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.
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.
Metallic nanowire arrays have been used for discriminating linear polarization and used as a wire-grid polarizer in the simplest form. For their linear geometry, metallic nanowire arrays distinguish p- (perpendicular) from s-polarization (parallel) with respect to the direction of nanowire arrays. In this study, we analyze opto-thermal response on metallic nanowire arrays, for which we performed simulation using finite element method to solve wave-coupled heat transfer equations on metallic nanowire arrays. From the analysis, it was shown that s-polarized light presents lower maximum temperature Tmax = 331.7 K than that of p-polarized light Tmax = 354.5 K under an incident power at 0.1 mW/μm2. In addition, thermal extinction which is defined as the ratio of maximum temperature between p- and s-polarization is measured as 4.78 dB which corresponds to a temperature difference of 54.3K. We have also investigated dispersive misregistration assuming that metallic nanowire arrays are integrated with an imaging detector placed at the focal length of a tube lens. The dispersive effect of a convex tube lens was evaluated using non-uniformity metrics that measure offaxis performance (NTroff, ERoff) and wavelength dependence (NTrλ, NERλ). Maximum non-uniformity was measured to be NTrλ.max = 0.84935, NERλ.max = 0.90139, NTroff.max = 0.93211, and NERoff.max = 0.93624. Image misregistration induced by dispersive effects was also assessed.
We have investigated the plasmonic effect of a gold thin film on the optical properties under a range of combinations of incident wavelengths, incident angles and polarization states, while assuming various film thicknesses. Theoretical calculation was performed with rigorous coupled-wave analysis based on the temperature-dependent Drude-Lorentz dispersion model. The calculation method considers the effects of absorption, which is converted to heat in a gold thin film and can affect material parameters such as permittivity. Experimentally, light absorption and field enhancement factor were directly measured using near-field scanning optical microscopy. We have also measured the near-field distribution and thermal effects in the gold thin film. Absorption and field enhancement experimentally measured using three incident wavelengths of 488, 532, and 721 nm for a thin gold film with thicknesses 20, 50, and 70 nm showed good agreement with calculated data. Also observed was the disparity between the incident angles that correspond to maximum absorption and highest field enhancement. The results can help understand the thermal effects on optical properties of plasmonic nanostructures for applications in biological imaging and sensing techniques.
In this study, we investigate dispersive effects of a wire-grid polarizer (WGP) in imaging polarimetry. Dispersion in periodic structures such as WGP may cause dispersive misregistration in images captured in the far-field imaging system. As a measure of performance, we defined off-axis non-uniformity (NToff) and extinction ratio (ERoff) to evaluate non-uniformity induced by off-axis imaging scene. Similarly, we considered metrics to evaluate dispersive effects as wavelength-dependent non-uniformity in extinction ratio (NERλ) and transmittance (NTλ). Significant non-uniformity in the performance of a WGP was measured: the highest non-uniformity was obtained as |NToff|max = 0.93211 at Λ = 400 nm and |NERoff|max = 0.93624 at Λ = 600 nm, while |NTλ|max = 0.84935 at Λ = 400 nm and |NERλ|max = 0.90139 at Λ = 300 nm. We also present images in imaging polarimetry that suffers from dispersive effects.
We demonstrated gold nanodimer arrays could improve the signal-to-noise ratio (SNR) of fluorescence correlation spectroscopy (FCS). In this research, we explore the feasibility of plasmon-enhanced FCS for biomolecular study using a nanodimer array whose gap size was 18 nm. Fluorescence nanobead with a diameter of 40 nm was first examined to verify if gold nanodimer arrays can enhance SNR of fluorescence and scattering intensities. We emphasize that plasmon-enhanced FCS can improve the precision for analyzing the dynamics of the particle by combining scattering characteristics of nanodimer arrays to surface plasmon resonance imaging technique. We have also observed the fluorescence enhancement and plasmon scattering in the movement of lysosome in HEK293 cells. It was found that we could measure diffusion properties such as diffusion coefficients and anomalous exponents with a low standard deviation.
In this study, we have used scanning probe microscopy (SPM) to validate temperature-dependent thermo-plasmonic calculation which is called iterative opto thermal analysis (IOTA). We have applied temperature-dependent Drude-Lorentz model to IOTA. To solve wave-coupled heat transfer equation, finite element method based multi-physics analysis tool has employed with wave optics module and heat transfer module under the proper boundary and initial conditions. For this study, various plasmonic structures were considered to acquire temperature and plasmonic field enhancement using IOTA and SPM experiments. As a result, we have improved plasmonic analysis with consideration of temperature-dependence.
We have investigated the excitation of fluoresce molecules using nanoscale light confinement on the plasmonic nanostructures. We have fabricated gold nano-dimer arrays whose diameter and height were 100 and 20 nm respectively on 20-nm gold film with BK7 substrate with a period of 746 nm. We have calculated the field distribution by three dimensional finite-difference time-domain (FDTD) method and confirmed the field localization on the dimer’s gap whose size was 18 nm. The field confinement was induced by a light source at 671 nm and experimentally measured by near-field scanning optical microscopy (NSOM) under same incident condition given at FDTD calculation. The angle of surface plasmon resonance (SPR) was chosen to enable both fluorescence microscopy and SPR microscopy simultaneously. Given a resonance angle, a dimer could provide a subdiffraction-limited observation volume to study the dynamics of fluorescence molecules and a highly sensitive light scattering probe as a nanoantenna. By employing dual microscopic images, we could separate the fluorescence excitations within a subdiffracion-limited volume from those outside the volume. We calculated scattering intensities of fluorescence nanobeads on the dimer nanoantenna to assure the presence within an observation volume when they diffuse near the dimer nanoantenna with Brownian motion. We have applied the subdiffraction-limited volume to fluorescence correlation spectroscopy (FCS) to measure the dynamics of fluorescence molecules with high signal-to-noise ratio, with additional spatial analysis from SPR microscopy. The result of imaging FCS using gold nano-dimer with SPR microscopy shows novel applications for nanoscale sensing and imaging methods.
Heat induced by electromagnetic absorption affects optical properties and experimental conditions. For this reason, thermal effects in optics remain important. In this work, we investigate thermal properties of a wire-grid polarizer (WGP). A WGP is a well-known optical polarizing device and easy to combine with planar structures such as microfluidic channel and other optical components. We analyzed thermal characteristics of a WGP by considering the effects of various geometric parameters: wire-grid period, height, and fill factor. For far-field calculation of optical characteristics, rigorous-coupled wave analysis (RCWA) has been used with 40 spatial harmonics. Together, we solved wave-coupled heat transfer equation by 2D finite element method (FEM) for computing electromagnetic-thermal characteristics. 2D FEM calculation was verified with RCWA and 3D FEM. From the analysis, it was shown that a fill factor was the most dominant geometrical parameter for near-field thermal extinction. In addition, TM polarized light has higher local temperature Tmax = 354.5 K than that of TE polarized light Tmax = 331.7 K with an incident power at 0.1 mW/μm2. Polarization switching was found to induce thermal extinction of 4.78 dB with a temperature difference ▵T = 54.3 K in an identical WGP structure. Furthermore, the ratio of steady-state time was almost uniform within 15%, because the heat transfer mechanism is almost identical for TE and TM polarization. Time scale was on the order of μs. We expect this result to be useful for the integration of WGPs in polarization-sensitive thermal switching applications.
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