Nanophotonics for dark materials, filters, and optical magnetism

Mengren Man, Purdue University

Abstract

Research on nanophotonic structures for three application areas is described, a near perfect optical absorber based on a graphene/dielectric stack, an ultraviolet bandpass filter formed with an aluminum/dielectric stack, and structures exhibiting homogenizable magnetic properties at infrared frequencies. The graphene stack can be treated as a effective, homogenized medium that can be designed to reflect little light and absorb an astoundingly high amount per unit thickness, making it an ideal dark material and providing a new avenue for photonic devices based on two-dimensional materials. Another material stack arrangement with thin layers of metal and insulator forms a multi-cavity filter that can effectively act as an ultraviolet filter without the usual sensitivity of the incident angle of the light. This is important in sensing applications where the visible part of the spectrum is to be removed, allowing detection of ultraviolet signals. Finally, achieving a magnetic material that functions at optical frequencies would be of enormous scientific and technological impact, including for imaging, sensing and optical storage applications. The challenge has been to find a guiding principle and a suitable arrangement of constituent materials. A lattice of dielectric spheres is shown to provide a legitimately homogenized material with a magnetic response. This should pave the way for experimental studies. More specifically, a graphene stack is designed, fabricated and characterized. The structure shows strong absorption of light. Spectroscopic ellipsometry is used to obtain the complex sheet conductivity of graphene. Further modeling results establish the graphene stack as the darkest optical material, with lower reflectivity and higher per-unit-length absorption than alternative light-absorbing materials. An optical bandpass filter based on a metal/dielectric structure is modeled, showing performance that is largely independent of the angle of incidence. Parametric evaluations of the reflection phase shift at the metal-dielectric interface provide insight and design information. Filter passbands in the ultraviolet (UV) through visible or longer wavelengths can be achieved by engineering the dielectric thickness and selecting a metal with an appropriate plasma frequency, as demonstrated in simulations. A lattice of suitable dielectric particles is shown to fulfill the requirements for a magnetic optical material. Using Mie theory, the microscopic origin of the magnetic response is explicitly identified as being due to the magnetic dipole resonance of an isolated sphere. This provides a design basis, and dielectric and lattice requirements with candidate dielectrics that will allow magnetic materials to be designed and fabricated for optical applications are presented.

Degree

Ph.D.

Advisors

Webb, Purdue University.

Subject Area

Electromagnetics|Nanotechnology|Optics

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