Localized surface plasmon was formed with plasmonic nanohole arrays for measurement of the Raman spectrum in polystyrene beads. For the selective acquisition of enhanced Raman signals, we used the surface-enhanced Raman spectroscopy (SERS) system employing particle manipulation within the electrode chip. This integration involves the amalgamation of metal nanostructures and microelectrodes to assess the electrophysiological characteristics of microparticles. The manipulation of particle movement, contingent upon their electrical properties such as conductivity and permittivity, can be controlled through modulation of the frequency of the applied alternating current field. In our integrated system, Raman spectroscopy can be conducted with enhanced intensity. Real-time measurement of enhanced Raman scattering is achieved by controlling the direction of force exerted on single particle and selectively trapping it on the nanostructured surfaces.
KEYWORDS: Light sources and illumination, Speckle, Microscopy, Reflection, 3D image processing, Super resolution, Biological imaging, 3D image reconstruction
Optical super-resolution microscopy has revolutionized imaging in the lateral axis, enabling nanoscale structure visualization with unprecedented detail. However, achieving high axial resolution along the z-axis remains challenging. In this study, nanoscale fluctuation-enhanced axial localization microscopy addresses this issue by incorporating metallic structures, specifically reflective optical devices, into dynamic speckle illumination microscopy. By controlling light waves within the fluidic chip, these metallic devices enable super-resolution to be achieved not only in the lateral direction but also along the z-axis, all in a cost-effective manner. Experimental investigation using 100-nm fluorescent beads and the U-87 MG cell membrane demonstrates axial-resolving performance of fluctuation-enhanced imaging compared to conventional methods. The application of an optical fluctuation-based reconstruction algorithm further allows the extraction of 4-fold enhanced axial information over diffraction-limited system.
When a microparticle is exposed to an alternating current (AC) electric field, the polarized particle is moved by the interaction between the dipoles and the AC electric field under dielectrophoresis (DEP). The DEP force is widely used for manipulation of microparticles in diverse practical applications such as 3D manipulation, sorting, transfer, and separation of various particles such as living cells. In this study, we propose integration of surface-enhanced Raman spectroscopy (SERS), an extremely sensitive and versatile technique based on Raman scattering of molecules supported by nanostructured materials, with DEP using a microfluidic device, i.e., the microfluidic device combines metal nanostructures and microelectrodes to characterize electrophysiological and biochemical properties of biological cells. The movement of particles, which varies depending on the electrical properties such as conductivity and permittivity of particles, can be manipulated by the cross-frequency change, which is one of the DEP properties. Raman spectroscopy using this DEP-SERS integrated system can be performed with an improved signal-to-noise ratio by determining the direction of the DEP force applied to the cells with respect to the applied AC power and collecting them on the nanostructure.
Metallic nanostructures have the potential to be used in a variety of applications related to sensing and imaging biological molecules due to their ability to enhance the way molecules absorb and emit light. However, the interaction between metallic nanostructure and molecules can give rise to difficulty with determining precise molecular positions and orientations and therefore pose major challenges in the field of super-resolution imaging. In this work, we used axially defocused imaging to analyze the interaction between a single fluorescent molecule and a metallic nanostructure. In addition, a pattern matching algorithm was used to analyze the images, explore the interaction between the molecule and the nanostructure and thereby determine the lateral position. The accuracy was found to improve while the degree was dependent on the dipolar orientation and the distance between dipole and nanostructure. This approach has the potential to improve the reliability of using metallic nanostructures for imaging and sensing in the future and opens up new possibilities for various imaging and sensing methods.
Surface plasmon, collective electron oscillation induced by light absorption in noble metals, has received renewed attention that opens a new area of photonics research in what is known as thermoplasmonics. As thermoplasmonics develops, opto-thermal response measurement of a single nanostructure becomes essential. In this study, we propose a collection-type near-field scanning optical microscopy (NSOM) that can simultaneously measure light absorption and near-field enhancement on a single nanostructure. We analyzed light absorption from optically induced thermal expansion while measuring a near-field coupled with the NSOM tip. We have observed discrepancy and nonlinearity of angular spectrum between light absorption and near-field enhancement on gold thin films and compared with simulation results based on iterative opto-thermal analysis. We were able to determine the cause of the axial shift on the NSOM and the mechanisms by which the discrepancy may ariss. The proposed technique can also acquire optical characteristics of a single disk in a periodic array of gold nanodisks, and even measure the gaps between the disks. Furthermore, we expect the proposed technology to be extended to measuring near-field thermal characteristics of more complicated structure such as metamaterials.
Surface plasmon resonance (SPR) has been widely exploited for label-free molecular detection in various applications. Despite significant advances in SPR technology, most uses are based on the flat device platform, limiting their scopes of applications. This is because plasmonic devices with a finite radius of curvature are difficult to experimentally implement and require heavy computing resources to analyze. In this report, we carried out calculation of SPR characteristics for curved structures by using segmented-wave analysis which enables calculation of curved plasmonic structures efficiently. We conducted calculation of Au thin film on a curved substrate for the curvature radius in a range of 100 to 3000 um for parallel and perpendicular light incidence in which chord length is fixed regardless of the curvature radius. Reflectance spectra by each segment were obtained using the transfer matrix method based on thin-film optics. This is followed by the results expressed as a discrete sum of segmental results. Then we compared the segmented-wave analysis with the finite element method (FEM) model for validation of results and prove the efficiency of the suggested method. The calculation time of segmented-wave analysis took less than 5 seconds on the personal computer whereas FEM took about 25 hours on the workstation. It was found that the results from the segmentation were in excellent agreement, resonance wavelength in particular, while other parameters such as reflectance and resonance width under parallel incidence showed disparity between the two models in the case of the short segmentation.
Surface plasmon resonance (SPR) has been heavily used as biosensors and studied dominantly on a flat surface. Recently, flexible sensor platforms have emerged, for example, as wearable devices. Here, we report investigation of SPR characteristics on a curved film structure. A rigorous 3D computational model requires extremely heavy calculation time and resources. Therefore, we adopted segmentation analysis in which curved surface is divided into an array of flat segments. Such analysis allows fast and efficient calculation. The results indicate that increased curvature produces broader SPR due to wider momentum-matching. The segmentation analysis is expected to play a critical role for diverse optical elements on curved surface.
Surface plasmon represents oscillations of electrons at the interface between metal and dielectric layers. Surface plasmon resonance (SPR) is influenced by the environment near the surface, which has been the basis for label-free biosensor structure for various applications of molecular detection. An important aspect of SPR biosensing is that its characteristics are affected by the geometrical structure. Yet most research has focused largely on a structure using flat surface. Although flat structure is suitable for typical sensor applications, it may not be appropriate for wearable or in vivo applications. In this study, we analyzed the effects of surface curvature on flexible SPR biosensors. Curved surface was approximated using a segmented model in which each segment is treated as a flat surface with a different incident angle and then optical characteristics of the overall model were calculated by rigorous coupled wave analysis in two different configurations of light incidence. We calculated curvature effects on SPR with curvature radius larger than 255 μm. It was found that regardless of the incident configurations, resonance curves tend to broaden with increased curvature due to larger momentum dispersion. Resonance shifts as a result of DNA immobilization and hybridization decrease with curvature. The analysis was extended to multi-curvature structure and finds significant fluctuation of resonance shift for parallel light incidence. The study can be of profound importance for plasmonic devices using flexible substrates.
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