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Applied Optics

Geometric determination of saturation in fluorescence spectroscopy

Michele Marrocco

Published May 10, 2005• 2 citations
Publisher

Abstract

In laser spectroscopy, saturation of atomic or molecular transitions cannot be ignored, even at modest laser intensities. The saturation status is customarily diagnosed from measurements of saturation curves describing the dependence of spectroscopic signals on laser intensity. I propose an alternative method that relies on a geometric comparison of the spatial laser profile with images of the spectroscopic quantity under investigation. A single image can be used to determine the saturation status and its associated saturation laser intensity.

DOI
10.1364/ao.44.002879

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In laser spectroscopy, saturation of atomic or molecular transitions cannot be ignored, even at modest laser intensities. The saturation status is customarily diagnosed from measurements of saturation curves describing the dependence of spectroscopic signals on laser intensity. I propose an alternative method that relies on a geometric comparison of the spatial laser profile with images of the spectroscopic quantity under investigation. A single image can be used to determine the saturation status and its associated saturation laser intensity.
Ultrafast saturation of signals from an optical process occurs when intense laser pulses, with the duration of the driving laser pulse shorter than the effective decay timescales, interact with the target system. Recently, it has been shown that such saturation is marked by transition from transient-absorption to the transient-gain regime, unlike in continuous-wave or long-duration excitations where saturation occurs once the underdamping regime is achieved. The threshold intensities of the drive pulse that drives the optical signal to saturate can be estimated by calculating the area of the pulse. We demonstrate that depending on the shape of the pulse, the saturation behavior differs. The lineshape dependence of ultrafast saturation of absorption and fluorescence is studied for a variety of pulse shapes. We demonstrate that the saturation threshold is lower for super-Gaussian pulses and almost an order of magnitude lower for sech pulses compared to a Gaussian pulse.
Absorption and fluorescent spectra are obtained in saturation regime of a single beam laser spectroscopy of Cesium D2 line, in a vapor layer with thickness close to the light wavelength. We compare experimentally and theoretically obtained spectra distinguishing between open and closed, in terms of optical pumping, hyperfine transitions. In absorption, we observe a persisting difference between open and closed transitions, in terms of Dicke narrowing, when increasing laser light intensity. In fluorescence, for open transitions we note saturation dips which do not change significantly when increasing intensity. In case of closed transition, a small feature at the fluorescence line center appears for relatively high light intensity.
An ongoing spectroscopic investigation of atomic rubidium utilizes a two-photon, single-laser excitation process. Transitions accessible with our tunable laser include 5P1/2 (F ′ ) ← 5S1/2 (F) and 5P3/2 (F ′ ) ← 5S1/2 (F). The laser is split into a pump and probe beam to allow for Doppler-free measurements of transitions between hyperfine levels. The pump and probe beams are overlapped in a counter-propagating geometry and the laser frequency scans over a transition. Absorption, fluorescence and polarization spectroscopy techniques are applied to this basic experimental setup. The temperature of the vapor cell and the power of the pump and probe beams have been varied to explore line broadening effects and signal-to-noise of each technique. This humble setup will hopefully grow into a more robust experimental arrangement in which double resonance, two-laser excitations are used to explore hyperfine state changing collisions between rubidium atoms and noble gas atoms. Rb-noble gas collisions can transfer population between hyperfine levels, such as 5P3/2 (F ′ = 3) Collision ←− 5P3/2 (F ′ = 2), and the probe beam couples 7S1/2 (F ′′ = 2) ← 5P3/2 (F ′ = 3). Polarization spectroscopy signal depends on the rate of population transfer due to the collision as well as maintaining the orientation created by the pump laser. Fluorescence spectroscopy relies only on transfer of population due to the collision. Comparison of these techniques yields information regarding the change of the magnetic sublevels, mF , during hyperfine state changing collisions.
Resonant frequency modulation spectroscopy has been previously used as a highly sensitive method for measuring the output of cold atom interferometers. Using a detailed model that accounts for optical saturation, laser intensities and atomic densities that vary spatially, and radiation pressure on the atoms, we theoretically investigate under what parameter regimes the optimum signal-to-noise ratio is found under experimentally realistic conditions. We compare this technique to the standard method of fluorescence imaging and find that it outperforms fluorescence imaging for compact interferometers using condensed atomic sources or where the photon collection efficiency is limited. However, we find that fluorescence imaging is likely to be the preferred method when using squeezed atomic sources due to limited atom number.