• modulazione di polarizzazione
• porfirine
• J-aggregati

Near-field microscopy of circular-dichroic nanostructures



Circular dichroism (CD), i.e., the response of an absorbing material to left and right-handed light polarization, is one of the most important optical properties in artificial and natural systems. CD-spectroscopy is customarily carried out in liquid samples such as solutions containing chromophores with a supra-molecular structure.

Investigation of CD in solid-state samples is a very challenging task, due to the simultaneous occurrence of linear dichroism (LD) and to the need for highly sensitive and spatially-resolved optical diagnostic tools in order to detect the weak absorption from thin material layers.

In this thesis, CD and LD are studied in samples consisting of porphyrin (TPPS3) nanostructures deposited onto glass through a solution casting method. In particular, nanoscopy measurements are carried out on such samples using a Scanning Near-field Optical Microscope working in the emission mode and equipped with Polarization-Modulation (PM-SNOM).

Results indicate the formation of ribbon-like nanostructures, with a transverse size on the order of a few hundreds nanometers and a variable length (up to the micrometer range). Such nanostructures exhibit remarkable optical activity, as determined through polarization-modulated excitation at 473 nm and demodulation to the first and second harmonics of the transmitted signal.

Results interpretation has been carried out based on a comprehensive model of the optical setup and of the material, developed within the frame of the Mueller matrix formalism. The model allows disentangling LD and CD contributions to the optical activity, which are demonstrated to be related to different harmonic components. Moreover, an evaluation of the relevant dichroic coefficients has been made.

The work demonstrates the abilities of PM-SNOM for sensitive and spatially-resolved detection of the optical activity at the nanoscale in solid-state nanostructure.