Plasmon-Enhanced Frequency Upconversion

The strong local field in plasmonic nanostructures can enhance many optical processes including absorption, emission, and energy transfer. Frequency upconversion is of particular interest because the nonlinear nature of the process can lead to dramatic enhancement. We developed an 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 lithography and nanoimprint lithography. Conducting photoluminescence spectroscopy and time-resolved spectroscopy as well as theoretical study based on quantum electrodynamics, 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. Based on this fundamental understanding, we were able to design a plasmonic nanostructure that resulted in more than 1000-fold enhancement of upconversion luminescence. Plasmon enhancement of frequency upconversion has many promising applications which include solar energy harvesting, nano-medicine, sensing, and security.

Selected publications on this topic are here.

Plasmon enhanced upconversion

Metal-insulator-metal (MIM) enhanced upconversion

MIM-UCNP cross section

Fabrication of MIM structure

MIM-UNCP power dependence

Over 1000-fold enhancement of upconversion

Nonlinear and Quantum Optical Devices

We develop integrated micro- and nano-photonic devices based on novel materials such as chalcogenide glass. Known for high nonlinearity and excellent transparency in the infrared region, these devices offer excellent platform to build nonlinear and quantum optical devices. Also, the material can be deposited at low temperatures, making it COMS-compatible. High-quality waveguides and resonators based on chalcogenide materials would open doors to a wide range of applications including sensing and communications. Furthermore, operating in the mid-infrared region has many unique advantages. It is a natural spectral window to expand into for optical communications which are currently performed 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. It also provides new advantages for quantum optical devices that have not been fully exploited yet. Our current research is focused on developing high quality micro-optical devices that could realize the full potential of mid-infrared integrated photonic device platform for nonlinear and quantum optics.

Selected publications on this topic are here.

Ring resonator

Chalcogenide ring resonator

Wedge resonator

Chalcogenide wedge resonator

Nanophotonics for Biology and 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 nanorods, 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. Alternatively, irradiation with femtosecond laser induces nano-cavitation, creating a hole on the cell membrane. A subsequent intake of chemotherapy drug results in effective cell killing at very low drug concentrations, enabling highly targeted chemotherapy. 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.

Selected publications on this topic are here.

Optoporation of bladder cancer

Optoporation and Targeted Chemotherapy of Bladder Cancer Cells

Nanophotonics for Energy Technology

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 radiation law. These studies are expected to pave ways for new energy technologies.

Selected publications on this topic are here.

Scanning electron micrograph of gold nanowire forest

Scanning electron micrograph of gold nanowire forest

Schematic of metamaterial structure

Schematic of metamaterial structure

Photonic Nanostructures

Nanostructures such as photonic crystals, plasmonic nanostructures and metamaterials can dramatically impact light-matter interaction and induce novel optical properties. Furthermore, they allow unprecedented engineering freedom, offering new avenues to novel photonic device applications. Photonic crystal is a structure with a 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 structures consisting of deep subwavelength-scale features, which may be considered artificial 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.

Selected publications on this topic are here.

2D periodic nanostructure

2D Periodic Nanostructure

Plasmonic Molecule

Plasmonic Molecule