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High resolution laser spectroscopy of cesium and rubidium molecules with optically induced coherence

Hui Chen

Published Oct 30, 2006• 0 citations
OpenAlex

Abstract

This work is devoted to the study of the quantum coherent effects in diatomic molecular systems by using high resolution laser spectroscopy. In particular, we have studied\nthe rubidium diatomic molecular gaseous medium's absorption spectrum with high\nresolution single mode laser spectroscopy. The derived electronic and rotational vibrational constants were used in the backward Raman amplification experiment of\nRb diatomic molecule. Both experimental results and theoretical calculation confirms that there is strong backward directionally dependent radiation. This effect can\nfurther be utilized in remote detection of chemical material.\nIn the saturated spectroscopy experiment of the cesium diatomic molecule, long-lived ground state coherence was observed. The coherence would decay at a rate less\nthan the natural life time of the excited states, which indicates great possibility for\nperforming the quantum optics experiments previously performed in atomic systems\nonly.\nElectromagnetically induced transparency has been observed in many atomic\nsystems for many years, while it has been seldom realized in molecular systems.\nIn our experiment of electromagnetically induced transparency in cesium diatomic\nmolecules, we utilized  energy levels, and observed subnatural linewidth. This is the first time to realize a  type EIT in a molecular ensemble. This experiment will\nlead to many other experiments of quantum effects in a molecular system, such like\nmagnetic optical rotation, light storage in ensemble of molecules.\nMagnetically induced chirality in an atomic ensemble is also investigated in my\nresearch.

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High-Resolution Spectroscopy in Atoms Using Coherent Control

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Coherent spectroscopy of degenerate two-level systems of the Doppler-broadened ${D}_{2}$ cesium line has been performed in vacuum and in buffer gas cells. Subnatural-width (SNW) electromagnetically induced transparency and electromagnetically induced absorption (EIA) resonances have been observed with single-frequency laser excitation for different light polarizations and collision regimes. SNW-EIA resonances with linearly polarized excitation of the ${F}_{g}=4$ level are observed, and they increase their contrast with power density increase and reach a contrast of 20% at $200 \mathrm{mW}{\mathrm{cm}}^{\ensuremath{-}2}.$ A theoretical model has been elaborated, taking into account the Doppler broadening of the transitions, the linewidth of the laser light, and the experimental conditions for observation of the fluorescence. Here the theoretical results are in agreement with those of the experiment. In a buffer gas cell, the SNW resonances in the fluorescence, in degenerate two-level systems are observed for the first time and they are only obtained with circularly polarized light. A theoretical explanation of this result is proposed. At the expense of contrast, in the presence of a buffer gas the coherent resonances are significantly narrower than in a vacuum cell for the same power density. The observation of SNW resonances in buffer gas is significantly less sensitive to the laser frequency drift than in a vacuum cell, which could be advantageous for applications in metrology and in the measurement of weak magnetic fields.

Physical Review A•Jul 11, 2002•96 citations
This work is devoted to the study of the quantum coherent effects in diatomic molecular systems by using high resolution laser spectroscopy. In particular, we have studied\nthe rubidium diatomic molecular gaseous medium's absorption spectrum with high\nresolution single mode laser spectroscopy. The derived electronic and rotational vibrational constants were used in the backward Raman amplification experiment of\nRb diatomic molecule. Both experimental results and theoretical calculation confirms that there is strong backward directionally dependent radiation. This effect can\nfurther be utilized in remote detection of chemical material.\nIn the saturated spectroscopy experiment of the cesium diatomic molecule, long-lived ground state coherence was observed. The coherence would decay at a rate less\nthan the natural life time of the excited states, which indicates great possibility for\nperforming the quantum optics experiments previously performed in atomic systems\nonly.\nElectromagnetically induced transparency has been observed in many atomic\nsystems for many years, while it has been seldom realized in molecular systems.\nIn our experiment of electromagnetically induced transparency in cesium diatomic\nmolecules, we utilized  energy levels, and observed subnatural linewidth. This is the first time to realize a  type EIT in a molecular ensemble. This experiment will\nlead to many other experiments of quantum effects in a molecular system, such like\nmagnetic optical rotation, light storage in ensemble of molecules.\nMagnetically induced chirality in an atomic ensemble is also investigated in my\nresearch.
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.
The subject of this thesis is the study of coherent interaction of light with matter (atoms) to improve the precision measurements and techniques. Special attention is drawn to get the narrow subnatural electromagnetically induced transmission (EIT), electromagnetically induced absorption (EIA) and nonlinear magneto-optic rotation (NMOR) caused by alkali atoms contained in a vapor cell. Subnatural polarization rotation introduces by a strong circularly polarized light in the absence of any external magnetic field was also studied. A detailed theoretical treatment, given in this dissertation, allows to associate each of the features of the spectra with a special physical mechanism. Many quantum phenomena related to interferences, coherences, optical pumping etc. experiments are studied using home-built diode lasers. This thesis also describes laser cooling and trapping of rubidium atoms using two techniques. Deflection of cold atoms horizontally from MOT using pushing beams are discussed in close consideration for the improvements in the precision measurements. This thesis is organized as follows. In Chapter 1, an introduction to the importance of sub-natural narrow resonance and simplified technique in the precision measurement will be discussed. In Chapter 2, an introduction to EIT, EIA and NMOR resonance are discussed. This chapter will provide a basic theoretical background of atom-field interactions, especially for Λ-type and N -type systems and its steady state solution using density matrix analysis and experimental tools. The most important notion of laser cooling of atoms, ions or molecule i.e., exchange of momentum between light and atoms combining with the Doppler effect will be discussed. In Chapter 3, the observation of subnatural EIT and subnatural EIA in closed and open degenerate two-level system using room temperature vapor cell filled with Rb will be presented. Physical mechanisms that contribute to EIT and EIA, and the contrast of our results from the coherent population trapping (CPT)–type resonances are discussed in detail in appendix A.2 according to our experimental results. In Chapter 4, the narrowing of subnatural EIT and subnatural EIA linewidth in closed and open systems again in degenerate two level transition, using the “Laguerre-Gaussian” control beam instead of generally studied Gaussian beam, will be discussed in detail. In Chapter 5, the conversion between subnatural EIT to subnatural EIA in a degenerate Λ system will be discussed. The physical mechanism created by the introduction of a coherent counter propagating control laser to the co-propagating probe and the control laser are studied. The effect of polarization and axial velocity Doppler averaging will also be studied. In Chapter 6, we will discuss the sensitive technique for precise measurement of small magnetic fields using the NMOR, by chopping the resonant laser beam. We will study the sensitivity and the potential application of this technique in the measurement of an atomic electric-dipole moment. In Chapter 7, we will…
A high sensitivity spectroscopy of Rydberg atoms is presented by using electromagnetically induced transparency (EIT) in the 6S(1/2)-6P(3/2)-n D ladder-type system of cesium vapor cell at room temperature. The EIT spectra of 40 D Rydberg state are measured and the dependences of the EIT magnitude and linewidth on the coupling laser power are investigated in detail. The Rydberg EIT linewidth is measured to be about 5.6 MHz when the powers of probe and coupling lasers are 50 microW and 5.2 mW, respectively, and which is close to the natural linewidth of cesium atoms. The effect of double resonance optical pumping on EIT is also investigated. The fine structures of nD (n = 39-55) are measured and the experimental result is in agreement with quantum defect theory.
In the sealed-off cesium vapor cell studied in this work, a residual rubidium fraction of approximately $\sim1%wasobserved.WeinvestigatetheopticalresponseofthesetraceRbatomsinasealed1 cmlongCs−filledvaporcell.Despitethelowconcentration,laserexcitationat795 nmallowstheobservationofsaturatedabsorptionandelectromagneticallyinducedtransparency(EIT)resonances.ThesurroundingCsvaporeffectivelyactsasabuffermedium,reducingtheRbatomicvelocityandincreasingtheinteractiontimewiththelaserfield,whichimprovestheEITsignal.Theexperimentsareperformedinanall−sapphirecellthatcanbeheatedupto5001\% was observed. We investigate the optical response of these trace Rb atoms in a sealed 1~cm long Cs-filled vapor cell. Despite the low concentration, laser excitation at 795~nm allows the observation of saturated absorption and electromagnetically induced transparency (EIT) resonances. The surrounding Cs vapor effectively acts as a buffer medium, reducing the Rb atomic velocity and increasing the interaction time with the laser field, which improves the EIT signal. The experiments are performed in an all-sapphire cell that can be heated up to 5001%wasobserved.WeinvestigatetheopticalresponseofthesetraceRbatomsinasealed1 cmlongCs−filledvaporcell.Despitethelowconcentration,laserexcitationat795 nmallowstheobservationofsaturatedabsorptionandelectromagneticallyinducedtransparency(EIT)resonances.ThesurroundingCsvaporeffectivelyactsasabuffermedium,reducingtheRbatomicvelocityandincreasingtheinteractiontimewiththelaserfield,whichimprovestheEITsignal.Theexperimentsareperformedinanall−sapphirecellthatcanbeheatedupto500^{\circ}$C without window blackening, unlike conventional glass cells. From the measured spectra, Cs--Rb collisional cross sections are estimated. These results show that residual atomic species in high-temperature vapor cells can be exploited for spectroscopic and nonlinear-optical studies.
Coherent spectroscopy of degenerate two-level systems of the Doppler-broadened D2{D}_{2}D2​ cesium line has been performed in vacuum and in buffer gas cells. Subnatural-width (SNW) electromagnetically induced transparency and electromagnetically induced absorption (EIA) resonances have been observed with single-frequency laser excitation for different light polarizations and collision regimes. SNW-EIA resonances with linearly polarized excitation of the Fg=4{F}_{g}=4Fg​=4 level are observed, and they increase their contrast with power density increase and reach a contrast of 20% at $200 \mathrm{mW}{\mathrm{cm}}^{\ensuremath{-}2}.$ A theoretical model has been elaborated, taking into account the Doppler broadening of the transitions, the linewidth of the laser light, and the experimental conditions for observation of the fluorescence. Here the theoretical results are in agreement with those of the experiment. In a buffer gas cell, the SNW resonances in the fluorescence, in degenerate two-level systems are observed for the first time and they are only obtained with circularly polarized light. A theoretical explanation of this result is proposed. At the expense of contrast, in the presence of a buffer gas the coherent resonances are significantly narrower than in a vacuum cell for the same power density. The observation of SNW resonances in buffer gas is significantly less sensitive to the laser frequency drift than in a vacuum cell, which could be advantageous for applications in metrology and in the measurement of weak magnetic fields.