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

Engaging new dimensions in nonlinear optical spectroscopy using auxiliary beams of light

Jack Ford, David S. Bradshaw, Davıd L. Andrews

Published May 1, 2014• 0 citations
Publisher

Abstract

By applying a sufficiently intense beam of off-resonant light, simultaneously with a conventional excitation source beam, the efficiencies of one- and two-photon absorption processes may be significantly modified. The nonlinear mechanism that is responsible, known as laser modified absorption, is fully described by a quantum electrodynamical analysis. The origin of the process, which involves stimulated forward Rayleigh-scattering of the auxiliary beam, relates to higher order terms which are secured by a time-dependent perturbation treatment. These terms, usually inconsequential when a single beam of light is present, become prominent under the secondary optical stimulus – even with levels of intensity that are moderate by today’s standards. Distinctive kinds of behaviour may be observed for chromophores fixed in a static arrangement, or for solution- or gas-phase molecules whose response is tempered by a rotational average of orientations. In each case the results exhibit an interplay of factors involving the beam polarisations and the molecular electronic response. Special attention is given to interesting metastable states that are symmetry forbidden by one- or two-photon absorption. Such states may be accessible, and thus become populated, on input of the auxiliary beam. For example, in the one-photon absorption case, terms arise that are more usually associated with three-photon processes, corresponding to very different selection rules. Other kinds of metastable state also arise in the two-photon process, and measuring the effect of applying the stimulus beam to absorbances of such character adds a new dimension to the information content of the associated spectroscopy. Finally, based on these novel forms of optical nonlinearity, there may be new possibilities for quantum non-demolition measurements.

DOI
10.1117/12.2051333

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By applying a sufficiently intense beam of off-resonant light, simultaneously with a conventional excitation source beam, the efficiencies of one- and two-photon absorption processes may be significantly modified. The nonlinear mechanism that is responsible, known as laser modified absorption, is fully described by a quantum electrodynamical analysis. The origin of the process, which involves stimulated forward Rayleigh-scattering of the auxiliary beam, relates to higher order terms which are secured by a time-dependent perturbation treatment. These terms, usually inconsequential when a single beam of light is present, become prominent under the secondary optical stimulus – even with levels of intensity that are moderate by today’s standards. Distinctive kinds of behaviour may be observed for chromophores fixed in a static arrangement, or for solution- or gas-phase molecules whose response is tempered by a rotational average of orientations. In each case the results exhibit an interplay of factors involving the beam polarisations and the molecular electronic response. Special attention is given to interesting metastable states that are symmetry forbidden by one- or two-photon absorption. Such states may be accessible, and thus become populated, on input of the auxiliary beam. For example, in the one-photon absorption case, terms arise that are more usually associated with three-photon processes, corresponding to very different selection rules. Other kinds of metastable state also arise in the two-photon process, and measuring the effect of applying the stimulus beam to absorbances of such character adds a new dimension to the information content of the associated spectroscopy. Finally, based on these novel forms of optical nonlinearity, there may be new possibilities for quantum non-demolition measurements.
It is shown that conventional one-photon and two-photon absorption processes can be made subject to nonlinear optical control, in each case significantly modifying the efficiency of absorption, through the effect of a secondary, off-resonant stimulus laser beam. The mechanistic origin of these laser-modified absorption processes, in which the stimulus beam emerges unchanged, is traced to higher-order terms in standard perturbation treatments. These normally insignificant terms become unusually prominent when the secondary optical stimulus is moderately intense. Employing a quantum formulation, the effects of the stimulus beam on one-photon and two-photon absorption are analyzed, and calculations are performed to determine the degree of absorption enhancement, and the form of spectral manifestation, under various laser intensities. The implications of differences in selection rules are also considered and exemplified, leading to the identification of dark states that can be populated as a result of laser-modified absorption. Attention is also drawn to the possibility of quantum nondemolition measurements, based on such a form of optical nonlinearity.
The efficiencies of one- and two-photon absorption by chromophores in solution may be significantly modified by a sufficiently intense beam of off-resonant light. A molecular analysis based on quantum electrodynamics (QED) fully accounts for this phenomenon of laser-modified absorption. A time-dependent perturbation-theory treatment describes the process in terms of stimulated forward Rayleigh-scattering of the auxiliary beam occurring simultaneously with the absorption interaction(s). Our formulation accommodates media modifications to the basic character of light-matter interactions, taking into account the refractive and dispersive properties of a solution-phase environment. This introduces the bulk refractive index of the solvent directly into the QED framework. The measurable electronic response of molecules freely rotating in solution is defined by an average of all orientations. We explicitly derive fixed-orientation and rotationally averaged calculations for the Fermi-rule rate of laser-modified one- and two-photon absorption. For a given beam polarization geometry, the solution-phase molecular response is expressible as a set of natural invariant scalars. These results reveal details of the dependence on the beam polarisations and on the rotationally averaged molecular response: we illustrate the breadth of variation available via geometric manipulation of beam polarization, and raise new possibilities for quantum weak measurements of laser states.
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.
The theory of non-resonant optical processes with intrinsic optical nonlinearity, such as harmonic generation, has been widely understood since the advent of the laser. In general, such effects involve multiphoton interactions that change the population of each input optical mode or modes. However, nonlinear effects can also arise through the input of an off-resonant laser beam that itself emerges unchanged. Many such effects have been largely overlooked. Using a quantum electrodynamical framework, this review provides detail on such optically nonlinear mechanisms that allow for a controlled increase or decrease in the intensity of linear absorption and fluorescence and in the efficiency of resonance energy transfer. The rate modifications responsible for these effects were achieved by the simultaneous application of an off-resonant beam with a moderate intensity, acting in a sense as an optical catalyst, conferring a new dimension of optical nonlinearity upon photoactive materials. It is shown that, in certain configurations, these mechanisms provide the basis for all-optical switching, i.e., the control of light-by-light, including an optical transistor scheme. The conclusion outlines other recently proposed all-optical switching systems.
The electronic transfer of energy from a donor particle to an acceptor is a mechanism that plays a key role in a wide range of optical and photophysical phenomena. The ability to exert control on this transfer is of immense importance. It now emerges that there are all-optical routes which can be introduced to achieve this very purpose. We demonstrate the possibility of promoting energy transfer, in the optical near field, that is rigorously forbidden (on geometric or symmetric grounds) in the absence of laser light. The effect operates through coupled stimulated Raman scattering by the donor-acceptor pair. The absorption of a photon takes place at one particle and stimulated emission at either, coupled with energy transfer between the pair. At this fundamental level, transfer phenomena arise for both single and dual input auxiliary beams. In the latter case the emitted photon may differ from the absorbed photon. In either situation energy transfer will not occur in the absence of auxiliary radiation, if either the donor or acceptor transition is single-quantum forbidden. By engaging input laser light, energy transfer may proceed through two or three quantum allowed transitions. The results for this novel type of optical control suggest transfer efficiency levels comparable to Förster transfer. Many applications are envisaged, chief of which is the potential for all-optical switching.
In the optical excitation of many nanoscale systems, the primary result of photon absorption is an electronic excitation that is typically followed by ultrafast relaxation processes. The losses associated with such relaxation generally produce a partial degradation of the optical energy acquired, before any ensuing photon emission occurs. Recent work has shown that the intensity and directional character of such emission may be significantly influenced through engagement with a completely off-resonant probe laser beam of sufficient intensity: the mechanism for this optical coupling is a secondorder nonlinearity. It is anticipated that the facility to actively control fluorescent emission in this way may lead to new opportunities in a variety of applications where molecular chromophores or quantum dots are used. In the latter connection it should prove possible to exploit the particle size dependence of the nonlinear optical dispersion, as well as that of the emission wavelength. Specific characteristics of the effect are calculated, and suitable experimental implementations of the mechanism are proposed. We anticipate that this all-optical control device may introduce significant new perspectives to fluorescence imaging techniques and other analytical applications.