Youngkyu Lee

Tunable directive radiation of surface-plasmon diffraction gratings

We experimentally demonstrate tunable radiation from a periodic array of plasmonic nanoscatterers, tailored to convert surface plasmon polaritons into directive leaky modes. Extending our previous studies on efficient directional beaming based on leaky-wave radiation from periodic gratings driven by a subwavelength slit, we experimentally show dynamic beam sweeping by tuning the directional leaky-wave mechanism in real-time. Two alternative tuning mechanisms, wavelength- and index-mediated beam sweeping, are employed to modify the relative phase of scattered light at each grating edge and provide the required modification of the radiation angle.

Efficient directional beaming from small apertures using surface-plasmon diffraction gratings

We demonstrate efficient optical directional beaming using an array of sub-wavelength patterns on a metallic surface. Specifically, a sub-wavelength sized slit placed next to a periodic grating is designed and optimized to realize maximum coupling efficiency and directional radiation into a leaky-wave plasmonic mode. Collective scattering from the corrugations forming the grating is synthesized to radiate towards the desired direction, and efficient beaming is achieved through tailoring the design parameters with a simple analytical model. We also prove that directivity can be further enhanced by improving the slit-grating coupling efficiency through efficient plasmon generation, showing improved angular response in far-field radiation.

Efficient apertureless scanning probes using patterned plasmonic surfaces

We present a novel concept to design apertureless plasmonic probes for near-field scanning optical microscopy (NSOM) with enhanced optical power throughput and near-field enhancement. Specifically, we combine unidirectional surface plasmon polariton (SPP) generation along the tip lateral walls with nanofocusing of SPPs through adiabatic propagation towards an apertureless tip. Three key design parameters are considered: the nanoslit width, the pitch period of nanogrooves for unidirectional plasmonic excitation and the pyramidal geometry of the NSOM probe for SPP focusing. Optimal design parameters are obtained with 2D analysis and two realistic probe geometries with patterned plasmonic surfaces are proposed using the optimized designs. The electromagnetic properties of the designed probes are characterized in the near-field and compared to those of a conventional single-aperture probe with same pyramidal shape. The optimized probes feature FWHM around 150nm, comparable with conventional NSOM designs, but over 3 orders of magnitude larger field enhancement, without degrading its spatial resolution. Our ideas effectively combine the resolution of apertureless probes with throughput levels much larger than those available even in aperture-based devices.

A thin-film piezoelectric PVDF-TrFE based implantable pressure sensor using lithographic patterning

We report a thin PVDF-TrFE (polyvinyledenedifluoride-tetrafluoroethylene) copolymer film pressure sensor, fabricated using the standard lithography process for cost-effective batch process, film uniformity, and high resolution of polymer patterning. PVDF-TrFE copolymer, a semi-crystalline material, was spin-coated into thin films (1 µm thick or less) to tap the near βphase formation. Such thin films, curing in the vacuum oven, offer significant piezoelectricity owing to the residual stress between the thin film and the substrate. Pressure measurements demonstrated that the dual film can achieve 0.6 – 1 V output voltages for 0 – 5 psi pressures, as normal physiological pressure range, with fast recovery time of around 0.2 second, and 9 % low output variation.

Plasmonic nanograting enhanced quantum dots excitation for cellular imaging on-chip.

We present the design and integration of a two-dimensional (2D) plasmonic nanogratings structure on the electrode of colloidal quantum dot-based light-emitting diodes (QDLEDs) as a compact light source towards arrayed on-chip imaging of tumor cells. Colloidal quantum dots (QDs) were used as the emission layer due to their unique capabilities, including multicolor emission, narrow bandwidth, tunable emission wavelengths, and compatibility with silicon fabrication. The nanograting, based on a metal-dielectric-metal plasmonic waveguide, aims to enhance the light intensity through the resonant reflection of surface plasmon (SP) waves. The key parameters of plasmonic nanogratings, including periodicity, slit width, and thicknesses of the metal and dielectric layers, were designed to tailor the frequency bandgap such that it matches the wavelength of operation. We fabricated QDLEDs with the integrated nanogratings and demonstrated an increase in electroluminescence intensity, measured along the direction perpendicular to the metal electrode. We found an increase of 34.72% in QDLED electroluminescence intensity from the area of the pattern and an increase of 32.63% from the photoluminescence of QDs deposited on a metal surface. We performed ex vivo transmission-mode microscopy to evaluate the nucleus-cytoplasm ratios of MDA-MB 231 cultured breast cancer cells using QDLEDs as the light source. We showed wavelength dependent imaging of different cell components and imaging of cells at higher magnification using enhanced emission from QDLEDs with integrated plasmonic nanogratings.

Colloidal Quantum Dot-Based Light Emitting Diodes With Solution Processed Electron Transporting Layer for Cellular Imaging

We report a quantum dot (QD)-based light emitting diode (LED) structure with a solution processed zinc oxide electron transporting layer, with patterning of anode and cathode on the substrate toward cellular imaging applications. Compared with the use of sputtered thin films, solution processed electron transporting layer improves robustness of the device, as crystalline ZnO shows low CB and VB edge energy levels, efficiently suppressing hole leakage current resulting in LEDs with longer lifetimes. We demonstrate a working lifetime of more than 12 h and a shelf-life of more than 180 days for the devices. Our solution-based process is applicable to microcontact printed and also spin-coated QD films. Further, we demonstrate electroluminescence of QD LEDs at 580and 600-nm emission wavelengths, with typical turn-ON voltage of 12 V. We compare operating characteristics of an array of individually addressable LEDs fabricated on a single substrate, with average size of each element being 800 μm × 200 μm. The LEDs with spin-coated ETL show a lifetime increase of more than three orders of magnitude compared with devices made using sputtered ETL layer. We find marginal increases in the intensity and uniformity of devices fabricated using a spin-coated ZnO layer. Finally, we demonstrate a compact multicolor light source for cell characterizations by imaging HEMA 3 stained MDA-MB 231 breast cancer cells on-chip. We demonstrate the capability of the devices as a light source by measuring intensity across stained cells with QDLEDs of two different wavelengths and show the correlation as expected with the absorption profile of the fluorescent dye.

Micro-patterned quantum dots excitation for cellular microarray imaging

We present a compact light source designed for arrayed lab-on-chip cell imaging with the motivation of creating a microchip based system for detection of tumor cells. We aim at creating a multicolor light source that can be integrated for on-chip imaging. Colloidal quantum dots (QDs) were used as the emission layer due to their unique capabilities like multicolor emission, multiple available methods of electrical and photo excitation and compatibility with silicon fabrication were achieved. Micropatterning of QDs was used to create both electrically and photo excited light sources. We study the photo activated source as a robust, high intensity light source which can be easily integrated with lab-onchip systems while requiring additional filters and excitation systems and compare it with an electrically excited source with the capability of individually addressable, multicolor sources on a single substrate eliminating the need for additional optical components. To demonstrate the efficacy of our design, we performed ex vivo transmission mode microscopy to evaluate the nucleus-cytoplasm ratios of cancer cells. We showed the capability of imaging of inner cell structures using multiple wavelengths to perform high contrast imaging and observation. We performed immunofluorescence excitation of MDA-MB 231 cancer cells, cultured in a microwell array. Our method provides patterned multicolor light sources and low cost which are suitable for high-throughput microarray cellular imaging.

Multicolor colloidal quantum dot based light emitting diodes using a solution processed electron transporting layer

We report a multilayered Quantum Dot (QD) based Light Emitting Diode structure with a solution processed zinc oxide nanoparticle based electron transporting layer, with subsequent patterning of the anode and cathode on the same substrate. The solution processed electron transporting layer improves the robustness of the device as compared to the use of sputtered thin films, resulting in LEDs with longer lifetimes. The use of spin coated zinc oxide layer and an insulating aluminum oxide layer allows for fabrication of LEDs using spin coating of quantum dots while patterning the anode and cathode. Zinc oxide shows low CB and VB edge energy levels, efficiently suppressing the hole leakage current. We have shown successful electroluminescence at 580nm and 600nm wavelengths, with a typical turn on voltage of about 12V. An array of individually addressable LEDs was fabricated on a single substrate, with the average size of each element of 800um × 200um. A potential application of such compact multicolor light source for cell characterizations is demonstrated by imaging of HEMA 3 stained MDA-MB 231 breast cancer cells on-chip.

Tunable plasmonic substrates with ultrahigh Q-factor resonances

Precisely tailored plasmonic substrates can provide a platform for a variety of enhanced plasmonic applications in sensing and imaging. Despite the significant advances made in plasmonics, most plasmonic devices suffer critically from intrinsic absorption losses at optical frequencies, fatally restricting their efficiency. Here, we describe and engineer plasmonic substrates based on metal-insulator-metal (MIM) plasmon resonances with ultra-sharp optical transmission responses. Due to their sharp transmission spectrum, the proposed substrates can be utilized for high quality (Q)-factor multi-functional plasmonic applications. Analytical and numerical methods are exploited to investigate the optical properties of the substrates. The optical response of the substrate can be tuned by adjusting the periodicity of the nanograting patterned on the substrate. Fabricated substrates present Q-factors as high as ∼40 and refractive index sensing of the surrounding medium as high as 1245 nm/RIU. Our results indicate that by engineering the substrate geometry, the dielectric thickness and incident angle, the radiation losses can be greatly diminished, thus enabling the design of plasmonic substrates with large Q factor and strong sensitivity to the environment.