Hongki Lee

Molecular overlap with optical near-fields based on plasmonic nanolithography for ultrasensitive label-free detection by light-matter colocalization

In this work, we investigate the detection sensitivity of surface plasmon resonance (SPR) biosensors by engineering spatial distribution of electromagnetic near-fields for colocalization with molecular distribution. The light-matter colocalization was based on plasmonic nanolithography, the concept of which was confirmed by detecting streptavidin biotin interactions on triangular nanoaperture arrays after the structure of the aperture arrays was optimized for colocalization efficiency. The colocalization was shown to amplify optical signature significantly and thereby to achieve detection on the order of 100 streptavidin molecules with a binding capacity below 1fg/mm2, an enhancement by more than three orders of magnitude over conventional SPR detection.

Sub-10 nm near-field localization by plasmonic metal nanoaperture arrays with ultrashort light pulses.

Near-field localization by ultrashort femtosecond light pulses has been investigated using simple geometrical nanoapertures. The apertures employ circular, rhombic, and triangular shapes to localize the distribution of surface plasmon. To understand the geometrical effect on the localization, aperture length and period of the nanoapertures were varied. Aperture length was shown to affect the performance more than aperture period due mainly to intra-aperture coupling of near-fields. Triangular apertures provided the strongest spatial localization below 10 nm in size as well as the highest enhancement of field intensity by more than 7000 times compared to the incident light pulse. Use of ultrashort pulses was found to allow much stronger light localization than with continuous-wave light. The results can be used for super-localization sensing and imaging applications where spatially localized fields can break through the limits in achieving improved sensitivity and resolution.

Message and its origin authentication protocol for data aggregation in sensor networks

In distributed sensor networks, the researches for authentication in sensor network have been focused on broadcast authentication. In this paper, we propose a message and its origin authentication protocol for data aggregation in sensor networks, based on one way hash chain and Merkle tree authentication with pre-deployment knowledge. Proposed protocol provides not only for downstream messages but also for upstream messages among neighbors, and it solves the secret value update issue with multiple Merkle trees and unbalanced energy consumption among sensor nodes with graceful handover of aggregator. In treating compromised node problem, our protocol provides an equivalent security level of pair-wise key sharing scheme, while much less memory requirements compared to pair-wise key sharing scheme.

Super-resolution photoacoustic microscopy using a localized near-field of a plasmonic nanoaperture: a three-dimensional simulation study

Super-resolution microscopy has been increasingly important to delineate nanoscale biological structures or nanoparticles. With these increasing demands, several imaging modalities, including super-resolution fluorescence microscope (SRFM) and electron microscope (EM), have been developed and commercialized. These modalities achieve nanoscale resolution, however, SRFM cannot image without fluorescence, and sample preparation of EM is not suitable for biological specimens. To overcome those disadvantages, we have numerically studied the possibility of superresolution photoacoustic microscopy (SR-PAM) based on near-field localization of light. Photoacoustic (PA) signal is generally acquired based on optical absorption contrast; thus it requires no agents or pre-processing for the samples. The lateral resolution of the conventional photoacoustic microscopy is limited to ~200 nm by diffraction limit, therefore reducing the lateral resolution is a major research impetus. Our approach to breaking resolution limit is to use laser pulses of extremely small spot size as a light source. In this research, we simulated the PA signal by constructing the three dimensional SR-PAM system environment using the k-Wave toolbox. As the light source, we simulated ultrashort light pulses using geometrical nanoaperture with near-field localization of surface plasmons. Through the PA simulation, we have successfully distinguish cuboids spaced 3 nm apart. In the near future, we will develop the SR-PAM and it will contribute to biomedical and material sciences.

Ultrasensitive colocalization detection based on plasmonic nanolithography with molecular-overlapped optical near-fields

The detection sensitivity of surface plasmon resonance (SPR) biosensors has been improved by employing colocalization of spatial distribution of electromagnetic near-fields and detection molecules. We have used plasmon nanolithography to achieve light-matter colocalization on triangular nanoaperture arrays and optimized array configurations to improve colocalization efficiency. Streptavidin-biotin interactions were measured to validate the concept. It was confirmed that colocalized distributions of target binding and localized near-fields produced larger optical detection sensitivity. The colocalized detection was also shown to come with wider dynamic range than noncolocalized detection. The effective limit-of-detection of colocalized measurements was on the order of 30 pM. The colocalized detection sensitivity was estimated to be below 1 fg/mm2 in a 100-nm deep evanescent area, an enhancement by more than three orders of magnitude over conventional SPR sensor.

Nanoscale light confinement and fluorescence excitation using plasmonic metal nanostructures (Conference Presentation)

It is required to reduce the excitation volume of fluorescence with enhanced field intensity to apply fluorescence correlation spectroscopy (FCS) technique to the investigation of biological molecules. Field localization induced by plasmonic nanostructures enables measurements of molecular dynamics under high concentration exhibiting high signal-to-noise (SNR) ratio. To achieve this goal, we have investigated the feasibility of plasmonic monomer and dimer nanostructures for FCS techniques. We have studied field enhancement and localization induced by different gold monomer arrays whose shapes were circle, rhombus and triangle and also gold dimer arrays which had a gap of 10 nm. These plasmnoic nanostructures were considered to be on the gold film and glass substrate with chrome adhesion layer. We could shift the peak wavelength of field enhancement by changing the dimensions of nanostructures to spectrally overlap the field enhancement to excitation and emission spectrum of fluorophores. In the case of dimer configuration having a 90-nm diameter and a 20-nm height, we have induced the near-field localization with a light source at 671 nm whose dimension was 18×6×6 nm^3 with an enhanced field intensity by 500 times in comparison with a incident light. The field distribution was analyzed numerically and experimentally using finite-difference-time-domain method and near-field scanning optical microscope. We could measure the diffusion coefficients of 50-nm fluorescent beads with improved SNR which was found to be 44.6×10-14 m^2/s. Results would include the diffusion mapping of fluorescence molecules using imaging FCS technique to show the plasmnoic nanostructures is applicable to nanoscale FCS for study of molecular biology.

Plasmonic super-localization using nano-post arrays for biomedical spectroscopy

Plasmonic nanostructures enable field confinement which is locally amplified within sub-diffraction limited volume. The localized near-field can be useful in many biomedical sensing and imaging applications. In this research, we present the near-field characteristics localized by plasmonic nano-post arrays for biomedical spectroscopy. Circular gold nano-post arrays were modeled on gold and chrome films fabricated on a glass substrate whose thickness was 50, 20 and 2 nm, respectively. The nano-post arrays were fabricated with an e-beam lithography and a diameter of the post was 250 nm with periods varied as 500, 700, and 900 nm. The field localization produced by nano-posts was induced by angled illumination with a total internal reflection fluorescence microscope objective lens and measured by a near-field scanning optical microscope (NSOM). The NSOM has a tapered fiber probe with a 70-nm aperture and was a continuous-wave laser whose wavelength is 532 nm as light source. Incident TM-polarized light exhibited field localization on one side of an individual gold nano-post. When the direction of light incidence was changed opposite, localized field was switched to the opposite edge of the circular nano-post. We performed 3D finite difference time domain s for the field calculation and confirmed the localized field distribution at given illumination angles. We also discuss the potential applications of plasmonic field localization for analysis of biomolecules, cells, and tissues.

Near-field localization by two dimensional metallic nano-post arrays with ultrashort light pulses

Locally amplified near-fields can be induced with nanostructures within a sub-diffraction-limited volume, which is useful for biomedical imaging and sensing applications. Employment of field localization in the biomedical applications where the pulsed light is used necessitates the spatial and temporal characteristics of fields near nanostructures. We considered the gold nano-post arrays of three different shapes to localize the near-fields which are circular, rhombic, and triangular. They were modeled to be located on an ITO film and a quartz substrate with periods changing from 300 to 900 nm by 200 nm. Their size changes from 50 to 250 nm which corresponds to the radius for the case of circular nanoposts and the distance between the center and the vertices for equilateral rhombic and triangular nanoposts. Numerical calculation of near-fields at the top of nanoposts was performed with finite difference time domain method when the Gaussian pulses at center wavelengths of 532, 633, and 850 nm were normally incident. Near-fields localization occurred mainly at vertices of the nanoposts, which makes the triangular nanoposts of primary interest with an observation of the strongest field intensity within a diffraction limited field-of-view. The observed fields on the triangular vertices were enhanced by 7.85, 51.54, and 7268 when the center wavelengths were 532, 633, and 850 nm respectively. Their temporal peaks were delayed by 2.05, 4.03, and 14.49 fs, which indicates the correlation between field enhancement and time delay associated with electron damping process. It was shown that with rhombic and triangular nanoposts fields can be localized below 10 nm on vertices and their signal-to-noise ratio increased with a larger period.