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Physical Review A

Observation of gain due to coherence effects in a potassium-helium mixture

Jeffrey A. Kleinfeld, Alan D. Streater

Published Jun 1, 1994• 52 citations
Publisher

Abstract

We report steady-state gain due to coherence effects in a potassium-helium mixture, for strong pumping near the ${\mathit{D}}_{2}$ line and probing near the ${\mathit{D}}_{1}$ line. The absorption-gain spectra are in good agreement with a four-level Raman-driven, lasing with inversion model, where collisional transfer populates the probed excited state. Using parameters corresponding to the experimental conditions, the model predicts gain without bare-state population inversion by a large margin.

DOI
10.1103/physreva.49.r4301

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We describe theoretical and experimental investigations of steady-state gain without population inversion that can occur on the ${\mathit{D}}_{1}$ transition of potassium, upon strong cw-laser excitation of the ${\mathit{D}}_{2}$ transition. A Raman-driven four-level model, in which a noninverted population in the 4${\mathrm{}}^{2}$${\mathit{P}}_{1/2}$ level is established via collisional transfer from the 4${\mathrm{}}^{2}$${\mathit{P}}_{3/2}$ level, is in good qualitative agreement with a variety of experimental results. These results include, among other studies, an examination of the impact of coherence dephasing on the realizable gain, as well as measurements of atomic energy-level populations for a case of line-center pumping that clearly displays gain. The measured relative populations for this case are n(4${\mathit{P}}_{1/2}$)/${\mathit{n}}_{\mathrm{total}}$=0.098\ifmmode\pm\else\textpm\fi{}0.034 and n(4${\mathit{P}}_{3/2}$)/${\mathit{n}}_{\mathrm{total}}$=0.19\ifmmode\pm\else\textpm\fi{}0.08, where ${\mathit{n}}_{\mathrm{total}}$ is the total population. These measurements indicate that the probed level 4${\mathit{P}}_{1/2}$ population is not inverted with respect to the ground state. \textcopyright{} 1996 The American Physical Society.

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Atomic coherence gives rise to a series of interesting phenomena in quantum optics.1 In this work we investigate coherence effects within the Na Dl line, demonstrating coherent trapping and lasing without inversion, and study the dependence of these phenomena on buffer gas pressure. Two pump laser beams (I, and I2) are tuned to the frequencies υ1 and υ2 that correspond to the 3S1/2 (F=1, 2) → 3P1/2 transition and are right circularly polarized. Pockels cells are used to turn on or/and off the beams within approximately one nanosecond. To demonstrate lasing without inversion, a weak pulse of left polarized light tuned to t)2 was used to create a small population in the upper level (3P1/2) and the amplification of I, was observed. We found a significant gain in the absence of population inversion, which was verified by a direct experimental observation. These results were observed as a function of buffer gas pressure. The atomic coherence effects are shown to disappear in the presence of a helium buffer gas at a pressure of ≈ 1 Torr; whereas, complete optical pumping is only achieved at helium buffer gas pressures >10 Torr.

Annual Meeting Optical Society of America•Jan 1, 1993•0 citations
We report steady-state gain due to coherence effects in a potassium-helium mixture, for strong pumping near the D2{\mathit{D}}_{2}D2​ line and probing near the D1{\mathit{D}}_{1}D1​ line. The absorption-gain spectra are in good agreement with a four-level Raman-driven, lasing with inversion model, where collisional transfer populates the probed excited state. Using parameters corresponding to the experimental conditions, the model predicts gain without bare-state population inversion by a large margin.
We describe theoretical and experimental investigations of steady-state gain without population inversion that can occur on the D1{\mathit{D}}_{1}D1​ transition of potassium, upon strong cw-laser excitation of the D2{\mathit{D}}_{2}D2​ transition. A Raman-driven four-level model, in which a noninverted population in the 42{\mathrm{}}^{2}2P1/2{\mathit{P}}_{1/2}P1/2​ level is established via collisional transfer from the 42{\mathrm{}}^{2}2P3/2{\mathit{P}}_{3/2}P3/2​ level, is in good qualitative agreement with a variety of experimental results. These results include, among other studies, an examination of the impact of coherence dephasing on the realizable gain, as well as measurements of atomic energy-level populations for a case of line-center pumping that clearly displays gain. The measured relative populations for this case are n(4P1/2{\mathit{P}}_{1/2}P1/2​)/ntotal{\mathit{n}}_{\mathrm{total}}ntotal​=0.098\ifmmode\pm\else\textpm\fi{}0.034 and n(4P3/2{\mathit{P}}_{3/2}P3/2​)/ntotal{\mathit{n}}_{\mathrm{total}}ntotal​=0.19\ifmmode\pm\else\textpm\fi{}0.08, where ntotal{\mathit{n}}_{\mathrm{total}}ntotal​ is the total population. These measurements indicate that the probed level 4P1/2{\mathit{P}}_{1/2}P1/2​ population is not inverted with respect to the ground state. \textcopyright{} 1996 The American Physical Society.
We consider a four-level atomic system with a degenerate or quasidegenerate ground-state doublet and two excited states. One of the upper levels is coupled to the ground doublet by a coherent field of variable strength, which we refer to as the Raman field, whose role is to create atomic coherence between the driven atomic states. The other higher lying state is coupled to the ground states when laser action develops. This model includes an incoherent pump mechanism to create some population in the top lasing state. In appropriate conditions, for the incoherent pump rate and the Raman field strength, the absorption profile of the laser transition develops gain features. We discuss the physical origin of this gain which we find to be the consequence of a population inversion between dressed atomic states. It is interesting to compare the results of this analysis with a similar study of a three-level lambda system which was previously recognized to yield lasing without inversion.1 In this case the appearance of gain can be attributed to the induced atomic coherence.
We demonstrate amplification without inversion in the 87Rb D1-line pumped by an incoherent light. A closed N-type, four-level atomic system has been considered to obtain an insight into the origin of the gain as well as the conditions under which the system exhibits gain without population inversion in bare states. We investigate the gain as a function of the incoherent pumping rate, the coupling laser detuning, and Rabi frequency. The experimental results are in good agreement with the numerical calculations considering the velocity distribution of atoms.
For a four-level atomic system with a doublet of closely spaced levels, we find that, owing to the coherence that results from the vacuum of the radiation field, population trapping at excited levels and probe gain with population inversion can be achieved with weak incoherent pumping. This gain is different from both the conventional lasing gain and gain without inversion in that there exists population inversion on probe transitions but the inversion is achieved by the vacuum-induced coherence.
Atomic coherence gives rise to a series of interesting phenomena in quantum optics.1 In this work we investigate coherence effects within the Na Dl line, demonstrating coherent trapping and lasing without inversion, and study the dependence of these phenomena on buffer gas pressure. Two pump laser beams (I, and I2) are tuned to the frequencies υ1 and υ2 that correspond to the 3S1/2 (F=1, 2) → 3P1/2 transition and are right circularly polarized. Pockels cells are used to turn on or/and off the beams within approximately one nanosecond. To demonstrate lasing without inversion, a weak pulse of left polarized light tuned to t)2 was used to create a small population in the upper level (3P1/2) and the amplification of I, was observed. We found a significant gain in the absence of population inversion, which was verified by a direct experimental observation. These results were observed as a function of buffer gas pressure. The atomic coherence effects are shown to disappear in the presence of a helium buffer gas at a pressure of ≈ 1 Torr; whereas, complete optical pumping is only achieved at helium buffer gas pressures >10 Torr.