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Optics Letters

Coherent transient amplification in inhomogeneously broadened rubidium atoms by diode-laser frequency switching

Yongqing Li, W. H. Burkett, Min Xiao

Published Jul 1, 1996• 16 citations
Publisher

Abstract

We report observation of coherent transient amplification that is due to free-induction decay and optical nutation in inhomogeneously broadened rubidium atoms by a sudden switch of diode-laser frequency. An amplification coefficient of 2.2 x 10(-2) cm(-1) is observed when the diode-laser frequency, initially tuned to the blue side of the Doppler-broadened absorption profile, is switched to be out of resonance by application of a step-function pulsed injection current. The transition from free-induction decay to optical nutation is observed and discussed.

DOI
10.1364/ol.21.000982

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Oct 8, 2010•1 citation
We report observation of coherent transient amplification that is due to free-induction decay and optical nutation in inhomogeneously broadened rubidium atoms by a sudden switch of diode-laser frequency. An amplification coefficient of 2.2 x 10(-2) cm(-1) is observed when the diode-laser frequency, initially tuned to the blue side of the Doppler-broadened absorption profile, is switched to be out of resonance by application of a step-function pulsed injection current. The transition from free-induction decay to optical nutation is observed and discussed.
We have observed optical free induction decay on the (3)H(6)(1) ? (3)H(4)(1) transition of Tm(3+) ions in LaF(3), using an external-cavity diode laser. The diode-laser frequency was switched by the application of a current step, and the resulting transient was detected by heterodyne beating with the laser at its new frequency. This provides a simple new way to measure coherent transients. The beat note shows a frequency chirp that gives a real-time measure of the diode-laser frequency. This result is attributed to changes in the laser lattice temperature on the microsecond time scale.
We describe transient spectroscopic measurements with a current-switched semiconductor diode laser. Following a fast pulse modulation in the injection current of the diode laser, the diode laser frequency is switched by a few hundred MHz and, then, is scanned over 10 GHz range in a short-time evolution (<0.5 µs). Free-induction decay based on the frequency switching and transient absorption spectroscopy of rubidium D1 lines based on the fast frequency-scan are experimentally demonstrated. These fast spectroscopic measurements are accomplished in a single-shot pulse modulation and are, thus, applicable to short-lived samples such as in flowing or combustion systems.
A simple method for observing coherent optical-transient phenomena is described. Effects such as photon echoes and free induction decay (FID) should allow studies of the dynamic properties of atoms, molecules, and solids on a time scale ranging from milliseconds to \ensuremath{\sim} 50 ps. The frequency of a cw dye laser is switched with an intracavity electro-optic modulator, producing in an external sample coherent transients which are detected in the forward beam. Measurements of long dephasing times are possible, as in FID, because the laser can oscillate indefinitely at the new frequency following a step-function switching pulse. Measurements of short decay times are also practical because the switching time is not restricted by the laser gain or cavity ringing time. In addition, the dye's dephasing time of a few picoseconds or less is too rapid to interfere with these observations. Since the pulse sequence can be preselected, the entire class of coherent optical transients is accessible. The method thus incorporates all of the advantages inherent in Stark switching, including heterodyne detection and high sensitivity, without being restricted to Stark-tunable systems. Laser frequency switching is discussed also in terms of a theoretical model which exposes the time-dependent properties of a phase-modulated cavity mode. Dephasing studies have been carried out thus far in atomic and molecular gases and low-temperature solids containing inorganic impurity ions or large organic molecules. The utility of the FID effect is emphasized where precise dephasing times are obtained from time-frequency Fourier transformation of FID signals using a digital computer. The first quantitative and detailed test of FID theory is provided by these experimental techniques.
When an intense and short laser pulse or a fast-switching-on/off laser field interacts with a resonant absorbing medium, several optical coherent transient effects can be observed, including the self-induced transparency effect, photon echo effect, optical nutation effect, and the optical free induction decay effect. The study of these effects enables us to gain a deeper understanding of the coherent transient interaction of an intense pulsed or rapidly switched optical field with the resonant medium, and to measure the magnitude of the dipole-moment matrix elements, relaxation parameters, and spectral broadening mechanisms of the investigated materials. On the other hand, these optical coherent transient effects can also be applied for developing special techniques of optical pulse delay, pulse reshaping, and optical pulse manipulation.
A 64.4% reduction in absorption at the rubidium D2 line is observed when a pumping field at 775.8 nm is tuned on resonance to the transition between the excited states 5P3/2{\mathit{P}}_{3/2}P3/2​ and 5D5/2{\mathit{D}}_{5/2}D5/2​. As the pumping field is tuned off resonance, an absorption peak appears at the side of the Doppler-broadened D2 line. This modification in absorption is related to pumping-induced atomic coherence in this three-level ladder-type system. This experiment is done in a Rb vapor cell at room temperature and with cw diode lasers for both pumping and probe beams in a Doppler-free configuration.
Laser spectroscopy experiments are reported on rubidium atoms by using two or three external cavity diode lasers to study multiple resonance transitions in Λ- and V-type systems as well as on (Λ+V)-type system. Electromagnetically induced transparency (EIT) peaks having sub-natural line-widths are found on the Doppler broadened transmission backgrounds with velocity selective enhanced absorption dips. In the presence of a pump laser, probe spectrum shows enhancement of EIT signal by tuning the control laser frequency when the Λ- and V-type EIT signals are made to overlap. Theoretical analysis is carried out by solving the optical Bloch equations for five-level atomic model under Λ configuration. With the numerically simulated spectra, variation of EIT transmission peak with ground state decay rate and excited state spontaneous decay rate are investigated. Effect of pump Rabi frequency on the transmission peak is also shown.