Photoinduced phase transition in an organic radical crystal with room-temperature optical and magnetic bistability

A phase control by photoirradiation is successfully achieved in a spin-Peierls system of the organic radical crystal, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which exhibits optical and magnetic bistability around room temperature with a large hysteresis loop. A nanosecond laser pulse is found to induce a transition from a diamagnetic low-temperature phase to a paramagnetic high-temperature phase both inside (296 K) and outside (11 K) the hysteresis loop. Comparison of the excitation energy dependence between transition efficiency and photoconductivity suggests that the photoinduced transition is driven by suppression of the spin-Peierls instability by the accumulation of photocarriers.     

Novel optical and magnetic bistability and photoinduced transition in a one-dimensional halogen-bridged binuclear pt complex

In iodine-bridged binuclear Pt compounds, R(4)[Pt(2)(pop)(4)I]nH(2)O and R(')(2)[Pt(2)(pop)(4)I]nH(2)O (pop=P(2)O5H2-2), electronic structures on the PtPtI chains have been controlled between a diamagnetic charge-density-wave (CDW) state and a paramagnetic charge-polarization (CP) state by modification of the counterions (R, R') located between chains. In the R=(C2H5)2NH+2 compound, a pressure-induced CP to CDW transition with a drastic color change is identified. This transition is accompanied by a large hysteresis loop within which photoinduced transition between CDW and CP can be driven by selecting the excitation photon energy.     

Photoinduced phase transition accompanied with the changes in magnetic properties

Recently, many studies have been started in search for materials which show a photoinduced phase transition (PIPT). In this work, we review two systems as typical examples of PIPT accompanied with changes in magnetic characteristics; (1) organo-metal complex [Fe(2-pic)₃]Cl₂ EtOH (2-pic = 2-amino-methyl-pyridine) and (2) III-V based magnetic semiconductors (In_1-x , Mn _x )As. In the former case, we show several nonlinear characteristics in dynamical process of photoinduced spin state transition from low-spin to high-spin states. In the latter one, photocarrier-induced ferromagnetic order has been observed by both magnetic and transport measurements.     

The primary stages of the charge carrier photogeneration in C60 films studied by the 100-fs laser pulse pump-probe method

The primary stages of photoinduced processes are studied in thin C₆₀ films by the femtosecond laser pump-probe method. The films were excited by 100-fs laser pulses with photon energies above (wavelengths 345 and 367 nm) and below (645 nm) the mobility threshold, the fraction of excited molecules being no more than several percent. Upon probing in the spectral range from 400 to 1100 nm, several regions with substantially different decay kinetics were observed in the difference spectrum, which is caused by the simultaneous presence of several relaxing components. The appearance of the 465-and 500-nm bleaching bands in the difference spectrum upon excitation by photons with energies both above and below the mobility threshold, which are typical for electroabsorption spectra, suggests that charge carriers are produced in both these cases. The observed dependence of relaxation on the oxygen amount in the sample volume suggests that during excitation both charged (electrons and holes) and neutral (excited molecules) components are produced. The fraction of charged components is greater upon excitation into the fundamental band. The appearance of the 500-nm absorption band delayed by 10^?13–10^?14 s, the delay being increased in the presence of oxygen, was attributed to the formation of excited anions due to the capture of electrons by C₆₀ molecules. It is concluded that upon excitation of the films by photons with the energy below the mobility threshold, charge carriers are produced due to two-photon absorption rather than due to singlet-singlet annihilation. When the films are excited by photons above the mobility threshold, the primary charge carriers are produced by direct optical excitation.     

Low-energy theorems for nuclear compton and raman scattering and 0+ → 0+ two-photon decays in nuclei

Low-energy theorems for elastic photon scattering (nuclear Compton scattering) from a nucleus of arbitrary spin are derived in the nonrelativistic approximation through terms quadratic in the photon frequency. The same derivation is made for the special case of 0⁺ → 0⁺ nuclear excitation by inelastic photon scattering (nuclear Raman scattering). Use is made of the general principle of gauge invariance, which bypasses the need to specify the form of the current operator explicitly. A general discussion of the contribution of mesonic exchanges is made and their effect is isolated. The center-of-mass correction to the nuclear diamagnetic susceptibility is calculated. The 0⁺ → 0⁺ two-photon decay amplitude is obtained from the nuclear Raman amplitude and the transition rate is calculated.     

Ultrafast carrier dynamics of Cu2O thin film induced by two-photon excitation

Cuprous oxide (Cu₂O) has attracted plenty of attention for potential nonlinear photonic applications due to its superior third-order nonlinear optical property such as two-photon absorption. In this paper, we investigated the two-photon excitation induced carrier dynamics of a Cu₂O thin film prepared by radio-frequency magnetron sputtering, using the femtosecond transient absorption experiments. Biexponential dynamics including an ultrafast carrier scattering (< 1 ps) followed by a carrier recombination (> 50 ps) were observed. The time constant of carrier scattering under two-photon excitation is larger than that under one-photon excitation, due to the different transition selection rules and smaller absorption coefficient of the two-photon excitation.     

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation

Improving the up-conversion luminescence efficiency of rare-earth ions via the multi-photon absorption process is crucial in several related application areas. In this work, we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er~(3+) ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components.So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. Moreover, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width(or laser spectral bandwidth), final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earthions.     

Photobleaching and stabilization of. fluorophores used for single-molecule analysis. with one- and two-photon excitation

Fluorescence spectroscopic measurements at the single-molecule level usually require large absorption cross sections and fluorescence quantum yields for the dyes under study. In addition to these parameters, the collectable number of fluorescence photons and, thus, the signal-to-noise ratio of the measurement, is influenced by processes like triplet-state population and photobleaching, shifting the saturation threshold of the dye to lower excitation intensities. Confocal fluorescence correlation spectroscopy (FCS) is a versatile method to precisely determine photon emission rates of single molecules but also gives access to rate constants of dynamic bleaching and intersystem crossing. In recent FCS studies in solution and living cells, two-photon excitation with its inherent spatial sectioning has proven to be a very valuable alternative to minimize background and cumulative signal loss. However, there is evidence that in many dye systems, the photobleaching rates with two-photon excitation are significantly enhanced with respect to one-photon excitation at comparable photon-emission yields. The reasons have so far remained mainly speculative. In the present study, potential photobleaching pathways are investigated by adding chemical stabilizers and by working at different oxygen concentrations. The results suggest that the population of triplet states does not appear to be responsible for the limited emission rate with two-photon excitation. Rather, photobleaching pathways via the formation of radicals seem to be plausible causes for the signal limitation. Favorable conditions are discussed to maximize the overall photon-collection yield in two-photon experiments. Received: 4 July 2001 / Published online: 23 November 2001     

Ultrafast optical nonlinearity in the quasi-one-dimensional mott insulator Sr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching.     

Primary processes of laser-induced selective dimer-layer removal on Si(001)-(2x1)

Excitation with nanosecond-laser pulses at fluences well below the melt threshold removes Si dimers on the Si(001)-(2x1) surface and induces atomic-Si desorption through an electronic mechanism. The rate of this photoinduced reaction depends superlinearly on the excitation intensity, and is enhanced resonantly at the photon energy where the optical transition injects holes into the dimer backbond surface-band state. The results reveal the crucial role of surface holes and their nonlinear localization in the bond rupture of Si dimers on this surface.     

Initial-Condition Sensitivity and Nonlinear Relaxation in Photo-Induced Ionic-to-Neutral Phase Transition in Tetrathiafulvalen- p -Chloranil Crystals

Recent experimental results of the photoinduced ionic-to-neutral (NI) phase transition in tetrathiafulvalen- p -chloranil (TTF-CA) crystals are reviewed with emphasis on the state-sensitive features and nonlinear properties. Frenkel-type and charge transfer (CT)-type excited states can induce the NI transition but with different characteristics; the transition can be induced only above threshold-excitation intensities in the case of CT excitation, whereas it is induced without any threshold for Frenkel-type excitation. The threshold that implies nonlinear processes of CT excited states is strongly state-dependent and temperature-dependent. Femtosecond time-resolved studies have resolved three distinctive sequential steps in the dynamics; formation of the precursor of N-phase domains, the local proliferation of photoinduced changes, and the process of forming N-phase orders. Origin of the nonlinear processes and the mechanism of the NI transition have been discussed based on these results.     

Interplay between electronic correlations and coherent structural dynamics during the monoclinic insulator-to-rutile metal phase transition in VO2

We report direct observations of the structural and electronic dynamics of the photoinduced insulator-metal transition in VO(2), by means of time-resolved photoemission spectroscopy. These observations provide new insights into the processes involved in this transition. Slightly above the threshold of the photoinduced phase transition, the different response times of the electrons and the lattice reveal the electronic nature of the band gap collapse. At high excitation densities, we find that the phase transition is induced nonthermally in an ultrashort time scale. Moreover, we identify different V 3p dynamics indicating the existence of different structural pathways. These results represent a clear demonstration of the potential of time-resolved core level photoelectron spectroscopy to study ultrafast dynamics in condensed matter.     

Fast bistable all-optical switch and memory on a silicon photonic crystal on-chip

We demonstrate extremely low-power all-optical bistability by utilizing silicon photonic crystal nanocavities, based on the plasma effect of carriers generated by two-photon absorption. Owing to the high quality factor and the small volume of the nanocavities, the photon density inside the cavity becomes extremely high, which leads to a large reduction in operation power. Optical bistable operation in a single nanocavity permits optical read-write memory operation, which opens the possibility of an integrated optical logic circuit on a single chip, based on photonic crystals. The demonstrated bistable threshold power is 0.4 mW with a set pulse energy of 74 fJ, at a switching speed of <100 ps.     

低强度飞秒激光激发下CdTe和CdTe/CdS核壳量子点的荧光上转换研究

利用400 nm和800 nm不同波长的低强度飞秒激光,对CdTe和CdTe/CdS核壳量子点溶胶进行激发,研究其稳态和时间分辨荧光性质.800 nm飞秒激光激发下,CdTe和CdTe/CdS核壳量子点产生上转换发光现象,上转换荧光峰与400 nm激发下的荧光峰相比蓝移最多达15 nm,而且蓝移值与荧光量子产率有关.变功率激发确认激发光功率与上转换荧光强度间满足二次方关系,时间分辨荧光的研究表明荧光动力学曲线服从双e指数衰减.提出表面态辅助的双光子吸收模型是低激发强度上转换发光的主要机理.CdTe和CdT 关键词： CdTe量子点 CdTe/CdS核壳量子点 时间分辨荧光 上转换荧光     

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.     

双光子吸收光谱

本文报道以皮秒脉冲激光抽运的光参量放大器为宽带可调谐激发光源，直接测量非线性光学材料的双光子吸收谱的方法。光参量放大器不但可调谐谱带很宽，而且可用一对尼科尔棱镜很容易地实现全波段功率足够强和相等，使此方法的测量结果准确可靠。用此方法研究了一个新有机化合物的双光子吸收谱。实验结果表明所测的新有机化合物在很宽的波长范围内呈现出很大的双光子吸收截面。     

单光子和双光子激发的超歧化聚合物的光谱学特性研究

研究了二种新型超歧化聚合物在氯仿和二甲基甲酰胺溶剂中的飞秒激光单光子和双光子激发的光谱学特性．这二种聚合物是以苯为核、以对二甲氧基苯为连接单元，分别以二甲基苯胺为端基或以特丁基苯为端基的超歧化共轭聚合物．结果表明这种超歧化聚合物具有较大的溶剂极性相关的飞秒双光子吸收截面，不同的端基不仅影响分子内的电荷转移，而且导致超歧化分子共轭程度的变化．作为一种潜在的光学非线性材料，利用这种大的双光子吸收截面的超歧化聚合物可实现了飞秒双光子直写的三维高密度数据存储．     

Holographic recording using two-photon-induced photopolymerization

Molecular excitation via the simultaneous absorption of two photons can lead to improved three-dimensional control of photochemical or photophysical processes due to the quadratic dependence of the absorption probability on the incident radiation intensity. This has lead to the development of improved three-dimensional fluorescence imaging, optical data storage, and microfabrication. The latter of these involves the fabrication of three-dimensional structures using a spatial variation in the incident intensity within a photopolymerizable resin. In the past, the translation of the focal plane of a tightly focused laser beam was used to induce localized photopolymerization and fabrication of three-dimensional structures. Here we report the first successful demonstration of large-area patterning via ultrafast holography-based two-photon polymerization of a commercially available optical resin and a large two-photon cross-section dye (AF380). This opens tremendous possibilities for the wide-spread use of two-photon absorption for the three-dimensional control of photoinduced processes. Received: 21 June 1999 / Accepted: 23 June 1999 / Published online: 8 September 1999     

Photoelectron Spectroscopic Study on Photo-Induced Phase Transition of Spin-Crossover Complex

Valence band structures in various phases of an organometal spin-crossover complex [Fe(2-pic) 3 ]Cl 2 - EtOH have been measured by means of photoelectron spectroscopy based on the combination of synchrotron radiation and laser light. The valence band structure showed the remarkable change due to the photo-induced (PI) phase transition (PIPT) at low-temperatures (LTs) as well as the thermally induced phase transition (TIPT), but the structure of the PI phase was very different from that of the high-temperature (HT) phase. The DV-X f calculation based on the [Fe-(NH 3 ) 6 ] octahedron was also carried out for comparison, indicating that the electronic structure originating from Fe and N ions is closely related to the PIPT.     

Two-photon correlations as the signal of a sharp transition in quark-gluon plasma

The photon production arising due to time variation of a medium has been considered. The Hamilton formalism for photons in a time-variable medium (plasma) has been developed with application to inclusive photon production. The results have been used for calculation of the photon production in the course of the transition from a quark-gluon phase to a hadronic phase in relativistic heavy-ion collisions. The relative strength of the effect and the specific two-photon correlations have been evaluated. It is demonstrated that the opposite-side two-photon correlations are indicative of a sharp transition from the quark-gluon phase to hadrons.