A 10 ps transform limited optical pump pulse two-photon-resonant with the 6 S1/2-6 D32 442 nm transition in Cs vapor generates a coherent state superposition which, after a delay exceeding the pulse width, gives rise to second harmonic emission (when a transverse magnetic field is present) and superfluorescence (SF) directed against the pump on the upper 6 D3/2-6 P1/2 transition, followed by cooperative SF along the pump simultaneously on both the upper and lower 6 P3/2 − 6 P1/2 transitions, followed in turn by SF against the pump on the lower transition. Phase locked quantum beats associated with the 1.17 GHz hyperfine splitting of the 6 P1/2 appear on all SF emissions.
We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the
5D5/2 level through the
6P3/2 level to the
5S1/2 ground level. The atomic system is initially excited to the
5D5/2 level from the ground state via two-photon excitation through the intermediate
5P3/2 level. The fluorescence on the $6P\text{\ensuremath{-}}5S$ transition at 420 nm is recorded using time-resolved measurements. The time delays of the observed SF emission peaks typically scale as $\ensuremath{\sim}{N}^{\ensuremath{-}1}$, where
N is the atom number, and are much smaller than the time delay expected for uncorrelated cascade fluorescence. Since
N is significantly smaller than the threshold number for SF on the 420 nm transition, and larger than the threshold number for the $5D\text{\ensuremath{-}}6P$ transition at $5.2\text{ }\ensuremath{\mu}\text{m}$, our observations suggest that the 420 nm SF emission is triggered by rapid deexcitation of the
5D to the
6P level via SF at $5.2\text{ }\ensuremath{\mu}\text{m}$. The observed SF time delays for 420 nm emission agree with SF time-delay estimates for the $5.2\text{ }\ensuremath{\mu}\text{m}$ transition. For spherical clouds, the SF is isotropic. For cigar-shaped clouds, the SF is highly anisotropic. Along the long axis of cigar-shaped atom clouds, SF and incoherent cascade fluorescence produce temporally well-resolved peaks in the detected signal. In this case, the SF component of the signal is highly concentrated along a direction in between the directions of the two almost parallel excitation beams. The observed SF intensities scale as
N, suggesting that the
5D level is regeneratively pumped during the SF decay.
A dense sample of atomic rubidium vapor excited by a short (4 ps) transform-limited laser pulse two-photon resonant with the 5S-5D transition in a large Fresnel number geometry emits a delayed omnidirectional infrared superfluorescence pulse (coherent flash) on the 5D-6P transition. This superfluorescence emission involves a coherent population transfer to the 6P level and results in prompt directional yoked UV emission and coherent population transfer back to the ground 5S state. This emission completes the process of parametric four-wave mixing, selectively involving only those atoms on the 6P level whose 5D-6P emission satisfies the phase-matching condition. The unique feature of this four-wave mixing radiation is that it occurs after the termination of the pump pulse, as evidenced by direct measurement of emission timing. The multidirectional character of superfluorescent emission is confirmed by the observation of non-collinear parametric four-wave mixing when exciting the sample by two simultaneous overlapping pulses, for a wide range of beam angles. Also observed and investigated is superfluorescent transient diffraction that appears when the sample is excited by a sequence of three laser pulses.
Summary form only given.We have observed yoked superfluorescence (YSF) on the 5D-6P-5S channel in atomic rubidium after two-photon excitation of the 5D-5S transition by a short (3 ps), 778-nm pulse from a Ti:sapphire laser system. The excitation pulse is formed by a pair of spatially overlapping pulses of wave vectors k/sub 1/ and k/sub 2/ where |k/sub 1/|=|k/sub 2/|=2/spl pi///spl lambda/ and /spl lambda/ is the wavelength of the 5D-5S transition. When the atoms are excited by taking one photon from along k/sub 1/ and one from along k/sub 2/, then phase matching requires that the yoked transition consists of photon pairs along k/sub DP/ and k/sub PS/ where k/sub 1/+k/sub 2/=k/sub DP/+k/sub PS/ and k/sub DP/ and k/sub PS/ are the wave vectors of, respectively, the upper (5D-6P) and the lower (6P-5S) transition components of the YSF. The YSF pulses are emitted in a cone determined by Brownell et al. (1995). This conical emission of YSF was observed on the 420 nm lower transition. The measured behavior of the conical apex angle /spl beta/ as a function of the angle between the excitation pulses /spl alpha/ is given as a series of points and shows good agreement with the solid line based on Brownell et al.
Two closely spaced 778 nm, 4-ps pulses, two-photon resonant with the 5S-5D transition in Rb vapor generate a ground state grating, which later is excited by a similar third pulse, producing conical yoked superfluorescence echo on the 5S-6P, 420-nm transition. The intensity of this emission as a function of the relative delay between the leading excitation pulses is governed by the dynamics of Doppler dephasing and rephasing. This is the first observation of an echo effect induced by a spontaneous relaxation process.