We investigate local-field effects in nonlinear optical materials composed of two species of atoms. One species of atom is assumed to be near resonance with an applied field and is modeled as a two-level system while the other species of atom is assumed to be in the linear regime. If the near dipole-dipole interaction between two-level atoms is negligible, the usual local-field enhancement of the field is obtained. For the case in which near-dipole-dipole interactions are significant due to a high density of two-level atoms, local-field effects associated with the presence of a optically linear material component lead to local-field enhancement of the near dipole-dipole interaction, intrinsic cooperative decays, and coherence exchange processes.
We investigate the effect of a linear host medium on the dynamics of a dense collection of resonant atoms. We find that near dipole-dipole interaction is enhanced by the presence of the host dielectric, resulting in a lower threshold density and a greater hysteresis area for intrinsic optical bistability. Cooperative decay terms appear that represent the interaction of near dipoles mediated by the absorptive component of the dielectric function of the host medium. These terms are part of the local response and correspond to local cooperative decay effects. \textcopyright{} 1996 The American Physical Society.
It is well known that, for static fields, the local microscopic electric field EL, that couples to the atomic dipole moment p, is related to the macroscopic field E and volume polarization P, in a dense dielectric medium, by the Lorentz-Lorenz (LL) relation.1 For dense media, this relation implies a nonlinear relationship between the microscopic molecular polarizability α and the macroscopic susceptibility χ, for a material volume density N. The LL relation also holds for arbitrary time-dependent fields in a nonlinear medium.2 What this implies is that all of quantum optics that uses an external electric field rather than the local field is internally inconsistent and will yield only approximate results for an optically dense medium. We consider the effect of the local field interaction on a simple three-level A system predicted to exhibit lasing without inversion and a high index of refraction at zero absorption.3 A threshold transition allows us to choose a value of this parameter that enhances inversionless gain and absorptionless dispersion by at least two orders of magnitude over that foundin usual lasing without inversion.4
Interactions between light and molecular matter featuring photon absorption are commonly associated with excitation of individual chromophores. Subsequent relaxation is achieved through numerous mechanisms, such as scattering and energy transfer to neighbouring chromophores. The efficiency of such processes depends on many factors, including the intensity and wavelength of the optical input, the absorption cross-section of the molecule and the relative orientation of molecular components. New photonic materials are developed on the principle that such factors are controllable, duly tailoring the system to suit new technological applications. The parameters that determine the linear response in multicomponent materials are typically assessed within the theoretical framework of classical optics. However, with improved interest and capability in fabricating nanoscale structures (especially those that exhibit quantum effects), it is becoming increasingly relevant to develop theory that more faithfully represents how individual photons engage with matter. In such constructs, the interaction of materials with optical waves and photons is highly dependent on local molecular environment, therefore surrounding structures and ancillary species exert forms of influence on the photophysical processes inherent within dielectric media. At optical wavelengths where the secondary structure displays little intrinsic optical absorption, the role of such components is often interpreted as modifying the input through a corrective local index of refraction. Although expedient in the discussion of bulk properties of a macroscopic medium, it is reasonably supposed at a local chromophore-photon level that optical mechanisms operate in a different fashion. Using a fully quantized approach to the representation of local molecular electronic structure and electrodynamics, this research develops rigorous theory and a corresponding physical interpretation of how photon absorption, scattering and energy transfer are modified by vicinal, non-absorbing chromophores. The results provide insight into the mechanisms achieved within multi-chromophore systems, highlighting factors that assist in the optimization of optical characteristics in designer materials.
Summary form only given. The dynamics of a medium consisting of a dense collection of two-level atoms interacting via the electromagnetic field has been derived from first principles. In dense media and condensed matter, the microscopic local field that couples with a single atom is related to the macroscopic field and volume propagation by the Lorentz-Lorenz relation written for an isotropic homogeneous medium. The dynamical equations for the medium, coupled self-consistently with the Maxwell inhomogeneous wave equation, are given in the rotating wave and dipole coupling approximations. The microscopic development is used within the framework of full quantization, and the explicit effects of spontaneous relaxation for the host as well as the atoms are treated, together with corresponding fluctuations. Particular effects upon intrinsic adiabatic inversion, self-induced transparency, and lasing without inversion, are presented and discussed. In particular, the equilibrium, stationary, and dynamical effects of the host medium, and medium and field fluctuations upon local field effects are discussed.
Spectroscopy, the study of matter via its interactions with light, is a vast subject that includes many of humanity's principle means of probing the fundamental structure of matter. Though usually understood from the perspective of classical light, important effects arise due to the quantum nature of the electromagnetic field. Additionally, the quantum vacuum can serve as an effective couple between molecules in the system, generating many-body interactions and cooperative processes in the material. While many techniques are well-described classically, quantum-field corrections can dominate the desired signals in certain cases, such as cascading in higher-order n-wave mixing signals, and are key for experimental interpretation. Even spontaneous emission and light scattering are best understood from the perspective of quantum fields. In this thesis, we utilize a systematic procedure, based on the density matrix in Liouville space, to give a rigorous description of elementary quantum-field effects in nonlinear spectroscopy, with specific examples made to ultrafast x-ray and optical techniques.