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arXiv:1406.2198 · Progress of Theoretical and Experimental Physics

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

Y. Miyamoto, H. Hara, S. Kuma, T. Masuda, I. Nakano, C. Ohae, N. Sasao, M. Tanaka, S. Uetake, A. Yoshimi, K. Yoshimura, M. Yoshimura

Published Nov 24, 2014• 28 citations
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Abstract

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.

DOI
10.1093/ptep/ptu152

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Journal of Physics B Atomic Molecular and Optical Physics•Nov 1, 2017•3 citations
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.
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.
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 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 present a multi-color ladder excitation scheme that exploits Stark-induced adiabatic Raman passage to selectively populate a highly excited vibrational level of a molecule. We suggest that this multi-color coherent ladder excitation provides a practical way of accessing levels near the vibrational dissociation limit as well as the dissociative continuum, which would allow the generation of an entangled pair of fragments with near-zero relative kinetic energy. Specifically, we consider four- and six-photon coherent excitation of molecular hydrogen to high vibrational levels via intermediate vibrational levels, which are pairwise coupled by two-photon resonant interaction. Using a sequence of three partially overlapping, single-mode, nanosecond laser pulses we show that the sixth vibrational level of H 2 , which is too weakly coupled to be easily accessed by direct two-photon Raman excitation from the ground vibrational level, can be efficiently populated without leaving any population stranded in the intermediate level. Furthermore, we show that the fourteenth vibrational level of H 2 , which is the highest vibrational level in the ground electronic state with a binding energy of 22 meV, can be efficiently and selectively populated using a sequence of four pulses. The present technique offers the unique possibility of preparing entangled quantum states of H atoms without resorting to an ultracold system.
The solid para-hydrogen film was prepared by depositing the para-hydrogen vapor onto a sapphire substrate. From the stimulated Raman scattering experiment using two ns-lasers whose laser-frequency difference is equal to the fundamental vibrational transition of the solid para-hydrogen, we observed the collinear generation of Raman sidebands. The high-order Raman sidebands were obtained under the strong pumping intensity (≥230 MW/cm 2 ). The generated five anti-Stokes(AS)-Raman sidebands extend from ultraviolet (292 nm for AS 5 band) to visible (565 nm for AS 1 band) region. Also, we found that the first AS-Raman signal shows weak rotational components as well as a strong vibrational line. From the experiment that makes the multimode probe beam beat with the prepared Raman coherence, we found that the prepared Raman coherence replicates the probe beam to its Raman sidebands. The frequency conversion efficiency from the probe beam to the first AS-Raman sideband rises to the maximum (4.7%) at 284 MW/cm 2 , and then falls with the increase of the pumping intensity. This is thought to be due to the fact that some molecules in the first vibrational level incline to go to the higher vibrational level under the higher pumping intensity. For the strong pumping intensity (290 MW/cm 2 ), four anti-Stokes and one Stokes Raman sidebands of the probe beam were generated.
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.