Hot Atom High-resolution Spectra And Polarization Modulation Of The Four-wave Mixing

Thermal atomic vapor is usually used as optical medium for laser spectroscopy experiments.The line broadening occurs due to the collision and the Doppler effect of moving atoms,thus high resolution spectroscopy is limited.The high resolution spectrum can be obtained through many methods,such as saturated absorption spectroscopy,Doppler free methods of two-photon spectroscopy,and etc.In this thesis,we devote to Dicke-narrowing spectroscopy and polarization modulated four-wave mixing(PMFWM) spectroscopy of thermal atoms.There are five chapters in this thesis:Chapter 1:Introduction.The development of laser spectroscopy,electromagnetically induced transparency(EIT) and four-wave mixing(FWM) is discussed in this chapter.Chapter 2:Basic theory.In this chapter,we mainly give some basic theory of absorption, dispersion,dipole approximation and the equation of density matrix elements.Chapter 3:High resolution spectroscopy of confined atoms.By designing an extremely thin cell(ETC) with two parallel dielectric planes,high resolution spectroscopy can be achieved.In such a tnin cell,the dilute atomic vapor is confined,the average free path of atoms corresponding to the shape of the cavity is anisotropic.Atoms flying along the surface of the plane do not contribute to the Doppler broadening,while atoms flying to or away from the surfaces interact with the light in a transient regime.Thus one-photon sub-Doppler absorption and selective reflection spectroscopy(Dicke-narrowing spectroscopy) are achieved for many cases.In a two-photon situation,much higher resolution Dicke-narrowing spectroscopy can be obtained as Doppler free configuration is satisfied.It is feasible to implement an experiment instead of a cold atomic system for high resolution spectroscopy.Chapter 4:Electromagnetically induced transparency(EIT).Doppler free can be achieved due to de-excited and slow thermal atoms in ETC.Stationary atoms in a ground state evolve into the population of another ground state induced by the probe laser as the pump fields are applied to the system.When a pump laser is applied to the system,the dipole from the coherent population has the same magnitude and opposite sign for the probe laser applying to only.Thus EIT is interpreted as ac-Stark splitting and quantum interference of two dressing states.The high resolution dips of EIT and the effects of supper slow group velocity occur due to the effect of slow multi-level atoms in ETC.Chapter 5:Polarization modulated four-wave mixing(PMFWM).FWM signal propagates with less loss in thermal atomic cell in the windows of EIT,thus high efficient FWM signal is achieved.EIT,especially FWM signal can be modulated by the polarization of the probe and the pump fields.The magnitude of FWM signal is larger than other situation as the horizontally polarized probe beam and the vertical polarized pump beam are applied to the system.The strength of FWM signal can be changed through different polarization of the probe or the pump fields.Comparing with FWM without dressing fields(NDFWM),the strength of the singly-dressing FWM(SDFWM) signal is suppressed when the modulation-field is linear or elliptically polarized.In doubly-dressing FWM(DDFWM) situation,the suppression occurs in all polarized situation of the modulated fields.We also find that both the strength and the polarization of FWM signal can be modulated by the polarization of the dressing fields,and thus FWM signal is successfully suppressed or enhanced by the methods of polarized modulation.It is similar to the situation of a macroscopic cell,ac-Stark(Autler-Townes(AT)) splitting and the coherent effect from the dressing state created by the pump laser in doubly-dressing EIT(DDEIT) and singly-dressed FWM(SDFWM) are observable in ETC. The enhancement of the contribution from slow atoms in dark state and the increase of coherent relaxation time of ground state atoms result in multi-dip EIT lines in multi-level confined atoms.Dicke-narrowing appears both in DDEIT and SDFWM lines due to Doppler free configuration and the de-excited effects of atoms in ETC.Suppression and enhancement of FWM suggest that multi-wave mixing(MWM) can be controlled.It is possible to apply ETC experiment to the design of nonlinear optical devices,wavelength conversion and quantum information processing.Finally,we give a summarization of the whole thesis and some of the possible new research topics that followed,such as the theory and experiment of two-photon spectroscopy, FWM and atom-surface van der Waals interaction(vdW) in ETC.