EFFICIENT LIGHT COUPLING TO PLASMONIC NANOANTENNAS VIA OPTIMIZING THE NEAR-FIELD POLARIZATION DISTRIBUTION

Plasmonic nanoantennas have found recent use as near-field transducers (NFTs) in heat assisted magnetic recording (HAMR). A critical component to their efficiency as NFTs is that the applied electric field (or polarization) of the light be precisely aligned with the apex of the sharp tips comprising the nanoantennas. However, a tip may not be perfectly fabricated and as such its apex may not be normal to the surface of the recording medium. Moreover, future nanoantenna design alternatives may require that the applied electric field be oriented in a general direction in three-dimensional space. Therefore, the long-term objective of the proposed research is to facilitate the optimization of light coupling to NFTs. The 1-year pilot study will lay the groundwork for this by developing appropriate methods to efficiently align the input optical polarization to a plasmonic nanoantenna located in the focal plane of a strongly focusing optical system. This will be achieved by exercising full control of the near-field optical polarization distribution in three spatial dimensions, and by measuring the response of a near-field antenna excited by such a field. To this end, the PI has previously developed the theory for achieving 3D polarization control with relatively high efficiency, without the use of any polarization optics. Specifically, the polarization at the focal spot of a strongly focusing system is tuned through modifying the scalar complex field distribution of the input light. Three low-order azimuthal spatial harmonics have been identified to each produce a focused linearly polarized component. Thus, controlling the relative complex weights of these azimuthal harmonics, which in the proposed work will be done using low-cost liquid crystal display technology, generates any desired 3D state of polarization. In addition, a recently proposed method of using the dipolar response of a nanoparticle to measure the 3D polarization state of strongly focused unpolarized light will be adapted to map the polarization response of a nanoantenna. For this pilot study, the 2D waveguide (e.g., planar solid immersion mirror) used in HAMR to create a tightly focused longitudinal field at the NFT will be replaced by a high-numerical-aperture (NA) objective lens (typically NA ~ 1.3-1.4) and various nanoantenna designs will be explored to test the proposed methodology. The project deliverables include: 1) a method to convert a standard laser-illuminated inverted microscope to a NFT metrology platform capable of determining the optimum field distribution for an NFT; and 2) a Matlab graphical user-interface to facilitate analysis. Principal Investigator/Designated Contact: Dr. Kimani C. Toussaint, Jr. Affiliation: University of Illinois at Urbana-Champaign Assistant Professor, Department of Mechanical Science and Engineering Affiliate Faculty, Dept. of Electrical & Computer Engineering, Dept. of Bioengineering, and the Beckman Institute for Advanced Science and Technology Phone: (217) 244-4088 ; Fax: (217) 333-1942 E-mail: ktoussai@illinois.edu; Lab Webpage: http://probe.mechse. illinois.edu