Photoinduced Transition Research Articles

Model for a photoinduced insulator-metal transition in a one-dimensional quarter-filled organic salt: Evidence for a nonadiabatic charge-phonon coupling

Model for a photoinduced insulator-metal transition in a one-dimensional quarter-filled organic salt: Evidence for a nonadiabatic charge-phonon coupling

Photo-Stimulated and Photo-Suppressed Phase Transitions

Temperature, concentration of the solvent and pressure are the parameters that are well known to bring about phase transitions in liquid crystalline systems. In recent years a new parameter has been added to this list: light. The ability of light to alter/stabilize a particular thermodynamic phase via the photo-isomerization of the constituent molecules is an interesting tool to investigate condensed matter from a new dimension. In this talk I will describe our recent results on just two aspects of these non-equilibrium phase transitions: (i) In the case of the photo-induced isothermal nematic-isotropic transition, it is found that the time required for the system to achieve the photo-stationary state as well as to recover the original state after photo-irradiation, is a smooth function of the absolute temperature except in the vicinity of the transition; the behaviour is explained in terms of the order parameter excess between the equilibrium and photo-stimulated states. It is also found that the effect induced by photoisomerization can be suppressed by applying an external field. This novel method provides an accelerated means of recovering the equilibrium from the photo-driven state. The applied electric field is seen to make the recovery at least two orders of magnitude faster. (ii) Recently, we reported the first exception to an established phenomenon that the photo-induced transition always leads to a phase that in any case exists in the thermal cycle. A guest-host ternary mixture consisting of the photoactive guest molecules and not exhibiting smectic A phase, is seen to induce and stabilize the smectic A phase only in the presence of UV light. We have also mapped out hitherto unexplored temperature vs. UV intensity phase diagrams for various mixtures which illustrate that light mimics, in a limited sense, the role of a thermodynamic parameter like, e.g., pressure; these studies also suggest the possibility of observing a double critical point by employing the UV intensity as a fine-tuning parameter.

Metastability of defects, potential fluctuations, and percolation transition in GaAs

Potential fluctuations due to donor–acceptor compensation have been used to observe localization–delocalization transition in semi-insulating GaAs. Photoinduced transients, resulting from relaxation of stored charges in potential valleys, have two components. The long-lived power law decay at low temperature signifies a microscopically inhomogeneous disordered phase, and single exponential decay at a higher temperature signifies a homogeneous ordered phase. Temperature dependence of steady state photocurrent and kinetics of photocurrent decay suggest percolation as a possible mechanism for photoinduced transition in semi-insulating GaAs.

Alternative pathways of photoinduced electron transport in chloroplasts

The influence of the alternative pathways of electron transport in photosynthetic systems of the oxygen type on the kinetics of the photoinduced redox transitions of P700, ferredoxin, NADP, pH of the intrathylakoid space or lumen, and relative concentration of ATP was studied. The oxygen effect on the kinetics of photooxidation of P700 was analyzed. The retardation of the photooxidation of P700 at low oxygen concentrations can be explained by the “over-reduction” of the acceptor side of PS1 as a result of a decrease in the electron outflow from PS1 to oxygen during hypoxia. The results of numerical experiments are in good agreement with known experimental data that the withdrawal of electrons from PS1 (on the ferredoxin-NADP segment of the chain) can be the limiting stage in the noncyclic electron transport chain. The functioning of the cyclic electron transport chain provides additional synthesis of ATP molecules and weakens the excess reduction of the acceptor segment of PS1. The alternative pathway of electron transport, namely, electron outflow from PS1 to oxygen also favors the optimum conditions for the functioning of the photosynthetic electron transport chain.

Photoinduced ultrafast local volume changes in intermediate-valence solids

We present a theoretical model for the description of the ultrafast structural response of intermediate-valence solids to femtosecond laser excitation. The approach includes the calculation of the free energy of the hot electrons produced by the laser pulse and the determination of the changes in the thermodynamic and elastic properties of the solid as a consequence of the excitation. In particular and based on the promotional Ramirez-Falicov model, we consider the femtosecond laser heating of $\ensuremath{\alpha}$ cerium and the subsequent ultrafast lattice expansion dynamics. For this purpose, we determine the thermodynamic properties of cerium at very high electronic temperatures (simulating the laser excitation). The possibility for a nonequilibrium photoinduced inverse volume collapse transition is discussed. We consider both the laser-excited and the unexcited parts of the system in order to account for inertial confinement. The thermodynamic properties are obtained as function of time and used to calculate the shock velocity variation and the time scale for expansion of the heated spot into the surrounding (unheated) part of the sample. A transition on a subpicosecond time scale is predicted.

Photoinduced insulator-metal transition in correlated electrons — a Floquet analysis with the dynamical mean-field theory

In order to investigate photoinduced insulator-metal transitions observed in correlated electron systems, we propose a new theoretical method, where we combine a Floquet-matrix method for AC-driven systems with the dynamical mean-field theory. The method can treat nonequilibrium steady states exactly beyond the linear-response regime. We have applied the method to the Falicov-Kimball model coupled to AC electric fields, and numerically obtained the spectral function, the nonequilibrium distribution function and the current-voltage characteristic. The results show that intense AC fields indeed drive Mott-like insulating states into photoinduced metallic states in a nonlinear way.

Evidence for complex multistability in photomagnetic cobalt hexacyanoferrates from combined magnetic and synchrotron x-ray diffraction measurements

Cobalt hexacyanoferrates, which exhibit thermally induced and photo-induced spin transitions, have been extensively studied as model materials for molecular-based photomagnetic solids. Synchrotron x-ray diffraction (XRD) experiments combined with magnetic measurements were used here to get some new insights in their diagram of phase transformation. For a ${\text{Na}}_{0.32}\text{Co}{[\text{Fe}{(\text{CN})}_{6}]}_{0.74}\ensuremath{\cdot}3.4{\text{H}}_{2}\text{O}$ sample composition, the high-temperature (HT) phase could be efficiently trapped by rapid cooling, thus allowing a comparison with the photo-induced phase directly formed at low temperature by light irradiation. In terms of magnetic behavior, the two metastable phases strongly differ with lower critical temperature and magnetization values but higher decay temperature for the HT quenched phase. As only a slight change in the lattice constant with no symmetry breaking is reported from XRD, the differences between the two phases are discussed on the basis of local and disordered tilting of the $[\text{Fe}{(\text{CN})}_{6}]$ polyhedral units.

Ultrafast photoinduced transitions in charge density wave, Mott insulator, and metallic phases of an iodine-bridged platinum compound

Ultrafast photoinduced transitions among three phases\char22{}charge density wave (CDW), Mott-Hubbard insulator, and metal\char22{}were investigated in an iodine (I)-bridged platinum (Pt) compound by femtosecond reflection spectroscopy. For a weak photoexcitation, CDW phase over 70 Pt sites is converted to Mott-Hubbard phase by one photon. Such large conversion efficiency makes possible the complete and transient transition from CDW to Mott-Hubbard phase. For a strong excitation, the low-energy spectral weight is increased, suggesting a conversion to a metallic state.

Dynamics of photoinduced insulator-metal transition in one-dimensional Mott insulators

We theoretically investigate the dynamics of insulator-metal transition induced by light-pulse excitation in one-dimensional (1D) and two-dimensional (2D) Mott insulators. We adopt the Pariser-Parr-Pople model, and pump-probe signal is obtained by numerically calculating the time development of the state excited by a light pulse. In the 1D case, when photoexcitation density is below about 10%, an extremely small portion of the energy eigenstates dominates the optical process, and the spin-charge separation holds in these dominant energy eigenstates. As a result, the Mott gap and the short-range antiferromagnetic (AF) spin order are preserved, and the spin relaxation does not occur. When the density is above the value, a metallic state without the Mott gap is photogenerated, and the magnitude of the short-range AF spin order is significantly reduced by photoexcitation. This is consistent with the experimentally observed photoinduced Mott transition. Furthermore, the spin relaxation occurs in the metallic state. This photoinduced Mott transition is a manifestation of the spin-charge coupling in the intensely photoexcited states. In the 2D case, the spin and the charge degrees of freedom are coupled even in the weakly photoexcited states, and the characteristic crossover associated with the excitation density is not observed.

Photoinduced insulator-metal transition and nonlinear optical response of correlated electrons – a DMFT analysis

We investigate a photoinduced insulator-metal transition in the Falicov-Kimball model with the dynamical mean-field theory (DMFT) extended to nonequilibrium systems in periodic modulations in time. When the photon energy of the pump light is Ω≈ (U: the interaction strength), a Drude-like peak is found to grow in the optical conductivity spectrum, which is an evidence that the system is driven into a metallic state. During the transition, the band gap does not collapse, whereas the distribution function exhibits a non-monotonic behaviour away from the Fermi distribution. This indicates that the transition cannot be accounted for by heating effects, but creation of photo-carriers is responsible.

Imaging Technique with Liquid Crystals of Halogen‐Containing 3,6‐Distyrylpyridazines (=3,6‐Bis(2‐phenylethenyl)pyridazines)

Abstractmagnified imageThe 3,6‐bis{(1E)‐2‐[4‐(dodecyloxy)phenyl]ethenyl}pyridazine (1a) was regio‐ and stereoselectively brominated or chlorinated to the (Z,Z)‐compounds 3 and 6, respectively. The conjugated core of these systems represents a mesogen for smectic (and nematic) liquid crystals and, moreover, a chromophore with an absorption at the UV/VIS border. Irradiation and efficient intersystem crossing to the triplet state leads to isomerization reactions (Z,Z)→(E,Z) in solution as well as in the thermotropic LC phases. Since (E,Z)‐configurations are only tolerated to a very small extent by the mesophases, the photoinduced transition S→N→I of 6 can be used as an imaging technique.

Ultrafast optical responses in one-dimensional Mott insulators of halogen-bridged Ni compounds

Halogen-bridged nickel compounds have attracted much attention because they show large optical nonlinearity and photoinduced insulator-metal transition. We report a new fabrication method for thin film samples of the Ni compounds. Using these thin film samples, we have performed femtosecond pump-probe spectroscopy. Photomodulated absorption induced by resonant and nonresonant photoexcitation on charge-transfer band have revealed that excitons decay very fast with the time constant τ less than 2 picoseconds and charge carriers decay relatively slow with τ > 20 ps. The slower decay of the charge carriers is attributable to the formation of polarons and the resultant decrease of their mobility.

Photoinduced insulator-metal transition in Pr0.5Ca0.5CoO3 as studied by femtosecond spectroscopy

Ultrafast optical response has been investigated using fs laser system on a perovskite-type cobalt oxide, Pr0.5Ca0.5CoO3 which undergoes the insulator-metal transition at 89 K. After the photo-irradiation at 30 K, the reflectivity shows a sudden increase. The ultrafast reflectance changes are quantitatively reproduced by the numerical calculation considering the linear combination of the dielectric functions of the initial insulating and the final metallic state, indicating ultrafast phase transition on the time scale of femtosecond.

Multi-phonon dynamics of the ultra-fast photoinduced transition of (EDO-TTF)2SbF6

We report here the first observation of the photoinduced insulating-to-metal phase transition in the (EDO-TTF)2SbF6 salt, which occurs on the picosecond time-scale. The time-resolved optical experiments performed with 80 fs time-resolution demonstrate that the dynamical process involves several low-frequency phonons, as the crystalline structure is destabilized upon laser excitation.

Reversible Photoinduced Wettability Transition of Hierarchical ZnO Structures

Control over the wettability of solids as well as the manufacturing of functional surfaces with special wetting properties has aroused much interest because of great advantages given to various applications in daily life, industry, and agriculture. We report here the dynamic optical control of the wetting behavior of liquids on hierarchically structured ZnO surfaces produced by irradiating silicon (Si) wafers with femtosecond laser pulses and subsequently coating them with ZnO by pulsed laser deposition. The final surface exhibits roughness at two length scales, comprising micrometer-sized Si spikes decorated with ZnO nanoprotrusions. It is shown that a liquid droplet on these surfaces can be rapidly and reversibly switched between hydrophobicity and superhydrophilicity by alternating UV illumination and dark storage or thermal heating. By studying the magnitude and the rate of the photoinduced transitions, we investigated the contribution of roughness at different scales in the framework of two theoretic...

Nematic-to-isotropic photo-induced phase transition in azobenzene-doped low-molar liquid crystals

The photo-induced phase transition generated by several 4,4'-disubstituted azobenzenes in host nematic low-molar mass liquid crystals was quantified by means of the efficiency, DeltaT(N-I), using polarized optical microscopy (POM) at variable temperature. Non-covalent interactions established between mesogen and azobenzene were the main determinants of the photo-induced phase transition behaviour. The correct design of the chemical structure of the system components allows controlling the temperature range in which the photo-induced mesophase transition occurs. Therefore, more efficient systems were obtained when azobenzenes with an extended aromatic core were used.

Intraband absorption in solar cells with an intermediate band

This article presents a thermodynamic treatment of an intermediate band solar cell that includes photoinduced electronic transitions between two distinct states of the intermediate band. The treatment also allows for two black-body sources, interband photoinduced electronic transitions, overlapping absorption coefficients, multiple electron-hole pair generation, and nonradiative processes. A schematic of the device’s thermodynamic configuration shows that the solar cell is composed of three particle engines operating in tandem. The authors present detailed-balance results where it is assumed that when there is the physical possibility of both photoinduced intra- and interband electronic transitions at intermediate levels that the latter predominates. Results indicate that as the intermediate band’s width increases, the efficiencies saturate to those of two-stack tandem solar cells while the band structures approach that of a material that should operate as a black body. The authors conclude that the assumption that interband transitions predominate over intraband transitions, which is equivalent to ignoring or excluding intraband transitions, may yield results inconsistent with physical reality. The larger the difference between the intermediate band width and the smallest band gap in the system, the more pronounced will be the inconsistency.

Solar cell with an intermediate band of finite width

This article considers idealized solar cells whose absorbers are intermediate band (IB) media with finite bandwidths that permit both interband and intraband photoinduced electronic transitions at states within the IB. To comprehend the effect of the IB width, three classes of IB absorbers are constructed where each class is distinguished from the others by its spectral selectivity. It is shown that (i) the maximum-power efficiency tends gradually toward zero with increasing bandwidth when photoinduced interband transitions and intraband transitions are equally likely; (ii) with respect to the former, a relative efficiency enhancement may occur when photoinduced intraband transitions dominate interband transitions; and (iii) although thermodynamically consistent, efficiencies may be physically inconsistent without including photoinduced intraband transitions. Resulting from the solar surface temperature of 6000 K, the authors conclude that the largest efficiencies result when the IB width is roughly equal to or less than 800 meV.

Oxygen as an alternative electron acceptor in the photosynthetic electron transport chain of C3 plants

This study deals with effects of oxygen on the kinetics of P(700) photoinduced redox transitions and on induction transients of chlorophyll fluorescence in leaves of C(3) plants Hibiscus rosa-sinensis and Vicia faba. It is shown that the removal of oxygen from the leaf environment has a conspicuous effect on photosynthetic electron transport. Under anaerobic conditions, the concentration of oxidized P700 centers in continuous white light was substantially lower than under aerobic conditions. The deficiency of oxygen released non-photochemical quenching of chlorophyll fluorescence, thus indicating a decrease in the trans-thylakoid pH gradient (DeltapH). Quantitative analysis of experimental data within the framework of an original mathematical model has shown that the steady-state electron flux toward oxygen in Chinese hibiscus leaves makes up to approximately 40% of the total electron flow passing through photosystem 1 (PS1). The decrease in P700+ content under anaerobic conditions can be due to two causes: i) the retardation of electron outflow from PS1, and ii) the release of photosynthetic control (acceleration of electron flow from PS2 to P700+) owing to lower acidification of the intra-thylakoid space. At the same time, cyclic electron transport around PS1 was not stimulated in the oxygen-free medium, although such stimulation seemed likely in view of possible rearrangement of electron flows on the acceptor side of PS1. This conclusion stems from observations that the rates of P700+ reduction in DCMU-poisoned samples, both under aerobic and anaerobic conditions, were negligibly small compared to rates of electron flow from PS2 toward P700+ in untreated samples.

Photo-EPR Study of Vacancy-Type Defects in Irradiated n-Type 4H-SiC

We report photo-induced electron paramagnetic resonance (photo-EPR) data for irradiated n-type 4H-SiC. Energy levels and associated photo-induced transitions are discussed for silicon vacancy (VSi), carbon vacancy (VC), carbon antisite-vacancy pair (CSiVC), and divacancy (VSiVC).