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NEWS

Dr. Michele Celebrano

Physics Dept., Politecnico di Milano

Monday, July 22nd, 2013 at 11:30:00 AM  

Conference room Querzoli - LENS - via Nello Carrara 1 - Sesto Fiorentino (Florence)

Published on-line at 12:08:40 PM on Thursday, July 4th, 2013

Linear and non-linear optical sensing down to a single molecule

High-sensitivity detection devices in optical diagnosis systems for pharmacology, metrology and medicine.

In the last decade many efforts have been dedicated to conceive and develop high-sensitivity devices for applications in pharmaceutical and medical diagnostics able to detect small amounts of matter and to monitor chemical reactions down to the single molecule level. In particular, thanks to its non-invasive character, optical sensing established itself as one of the most promising approaches to detect and identify chemical and biological quantities. In this framework, single molecule detection demonstrated to be a powerful tool for a wide range of studies, ranging from biophysics to quantum optics. However, this approach has been limited so far to species with a high fluorescence quantum yield.

Over the past few years, extinction detection and spectroscopy have been thoroughly developed as an alternative to fluorescence for investigating single nano-objects such as metallic nanoparticles, viruses, dye molecules, and quantum dots. The contrast mechanism at work in this coherent scheme can be understood as the interference between the incident beam and the light scattered by the nano-object. The sensitivity of this technique can be pushed to detect single semiconductor nanocrystals at room temperature and measure the absorption cross section of a single quantum dot before and after photo-bleaching, as well as during the on and off photo-blinking times. Here, we will focus on the recent improvements of such a technique which allowed the direct detection and imaging of a single molecule by optical absorption at room temperature (10.1038/nphoton.2010.290). These efforts open the door to understanding the photophysics of a large class of nano-objects, which do not fluoresce or do so with very low quantum efficiency.

TEM image of a Ag sample grown at 10 Pa of Ar. Different morphologies are evident, corresponding to different stages of film growth. Credit: fisi.polimi.it.

Concurrently, high-sensitivity detection devices exploiting propagating surface plasmons rapidly found their way in pharmacology, metrology and medicine given their reliability and simple implementation in optical diagnosis systems. A recent development of this sensing technique focuses on the use of metal nanostructures wherein the resonance frequency and linewidth, which determine the device sensitivity, can be tuned by engineering the nanostructure geometry. A remarkable feature of metal nanoparticle-based platform resides in the dramatic shrinking of the probing volume with respect to thin films.

Here we envision further advances of these sensing techniques exploiting nonlinear optical processes (i.e. surface Second Harmonic Generation, SHG) occurring at noble metal nanostructures, which possess the outmost advantage of being background-free. We will show how to exploit a metal nanostructure as a near-field source for SHG imaging (10.1117/12.626337) and discuss the main key features of SHG emission from metal nanoparticles. We will eventually introduce a novel nanostructure concept that helps overcoming the relative low efficiency of SHG, which has so far hindered the realization of efficient nonlinear plasmonic devices for high sensitivity detection.

For further informations, please contact Prof. Paolo Foggi.