Probing the chiral nature of electromagnetic fields surrounding plasmonic nanostructures

Abstract

We investigate the chiral properties of near fields around plasmonic nanostructures and their relation to the electromagnetic chirality C. By combining chiral metal nanoparticles with achiral dye molecules and measuring the circular polarization dependence of the enhanced photoluminescence, we find a correlation between the dissymmetry of the luminescence enhancement and the calculated values of C. These effects are strong (∼10 −1 ), despite the weak circular dichroism of the particles (∼10 −5 ). We further show that C represents the chiral selectivity of the near-field coupling between an emitter and a nanoantenna. Structural chirality, i.e., the handedness of an object that makes it incongruent with its mirror image, is a common property of many biologically relevant molecules, ranging from simple amino acids to complex helical DNA strands. This quality is often characterized by optical techniques that exploit the enantioselective interaction with circularly polarized light, e.g., by measuring circular dichroism (CD), 1,2 or by detecting molecular transitions in chiral molecules. 3,4 Such studies of chiroptical effects require an understanding of the chirality-or helicity-of light, which is not as simple a concept as structural chirality: It is, more fundamentally, associated with angular momentum. However, determining the connection between helicity and angular momentum is not trivial, 5 not even for a plane electromagnetic wave, and it breaks down entirely for evanescent fields.6 Nevertheless, it has been shown that the evanescent near fields around certain plasmonic nanostructures can undergo chiroptical interactions.7-12 However, the chiral properties of these electromagnetic near fields are very different from circularly polarized light; indeed, the concept of polarization is not applicable to evanescent fields: They exhibit phase-shifted electric and magnetic field vector components without temporal vector rotation. Such solutions to the Maxwell equations are only allowed in local regions of space, such as in the vicinity of scattering nanostructures or at the center of Laguerre-Gaussian beams. 13A more general description of the chiral symmetry of arbitrary electromagnetic fields is therefore needed and several theoretical approaches have been proposed. a pseudoscalar that serves as a measure of the chiral nature of an electromagnetic field with E and B denoting complex electric and magnetic field vectors, respectively. The formalism developed in Ref.15 is based on the interaction of a time-varying electromagnetic field with the electric and magnetic dipole moments that describe a chiral probe molecule. The electromagnetic chirality is then linked to the dissymmetry in the excitation rate of this probe for left-and right-handed systems. It contains only the chirality intrinsic to the electromagnetic field and is independent of the probe.Chirality is usually quantified through far-field measurements, more specifically, of the CD. Such a measurement does not probe the near-field chirality C. Here, we dire...

Plasmonic meta-atoms and metasurfaces

“…Going beyond a simple point dipole picture one can elicit an entirely different response from meta--atoms. By suitable structural design one can arrange for spatially and spectrally overlapped electric and magnetic field components to couple 36 , enabling near fields to take on a chiral character [36][37][38][39] . (Fig.…”

Time-harmonic optical chirality in inhomogeneous space

“…10 This expectation is supported by recent experiments claiming to measure the chirality of optical near-fields. 11 Additionally, there have been various studies of the enhancement of optical chirality in the context of time-harmonic simulations 12 as well as theory. [13][14][15] These theoretical analyses have been mostly limited to homogeneous media 16 or vacuum only.…”

Optical Chirality Flux as a Useful Far-Field Probe of Chiral Near Fields

“…Further, a significant increase in enantioselectivity was found when chiral molecules interact locally with electromagnetic fields with χ greater than in circularly polarized light [5]. Consequently, χ has since been used to study chiral plasmonic nanostructures [6,18,19,27]. It has identified near fields of high optical chirality enhancement, defined as the time-averaged optical chirality density [Eq.…”