Sasani Jayawardhana

Sub‐15nm Optical Fiber Nanoimprint Lithography: A Parallel, Self‐aligned and Portable Approach

We demonstrate the parallel patterning of multiple optical-fiber facets using nanoimprint lithography on a novel platform. A resolution of better than 15 nm is demonstrated and up to 40 optical-fiber facets have been imprinted in parallel. The lithography platform features a self-alignment mechanism (see figure) that greatly relaxes the mechanical requirements, allowing for the demonstration of a compact, portable imprinting-module and the accommodation of non-planar, biological molds. The imprinted fibers are metalized and employed as bi-directional probes for surface-enhanced Raman scattering.

Additional Enhancement of Electric Field in Surface-Enhanced Raman Scattering due to Fresnel Mechanism

Surface-enhanced Raman scattering (SERS) is attracting increasing interest for chemical sensing, surface science research and as an intriguing challenge in nanoscale plasmonic engineering. Several studies have shown that SERS intensities are increased when metal island film substrates are excited through a transparent base material, rather than directly through air. However, to our knowledge, the origin of this additional enhancement has never been satisfactorily explained. In this paper, finite difference time domain modeling is presented to show that the electric field intensity at the dielectric interface between metal particles is higher for “far-side” excitation than “near-side”. This is reasonably consistent with the observed enhancement for silver islands on SiO2. The modeling results are supported by a simple analytical model based on Fresnel reflection at the interface, which suggests that the additional SERS signal is caused by near-field enhancement of the electric field due to the phase shift at the dielectric interface.

Optical fiber sensor based on oblique angle deposition

The technique of oblique angle deposition has been extended to the fabrication of nanostructured metal coatings on the tips of standard silica optical fibers by thermal evaporation. The coatings are initiated as metal island films, which grow into extended rodlike structures as the deposition continues. The nanorod coatings demonstrate excellent surface-enhanced Raman scattering performance with variability of less than 10% as shown by direct measurements off the fiber tip with thiophenol as a test analyte. However, in the remote sensing configuration, the nanorod structures perform no better than thin metal island films. This appears to be mainly due to reduced transmission when nanorod lengths exceed ~100 nm. Moreover, the variability of remote measurements is increased to 18%. This is believed to be due to variations in coupling efficiency.

Influence of Electric Field on SERS: Frequency Effects, Intensity Changes, and Susceptible Bonds

The fundamental mechanism proposed to explain surface-enhanced Raman scattering (SERS) relies on electromagnetic field enhancement at optical frequencies. In this work, we demonstrate the use of microfabricated, silver nanotextured electrode pairs to study, in situ, the influence of low frequency (5 mHz to 1 kHz) oscillating electric fields on the SERS spectra of thiophenol. This applied electric field is shown to affect SERS peak intensities and influence specific vibrational modes of the analyte. The applied electric field perturbs the polar analyte, thereby altering the scattering cross section. Peaks related to the sulfurous bond which binds the molecule to the silver nanotexture exhibit strong and distinguishable responses to the applied field, due to varying bending and stretching mechanics. Density functional theory simulations are used to qualitatively verify the experimental observations. Our experimental and simulation results demonstrate that the SERS spectral changes relate to electric field induced molecular reorientation, with dependence on applied field strength and frequency. This demonstration creates new opportunities for external dynamic tuning and multivariate control of SERS measurements.

Collection efficiency of scattered light in single-ended optical fiber sensors.

Optical fibers allow a variety of spectroscopic sensing methods to be implemented in a single-ended backscattering geometry. Taking multimode fibers with surface-enhanced Raman scattering active tips as a model system, it is shown that the remote single-ended collection geometry can be relatively inefficient in comparison to the performance of the underlying sensor structure. Therefore the performance of the single-ended geometry has been compared to the analogous sensor structure on a nonguiding silica glass substrate. While part of the reduction in collection efficiency can be attributed to mismatches between the numerical aperture of the collection optics and that of the fiber, this study suggests that there can be an additional loss due to a mismatch between the confocal area of the collection optics and the area of the fiber core. This effect is most significant for high numerical aperture objectives. However, the collection efficiency is somewhat higher than would be expected from a simple area ratio analysis. This can be attributed to the graded-index fiber used in the model system and the relaxation of confocal requirements in the longitudinal direction.

Nanofabrication of surface-enhanced Raman scattering substrates for optical fiber sensors

Surface-enhanced Raman scattering (SERS) allows the detection of sub-monolayer adsorbates on nanostructured metal surfaces (typically gold or silver). The technique has generated interest for applications in biosensing, high-resolution chemical mapping and surface science. SERS is generated by the localized surface plasmon resonance that occurs when the nano-metal is exposed to laser light. These plasmonic effects rely on features as small as ~1 nm, which poses a challenge for the fabrication of sensitive and reproducible substrates. Consequently a wide range of nanofabrication techniques have been used to make SERS substrates. Further challenges are encountered when transferring wafer-scale techniques to the tips of optical fibers in order to produce devices for in vivo SERS sensing. Here we describe fiber tip substrates based on miniaturization by fiber drawing, physical vapor deposition and nanoimprint lithography. Despite recent progress, the fabrication of sensitive, reproducible and affordable SERS fiber sensors remains an unresolved problem.

Angle cleaving optical fibers using a CO 2 laser

A CO2 laser at λ = 9.3 μm was used to cleave optical fibers at a given angle. The defects that might occur during the process are explained and resolved. The good surface quality obtained on the cleaved surface is demonstrated by SEM and AFM images.