Abstract

In medical diagnostics at wavelengths between 900 and 1200 nm, where biological tissues are transparent and fluorescence-free, Raman micro-spectroscopy has traditionally relied on the Fourier-transform technique and only in the past few years did technological advances allow the more widespread use of the dispersive technique. At such long wavelengths, the reduced Raman scattering cross-sections and non-silicon-based detector sensitivities require long measurement times, specifically in dispersive Raman spectroscopy. In contrast, surface enhanced Raman scattering (SERS) can provide chemical fingerprints at the single molecule level with extremely high enhancement factors in the local electromagnetic field of plasmonic nanostructures [1], therefore fast imaging is possible with this method. For surface enhanced hyper-Raman scattering (SEHRS), where a multi-photon process is at work, the detection wavelength range falls into the visible even though near-infrared excitation is used. In addition, the inherently low multi-photon cross sections can easily be overcome near metal surfaces by exploiting the nonlinear scaling of the signal with the intensity of the excitation field [1]. Further advantages of SEHRS compared to SERS are the reduced interaction volume leading to better spatial resolution and the different selection rules, which provide complementary structural and chemical information. Here we present a novel, simultaneous SERS and SEHRS-based confocal microspectroscopic approach operating at 1064 nm and demonstrate its excellent utility for all-optical pH nanosensing over a wide pH-range.

© 2015 IEEE