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Dipartimento di Fisica - Politecnico di Milano

Scanning near-field optical microscopy (SNOM)

The SNOM laboratory is founded in 2005 to support  the nano-optics activity of the Surface Physics group at the Physics Department. Its research is focused on the development and application of sub-diffraction microscopy techniques and on the study of the interaction between light and nanostructured materials (thin films, metal and semiconducting nanoparticles and nanocrystals, molecular systems).

The core of the laboratory is constituted by the WITec AlphaSNOM© microscope, which features a confocal optical microscope, an atomic force microscope (AFM), and a scanning near-field optical microscope (SNOM).

Scanning near-field optical microscopy is a technique first developed in the Eighties, where an illuminated subwavelength aperture is brought into close proximity to a sample and an optical image is obtained by raster scanning the probe and collecting the optical signal after interaction with the sample. Our system is based on hollow-pyramid cantilevers, with an aperture in the 50-100 nm range, which roughly defines the achievable lateral resolution.

Left: sketch of a SNOM setup based on hollow-pyramid probes. Right: electron microscopy images
of the pyramid probe at the apex of the cantilever.

Over the years, the SNOM activity has been focused mainly on the development of sub-diffraction techniques for nonlinear optics (second-harmonic generation and two-photon photoluminescence) and of polarization modulation techniques for molecular dichroism and magneto-optics investigations. In this respect, premium features of hollow-pyramid probes, compared to more common tapered-fiber probes, are:

(i) the possibility of coupling a larger optical power (10 mW or higher) into the probe before reaching the onset of thermal damage;
(ii) the maintenance of the polarization properties of the optical field at the aperture;
(iii) the absence of temporal dispersion, since the optical beam travels in air before reaching the subwavelength aperture.

The SNOM apparatus is complemented by:

(i) a tunable femtosecond laser source (Chameleon© from Coherent Inc.), which delivers 140 fs pulses with full tunability in the 680-1080 nm range;
(ii) solid-state and diode laser sources covering the whole visible and near-infrared spectrum;
(iii) single-photon Si photodiodes, InGaAs photodiodes, and photomultiplier tubes for the detection of visible and near-infrared light;
(iv) a prism-based spectrometer for the acquisition of spectra at extremely low signal levels in the visible and near-infrared spectrum.


Selected publications

• M. Savoini, P. Biagioni, S. C. J. Meskers, L. Duò, B. Hecht, and M. Finazzi
Spontaneous formation of left- and right-handed cholesterically ordered domains in an enantiopure chiral polyfluorene film” - J. Phys. Chem. Lett. 2, 1359 (2011)

• M. Savoini, F. Ciccacci, L. Duò, and M. Finazzi
Apparatus for vectorial Kerr confocal microscopy” - Rev. Sci. Instrum. 82, 023709 (2011)

• M. Savoini, P. Biagioni, L. Duò, and M. Finazzi
All-optical subdiffraction multilevel data encoding onto azo-polymeric thin films” - Opt. Lett. 34, 761 (2009)

• M. Savoini, P. Biagioni, G. Lakhwani, S. C. J. Meskers, L. Duò, and M. Finazzi
Near-field circular polarization probed by chiral polyfluorene” - Opt. Lett. 34, 3571 (2009)

• M. Celebrano, M. Savoini, P. Biagioni, M. Zavelani-Rossi, P.-M. Adam, L. Duò, G. Cerullo, and M. Finazzi
Retrieving the complex polarizability of single plasmonic nanoresonators” - Phys. Rev. B 80, 153407 (2009)

• M. Zavelani-Rossi, M. Celebrano, P. Biagioni, D. Polli, M. Finazzi, L. Duò, G. Cerullo, M. Labardi, M. Allegrini, J. Grand, and P.-M. Adam
Near-field second-harmonic generation in single gold nanoparticles” - Appl. Phys. Lett. 92, 093119 (2008)

• P. Biagioni, M. Celebrano, M. Zavelani-Rossi, D. Polli, M. Labardi, G. Lanzani, G. Cerullo, M. Finazzi, and L. Duò
High-resolution imaging of local oxidization in polyfluorene thin films by nonlinear near-field microscopy” - Appl. Phys. Lett. 91, 191118 (2007)

• P. Biagioni, D. Polli, M. Labardi, A. Pucci, G. Ruggeri, G. Cerullo, M. Finazzi, and L. Duò
Unexpected polarization behavior at the aperture of hollow-pyramid near-field probes” - Appl. Phys. Lett. 87, pp. 223112 (2005)