Chiaki Ohae

Observation of coherent two-photon emission from the first vibrationally excited state of hydrogen molecules

Yuki Miyamoto1, Hideaki Hara1, Susumu Kuma1,†, Takahiko Masuda1, Itsuo Nakano1, Chiaki Ohae2,‡, Noboru Sasao1,∗, Minoru Tanaka3, Satoshi Uetake4,∗, Akihiro Yoshimi1, Koji Yoshimura1, and Motohiko Yoshimura4 1Research Core for Extreme Quantum World, Okayama University, Okayama 700-8530, Japan 2Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan 3Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan 4Research Center of Quantum Universe, Okayama University, Okayama 700-8530, Japan ∗E-mail: sasao@okayama-u.ac.jp, uetake@okayama-u.ac.jp

A streak camera study of superfluorescence at multiple wavelengths from helium atoms excited using free electron laser pulses

We report the observation of superfluourescence at wavelengths (in air) of 501.6 nm, 667.8 nm, and 728.1 nm following the excitation of helium atoms with free-electron laser pulses at wavelengths of 53.7 nm (n = 3 excitation) and 52.2 nm (n = 4 excitation). The observed wavelengths of the superfluorescence pulses correspond to 1s3p?1s2s, 1s3d?1s2p, and 1s3s?1s2p transitions. Observation of superfluorescence on these transitions implies either competing cascade decays, or direct excitation of non-dipole allowed transitions. We have studied the time structure of the emitted pulses using a streak camera, and the results cannot be explained by straightforward considerations using the usual model for two-level superfluorescence.

Generation of five phase-locked harmonics by implementing a divide-by-three optical frequency divider.

We report the generation of five phase-locked harmonics, f₁:2403  nm, f₂:1201  nm, f₃:801  nm, f₄:600  nm, and f₅:480  nm with an exact frequency ratio of 1:2:3:4:5 by implementing a divide-by-three optical frequency divider in the high harmonic generation process. All five harmonics are generated coaxially with high phase coherence in time and space, which are applicable for various practical uses.

Simultaneous measurements of super-radiance at multiple wavelengths from helium excited states: I. Experiment

In this paper, we report the results of measurements of the intensities and delays of super-radiance decays from excited helium atoms at multiple wavelengths. The experiment was performed using extreme ultraviolet radiation produced by the free electron laser at the SPring-8 Compact SASE Source test accelerator facility as an excitation source. We observed super-radiant transitions on the 1s3p → 1s2s (λ = 502 nm), 1s3d → 1s2p (λ = 668 nm), and 1s3s → 1s2p (λ = 728 nm) transitions. The pulse energy of each transition and its delay time were measured as a function of the target helium gas density. Several interesting features of the data, some of which appear to contradict with the predictions of the simple two-level super-radiance theory, are pointed out.

Production of Ba Metastable State via Superradiance

We describe in this paper a fast and efficient method of producing a high density of barium (Ba) atoms in the 1D2 metastable state, which is a candidate initial state for a new class of cooperative and coherent optical process called paired superradiance (PSR). In the experiment, Ba atoms are excited first by laser light to the 1P1 state and then brought to the desired state 1D2 via radiative transition in the superradiance (SR) mode. It is found that a production efficiency (from 1S0 to 1D2) of more than ∼30% is achieved within a time interval of a few nanoseconds for a Ba density n > 1019 m−3. Several key features of SR important for future PSR experiments are also studied.

Simultaneous Measurements of Superradiance at Multiple Wavelength from Helium Excited States: II. Analysis

Previous experimental studies of superradiance (SR) in multi-level systems have been explainable using the predictions of the well-known simple two-level SR model. However our recent study [K. Nakajima et al., J. Phys. Soc. Jpn. 84, 054301 (2015)] using EUV free-electron laser excitation of helium atoms, where SR was observed at wavelengths of 502, 668, and 728 nm, revealed behaviour which necessitates a full multi-level treatment of the SR development. In this paper, we report simulations of the initial excitation by the FEL pulses, and the subsequent development of multi-level SR. The results of the simulation reproduce the experimental findings, and reveal that competitive SR on two transitions with a common upper level plays an important role in the development of the system.

Generation of five phase-locked harmonics in the continuous wave regime and its potential application to arbitrary optical waveform synthesis

We have successfully generated a new broadband coherent light source in the continuous wave (CW) regime which is an ensemble of multi-harmonic radiations (2403, 1201, 801, 600 and 480 nm) by implementing a frequency dividing technology. The system is uniquely designed that all the harmonics are generated and propagate coaxially which gives the advantage of robustly maintaining the phase coherence among the harmonics. The highlight is its huge potential for the arbitrary optical waveform synthesis in the CW regime which has not been performed yet due to the limitation of the existing light source.

Tailored nonlinear optical process by manipulating the relative phases among the relevant electromagnetic fields

We describe tailoring nonlinear optical process by manipulating the spectral phases among the relevant electromagnetic fields. We demonstrate, as a typical example, an efficient generation of Stokes or anti-Stokes component in the Raman-resonant four-wave-mixing process.

Dual-frequency injection-locked continuous-wave near-infrared laser

We report a dual-frequency injection-locked continuous-wave near-infrared laser. The entire system consists of a Ti:sapphire ring laser as a power oscillator, two independent diode lasers employed as seed lasers, and a master cavity providing a frequency reference. Stable dual-frequency injection-locked oscillation is achieved with a maximum output power of 2.8 W. We show its single longitudinal/transverse mode characteristics and practical power stability, as fundamental performance features of this laser system. We also demonstrate arbitrary selectivity of the two frequencies and flexible control of their relative powers by simply manipulating the seed lasers, as advanced features.