Optical fields possess energy, momentum, and helicity. For the plane waves and paraxial fields of standard, classical optics, the spin angular momentum and optical helicity are well understood, both being proportional to the degree of circular polarization. In contrast, 3D non-paraxial optical fields generated by spatial confinement, such as tight focusing and evanescent waves, may possess spin angular momentum and optical helicity when the degree of 2D-circular polarization is zero. In this work using quantum electrodynamics we highlight two novel properties of non-paraxial optical vortices 1) a non-zero optical helicity (chirality) density for 2D-unpolarized (and 2D-linearly polarized) 3D optical vortices and 2) a longitudinal spin angular momentum density for 2D-linearly polarized 3D optical vortices.
Longitudinal fields of quantized Laguerre-Gaussian modes are derived, revealing their importance even for light that may be considered to be propagating in the paraxial regime. This is in contrast to unstructured laser light, e.g. a Gaussian beam, where the magnitude of longitudinal components only become important under strong-focusing of the source. The unique effects stem specifically from the optical angular momentum, both orbital and spin, of optical vortex light, and include spin-orbit interactions in freely-propagating circularly-polarized vortices in free-space. The contribution that longitudinal fields make to the rate of single-photon absorption is calculated, highlighting that for optical vortices they cannot be neglected in general.
It is well known that Fluorescence Resonance Energy Transfer (FRET), the most common mechanism for electronic energy to migrate between molecular chromophores, has a predominantly inverse sixth power dependence on the rate of transfer as a function of the distance R between the chromophores. However, the unified theory of electronic energy transfer, derived from quantum electrodynamics, predicts an additional contribution with an R-4 dependence on distance. This intermediate-zone term becomes especially important when the chromophore spacing is similar in magnitude to the reduced wavelength (ƛ= λ 2π ) associated with the mediated energy. In previous theoretical studies we have suggested that inclusion of the intermediate term, through rate equation and quantum dynamical calculations, may be important for describing the exciton diffusion process in some circumstances, and in particular when the distance between the chromophores exceeds 5 nm. In this paper, we focus of the role of the intermediate-zone contribution to distance measurements between chromophores made through the application of spectroscopic ruler techniques. One of the major assumptions made in employing these experimental techniques is that the R−6dependence is valid. In this work, we reformulate the spectroscopic ruler principles for intermediate distances to include the inverse fourth power rate component, and compare the results of this reformulation to experimental FRET results from the literature.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.