Laser-atom interactions create atomic coherence and large nonlinear atomic polarization.\nWe investigate resonant laser-atom interactions to generate large nonlinearities\nand control them using magneto-optical fields. Coherent control of high-order\nsusceptibilities and magneto-optical rotation are demonstrated. Experiments are supported\nby theoretical studies that effectively describe the observed phenomena.\nIt is shown that a new coherent field, with polarization orthogonal to a weak\nsignal field, can be parametrically generated via an all-resonant four-wave-mixing\nprocess. This is demonstrated in a double-ladder system having two intermediate\nstates between a ground and an excited state. It is shown that the parametricgeneration\nprocess can be coherently controlled by coupling lasers and magnetic fields.\nIt is theoretically established that the underlying physics is a resonant three-photon\nprocess with a wide domain of control parameters.\nElectromagnetically induced transparency (EIT), where absorption of a weak\nprobe is suppressed via quantum interference, is demonstrated in a usual three-level\nladder system. It is observed that in contrast with EIT in a usual ladder system,\naddition of a second channel helps to suppress the absorption of two weak probe\nfields in the double-ladder system. The resulting enhancement of transmission in two\ndifferent channels is due to gain caused by three-photon processes.\nCoherent control is strongly limited by coherence lifetime, which is the inverse of\nthe dephasing rate. A lambda-system, having two ground states coupled to a common\nexcited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing\nperturbs the dark state. A new method for measuring the ground-state dephasing\nrate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated.\nWhile two laser fields in a lambda-system are resonant with their respective\ntransitions, the atomic polarizations are very sensitive to an applied magnetic field.\nThis effect can be used for optical magnetometry. The degree of sensitivity of the\nmagnetometer is determined by two competing parameters–atomic density and laser\nintensity. It is shown experimentally that the optimal sensitivity reaches saturation,\nwhich is contrary to the idea that sensitivity increases indefinitely with an increase\nin the above parameters.
Laser-atom interactions create atomic coherence and large nonlinear atomic polarization.\nWe investigate resonant laser-atom interactions to generate large nonlinearities\nand control them using magneto-optical fields. Coherent control of high-order\nsusceptibilities and magneto-optical rotation are demonstrated. Experiments are supported\nby theoretical studies that effectively describe the observed phenomena.\nIt is shown that a new coherent field, with polarization orthogonal to a weak\nsignal field, can be parametrically generated via an all-resonant four-wave-mixing\nprocess. This is demonstrated in a double-ladder system having two intermediate\nstates between a ground and an excited state. It is shown that the parametricgeneration\nprocess can be coherently controlled by coupling lasers and magnetic fields.\nIt is theoretically established that the underlying physics is a resonant three-photon\nprocess with a wide domain of control parameters.\nElectromagnetically induced transparency (EIT), where absorption of a weak\nprobe is suppressed via quantum interference, is demonstrated in a usual three-level\nladder system. It is observed that in contrast with EIT in a usual ladder system,\naddition of a second channel helps to suppress the absorption of two weak probe\nfields in the double-ladder system. The resulting enhancement of transmission in two\ndifferent channels is due to gain caused by three-photon processes.\nCoherent control is strongly limited by coherence lifetime, which is the inverse of\nthe dephasing rate. A lambda-system, having two ground states coupled to a common\nexcited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing\nperturbs the dark state. A new method for measuring the ground-state dephasing\nrate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated.\nWhile two laser fields in a lambda-system are resonant with their respective\ntransitions, the atomic polarizations are very sensitive to an applied magnetic field.\nThis effect can be used for optical magnetometry. The degree of sensitivity of the\nmagnetometer is determined by two competing parameters–atomic density and laser\nintensity. It is shown experimentally that the optimal sensitivity reaches saturation,\nwhich is contrary to the idea that sensitivity increases indefinitely with an increase\nin the above parameters.
We report on experimental and theoretical study of the nonlinear Faraday effect under conditions of electromagnetically induced transparency at the $5{S}_{1/2}\ensuremath{\rightarrow}5{P}_{3/2}\ensuremath{\rightarrow}5{D}_{5/2}$ two-photon transition in rubidium vapor. These transitions realize the inverted Y model which combines the $\ensuremath{\Lambda}$ and ladder systems. Strong nonlinearity allowing for large rotation angles of a probe beam tuned to the $S\ensuremath{\rightarrow}P$ transition was obtained by creation of quantum superpositions of magnetic sublevels (Zeeman coherences) in the rubidium ground state ($\ensuremath{\Lambda}$ scheme). Additionally, electromagnetically induced transparency was accomplished in a ladder scheme by acting with a second strong coupling laser on the $P\ensuremath{\rightarrow}D$ transition. Under conditions of a two-photon resonance the rotation was significantly reduced, which is interpreted as a competition between the coherent processes. The effect was observed in sub-Gauss magnetic fields and could be used for efficient coherent control of generation of the ground-state coherences, e.g., for controlling the polarization state of the probe light.
In this thesis, atomic coherence is used to enhance nonlinear optical processes in multi-level atoms. The multi-photon transitions are driven resonantly, and at the same time without absorptive losses, by using electromagnetically induced transparency (EIT), thereby allowing the study of χ(3) and χ(5) nonlinearities using weak driving fields. The coherently modified probe beam(s) and the atom-radiated signal fields arising from four- and six- wave- mixing (FWM and SWM) processes are measured in the spectral, temporal and spatial domains. In a three-level ladder-type atomic system, multiple peaks having spectral asymmetries are observed in the EIT window as well as in the FWM signal waveforms due to the diverse multiplicities of the participating atomic states. Using phase control tailored in the frequency domain, we demonstrate all-optical methods to control these spectral waveforms and discuss applications involving waveform-shaping and metrology. For the EIT study we demonstrate a switching of multiple dark peaks into bright peaks via phase-control of interferences in the underlying dark-states. In the FWM study we demonstrate all-optical spectral line shape symmetrization, linewidth narrowing and bandwidth switching. In a four-level inverted-Y-type atomic system, we drive and measure coexisting and phase-matched FWM and SWM signals. By using precision control of the relative phase and amplitude between these two processes of different nonlinear orders, we demonstrate phase coherence between them. First, a single-phase measurement is performed in the temporal and spatial domains, and the interferogram is used to measure the resonant frequency of the 5D5/2-5P3/2 atomic transition in 85Rb. Second, the method is extended to realize a capacity for two-phase measurement. In this case, the spectral bandwidth of the signal is modified in order to measure the phase-shift occurring in one Mach-Zehnder interferometer, while the intensity of the total signal waveform measures the phase-shift occurring in a second interferometer. Finally, we demonstrate phase-dependent spatial fusion between two ultra-weak optical fields by using a strong coupling field to first convert the weak fields into bosonic dark-state polaritons, which are then steered into a common all-optical waveguide mode arising due to the coupling field's intensity distribution and the resulting cross-Kerr refractive index gradient.
This dissertation reports experimental studies of nonlinear optical effects manifested by electromagnetically induced transparency (EIT) in cold Rb atoms. The cold Rb atoms are confined in a magneto-optic trap (MOT) obtained with the standard laser cooling and trapping technique. Because of the near zero Doppler shift and a high phase density, the cold Rb sample is well suited for studies of atomic coherence and interference and related applications, and the experiments can be compared quantitatively with theoretical calculations. It is shown that with EIT induced in the multi-level Rb system by laser fields, the linear absorption is suppressed and the nonlinear susceptibility is enhanced, which enables studies of nonlinear optics in the cold atoms with slow photons and at low light intensities. Three independent experiments are described and the experimental results are presented. First, an experimental method that can produce simultaneously co-propagating slow and fast light pulses is discussed and the experimental demonstration is reported. Second, it is shown that in a three-level Rb system coupled by multi-color laser fields, the multi-channel two-photon Raman transitions can be manipulated by the relative phase and frequency of a control laser field. Third, a scheme for all-optical switching near single photon levels is developed. The scheme is based on the phase-dependent multi-photon interference in a coherently coupled four-level system. The phase dependent multi-photon interference is observed and switching of a single light pulse by a control pulse containing ∼20 photons is demonstrated. These experimental studies reveal new phenomena manifested by quantum coherence and interference in cold atoms, contribute to the advancement of fundamental quantum optics and nonlinear optics at ultra-low light intensities, and may lead to the development of new techniques to control quantum states of atoms and photons, which will be useful for applications in quantum measurements and quantum photonic devices.
Summary form only given. Electromagnetically induced transparency (EIT) refers to a phenomenon, whereby the optically resonant transition is rendered nearly transparent by applying a strong coupling laser field. The mechanism of EIT is usually explained in terms of quantum coherence or quantum interference. This EIT has attracted increasing interest because absorption associated with nonlinear optical processes can be removed. In the typical three-level /spl lambda/-system, which includes the ground state and the metastable state, when the atomic population is prepared in the superposition of the states by the coupling fields, the optical response of the media to the probe field is significantly modified. Since this is a coherent control of an electric field by another field via the atomic coherence, several schemes to control light coherently have been suggested. We propose an optical coherent control scheme of the polarization of optical pulses using the EIT quantum coherence, and numerically study its applicability to fast optical switching.
We present the results of an investigation of the different physical processes that influence the shape of the nonlinear magneto-optical signals both at small magnetic field values (~ 100 mG) and at large magnetic field values (several tens of Gauss). We used a theoretical model that provided an accurate description of experimental signals for a wide range of experimental parameters. By turning various effects "on" or "off" inside this model, we investigated the origin of different features of the measured signals. We confirmed that the narrowest structures, with widths on the order of 100 mG, are related mostly to coherences among ground-state magnetic sublevels. The shape of the curves at other scales could be explained by taking into account the different velocity groups of atoms that come into and out of resonance with the exciting laser field. Coherent effects in the excited state can also play a role, although they mostly affect the polarization components of the fluorescence. The results of theoretical calculations are compared with experimental measurements of laser induced fluorescence from the D2 line of atomic rubidium as a function of magnetic field.
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…