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Journal of the Optical Society of America B

Conical emission by 2-ps excitation of potassium vapor

David G. Sarkisyan, B. D. Paul, Steven T. Cundiff, Emily A. Gibson, Alan C. Gallagher

Published Feb 1, 2001• 11 citations

Abstract

Conical emission, redshifted from the atomic resonance, is observed when a ∼2-ps laser beam, blueshifted from resonance, propagates through potassium vapor. In contrast, no cone is visible when a 150-fs pulse is used. The 2-ps-pulse-induced cone has a unique, small, angular spread of Δθ/θ≈0.1, compared with Δθ/θ≈0.3 from other pulsed experiments, which used 25-ps–5-ns-excitation pulses. In addition, the fraction of the 2-ps-pulse power emitted in the cone reached 7.5%, far exceeding this fraction from the longer-pulse experiments. An additional, unexpected result is that the spectral width of the laser pulse is narrower, without significant intensity loss, after propagation through the vapor. The vapor densities and laser detunings that yield cones in the present experiment are similar to those from the longer-pulse experiments. Although the 2-ps-pulse spectrum partially overlaps the resonance, the cone spectrum and cone angles versus density and laser detuning are similar to those previously reported for nanosecond pulses. The fourth (blue-detuned) wave required for satisfying four-wave mixing is absent, again equivalent to what is observed in the nanosecond experiments. The physical origin of cone emission by pulsed excitation of atomic vapors is still largely unknown, and this high efficiency and narrow angular spread of a spectrally broad cone further contradicts existing models, perhaps ultimately providing a clue to a correct explanation.

DOI: 10.1364/josab.18.000218

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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)•Jan 20, 2003•0 citations

Conical emission, redshifted from the atomic resonance, is observed when a ∼2-ps laser beam, blueshifted from resonance, propagates through potassium vapor. In contrast, no cone is visible when a 150-fs pulse is used. The 2-ps-pulse-induced cone has a unique, small, angular spread of Δθ/θ≈0.1, compared with Δθ/θ≈0.3 from other pulsed experiments, which used 25-ps–5-ns-excitation pulses. In addition, the fraction of the 2-ps-pulse power emitted in the cone reached 7.5%, far exceeding this fraction from the longer-pulse experiments. An additional, unexpected result is that the spectral width of the laser pulse is narrower, without significant intensity loss, after propagation through the vapor. The vapor densities and laser detunings that yield cones in the present experiment are similar to those from the longer-pulse experiments. Although the 2-ps-pulse spectrum partially overlaps the resonance, the cone spectrum and cone angles versus density and laser detuning are similar to those previously reported for nanosecond pulses. The fourth (blue-detuned) wave required for satisfying four-wave mixing is absent, again equivalent to what is observed in the nanosecond experiments. The physical origin of cone emission by pulsed excitation of atomic vapors is still largely unknown, and this high efficiency and narrow angular spread of a spectrally broad cone further contradicts existing models, perhaps ultimately providing a clue to a correct explanation.

We report observations of conical emission from a pulsed laser beam with incident laser power, detuning, and beam diameter matched to a range of single, steady-state filaments. The beam thus propagates with nearly constant diameter through 5 cm of strontium (Sr) vapor. Emission angles $(\ensuremath{\theta})$ are imaged as the height on the input slit of a 1.5 m spectrometer; a charge-coupled device camera in the spectrometer exit plane measures energy as a function of $\ensuremath{\theta}$ and frequency. Data are reported versus laser-pulse energy, diameter and detuning, and the collisional dephasing rate. Spectrally broad, red-detuned cones are observed, but only when the laser beam is predominately self-trapped. As previously observed in experiments where the beam breaks up into multiple filaments, forward emission at the blue-detuned Rabi sideband is much weaker than at the red (cone) sideband. The cone angle is largely independent of the phase velocity of the self-trapped laser beam, which varies from the vapor to the vacuum speed. The efficiency of the cone emission shows incomplete saturation, as well as strong dependence on self-trapped filament parameters.

A time-dependent theory for conical emission during near-resonant propagation of laser light in an atomic vapor, which includes full propagation for the laser and frequency sidebands in a nonlinear two-level medium is presented. The density-matrix equations for the dipole moment and population are solved in the dressed atomic frame. The polarization source terms are accurate to order \ensuremath{\gamma}/R, where \ensuremath{\gamma} is a damping constant and R is the generalized Rabi frequency. Analytical plane-wave solutions and numerical, cylindrically symmetric propagation simulations including diffraction are presented. It is shown that the calculations with cylindrically symmetric fields and atomic excitation profiles are incapable of accounting for the high levels of optical gain that are responsible for the intense conical emission observed in experiments. This result is at first surprising, since the model accounts rigorously for all of the physical phenomena that have been previously proposed as being responsible for generating large gains, and the calculation matches the symmetry of the observations. The lack of large calculated gain seems to imply the existence of higher-order

(m>0)

radial modes in the field for the experimental conditions that give rise to cone emission. In the simulations, however, the cylindrically symmetric fields do produce weak red-detuned cones with angular-frequency distributions similar to those seen in experiments.

Conical light emission is observed when an intense quasiresonant laser beam propagates in an atomic vapor.1-4 A parametric four-wave mixing process was suggested to explain this effect.1 However, this model is inconsistent with experiments both quantitatively in the value of the predicted cone angle and qualitatively due to the absence of the obligatory blue counterpart to the red-shifted cone photon.2-4 A model of Cherenkov radiation by a laser-induced moving polarization was proposed to account for the origin and angular/spectral dependencies of the conical emission.3

In our experiment conical emission (CE) has no wavelength shift and the same polarization as the original beam. Varying laser power, detuning, and atomic density, the images of CE were captured and analyzed. Typical far-field images are presented. Under the blue detuning condition, manifest CE patterns were observed, and under red detuning, original beam blurs due to not only to self-defocusing but also the conical pattern were observed. By comparing laser power and atomic density dependence of cone angle and theoretical curve, we could estimate spectroscopic data such as nonlinear refractive index. From the density dependence data, we get the fact that cone angle is proportional to atomic density which supports the self-phase-modulation (SPM) model. In conclusion, we observed and analyzed CE from near-resonant Sm vapor. With this simple setup, we could measure spectroscopic data.