hybrid system developed through the noncovalent interaction of C-2-phenylethenilcalix [4]resorcinarene (CPECR) with the silver nanoparticles (Ag NPs). The modified absorption spectrum and significantly reduced emission spectrum confirm the hybrid system formation. The characteristic shift in the Raman peaks ensures the charge transfer between CPECR and Ag. The absorptive nonlinearity of the hybrid system at the surface plasmon resonance (SPR) band of Ag NPs was studied by the z-scan method using a Q-switched Nd: YAG laser with nanosecond pulsed beams having a repetition rate of 10 Hz at 532 nm. The dominance of the hybrid system formation over the pure system is the emergence of impressive nonlinear response at low input intensity resulting from the synergistic effect of effective two-photon absorption, the photo-induced charge transfer between CPECR and Ag and the local field effect produced by the SPR of Ag NPs.

Growth and characterization of L - Methionine Barium Bromide (LMBB) semi-organic crystal for optical limiting applications

The semi-organic nonlinear optical single crystal was grown from an aqueous solution by a slow evaporation solution technique (SEST) at room temperature. The characterization of the crystal was made using single crystal x-ray diffraction analysis and it was found to be L-Methionine Barium Bromide (LMBB) crystallized in a monoclinic system with space group C2/c and the cell parameter values are a= 10.74Å, b= 7.39Å, c= 8.69Å, α = γ = 90°, β = 113.53°. The UV-Vis-NIR spectroscopic study revealed that the crystal has good optical transparency and a lower cut-off wavelength was found to be 246 nm and also evaluated linear optical properties such as extinction coefficient and reflectance. The luminescence behavior of LMBB has been analyzed by fluorescence (FL) spectral study. The presence of functional groups in the LMBB is confirmed by FT-IR and FT-Raman vibrational patterns. The hardness (Hv), Meyer’s index (n), elastic stiffness constant (C11), yield strength (σy), knoop hardness (Hk), fracture toughness (Kc), and brittleness index (Bi) were analyzed using Vickers microhardness tester. Dielectric behavior as a function of frequency was examined. The laser-induced damage threshold value was found to be higher than that of KDP. The surface morphology of the as-grown crystal was determined using the SEM technique. Energy dispersive X-ray analysis was used to determine the chemical composition of the LMBB crystal. The nonlinearity coefficient (β) is measured as 1.7 × 10−11 m/W using the open aperture Z-scan technique (Nd: YAG, 532 nm, 9 ns), which results from an effective two-photon absorption phenomenon. The resulting optical limiting behavior has a fluence threshold of 8.26 × 1012 W/m2 making the LMBB a potential candidate for optical limiting devices.

Photonic crystal cavity-mediated improved absorptive nonlinearity of C-4-hydroxy-3-methoxphenilcalix[4]resorcinarene.

The insertion of a nonlinear material at the photonic crystal cavity leads to a strong interaction between localized photonic modes and the introduced material's electronic state, leading to exceptionally improved nonlinear optical properties at low input power. We report the enhanced nonlinear absorption and optical limiting properties of C-4-hydroxy-3-methoxphenilcalix[4]resorcinarene (CHMPCR) in a one-dimensional polymeric photonic crystal cavity. Open aperture z-scan measurements with a nanosecond pulsed laser beam having a repetition rate of 10 Hz at 532 nm from a polymeric-based microcavity demonstrate a 4.5 times enhancement in the nonlinear absorption coefficient of CHMPCR relative to its reference film. The phenomenal increase in nonlinear response could be due to the enhanced local field-assisted effective two-photon absorption arising from the interaction of CHMPCR with intensified confined light emerging from the resonant excitation at the cavity. The low optical limiting threshold and enhanced nonlinear absorption of the CHMPCR-based photonic crystal cavity make it a promising candidate for realizing cost-effective optoelectronic devices.

Linear and nonlinear absorption properties of hemicyanine dyes containing 5-(4H)-oxazolone as heterocyclic ring

A series of dyes containing electron-donating (D) and electron-accepting (A) groups connected through a π-conjugated linker were designed and their linear and nonlinear absorption properties were studied. The dyes combined 1,3-oxazol-5(4H)-one core with methoxy, methyl, phenyl, cyano, and nitro group in the para-position of a phenyl ring. Spectrally resolved two-photon absorption cross-section measurements carried out by the femtosecond Z-scan technique showed wide wavelength regions of nonlinear optical absorption. The studied compound containing the phenyl ring exhibits relatively high effective two-photon absorption cross-section equal to 772 ± 100 GM at 600 nm. The experimental optical characterization is supported by the results of electronic-structure calculations.

Two quinoline-based two-photon fluorescent probes for imaging of viscosity in subcellular organelles of living HeLa cells

Two viscosity-sensitive two-photon fluorescent probes (QL and QLS) were designed and synthesized, which can be localized in lysosome and mitochondria in living HeLa cells, respectively. As the increases of viscosity from 2.55 to 1150 cP, the fluorescence quantum yield (Φ) of QL and QLS was increased by 28-fold and 37-fold, respectively. At the same time, its effective two-photon absorption cross section (ΦδTPA) was enhanced by 15-fold and 16-fold, respectively. Fluorescence lifetime imaging (FLIM) of living HeLa cells stained with QL and QLS, revealed that lysosomal viscosity ranged from 100.76 to 254.74 cP and mitochondrial viscosity ranged from 92.21 to 286.79 cP. This type of fluorescent probe is helpful in the design and application of materials for monitoring diseases associated with abnormal viscosity.

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.

Solvent-conjugation-induced ultrafast dynamics of enhanced reverse saturable absorption in lead phthalocyanine derivatives solutions

With the continued development of high-power laser technology, research on laser protection has become extremely important. In this study, an analysis of Z-scan and femtosecond transient absorption spectra data for lead (II) tetrakis (4-cumylphenoxy) phthalocyanine (4-PbPc) solutions prepared using different concentrations and solvents established that charge transfer plays an important role in the optical limiting dynamics of this molecule. Based on the correlation of decay-associated difference spectra (DADS) for different decay channels, it was confirmed that greater charge transfer efficiency was associated with more highly conjugated solvent molecules. In a dilute solution, the polarity of the solvent molecules will influence the optical limiting performance of the solution to an extent, whereas in a highly concentrated solution, the degree of conjugation of the solvent molecules is the main influence. Therefore, the more highly conjugated the solvent and the higher the solution concentration, the higher the excited state absorption cross-section and effective two-photon absorption coefficient, and the longer the triplet lifetime. These results should provide theoretical guidance and experimental clues for the design of materials with better optical limiting performance and promote the further development of laser protection.

Geometry-dependent two-photon absorption followed by free-carrier absorption in AlGaAs waveguides

Nonlinear absorption can limit the efficiency of nonlinear optical devices. However, it can also be exploited for optical limiting or switching applications. Thus, characterization of nonlinear absorption in photonic devices is imperative for designing useful devices. This work uses the nonlinear transmittance technique to measure the two-photon absorption coefficients ( α 2 ) of AlGaAs waveguides in strip-loaded, nanowire, and half-core geometries in the wavelength range from 1480 to 1560 nm. The highest α 2 values of 2.4, 2.3, and 1.1 c m / G W were measured at 1480 nm for 0.8-µm-wide half-core, 0.6-µm-wide nanowire, and 0.9-µm-wide strip-loaded waveguides, respectively, with α 2 decreasing with increasing wavelength. The free-carrier absorption cross section was also estimated from the nonlinear transmittance data to be around 2.2 × 10 − 16 c m 2 for all three geometries. Our results contribute to a better understanding of the nonlinear absorption in heterostructure waveguides of different cross-sectional geometries. We discuss how the electric field distribution in the different layers of a heterostructure can lead to geometry-dependent effective two-photon absorption coefficients. More specifically, we pinpoint the third-order nonlinear confinement factor as a design parameter to estimate the strength of the effective nonlinear absorption, in addition to tailoring the bandgap energy by varying the material composition.

Strong increase in the effective two-photon absorption cross-section of excitons in quantum dots due to the nonlinear interaction with localized plasmons in gold nanorods.

Excitons in semiconductor quantum dots (QDs) feature high values of the two-photon absorption cross-sections (TPACSs), enabling applications of two-photon-excited photoluminescence (TPE PL) of QDs in biosensing and nonlinear optoelectronics. However, efficient TPE PL of QDs requires high-intensity laser fields, which limits these applications. There are two possible ways to increase the TPE PL of QDs: by increasing their photoluminescence quantum yield (PLQY) or by further increasing the TPACS. Plasmonic nanoparticles (PNPs) may act as open nanocavities for increasing the PLQY via the Purcell effect, but this enhancement is strictly limited by the maximum possible PLQY value of 100%. Here we directly investigated the effect of PNPs on the effective TPACS of excitons in QDs. We have found that effective TPACS of excitons in a QD-PMMA thin film can be increased by a factor of up to 12 near the linearly excited gold nanorods (GNRs). Using gold nanospheres (GNSs), in which plasmons cannot be excited in the infrared range, as a control system, we have shown that, although both GNSs and GNRs increase the recombination rate of excitons, the TPACS is increased only in the case of GNRs. We believe that the observed effect of TPACS enhancement is a result of the nonlinear interaction of the plasmons in GNRs with excitons in QDs, which we have supported by numerical simulations. The results show the way to the rational design of the spectral features of plasmon-exciton hybrids for using them in biosensing and nonlinear optoelectronics.

Synthesis, physicochemical properties and computational study of donor–acceptor polymer for optical limiting application

A new donor–acceptor configured π-conjugated polymer P1 with alkoxy pendant groups having cyclic diimide and thiophene core moieties in polymer backbone were efficiently synthesized via polycondensation reaction. The incorporation of cyclic diimide in polymer increased the rigidity and thermal stability of polymer backbone aided by its high Tg value. These chromophores assisted in reducing the steric interaction of bulky alkoxy pendants which supported lowering the bandgap. The donor and acceptor moieties along with π spacers were particularly chosen to enhance the π-conjugation length in the polymer thereby increasing its nonlinear optical absorption i.e. two-photon absorption. The various structure–property relationships of the polymer were characterized by UV–Vis absorption, fluorescence emission, cyclic voltammetry, and density functional theory studies. The molecular nonlinear properties were theoretically evaluated through the calculation of polarizabilities and hyperpolarizabilities using time-dependent Hartree–Fock method. The polymer showed enhanced effective two-photon absorption with an absorption coefficient ( $$\beta_{eff}$$ ) of 2.031 × 10−10 m/W obtained from open aperture Z-scan analysis which is in good agreement with theoretical study.

Enhanced third order optical nonlinearity in ultrathin amorphous film of tetraphenyl-porphyrin in picosecond regime

The third order optical nonlinearity of ultrathin 5, 10, 15, 20-Tetraphenyl-21H, 23H-porphine (TPP) films fabricated on an ultrasonically cleaned glass-substrate, by high vacuum thermal evaporation method is investigated. The powder X-ray diffraction (XRD) pattern demonstrates the amorphous nature of the fabricated thin film. The atomic force microscopy (AFM) and the field emission scanning electron microscope (FESEM) images show that the surface morphology of thin film composes of randomly oriented particles with the mean surface roughness of 17.73 nm. The structure of TPP thin film portrays a characteristic UV–Visible spectrum due to π-π* transitions in the porphyrin molecule. The photoluminescence spectroscopic study reveals that the TPP exhibits excellent fluorescence emission from S1 singlet excited state. The third order optical nonlinearity is studied using single beam Z-scan technique at 532 nm with picosecond pulsed (Δτ = 30 ps) laser. The TPP thin film exhibits saturation absorption property, with the effective two-photon absorption coefficient (βeff) of the order of 10−6 m/W. The observed nonlinear saturation absorption behavior is largely influenced by one-photon absorption due to the filling effect of the surface states and the localized defect states in the thin film. The closed aperture Z-scan study highlights the self-defocusing nature of the TPP thin film with negative nonlinear refractive index (n2) of the order of 10−13 m2/W. The enhanced n2 value is attributed to the highly polarizable structure of free base TPP molecule and modified electronic band structure due to the strong intermolecular interactions observed in the condensed state.

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.

Studies on NiO-PVA Composite Films for Opto-Electronics and Optical Limiters

We report optical, excited-state lifetime, and nonlinear optical (NLO) properties of nickel oxide (NiO)/poly vinylalcohol (PVA) polymer free-standing films for opto-electronics and optical limiting applications. Optical absorption studies of NiO-loaded PVA films show that the exciton peaks are centered at 330 nm. Fluorescence studies reveal that the emission is due to exciton recombination and defects in NiO. Time-resolved fluorescence studies show that the recombination in PVA happens through NiO, where NiO has a faster decay time than the pure PVA. NLO measurements carried out using a 532 nm, 10-ns Nd: YAG laser, show that for a given energy, NiO/PVA shows good optical limiting, and the obtained NLO behavior is due to effective two-photon absorption.

Enhanced Ultrafast Nonlinear Optical Response in Ferrite Core/Shell Nanostructures with Excellent Optical Limiting Performance.

Nonlinear optical nanostructured materials are gaining increased interest as optical limiters for various applications, although many of them suffer from reduced efficiencies at high-light fluences due to photoinduced deterioration. The nonlinear optical properties of ferrite core/shell nanoparticles showing their robustness for ultrafast optical limiting applications are reported. At 100 fs ultrashort laser pulses the effective two-photon absorption (2PA) coefficient shows a nonmonotonic dependence on the shell thickness, with a maximum value obtained for thin shells. In view of the local electric field confinement, this indicates that core/shell is an advantageous morphology to improve the nonlinear optical parameters, exhibiting excellent optical limiting performance with effective 2PA coefficients in the range of 10-12 cm W-1 (100 fs excitation), and optical limiting threshold fluences in the range of 1.7 J cm-2 . These values are comparable to or better than most of the recently reported optical limiting materials. The quality of the open aperture Z-scan data recorded from repeat measurements at intensities as high as 35 TW cm-2 , indicates their considerably high optical damage thresholds in a toluene dispersion, ensuring their robustness in practical applications. Thus, the high photostability combined with the remarkable nonlinear optical properties makes these nanoparticles excellent candidates for ultrafast optical limiting applications.

Bi-directional top-hat D-Scan: single beam accurate characterization of nonlinear waveguides

The characterization of a third-order nonlinear integrated waveguide is reported for the first time by means of a top-hat dispersive-scan (D-Scan) technique, a temporal analog of the top-hat Z-Scan. With a single laser beam, and by carrying two counterdirectional nonlinear transmissions to assess the input and output coupling efficiencies, a novel procedure is described leading to accurate measurement of the TPA figure of merit, the effective two-photon absorption (TPA), and optical Kerr (including the sign) coefficients. The technique is validated in a silicon strip waveguide for which the effective nonlinear coefficients are measured with an accuracy of ±10%.