Investigation Of Effects In Thermal Atomic Ensemble And All Optical Switching

In the Past decade, researchers have found that the optical properties of the atomic media can be dramatically altered by the optically inducing the atom into a coherent superposition. An otherwise opaque medium on a probe resonance is made highly transparent, accompanying with large susceptibility. The atomic medium also behaves large nonlinear susceptibility. In this thesis, the related nonlinear optical effects in thermal rubidium vapor are discussed systematically. And a fast optical switching based on the phase controlled quantum interference is implemented.1. Double dark resonance and large cross-Kerr nonlinearity in a four-level inverted-Y atomic systemSince the phenomenon of electromagnetically induced transparency (EIT) has been observed in a variety of three-level atomic systems, it has attracted wide interest and attention. Light manipulation techniques based on EIT allow us to well control the storage, generation, and interaction of classical and quantum light signals in atomic gases and solid samples through coherently enhanced optical nonlinearities with negligible absorption. EIT-based resonant four wave mixing and enhanced cross-phase modulation coefficient are also reported.The studies of these coherent effects are concentrated in theⅤ-type,Λ-type and (?)-type three-level atomic systems. With the development of the experimental and theoretical research, new requirements of multi-level quantum interference effect are presented, because of the potential applications in bipolar quantum well laser and the quantum computers, the phenomenon of double dark resonance has been given even more attention.The electromagnetically induced transparency (EIT) and its dispersion properties in a four-level inverted-Y atomic system are investigated. The absorption spectrum of a weak probe field shows two EIT windows (dark resonances) whose location, width, and depth can be controlled by manipulating the parameters of the coupling fields; the corresponding dispersion properties are also measured by using a Mach-Zehnder interferometer. This kind of system can find important applications in two-channel quantum communication and information storage.Further more, we also studied the EIT enhanced Kerr nonlinearity with reduced linear absorption in a four-level inverted-Y atomic scheme. When detunings of the coupling and control fields are appropriately set, an enhanced EIT window is observed, and the induced phase shift of the probe field due to cross-phase modulation (XPM) is obtained by measuring the dispersive property of the probe transition. The maximal XPM phase shift is about 12°under the current experimental conditions. The experimental measurements agree well with the theoretical calculations. This work could be useful for future experiments on the generation of crossed-Kerr nonlinearities in multilevel systems. The enhanced XPM phase shift in such an atomic system has applications in quantum nonlinear optics and quantum information science.2. Measurement of coherence dynamics based on coherent anti-Stokes Raman scatteringThe atomic coherence has great significance in basic research of quantum optics and the practical applications, several methods has been proposed to achieve atomic coherence, including the strong magnetic field induced atomic coherence, the microwave field induced atomic coherence, Spontaneous Coherent Generation (SGC) and so on.In recent years, the establishment of atomic coherence via adiabatic passage attracted much attention. The adiabatic process techniques, such as stimulated Raman adiabatic passage, Stark-chirped rapid adiabatic passage, and piecewise adiabatic passage, are used to prepare superposition in atomic system. Under such a background, people start to focus on the temporal evolution of the coherence in the system undergoing adiabatic process.In this part, by using STIRAP and fractional STIRAP, a time-dependent coherence is prepared and indirectly monitored by the generated coherent Raman scattering (CARS) signal, the experiment results fit very well with numerical simulations. It is demonstrated that the CARS can be applied to measure the coherence dynamics between two particular levels during adiabatic passage. Such an efficient technique can be used to explore the hypostasis of coherence control adiabatic passage (CCAP), which has attracted much attention recently, and have potential applications in nonlinear process based on dynamical atomic coherence.3. Phase-dependent coherent population trapping and fast all-optical switchingWe demonstrate a new scheme for achieving phase-dependent coherent population trapping and a fast optical switching based on the phase controlled quantum interference is implemented. Two strong coupling lasers establish a coherent superposition (dark state) of two ground states in rubidium atoms.When the coupling lasers are turned off simultaneously, the amplitude and phase information is stored in the atomic coherence. Then the switch and probe pulses are turned on, the optical fields and atomic superposition form a quasi loop configuration, where the quantum interference is dependent on the relative phase of the whole ensemble.The relative phase of the system can be manipulated by adjusting the phase of any optical field coupling the atomic ensemble. So we choose the left circular polarized weak field as the switch field, the other weak field can be absorbed or transmit through the medium. The phenomena are considered as "on" and "off' state of the all optical switching. We experimentally realized this all-optical switching, and discussed the effects induced by the decoherence between the atomic ground state and four-wave mixing on the optical switching.The all-optical switching based on the phase dependent coherent population trapping has many advantages, for example, the switching speed is not restricted by the relaxation rates, there is no response time delay caused by slow-light propagation effect and the width of probe pulse can be in the nanosecond time domain without shape distortion, and the switch pulse can be achieved at an energy density of 10-2 photon per atomic cross sectionλ2/2π, The all-optical switching will have potential application in high-speed optical communications and quantum information systems.