The two-photon-driven can be used to build up the atomic coherence since the Yb+3 has the simple two-level structure.Atomic transitions can take place in many Stark sublevels because of the characteristics of its energy level with up-and-down multi-state structure.The coherent overlap of these atomic transitions can determine the nonlinear absorption and dispersion in the medium and the results of the coherent overlap depend on the Rabbi frequency of driving field.The results show that by changing the Rabbi frequency of driving field,we can effectively control the existence of bi-stable or multi-stable system and threshold level of intensity.
The two-photon-driven can be used to build up the atomic coherence since the Yb+3 has the simple two-level structure.Atomic transitions can take place in many Stark sublevels because of the characteristics of its energy level with up-and-down multi-state structure.The coherent overlap of these atomic transitions can determine the nonlinear absorption and dispersion in the medium and the results of the coherent overlap depend on the Rabbi frequency of driving field.The results show that by changing the Rabbi frequency of driving field,we can effectively control the existence of bi-stable or multi-stable system and threshold level of intensity.
The resonant absorption, emission, and scattering of light are the fundamental optical processes that have been used both to probe matter and to manipulate light itself. In the last decade there has been essential progress in coherent control of both linear and nonlinear optical responses based on resonant excitation of atomic coherence in multilevel quantum systems. Some interesting and useful phenomena, resulting from coherent control of absorption and the group index, such as electromagnetically induced transparency, lasing without inversion, and ultra-slow group velocity of light have been widely studied. This work is focused on coherent control of refractive index and resonant fluorescence in multilevel medium. \n \nWe suggest two promising schemes for resonant enhancement of the refractive index with eliminated absorption and propose their implementation in transition element doped crystals with excited state absorption and in a cell of Rb atoms at natural abundance. We show how to use one of these schemes for spatial variation of the refractive index via its periodic resonant increase/decrease, remarkably keeping at the same time zero absorption/gain. It opens the way to production of transparent photonic structures (such as distributed Bragg reflectors, holey fibers, or photonic crystals) in a homogeneous resonant atomic media such as dielectrics with homogeneously distributed impurities, atomic, or molecular gases. These optically produced photonic structures could easily be controlled (including switching on/off, changing amplitude and period of modulation) and would be highly selective in frequency, naturally limited by the width of the optical resonance. \n \nWe also derive the optical fluorescence spectra of a three-level medium driven by two coherent fields at the adjacent transitions in a general case when all three transitions are allowed. We show that coherent driving can efficiently control the distribution of intensities between the fluorescent channels. In particular, the total intensity of fluorescence at the transition which is not driven by the optical fields may essentially exceed the fluorescence intensity at the driven transitions under the condition of two-photon resonance. This counter-intuitive effect is due to depletion of the intermediate state via atomic interference.
In this thesis, atomic coherence is used to enhance nonlinear optical processes in multi-level atoms. The multi-photon transitions are driven resonantly, and at the same time without absorptive losses, by using electromagnetically induced transparency (EIT), thereby allowing the study of χ(3) and χ(5) nonlinearities using weak driving fields. The coherently modified probe beam(s) and the atom-radiated signal fields arising from four- and six- wave- mixing (FWM and SWM) processes are measured in the spectral, temporal and spatial domains. In a three-level ladder-type atomic system, multiple peaks having spectral asymmetries are observed in the EIT window as well as in the FWM signal waveforms due to the diverse multiplicities of the participating atomic states. Using phase control tailored in the frequency domain, we demonstrate all-optical methods to control these spectral waveforms and discuss applications involving waveform-shaping and metrology. For the EIT study we demonstrate a switching of multiple dark peaks into bright peaks via phase-control of interferences in the underlying dark-states. In the FWM study we demonstrate all-optical spectral line shape symmetrization, linewidth narrowing and bandwidth switching. In a four-level inverted-Y-type atomic system, we drive and measure coexisting and phase-matched FWM and SWM signals. By using precision control of the relative phase and amplitude between these two processes of different nonlinear orders, we demonstrate phase coherence between them. First, a single-phase measurement is performed in the temporal and spatial domains, and the interferogram is used to measure the resonant frequency of the 5D5/2-5P3/2 atomic transition in 85Rb. Second, the method is extended to realize a capacity for two-phase measurement. In this case, the spectral bandwidth of the signal is modified in order to measure the phase-shift occurring in one Mach-Zehnder interferometer, while the intensity of the total signal waveform measures the phase-shift occurring in a second interferometer. Finally, we demonstrate phase-dependent spatial fusion between two ultra-weak optical fields by using a strong coupling field to first convert the weak fields into bosonic dark-state polaritons, which are then steered into a common all-optical waveguide mode arising due to the coupling field's intensity distribution and the resulting cross-Kerr refractive index gradient.
The atomic optical bistability (AOB) in multi-level systems has advantages over the two-level system as absorption, dispersion, and nonlinear optical properties of one optical field coupled to one atomic transition can be greatly modified by other optical fields coupled to other connected nearby atomic transitions in multi-level atomic systems due to induced atomic coherences. By making use of such changes in linear and nonlinear optical properties around resonance, which are related to the electromagnetically induced transparency (EIT), it is easy to manipulate and control nonlinear optical processes in the multi-level atomic systems. We review earlier two-level AOB theory, as well as experiments and some recent studies that made use of multi-level atomic EIT systems in achieving controls of nonlinear optical processes such as instabilities and stochastic resonance associated with the coupled atom–cavity systems.
In this paper, two-photon absorption resonance has been investigated . We studied this phenomenon in three-level system with common upper level, two lower long-life levels and two quasi-resonant laser fields with different frequency spectrum. It has been shown that, the absorption coefficient in the far distance from absorption line at the frequency of two-photon resonance has considerable amplitude and narrow width. Frequency difference between two lower levels is transition frequency for optical clocks. This resonance is much less sensitive to the effects of noise components and probing fields compared with electromagnetically induced transparency. Stark shift of the two-photon resonance can be offset and reduced by the changes in the probing field amplitude. We recommended to use an additional laser with narrow line to stabilize the frequency at optical clock to get a line in sub-Hertz range.
A detailed analytical description of a Doppler-free probe response (absorption and dispersion) is presented for a coherently prepared four-level atomic tripod driven by two coupling fields. Under the density matrix formalism the field-dependent and field-independent coherence are introduced through the off-diagonal matrix elements. Field-dependent coherence appears due to the interference between the probability amplitudes of finding the atom in energy levels associated with the allowed transition, while field-independent phases measure the atomic coherence for a coherently prepared lower state triplet. The double-control coherent effects on the probe response are investigated by controlling the relative driving contribution (coherence) of the two coupling fields, whereas the driving contribution can be controlled by the use of field-independent coherence, along with two coupling parameters like Rabi frequency and detuning of the respective coupling field. We report on the possibility of modifying the Rabi splitting into a perfect electromagnetically induced transparent window at probe resonance by a coherent double-control mechanism in an on-resonance coupling situation. Moreover, for off-resonance coupling, we exhibit the enhancement of multiphoton (two- or three-photon) absorption via coherent control of multiresonance processes.
A method of laser control of absorption and dispersion of two-level quantum system (atoms, ions, quantum dots) embedded into photonic band gap (PBG) material via variation of intensity and frequency of an external laser field is proposed. It is shown that in order to provide an effective control, the frequency of atomic transition should be located at the photonic band edge near the bottom of the PBG. The method is based on the laser induced Mollow splitting of the atomic energy levels and strong dependence of the decay rates of the so-called dressed states on the frequency of the Mollow splitting due to photonic band edge. This enables us to control absorption and dispersion of a signal laser field, which is near-resonant to the quantum transition of dopant.