Force Detection of Electromagnetic Chirality of Tightly Focused Laser Beams

Abstract

We theoretically show that the optical chiral properties of tightly focused laser beams can be characterized by means of force detection. To measure the chiral properties of a beam of given handedness in the microscopic focal volume, we determine the photoinduced force exerted on a sharp tip, which is illuminated first by the beam of interest and second by an auxiliary beam of opposite handedness, in a sequential manner. We show that the difference between the force measurements is directly proportional to the chiral properties of the beam of interest. In particular, the gradient force difference Δ⟨F grad ,z ⟩ is found to have exclusive correspondence to the time-averaged helicity density of the incident light, whereas the differential scattering force provides information about the spin angular momentum density of light. We further characterize and quantify the helicity-dependent Δ⟨F grad ,z ⟩ using a Mie scattering formalism complemented with full wave simulations, underlining that the magnitude of the difference force is within an experimentally detectable range.

Optical chirality of vortex beams at the nanoscale

“…However, the spatial distribution of the optical chirality stemming from longitudinal fields is distinctly different, being a doughnut shape and not a typical Gaussian spot. A proposal to directly measure the optical chirality of tightly-focused Gaussian beams has recently been put forward 44 . It is worth noting that this method would be readily applied to all the structured light chirality in this work 45 .…”

“…A proposal to directly measure the optical chirality of tightly-focused Gaussian beams has recently been put forward. 44 It is worth noting that this method would be readily applied to all the structured light chirality in this work. 45 The behaviour of the optical chirality density C ˉG as a function of propagation distance z is given in Fig.…”

Chiral nanoparticle separation and discrimination using radially polarized circular Airy vortex beams with orbital-induced spin angular momentum

“…Tight focusing of light provides a common way to realize SOI, without the need for specialized media. It has led to a variety of applications, including optical trapping, 15 microscopy, 16 and detection, 17 thus having emerged as a research frontier and popular topic in optics. In the tight focusing process, SOI effects offer a flexible and effective approach to manipulate the AM behavior of the optical field.…”

Spheres of maximum electromagnetic chirality

“…In electromagnetism, the most usual magnitude to unravel the chirality of a sample is the handedness of circularlypolarized (CPL) fields [9,10], commonly referred to as helicity [11]. In addition to the potential applications of helicity in the context of optical forces [12][13][14][15][16][17][18][19][21][22][23][24], spin-orbit interactions of light [25][26][27][28][29][30][31][32], or its fundamental connection to Kerker conditions [33][34][35], helicity can also be used to probe chiral and achiral objects [36][37][38][39][40]. Now, an object is electromagnetically achiral if all components of the scattered field under the illumination with CPL electromagnetic waves of one sign can also be mimicked with incident CPL waves of the opposite sign.…”