Investigations On The Atomic Coherence Effects Induced By Standing-wave Fields

The manipulation of light via quantum coherence and interference effects in optical medium interacting with electromagnetic fields has been studied extensively both theoretically and experimentally. In the coherent fields, the bare state of the atom splits into two dressed states which refers to Altler-Townes splitting or AC-Stark effect, then quantum interferences occur between the multichannel in the absorption or spontaneous emission. The atomic coherence effect can efficiently modify the optical properties, such as absorption, refraction and spontaneous emission, and has many applications in precision spectroscopy, nonlinear optics, quantum optics, quantum information and quantum control. Usually, traveling-wave fields are used in the investigations on atomic coherent effect. However, when the coherent field has a standing-wave (SW) pattern, both the probe absorption and refraction are modulated periodically along the direction of the standing-wave. Therefore, it can show many fascinating phenomena that are different from the traveling-wave driven schemes. Based on the standing-wave induced atomic coherence effects, electromagnetically induced double photonic band gaps (D-PBG), electromagnetically induced grating (EIG), and two-dimensional atom localization are investigated in this thesis.1. In an SW driven system with double-dark resonances, the absorption and refraction of the atomic media are periodically modulated along the direction of SW, then the probe field propagates as in a one-dimensional photonic crystal. When the Bragg condition is fulfilled, well developed D-PBG can be opened up in the two transparency windows. In the microwave coupling scheme, the coherently induced D-PBG with high reflectivity can be obtained, on the side of the D-PBG there are two regions with high transmissivity. Theses features can be used to control the propagation of two light pulses with different centre frequencies simultaneously. In the N-type system, we investigate the effect of interacting double-dark resonances on the position and width of D-PBG. By changing the Rabi frequencies and detunings of the fields, the structure of D-PBG can be tuned efficiently. In the coherently driven Fe=0(?)Fg=1 transition, the PBG structure depends on the magnitude of the magnetic field. In the absence of the magnetic field, single dark resonance is obtained which corresponds to single PBG. While in the presence of the magnetic field, single dark resonance splits into double ones, hence D-PBG occurs. By control the magnetic field, we can simultaneously control the flow of two light pulses with different center frequencies, and therefore double-channel magneto-optical switching and routing can be realized. Compared with single PBG, D-PBG has significant advantages in the quantum control processes. We expect it may has applications in the trapping of two light pulses, i.e. stationary light pulses, and enhancing their nonlinear interaction which can be exploited to quantum logic.2. An EIG scheme is proposed in an SW driven four-level atomic system with two closely lying intermediate states. By investigating the third order nonlinearity of the four-level ladder-type atomic system, it is found that, in the presence of spontaneously generated coherence (SGC), the nonlinear absorption or refraction can be significantly enhanced with linear absorption suppressed. We attribute the enhancement of nonlinearity to the quantum interference effect in the two decay pathways from the two intermediate closely lying levels to the ground level. When the trigger field is resonant, the two-photon absorption is enhanced due to the constructive quantum interference. With the large off-detuned trigger field, the cross Kerr-type refractivity is enhanced with neglectable nonlinear absorption. Therefore, when the trigger field has an SW pattern, absorption-or phase-grating, which effectively diffracts a weak probe into high-order direction, can be induced by the SGC enhanced absorptive or refractive nonlinear modulation. In the phase grating, the maximal diffraction efficiency is 31% which is close to the idea sinusoidal phase grating. The EIG can diffract light into different directions, and has applications in all optical switching and routing.3. In recent years, high-precision position measurement of an atom has been attracted many interests in laser physics and quantum optics. Optical methods provide high precision and resolution. Several atom localization schemes have been proposed using the SW light field due to the position-dependent atom-field interaction. Therefore, the resultant position-dependent spontaneous emission and absorption can provide the position information of an atom when it is still in the light fields. In this thesis, we have proposed three schemes for two-dimensional (2D) subwavelength atom localization, in which the atom interacts with two orthogonal SW lasers. Generally, in a wavelength domain, the interaction between the SW fields and the atom is the same in the four quadrants of the SW plane. Then when a spontaneous emission photon is detected or the probe field is absorbed, the conditional position distribution of the atom is the same in the four quadrants, and the maximum probability of finding an atom at a particular position is 1/4 when the atom localizes at (k1x,k2y)=(±π/2,±π/2). However, an improvement by a factor of 2 in the detecting probability of an atom can be achieved by initially preparing the atom in a superposition state or using the two standing-waves to couple one transition, in such cases the atom localizes at the centers of I and III (II and IV) quadrants. Qualitatively, the high precision and resolution of atom localization can be attributed to the quantum interference effect. It has potential applications in the atom lithograph.