In pursuit of photo-induced magnetic and chiral microscopy

Abstract

Light-matter interactions enable the perception of specimen properties such as its shape and dimensions by measuring the subtle differences carried by an illuminating beam after interacting with the sample. However, major obstacles arise when the relevant properties of the specimen are weakly coupled to the incident beam, for example when measuring optical magnetism and chirality. To address this challenge we propose the idea of detecting such weakly-coupled properties of matter through the photo-induced force, aiming at developing photo-induced magnetic or chiral force microscopy. Here we review our pursuit consisting of the following steps: (1) Development of a theoretical blueprint of a magnetic nanoprobe to detect a magnetic dipole oscillating at an optical frequency when illuminated by an azimuthally polarized beam via the photo-induced magnetic force; (2) Conducting an experimental study using an azimuthally polarized beam to probe the near fields and axial magnetism of a Si disk magnetic nanoprobe, based on photo-induced force microscopy; (3) Extending the concept of force microscopy to probe chirality at the nanoscale, enabling enantiomeric detection of chiral molecules. Finally, we discuss difficulties and how they could be overcome, as well as our plans for future work.

Photo induced force microscopy: chemical spectroscopy beyond the diffraction limit

“…Theoretically, if a chiral sample and the nanoprobe were illuminated with chiral light the photoinduced force would be different depending on the chirality of the light. 95 This principle has been shown to have credibility through the modelling of a chiral sphere in the presence of chiral light.…”

Helicity maximization below the diffraction limit

“…The APB and RPB are obtained by superposing Laguerre Gaussian beams with opposite angular momenta [29], [31], [32], [37], [68]. The electric field vector in an APB propagating along the +z-direction (optical axis of the beam) is polarized along the azimuthal direction  in the transverse plane that, after suppressing the time dependence exp( ) , where 0 w and  are, respectively, the beam parameter and the excitation wavelength.…”

Unscrambling Structured Chirality with Structured Light at the Nanoscale Using Photoinduced Force

“…. Furthermore, neglecting the field's phase difference between the tip-apex and sample, due to their subwavelength distance, it is shown that the difference between the forces exerted on the tip-apex for two CP plane wave illuminations with opposite handedness reads 27,47,48  …”

“…In this regard, the z -component of the general force expression of eq reduces to (see eq 2.2 of ref and eq of ref ) . Furthermore, neglecting the field’s phase difference between the tip-apex and sample, due to their subwavelength distance, it is shown that the difference between the forces exerted on the tip-apex for two CP plane wave illuminations with opposite handedness reads ,, in which ε 0 is the vacuum permittivity and d is the center-to-center distance between the sample and tip-apex. Note that in deriving this formula we assume CP plane waves with electric field magnitude | E 0 | on the beam axis since it simplifies our analytical calculations.…”