Enhancement Of Two-photon Absorption Research Articles

Two-Photon-Excited Fluorescent Tetrabranched Triphenylborane Featuring the Cooperative Effect of Branching in Two-Photon Absorption.

We here disclose a new type of two-photon-excited fluorescent triarylborane, tetrabranched triphenylborane 1, which contains four electron-donating [4-(N,N-diphenylamino)phenyl]ethynyl branches at 2,6-positions of two phenyl rings. The cross section of 1 reaches 275 GM (1 GM = 10-50 cm4 s photon-1) in tetrahydrofuran. Compared with dibranched triphenylborane 2, the 2-fold increase in the number of electron-donating branches induces a 3.6-fold increase in the two-photon absorption cross section, suggesting the great cooperative effect of branching in the enhancement of two-photon absorption.

Ultrafast response of diketo-pyrrolo-pyrrole dyes with large two-photon absorption enhancement as well as AIE property

To study the effect of terminal groups substituent on the two-photon absorption (TPA) properties of [Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text] structural dyes with diketo-pyrrolo-pyrrole (DPP) as central electron acceptor group, linear/transient absorption spectroscopy, one/two-photon fluorescence spectra, open-aperture [Formula: see text]-scan, and excited states quantum chemical calculations have been performed to systematically study the intermediate product electron acceptor J0 as well as three [Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text] structural dyes using benzene, triphenylamine and 2,3-methoxy-triphenylamine as terminal electron donors, named as J1, J2, and J3, respectively. The intermediate product electron acceptor itself exhibits a relatively large TPA response, which indicates the DPP group has a large conjugated system. J2, with strong electron-donating ability groups triphenylamine as terminal groups, shows a large TPA cross-section of 534[Formula: see text]GM, which is approximately 10 times larger than that of J1 with weak electron pushing/pulling abilities groups benzene as terminal groups (51[Formula: see text]GM). The result indicates that the electron-donating abilities of terminal groups do play a key role in TPA response. Furthermore, methoxy substituent on the terminal triphenylamine groups can further effectively increase the TPA response, J3exhibits a 15-fold enhancement compared with that of J1. Transient absorption spectra and quantum chemical calculation results are well accorded with the experimental results. Terminal substituents with strong electron donor groups are beneficial for the formation of intramolecular charge transfer (ICT) process. Besides, methoxy substituent in the terminal of triphenylamine groups can further enhance the degree of ICT and induce AIE characteristics in the molecule.

Mechanism of Two-photon Absorption Enhancement for a Piperazine-based Zinc Ion Probe

Mechanism of Two-photon Absorption Enhancement for a Piperazine-based Zinc Ion Probe

Enhancement of Two-Photon Absorption in Boron-Dipyrromethene (BODIPY) Derivatives

The linear and nonlinear optical properties of two BODIPY derivatives, 1,7-Diphenyl-3,5-bis(9,9-dimethyl-9H-fluoren-2-yl)-boron-diuoride-azadipyrromethene (ZL-61) and 1,7-Diphenyl-3,5-bis(4-(1,2,2-triphenylvinyl)phenyl)-boron-diuoride-azadipyrromethene (ZL-22), were comprehensively investigated based on experimental and theoretical studies. It was found that both compounds show a strong two-photon absorption response in the near-infrared regime, and the two-photon-absorption cross-section values of ZL-61 and ZL-22 were determined to be 8321 GM and 1864 GM at 800 nm, respectively. The improvement of the two-photon absorption cross section in ZL-61 was attributed to the enhancement of the donor group, which was confirmed by transient absorption measurements and DFT calculation. Our results indicate that these BODIPY derivatives are a promising candidate for optical limiting and two-photon imaging applications.

Resonant enhancement of two-photon absorption in rubidium with crossed polarizations

The rubidium 52S1/2→52D3/2 and 52S1/2→52D5/2 two-color two-photon transitions were examined with a variety of detunings in the region 758–798 nm to reveal the transition rates. Two cw lasers with crossed linear polarizations were overlapped and counterpropagated through a vapor cell, with the transition rate monitored by side fluorescence from 6P. Resonant enhancement via the 5P states results in intensity variations of over 6 orders of magnitude across the detuning range. No destructive interference zeros were seen, as expected from the polarization dependent calculation of the third-order susceptibility. Zeros occur from interference between two intermediate states, but due to the contributions from both symmetric and antisymmetric susceptibility tensors the zero of one is hidden by a non-zero value of the other. The polarization dependence was explicitly seen by fixing the detuning and iterating the linear polarizations from crossed to parallel.

Nonlinear loss characterization of continuous wave guiding in silicon wire waveguides

Accurate propagation loss characterization of silicon waveguides is increasingly demanded for silicon-photonics-(Si-Ph) applications with high-power continuous-wave-(CW) light sources. We report on nonlinear loss parameters of silicon wire waveguides for 1.31 μm wavelength CW light extracted from transmission data measured for different lengths and polarizations. Such parameters were, so far, unavailable, although they are required for accurately modeling Si-Ph optical circuits. More-than-ten-times enhancement of two-photon absorption from prior results for short pulse light was observed at power densities ranging up to 4.7 × 1011 W m−2 while free carrier absorption was suppressed. We estimate the nonlinear loss of the waveguides using the parameter values obtained.

Quantifying the enhancement of two-photon absorption due to spectral-temporal entanglement.

When a low flux of time-frequency-entangled photon pairs (EPP) illuminates a two-photon transition, the rate of two-photon absorption (TPA) can be enhanced considerably by the quantum nature of photon number correlations and frequency correlations. We use a quantum-theoretic derivation of entangled TPA (ETPA) and calculate an upper bound on the amount of quantum enhancement that is possible in such systems. The derived bounds indicate that in order to observe ETPA the experiments would need to operate at a combination of significantly higher rates of EPP illumination, molecular concentrations, and conventional TPA cross sections than are achieved in typical experiments.

Anisotropic luminescence and third-order electric susceptibility of Mg-doped gallium oxide under the half-bandgap edge.

Strong anisotropy of photoluminescence of a (100)-cut β-Ga2O3 and a Mg-doped β-Ga2O3 single crystals was found in UV and visible spectral range, the bands of which were attributed to different types of transitions in the samples. Green photoluminescence in the Mg-doped sample was enhanced approximately twice. A remarkable enhancement of two-photon absorption and self-focusing in β-Ga2O3 after doping was revealed by 340-fs laser Z-scanning at 515 nm. The absolute value of complex third order susceptibility χ(3) determined from the study increases by 19 times in [001] lattice direction. Saturable absorption and associated self-defocusing were found in the undoped crystal in the [010] direction, which was explained by the anisotropic excitation of F-centers on intrinsic oxygen defects. This effect falls out of resonance in the Mg-doped crystal. The χ(3) values which are provided by a decrease of bandgap in Mg-doped β-Ga2O3 are χ(3) [001] = 1.85·10-12 esu and χ(3) [010]=χ(3)yyyy = 0.92·10-12 esu. Our result is only one order of magnitude lower than the best characteristic in green demonstrated by a Mg-doped GaN, which encourages subsequent development of Mg-doped β-Ga2O3 as an effective nonlinear optical material in this region.

How large is the quantum enhancement of two-photon absorption by time-frequency entanglement of photon pairs?

We present a theoretical proof that the “quantum enhancement” of two-photon absorption, thought to be a means to improve molecular spectroscopy and imaging, is tightly bounded by the physics of photonic entanglement and nonlinear response.

Benzothiadiazole-Substituted Aza-BODIPY Dyes: Two-Photon Absorption Enhancement for Improved Optical Limiting Performances in the Short-Wave IR Range.

Aza-boron dipyrromethenes (aza-BODIPYs) presenting a benzothiadiazole substitution on upper positions are described. The strong electron-withdrawing effect of the benzothiadiazole moiety permits enhancement of the accepting strength and improves the delocalization of the aza-BODIPY core to attain a significant degree of electronic communication between the lower donating groups and the upper accepting groups. The nature of the intramolecular charge transfer is studied both experimentally and theoretically. Linear spectroscopy highlighted the strongly redshifted absorption and emission of the synthesized molecules with recorded fluorescence spectra over 1000 nm. Nonlinear optical properties were also investigated. Strong enhancement of the two-photon absorption of the substituted dyes compared with the unsubstituted one (up to 4520 GM at 1300 nm) results in an approximately 15-20 % improvement of the optical power limiting performances. These dyes are therefore a good starting point for further improvement of optical power limiting in the short-wave IR range.

Two-photon absorption enhancement for organic acceptor molecules with QD antennas.

The photophysics of an inorganic/organic hybrid system was studied by time-resolved optical spectroscopy, focusing on the goal of increasing the two-photon efficiency of photoresponsive systems. The hybrid system consists of CdS/ZnS core/shell quantum dots (QDs) as energy donor and coumarin derivatives as energy acceptor molecules. The spectral overlap of QD emission and coumarin 343 absorption promotes a Förster resonance energy tranfer (FRET) mechanism leading to a FRET efficiency up to nearly 90%. Additionally, time-correlated single photon counting showed a faster fluorescence decay while acceptor molecules were attached to the QD surface. Femtosecond transient absorption measurements demonstrated an ultrafast FRET reaction. Importantly, FRET was observed also after two-photon excitation of the QDs indicating that the chosen QDs can act as two-photon antennas.

Nondegenerate two-photon absorption in GaAs/AlGaAs multiple quantum well waveguides

We present femtosecond pump-probe measurements of the nondegenerate (1960$\,$nm excitation and 1176--1326$\,$nm probe) two-photon absorption spectra of 8 $\,$nm GaAs/12$\,$nm Al$_{0.32}$Ga$_{0.68}$As quantum well waveguides. Experiments were performed with light pulses co-polarized normal and tangential to the quantum well plane. The results are compared to perturbative calculations of transition rates between states determined by the $\mathbf{k}\cdot\mathbf{p}$ method with an 8 or 14 band basis. We find excellent agreement between theory and experiment for normal polarization, then use the model to support predictions of orders-of-magnitude enhancement of nondegenerate two-photon absorption as one constituent photon energy nears an intersubband resonance.

Plasmon coupling interactions and inhibition of nonlinear absorption in a complex system with Ag and Pt nanoparticles in silica.

The optical response exhibited by a complex hybrid system integrated by Pt ultrasmall fluorescent particles and plasmonic Ag nanoparticles is reported. The system was synthesized by coimplantation of Ag and Pt ions into a silica matrix followed by a proper thermal annealing. The energies and fluences were chosen in order to overlap the spatial regions of the Ag and Pt ion distributions below the silica surface. Optical absorption and emission spectroscopies show that the complex nanostructures exhibit an important plasmonic response, together with photoluminescence excited at 355 nm, which is enhanced when compared to the reference sample with only Pt particles. Off-resonance nonlinear transmission and Z-scan measurements were undertaken using ultrafast pulses. High-irradiance excitation at 1064 nm with picosecond pulses shows that the Pt or Ag nanoparticles exhibit a two-photon absorption effect, while the complex system shows the absence of any nonlinear absorption. Similar observations were made using femtosecond pulses at 800 nm wavelength. This inhibition of the two-photon absorption effect and enhancement in the emission of the complex hybrid samples by the synergic participation of Ag and Pt particles can be explained as a result of a plasmon coupling via the near-field interaction between plasmonic and emitting sources.

Unexpected disruption of the dimensionality-driven two-photon absorption enhancement within a multipolar polypyridyl ruthenium complex series

The dimensionality-driven two-photon absorption (2PA) enhancement effect is investigated in a series of functionalized bipyridyl Ru-complexes. Our design strategy leads to very high 2PA responses up to ∼1500 GM. However, we highlight that the 2PA performance vs. dimensionality correlation reaches an unexpected limit stemming from 'anti-cooperative' interchromophoric couplings.

Ultrafast nonlinear absorption enhancement of monolayer MoS2 with plasmonic Au nanoantennas.

In this work, we experimentally study the nonlinear absorption enhancement of saturable absorption and two-photon absorption on a hybrid structure comprising a monolayer MoS2 and Au nanoantennas via femtosecond I-scan measurement. Specifically, a 13-fold increment in the linear absorption coefficient is attained at 1.85eV, along with an 8-fold enhancement of the two-photon absorption coefficient at 1.65eV, which is attributed to exciton-plasmon coupling resonance and plasmonic hot electron transfer. The exciton-plasmon coupling effect is characterized by stable photoluminescence experiments. Furthermore, the exciton recombination time is extracted from the pump-probe measurement, whose value in the hybrid structure is shortened from 18.5ps (pure MoS2) to 1.84ps. Our findings facilitate a new perspective to modulate the nonlinear optical response and to promote the performance of nonlinear photonic devices.

Higher-order contributions and nonperturbative effects in the nondegenerate nonlinear optical absorption of semiconductors using a two-band model

The semiconductor Bloch equations for a two-band model including inter- and intraband excitation are used to study the nonlinear absorption of single and multiple light pulses by direct-gap semiconductors. For a consistent analysis the contributions to the absorption originating from both the interband polarization and the intraband current need to be included. In the Bloch equation approach theses contributions as well as different excitation pathways in terms of sequences of inter- and intraband excitations can be evaluated separately which allows for a transparent analysis, the identification of the dominant terms, and analyzing their dependence on the excitation conditions. In the perturbative regime, we obtain analytical expressions for the multi-photon absorption coefficients for continuous-wave excitation. These results are shown to agree well with numerical results for short pulses and/or finite dephasing and relaxation times and we confirm the previously predicted strong enhancement of two-photon absorption for non-degenerate conditions for pulsed excitation. We discuss the dependencies on the light frequencies, initial band populations, and the time delay between the pulses. The frequency dependence of the two-photon absorption coefficient for non-degenerate excitation is evaluated perturbatively in third-order. The higher-order contributions to the optical absorption include three- and four-photon absorption and show a rich frequency dependence including negative regions and dispersive lineshapes. Non-perturbative solutions of the Bloch equations demonstrate a strongly non-monotonous behavior of the intensity-dependent optical absorption for a single incident pulse and in a pump-probe set-up.

Ultrafast pump-probe and time-resolved fluorescence anisotropy study of the intramolecular interaction of branched styryl derivatives based on 1,3,5-triazine

A series of branched styryl derivatives based on 1,3,5-triazine were studied by nonlinear optical property measurement, degenerated pump-probe, and time-resolved fluorescence anisotropy methods to elucidate the two-photon absorption (TPA) properties and intramolecular interactions between branches. Significant enhancement of the TPA cross-section was observed in multi-branched derivatives. The anisotropy of multi-branched compounds shows faster decay and small residual values, indicating strong intramolecular interactions between branches, which further confirmed the TPA enhancement mechanism.

The remarkable enhancement of two-photon absorption in pyrene based chalcone derivatives

Two new isomeric pyrene based chalcone derivatives (E)-3-(3,4-dimethoxy-5-(methoxy-methyl)phenyl)-1-(pyren-1-yl)prop-2-en-1-one (1) and (E)-1-(3,4-dimethoxy-5-(methoxy-methyl)phenyl)-3-(pyren-1-yl)prop-2-en-1-one (2) were designed and synthesized for ultrafast broadband nonlinear optics. Z-scan and transient transmission measurements were performed to investigate the differences of regioisomer on nonlinear optics. Both derivatives display reverse saturable absorption as a result of two-photon absorption (TPA). It was found that both π-electron delocalization and intramolecular charge transfer contribute to the TPA and thus influence the nonlinear properties. What's more, remarkable enhancement of TPA cross-section (2400 GM) is observed in 2 as a result of intermediate state resonance enhancement (ISRE). All the studies suggest that these derivatives possess the potential for optical data storage and optical limiting. Moreover, our study further proved the superiority of ISRE on enhancing the TPA of nonlinear materials.

The influence of illumination on two-photon absorption of quantum dots

The influence of illumination with 365[Formula: see text]nm ultraviolet light on two-photon absorption (TPA) of aqueous Co[Formula: see text] doped CdTe (CdTe:Co) quantum dots (QDs) is studied. The results show that with the increase of illumination time, the TPA cross section of the CdTe:Co QDs increases. The experimental results suggest that this enhancement is due to the reduction of surface nonradiative traps which is consistent with the formation of CdS shell on the surface of CdTe:Co QDs. Meanwhile, the results indicate that aqueous CdTe:Co QDs have good optical limiting and optical stabilization performances. This provides a strategy of TPA enhancement by illumination.

Enhancement of NLO Properties in OBO Fluorophores Derived from Carbazole–Coumarin Chalcones Containing Carboxylic Acid at the N-Alykl Terminal End

The nonlinear optical (NLO) properties of carbazole–coumarin-based chalcones and their OBO complexes of D−π–A and D−π–A−π–D types with carbazole as donor substituted with N-alykl chain at the 9 position with carboxylic acid end group are studied in detail. These dyes exhibit good emission solvatochromism. The NLO properties of dyes 5–8 were studied by two methods, namely, solvatochromic and computational method. The linear polarizability (α), hyperpolarizability (β), and second-order hyperpolarizability (γ) of dyes 5–8 were studied by time-dependent density-functional theory computational method using five different range-separated functionals, namely, CAM-B3LYP, HISSbPBE, HSEH1PBE, wB97, and wB97X, with triple ζ basis set 6-311++G(d,p). The correlation between the NLO properties is well established with the geometry of the dyes by understanding the enhancement of hyperpolarizability, second-order hyperpolarizability, and two-photon absorption (TPA) properties of complexed and uncomplexed carbazole–coumar...