The observation of Raman-active mode softening is a powerful tool to understand phase transition mechanisms in solids. The evolution of the phonon spectrum of technologically important correlated oxide V 3O 5 attracts much attention for the steady-state regime and for ultrashort temporal scales since thermal contribution to the insulator-to-metal (IMT) transition is excluded during the first few picoseconds. Here, we show theoretical modeling of V 3O 5 phonon spectrum and experiment. It is shown that the IMT is a first-order phase transition. Photoinduced transition was observed by a elastic light scattering technique, showing the principal difference in V 3O 5 surface dynamics across IMT for thermally and ultrafast light-induced phase transitions. The ultrafast IMT is accompanied primarily by short-range optical phonon interactions with a negligible contribution by acoustic phonons, while the acoustic phonons provide a key contribution to the transformation of surface geometry upon thermally induced IMT. Scattering imaging of surface autocorrelation function upon IMT shows the rise of optical homogeneity in the lateral direction of the surface in the steady-state regime and on a subpicosecond time scale.

Ultrafast transition in V3O5 shows complex dynamics of the surface autocorrelation function. Phase-retrieval algorithms with filtering of stochastic scattering component provide reconstruction of specific/hidden features of photoexcited surface.

Angle-resolved reflection, light scattering and ultrafast pump-probe spectroscopy combined with a surface plasmon-polariton (SPP) resonance technique in attenuated total reflection geometry was used to investigate the light-induced plasmonic switching in a photorefractive VO2/Au hybrid structure. Measurements of SPP scattering and reflection shows that the optically-induced formation of metallic state in a vanadium dioxide layer deposited on a gold film significantly alters the electromagnetic field enhancement and SPP propagation length at the VO2/Au interface. The ultrafast optical manipulation of SPP resonance is shown on a picosecond timescale. Obtained results demonstrate high potential of photorefractive vanadium oxides as efficient plasmonic modulating materials for ultrafast optoelectronic devices.

Photoinduced phase transitions in complex correlated systems occur very rapidly and involve the interplay between various electronic and lattice degrees of freedom. For these materials to be considered for practical applications, it is important to discover how their phase transitions take place. Here we use a novel ultrafast diffraction conoscopy technique to study the evolution of vanadium dioxide (VO2) from biaxial to uniaxial symmetry. A key finding in this study is an additional relaxation process through which the phase transition takes place. Our results show that the biaxial monoclinic crystal initially, within the first 100-300 fs, transforms to a transient biaxial crystal, and within the next 300-400 fs converts into a uniaxial rutile crystal. The characteristic times for these transitions depend on film morphology and are presumably altered by misfit strain. We take advantage of Landau phenomenology to describe the complex dynamics of VO2 phase transition in the femtosecond regime.

The ultrafast elastic light scattering technique is applied to reveal the strong nonlinearity of V_{3}O_{5} associated with a photoinduced insulator-metal phase transition. Observation of time-domain relaxation dynamics suggests several stages of structural transition. We discuss the nonequilibrium processes in V_{3}O_{5} in terms of photoinduced melting of a polaronic Wigner crystal, coalescence of V-O octahedra, and photogeneration of acoustical phonons in the low-T and high-T phases of V_{3}O_{5}. A molecular dynamics computation supports experimentally observed stages of V_{3}O_{5} relaxation dynamics.

Distinct contribution of acoustic and optical phonons in light-induced lattice transformation was resolved at different time scales by monitoring the insulator-to-metal phase transition in epitaxial and nonepitaxial VO2 films. Applying the ultrafast angle-resolved light scattering technique we demonstrate a significant influence of internal misfit strain in epitaxial films on subpicosecond phase transition dynamics. This technique also allows for observing a contribution of structural defects in the evolution of the transient state. The ultrafast structural phase transition dynamics is discussed in terms of the Ginzburg-Landau formalism. Using a set of experimental data we reconstruct the thermodynamic potential of photoexcited VO2 and provide a phenomenological model of the ultrafast light-induced structural phase transition.

The compound V3O5, a member of the vanadium oxide Magnéli series, exhibits a metal-insulator transition near 430 K, the highest known temperature value among all vanadium oxides. It has been studied before mainly in single-crystal form, and for the very few cases in which thin films have been fabricated before, the procedure has required extensive post-deposition annealing of other oxides or vanadium metal at high temperatures in tightly controlled atmospheres. For the present work, V3O5 films were deposited directly on SiO2 glass substrates, without subsequent annealing, by DC magnetron sputtering. X-ray diffraction study of the samples evidenced oxygen deficiency, accommodated by oxygen vacancies. Resistivity measurements from 300 to 500 K revealed the metal-insulator transition by Tc ∼ 430 K, with an associated resistivity change by a factor of 20, and no detectable hysteresis in heating-cooling cycles, in agreement with most single-crystal studies. Resistivity values obtained were, however, lower than published results for bulk crystal values, particularly at temperatures below Tc. This was attributed to conduction electrons generated by the oxygen vacancies. Gradual resistivity increase in a very thin sample, through heating in air at temperatures up to 500 K, lends support to this argument. Using a pump-probe scattering technique, the V3O5 films were also probed for ultrafast nonlinear optical response. A reduction in the transient relative scattered light signal was recorded, which reached –10% within ∼800 fs. This observed response, likely related to the photoinduced insulator-to-metal phase transition, should stimulate additional interest in this material.

Using time- and angle-resolved hemispherical elastic light scattering technique we reveal complex pathways of photoinduced nonlinear optical dynamics in VO2, V2O3 and V3O5 thin films. The structural dynamics was monitored by using an ultrafast diffraction conoscopy technique. The evolution of phases in these correlated oxides is substantially different and significantly depends on optical excitation, temperature and size of grains and domains. Strong optical nonlinearity along with its complex transient dynamics makes vanadium oxides attractive for high-contrast all-optical switches, high-speed optical data storage and holographic devices. The characteristic time of optical nonlinearity can be tuned from several femtoseconds to picoseconds by altering the excitation fluence and size of grains and domains. Additional control of ultrafast phase transition dynamics can be achieved by photoacoustical generation of strain waves. Depending on material morphology and level of optical excitation, the optical signal shows coherent oscillations caused by photoacoustic wave at picosecond and nanosecond time scales. Complex nonlinear dynamics of correlated vanadium oxides can provide a way for precise tuning of transient optical and electronic properties in photonic devices.

Using ultrafast angle-resolved light scattering technique, we were able to trigger photoinduced phase transition processes in V_2O_3 film grown on a glass substrate. The phase transition is caused by photoacoustic wave in the film and appears as coherent oscillations of scattering signal at various time scales. These processes strongly depend on the size of microstructures constituting the V_2O_3 film. One of the key findings of our study is the presence of a size dependent phase transition threshold for V_2O_3 microstructures, where small size structures (<0.5µm) have lowest contribution to the phase transition. The presence of this threshold can be well described by considering uneven internal strain in the films which is one of the key parameters controlling phase transition dynamics in various vanadium oxides.

KU ScholarWorks is a service provided by the KU Libraries’ Office of Scholarly Communication & Copyright. This is the published version of the article, made available with the permission of the publisher. The original published version can be found at the link below. Maogang Gong et al. (2013). Ionic-Passivated FeS2 Photocapacitors for Energy Conversion and Storage. Chemical Communications Published version: http://www.dx.doi.org/10.1039/C3CC45088K Terms of Use: http://www2.ku.edu/~scholar/docs/license.shtml Please share your stories about how Open Access to this article benefits you.

The newly discovered two-dimensional materials can be used to form atomically thin and sharp van der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements: electron transfer across the interface and interlayer coupling. Here we show, using graphene–tungsten disulfide heterostructures as an example, evidence of ultrafast and highly efficient interlayer electron transfer and strong interlayer coupling and control. We find that photocarriers injected in tungsten disulfide transfer to graphene in 1 ps and with near-unity efficiency. We also demonstrate that optical properties of tungsten disulfide can be effectively tuned by carriers in graphene. These findings illustrate basic processes required for using van der Waals heterostructures in electronics and photonics. Two-dimensional materials get their unusual properties because the motion of their electrons is confined to a single plane, but combining two such materials adds an extra degree of freedom: interlayer coupling. Here, the authors use ultrafast spectroscopy to show that this electron motion is highly efficient.

The exciton dynamics in monolayer and bulk MoSe2 samples are studied by transient absorption microscopy with a high spatiotemporal resolution. Excitons are injected with a point-like spatial distribution using a tightly focused femtosecond pulse. The spatiotemporal dynamics of these excitons are monitored by measuring transient absorption of a time-delayed and spatially scanned probe pulse. We obtain the exciton diffusion coefficients of 12 ± 3 and 19 ± 2 cm(2) s(-1) and exciton lifetimes of 130 ± 20 and 210 ± 10 ps in the monolayer and bulk samples, respectively. These values are useful for understanding excitons and their interactions with the environment in these structures and potential applications of MoSe2 in optoelectronics and electronics.

We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by femtosecond transient absorption. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by measuring a differential reflection of a probe pulse tuned in the range 790-820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in a bulk sample under comparable conditions. We also observe the saturated absorption induced by excitons in both monolayers and the bulk in the differential reflection measurements, which indicates their potential applications as saturable absorbers.

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.

We present an experimental investigation on the exciton dynamics of monolayer and bulk WSe2 samples, both of which are studied by femtosecond transient absorption microscopy. Under the excitation of a 405 nm pump pulse, the differential reflection signal of a probe pulse (tuned to the A-exciton resonance) reaches a peak rapidly that indicates an ultrafast formation process of excitons. By resolving the differential reflection signal in both time and space, we directly determine the exciton lifetimes of 18 ± 1 and 160 ± 10 ps and the exciton diffusion coefficients of 15 ± 5 and 9 ± 3 cm2/s in the monolayer and bulk samples, respectively. From these values, we deduce other parameters characterizing the exciton dynamics such as the diffusion length, the mobility, the mean free path, and the mean free length. These fundamental parameters are useful for understanding the excitons in monolayer and bulk WSe2 and are important for applications in optoelectronics, photonics, and electronics.

We study valley and spin dynamics in monolayer molybdenum diselenide by polarization-resolved femtosecond transient absorption spectroscopy. Valley- and spin-polarized excitons are injected by a circularly polarized laser pulse, with an excess energy of 120 meV. Relaxation of the valley polarization is time-resolved by measuring dynamical circular dichroism of a linearly polarized probe pulse tuned to 790 nm, the peak of the exciton resonance of monolayer MoSe2. We obtain a valley relaxation time of 9 ± 3 ps at room temperature, which is at least one order of magnitude shorter than the simultaneously measured exciton lifetime. The results illustrate potential applications of MoSe2 in room-temperature valleytronic and spintronic devices.

We show that the lack of inversion symmetry in monolayer MoS2 allows strong optical second harmonic generation. Second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 1E-7 m/V. The susceptibility reduces by a factor of seven in trilayers, and by about two orders of magnitude in even layers. A proof-of-principle second harmonic microscopy measurement is performed on samples grown by chemical vapor deposition, which illustrates potential applications of this effect in fast and non-invasive detection of crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size of atomically thin films of MoS2 and similar materials.