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arXiv:physics/0505051 · Physical Review A

Coherent population trapping in a crystalline solid at room temperature

Roman L. Kolesov

Published Nov 1, 2005• 26 citations
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Abstract

Observation of coherent population trapping (CPT) at ground-state Zeeman sublevels of ${\mathrm{Cr}}^{3+}$ ion in ruby at room temperature is reported. A mechanism of CPT, not owing to optical pumping, is revealed in a situation when the optical pulse duration is shorter than the population decay time from the excited optical state.

DOI
10.1103/physreva.72.051801

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Coherent population trapping (CPT) in a three-level atomic medium pumped by two subsequent short optical pulses is considered under the condition of negligible population decay from the excited optical state. It is shown that the amount of atomic population transferred to the excited state by the combined action of the pulses strongly depends on the phase of the ground-state coherence excited by the first pulse at the arrival time of the second pulse. Oscillatory behavior of optical excitation efficiency on the time delay between the pulses is predicted. It is also shown that saturating optical pulses can produce population inversion in a resonantly pumped quasi-two-level system. A class of solid materials in which the predicted phenomena can be observed at room temperature is found. It includes some rare-earth and transition-metal doped dielectric crystals where Orbach relaxation between ground-state Zeeman states is suppressed: ruby, alexandrite, and several others. On the basis of the theoretical predictions, experimental observation of Ramsey fringes in CPT spectrum of ruby is reported.

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Observation of coherent population trapping (CPT) at ground-state Zeeman sublevels of Cr3+{\mathrm{Cr}}^{3+}Cr3+ ion in ruby at room temperature is reported. A mechanism of CPT, not owing to optical pumping, is revealed in a situation when the optical pulse duration is shorter than the population decay time from the excited optical state.
Coherent population trapping (CPT) in a three-level atomic medium pumped by two subsequent short optical pulses is considered under the condition of negligible population decay from the excited optical state. It is shown that the amount of atomic population transferred to the excited state by the combined action of the pulses strongly depends on the phase of the ground-state coherence excited by the first pulse at the arrival time of the second pulse. Oscillatory behavior of optical excitation efficiency on the time delay between the pulses is predicted. It is also shown that saturating optical pulses can produce population inversion in a resonantly pumped quasi-two-level system. A class of solid materials in which the predicted phenomena can be observed at room temperature is found. It includes some rare-earth and transition-metal doped dielectric crystals where Orbach relaxation between ground-state Zeeman states is suppressed: ruby, alexandrite, and several others. On the basis of the theoretical predictions, experimental observation of Ramsey fringes in CPT spectrum of ruby is reported.
We report our study on coherent population trapping (CPT) in a $\ensuremath{\Lambda}$ configuration coupled by magnetic dipole interactions. The $\ensuremath{\Lambda}$ configuration is formed by bichromatic radio-frequency fields coupling a pair of Zeeman sublevels in one hyperfine level of an alkali-metal atom to another sublevel in the other hyperfine level. The Zeeman sublevel at the apex is coupled to a state in the nPnPnP manifold via an optical transition on one of the spectroscopic DDD lines to induce optical pumping to a dark state. The configuration is closed without leakage to other Zeeman sublevels. We use lithium atoms in an optical trap for our experimental study. The system allows independent control of the main parameters characterizing CPT, which include the upper-state decay rate and the decoherence rate as well as the Rabi frequencies. By turning off the applied fields, the system is frozen so that its quantum state can be measured precisely. We studied the line shapes and dynamics of the CPT system, and measured the phase relation of the dark superposition state to find excellent agreement with theory. The possible application of the scheme as a method to cool optically trapped atoms below the recoil limit in a manner analogous to velocity-selective coherent population trapping is discussed.
The excitation of room-temperature four-level 87Rb^{87}\mathrm{Rb}87Rb atoms by a train of ultrashort pulses is investigated theoretically in the conditions when the pulse repetition period is shorter than the characteristic atomic relaxation times. It is shown that coherent accumulation of excitation leads to coherent population trapping and electromagnetically induced transparency of the excitation pulses when the pulse repetition rate is a subharmonic of the ground-state hyperfine splitting. It is illustrated how the judicious choice for the frequency comb parameters can provide a means to effectively control the degree of coherence between the ground-state hyperfine levels for selective atomic groups, and even transfer the whole atomic distribution to the dark state with up to 9595%95 efficiency.
Population trapping in the ground state spin sublevels of Cr 3+ in ruby at liquid helium temperature is considered. It is proposed to excite the R 1 -transition by a bichromatic laser beam with an optical frequency difference matching the ground state spin sublevels splitting. If the excitation is strong enough to decouple Cr 3+ from its environment, responsible for dephasing, an appreciable amount of the population is trapped in a particular superposition of the spin sublevels. This superposition state does not interact with the laser beam because of the destructive interference of the quantum paths constituting the transition. Population trapping can be detected by the reduced luminescence from the excited state and by the magnetization ringing originating from the ground state spin coherence.
Coherent population trapping is studied in 87Rb{}^{87}\mathrm{Rb}87Rb in a buffer gas. Experimental results are reported on the contrast of the field independent hyperfine resonance line and on its linewidth. A physical model, which includes an optical pumping process that traps atoms in a Zeeman sublevel of the ground state, is developed. The model explains the general behavior of the experimental observations.