Chiral optical fields: a unified formulation of helicity scattered from particles and dichroism enhancement

Abstract

We establish a general unified formulation which, using the optical theorem of electromagnetic helicity, shows that dichorism is a phenomenon arising in any scattering—or diffraction—process, elastic or not, of chiral electromagnetic fields by objects either chiral or achiral. It is shown how this approach paves the way to overcoming well-known limitations of standard circular dichroism, like its weak signal or the difficulties of using it with magnetodielectric particles. Based on the angular spectrum, representation of optical fields with only right circular or left circular plane waves, we introduce beams with transverse elliptic polarization and possessing a longitudinal component. Then, our formulation for general optical fields shows how to enhance the extinction rate of incident helicity (and therefore the dichroism signal) versus that of energy of the light scattered or emitted by a particle, or vice versa. This article is part of the themed issue ‘New horizons for nanophotonics’.

Resonant optical gradient force interaction for nano-imaging and -spectroscopy

“…where E i is the ith component of the electric field, B i is the ith component of the magnetic flux field. Under a plane wave incidence with vertical polarization, it is well known that the gradient components of the force dominate the vertical direction (along the tip axis), while the radiation pressure acts in the transverse direction [4,22]. Thus only the gradient force, which is responsible for the tip-sample interaction in the vertical direction, is considered.…”

“…Since the radius of the tip apex  r 10 nm is much smaller than the laser wavelength λ, the nearfield tip and sample interaction under plane wave illumination can be analyzed in the quasistatic approximation by assuming the probe to be a polarizable sphere with radius r. The resulting field distribution on the sample can be effectively reduced to an image sphere of radius r, as shown in figure 1(a). This coupled nanoparticle geometry has been used extensively and with great success, predicting general behaviors accurate to within an order of magnitude [4,5,29]. While the exact geometry of tip and sample affects both the details of magnitude and spectral response of the optical gradient force [49,50], especially for strong polaritonic resonances, the limiting case of two finite spheres provides enough general insight into the spectral variation of the force spectrum and its distance dependence.…”

“…Under light illumination, the induced coupled optical polarization between scanning probe and sample forms the basis of scanning near-field optical microscopy (SNOM) [1,2], with the near-field signal typically detected by far-field scattering. The coupled optical polarization is expected to also give rise to an optical gradient force between the tip and the sample as illustrated in figure 1(a) [3,4].…”

“…Optically induced forces, associated with gradient, scattering, and thermal expansion, have been studied in scanning probe microscopy for topographic and near-field optical imaging with a focus on the spatial dependence, as well as for particle trapping and manipulation [4][5][6]. In scanning probe microscopy, the optically induced forces, as competing factors to the van der Waals force, have been studied in the context of topographic artifacts in SNOM imaging [7][8][9][10][11][12].…”

“…In addition, a trapped particle as a probe was used for near-field optical or force mapping in photonic force microscopy [17][18][19][20]. However, despite several theoretical studies [4,[21][22][23][24][25], these works provided limited insight into the spectral characteristics of the optical gradient force and its dependence on electronic and vibrational material resonances of the sample. While the frequency dependence of the optically induced force is often explored in optical trapping of micro-/nanoparticles [5,[26][27][28][29][30][31][32], and in the optically induced mechanical response of plasmonic structures [33,34], the extension for spectroscopic nano-imaging was not explored.…”