Fig. 3: Initial preparation for the subwavelength spin-orbit photonic state.
From: Precise detection of tiny birefringence with accuracy reaching 10−11 level
a An experimental setup. A linearly polarized He-Ne laser operating at the fundamental Gaussian mode with wavelength λ = 632.8 nm is separated into two paths by a beam splitter (BS). One path as the signal beam passes through a combined optical element (inhomogeneous wave plate plus ultrathin metallic disc, see inset I), generating the expected equatorial spin-orbit state at the subwavelength scale. The initially prepared state is then sent to an electrically engineered photonic crystal (see inset II), whose birefringence can be transversely modulated through an external voltage U. A microscopic system [comprising objective lens (OB), tube lens (TL), polarizer (P) and charge-coupled device (CCD)], together with a precision position tracking system (inset III), is built for observing the birefringence-sensitive spin-orbit topological transition. Another path as a reference beam interferes with the emerging beam from the crystal, detecting the phase wavefront of the output photonic state. b–d Experimental characterization of the initial state at the input end of the crystal: b the experimentally recorded BG envelope field with envelope parameter measured as r0 = 130 nm (the peak-to-peak size 466 nm); c the horizontal polarization; and d the vertical polarization. e–g Simulation results corresponding to b–d. b–g share identical scale with scale bar being 500 nm.
