Nanooptomechanical silicon devices for sensing applications

The interest in the field of plasmonics is growing steadily in the last two decades; from the ability to transmit and receive information at very high speed with greatly reduced losses to the enhancement of very weak signals for chemical and biological analysis, its range of applicability is boundless. The understanding of plasmonic phenomena is also increasing, and with this comes the ability to tune plasmonic properties to the designer’s will. Among other approaches, the use of MicroElectroMechanical Systems (MEMS) for the modulation of plasmonic properties has been recently reported in literature, but no practical application, such as Raman spectroscopy, has been reported beside the field of Tip-Enhanced Raman Spectroscopy, a powerful approach that demonstrated nanometric chemical spatial resolution, but which remains confined the research laboratory benches due to its intrinsic experimental complexity. In this project, we propose to explore the coupling between mechanical and plasmonic properties of micro and nanosensors in order to realize a mechanical resonator capable of turning on and off a frequency modulated hot spot. Different strategies have been tried to achieve the desired result: the first version of the optomechanical device was based on a vertical resonator (pillar) put in close proximity with a steady structure, 100 nm apart from each other. The devices are fabricated using electron-beam lithography for the high resolution required for the sub-micron gap and ICP-RIE to obtain an inverted tapered structure of the pillar walls; the evaporation of a gold layer on top of the devices ensures a plasmonic activity of the upper surface during the actuation of the devices. Optical lever techniques and Rayleigh scattering mapping have been used for the mechanical characterization and the onset of an impact oscillation condition is discussed. The Raman scattering intensification due to the formation of a plasmonic hot spot in the contact region has been studied functionalizing the devices with pentacene and an enhancing factor for the Raman signal during actuation can be estimated. However, severe drawbacks have been identified in this configuration, since pillars tilt and bend nanometrically during the motion, causing the hot spots to be randomly localized along the gap and reducing their field enhancement capabilities. The problems arisen with the first version of the device have been solved through the careful design of a new geometry: the vertical resonator has been changed into a horizontally-oscillating cantilever tuning the width-to-height ratio, and a tip has been added to the design for a well-defined contact point. This new device has been characterized using sample scanning confocal microscopy, both in its mechanical properties and in the surface distribution of the chosen Raman dye, benzotriazole azo (BT-Azo), after the functionalization. Finally, the plasmonic behaviour has been investigated and the signal coming from the hot spot has been successfully isolated using a combination of polarization-dependent excitation light and lock-in deconvolution of the signal at higher harmonics, thus demonstrating the successful realization of a frequency modulated hot spot for Raman spectroscopy applications. As a side activity, a wire scanner sensor with nanofabricated bridges suspended over a wide window has been fabricated, for the characterization of high-energy electron or light beams. The test of this device has been performed at the BEAR beamline of the Elettra Synchrotron and in the FERMI FEL-1 Free-Electron Laser facility; the performances of this sensor have been proved to be comparable, when not superior, to those of the commercially available devices.