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Qiwen Zhan

Prof. Qiwen  Zhan

Associate Professor
Electro-Optics Program & Electrical and Computer Engineering
University of Dayton
Electro-Optics Program & Electrical and Computer Engineering
The University of Dayton
300 College Park
Dayton OH 45469-2951
United States

tel: 937-229-5590
fax: 937-229-2097
E-mail: qiwen.zhan@notes.udayton.edu
Web: http://academic.udayton.edu/QiwenZhan

Area of Expertise

Polarization, nano-metallic photonics, plasmonics, metamaterials and optical metrology

Biography

Prof. Qiwen Zhan received his BS degree in Physics (optoelectronics) from the University of Science and Technology of China (USTC) in 1996 and Ph.D. in Electrical Engineering from the University of Minnesota - Twin Cities in 2002. He is now an Associate Professor of Electro-Optics and an Associate Professor of Electrical & Computer Engineering at the University of Dayton. Prof. Zhan's research interests are mainly in the general area of physical optics and quantum electronics. The current research in his group focuses on utilizing modern micro- and nano-fabrication tools to achieve subwavelength spatial engineering of light wave properties (such as polarization, intensity and phase) and subsequently manipulate the light matter interactions on the nanometer scale. The localized light matter interactions are exploited in various applications, including the development of new nanoscale imaging capabilities for nanophotonics, biophotonics, nanomaterials characterization and metrology.

Lecture Title(s)

Spatial Polarization Engineering and Its Applications in Optical Instrumentation: With recent advances in nanofabrication capabilities and spatial light modulators, it is possible to spatially manipulate and engineer the state of polarization across a laser beam. In this talk, I will explore new effects and phenomena that can benefit optical instrumentations by spatially arranging the local polarization of an optical beam purposefully.

A special type of spatially variant polarized beam that has received lots of attention is the so-called cylindrical vector beams. Radial and azimuthal polarizations are the two special cases of the CV beams that have the local polarization aligned in the radial and azimuthal directions. These beams can be generated via different active and passive methods. Radial polarization can be used to match the optical illumination to the surface plasmon generation condition for optimal focusing effect. Evanescent Bessel beam can be generated with plasmonic excitation under high focused radial polarization illumination on homogeneous metal film. The created evanescent Bessel beam has been utilized in the imaging of cell-substrate interface of biological cell in aqueous media. Radial polarization can also be used to improve the excitation and focusing for flat plasmonic lens with concentric rings carved into metal film. In addition, it is found that the electric field near the vicinity of the focus are purely polarized along the longitudinal direction that is normal to the interface, generating an optical "needle" field without the use of a high numerical aperture objective lens. Plasmonic focusing with a fully metalized conical antenna under internal illumination of radial polarization has also been investigated. At the optimized cone angle, intensity enhancement as high as 105 can be achieved with a sharp glass cone completely coated with 50 nm silver film. The strong field enhancement effect is associated with tight spatial confinement of a few tens of nanometers for the plasmonic field. A near-field Raman spectroscopic imaging system for semiconductor metrology will be presented as an example of its application.

A new approach that enables full control over three-dimensional state of polarization and field distribution near the focal point of a high NA objective lens will be presented. By combining the electric dipole radiation and the Richards-Wolf vectorial diffraction theory, the required input field at the pupil plane of the high NA objective lens for generating arbitrary three dimensionally linear and elliptical states of polarization at the focal point with an optimal spot size is found analytically. Such a focal field polarization tailoring technique may find important applications in single molecule imaging, tip enhanced Raman spectroscopy, high resolution optical microscopy, particle trapping and manipulation.

Antenna radiation and plasmonic nanofocusing: Surface plasmon polaritons are electron density oscillations at metal/dielectric interface. As a surface wave phenomenon, surface plasmon can be focused to produce strongly localized and enhanced electromagnetic field in a controllable manner. The challenges in plasmonic nanofocusing are to optimize the focus shape, size and strength. In this talk, I will discuss new perspectives of plasmonic nanofocusing from the antenna theory point of view and show that optimal plasmonic focusing can be achieved through matching the plasmonic structure to the spatial polarization distribution of the illumination. Such an optimal excitation can be understood using the antenna radiation theory. Essentially, the excitation polarization pattern needs to match the corresponding polarization pattern that would radiate from the plasmonic structure if the plasmonic structure were used as a transmitter. Examples using spatially homogeneous and inhomogeneous polarizations will be shown with their matching plasmonic structures. Numerical modeling and experimental confirmation of the optimal plasmon focusing with strongly focused radial polarization on planar, bulls eye and conical plasmonic structures will be presented. Numerical and experimental results of spiral plasmonic lens and its spin dependence will be demonstrated. Numerical simulation of optimal nanofocusing using conical tip integrated with a bull's eye and a spiral plasmonic lens will be shown. Potential applications of these plasmonic focusing structures will be discussed. Such a connection provides further insights into future plasmonic focusing and manipulation studies.

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