Theoretical Studies On Spontaneous Emission And Related Properties In Quantum Coherent Media

The spontaneous emission refers to the atom which is in the excited state spontaneously de-cays to the lower level process. In daily life, the light of various light sources, such as incandescent lamps, fluorescent lamps, neon lights, and fluorescent materials, originated from the spontaneous emission. And the spontaneous emission has an important influence on many physical processes and practical applications. For example, in semiconductor lasers, spontaneous emission is the major sink for threshold current, which must be surmounted in order to initiate lasing. As a result, ef-ficient control and modification of spontaneous emission based on atomic coherence and quantum interference has attracted much attention in quantum optics, mainly because of its wide and potential applications in lasing without inversion, coherent population trapping, transparent high-index mate-rials, high-precision spectroscopy and magnetometry, quantum information and computing, and so on. It has been demonstrated that spontaneous emission from an excited atom depends not only on the properties of the atomic system but also on nature of the surrounding environment. The usual way to modify the spontaneous emission of an atom is to place it in different circumstances, such as in the free space, an optical cavity, an optical waveguide, a photonic-band-gap (PBG) material, or an otherwise modified reservoir. In this thesis, we mainly investigate the control and modification of the spontaneous emission and optical properties of the atom in the free space and in the photonic crystals (PCs). Moreover, we realize the high-precision and high-resolution two-dimensional atom localization via the controlled spontaneous emission and the probe absorption measurement. The main contents are as follows:(1) The spontaneous emission properties of radio-frequency (RF)-driven multilevel atomic sys-tems with three different configurations are investigated in details. It is demonstrated that, due to the presence of the RF-induced coherence in the multilevel system, a few interesting phenomena such as spectral-line narrowing, spectral-line enhancement, spectral-line suppression, and fluorescence-quenching can be realized under realistic experimental conditions. By inspecting single-RF-driven, double-RF-driven, and triple-RF-driven atomic systems, we find that (i) when only one RF-driven field is applied, there are two fluorescence-quenching points and four spectral lines;(ii) in the case of applying two RF-driven fields, there arc three fluorescence-quenching points and five spectral lines;(iii) when applying three RF-driven fields, there arc four fluorescence-quenching points and six spec- tral lines; and (iv) as expected, when the atomic system coupled by N RF-driven fields, there will be N+1fluorescence-quenching points and N+3spectral lines. Interestingly enough, the spectral-line enhancement, the spectral-line suppression, and the selective cancellation of fluorescence-quenching can be well controlled just by appropriately modulating the intensities and frequencies of the applied RF fields, respectively. The proposed schemes can be achieved by use of RF-driven fields into hy-perfine levels in rubidium atomic systems. These investigations may have potential applications.(2) The spontaneous emission behaviors of a double A-type four-level atom and an M-type five-level atom embedded in a PC with anisotropic dispersion relations arc investigated. There exist two types of quantum interference:reservoir-induced interference and laser-induced interference. It is shown that we can control the behavior of the spontaneous emission by adjusting the density of states (DOSs) of the PBG reservoir and the intensity of the laser fields, the modification of the emission spectra originates from the quantum interference effects and the control of external laser fields. These theoretical investigations may provide more degrees of freedom to manipulate the atomic spontaneous emission.(3) The spontaneous emission and optical properties of an RF-driven five-level atom embedded in a three-dimensional PC are discussed by considering the anisotropic double-band PBG reservoir. In our proposed model, the two transitions are, respectively, coupled by the upper and lower bands in such a PBG material, thus leading to some different phenomena. These investigations show that the appearance of the spontaneous-emission enhancement, suppression, and the spectral-line narrowing, as well as the absorption-dispersion properties of a probe laser field depends strongly on the RF-induced quantum interference and the DOSs of the PBG reservoir.(4) It should be pointed out that when an artificial defect is introduced into the PC by disturbing the crystal’s periodicity, an artificial cmission channel can be generated in the PBG region. As a result, we can easily control the emission spectra and optical properties of nanoparticles which are placed in such a PC structure. If the transition frequency matches the frequency of the defect modes, the rate of spontaneous emission can be enhanced. Some interesting phenomena such as absorption, transparency, normal or anomalous dispersion, and large amplification of a weak probe field can be observed in the optical spectra by modulating system parameters.(5) The schemes are proposed for two-dimensional (2D) atom localization in different atomic systems via controlled spontaneous emission and probe absorption measurement. The interaction of the atom with space-dependent standing-wave fields can provide information about the position of the atom passing through, thus leading to atom localization. We find that when the two orthogonal standing-wave laser fields are applied to couple the same atomic transition, the atom can be localized at a particular position by properly varying the system parameters. Thus, high-precision and high-resolution2D atom localization is, indeed, achieved.This thesis deepens our awareness and understanding of spontaneous emission and related properties in quantum coherent media, and helps us to further control the atomic spontaneous emission and optical properties. The results of this thesis may provide more degrees of freedom to manipulate the atomic spontaneous emission. Moreover, we obtain high-precision and high-resolution2D atom localization via the controllable spontaneous emission and the measurement of the probe absorption. These investigations may have many potential applications in optical communications and in the fabrication of novel optoelectronic devices, as well as laser cooling and trapping of neutral atoms, atom nanolithography, Bose-Einstein condensation.