Two-photon Mechanism Research Articles

Two-Photon Ionization Induced Stable White Organic Long Persistent Luminescence.

Organic long persistent luminescence (OLPL) materials with afterglow duration in the scale of minutes or even hours are still rare. Most OLPL systems are based on exciplexes, which require complicated multi-component system in order to realize white afterglow but with slightly compromised duration and color stability. In this work, OLPLs lasting from 20 to 40 minutes are realized in a simple binary system based on two-photon ionization mechanism, which can simultaneously harvest excitons from both singlet and triplet excited states, making it potentially one of the most promising candidates to achieve stable white OLPL. Through modulation and optimization of dopant molecules in dibenzo[b,d]thiophen-2-yldiphenyl phosphine oxide host, the emission profiles of afterglow can be readily tuned from cyan (0.19, 0.22), cold white (0.31, 0.35), standard white (0.33, 0.33) to warm white (0.31, 0.46), with excellent color consistency.

The muon pair production in the process $$e^{+}e^{-}\rightarrow e^{+}e^{-}\mu ^{+}\mu ^{-}$$ through the two-photon mechanism at high energies: transverse momentum, rapidity and angular distributions

In the present paper, we investigate the muon pair production in the interaction between two quasi-real photons in $$e^+e^-$$ collision. We calculate the total and differential cross section of the process $$\gamma \gamma \rightarrow \mu ^+\mu ^-$$ at a beam energy of photons from 3 to 40 GeV in the center-of-mass and for different values of the muon transverse momentum, the muon rapidity and the muon angle. In the $$\gamma \gamma \rightarrow \mu ^+\mu ^-$$ process we also take the contribution of the Box+Vertex correction+ Soft photon emission+Vacuum polarization diagrams. We also study the cross section of the $$e^+ +e^- \rightarrow e^+ + e^- +\mu ^+ + \mu ^-$$ process as a function of the $$e^+ e^-$$ center-of-mass energy $$\sqrt{s}$$ in the region 5 GeV $$\le \sqrt{s} \le $$ 209 GeV using the two-photon mechanism. The obtained results are in satisfactory agreement with experimental data.

Populating 229mTh via two-photon electronic bridge mechanism

Populating 229mTh via two-photon electronic bridge mechanism

Ellipsometric and third-order nonlinear optical studies of pulsed laser deposited aluminium doped zinc oxide thin films

In the present work, an attempt is made to study the optical properties of aluminium doped zinc oxide (AZO) thin films fabricated by pulsed laser deposition (PLD) technique at different laser fluences. The non-polar ‘a-plane’ orientation of the AZO nanostructure is confirmed by x-ray diffraction technique. The linear optical parameters such as refractive index (n), extinction coefficient ( k ), and dielectric constants ( ε ) are studied by spectroscopic nulling ellipsometer. An increase in extinction coefficient with increase in wavelength is observed. This rare phenomenon is observed for the film prepared at highest laser fluence. The nonlinear optical properties of prepared AZO thin films are evaluated by employing an open aperture (OA) and a closed aperture (CA) Z-scan measurements at a continuous 532 nm wavelength laser source. The presence of reverse saturable absorption (RSA) is observed in all the films; which is attributed to the two-photon absorption mechanism. CA measurements revealed, a switching behaviour from self-de-focusing to a self-focusing mechanism. Third-order nonlinear optical susceptibility, χ ( 3 ) values of AZO films are found be in the order of 10 −6 e.s.u . Further, a-plane oriented AZO films showed optical limiting behaviour with promising optical threshold value (2.964 kJ/cm 2 ) .

Synthesis of NaLn(WO4)2 phosphors via a new phase-conversion protocol and investigation of up/down conversion photoluminescence

The two-dimensional (2D) crystallite morphology and low OH−/Ln3+ molar ratio of Ln2(OH)4SO4·2H2O (Ln = lanthanide) make it an ideal precursor for materials synthesis via phase conversion, which was manifested in this work by the direct generation of well-defined NaLn(WO4)2 phosphor particles via hydrothermal reaction with Na2WO4. Kinetics study showed that pure NaLn(WO4)2 can be produced by reaction at 180 °C for ~24 h or at 200 °C for ~6 h under WO42−/Ln3+ = 10 M ratio. Morphology analysis revealed that, though NaLn(WO4)2 evolved via re-precipitation, the layered crystal structure and 2D crystallite morphology of the precursor could have templated the nucleation/growth of NaLn(WO4)2, leading to uniform particles (~4–5 μm) of a unique microdisc-like morphology. Under 394 nm excitation, the Ln = La0.95Eu0.05 phosphor showed down-conversion luminescence having an absolute quantum yield of ~35.4%, a fluorescence lifetime of ~1.13 ms for its 616 nm main emission, and color coordinates of around (0.63, 0.37). Under 978 nm laser excitation, the Ln = La0.97Yb0.02Ho0.01 and Ln = La0.97Yb0.02Er0.01 phosphors exhibited the strongest up-conversion (UC) luminescence at ~660 nm (the 5F5 → 5I8 transition of Ho3+) and 551 nm (the 4S3/2 → 4I15/2 transition of Er3+), average fluorescence lifetimes of ~178.3 and 82.3 µs for the above emissions, and chromaticity coordinates of around (0.62, 0.38) and (0.25, 0.72), respectively. The two UC phosphors were also analyzed to exhibit UC luminescence through a two-photon mechanism.

Phase-conversion synthesis of LaF3:Yb/RE (RE = Ho, Er) nanocrystals with Ln2(OH)4SO4·2H2O type layered compound as a new template, phase/morphology evolution, and upconversion luminescence

Layered hydroxide sulfate (Ln2(OH)4SO4·2H2O) was employed as a new template for (La0.97Yb0.02RE0.01)F3 nanocrystal synthesis via fluorination (RE=Ho, Er), with the course of phase/morphology evolution being studied in detail via systematically varying the reaction temperature (up to 180 °C) and reaction duration (up to 24 h). It was shown that phase conversion can be completed by reaction at room temperature for 24 h or at 180 °C for ∼2 h and the hexagonal structure of LaF3:Yb/RE preferentially develops with ab instead of (00l) basal planes. The upconversion (UC) performance under 978 nm laser excitation was examined with the nanocrystals synthesized via 24 h of reaction at 180 °C (average crystallite size ∼31 nm), and it was found that LaF3:Yb/Ho exhibits luminescence at ∼540 nm (5S2 → 5I8 transition, medium strong), 642 nm (5F5 → 5I8, strong) and 749 nm (5F4 → 5I7, weak), with chromaticity coordinates of about (0.47, 0.52), and LaF3:Yb/Er shows strong green (∼520/540 nm, 2H11/2,4S3/2 → 4I15/2) and even stronger red (∼656/668 nm, 4F9/2 → 4I15/2) luminescence, with color coordinates of around (0.45, 0.54). Analyzing the UC intensity against excitation power (0.6-1.5 W) via logarithmic data transformation found approximately two laser photons are needed to populate the emitting state, implying a two-photon mechanism for the observed UC luminescence of the two types of fluoride nanocrystals. The 642 and 656 nm main emissions of Ho3+ and Er3+ were found to decay in a single exponential manner and have fluorescence lifetimes of ∼742 and 1453 μs, respectively.

Enhancement of nonlinear optical response of methylene blue and azure a during association with colloidal CdS quantum dots

The nonlinear optical response of thiazine dyes during association with colloidal CdS quantum dots (QDs) applying the z-scan technique at wavelength of 532 nm with use 10 ns pulses of Nd:YAG laser second harmonic were studied. Increasing in nonlinear absorption intensity in hybrid associates in comparison with the initial components were found. Moreover, UV–vis absorption spectrum shows the effective formation of dyes H-aggregates during association with colloidal QDs. Two-photon mechanism of observed nonlinear absorption in hybrid associates is proposed, based on a sharper dependence of the normalized transmittance on the incident intensity (distance between the sample and focal plane of focusing lens). And for pure solutions, the reverse saturated absorption is preferred nonlinear absorption mechanism. The nonlinear absorption coefficients for MB and AzA in the hybrid associates were found. They are β ≈ 8.4 × 10−9 cm W-1 и ≈ 1.34 × 10-8 cm W-1, respectively. It was shown that it is possible to control the parameters of nonlinear absorption of thiazine dyes via the hybrid association.

Nonlinear optical absorption properties of tellurium glasses containing different network modifiers

Linear- and nonlinear-optical absorption and carrier dynamic properties of binary tellurite glasses modified with different amount of WO3, Nb2O5, and ZnO were investigated via UV–Vis spectrophotometer, open aperture Z-scan and pump-probe techniques. Glasses were synthesized using conventional melt quenching technique. The vitreous nature of the samples were confirmed by conducting XRD measurements. The studied glasses have got absorption spectra similar to that of the semiconductor materials and it is found that the absorption edges of the samples blue-shifted with the changing amount and the type of the network modifier. Nonlinear absorption measurements of samples were carried out using a laser source with 532 nm wavelength, 10 Hz repetition rate and 4 ns pulse duration. All of the samples showed nonlinear absorption behavior. The transmission pump-probe spectroscopy experiments were carried out to investigate the ultrafast carrier dynamics of studied glasses with the help of Ti:sapphire laser amplifier optical parametric amplifier system with 100 fs pulse duration. The experiments were established by excitation of glasses with 400 nm pump and 500 nm probe lights. The results of the measurements indicated that the two-photon absorption and two-photon assisted free carrier absorption mechanisms are responsible for the nonlinear optical processes. In the light of obtained linear and nonlinear optical results, it can be easily concluded that these glasses are very useful for LED, optical amplifier, data storage, and laser host applications

A third-order nonlinear optical single crystal of 3,4-dimethoxy-substituted chalcone derivative with high laser damage threshold value: a potential material for optical power limiting

Third-order nonlinear optical material 4-[(1E)-3-(3,4-dimethoxyphenyl)-3-oxoprop-1-en-1-yl]phenyl 4-methylbenzene-1-sulfonate (DMPMS) is crystallized by slow solvent evaporation technique. The crystal has inversion symmetry and belongs to monoclinic system with {P2}_{1}/c space group. The C–H⋯O/C–H⋯π intermolecular interactions will be large complementarity for molecular density/crystal packing. A comprehensive investigation for absorbance and emission properties has been performed. Thermal stability is up to 258 °C without any weight loss and calculated value of laser damage threshold is ≈ 12 GW/cm2. The DMPMS shows low dielectric constant value, about 4.42 at 1 MHz and electronic polarizability values in the order of {10}^{-23} cm3. Furthermore, theoretical calculation has been performed using B3LYP and M06-2X functional. The static first-order hyperpolarizability parameter is 55 (B3LYP) and 34 (M06-2X) times that of urea. The total contribution of second-order hyperpolarizability is − 37.9 times {10}^{-40 }mathrm{esu} (in B3LYP functional) and − 25.77 times {10}^{-40 }mathrm{esu} (in M06-2X functional), respectively. Here, two-photon absorption mechanism is responsible for nonlinear absorption and co-efficient is found to be 28.3 times {10}^{-12} m/W. In optical limiting study, limiting threshold is found to be 65 µJ. The real and imaginary third-order nonlinear optical susceptibility is of the order {10}^{-12} mathrm{esu}.

Multistep synthesis and upconversion luminescence of spherical Gd2O3:Er and Gd2O3:Er @ silica

Spherical Gd2O3:Er3+ and Gd2O3:Er3+ @ silica nanoparticles have been successfully synthesized by a multistep procedure including precipitation and sol–gel processes from rare-earth nitrates, TEOS and urea as starting precursors. The structure, morphology and chemical composition of the products were characterized by X-ray diffraction, field emission scanning electron microscopy, and fourier transform infrared spectroscopy. The results indicated that the obtained phosphors consist of separated, non-agglomerated spheres in nanoscale with an uniform, ideal spherical shape. The suitable amount of TEOS was addressed to obtain the spherical Gd2O3:Er3+ @ silica nanocomposites with the luminescence intensity almost unchanged with respect to the uncoated Gd2O3:Er3+ sample. Besides, the upconversion emission spectra showed intense green and red emission bands under 980-nm laser diode excitation with remarkable increase in the red emissions. The upconversion luminescence followed the two-photon mechanism for green and red emissions. With obtained results, the prepared nanospheres are expected to be a potential material to create functional groups for subsequent bio-conjugation with various biomolecules in medical diagnostics.

"Roller coaster"-like thermal evolution of the Er3+ ion's red photoluminescence in CaWO4:Yb3+/Er3+ phosphors.

The abnormal "roller coaster"-like thermal evolution of the Er3+ ion's red photoluminescence (corresponding to the F9/24-I415/2 transition) in CaWO4:Yb3+/Er3+ phosphors is observed. This red emission suffers from a strong thermal quenching in the 293-573K temperature range, followed by a sharp increase on further increasing the temperature. The mechanism behind this phenomenon is confirmed to be from the dynamic temperature-dependent multiple mechanisms imposed on the F9/24 state. At relatively low temperatures, the two-photon upconversion mechanism plays a leading role while, with the increasing of temperature, the one-photon channel, ascribed to the thermal population from the lower I9/24 state, gradually takes a dominant place.

Strategy for highly sensitive optical ratiometric temperature measurement

The Er3+ ion’s 4F7/2-4I15/2 transition, which has been less reported in the past, is studied in CaWO4:Yb3+/Er3+ phosphors. It has been confirmed that this transition is from the two-photon upconversion mechanism when the 980 nm laser diode is used as the excitation source. Moreover, the transition has the same lifetime with the neighboring lower 2H11/2/4S3/2 states, suggesting that the 4F7/2/4S3/2 states are thermally linked. It is shown that the 4F7/2/4S3/2-4I15/2 emissions can be used for ratiometric temperature measurement. Their relative thermal sensitivity is up to 2820/T2, one of the largest sensitivities reported so far. It is very close to the theoretical maximum 2877/T2.

Exclusive double Bc meson production from e+e− annihilation into two virtual photons

We calculate cross sections of a pair Bc meson production on the basis of two-photon mechanism from electron-positron annihilation. We investigate the production cross sections in nonrelativistic approximation and with the account of relativistic corrections. Relativistic production amplitudes of S-wave pair pseudoscalar, vector and pseudoscalar+vector Bc-mesons are constructed on the basis of relativistic quark model. Numerical values of the production cross sections are obtained at different center-of-mass energies. The comparison of one-photon and two-photon annihilation contributions is presented.

Enhanced up-conversion behavior in LiNbO3Er2O3 ceramics

Abstract For the sake of integrating some novel function in existing functional materials, LiNbO3-xEr2O3 ceramics were synthesized through a conventional solid-state reaction. As-obtained XRD patterns indicated that pure perovskite structure had been achieved within the designed composition region. Room temperature Raman spectra illustrated that an abnormal Raman spectra was induced by Er3+ doping. The photoluminescence performances indicated that the relative intensity of up-conversion emission showed a first increasing and then decreasing trend, achieving a maximum value with LiNbO3-0.75%Er2O3 ceramics. In addition, the ratio for the emission intensity of red and green band can be manipulated through adjusting the doping level of Er3+. A two-photon absorption mechanism has been come up with to elaborate this observed up-conversion emission behavior.

Angularly resolved two-photon above-threshold ionization of helium

Two-photon single ionization of helium is studied for photon energies from the two-photon ionization threshold up to 70 eV by numerically solving the full-dimensional time-dependent Schr\odinger equation. Analysis of the photoelectron angular distributions reveals the dominant two-photon mechanisms and the relevance of radial and electron correlation terms as a function of energy and pulse parameters, as well as the breakdown of a single-active-electron picture, qualitatively valid at low photon energies, at energies above ca. 50 eV.

Nonlinear optical properties of benzylamine lead(II) bromide perovskite microdisks in femtosecond regime

Organometal halide perovskites, an emerging class of direct bandgap semiconductors, are attractive candidates for many optoelectronic device applications. Herein, we have reported the nonlinear optical (NLO) properties of layered benzylamine lead(II) bromide perovskite microdisks (MDs) having a lateral dimension of a few micrometers and an average thickness of 35 nm, featuring narrow deep blue emission using the Z-scan technique. The NLO behavior switches over from saturable absorption to reverse saturable absorption under femtosecond laser pulse excitation. Our NLO studies have demonstrated tunable nonlinear behavior, which is attributed to the interplay between single and two-photon absorption by the carriers in the conduction band. Perovskite MDs exhibit an optical limiting behavior originating from the two-photon absorption mechanism.

Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis.

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups.

Reactivity control of a photocatalytic system by changing the light intensity

We report a novel light-intensity dependent reactivity approach allowing us to selectively switch between triplet energy transfer and electron transfer reactions, or to regulate the redox potential available for challenging reductions. Simply by adjusting the light power density with an inexpensive lens while keeping all other parameters constant, we achieved control over one- and two-photon mechanisms, and successfully exploited our approach for lab-scale photoreactions using three substrate classes with excellent selectivities and good product yields. Specifically, our proof-of-concept study demonstrates that the irradiation intensity can be used to control (i) the available photoredox reactivity for reductive dehalogenations to selectively target either bromo- or chloro-substituted arenes, (ii) the photochemical cis-trans isomerization of olefins versus their photoreduction, and (iii) the competition between hydrogen atom abstraction and radical dimerization processes.

Signal and pointing accuracy of ultraviolet laser in micro-pattern gaseous detector

In the study of the gas detectors, it is an important calibration method to use the ultraviolet (UV) laser with two-photon ionization mechanism for producing ionized signal. In the last decades, micro pattern gas detector, especially gaseous electron multiplier and micromesh gaseous detector, has been widely used in high energy experiments. These kinds of gaseous detectors have the advantages of higher ion backflow suppression ability, smaller <b><i>E</i></b> × <b><i>B</i></b> effect and good radiation resistance under the relatively higher count rate environment. To obtain a higher spatial resolution with a UV laser calibration system in gaseous electron multiplier detector, two critical technical issues remain to be resolved: the measurability of the laser signal and the accuracy of the laser beam position. In this paper, the studies in simulation and experiment are conducted to discuss these two critical questions. In the simulation section, the simulation results provide an estimation of signal in the gaseous electron multiplier detector with UV laser of 266 nm wavelength in the mixture working gases of Ar/CO<sub>2</sub> (70/30), and give an evaluation of the laser pointing accuracy and the possible relative error of the electron drift velocity. In the experiment section, a UV laser calibration prototype is designed and developed. A pulsed laser of 266 nm wavelength is used as a signal source, which has a Gaussian-like cross section with a frequency of 10 Hz. The experimental results indicate that the signal of the UV laser in a triple gaseous electron multiplier detector reaches 400 mV for a readout strip width of 6 mm, a gain of detector of 5000, and a gain of amplifier of 10 mV/fC. For the calibration laser, the angle accuracy is discussed and tested. The angle uncertainty of the laser can be kept under 5′, and the accuracy of the drift velocity can reach 6.4 × 10<sup>−4</sup> with a shift of 0.33 mm in the <i>z</i> direction when the laser beam transmits a distance of 400 mm in the gas chamber. All of these results show that the laser beam specific parameters are the main reference for designing the prototype detector. According to the optimal parameters, a gaseous prototype detector will be tested in the next study.

Complete reconstruction of bound and unbound electronic wavefunctions in two-photon double ionization

To be complete, the characterization of the photoionization process of atoms and molecules requires the extraction of all quantum-mechanical phases and amplitudes. So far, complete experiments have accessed only the ionization process of neutral atoms and molecules. Here we report the quantum-mechanically complete characterization of the single and double ionization of neon to yield doubly charged ions. The first ionization step by intense, polarized extreme ultraviolet light from a free-electron laser leaves the ion in a polarized state (that is, one in which the angular momentum of the ion is aligned in space). By controlling the polarization of the light, we determine the bound and continuum components of the system in the first and second ionization steps leading to the formation of doubly charged neon ions. We test the validity of our approach by characterizing the influence of autoionizing ionic states on the two-photon double-ionization mechanism. Our results are important for understanding the physics of the interaction of extreme ultraviolet radiation with ions. Photoionization is one of the most important photophysical events. This process can now be characterized in a quantum-mechanically complete manner by use of polarization-controlled extreme-ultraviolet light derived from a free-electron laser