Picosecond Region Research Articles

Water dynamics on the structural properties of some NSAID’s with leucine in the picosecond region using time domain spectroscopy

Concentration-dependent dielectric response for non-steroidal anti-inflammatory drugs (NSAIDs): Aceclofenac (ACF) and Diclofenac (DCF) in the aqueous leucine solution have been reported at different concentrations and temperatures (298.15 K to 283.15 K). The time domain reflectometry technique in the frequency region of 1 GHz to 30 GHz was used for the present study. Complex permittivity (ε*), static dielectric constant (ε), dielectric relaxation time (τ), dipole moment (μ) and Kirkwood correlation factor (g) have been calculated and discussed in terms of the molecular interaction of water and the used drugs. To give more insights into the structural dynamics of drug-induced amino acids, the study includes molar enthalpy of activation (ΔH), entropy of activation (ΔS), and free energy of activation (ΔF). The overall study concludes that the drug (DCF) having a potent inhibitor of cyclooxygenase found a higher static dielectric constant (ε0) than that of the drug (ACF) having more carbon (C), hydrogen (H), and oxygen (O) in the chain, which is more efficient in controlling pain. Communicated by Ramaswamy H. Sarma

Laser-combined multiprobe microscopy and its application to the materials with atomic layer thickness

We have developed a method of evaluating the photo-induced carrier dynamics of nanostructures by combining optical technologies with multiprobe microscopy techniques. Using multiple probes, measurement can be carried out even for a small sample without complicated pretreatments, such as attaching electrode structures. Using transition metal dichalcogenides as a sample and a continuous laser or an ultrashort pulse laser as the light source, we demonstrated analyses of the carrier dynamics related to trap levels in a millisecond to second time domain and the ultrafast photoexcited carrier dynamics in the picosecond region.

Third-Order Nonlinear Optical Properties of CDTE Quantum Dots

Size-dependent optical properties of semiconductor cadmium tellurite (CdTe) quantum dots (QDs) were examined by UV–Visible absorption, photoluminescence, Transmission electron microscope (TEM) and [Formula: see text]-scan. Red shift in the wavelength was observed in absorption and photoluminescence as the size increased. The size of the CdTe QDs is in the range between 3[Formula: see text]nm and 6[Formula: see text]nm. Two-photon absorption with reverse saturable absorption varied with size in picosecond region due to exciton oscillator. CdTe QDs can be used as optical limiters in the region of visibility.

Core modified freebase porphyrins in glass matrix for nonlinear optical properties

Two different types of freebase porphyrins with variation in the central core, namely H2TPP (meso-5,10,15,20- tetrakis phenyl porphyrin) and HSTPP (meso-tetrakis 5,10,15,20-tetra-phenyl-23-H-21-thiaporphyrin) were introduced into the borate glass matrix and developed the thin films by melt quenching technique. Due to core modification and extended non-planarity in the glass matrix, the absorption spectrum of HSTPP shows a strong redshift in the soret and Q bands. Emission spectra of HSTPP show an intense emission at 739 nm (S1 → S0) and ~ 505 nm due to coordination of boron atom at the central core and freezing of N-BO3 vibrations furnishes slow relaxation from higher energy states. Comparison studies are made by treating with BF3O(C2H5)2 to mimic structure in the borate glass matrix due to boron coordination at the central core. The excited state lifetimes are shortened due to BF3 interaction in the solution as well as in the borate glass matrix endowing its potential to develop the optical limiting devices. Nonlinear optical studies reveal reverse saturable absorption (RSA) behaviour in the picosecond (ps) region and show enhanced two photon absorption (TPA) by core modification. Optical power limiting measurements on HSTPP in glass matrix delivers a better limiting threshold value of 15.2 mJ/cm2.

Principle and progress of attosecond pulse generation

<p indent=0mm>With the rapid development of laser technology, the vision of attosecond world has been unfolded. Since the first isolated attosecond pulses produced by high-order harmonic generation in 2001, the time-resolved spectroscopy has entered the attosecond domain and greatly strengthened our understanding of ultrafast electron dynamics in atoms, molecules and condensed matter. Since it approaches the time scale of inner shell electron motion, attosecond pulses become one of the most important tools to explore the electron dynamics. Therefore, developing techniques of attosecond pulse generation is not only essential in optical research field but also imperiously demanded in atomic and molecular physics, which has attracted great attention of researchers worldwide. After years of continuous innovation and breakthroughs, the shortest pulse of 43 attosecond has been achieved. Comparing with the ultrafast laser pulse generation techniques in picosecond or femtosecond regions, the generation of attosecond pulses is rather complex and challenging. From high-order harmonic generation to single attosecond pulse characterization, several crucial technical details need to be considered and investigated carefully, while attosecond gating is a vital link in the chain of technics required by attosecond pulse generation. Up to now, various gating methods have been developed, such as amplitude gating, which, with spectral filtering, takes advantage of the continuous spectrum near the cut-off. In addition, new gating techniques, such as polarization gating, double optical gating, interference polarization gating, polarization-assisted amplitude gating, and phase-matched time gating, also have been proposed or demonstrated to generate isolated attosecond pulses. Recently, spatial gating, which takes advantage of the variation on the driving laser wave front to produce isolated attosecond pulses, has also been developed. Meanwhile, as the intensity at a femtosecond laser focal point now easily exceeds <sc>10¹⁶ W/cm²,</sc> attosecond pulse generation via laser-plasma interaction has been investigated more intensively. Combined with atto-chirp compensation techniques, these methods are pushing the frontier of ultrafast optics toward shorter and stronger attosecond pulses, and achieving even better time resolution in a broader range of applications. In this article, starting with a general introduction of the principle of attosecond pulse generation, we review various gating techniques for attosecond pulse generation in gas targets, new development of attosecond pulses generated by laser-plasma interaction, as well as atto-chirp compensation for even shorter pulses, which are all crucial for the advancement of attosecond science.

Time-Resolved Spectroscopy of Defect-Induced Effects on Photocarrier Dynamics in SrTiO3 Powder

Photocatalytic water splitting has attracted much attention as an environment-friendly way to produce hydrogen gas as energy resources. Photocatalysts are usually used in powder form in view of their low cost and large surface area. Powder particles are rich in surface defects. It has been widely believed that defects work as electron-hole recombination centers and decrease the photocatalytic activity. Nevertheless, in fact, photocatalytic reactions proceed well with powders. Moreover, it has been reported that introducing more defects can sometimes elongate the photocarrier lifetime and improve the photocatalytic activity. However, the effects of powder defects on photocarrier dynamics have not been fully understood yet. Herein, we studied the effects of defects on photocarrier recombination processes by comparing powder and a defect-less single-crystalline SrTiO3. We also compared photocarrier dynamics in undoped and Na-doped SrTiO3 to investigate the roles of defects on photocatalytic reaction.A broadband time-resolved absorption spectroscopy was conducted to observe the photocarrier dynamics. In the case of the single-crystalline SrTiO3, most of the photoexcited electrons survive as free electrons without being trapped by defects. The absorptions by free electrons hardly decayed in the picosecond region. However, the absorption decayed almost single-exponentially in the nanosecond region, and the intensity decreased to less than one-hundredth the original value by 150 ns. On the other hand, in the case of the powder SrTiO3, a large portion of the photogenerated carriers are trapped by defects. The free and trapped electrons decayed rapidly even within a few picoseconds, where trapped electrons decreased more rapidly than free electrons. This result indicates that defects accelerate the recombination just after the excitation. However, the electrons in the powder decayed not in a single-exponential but a multi-exponential manner, and their decay decelerated gradually. Especially, the decay of the trapped electrons became slower than that of free electrons within ~200 ps. Some of the photocarriers survive into the millisecond region. When the decay of electrons in the single-crystalline and powder SrTiO3 were compared, the lifetime of photocarriers in the single crystal was longer than that in powder in a few nanoseconds. However, the photocarrier lifetime in powder became longer in the nanoseconds to milliseconds region than that in the single crystal.These results suggest that defects in powders initially accelerate the photocarrier recombination, but later they decelerate it. This dualistic property of defects in powder is considered to depend on whether electrons and holes are trapped in the vicinity or far away from each other. Powders contain a lot of defects, and most of the photoexcited carriers in powder are captured by these defects. Since the charge separation is not proceeded enough just after the excitation, electrons and holes will tend to be captured when in the vicinity of one another. The closely trapped electrons and holes can hop to approach each other. Therefore, the recombination will be accelerated in the picosecond region. However, once the electrons and holes escape from this initial recombination process, they can spread and get separated by longer distances. In this case, the electrons and holes need to travel long distances by repeated hopping or tunneling for recombination to come closer. This makes the photocarrier lifetime in powder longer than that in single crystal after the nanosecond region.To further investigate the effects of defects in photocatalysts, we next compared the transient absorption of 4 mol% Na-doped SrTiO3 with that of undoped SrTiO3. We found that the trapped electrons in Na-doped SrTiO3 have longer lifetime than those in undoped one, and the number of surviving electrons in microseconds increased ~15 times. It is well known that substitution of cations by lower valence cations increases oxygen vacancies to keep the charge neutrality. When Na ions are doped into SrTiO3, Sr ions tend to be substituted by Na ions. This results in increasing the number of oxygen vacancies, which works as electron trapping sites. Moreover, the decay of trapped electrons was accelerated by introducing O2 gas. This indicates that the trapped electrons can still keep the reactivity with reactant molecules. Finally, we measured the photocatalytic activities of these photocatalysts. The rate of hydrogen evolution from pure water increased by ~200 times by Na-doping. This drastic enhancement could be accounted for by the increase of trapped electrons.The results obtained in this study demonstrate that the trapped electrons are contributing to enhance the steady-state water splitting reactions. This confirms that appropriate defect control is important on improving the photocatalytic activity, because the long photocarriers lifetime increases the probability of photocarriers to meet with the reactant molecules.

Large third‐order optical nonlinearity of chalcogenide glasses within gallium‐tin‐selenium ternary system

AbstractWe explore the third‐order optical nonlinear properties of arsenic‐ and germanium‐free chalcogenide glasses (ChGs) within a gallium‐tin‐selenium (Ga‐Sn‐Se, GSS) ternary system at wavelength of 1.064 μm in the picosecond region. A classical characterization showing the values of the absorption and the linear index will be given. Then, using D4σ‐Z‐scan technique, both nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of the GSS ChGs will be measured. A comparison of the results is made with other families of chalcogenide glasses considering the figure of merit calculation. The first results show that GSS system would give better NL performance promising high potential for n2(β)‐based photonic devices performed under clean production and green/sustainable chemistry.

Generation of sub-nanosecond pulse in dual-wavelength praseodymium fluoride fibre laser

In this paper, we demonstrate a stable dual-wavelength pulsed output from a praseodymium doped fluoride fiber laser. These dual functions are induced by intensity-dependent transmission and loss due to the cavity birefringence. The two lasing wavelengths centered at 1298.26 nm and 1299.91 nm with a spacing of 1.65 nm and a 3 dB bandwidth of 0.11 nm and 0.13 nm respectively are obtained. The pulse width induced by the dual-wavelength laser falls within the picosecond region, with a repetition rate of 389 KHz and a maximum average pulse energy of 5.4 nJ.

Propagation of a laser pulse under electromagnetically induced transparency conditions

We investigate the propagation dynamics of a pair of probe and coupling laser pulses in a three-level type-V atomic medium under the condition of electromagnetically induced transparency (EIT) by numerical solving the Maxwell-Bloch equations for atoms and fields. Influences of the intensity and pulse area of the coupling laser on the EIT formation of probe laser pulse are studied in a wide region from micro- to pico-second of pulse duration. It is found that a nearly solition pulse can be established at the smallest pulse area in the pico-second region and with the largest pulse area in the micro-second region. These results can be important for applications in all-optical switching, and quantum information processing and transmission.

Simulations of Frenkel to Wannier–Mott Exciton Transitions in a Nanohybrid System

Excitation energy transfer at a prototypical organic/inorganic interface is described theoretically. The nanohybrid system to be investigated is built up by a vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nanocrystal of 3903 atoms. To determine the time scale of excitation energy transfer all relevant electronic excitations of the organic and inorganic part are computed together with the related excitation energy transfer couplings. Values of the coupling lie in the millielectronvolt range or less. This motivates a Golden Rule description of the excitation energy transfer. Different Frenkel excitons are chosen as excitation energy donor levels. Due to the H-aggregate configuration of the organic part, the number of exciton wave function nodes increases with decreasing exciton energy. As a result, the couplings of the individual molecules to a certain ZnO electron–hole pair cancel each other more intensively and the overall transfer rate gets smaller. The highest exciton levels decay most rapidly and are characterized by lifetimes in the picosecond region. The lower part of the exciton band, however, has lifetimes in the nanosecond region. The Golden Rule description is finally compared to a direct solution of the time-dependent Schrödinger equation. The obtained transfer dynamics confirm those of the rate equation approach when the higher part of the Frenkel exciton band is considered. In the lower part, the reduced number of final electron–hole pair states in the inorganic part blocks the Frenkel exciton decay.

D-π-A type conjugated pyrene derivatives: Third-order nonlinear absorption and excited-state dynamics

Two novel conjugated pyrene derivatives (C1 and C2) with D-π-A structures were synthesized to realize broadband reverse saturable absorption (RSA) under femtosecond laser excitation (ranging from 515 to 700 nm) resulting from two-photon absorption (TPA) and TPA-induced excited-state absorption. Furthermore, these two derivatives showed RSA on the picosecond and nanosecond time regions (532 nm). Quantum chemical calculations indicated that two mechanisms for the HOMO-LUMO excitation of the two molecules exist simultaneously: intramolecular charge transfer (which is considered to trigger the nonlinear optical response) and π-π* transition. It was found that the π-π* transition localized in the pyrene group can contribute to the increase in the nonlinear absorption of pyrene derivatives. Femtosecond time-resolved transient absorption spectrum provided information on the dynamics of the locally excited singlet state (1LE) and the charge-transfer state (1CT) for both compounds. The results show that these pyrene derivatives are promising candidates for future optoelectronic and RSA applications.

Special Issue on Printed Electronics 2017

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The boundary of the N=90 shape phase transition: 148Ce

The even-even N=90 isotones with Z=60-66 are known to undergo a first order phase transition. Such a phase transition in atomic nuclei is characterized by a sudden change of the shape of the nucleus due to changes in the location of the potential minimum. In these proceedings we report a measurement of the B4/2 ratio of 148Ce, which will probe the location of the low-Z boundary of the N=90 phase transitional region. The measured B4/2 value is compared to the prediction from the X(5) symmetry within the interacting boson model at the critical point between the geometrical limits of vibrators and rigid/axial rotors. The EXILL&FATIMA campaign took place at the high-flux reactor of the Institut Laue Langevin, Grenoble, were 235U and 241Pu fission fragments were measured by a hybrid spectrometer consisting of high-resolution HPGe and fast LaBr3(Ce)-scintillator detectors. The fast LaBr3(Ce) detectors in combination with the generalized centroid difference method allowed lifetime measurements in the picosecond region. Furthermore, this kind of analysis can serve as preparation for the FATIMA experiments at FAIR.

Picosecond relaxation of band-gap renormalization induced by the Coulomb interaction of charge carriers in GaAs

The dynamics of the long-wavelength edge of the spectrum of intrinsic stimulated picosecond emission occurring upon the picosecond pumping of GaAs is studied experimentally. The shortage of bandgap renormalization induced by the Coulomb interaction of charge carriers, compared to renormalization in the quasi-steady state, is observed. It is conceived that the shortage is caused by the fact that, under the experimental conditions of the study, the renormalization relaxation time is in the picosecond region.

Evolution ofE2transition strength in deformed hafnium isotopes from new measurements onHf172,Hf174, andHf176

The available data for E2 transition strengths in the region between neutron-deficient Hf and Pt isotopes are far from complete. More and precise data are needed to enhance the picture of structure evolution in this region and to test state-of-the-art nuclear models. In a simple model, the maximum collectivity is expected at the middle of the major shell. However, for actual nuclei, this picture may no longer be the case, and one should use a more realistic nuclear-structure model. We address this point by studying the spectroscopy of Hf. We remeasure the 2^+_1 half-lives of 172,174,176Hf, for which there is some disagreement in the literature. The main goal is to measure, for the first time, the half-lives of higher-lying states of the rotational band. The new results are compared to a theoretical calculation for absolute transition strengths. The half-lives were measured using \gamma-\gamma and conversion-electron-\gamma delayed coincidences with the fast timing method. For the determination of half-lives in the picosecond region, the generalized centroid difference method was applied. For the theoretical calculation of the spectroscopic properties, the interacting boson model is employed, whose Hamiltonian is determined based on microscopic energy-density functional calculations. The measured 2^+_1 half-lives disagree with results from earlier \gamma-\gamma fast timing measurements, but are in agreement with data from Coulomb excitation experiments and other methods. Half-lives of the 4^+_1 and 6^+_1 states were measured, as well as a lower limit for the 8^+_1 states. We show the importance of the mass-dependence of effective boson charge in the description of E2 transition rates in chains of nuclei. It encourages further studies of the microscopic origin of this mass dependence. New data on transition rates in nuclei from neighboring isotopic chains could support these studies.

Tuning the nonlinear optical absorption of reduced graphene oxide by chemical reduction.

Reduced graphene oxides with varying degrees of reduction have been produced by hydrazine reduction of graphene oxide. The linear and nonlinear optical properties of both graphene oxide as well as the reduced graphene oxides have been measured by single beam Z-scan measurement in the picosecond region. The results reveal both saturable absorption and two-photon absorption, strongly dependent on the intensity of the pump pulse: saturable absorption occurs at lower pump pulse intensity (~1.5 GW/cm2 saturation intensity) whereas two-photon absorption dominates at higher intensities (≥5.7 GW/cm2). Intriguingly, we find that the two-photon absorption coefficient (from 1.5 cm/GW to 4.5cm/GW) and the saturation intensity (from 1 GW/cm2 to 2 GW/cm2) vary with chemical reduction, which is ascribed to the varying concentrations of sp2 domains and sp2 clusters in the reduced graphene oxides. Our results not only provide an insight into the evolution of the nonlinear optical coefficient in reduced graphene oxide, but also suggest that chemical engineering techniques may usefully be applied to tune the nonlinear optical properties of various nano-materials, including atomically thick graphene sheets.

The Early Steps in the Photocycle of a Photosensor Protein Sensory Rhodopsin I from Salinibacter ruber

Light absorption by the photoreceptor microbial rhodopsin triggers trans-cis isomerization of the retinal chromophore surrounded by seven transmembrane α-helices. Sensory rhodopsin I (SRI) is a dual functional photosensory rhodopsin both for positive and negative phototaxis in microbes. By making use of the highly stable SRI protein from Salinibacter ruber (SrSRI), the early steps in the photocycle were studied by time-resolved spectroscopic techniques. All of the temporal behaviors of the Sn←S1 absorption, ground-state bleaching, K intermediate absorption, and stimulated emission were observed in the femto- to picosecond time region by absorption spectroscopy. The primary process exhibited four dynamics similar to other microbial rhodopsins. The first dynamics (τ1 ∼ 54 fs) corresponds to the population branching process from the Franck-Condon region to the reactive (S1(r)) and nonreactive (S1(nr)) S1 states. The second dynamics (τ2 = 0.64 ps) is the isomerization process of the S1(r) state to generate the ground-state 13-cis form, and the third dynamics (τ3 = 1.8 ps) corresponds to the internal conversion of the S1(nr) state. The fourth component (τ3' = 2.5 ps) is assignable to the J-decay (K-formation). This reaction scheme was further supported by the results of fluorescence spectroscopy. To investigate the protein response(s), the spectral changes of the tryptophan bands were monitored by ultraviolet resonance Raman spectroscopy. The intensity change following the K formation in the chromophore structure (τ ∼ 17 ps) was significantly small in SrSRI as compared with other microbial rhodopsins. We also analyzed the effect(s) of Cl(-) binding on the ultrafast dynamics of SrSRI. Compared with a chloride pump Halorhodopsin, Cl(-) binding to SrSRI was less effective for the excited-state dynamics, whereas the binding altered the structural changes of tryptophan following the K-formation, which was the characteristic feature for SrSRI. On the basis of these results, a primary photoreaction scheme of SrSRI together with the role of chloride binding is proposed.

Solvent Dependence of 7-Azaindole Dimerization

We have investigated 7-azaindole (AI) in a variety of solvents including CCl4, CHCl3, CH2Cl2, acetone, CH3CN, and DMSO by femtosecond Raman-induced Kerr effect spectroscopy. In differential low-frequency Kerr spectra between the solutions and the respective neat solvents, vibrational bands of the AI hydrogen-bonding (HB) dimer have been observed at ca. 90 and 105 cm(-1) in CHCl3 and CH2Cl2, as well as CCl4: the standard solvent for the AI dimer. In contrast, a broad monomodal band at ca. 80 cm(-1) characterizes an HB mode between the AI monomer and solvent in acetone, CH3CN, and DMSO. The overdamped Kerr transients in the picosecond region show evidence of both the AI monomer and dimer reorientations in CHCl3, CH2Cl2, acetone, and CH3CN, but only the monomer reorientation has been confirmed in DMSO. The clear intermolecular HB bands have not been observed in acetone, CH3CN, and DMSO because these solvents are sufficiently strong HB acceptors, which form HB AI-solvent complexes, thus preventing quantitative AI dimerization. In addition, it is plausible that the HB band of between AI and solvent obscures the intermolecular bands of the AI dimer when the concentration of the AI dimer is much lower than the AI monomer. For comparison, we have employed NMR to study the concentration-dependent chemical shift of the proton attached to the N at the 7-position of AI and to estimate the dimerization constant: 356, 13.3, 14.7, 0.727, and 0.910 M(-1) in CCl4, CHCl3, CH2Cl2, acetone, and CH3CN, respectively. The femtosecond Raman-induced Kerr effect spectroscopy and NMR results are in good agreement.

Dynamic Structure of a Molecular Liquid S0.5Cl0.5: Ab initio Molecular-Dynamics Simulations

The static and dynamic structures of a molecular liquid S0.5Cl0.5 consisting of Cl–S–S–Cl (S2Cl2) type molecules are studied by means of ab initio molecular dynamics simulations. Both the calculated static and dynamic structure factors are in good agreement with experimental results. The dynamic structures are discussed based on van-Hove distinct correlation functions, molecular translational mean-square displacements (TMSD) and rotational mean-square displacements (RMSD). In the TMSD and RMSD, there are ballistic and diffusive regimes in the sub-picosecond and picosecond time regions, respectively. These time scales are consistent with the decay time observed experimentally. The interaction between molecules in the liquid is also discussed in comparison with that in another liquid chalcogen–halogen system Se0.5Cl0.5.

Real-Time Observation of Tight Au–Au Bond Formation and Relevant Coherent Motion upon Photoexcitation of [Au(CN)2–] Oligomers

Structural dynamics involving tight Au-Au bond formation of excited-state oligomers [Au(CN)(2)(-)](n) was studied using picosecond/femtosecond time-resolved emission and absorption spectroscopy. With selective excitation of the trimer ([Au(CN)(2)(-)](3)) in aqueous solutions, transient absorption due to the excited-state trimer was observed around 600 nm. This transient exhibited a significant intensity increase (τ = 2.1 ps) with a blue shift in the early picosecond time region. Density functional theory (DFT) and time-dependent DFT calculations revealed that the observed spectral changes can be ascribed to a structural change from a bent to a linear staggered structure in the triplet excited-state trimer. The transient absorption also exhibited a clear modulation of the peak position, reflecting coherent nuclear wave packet motion induced by photoexcitation. The frequencies of the coherent motions are 66 and 87 cm(-1), in very good accord with the frequencies of two Au-Au stretch vibrations in the excited state of the trimer calculated by DFT. Time-resolved emission spectra in the subnanosecond time region showed that association of the excited-state trimer with the ground-state monomer proceeds with τ = 2.0 ns, yielding the excited-state tetramer.