Active probes for high resolution near field microscopy – SAMIRé

The Nanophotonics team at CEA, which coordinates the project, has recently demonstrated an original concept, called nanoSHG, using the electric field present at the junction of a scanning tunneling microscope (STM), to orient locally a small number of molecules, thereby generating a second harmonic signal inherently localised. The first images of samples structured at the nanoscale could be obtained from this so-called nanoSHG concept, the current problem being however the need to work in a liquid environment, which greatly limits the scope of this technique. In order to achieve the realization of solid self-sustained active probes, the idea within SAMIRé also exploits the expertise developed during a previous collaboration between LNIO, University of Technology of Troyes and the Institute of Materials Science at Mulhouse (IS2M). The LNIO and IS2M have shown that local field exaltation could be exploited to exceed locally the threshold required to start a photopolymerisation process, thereby leading to a highly localized anisotropic coating of metal nano-objects. The main idea in SAMIRé consists especially in changing the photopolymerisable formulation so that it can contain «optically active« molecules (such as fluorescent molecules or molecules generating a second harmonic signal, SHG) while maintaining its local photopolymerization properties. We will then polymerize locally this formulation to the end of a tip using a localized polymerization process in order to get a new generation of active SNOM probes, which performances for high resolution optical imaging will further be investigated.

The main characteristics for the SAMIRE probes are:
• A final size of polymer nanodot that will be generated at the tip apex : the size of this dot should be less than 20 or even 10 nm.
• An emission spectrally well separated from the excitation source: for this purpose, we will take profit of nonlinear effects (CEA expertise): 2-photon fluorescence or SHG, the optical coherence of the latter process appearing advantageous.
• An intense and stable emission, involving the need to:
select active molecules well known for their high photochemical stability (IS2M and CEA know-how); to benefit from a rigid and well-crosslinked photopolymer material (IS2M) in order to ensure a sustainable anchoring of the chromophores and fluorophores and allow to really block the orientation of the SHG chromophores.

From a scientific point of view, the SAMIRE project will more particularly lead to a better understanding of confined photopolymerization processes and optical properties of nanostructures and nano-plasmonic antennas…
Technologically, the development of new types of probes such as those that will be developed as part of SAMIRE is expected to expand the range of possible applications of local probes technologies leading to an increase of the local probe microscopy market. In particular, it is anticipated that new optical techniques should open new fields of application for the characterization of local optoelectronic processes, molecular processes, dynamic phenomena at surfaces, etc. Thus, the proportion of equipments including nanoscale optical measurements could increase considerably with the development of these new probes, which should generate consequently technical and economic benefits. In particular, this project could lead to the development of new components to be added to the catalog of LovaLite, thus expanding its offer regarding consumables for near field microscopy, possibly giving access to new international markets at the heart of Lovalite’s business.

CEA patent (February 2012) : «Active probe for near field optical microscopy and its manufacturing process« (PCT/IB2013/050986)
Extension to Europe and United staes under way

Nanotechnology requires the development of specific characterization tools enabling ever-increasing resolutions. Specific scanning probe microscopy techniques such as AFM or STM are now available already allowing to follow the full development process of certain components. However, highly resolved optical techniques appear still to be particularly needed especially in biology or towards the development of new application fields based on local optoelectronic processes or molecular processes, or dynamic phenomena on surfaces ...
Although SNOM (Scanning Near-Field Optical Microscopy) techniques have enabled to circumvent the limits imposed by diffraction, their “routine” resolution is currently limited to 50 or even 30 nm, which is insufficient.
The proposed project involves 3 academic partners, CEA, IS2M and LNIO, and an industrial partner LOVALITE. It concerns the implementation of an innovative concept to develop SNOM active probes using an original process of self-assembly requiring no delicate nanopositioning or gluing stage. This concept, which has been patented, takes advantage of the complementary expertise developed by the consortium partners in the fields of plasmonics, nonlinear optics of nano-objects, nanophotopolymérisation and implementation of nano-optical antennas.
In particular, the Nanophotonics Laboratory at CEA, which will coordinate the project, has recently demonstrated an original concept, called nanoSHG, using the electric field present at the junction of a scanning tunneling microscope (STM), to orient locally a small number of molecules, thereby generating a second harmonic signal inherently localised. The first images of samples structured at the nanoscale could be obtained from this so-called nanoSHG concept, the current problem being however the need to work in a liquid environment, which greatly limits the scope of this technique. In order to achieve the realisation of solid self-sustained active probes, the idea within SAMIRé also exploits the expertise developed during a previous collaboration between LNIO, University of Technology of Troyes and the Institute of Materials Science at Mulhouse (IS2M). The LNIO and IS2M have shown that local field exaltation could be exploited to exceed locally the threshold required to start a photopolymerisation process, thereby leading to a highly localized anisotropic coating of metal nano-objects. The main idea in SAMIRé consists especially in changing the photopolymerisable formulation so that it can contain "optically active" molecules (such as fluorescent molecules or molecules generating a second harmonic signal, SHG). We will then polymerize locally this formulation to the end of a tip using a localized polymerization process in order to get a new generation of active SNOM probes, which performances for high resolution optical imaging will further be investigated.
Studies, planned over 3 years, aim to go from proof of concept to the demonstration of the scientific and technical feasibility. The main features that will be seeked for the active multifunctional probes that will be developed in SAMIRé concern the ultimate size of the polymer nanodot generated at the tip end (<10 nm), the ensemble exhibiting an intense and stable emission spectrally well separated from the excitation source.
Beyond the realization of a such nanoprobes, SAMIRé also aims to demonstrate their potential for super-resolution optical imaging (expected final resolution <10 nm).
Beyond fundamental objectives in nanophotonics and nano-chemistry, and beyond the challenges associated to the realization of such nanoprobes, SAMIRé will primarily enables Lovalite, specialized in the fabrication of specific SNOM tips, to expand its range of tips. SAMIRé will more generally broaden the range of possible applications of nanocharacterisation techniques resulting in an increased market for local probe microscopy, which should lead to substantial technical and economic benefits.