Fig. 1: Optical magnetic field-associated trapping of Si nanoparticles with an azimuthally polarized beam.
From: Optical trapping with optical magnetic field and photonic Hall effect forces
a The intensity profile of a tightly focused azimuthally polarized beam decomposed into three constituent fields in the focal volume: E (reddish), Hρ (reddish), and Hz (blue), with black arrows representing their polarization directions. b The ratios of Hz to E and Hρ intensities as a function of numerical aperture (NA) of the focusing lens. The black dots in the plot indicate the ratios for an NA of 1.27, which corresponds to the microscope objective used for trapping Si nanoparticles in water (n = 1.33). c The scattering cross sections of Mie multipole resonances of Si nanoparticles are calculated at the 770 nm trapping laser wavelength. The first intersection between the MD and ED scattering cross-section curves occurs at Si nanoparticle dia. =182 nm. This nanoparticle size separates the optical trapping into E-field- and H-field-dominated trapping domains. The hatched and grey regions represent the E-H transition (182–190 nm) and higher order multipole dominated (>250 nm) regions, respectively. Si nanoparticles with sizes from about 180 to 250 nm in dia. manifest trapping forces with strong optical magnetic contributions. d The calculated potential energy profiles across the focused azimuthally polarized vector beam illustrate the expected (energetic) tendencies of Si nanoparticles to trap in the E- and H-field dominated portions of the optical beam.
