Transmission-mode geometric phases and photonic-spin-dependent splitting characteristics of circular Bragg phenomenon through quantum weak measurements

A chiral sculptured thin film (CSTF) is an assembly of identical upright nanohelixes grown on a planar substrate using physical vapor deposition. We report a comparative study of geometric phases and photonic spin-dependent splitting (PSDS) of monochromatic light transmitted through two distinct CSTFs, when the free-space wavelength (λ₀ = 632.8 nm) of the normally incident (θ = 0⁰) light lies in the circular Bragg regime of one CSTF but not in the circular Bragg regime of the other CSTF. The distinguishable features of the transmission-mode geometric phases originating due to the exhibition of the circular Bragg phenomenon by CSTFs were identified using the Stokes polarimetric method in conjunction with circular transmittance spectra. The sub-wavelength values of PSDS were amplified 750–7500 times to obtain 300λo of improved PSDS for |θ|≤ 4⁰, utilizing the advantages of weak-value amplification and a unique θ-dependent post-selection procedure. The PSDS does not have distinguishable features related to the circular Bragg phenomenon, although it does largely depend on the nanohelixes that induce the rotation of the vibration ellipse of plane-polarized light. The distinctive characteristics of PSDS for |θ|≤ 1⁰ and |θ|> 1⁰ indicate the occurrence of (i) vortex generation dependent on the intrinsic orbital angular momentum and (ii) oscillating beam centroid displacements dependent on the extrinsic orbital angular momentum––in longitudinal and transverse directions, originating from the presence of Pancharatnam–Berry phase and spin-redirection Rytov–Vladimirskii–Berry phase, respectively. These experimental findings arising from the architected nanohelical morphology of CSTFs can be useful for wavefront manipulation, optical tweezing, and optoelectronics.