Design and fabrication of focusing nearfield optical probe for biomolecule trapping application

There has been a tremendous interest about the trapping of a single biomolecule using nearfield optical trapping. The optical trapping of a biomolecule can be accomplished by controlling both scattering force on the molecule and field gradient force. In order to achieve nearfield optical trapping of the biomolecule, it seems that the radiant trapping force should be greater than the Brownian motion of the molecule in the liquid and the gravity. The radiation force is proportional to the nearfield intensity of the aperture. Though, the throughput of the conventional fiber probe is known to have weak light intensity due to the long, narrow waveguide. In order to better confine the molecule around the aperture, the greater throughput of the light intensity through the aperture is desirable due to wider tapered angle of waveguide. In this report, the nanosize circular metal shape around the subwavelength-size oxide aperture was designed and fabricated using physical metallic deposition of Au or bimetallic Al and Ti. The circular metallic shape (metallic nanoflower) around the subwavelength-size metallic aperture is supposed to focus the horizontal evanescent electromagnetic field toward the propagating direction. This can provide an enhanced evanescent field and an increased gradient force toward the axis of propagating direction. Therefore the nanoflower around the nano-aperture would be expected to better confine a bio-molecule in a nanoscale region.