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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Conical emission as a result of self-phase-modulation in near-resonant Sm vapor: spectroscopic application

Won‐Kyu Lee, Young-Chul Noh, Jin-Ho Jeon, Jai-Hyung Lee, Joon-Sung Chang, Kwang-Hoon Ko, Jongmin Lee

Published Jan 20, 2003• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleopr.1999.817935

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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.
We studied the conical emission in samarium (Sm) vapor under the near-resonant condition. The incident dye laser was tuned to the transition line, 4f66s2 7F→4f6(7F)6s6p(1P0)(λ=562.601 nm), of the Sm atom. Using a high-temperature oven, we could obtain the atomic density of 8×1014 atoms/cm3, large enough to observe the conical emission. We observed one to three rings around the original laser beam, and the number of the rings depended on the laser intensity, the laser wavelength, and the atomic density. These results are attributed to the self-phase modulation of the laser beam interacting with the near-resonant atomic medium. We give a simple expression that describes the dependence of the locations of the rings on the laser intensity. We compare the experimental results with this expression quantitatively and then estimate the nonlinear susceptibility of Sm vapor.
We studied the emission from dense rubidium vapor under 100-fs pulsed laser excitation in the 730--820 nm range. Self-focusing along with a conical emission (CE) was observed for the excitation in the blue wing of D2D2D2 atomic resonance. Self-defocusing occurred for the red-wing excitation. This is in accordance with the theory of the intensity dependent nonlinear refraction index. In the case of the CE observation, our experimental results are attributed to the self-phase modulation of the laser beam interacting with the near-resonant atomic medium.
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 detailed study on yellow conical emission observed in dense sodium vapor following near-resonant one-photon excitation of the Na 42P levels was performed. The origins of conical emission are suggested to be due to Raman resonant four-wave parametric mixing processes. In order to describe spectral and spatial features of off-axis emission, we introduce self-phase modulation and induced-phase modulation as important additional nonlinear processes, explaining the nonmonotonic detuning of the wavelength of the conical radiation with the exiting laser wavelength in the vicinity of resonance transitions. The spatial evolution of emission generated via four-wave mixing is modeled by solving the two-dimensional nonlinear Schrödinger equation describing the spatial evolution of light beams in a nonlinear media. Experimental and theoretical results are presented and discussed.
We report the observation of cone emission generated when $\ensuremath{\sim}10\text{\ensuremath{-}}\mathrm{nJ}$, 100-fs laser pulses in the 730--770 nm wavelength range are transmitted through a dense cesium vapor. The spatial and spectral characteristics of the observed cone emission were studied experimentally. Cone angle dependence on the laser wavelength, laser average power, and Cs atomic and molecular concentrations were investigated. The cone emission (CE) was only observed when the laser beam self-focused after passing through the medium. We identify the self-phase modulation as a dominant mechanism for CE generation in our experiment.
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.