Photonic Nanostructures

Nanostructures such as photonic crystals, plasmonic nanostructures and metamaterials can dramatically impact light-matter interaction and induce novel optical properties. Furthermmore, they allow unprecedented engineering freedom, offering new avenues to novel photonic device applications. Photonic crystal is a structure with periodic refractive index profile and modulates light propagation in much the same way as the electronic wave is modulated in a crystal. The resulting changes in the optical mode profiles and photon density of states can be used to enable a variety of novel photonic devices. Metallic nanostructures can support strong and highly localized optical resonance called surface plasmon. Surface plasmon can be characterized as combined excitation of photon and free electron gas. The hybrid nature allows localization and operation in the nanoscale and enables a wide array of novel optical phenomena. Metamaterials and metasurfaces are artificial structure consisting of deep subwavelength-scale features, which may be considered artifical atoms and can be designed to exhibit prescribed optical properties. Our research is focused on investigating novel optical properties in these photonic nanostructures and developing new applications.

Photonic nanostructure

 

Plasmon Enhanced Frequency Upconversion

The strong local field in plasmonic nanostructures can enhance many optical processes including absorption, emission and energy transfer. Frequency upconverion is of particular interest because the nonlinear nature of the process can lead to dramatic enhancement. We developed efficient synthesis process for NaYF4:Yb,Er upconversion nanoparticles, highly controlled layer-by-layer deposition technique, and scalable fabrication of plasmonic nanostructures using laser interference lithogrtaphy and nanoimprint lithography. Conducting photoluminescence spectroscopy and time-resolved spectroscopy as well as theoretical study based on quantum electrodynamics and rate equations analysis, we demonstrated that the absorption enhancement is the most important factor due to the nonlinearity and also that the plasmon can enhance the Förster energy transfer process as well as absorption and emission. Plasmon enhancement of frequency upconversion has many promising applications which include solar energy harvesting, nano-medicine, sensing and security.

plasmon enhanced upconversion

(Left) Transmission electron micrograph of NaYF4:Yb,Er nanoparticle
(Right) Scanning electron micrograph (SEM) of silver nanograting designed for
upconversion enhancement. Inset: Cross-sectional SEM showing the stack of
silver grating, Si3N4 spacer layer and NaYF4:Yb,Er nanoparticle layer

 

Thermoplasmonics - A New Way of Heat Engineering

In addition to the well-studied optical processes such as scattering and luminescence, surface plasmon strongly impacts heating and thermal radiation as well. In the newly emerging field often called thermoplasmonics, plasmonic structures are used to control heating and thermal radiation. We are developing gold nanowire forest which was found to be highly effective in water heating. The structure has hierarchical design in which microscale funnel structure interacts strongly with infrared light while the nanowires exhibit strong nanofocusing in the visible frequency. In another project, we are developing artificial structure that supports strong surface plasmon like resonance in the terahertz frequency region. When heated, the surface plasmon modes are thermally excited and exhibit local light intensities far exceeding the normal heat radiation governed by the Planck's blackbody radation law. These studies are expected to pave ways for new energy technologies

Thermoplasmonics

(Left) Scanning electron micrograph of gold nanowire forest exhibisting ultrabroadband absorption and consequently efficient water heating
(Right) Schematic of metamaterial structure exhibiting plasmon-like resonance in the tereahertz region and the thermally excited optical mode in an antenna coupled to the metamaterial


Nano-Medicine

Plasmonic nanostructures can be used to enable a new diagnostic and therapeutic approach for cancer. We study bladder cancer, which is the 4th most common non-skin cancer among men in the U.S. and is also prevalent in Asian counrties. We developed gold nanorod conjugated with antibody to epidermal growth factor receptor (C-225) which is known to be overexpressed on bladder cancer cells. Also, the gold nanorod is designed to exhibit strong plasmon resonance and thus strong absorption in the near infrared region where the normal tissue exhibits minimal absorption. Thus, the gold nanorod when injected into the bladder selectively and specifically bind to the cancer cells and, upon subsequent irradiation by infrared laser, creates local heating to the point of ablation. Furthermore, the gold nanorod may be further coupled with upconversion nanoparticles so that the resultant nanocluster can simultaneously perform imaging and killing. We have demonstrated these functionalities in both in vitro and in vivo settings and are working toward eventual human trials.

nano-medicine

(Top Left) Dark-field micrograph showing the gold nanorod coupled to bladder cancer cells
(Top Right) Optical micrograph of bladder cancer cell (c1) and fluorescence microgrpah under infrared illumination showing the bright signal from the upconversion nanoparticles
(Bottom) Cell viability assay results showing clear, selective therapeutic effect of gold nanorod and upconversion nanoparticle clusters

 

Mid-Infrared Photonics

Finally, mid-infrared region represents a frequency range with a variety of new opportunities. It is a natural spectral window for optical communications which are performed currently in the near-infrared region. Mid-infrared region also contains a wealth of molecular vibrational resonances. The rich absorption spectra can be used to precisely identify targeted molecules, enabling new sensing technologies. To realize these potential, we are developing photonic devices based on chalcogenide glass, which exhibits excellent transmission and high nonlinearity in the mid-infrared region. Also, the material can be deposited at low temperatures, making it compatible with the CMOS processes. High quality waveguides and resonators based on chalcogenide materials would open doors to a wide range ofmid-infrared photonic devices such as optical switches, couplers, sensors.

Mid-infrared photonics

(Left) Scanning electron micrograph of chalcogenide waveguide fabricated by
electron-beam lithography
(Bottom) Scanning electron micrograph of chalcogenide disk resonantor fabricated by photolithography