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AIP conference proceedings

Modulated emission in iodine vapor

Azriel Z. Genack, P. M. Mitra, Axel Schenzle

Published Jan 1, 1986• 0 citations
Publisher

Abstract

The response of emission in I2 vapor to intensity modulated light at radial frequency ω is measured and described in terms of the lifetime of the excited state, T1, the rate of phase interrupting collisions γ, and the inhomogeneously broadened absorption line. When the laser is tuned to resonance, the variation of amplitude of the modulated emission with ω corresponds to a single exponential decay with lifetime T1. Off resonance, the modulation is described as the sum of the above ‘‘delayed’’ component and a ‘‘prompt’’ in‐phase component which is independent of modulation frequency. The ratio of the delayed to prompt component for off resonant excitation is shown to be 2γT1. This relation is used to determine the rate of phase interrupting collisions in I2 vapor.

DOI
10.1063/1.35712

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The response of emission in I2 vapor to intensity modulated light at radial frequency ω is measured and described in terms of the lifetime of the excited state, T1, the rate of phase interrupting collisions γ, and the inhomogeneously broadened absorption line. When the laser is tuned to resonance, the variation of amplitude of the modulated emission with ω corresponds to a single exponential decay with lifetime T1. Off resonance, the modulation is described as the sum of the above ‘‘delayed’’ component and a ‘‘prompt’’ in‐phase component which is independent of modulation frequency. The ratio of the delayed to prompt component for off resonant excitation is shown to be 2γT1. This relation is used to determine the rate of phase interrupting collisions in I2 vapor.
We address the distinction between resonance Raman scattering and fluorescence as the excitation source is continuously tuned through resonance. The experiments are performed in iodine vapor using a cw laser, whose intensity is electrooptically modulated in the 0.1-50-MHz frequency range. A lock-in amplifier is used to detect both the in-phase and out-of-phase response of the emission. The modulated signal is found to be the sum of two components. One component varies with angular modulation frequency ω, as (1 + ω2τ2)-1/2, where τ is the lifetime of the excited state. The other is found to be flat within the range of modulation frequency used in the experiment. Since the results of the modulation experiment are the Fourier transform of the response to pulsed excitation, the first component corresponds to exponential decay with the fluorescence lifetime and the second corresponds to a prompt response. A density matrix calculation shows that the prompt response would be absent if the absorption were independent of laser frequency. It exists as a result of the finite inhomogeneous linewidth and the laser detuning from the center of the transition. The amplitude of the delayed and prompt components is a function of laser frequency. This gives a full description of the dynamical response to the emission to linear excitation as the laser is tuned continuously through resonance. The response of the sample to nonlinear excitation is also discussed.
We observed the transient oscillation in the spectra of electromagnetically induced transparency by rapidly sweeping the probe laser frequency. The spectra were measured with laser-cooled 87Rb atoms. We show that the amplitude of the transient oscillation decays exponentially at a rate of Ωc2/2Γ, where Ωc is the Rabi frequency of the coupling laser and Γ is the spontaneous decay rate of the excited state. Using the transient oscillation, we demonstrate a simple method to determine Ωc. This method is particularly useful with the low-intensity coupling laser.
We report a significantly shortened scattering lifetime in molecular iodine when the frequency of a single-mode 5145-\AA{}A incident laser is tuned away from resonance with a discrete transition. This observation of a shortened scattering time is indicative of what has been conventionally called a Raman process. Off-resonance lifetime measurements, as reported here, may provide a means of determining the relative contribution to the absorption linewidth of homogeneous and inhomogeneous broadening.
The lifetime of atoms and molecules can be measured by using modulated resonant single-mode laser field. A theoretical mode for this method is presented. It is shown that the laser induced fluorescence, a resonance flourescence is modulated at the same frequency, but phase shift from the exciting laser. The phase shift is mainly due to three processes (1) coherent excitation process characterized by transverse relaxation rate and line width of the laser, (2) population damping process characterized by the decay time of excited levels and other populated levels, (3) collision process characterized by the relaxation time by which the system to be in equilibrium state. The effect of these processes on measurement result is discussed. The Doppler broading is considered also.
Time characteristics of the secondary emission from iodine molecules excited with light pulses from a cavity-dumped, single-frequency Ar + ion laser at 514.5 nm have been compared with existing theories. It has been found that the experimental data are well explained quantitatively by the theories, provided the 21 hyperfine lines as well as collisional effects and Doppler broadening are taken into account. The cross sections for the phase-changing and population-changing collisions have also been determined. The contributions of coherent and incoherent secondary emission processes are discussed.
By sweeping the frequency of a diode laser beam near the cesium resonance frequency, followed by propagation through a short length of cesium vapor, we obtained periodic peaks with peak-to-average ratios near 5 at pulse repetition rates up to a gigahertz. Light in the wings of a strong atomic absorption line of an alkali metal vapor encounters large group velocity dispersion. A frequency modulation in an off-resonance cw beam passing through the vapor consequently develops an amplitude modulation. For certain combinations of laser and vapor parameters, a sinusoidally frequency modulated beam can be made to develop peaks. We generated pulse trains in a frequency modulated single mode diode laser tuned to the cesium absorption line near 852 nm, which we observed by using a fast p-i-n photodiode and a sampling oscilloscope. The frequency of a diode laser was sinusoidally modulated with a sweep of about a gigahertz and a period of a few nansoseconds by introducing a small modulation into the laser pump current. We observed the formation of sharp peaks with a width of about one-tenth of the modulation period and a peak-to-background ratio of approximately 5.