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Optics Letters

Continuous spatial photon echoes

C. Schnurr, K. D. Stokes, George R. Welch, John E. Thomas

Published Oct 1, 1990• 11 citations
Publisher

Abstract

We report the observation of continuous spatial photon echoes in a magnetically compensated supersonic beam of atomic samarium. Atoms cross two spatially separated cw laser pump fields to produce an intense cw echo downstream. In contrast to previous studies of continuous coherent radiation in separated fields, the signals are readily observable with the unaided eye. The echo signal, which is generated in a magnetic-field gradient, differs from usual rephasing phenomena owing to a quadratic time dependence for the optical phase in the frame of the moving atoms.

DOI
10.1364/ol.15.001097

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Journal of the Optical Society of America B•Nov 1, 1985•100 citations
We report the observation of continuous spatial photon echoes in a magnetically compensated supersonic beam of atomic samarium. Atoms cross two spatially separated cw laser pump fields to produce an intense cw echo downstream. In contrast to previous studies of continuous coherent radiation in separated fields, the signals are readily observable with the unaided eye. The echo signal, which is generated in a magnetic-field gradient, differs from usual rephasing phenomena owing to a quadratic time dependence for the optical phase in the frame of the moving atoms.
We apply for the first time a novel coherent transient, the cw photon echo. The propagation direction of the optical coherence prepared in a medium by a broadband cw laser beam is reversed by a pair of counterpropagating laser pulses, resulting in a continuous emission of the signal. The signal decays with a phase relaxation time T2{\mathrm{T}}_{2}T2​ and can have a quantum-beat-type modulation, giving the same wave form in one single shot as the plot of echo intensity versus pulse separation in a standard two-pulse echo experiment. Experimental demonstration is given in a Eu3+{\mathrm{Eu}}^{3+}Eu3+:YAlO3{\mathrm{YAlO}}_{3}YAlO3​ crystal.
We have used two sets of orthogonally polarized traveling-wave pulses separated in time to establish and, subsequently, rephase a spatially periodic coherence grating between the magnetic sublevels of the F=3F=3F=3 ground state in a cloud of laser-cooled 85Rb{}^{85}\mathrm{Rb}85Rb atoms. The rephasing results in a magnetic grating echo (MGE). We have used the shape of the signals to determine both transverse and longitudinal velocity distributions and to study the effects of magnetic fields. The amplitude of the MGE with counterpropagating pulses is modulated at the atomic recoil frequency, a consequence of matter wave interference.
A novel coherent transient spectroscopy, cw photon echo, is described. A medium is exposed to broadband cw laser radiation and then to a pulsed laser, resulting in a continuous emission signal from the medium. The temporal profile of this signal is studied using full density-matrix theory for the cases of a two-level system and a multilevel system. It is shown that such signal decays with a homogeneous dephasing time T2{\mathit{T}}_{2}T2​ and can have a quantum-beat-type modulation, reproducing in a single shot the conventional two-pulse photon echo plot versus first-to-second pulse separation. Experimental demonstrations are given for the 7^{7}7F0{\mathit{F}}_{0}F0​${\mathrm{\ensuremath{-}}}^{5}$D0{\mathit{D}}_{0}D0​ transition of Eu3+{\mathrm{Eu}}^{3+}Eu3+ and the 3^{3}3H4{\mathit{H}}_{4}H4​${\mathrm{\ensuremath{-}}}^{1}$D2{\mathit{D}}_{2}D2​ transition of Pr3+{\mathrm{Pr}}^{3+}Pr3+; both ions are doped in YAlO3{\mathrm{YAlO}}_{3}YAlO3​. The Eu ions exhibit a single exponential decay with a slight modulation due to the ground-state hyperfine splittings. The temperature dependence of the signal is also studied. The Pr ions show a very deep modulation pattern, which provides excellent agreement with the independent two pulse echo measurement. A numerical simulation based on the modulation theory also agrees with the observed decay curves and the nuclear axis configuration in this crystal is discussed.
We report observations of the rotation of the plane of polarization of photon echoes by a weak axial magnetic field. The echoes are generated on the 6S126{S}_{\frac{1}{2}}6S21​​ to 7P127{P}_{\frac{1}{2}}7P21​​ transition in cesium (4593 \AA{}) by a sequence of two collinear laser pulses polarized in the same direction. When a magnetic field is applied along the propagation axis the polarization vector of the echo is rotated from that of the exciting pulses. We measure the intensity of the photon echo signal polarized orthogonal to the excitation pulse polarization as a function of magnetic field and pulse separation. The theoretical analysis decribes cesium atoms as a nondegenerate multilevel system, consisting of ground and excited Zeeman-split hyperfine-structure levels. The theory presented predicts the observed dependence on magnetic field strength, pulse separation, and excited- and ground-state ggg factors. A novel method of observing photon echoes is described which uses the polarization rotation effect to replace electro-optical shutters often employed in photon echo experiments.
We consider the interference of atoms caused by the wave nature of their motion. Coherently interfering beams of particles may be obtained by using the diffraction of atoms from separated standing waves resonant with the transition between the ground and excited states of the atoms. Interference arises with and without spatial separation of the diffracted atomic beams. A problem concerning two-quantum perturbation of the wave function of an atom in the ground state by a standing-wave field has been solved. Both amplitude and phase perturbation are taken into account. We show that in spite of the absence of coherence in the incident beam and a rapid decay of the coherence induced by the standing wave for scattered spatially overlapping beams, the interference can be observed for the conditions of an echo. We first consider the phenomenon of an echo under the quantum-mechanical action of light on the translational degrees of freedom. We show that owing to this phenomenon phase memory can be transferred through the ground state over unlimitedly large distances. Owing to the interference of the density harmonics, a periodic structure in the spatial distribution of the atomic density with a period of less than one wavelength of the light is localized at distances of the order of the distance between the fields. The harmonic amplitude is calculated for instantaneous excitation in the case in which the duration of the excitation is comparable with the backward Doppler linewidth in the beam and for scattering under Bragg conditions. In the latter case we show that the effective temperature of the atoms participating in the interference is less than one recoil energy.