Masaki Hada

Direct observation of collective modes coupled to molecular orbital-driven charge transfer

The making of a molecular movie Phase transitions familiar from everyday life, such as boiling or melting, are caused by changing the temperature. In the laboratory, however, researchers can also change the phase of a material by shining intense light on it. During such transitions, changes occur in both the electronic and lattice structure of the material. Ishikawa et al. used ultrafast optical and electron diffraction probes to monitor both types of change simultaneously during a photo-induced phase transition in a molecular crystal. The resulting molecular movies showed expansion of the intermolecular distance, flattening of the molecules, and tilting of molecular dimers. Science, this issue p. 1501 Ultrafast spectroscopy and electron diffraction are used to create molecular movies of a phase transition in Me4P[Pt(dmit)2]2. Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

Cold ablation driven by localized forces in alkali halides

Laser ablation has been widely used for a variety of applications. Since the mechanisms for ablation are strongly dependent on the photoexcitation level, so called cold material processing has relied on the use of high-peak-power laser fluences for which nonthermal processes become dominant; often reaching the universal threshold for plasma formation of ~1 J cm(-2) in most solids. Here we show single-shot time-resolved femtosecond electron diffraction, femtosecond optical reflectivity and ion detection experiments to study the evolution of the ablation process that follows femtosecond 400 nm laser excitation in crystalline sodium chloride, caesium iodide and potassium iodide. The phenomenon in this class of materials occurs well below the threshold for plasma formation and even below the melting point. The results reveal fast electronic and localized structural changes that lead to the ejection of particulates and the formation of micron-deep craters, reflecting the very nature of the strong repulsive forces at play.

Photo-induced lattice softening of excited-state VO2

In this letter, we demonstrated the photoexcitation of metallic phase vanadium dioxide (VO2) with time-resolved x-ray diffraction measurements. Through the photoexcitation, the metallic phase VO2 transitioned to the similar transient state, which was presented in the insulator to metal phase transition in the time-scale of ∼10 ps. This transient state was accessed only by the photoexcitation and not through further thermal excitation. The presence of the transient state could be an important factor in any further application of the phase transition phenomena.

Ultrafast time-resolved electron diffraction revealing the nonthermal dynamics of near-UV photoexcitation-induced amorphization in Ge2Sb2Te5.

Because of their robust switching capability, chalcogenide glass materials have been used for a wide range of applications, including optical storages devices. These phase transitions are achieved by laser irradiation via thermal processes. Recent studies have suggested the potential of nonthermal phase transitions in the chalcogenide glass material Ge2Sb2Te5 triggered by ultrashort optical pulses; however, a detailed understanding of the amorphization and damage mechanisms governed by nonthermal processes is still lacking. Here we performed ultrafast time-resolved electron diffraction and single-shot optical pump-probe measurements followed by femtosecond near-ultraviolet pulse irradiation to study the structural dynamics of polycrystalline Ge2Sb2Te5. The experimental results present a nonthermal crystal-to-amorphous phase transition of Ge2Sb2Te5 initiated by the displacements of Ge atoms. Above the fluence threshold, we found that the permanent amorphization caused by multi-displacement effects is accompanied by a partial hexagonal crystallization.

Femtosecond electron diffraction: Preparation and characterization of (110)-oriented bismuth films

Here, we present a new approach to synthesize (110)-oriented ultrathin membranes of bismuth (Bi). This rather exotic orientation was achieved by directing the growth through rationale control of lattice matching. Bi films were hetero-epitaxially grown on the (100)-surface of freshly cleaved potassium chloride crystals. The sample orientation was characterized by x-ray and electron diffraction. In addition, high quality free-standing films were obtained after dissolution of the substrate in water and controlled evaporation. Femtosecond electron diffraction (FED) was, therefore, used to monitor the coherent shear acoustic phonons in (110)-oriented free-standing Bi films produced by impulsive femtosecond optical excitation. The small de Broglie wavelength (flat Ewald sphere) of keV-electrons combined with an off-Bragg detection scheme provided a magnified view of shear atomic motions, i.e., lattice distortions in the transverse direction. All-optical pump-probe experiments are usually insensitive to shear di...

Evaluation of Damage Layer in an Organic Film with Irradiation of Energetic Ion Beams

We characterized the thickness and surface damage layer of poly(methyl metacrylate) (PMMA) organic films irradiated with Ar cluster or monomer ion beam using ellipsometry. A heavily damaged layer was detected on the surface of the PMMA film irradiated with Ar monomer ion beam; more than 2–3 nm of the surface were completely metamorphosed into a carbon-like layer and damage had accumulated with irradiation. On the other hand, no significant damage was detected on PMMA films irradiated with Ar cluster ion beams. These results corresponded with measurements of the irradiated surface by X-ray photoelectron spectroscopy (XPS). The sputtering depth from PMMA film irradiated with Ar cluster/monomer ion beams can also be measured using the ellipsometry method at nanometer-order resolution. The optical method of ellipsometry may be a desirable tool for sputtering yield measurement and surface damage layer estimation for organic films.

Simple Technique of Exfoliation and Dispersion of Multilayer Graphene from Natural Graphite by Ozone-Assisted Sonication

Owing to its unique properties, graphene has attracted tremendous attention in many research fields. There is a great space to develop graphene synthesis techniques by an efficient and environmentally friendly approach. In this paper, we report a facile method to synthesize well-dispersed multilayer graphene (MLG) without using any chemical reagents or organic solvents. This was achieved by the ozone-assisted sonication of the natural graphite in a water medium. The frequency or number of ozone treatments plays an important role for the dispersion in the process. The possible mechanism of graphene exfoliation and the introduction of functional groups have been postulated. The experimental setup is unique for ozone treatment and enables the elimination of ozone off-gas. The heat generated by the dissipation of ultrasonic waves was used as it is, and no additional heat was supplied. The graphene dispersion was stable, and no evidence of aggregation was observed---even after several months. The characterization results show that well-dispersed MLG was successfully synthesized without any significant damage to the overall structure. The graphene obtained by this method has potential applications in composite materials, conductive coatings, energy storage, and electronic devices.

Dynamics of all the Raman-active coherent phonons in Sb2Te3 revealed via transient reflectivity

The phonon dynamics in Sb2Te3 is investigated using femtosecond time-resolved reflection measurements. Time evolution of the reflectivity shows the dynamics of all of the Raman-active optical phonons of Sb2Te3 (A1g1, A1g2, Eg1, and Eg2). The amplitude of these coherent phonons strongly depends on the polarization of the excitation pulse, which can be explained by the analysis based on the Raman tensors. Fine tuning of the polarization enables the observation of even Eg1 phonons with small amplitude ( ΔR/R≈ 10−7) and short decay time (<1 ps).

REGAE: New Source for Atomically Resolved Dynamics

In this paper, we show the design and theoretical calculation of our new femtosecond electron source based on rf-accelerator generating 2-5 MeV electron bunches with high electron density and high coherence length.

Structural Monitoring of the Onset of Excited-State Aromaticity in a Liquid Crystal Phase

Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Bairds rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials.