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Journal of the Optical Society of America B

Collision-induced superfluorescence

Anantharaman Kumarakrishnan, S. Chudasama, Xianming L. Han

Published Jul 1, 2005• 14 citations

Abstract

We have studied superfluorescence (SF) in Ca vapor evolving on the 3d4s ^3 D_J -4s4p ^3 P_J-1 transitions at 1.9 µm by exciting the 4s^21 S_0 -4s4p ^1 P_1 with a pulsed dye laser. SF is generated following population transfer by spin-changing collisions with an inert gas Ar from the 4s4p ^1 P_1 and 3d4s ^1 D_2 levels. We show for the first time to our knowledge that the time delay for SF evolution follows the 1/sqrt N dependence expected for the case of uniform excitation of the vapor column by collisional transfer. Here, N is the number of participating atoms that was measured directly from the photon yield. The measured photon yield for the signal as a function of Ar pressure was found to be consistent with rate equations that simulate the buildup of populations in the ^3 D_J levels based on known collisional rates. This suggests that collisional rates can be directly inferred on the basis of SF photon yields and the atomic level populations. The pulse shapes for SF show temporal oscillations that depend on two distinct factors. The first is the presence of a number of independently evolving regions in the gain medium, and the second is the presence of spatial modes. Temporal ringing is a well-known effect related to the exchange of energy between the atoms and the radiation field during pulse propagation. However, the temporal ringing observed in this experiment is far more pronounced than in previous SF experiments due to a particular choice of evolution parameters. This should make it feasible to compare our results with detailed numerical simulations that have been carried out previously.

DOI: 10.1364/josab.22.001538

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We have studied superfluorescence (SF) in Ca vapor evolving on the 3d4s ^3 D_J -4s4p ^3 P_J-1 transitions at 1.9 µm by exciting the 4s^21 S_0 -4s4p ^1 P_1 with a pulsed dye laser. SF is generated following population transfer by spin-changing collisions with an inert gas Ar from the 4s4p ^1 P_1 and 3d4s ^1 D_2 levels. We show for the first time to our knowledge that the time delay for SF evolution follows the 1/sqrt N dependence expected for the case of uniform excitation of the vapor column by collisional transfer. Here, N is the number of participating atoms that was measured directly from the photon yield. The measured photon yield for the signal as a function of Ar pressure was found to be consistent with rate equations that simulate the buildup of populations in the ^3 D_J levels based on known collisional rates. This suggests that collisional rates can be directly inferred on the basis of SF photon yields and the atomic level populations. The pulse shapes for SF show temporal oscillations that depend on two distinct factors. The first is the presence of a number of independently evolving regions in the gain medium, and the second is the presence of spatial modes. Temporal ringing is a well-known effect related to the exchange of energy between the atoms and the radiation field during pulse propagation. However, the temporal ringing observed in this experiment is far more pronounced than in previous SF experiments due to a particular choice of evolution parameters. This should make it feasible to compare our results with detailed numerical simulations that have been carried out previously.

We have studied superfluorescence (SF) under highly unfavorable conditions of rapid collisional and radiative distribution in a Doppler-broadened medium. Nanosecond SF pulses at 5.5 \ensuremath{\mu}m were generated on the Ca

4s4p{}^{1}{P}_{1}--3d4s{}^{1}{D}_{2}

transition from a column of calcium vapor buffered with Ar by optically pumping the

{4s}^{2}{}^{1}{S}_{0}--4s4p{}^{1}{P}_{1}

transition. The Rabi frequency associated with the intense pump pulse prevents the occurrence of SF while the pump laser is on. As a result, the predicted scaling laws that describe the properties of SF in a transversely excited system, such as peak heights, pulse widths, and delay times, are shown to apply in our situation in which the conditions resemble swept excitation. The delay times were found to be in agreement with a fully quantum mechanical calculation which describes the initiation of SF. Measurements of the densities of the three levels, the absolute SF photon yield, and the spatial distribution of the excited states indicate that the system has a quantum yield of unity. The SF intensity increases with an increase in Ar pressure due to collisional redistribution until the collisional dephasing rate inhibits SF. The conditions describing the transition of SF to amplified spontaneous emission allow us to measure the collisional broadening rate for the SF transition.

We have studied effects of magnetic sublevel degeneracy on the polarization of superfluorescent pulses generated on the Ca

4s4p{}^{1}{P}_{1}--3d4s{}^{1}{D}_{2}

transition at $5.5\ensuremath{\mu}\mathrm{m}.$ These pulses were generated from a cell of length 50 cm by optically pumping calcium vapor on the

{4s}^{2}{}^{1}{S}_{0}--4s4p{}^{1}{P}_{1}

transition in the presence of Ar gas. The axis of ellipticity of superfluorescence (SF) polarization is oriented parallel to the axis of the pump-laser polarization at large detunings, and undergoes an abrupt rotation through $90\ifmmode^\circ\else\textdegree\fi{}$ for detunings close to resonance. The distribution of populations in the magnetic sublevels of the

{}^{1}{P}_{1}

state can be estimated using a simple model based on previously calculated cross sections for collisionally aided absorption in the presence of an intense (pump) field. For large detunings, these estimates are consistent with the polarized SF intensity measured in the experiment. A direct measurement of the populations of the

{}^{1}{P}_{1}

magnetic sublevels also supports the collisional redistribution predicted by the calculated cross sections. We therefore suggest that SF polarization can be a useful signature of collisional redistribution. However, the change in ellipticity is unexpected, and probable causes for this effect are discussed.

We have studied Superfluorescence (SF) under highly unfavorable conditions of rapid collisional and radiative distribution in a Doppler broadened medium. Nanosecond SF pulses at 5.5 {mu} were generated from a 50 cm column of calcium vapor buffered with argon by optically pumping the Ca (4s{sup 2} {sup 1}S{sub 0} - 4s4p{sup 1}P{sub 1} - 3d4s{sup 1}D{sub 2}) Raman Amplifier scheme. Power broadening due to an intense 13 nsec pump pulse precludes the evolution of SF for essentially this duration. As a result, the scaling laws which govern the properties of SF in a transversely excited system are shown to apply in our situation in which the conditions resemble swept excitation.

We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the

5D

state from the

5S

state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the

6P

state and then to the

5S

state. The SF emission at 420 nm on the

6P

--

5S

transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.

We study superfluorescence (SF) from spherical and cigar-shaped clouds of laser-cooled Rubidium atoms from the

5{D}_{5/2}

level through the

6{P}_{3/2}

level to the

5{S}_{1/2}

ground level. The atomic system is initially excited to the

5{D}_{5/2}

level from the ground state via two-photon excitation through the intermediate

5{P}_{3/2}

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

5{S}_{1/2}

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 laser pulse two-photon resonant with the ${6S}_{1/2}\ensuremath{-}{6D}_{3/2}$ 442 nm transition in Cs vapor generates delayed superfluorescence (SF) on the upper ${6D}_{3/2}\ensuremath{-}{6P}_{1/2}$ 876 nm and lower ${6P}_{1/2}\ensuremath{-}{6S}_{1/2}$ 894 nm transitions. Yoked SF is observed in the forward (along the laser) direction as evidenced by the simultaneous appearance of SF on both transitions. Cascade SF is observed in the backward direction, where SF on the lower transition only evolves after it has terminated on the upper. The four SF signals exhibit quantum beats associated with the

{6P}_{1/2}

hyperfine splitting.