Optical magnetism has long been the elusive, missing component in light-matter interaction. Interesting applications may emerge if optical magnetism is effectively harnessed and exploited. Of particular interest is the possible manipulation of the optical magnetic force, in the form of photo-induced magnetic force microscopy. We propose an optical system for inducing magnetic forces in an axis-aligned Si disk under azimuthally polarized beam illumination. The designed Si disk can support a magnetic resonance in the visible range under azimuthal polarization by interacting with the longitudinal magnetic field at the overlapping axis. Such structure can serve as the unique magnetic probe to “feel” the magnetic force of light. In our current step, we use photo-induced force microscopy to characterize the near-field electric field distribution of this system. Measurements show a stronger electric field enhancement near the edge of the Si disk which indicates a longitudinal magnetic field enhancement at the overlapping axis. This measurement is in accordance with theoretical modeling, confirming the observed magnetic enhancement. This indirect measurement on the magnetic response of the Si disk defines an important step towards our final goal of achieving direct mapping of the local magnetic field with photo-induced magnetic force microscopy. Also, our methodology can be extended to the characterization of arbitrary nanostructures, including metamaterials and metasurfaces, under structured light illumination.
We propose a novel high resolution microscopy technique for enantio-specific detection of chiral samples based on force measurement down to sub-100 nm where spectroscopic techniques for chirality detection encounter major challenges due to the very weak interaction of light with chiral nanoparticles. Specifically, we delve into the differential photo-induced optical force exerted on an achiral plasmonic probe in the vicinity of a chiral sample, when left and right circularly polarized beams separately excite the sample-probe interactive system. We analytically prove that the differential force is entangled with the enantiomer type of the sample enabling enantio-specific detection of chiral inclusions. Moreover, we demonstrate that the aforementioned differential force is linearly dependent on both the chiral response of the sample and the electric response of the tip and is inversely related to the quartic power of probe-sample distance. We support our theoretical achievements by several numerical examples, highlighting the potential application of the derived analytic properties. Lastly, we demonstrate the sensitivity of our method to enantio-specify nanoscale chiral samples. By establishing this high resolution measurement technique for biomedical applications, we essentially advance the characterization of chiral samples for controlling constructive reaction between drugs and receptors.
Due to the weak magnetic responsibility of natural existing materials at optical frequency, optical magnetism remains a “dark state” of light which is largely unexplored. However, optical magnetism is also very desirable because of the many splendid possibilities it may lead to, including ultra-compact opto-magnetic storage devices, high speed magnetic imaging, magnetic tweezers etc. Here we design a Si nano-disk structure as the magnetic nanoprobe which supports magnetic resonance in visible range with the incident azimuthally polarized beam (APB). APB features a donut shape beam profile, with a strong longitudinal magnetic field and a vanishing electric field at the beam axis. Therefore, on the magnetic resonance while the probe is aligned to the APB axis, a longitudinal magnetic dipole is excited in the probe, and interacts with the incident APB inducing an exclusive magnetic force. Making such magnetic nanoprobe under APB illumination serves as an important first step to realize the proposed photoinduced magnetic force microscopy (PIMFM), which selectively exploits the interaction between matter and the magnetic field of light to characterize the optical magnetism in nanoscale. Such investigation of the optical magnetism in samples is dearly needed in many mechanical, chemical, and life-science applications.