Investigation On The Nonlinear Optical Effect In Thermal Atomic Ensemble

The interaction of atoms with light is an important research field in quantum optics, even in optics. Many novel and important quantum phenomena and quantum interference effects are produced in the interaction of multi-level atoms with coherent optical field. The coherently-prepared atomic medium can be applied extensively in many optics areas because of its ideal optical properties of low absorption, steep dispersion and enhanced nonlinearity. As the most typical example, electromagnetically induced transparency (EIT) effect and EIT medium have attracted a great deal of attention. And the EIT-related theory and experiments studies become very active.The dissertation introduces the basic concepts and physical properties of EIT effect around the interaction of multi-level atoms with coherent optical field and reviews the development of the research works on EIT and related effects. On this basis, the experimental and theoretical works we completed based on EIT effect were mainly introduced in this dissertation, including light speed reduction, the enhanced EIT effect and cross-Kerr nonlinear effect between two weak beams based on Double EIT in a tripod system, study on the resonance Bragg-scattering spectroscopy and so on. The main works are as following:1) The related theory and realization in experiment on EIT effect are briefly introduced, in particular the light speed reduction experiment. We observed the light group speed reduction due to EIT effect in the three-level atomic medium with degenerate ground states. The measured group velocity of optical pulse is slowed down to 10000m/s.2) The optical induced polarization rotation resulting from the asymmetry EIT system is observed in a multi-level EIT system in rubidium atoms. By choosing left-circularly-polarized coupling beam to interact with atoms, the asymmetry in the number of EIT subsystems seen by the left- and right-circularly-polarized components of the weak probe beam is produced, which makes the refractive indices of left- and right-circularly-polarized components of the probe beam different and leads to the polarization rotation of the probe beam. We designed a polarization measurement setup to eliminate the influence of absorption on the measurement of polarization rotation angle and measured the rotation angle with the higher measurement precision. In the experiment, a rotation angle of 45 degrees has been achieved with a coupling beam power of only 15mW.3) The polarization spectroscopy of a linearly-polarized optical (probe) field is studied and a light switch based on optical polarization changes is demonstrated in a multi-Zeeman-sublevel atomic system of ⁸⁷Rb D1 line. The polarization spectroscopy signals of the probe light result from the changes in its polarization, which are caused by a left-circularly polarized pumping beam. A theoretical analysis involving multiple Zeeman sublevels is presented and the results are in qualitatively agreement with the experimental observations. Based on this phenomenon, we demonstrated a light switch at low light level (～68 photons perλ/2Ï€) with a switching efficiency of～3%.4) The two EIT windows and enhanced EIT signals resulting from the interaction between the two EIT windows are observed in a tripod system. By choosing left circularly-polarized probe, left circularly-polarized coupling beam and right circularly-polarized trigger beam to interact with the energy levels of D1 line in ⁸⁷Rb atoms, a tripod system is formed. In the four-level tripod system, we observed the two EIT dips produced by a strong coupling beam and a weak trigger beam are in the absorption spectrum of the probe field when the frequency detuning of trigger beam is different from that of the coupling beam. And we studied the interaction between the two EIT windows. The results show that the EIT signal created by coupling beam was significantly enhanced when the frequency detuning of trigger beam is near or equal to that of the coupling beam. We made numerical calculation for the above phenomena and the results are agreement with experimental results.5) The Double EIT windows and the cross-Kerr nonlinear effect between two weak beams based on Double EIT are investigated in a four-level tripod system formed in D1 line of ⁸⁷Rb atoms. We observed the simultaneous EIT windows for probe and trigger fields (double EIT) produced by a strong coupling beam and measured the cross-phase modulation (XPM) phase shifts and nonlinear coefficient between the two fields using Mach-Zehnder interferometer. The experimental results show that the XPM coefficient of trigger beam is larger than 2×10^-5cm²/W when the accompanying transmissions of probe and trigger beams are higher than 60%. Meantime, we studied the dependence of XPM coefficients on the power of modulation beam and found that the XPM coefficients decrease with the increase of modulation power.6) A high-resolution backward Resonance Bragg-scattering (RBS) spectroscopy from a population difference grating (PDG) is demonstrated experimentally in a Rb atomic vapor cell. A stable standing-wave field, which is formed by a pair of counter-propagating pump fields, periodically modulates the space distribution of the population of the thermal two-level rubidium atoms and constructs a PDG. When a probe beam with the polarization orthogonal to that of the pump fields, propagates through the PDG, the backward RBS spectra were observed. The linewidth of ³.5MHz is obtained in the backward RBS spectra and high S/N up to～2000 is achieved. Using the scheme of the coherent superposition of the individual elastic Rayleigh scattering light emitted from the atomic dipole oscillators on each grating volume element of the PDG, the RBS spectroscopy is theoretically explained.The characterized works among the above are as follows:â… . We observed the polarization spectroscopy signals of the probe light resulting from the changes in its polarization, which are caused by a left-circularly polarized pumping beam. Based on this phenomenon, we demonstrated a light switch at low light level with a switching efficiency of～3%.â…¡. By choosing proper polarized light beams to interact with Zeeman-sublevel of atoms, we formed a four-level tripod atomic system. We observed the simultaneous EIT windows (Double EIT) for probe and trigger fields induced by coupling beams in the four-level tripod system. And for the first time, the cross-Kerr nonlinearity between the two weak fields based on the Double EIT windows were experimentally investigated. We observed the significant cross-Kerr nonlinearity (cross-phase modulation (XPM)) at low intensities and accurately measured the XPM phase shifts between the probe and trigger fields by using the new-style Mach-Zehnder interferometer formed by two beam displacing polarizers.â…¢. For the first time, we demonstrated experimentally a high-resolution backward Resonance Bragg-scattering (RBS) spectroscopy from a population difference grating in an atomic vapor cell and explained theoretically the RBS spectroscopy by using the scheme of the coherent superposition of the elastic Rayleigh scattering light. The linewidth of the obtained backward RBS spectra is～3.5MHz and the S/N up to～2000.