Surface enhancement in near-field Raman spectroscopy

Abstract

The intensity and selection rules of Raman spectra change as a metal surface approaches the sample. We study the distance dependence of the new Raman modes with a near-field scanning optical microscope (NSOM). The metal-coated NSOM probe provides localized illumination of a metal surface with good distance control. Spectra are measured as the probe approaches the surface, and the changes elucidated with difference spectra. Comparisons to a theoretical model for Raman excitation by evanescent light near the probe tip indicate that while the general trends are well described, the data show oscillations about the model. PACS Numbers: 78.30.-j, 07.79.Fc, 78.66.-w , 82.80.Ch Table of Contents Category: Optics.The proximity of sharp metallic structures to a sample has profound effects on the Raman spectra of that sample. It leads to surface enhanced Raman spectroscopy (SERS), for example see [1,2] and references within, and to differences between far-field and near-field Raman spectroscopy measured with a near-field optical microscope (NSOM). [3-6] Two aspects of the spectra, the selection rules and the mode intensities, are altered by the presence of the metal. We concentrate in this paper on the dependence of the intensity of the new modes with distance between the metal-coated probe and the dielectric surface. This enhancement originates from the evanescent light present near the probe tip. A model describing the fields near such a small aperture was described by Bethe [7] and later modified by Bouwkamp.[8] Betzig et al. [9] measured the fields near an NSOM tip using single fluorescent molecules as detectors, and found good agreement with this theory. We show in this paper that the theory can also explain the general trends of the experimental enhancements in Raman spectroscopy, but that the data show oscillations about the theoretical curve as the tip approaches the surface.Near-field scanning optical microscopy (NSOM) [10] provides a unique method to study the effects of metal in proximity to the sample under Raman scrutiny. The aluminum-coated probe, forming the NSOM aperture, can be moved with nanometer accuracy to and from the surface. Force feedback of the NSOM is used as an indicator of distance, [11,12] [13] in combination with a calibrated piezoelectric scanner. A cooled (-45° C) CCD camera in the photon counting mode is used in conjunction with a Jarrel-Ash Czerney-Turner spectrometer for the Raman signal detection. An Argon Ion laser operating at 514.5 nm provides the excitation. In this experiment, the sample is illuminated through a tapered fiber probe, which is mounted through the center of a 0.5 N.A. aspheric lens. The backscattered light from the sample is collected and collimated with this lens. The light then passes through a holographic notch filter to remove elastically scattered light before being focused into the spectrometer. The primary difficulty encountered in NSOM-Raman is that of low signal levels. This cannot be countered by increased input intensity, as input of more than a few milliwat...

Enhanced Raman scattering by self-assembled silica spherical microparticles

“…Thus, the near-field volume is around 1.13 ϫ 10 −3 m 3 . Thus, the enhancement factor for the 2.34 m particle was calculated to be as high as 1.4ϫ 10 4 . Figure 3͑b͒ shows the size dependence of the ERI obtained experimentally ͑diamonds͒ and the size dependence of ͉E͉ 2 ͑squared electrical field͒ obtained by simulation ͑tri-angles͒.…”

“…[16][17][18][19] However, no further investigation has been reported to authenticate this hypothesis. In this study, the enhancement factor of Raman signals, in the order as high as 10 4 , was realized using closely packed silica spherical microparticles.…”

“…1 Extensive studies, therefore, have been conducted in order to improve the sensitivity of Raman scattering for the last two decades. [2][3][4][5][6][7][8][9][10][11][12][13][14] Several mechanisms related to physics, chemistry, and optics can be used to achieve this goal, such as electromagnetic and chemical mechanisms for surface-enhanced Raman scattering ͑SERS͒, [2][3][4][5] interference mechanism for interferenceenhanced Raman scattering ͑IERS͒, 6,7 angle-dependent reflection for total internal reflection Raman spectroscopy ͑TIRRS͒, 8 resonance effect from microstructures or subwavelength particles for resonance Raman scattering ͑RRS͒, 9,10 near-field optics for high-resolution near-field Raman microscopy ͑NORM͒, 11 and the localized evanescent electric field and surface plasmon polariton for tip-enhanced Raman spectroscopy ͑TERS͒. [12][13][14][15] Metal particles, e.g., Ag and Au, or substrates with roughness as sample carriers are the best candidates for the realization of SERS due to the high enhancement factor ͑10 4 -10 14 ͒; however, they are irreproducible and unpredictable.…”

Collection mode nano‐Raman setup

“…Though this method relatively easy to implement using regular AFM scanners in combination with confocal Raman microscope, it has significant disadvantage. The processes of the SERS are not well understood and are not studied enough to use them broadly as an industrial tool for the measurements [6]. The confocal microscope takes scattered signal from larger area, wherefore to…”

Correlation between the local stress and the grain misorientation in the polycrystalline Al2O3 measured by near-field luminescence spectroscopy

“…On the other hand, a near-field technique enables spectroscopic investigation with a spatial resolution of 30-500 nm. [19][20][21][22][23] There is thus significant interest in the application of this method to sensing stress in a narrower region, such as stress within grains of polycrystalline Al 2 O 3 . Recent technological improvements of lens-based luminescence detection system also enable the detailed analysis of each stress component inside Al 2 O 3 .…”