Research
Three-dimensionally confined nanoparticles that harvest strong near-field interaction with light, such as colloidal quantum dots (QD) and Au nanoparticles (NP), have opened new directions in nanophotonic devices and integration, as well as applications in biomedicine such as cancer cell ablation, drug delivery, and bio-manipulation. Since its inception in January 2004, the UWEE Photonics Laboratory has ventured into the general directions of nanophotonics and biophotonics by "bridging nanoparticles with light." Utilizing the unique optoelectronic properties and surface chemistries of colloidal QDs, we have proposed QD nanophotonic integrated circuits using molecular self-assembly fabrication. The published work along this direction includes sub-diffraction limit QD waveguide, nanoscale QD photodetector with high sensitivity and spatial resolution, and "epitaxial" deposition of colloidal QDs through electrostatic layer-by-layer self-assembly for optoelectronic devices. Utilizing localized surface plasmon resonance in the Au NPs, we have proposed a new plasmonic tweezers for bio-manipulation and nano-fabrication. The platform allows versatile applications, and we have demonstrated (1) high-efficiency optical manipulation of micro/nano-particles and biological cells, (2) long-range trapping of nanowires with very low optical intensity, (3) laser-controlled micro-concentrator, micro-sorter, and micro-mixer for opto-fluidics. In addition, we have also been working on new dielectrophoresis-field flow fractionation devices for DNA separation.
Schematic drawing of the sub-diffraction limit quantum dot integrated circuit.
To create nano-scale waveguides and find an alternate means of sub-diffraction limit optical propagation, quantum dots are a potential route to success due to their high degree of quantum confinement and respective size. In particular, our group investigates QD behavior under optical stimulation in terms of absorption, emission and corresponding linear gain characteristics. A gain model for CW and pulse pumped QDs has been derived and applied to core and core/shell structures such as CdSe and CdSe/ZnS over a range of pump powers. Furthermore, the optical propagation for a 1D array of quantum dots forming a waveguide has been simulated and reveals a compensating relation between gain and coupling coefficient. In addition, work has been done to determine the field distribution between quantum dots and we have demonstrated fabrication through DNA-based and two-layer self-assembly methods. Testing of the waveguides waveguiding effect with raised pump light for straight waveguide and waveguide with 90-degree bend. The QD waveguide achieves lower loss compared to sub-diffraction limit plasmonic waveguides.
Quantum dot array nanophotonic waveguide. (IEEE J. of Select. Topics in Quantum Electron., 2005)
Poynting vector distribution of the quantum-dot waveguide. (Optics Letters, 2007)
Fluorescence, AFM, and SEM images of the quantum dot waveguides. (Nanoscale Reseach Letters, 2007
and Nanotechnology, 2008)
Observation of waveguiding effict in 500 nm-wide QD waveguides: (Left) 10 mm straight waveguide. (Right) 20 mm waveguide with 90 degree bend. (Nano Letters, 2006 and its highlight in
Nature Photonics)
A nanoscale photodetector plays an important role in interfacing nanophotonic integrate circuits with the outside electronic world. We proposed a nanoscale photodetector consists of a pair of electrodes spaced nanometers apart with colloidal QDs in between. The tunneling current between the metal electrodes is mediated by the QDs and can be modulated by optically exciting the QDs at different intensities. The detection area is defined by the electric field across the nanogap, thus achieving high spatial resolution. Through optimizing the material and fabrication, we demonstrated sensitivity and bandwidth higher than prior work in nanoscale photodetection.
Optics Express, 2007, its highlight in Nature
Photonics and Micro and Nano Letters, 2007
The CdTe quantum dots are synthesized and dispersed in aqueous solution with either 2-mercaptoethylamine (positively charged) or thioglycolic acid (negatively chargd) as capping stabilizers. By electrostatic attraction, the charged quantum dots are self-assembled layer by layer on an indium tin oxide substrate modified with (3-aminopropyl)ethoxysilane. This process allows control of active layer thickness by self-assembly, and can in principle be applied to a wide range of substrates.
Applied Physics Letters, 2008
Non-invasive manipulation of single micro- and nano-sized particles is an important tool for both biotechnology and nanotechnology. It allows cells, cellular components, synthetic marker particles treated with biochemical tags, and micro- and nano-sized particles to be collected, separated, concentrated, transported and assembled without damage to the objects themselves. Conventional optical tweezers require high optical intensity with tight focusing to achieve the necessary force for manipulation. In our research, we propose utilizing enhanced scattering and absorption cross section of metal nanoparticles under localized surface plasmon resonance to achieve optical manipulation with low intensity.
Opto-Plasmonic Tweezers.(Optics Letters, 2007)
IEEE J. of Select. Topics in Quantum Electron., 2007.
Optics Express, 2008.
Applied Physics Letters, 2008.
Optics Express, 2008.
