Junko Omachi

The OVAL experiment: A new experiment to measure vacuum magnetic birefringence using high repetition pulsed magnets

Abstract A new experiment to measure vacuum magnetic birefringence (VMB), the OVAL experiment, is reported. We developed an original pulsed magnet that has a high repetition rate and applies the strongest magnetic field among VMB experiments online. The vibration isolation design and feedback system enable the direct combination of the magnet with a Fabry-Pérot cavity. To demonstrate and benchmark the searching potential, a calibration measurement with dilute nitrogen gas and a prototype search for VMB are performed. Based on the results, a strategy to observe VMB is reported. Graphical abstract

UV-Light-Induced Vibrational Coherences: The Key to Understand Kasha Rule Violation in trans-Azobenzene

We combine sub-20 fs transient absorption spectroscopy with state-of-the-art computations to study the ultrafast photoinduced dynamics of trans-azobenzene (AB). We are able to resolve the lifetime of the ππ* state, whose decay within ca. 50 fs is correlated to the buildup of the nπ* population and to the emergence of coherences in the dynamics, to date unobserved. Nonlinear spectroscopy simulations call for the CNN in-plane bendings as the active modes in the subps photoinduced coherent dynamics out of the ππ* state. Radiative to kinetic energy transfer into these modes drives the system to a high-energy planar nπ*/ground state conical intersection, inaccessible upon direct excitation of the nπ* state, that triggers an ultrafast (0.45 ps) nonproductive decay of the nπ* state and is thus responsible for the observed Kasha rule violation in UV excited trans-AB. On the other hand, cis-AB is built only after intramolecular vibrational energy redistribution and population of the NN torsional mode.

High-power, narrow-band, high-repetition-rate, 5.9 eV coherent light source using passive optical cavity for laser-based angle-resolved photoelectron spectroscopy.

We demonstrate a scheme for efficient generation of a 5.9 eV coherent light source with an average power of 23 mW, 0.34 meV linewidth, and 73 MHz repetition rate from a Ti: sapphire picosecond mode-locked laser with an output power of 1 W. Second-harmonic light is generated in a passive optical cavity by a BiB(3)O(6) crystal with a conversion efficiency as high as 67%. By focusing the second-harmonic light transmitted from the cavity into a β-BaB(2)O(4) crystal, we obtain fourth-harmonic light at 5.9 eV. This light source offers stable operation for at least a week. We discuss the suitability of the laser light source for high-resolution angle-resolved photoelectron spectroscopy by comparing it with other sources (synchrotron radiation facilities and gas discharge lamp).

Formation control of electron-hole droplets in diamond by a weak pulse injection

We demonstrate a formation control of electron-hole droplets (EHD) in diamond by a weak pulse injection. At high temperatures, we find a large enhancement of the luminescence signal from EHD. On the other hand, the enhancement decreases when we lower the temperature, indicating a decrease in the droplet size and the instability of EHD. In this region, electron-hole ensembles might form a new phase consisting of multi-excitonic clusters.

Tracking azobenzene photoisomerization with sub-20-fs UV pulses

Azobenzene (AB) is a diazene derivative where both hydrogens are replaced by phenyl groups. Under irradiation with UV light, it undergoes ultrafast trans→cis isomerization; the inverse cis→trans isomerization can be driven by light or occurs thermally in dark. AB photochromic properties enable its application as a light triggered switch in numerous molecular devices and functional materials [1]. Here we use femtosecond pumpprobe and two-dimensional electronic spectroscopy in the UV range (2DUV) with sub-20-fs time resolution, combined with ab initio quantum mechanics simulations, to study the initial dynamics of trans-AB isomerization with unprecedented mechanistic detail.

High-power, narrow-width, high-repetition-rate, 5.9 eV light source using a passive optical cavity for laser-based photoelectron spectroscopy

Summary form only given. Angle-resolved photoelectron spectroscopy (ARPES) is a powerful tool to provide direct information on the electronic structures of materials in energy-momentum space. Recent instrumental developments offer us an opportunity to explore fine structures near the Fermi surface [1, 2]. To use the capability of high-resolution ARPES, we should develop vacuum ultraviolet (VUV) light sources with sub-meV linewidths. However, using the typical light sources of synchrotron radiation and gas discharge lamps, it takes much time to acquire photoelectron spectra with a sufficient signal-to-noise ratio, because of the shortage of the photon flux per wavelength. On the other hand, laser-based light sources have many advantages such as high photon flux densities, high spatial coherence, and easy polarization control. Generally, the conversion efficiency from a sub-meV, near-infrared mode-locked laser to VUV (e.g. fourth-harmonic (FH) generation) is low since the peak intensity of the fundamental is not enough, preventing widespread use of such laser-based sources. To demonstrate the efficient conversion from near infrared to VUV, we developed a narrow-width (<; 0.5 meV), 5.9 eV light source from a 1 W, Ti:sapphire mode-locked laser with the pulse duration and the repetition rate of 10 ps and 73 MHz, respectively, using a passive optical cavity (Ref. [4] demonstrates efficient generation of 205 nm light from a 1.3 ps Ti:sapphire oscillator) and commercially-available nonlinear crystals.

Enhanced formation of electron-hole droplets in diamond by a weak pulse injection

The electron-hole droplets (EHD) formation in diamond is remarkably enhanced due to the irradiation by a weak pulse hundreds of picoseconds after a stronger excitation pulse.