Here, we report on the enhancement of molecular vibrational transitions overtones due to the excitation of surface plasmon polariton waves. We show that, assuming a modified Kretschmann-Reather configuration with ultra-thin dielectric over-layer, the effective absorption cross section of higher harmonics of molecular vibrations is boosted by at least two orders of magnitude. Based on the experimental observations reported by Karabchevsky and Kavokin  on photonic waveguides, we calculate the differential absorption which appears to exhibit a Fano-like line shape. This manifests the interaction between a narrow molecular resonance and a broad plasmonic mode. In fact, the interaction occurs due to the highest enhancement of the vibrational transitions overtones when the vibration mode and plasmonic mode are detuned. The enhancement factor reported in this study points on feasibility of vibrational overtones detection using conventional spectrometers. In addition, having high signal-to-noise ratio opens a new route for molecular detection and sensing.
Surface-enhanced Raman scattering (SERS) from silver nanosculptured thin films (STF) was studied in detail for biosensing. The influences of the nanostructures' sizes, topology, the substrate features, and the preparation conditions on the enhancement were examined. Enhancement factors on the order of 10 7 were obtained from silver nanorods deposited on bare silicon substrates with respect to their dense counterparts, using 4-aminotheophenol (4-ATP) for the Raman emission. The low detection limit that can be achieved with STFs is below 1 μg/lit of the probe molecule 4-ATP in Ethanol solution. Theoretical modeling based on a single small spheroidal nanoparticle helped in explaining the main properties of SERS from STFs. Stability of the films was noticed over a period of one year without significant degradation.
Increasing the sensitivity of surface plasmon resonance (SPR) sensors is important to enable controlling small concentrations of materials in liquid solutions or for gas sensing. Upon using a 10-15 nm top layer of dielectric film with a high value of the real part 'of the dielectric function, on top of an SPR sensor in the Kretschmann configuration, the sensitivity is improved by few times. The imaginary part "of the top nano layer permittivity needs to be small enough in order to reduce the losses and get sharper dips. The stability of the sensor is also improved because the nano layer is protecting the silver from interacting with the environment. In the near infrared range, the sensitivity is enhanced contrary to the standard SPR mode without top nanolayer. The calculated evanescent field is enhanced near the top layer - analyte interface, thus the enhancement is partially due to this fact and partially due to an increase of the interaction length as a waveguiding effect.