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arXiv:cond-mat/0401280 · Europhysics Letters (EPL)

Observation of narrow fluorescence from doubly driven four-level atoms at room temperature

U. K Khan, J Sebastian, N Kamaraju, A Narayanan, R Srinivasan, H Ramachandran

Published Jul 1, 2004• 5 citations
arXivHTMLPDF

Abstract

Unusually narrow spectral features are seen in fluorescence peaks from 85 Rb atoms under the action of two driving laser fields that are in a three-dimensional molasses configuration. One of the lasers, L1, is held at a fixed detuning from the "cooling" transition, while the other, L2, is scanned across the "repumping" transitions. The fluorescence peaks are split into symmetric pairs, with the separation within a pair increasing with the detuning of the L1 laser. For large detunings, additional small peaks are seen. A simple model is proposed to explain these experimental observations.

DOI
10.1209/epl/i2003-10274-0

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Unusually narrow spectral features are seen in fluorescence peaks from 85 Rb atoms under the action of two driving laser fields that are in a three-dimensional molasses configuration. One of the lasers, L1, is held at a fixed detuning from the "cooling" transition, while the other, L2, is scanned across the "repumping" transitions. The fluorescence peaks are split into symmetric pairs, with the separation within a pair increasing with the detuning of the L1 laser. For large detunings, additional small peaks are seen. A simple model is proposed to explain these experimental observations.
The unusually narrow features in the fluorescence from Rubidium-85 driven by cooling and repumper laser fields, reported in an earlier experiment [1] are explained on the basis of a four-level density matrix calculation. Quantum effects alter the efficiency of atom transfer by the probe (repumper) laser to the levels connected by the pump (cooling) laser. This combined with the double resonance condition [1], results in velocity selection from co-propagating and counter propagating probe and pump beams resulting in narrow fluorescence peaks from a thermal gas at room temperature.
We describe the resonance fluorescence spectrum of an atomic three-level system where two of the states are coupled by a single monochromatic laser field. The influence of the third energy level, which interacts with the two laser-coupled states only via radiative decays, is studied in detail. For a suitable choice of parameters, this system gives rise to a very narrow structure at the laser frequency in the fluorescence spectrum which is not present in the spectrum of a two-level atom. We find those parameter ranges by a numerical analysis and use the results to derive analytical expressions for the additional narrow peak. We also derive an exact expression for the peak intensity under the assumption that a random telegraph model is applicable to the system. This model and a simple spring model are then used to describe the physical origins of the additional peak. Using these results, we explain the connection between our system, a three-level system in the V configuration where both transitions are laser driven, and a related experiment that was recently reported.
In two recent experiments Verkerk et al. and our group have observed quantization of the center-of-mass motion of laser cooled atoms in sub-wavelength sized potential wells formed by the light field of a 1-dimensional optical molasses. Our 1-dimensional optical molasses consists of a pair of counterpropagating laser beams of orthogonal, linear polarization, creating a series of potential wells separated by {lambda}/4. Polarization gradient laser cooling reduces the thermal energy of the atoms until they are trapped in well resolved vibrational levels near the bottom of the wells. Transitions between these vibrational levels are seen in a high resolution spectrum of fluorescence, where they give rise to a Rayleigh peak centered at the laser frequency, and well resolved Raman sidebands shifted by a frequency {plus_minus}{omega}, typically on the order of 100 kHz. We find quantitative agreement between the measured level separation h{omega} and the results of a calculation of the bandstructure in the periodic optical potential. Based on the observed ratio of the strength of the blue to the red sideband we determine the temperature of the atoms, and find agreement with theoretical predictions. Based on the level spacing and temperature we find the atoms to be localized to {approx}/15, withmore » up to 50-60% of the population of the trapped atoms in the vibrational ground state. We observe intriguing differences between fluorescence radiated by localized atoms, and the fluorescence radiated by non-localized atoms in a 1-dimensional optical molasses composed of a pair of counterpropagating laser beams of opposite circular polarization. The prospect of observing quantum motion and spatial order in 2 and 3 dimensions, and the consequences for the spectrum of fluorescence, will be discussed.« less
We have measured the emission spectrum of two-level-like Ba atoms driven by a continuous-wave bichromatic field containing a strong resonant component and a weaker component detuned from atomic resonance by the strong-field Rabi frequency. With the specified detuning, the weak field resonantly drives a transition of the atom--strong-field dressed states. Observed spectra show that each peak of the normal (single-driving-field) resonance fluorescence triplet is split into three subpeaks separated by one-half the weak-field Rabi frequency. Also seen is another triplet of peaks displaced from the atomic resonance by twice the strong-field Rabi frequency. Splitting of the normal triplet peaks can be explained through weak-field dressing of the strong-field dressed states. The origin of the additional triplet is less transparent. Comparison with theory is made.
We report observation of anomalous fluorescence spectral features in the environs of a two-photon transition in a rubidium vapor when excited with two different wavelength lasers that are both counterpropagating through the vapor. These features are characterized by an unusual trade-off between the detunings of the driving fields. Three different hypothetical processes are presented to explain the observed spectra: a simultaneous three-atom and four-photon collision, a four-photon excitation involving a light field produced via amplified spontaneous emission, and population pumping perturbing the expected steady-state spectra. Numerical modeling of each hypothetical process is presented, supporting the population pumping process as the most plausible mechanism.