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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Study of molecular Rydberg states and nonlinear spectral effects

H.-R. Xia, Zugeng Wang

Published Oct 28, 1992• 0 citations
Publisher

Abstract

The paper summarizes the authors' major results achieved recently with their collaborators on the study of molecular Rydberg states and nonlinear spectral effects. The results include the findings of diversified nonlinear spectral effects, the recognition of new spectral regulations and their identification methods for molecular two-photon excitation spectra simplified by the mechanism of near-resonant enhancement, and proposals and demonstrations of manifold mechanisms for generating stimulated and coherent radiation from infrared to ultraviolet wavelength regions out of simple molecules or via them.

DOI
10.1117/12.130459

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The paper summarizes the authors' major results achieved recently with their collaborators on the study of molecular Rydberg states and nonlinear spectral effects. The results include the findings of diversified nonlinear spectral effects, the recognition of new spectral regulations and their identification methods for molecular two-photon excitation spectra simplified by the mechanism of near-resonant enhancement, and proposals and demonstrations of manifold mechanisms for generating stimulated and coherent radiation from infrared to ultraviolet wavelength regions out of simple molecules or via them.
Various characteristics of molecules have been clarified by spectroscopic methods in which the interactions of molecules with the radiation field were investigated. A stationary response of a molecule excited by a radiation field appears as a spectral line. Information on molecules may be obtained from position (resonance frequency), intensity, and shape and width of the spectral line. So far the interaction of molecules with the radiation field has been studied primarily in a “frequency domain.” Although molecular spectroscopy is one of the research fields in modern science which has the longest history, a variety of important aspects have been demonstrated very recently in this field as radiation sources have been improved. When the radiation field is “weak,” the photon nature of the radiation field is dominant in the interaction with a molecule, such as in the cases of emission, absorption, and scattering of photons. When the radiation field is “strong,” in the sense that many photons exist in a proper mode of the radiation field, the wave nature of the field plays an important role in the interaction. In this case the radiation may be described in the form of the classical field, say, a sinusoidal function of the time and spatial coordinates, coherently interacting with the molecule, which is described quantum mechanically. Here nonlinear properties would appear in the interactions: saturation in absorption, induced scattering, multiphoton processes, coherent transients, etc.
Nonlinear effects by intense optical field on mid-infrared spectral lines (∼3 μm) are discussed. Recent development of intense mid-infrared coherent radiation makes nonlinear mid-infrared spectroscopy possible. In the mid-infrared region, the transversal relaxation process is important compared to the visible or near-infrared region. In the analysis, a two-energy-state system is discussed. The intense pump beam scattering spectral lines (Mollow triplet) and the absorption spectral lines of the probe beam with the intense pump beam are calculated.
We experimentally investigate the nonlinear transmission spectrum of coherent light fields propagating through a Rydberg EIT medium with strong atomic interactions. In contrast to previous investigations, which have largely focused on resonant control fields, we explore here the full two-dimensional spectral response of the Rydberg gas. Our measurements confirm previously observed spectral features for a vanishing control-field detuning, but also reveal significant differences on two-photon resonance. In particular, we find qualitative deficiencies of mean-field models and rate-equation simulations as well as a third-order nonlinear susceptibility that accounts for pairwise interaction effects at low probe-field intensities in describing the nonlinear probe-field response under EIT conditions. Our results suggest that a more complete understanding of Rydberg EIT and emerging photon interactions requires to go beyond existing simplified models as well as few-photon theories.
Cold atomic gases resonantly excited to Rydberg states can exhibit strong optical nonlinearity at the single-photon level. We observe that in such samples radiation trapping leads to an additional mechanism for Rydberg excitation. Conversely we demonstrate that Rydberg excitation provides an in situ probe of the spectral, statistical, temporal, and spatial properties of the trapped rescattered light. We also show that absorption can lead to an excitation saturation that mimics the Rydberg blockade effect. Collective effects due to multiple scattering may coexist with cooperative effects due to long-range interactions between the Rydberg atoms, adding a new dimension to quantum optics experiments with cold Rydberg gases.
Results are reported on coherent monochromatic multiphoton excitation in many-level systems, which are representative for some of the basic mechanisms for atomic and molecular multiphoton processes. Numerical solutions are discussed that use the Floquet and quasi-resonant approximations in the framework of the URIMIR program package. The excitation schemes include direct three-photon excitation, two-photon excitation with diagonal coupling, Göppert-Mayer-type two-photon processes, multiphoton excitation with off-resonant inter-mediates, and practically irreversible coherent excitation into dense spectral structures. Several interesting phenomena are observed, such as nonlinear line shifts and broadenings of multi-photon resonances of relevance for multiphoton spectroscopy and almost constant intermediate population inversions, potentially useful for laser design. The accurate numerical results are compared with approximate solutions from perturbation theory, and with simple analytical solutions from Rabi-type formulae.
The history of Rydberg states in atoms and molecules will be briefly summarized. If the ground state of a molecular system is unstable (as for example two He atoms) while most excited states are stable Rydberg states, we may describe the system as a Rydberg molecule. The simplest diatomic example is the He2He_{2}He2​ molecule, while the simplest polyatomic system would be the H3H_{3}H3​ molecule discovered only in 1979. In recent years many other examples have been studied and will be briefly summarized.