Proc. SPIE. 10350, Nanoimaging and Nanospectroscopy V
KEYWORDS: Super resolution, Microscopy, Optical microscopy, Medical research, Magnetism, Distortion, Near field scanning optical microscopy, Optical scanning systems, Polarized microscopy, Near field optics
A sharply focused azimuthally polarized beam (APB) presents a strong longitudinal magnetic field with a vanishing electric field at its beam axis, forming an effective magnetic dominant region at the vicinity. This magnetic dominance is extremely desirable in the proposed high-speed ultra-compact optical magnetic force manipulation and microscopy, where the interaction between matter and the magnetic field of light can be exclusively exploited. However, direct characterization of such beam is challenging due to its subwavelength features. Here we show for the first time a direct characterization on a sharply focused APB in nanoscale using the novel Photoinduced Force Microscopy (PIFM) technique, which simultaneously excites and detects incident beam in near-field. Comparing to the Scanning Near-field Optical Microscopy (SNOM) which has near-field excitation and far-field detection, PIFM boasts a much smaller background noise and a more robust system. Based on the measured force-map, we develop a theoretical model to retrieve the corresponding electric and magnetic field distribution, and correct the distortion caused by the imperfect probe-tip of the PIFM. This research pioneers the exploration in the experimental investigation on the sharply focused structured light, unveiling its potentials in a plethora of optoelectronics, chemical, or biomedical applications.
We investigate azimuthally E-polarized vortex beams with enhanced longitudinal magnetic field. Ideally, such beams possess strong longitudinal magnetic field on the beam axis where there is no electric field. First we formulate the electric field vector and the longitudinal magnetic field of an azimuthally E-polarized beam as an interference of right- and left-hand circularly polarized Laguerre Gaussian (LG) beams carrying the orbital angular momentum (OAM) states of -1 and +1, respectively. Then we propose a metasurface design that is capable of converting a linearly polarized Gaussian beam into an azimuthally E-polarized vortex beam with longitudinal magnetic field. The metasurface is composed of a rectangular array of double-layer double split-ring slot elements, though other geometries could be adopted as well. The element is specifically designed to have nearly a 180° transmission phase difference between the two polarization components along two orthogonal axes, similar to the optical axes of a half-wave plate. By locally rotating the optical axes of each metasurface element, the transmission phase profile of the circularly polarized waves over the metasurface can be tailored. Upon focusing of the generated vortex beam through a lens with a numerical aperture of 0.7, a 41-fold enhancement of the magnetic to electric field ratio is achieved on the beam axis with respect to that of a plane wave. Generation of beams with large magnetic field to electric field contrast can find applications in future spectroscopy systems based on magnetic dipole transitions, which are usually much weaker than electric dipole transitions.