KEYWORDS: Nanostructures, Atomic force microscopy, Magnetism, Silicon, Microscopy, Near field optics, Nanoprobes, Structured light, Polarization, Near field
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.
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.
Colloidal self-assembly combined with templated surfaces holds the promise of fabricating large area
devices in a low cost facile manner. This directed assembly approach improves the complexity of assemblies that
can be achieved with self-assembly while maintaining advantages of molecular scale control. In this work,
electrokinetic driving forces, i.e., electrohydrodynamic flow, are paired with chemical crosslinking between
colloidal particles to form close-packed plasmonic metamolecules. This method addresses challenges of obtaining
uniformity in nanostructure geometry and nanometer scale gap spacings in structures. Electrohydrodynamic flows
yield robust driving forces between the template and nanoparticles as well as between nanoparticles on the surface
promoting the assembly of close-packed metamolecules. Here, electron beam lithography defined Au pillars are
used as seed structures that generate electrohydrodynamic flows. Chemical crosslinking between Au surfaces
enables molecular control over gap spacings between nanoparticles and Au pillars. An as-fabricated structure is
analyzed via full wave electromagnetic simulations and shown to produce large magnetic field enhancements on the
order of 3.5 at optical frequencies. This novel method for directed self-assembly demonstrates the synergy between
colloidal driving forces and chemical crosslinking for the fabrication of plasmonic metamolecules with unique
electromagnetic properties.
Plasmonic nano antennas like dimers, have been investigated for their capability to provide a strong near-field enhancement when illuminated by external light. Traditionally these nano antennas, isolated or arrayed, are placed on a substrate and used in spectroscopy techniques. Surfaces made of such plasmonic nano antennas have been very useful for applications like surface enhanced Raman scattering in which it provides various orders of magnitude of enhanced sensitivity. These instruments however are not economic and are often not mobile since surfaces require an external beam illumination and the Raman scattering is detected by a large aperture microscope. The goal of this paper is to combine nano antennas made of gold dimers with integrated waveguide to make a spectrometer which has low cost and volume in comparison with the structure mentioned above. A technique in which optical plasmonic nano antennas are located in proximity of silicon nitride waveguide is proposed that is useful both for illumination and detection channels. The waveguide evanescent field, which is decaying outside of the waveguide, excites the dimer and causes it to resonate which results in a very strong electric field enhancement of approximately 25 times in the antenna gap. Also the coupling effect of dimer resonance on waveguide modes is investigated. To show the efficiency of the proposed structure, full wave analysis has been done and its results are compared with the multilayer structure case. The simulation results demonstrate that this structure can be designed and fabricated for the purpose of spectroscopy application.
Pseudomonas aeruginosa (PA), a biofilm forming bacterium, commonly affects cystic fibrosis, burn victims, and immunocompromised patients. PA produces pyocyanin, an aromatic, redox active, secondary metabolite as part of its quorum sensing signaling system activated during biofilm formation. Surface enhanced Raman scattering (SERS) sensors composed of Au nanospheres chemically assembled into clusters on diblock copolymer templates were fabricated and the ability to detect pyocyanin to monitor biofilm formation was investigated. Electromagnetic full wave simulations of clusters observed in scanning electron microcopy images show that the localized surface plasmon resonance wavelength is 696 nm for a dimer with a gap spacing of 1 nm in an average dielectric environment of the polymer and analyte; the local electric field enhancement is on the order of 400 at resonance, relative to free space. SERS data acquired at 785 nm excitation from a monolayer of benzenethiol on fabricated samples was compared with Raman data of pure benzenethiol and enhancement factors as large as 8×109 were calculated that are consistent with simulated field enhancements. Using this system, the limit of detection of pyocyanin in pure gradients was determined to be 10 parts per billion. In SERS data of the supernatant from the time dependent growth of PA shaking cultures, pyocyanin vibrational modes were clearly observable during the logarithmic growth phase corresponding to activation of genes related to biofilm formation. These results pave the way for the use of SERS sensors for the early detection of biofilm formation, leading to reduced healthcare costs and better patient outcomes.
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