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arXiv:1806.04005 · Journal of Physics B Atomic Molecular and Optical Physics

Coherent two-photon emission from hydrogen molecules excited by counter-propagating laser pulses

Takahiro Hiraki, Hideaki Hara, Yuki Miyamoto, Kei Imamura, Takahiko Masuda, N. Sasao, Satoshi Uetake, A. Yoshimi, Koji Yoshimura, Motohiko Yoshimura

Published Jan 3, 2019• 15 citations
arXivHTMLPDF

Abstract

Abstract We observed two-photon emission (TPE) signal from the first vibrationally excited state of parahydrogen gas coherently excited by counter-propagating laser pulses. A single narrow-linewidth laser source has roles in the excitation of the parahydrogen molecules and the induction of the TPE process. We measured dependences of the signal energy on the detuning, target gas pressure, and input pulse energies. These results are qualitatively consistent with those obtained by numerical simulations based on Maxwell–Bloch equations with one spatial dimension and one temporal dimension. This study of the TPE process in the counter-propagating injection scheme is an important step toward neutrino mass spectroscopy.

DOI
10.1088/1361-6455/aafbd0

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Abstract We observed two-photon emission (TPE) signal from the first vibrationally excited state of parahydrogen gas coherently excited by counter-propagating laser pulses. A single narrow-linewidth laser source has roles in the excitation of the parahydrogen molecules and the induction of the TPE process. We measured dependences of the signal energy on the detuning, target gas pressure, and input pulse energies. These results are qualitatively consistent with those obtained by numerical simulations based on Maxwell–Bloch equations with one spatial dimension and one temporal dimension. This study of the TPE process in the counter-propagating injection scheme is an important step toward neutrino mass spectroscopy.
We report on coherent enhancement of two-photon emission from the excited vibrational state of molecular hydrogen triggered by irradiating mid-infrared pulses externally. We previously observed the two-photon emission triggered by the internally generated fourth Stokes photons. By injecting independent mid-infrared pulses externally, it is possible to control the experimental parameters and investigate the mechanism in more detail. In this article, we describe the two-photon emission using the external trigger pulses. Its spectrum and dependence on the energy and timing of the trigger pulse are presented along with numerical simulations based on the Maxwell–Bloch equations. The measured number of emitted photons is |6×10116 \times 10^{11}6×1011|photons/pulse and the resulting enhancement factor from the spontaneous emission is more than 10|18^{18}18|⁠. This value is three orders of magnitude higher than that of the previous experiment. External control of the emission process is expected to be essential for observation of the weaker process of radiative emission of a neutrino pair.
We report the observation of two-photon emission from a coherently excited vibrational state of solid parahydrogen, which is also a known quantum solid. Coherence between the ground and the excited states is prepared by stimulated Raman scattering using two visible laser pulses. The two-photon emission is triggered by another mid-infrared laser pulse. It was observed that the two-photon emission persists even when the trigger pulse is injected long after the excitation. This is due to the long decoherence time of the vibrational states of solid parahydrogen. It is found that the emission intensity increases even after the excitation pulses pass through the target completely. This coherence development is highly suppressed at high target temperatures and high residual orthohydrogen concentrations. Effects of target annealing and laser-induced damage on the target are also observed.
Abstract We report the coherent excitation of the vibrational state of hydrogen molecules by two-photon absorption and the resultant third-harmonic generation (THG). Parahydrogen molecules cooled by liquid nitrogen are irradiated by mid-infrared nanosecond pulses at 4.8 μ m with a nearly Fourier-transform-limited linewidth. The first excited vibrational state of parahydrogen is populated by two-photon absorption of the mid-infrared photons. Because of the narrow linewidth of the mid-infrared pulses, coherence between the ground and excited states is sufficient to induce higher-order processes. Near-infrared photons from the THG are observed at 1.6 μ m. The dependence of the intensity of the near-infrared radiation on mid-infrared pulse energy, target pressure, and cell length is determined. We used a simple formula for THG with consideration of realistic experimental conditions to explain the observed results.
We investigate how the efficiency of coherently amplified two-photon emission depends on the frequency of one of the two emitted photons, namely the signal photon. This is done over the wavelength range of 5.048--10.21 $\ensuremath{\mu}\mathrm{m}$ by using the vibrational transition of parahydrogen. The efficiency increases with the frequency of the signal photon. Considering experimental errors, our results are consistent with the theoretical prediction for the present experimental conditions. This study is an experimental demonstration of the frequency dependence of coherently amplified two-photon emission, and also presents its potential as a light source.
In this paper, we describe an experiment which was conducted to explore the macro-coherent amplification mechanism using a two-photon emission process from the first vibrationally-excited state of para-hydrogen molecule. Large coherence in the initial state was prepared by the adiabatic Raman population transfer method, and the lowest Stokes sideband was used as a trigger field. We observed the coherent two-photon emission consistent with the expectation of the paired super-radiance master equation.
SPAN (Spectroscopy of Atomic Neutrino) project aims to determine the absolute neutrino mass. The process we plan to use is a cooperative de-excitation of atoms in a metastable level emitting a neutrino pair associated with a photon. The photon energy spectrum of this process contains information on the absolute mass of neutrino. Key items of this experiment are a rate amplification using macro-coherence in a target medium in case of plural particles emission and an external triggering of the emission in order to scan the spectrum. We have demonstrated the rate amplification in two-photon emission from para-hydrogen gas which was coherently excited to its first vibrationally excited state. The coherence in the medium was generated by irradiating two driving laser pulses. The emission was stimulated by irradiating a mid-infrared laser pulse. The enhancement factor of more than 10 18 with respect to the spontaneous emission was achieved. This paper briefly summarizes the results.