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Journal of the Physical Society of Japan

Production of Ba Metastable State via Superradiance

Chiaki Ohae, Atsushi Fukumi, Susumu Kuma, Yuki Miyamoto, Kyo Nakajima, I. Nakano, Hajime Nanjo, N. Sasao, Satoshi Uetake, Tomonari Wakabayashi, A. Yoshimi, K. Yoshimura, M. Yoshimura

Published Mar 13, 2014• 4 citations
Publisher

Abstract

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.

DOI
10.7566/jpsj.83.044301

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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.
We develop for dipole-forbidden transition a dynamical theory of two-photon paired superradiance (PSR). This is a cooperative process characterized by two photons emitted back to back with equal energies. By irradiating the trigger laser from two target ends, with its frequency tuned at the half energy between two levels, a macroscopically coherent state of medium and fields dynamically emerges as time evolves, and a large signal of amplified output occurs with a time delay. The basic semiclassical equations in 1+11+11+1 space-time dimensions are derived for the field-plus-medium system to describe the space-time evolution of the entire system and are numerically solved to demonstrate the existence of both explosive and weak PSR phenomena in the presence of relaxation terms. The explosive PSR event terminates accompanying a sudden release of most of the energy stored in the target. Our numerical simulations are performed using the vibrational transition ${X}^{1}{\ensuremath{\Sigma}}_{g}^{+}v=1\ensuremath{\rightarrow}0$ of a para-H2{}_{2}2​ molecule and taking many different excited atom number densities and different initial coherences between the metastable and the ground states. In an example with a number density close to O(1021O({10}^{21}O(1021 cm${}^{\ensuremath{-}3}$) and a high initial coherence, the explosive event terminates several nanoseconds after the trigger irradiation, when the phase relaxation time larger than OOO(10 ns) is taken. After PSR events the system is expected to follow a steady-state solution which is obtained by analytic means and is made of many objects of field condensates endowed with a topological stability.
The lowest optically metastable excited state of barium, Ba(5d 3DJ), was populated by laser excitation of the 6p 1P01←6s 1S0 (λ=553.5 nm) transition followed by the spontaneous emission 6p 1P01→5d 3DJ (λ∼1130 nm). No evidence for the normally spin allowed transition 6p 1P01 →5d 1D2 (λ=1500 nm) was obtained. The deactivation of Ba (5d3DJ) by Ba (6s 1S0) is extremely efficient proceeding with a thermal cross section of 1.3±0.1 nm2 at ?730 K. This is in sensible accord with recent calculations of Ba2 potential curves.
This is the first reported demonstration of the use of pair-absorption transitions for optically pumping lasers. In a mixture of barium and thallium metal vapors, single-photon absorption of 3867-A laser light caused simultaneous excitation of colliding ground-state atoms to the Ba(6s6p 1P1(o)) and Tl(6p 2P(3/2)(o)) excited states. Excited-state densities of about 10(14) cm(-3) were created, and subsequent laser emission on the Ba(6s6p 1PI(o)) - Ba(6s5d 1D2) atomic transition at 1.5 microm was observed.
We find that intense short pulses of XUV radiation can be produced by cooperative spontaneous emission from visible or IR laser pulses driving atoms or ions. The process depends on the generation and utilization of atomic coherence as is the case in lasing without inversion. However, the radiation process is not stimulated emission, but is rather cooperative spontaneous emission in the sense of Dicke. More precisely, the many atom mathematics of superradiance has much in common with coherent anti-Stokes Raman scattering. The generation of Raman coherence is considered from the perspectives of multi-photon excitation and breaking of adiabaticity. In particular, we find that the advent of an ultrashort (e.g. single cycle) pulse rewards rethinking of conventional (nanosecond) nonlinear optics with new insights.
The problem of light propagation of frequency corresponding to half of the energy difference between a metastable excited state and the ground state of atoms is examined, and solved for coherent medium by analytic means. We demonstrate that the non-linear system of Maxwell-Bloch equation for the effective model of the Λ -type three levels is integrable in the mathematical sense. Analytic solutions thus obtained describe pulse splitting accompanied by compression, indicating a kind of non-linear instability of propagating pulses. The instability is eventually terminated by coherent two photon emission (called paired superradiance or PSR in short). These results are displayed by numerical outputs for visual understanding, as well. It is further shown that the integrable system allows a new class of soliton solutions. Solitons, implying the phenomenon of self-induced transparancy at non-resonant frequencies, are stable against PSR. One of our goals of the present work is construction of a calculable theoretical framework for PSR rates associated with a trigger pulse propagation, which is achieved by combining analytic results with perturbative methods. PSR photon spectrum and its rate ∝(target number density) 2 , along with their time structure, are clarified this way. These results may open a new path for interesting technological applications such as quantum entanglement and for solving the remaining problems of the still mysterious neutrino. Some basic strategy for realistic experiments of PSR detection and soliton production is also outlined.