Coherent atomic states prepared by laser field can have quantum interference between the different transition amplitudes. Therefore, the medium susceptibility and optical response can be engineered, leading to many interesting phenomena, such as coherent population trapping (CPT), electromagnetically induced transparency (EIT), and lasing without inversion (LWI). We studied the coherence effects in various prototype atomic systems, and found many interesting applications. We solved the slow light bandwidth problem by decomposing the pulse and matching each frequency to its EIT window using a magnetic field gradient. We also considered the probe field deflection induced by the driving field distribution in EIT, and showed that even a broadband pulse can be deflected without serious spreading. In the fast light area, we examined the effects of noise and parameter deviations in a bichromatic Raman type white light cavity. Taking advantage of the adjustable absorption of EIT, we showed that EIT in a laser cavity can have either first-order or second-order phase transitions. Last but not least, we show that the adiabatic population transfer can be used to reverse the weak measurement of an arbitrary field with finite photon number.

Coherent effects are studied in a dense atomic vapor driven by laser fields. With optical properties dramatically modified by these effects, the medium can be used to manipulate some of the properties of laser field. Our experiments demonstrate the coherent control over transmission, spatial distribution and noise feature of the laser field interacting with coherent media. The results have potential applications in the field such as precision metrology, precision spectroscopy, optical imaging and lithography. We develop an experiment to investigate the atomic excitation by few-cycle radio frequency (RF) pulses interacting with Zeeman sublevels. The system provides the flexibility to fully control all parameters of RF pulses. Such a flexibility can not be achieved in optical domain. Based on this system, experiments can be conducted to simulate processes in ultra-short laser physics. In particular, we study the carrier-envelope effect of few-cycle pulses and the strong off-resonant excitation by short pulses. We also discuss the selective reflection spectrum on a highly dense atomic vapor in which the dipole-dipole interaction can not be neglected. The spectrum broadening due to dipole-dipole interaction is much broader than the Doppler broadening. Our experiments show that the excitation by a pump laser can reduce the dipole-dipole interaction, thus reduce the broadening and improve the spectral resolution. The excitation dependence is studied at various atomic densities.

This thesis presents the development of photonic microcells for use as the host for coherent optics phenomena and related applications. A photonic microcell consists of a length of hollow-core photonic crystal fibre (HC-PCF) with a gas-filled core that is spliced to conventional optical fibre at either end to seal the gas within the fibre. Towards the goal of demonstrating and assessing the coherence properties of quantum optical effects in photonic microcells, the fabrication of two types of HC-PCF is presented. The established photonic bandgap HC-PCF offers extremely low transmission loss of ~10 dB/km over kilometre distances. However, the fibre has a limited transmission bandwidth of ~50 THz and exhibits modal coupling unfavourable for many applications. Work is presented on the tailoring of this fibre by control and shaping of the core-surround in order to improve its modal properties. A second type of HC-PCF is based on a large-pitch lattice, whose guidance relies on a new mechanism. This fibre exhibits a much improved bandwidth (>1000 THz) and has a relatively higher but still practical loss of ~1 dB/m. The development of photonic microcells at microbar pressure level and with low optical insertion loss is shown, an important step in the improvement of the technology for coherent optics applications which will take advantage of the extreme gas-laser interaction efficiency achieved in HC-PCF. Finally, quantum optical effects are demonstrated in HC-PCF and photonic microcells loaded with both the molecular gas acetylene and atomic vapour rubidium. The observation of electromagnetically induced transparency (EIT) in acetylene-filled HC-PCF represents the first such observation in a molecular gas, while the use of a photonic microcell allows a comparison of many experimental configurations to explore the coherence properties of coherent optical systems in the core of a HC-PCF. Furthermore, EIT is observed unambiguously in a rubidium loaded HC-PCF for the first time, and the anti-relaxation effects of a polymer coating demonstrated in this configuration.

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This thesis explores the theoretical foundations of cross-phase modulation (XPM) between optical fields in the N-configuration atom. This is the process by which the refractive index experienced by one field can be modulated by controlling the intensity of another. The electro-optical version of this effect was first discovered by John Kerr in 1875 and found applications in photonics as a means of very rapidly modulating the phase and intensity of electromagnetic fields. Due to recent advances in experimental techniques there has been growing interest in generating nonlinear optical interactions in coherently prepared atomic ensembles. The use of coherently prepared media brings the possibility of achieving a much larger cross-phase modulation than is possible using classical materials. This is particularly useful when trying to create large optical nonlinearities between low-intensity electromagnetic fields. Much of the current research into cross-phase modulation is directed towards realising potential applications in the emerging field of quantum information processing. Above all, the possibility of constructing an all-optical quantum computer has been at the heart of much research and controversy in the field. In this thesis the theory of steady-state, transient and pulsed cross-phase modulation is developed. Moreover, care has been taken to relate all research back to experimentally feasible situations. As such, the relevance of the theory is justified by consideration of the situation present in rubidium-87. Due to the close relationship between XPM in the N-configuration atom and electromagnetically induced transparency in the Lambda-atom, many similarities and insights act as link between these two fields. Indeed, it is frequently demonstrated that the key to understanding the various properties of XPM in the N-configuration atom is by comparison with the situation in the corresponding Lambda-atom equivalent.

Laser-atom interactions create atomic coherence and large nonlinear atomic polarization. We investigate resonant laser-atom interactions to generate large nonlinearities and control them using magneto-optical fields. Coherent control of high-order susceptibilities and magneto-optical rotation are demonstrated. Experiments are supported by theoretical studies that effectively describe the observed phenomena. It is shown that a new coherent field, with polarization orthogonal to a weak signal field, can be parametrically generated via an all-resonant four-wave-mixing process. This is demonstrated in a double-ladder system having two intermediate states between a ground and an excited state. It is shown that the parametricgeneration process can be coherently controlled by coupling lasers and magnetic fields. It is theoretically established that the underlying physics is a resonant three-photon process with a wide domain of control parameters. Electromagnetically induced transparency (EIT), where absorption of a weak probe is suppressed via quantum interference, is demonstrated in a usual three-level ladder system. It is observed that in contrast with EIT in a usual ladder system, addition of a second channel helps to suppress the absorption of two weak probe fields in the double-ladder system. The resulting enhancement of transmission in two different channels is due to gain caused by three-photon processes. Coherent control is strongly limited by coherence lifetime, which is the inverse of the dephasing rate. A lambda-system, having two ground states coupled to a common excited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing perturbs the dark state. A new method for measuring the ground-state dephasing rate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated. While two laser fields in a lambda-system are resonant with their respective transitions, the atomic polarizations are very sensitive to an applied magnetic field. This effect can be used for optical magnetometry. The degree of sensitivity of the magnetometer is determined by two competing parameters–atomic density and laser intensity. It is shown experimentally that the optimal sensitivity reaches saturation, which is contrary to the idea that sensitivity increases indefinitely with an increase in the above parameters.

Coherent nonlinear effects such as Electromagnetically Induced Transparency (EIT), Coherent Population Trapping (CPT), and Slow light are studied in thermal Rb vapor by both continuous and pulsed laser fields. This work primarily includes three parts: (I) mode-locked rubidium laser and its applications (II) enhanced coupling between optical and sound waves in the forward direction via ultra-slow light (III) optical steering via ultra-slow light in rubidium vapor. In part(I), I describe the construction and study of a mode-locked rubidium laser operating at the Rb D1 line using an active mode-locking technique inside the laser cavity. The mode-locked laser field is used to observe coherent effects in a dense rubidium gas. In part(II), I experimentally demonstrate enhanced acoustic-optic coupling that occurs when the velocity of sound is close to the group velocity of light. Dragging of the light by effective motion of the gas in a Rb cell is the origin of enhanced coupling. Good agreement between theory and experiment is found. In part(III), I experimentally demonstrate optical beam deflection in coherently driven rubidium vapor due to the steep refraction index profile in the region of EIT.

Quantum coherence and interference(QCI) effects have been studied for decades and are widely exploited in many areas. For media with QCI effect, the optical properties can change drastically, which leads to many interesting effects, such as coherent population trapping, electromagnetically induced transparency(EIT), lasing without population inversion(LWI) and so on. We have theoretically studied the pulsed regime of EIT. In particular, simulations of propagation of gaussian and 0 - pi co-propagating laser pulses in a medium consisting of 3-level Lambda-atoms have been performed. It has been found that, even at the two-photon resonance, the length of propagation for the 0 - pi pulses is much smaller than that for the Gaussian probe pulses. We explained such a behavior using the dark and bright basis and the dressed state basis. Some possible applications are discussed. We also investigated the collision-induced coherence of two decay channels along two optical transitions. Quantum interference will suppress the spontaneous emission. The degree of this suppression is measured by the branch ratio of these two transitions. Our preliminary calculations show that a significant decrease of the branching ratio with increase of electron densities is reproduced in the theory. We have developed a new variant of Raman spectroscopy with shaped femtosecond pulses. It has several advantages to be applied in multiscatterd media. It is based on change of the spectra of femtopulses due to Raman scattering (stimulated or coherent). The technique can be used for a broad range of applications from atomic and molecular optical and IR spectroscopy to spore detection and tissue microscopy. Finally, we have shown that Fano interference in the decay channels of three levels system can lead to considerably different absorption and emission profiles. We found that a coherence can be built up in the ground state doublet whose strength depends on a coupling parameter that arises from Fano interference. This can in principle lead to breaking of the detail balance between the absorption and emission processes in atomic systems.

In this work, interference and coherence phenomena, appearing in atomic and molecular ensembles interacting with coherent light sources, as electromagnetically induced transparency (EIT), coherent population trapping (CPT), and slow group velocity of light are investigated. The goal of the project is to make the steps towards various applications of these phenomena, first, by studying them in solid media (which are the most advantageous for applications), second, by suggesting some novel applications such as CPT-based plasma diagnostics, and realization of new types of solid-state lasers (based on suppression of excited-state absorption via EIT). The third goal of the project is extension of coherence and interference effects well-known in optics to the gamma-ray range of frequencies and, correspondingly, from atomic to nuclear transitions. A particular technique of chirped pulse compression applied to M??ossbauer transitions is considered and the possibility of compression of M??ossbauer radiation into ultrashort gamma-ray pulses is analyzed. The theoretical treatment of the interference and coherence effects is based on the semiclassical description of atom-light interaction, which is sufficient for correct analysis of the phenomena considered here. Coherent media are considered in two-, three-, and four-level approximations while their interaction with light is studied both analytically and numerically using the Maxwell-Bloch set of equations.