Investigation Of EIT-ATS Crossover And Propagation Properties Of Light In Inhomogeneous Broadened Media

In recent years, much attention has been paid to the study of quantum coherent and incoherent phenomena in various multi-level systems, typical ex-amples include Auter-Townes splitting (ATS) and electromagnetically induced transparency (EIT). Such phenomena are not only important from viewpoint of basic research, but also very attractive for many practical applications, such as lasing without inversion, coherent population transfer, enhanced Kerr nonlinear-ity, slow light, quantum memory, atom and/or photon entanglement, precision spectroscopy, precision measurement, and so on.EIT effect means that the probe field can pass through a resonance absorp-tion medium via the quantum destructive interference induced by the control field. In addition, EIT can largely enhance the dispersion and nonlinear effects due to resonance interaction. The physical mechanism of ATS is ac-Stark effect, in which the absorption spectrum displays as a doublet Lorentzian line due to the change of the energy level in present of the additional field. Since the discov-ery of ATS effect, it has been widely used in atomic and molecular spectroscopy detection and other fields.EIT and ATS can be realized in similar or even same system, and the ab-sorption spectrum profiles of EIT and ATS are also similar, i.e. transparency window in absorption spectrum around the resonance frequency, but the physi-cal mechanism of EIT and ATS are completely different. Thus, the opening of the transparency window can not tell us the phenomena belongs to EIT or ATS, this has resulted in confusion on the people to understand EIT and ATS. People also tend to distinguish EIT and ATS depends on the experience, mostly qualitative analysis, which is no rigorous. Furthermore, during the research of EIT, A-, V-and Ladder-types three level systems are generally considered to have the same geometric topology nature, and can be solved by similar theoretical approach, thus, many people think that the quantum interference effect in those systems should be the same. But with new discoveries in both experimental and theoret-ical study, more and mere people realized that the quantum interference effect in EIT systems with different configurations should be different. With the advent of many contradictory views, people need to theoretically analyze the differences and relations between EIT and ATS, and EIT in several typical configuration systems.Recently, study on objectively discerning of EIT and ATS in cold atomic systems, and quantum interference effect in different configuration EIT systems has made considerable progress. However, in many systems which have prac-tical applications, inhomogeneous broadening must be taken into account, such as hot molecular and solid material systems. Experimental research has shown that the absorption spectrum of the probe field may be largely changed when the inhomogeneous broadening of the system can not be neglected, the physical reason is that the present of inhomogeneous broadening affects or even destroy the quantum interference effect in the system. In this situation, the quantum interference effect may have more interesting characteristics depending on differ-ent configurations. However, analysis of the quantum interference effect in such kind of systems are still absent, many phenomena measured in experiment need theoretical explanations.In this dissertation, we develop a systematic analytical spectrum decompo-sition method to analyze the quantum interference effect in EIT and ATS. Based on this method, we systematic detailed analyze the quantum interference effect in open inhomogeneous broadening A-, V-and Ladder-types molecular systems, and explicitly proposed the concept of EIT-ATS crossover, our work includes the following aspects:1, Study on EIT-ATS crossover and linear and nonlinear propagations of light in an open inhomogeneous broadening A-type molecular system. In linear case, by using residue theorem and a spectrum decomposition method, we prove that there exists a crossover from EIT to ATS for co-propagating configuration of probe and control fields. However, there is no EIT and hence no EIT-ATS crossover for counter-propagating configuration. We give various explicit formu-las, including probe-field spectrum decomposition, EIT condition, width of EIT transparency window, as well as a comparison with the result of cold molecules. Our analytical result agrees well with the experimental one reported recently. In nonlinear case, by using the method of multiple-scales, we derive a nonlinear envelope equation for probe-field propagation. We show that stable ultraslow solitons can be realized in the open A-type molecular system. This research has many potential applications in the field of coherent molecular spectroscopy, pre-cision measurement, molecular quantum state manipulation and nonlinear pulse propagation.2, Study on EIT, ATS and EIT-ATS crossover in an open inhomogeneous broadening V-type molecular system. Through detailed analytical calculations on the absorption spectrum of probe laser field by using residue theorem and spec-trum decomposition, we find that EIT may occur and there exists a crossover from EIT to ATS (i.e. EIT-ATS crossover) for hot V-type molecular system. Howev-er, there is no EIT and hence no EIT-ATS crossover for cold V-type molecular system. Furthermore, we prove that for hot V-type molecular system EIT is allowed even for a counter-propagating configuration due to the contribution of Doppler effect. We provide explicit formulas of EIT conditions and widths of EIT transparency windows of probe field when hot molecules work in co-propagating and counter-propagating configurations, respectively. In addition, we clarify the roles of optical hole burning and saturation effect in the system. Our theoretical result agrees well with the recently reported experimental result.3, Study on EIT-ATS crossover in open inhomogeneous broadening ladder-type systems. We show that when the wavenumber ratio kc/kp≈-1, EIT, ATS, and EIT-ATS crossover exist for both ladder-Ⅰ and ladder-Ⅱ systems, where kc (kp) is the wavenumber of control (probe) field. Furthermore, when kc/kp is far from -1 EIT can occur but ATS is destroyed if the upper state of the ladder-Ⅰ system is a Rydberg state. In addition, ATS exists but EIT is not possible if the control field used to couple the two lower states of the ladder-Ⅱ system is a microwave field. The theoretical scheme developed here can be applied to atoms, molecules, and other systems (including Na2 molecules, and Rydberg atoms), and the results obtained may have practical applications in optical information processing and transformation.The results presented above are not only conductive to the development of theoretical research on quantum interference effect, but also provide theoretical guidance on parameters selected in experimental research of atomic and molecular spectroscopy. In addition, we also make some progress in the realization of linear and nonlinear stable propagation of surface polaritons in dielectric-negative index metamaterial interface via gain mechanisms, our work includes the following aspects:4, Study on the possibility of generating nonlinear surface polaritons at the interface between a negative-index metamaterial and a dielectric doped with two-level quantum emitters by means of coherent population oscillation (CPO) working at room temperature. Based on the CPO and the strong confinement of optical field near the interface, we find that the system can possess a giant Kerr nonlinearity. We demonstrate that it is possible to obtain a new type of surface polaritonic solitons which are not only robust during propagation but also have ultraweak generation power and superluminal propagating velocity.5, Study on the realization of stable propagations of linear and nonlinear surface polaritons (SPs) by placing a N-type four-level quantum emitters at the interface between a dielectric and a negative-index metamaterial (NIMM). We show that in linear propagation regime SPs can acquire an active Raman gain (ARG) from a pump field, and a gain doublet appears in the gain spectrum of signal field induced by the quantum interference effect from a control field. The ARG can be used not only to completely compensate the Ohmic loss in the NIMM but also to acquire a superluminal group velocity for the SPs. We also show that in nonlinear propagation regime a huge enhancement of Kerr nonlinearity of the SPs can be obtained. As a result, ARG-assisted (1+1)- and (2+1)-dimensional superluminal surface polaritonic solitons with extremely low generation power may be produced based on the strong confinement of electric field at the dielectric-NIMM interface. The results presented here may have promising applications in light information processing and transmission at nanoscale level based on the NIMM.The research methods proposed in this dissertation and the theoretical find- ings presented above have a certain significance in the development of quantum interference and nonlinear optical effects theory in multi-level quantum systems, and also may have promising applications in precision spectroscopy of science and technology, optical information processing and transmission, quantum computing and quantum information, and so on.