The NLOM system consists in an upright microscope which uses an ultrashort laser system as excitation light source. The laser is a mode-locked Ti:Sapphire oscillator emitting at 800 nm, delivering 70 femtosecond pulses at a repetition rate of 80 MHz. A variable neutral density filter is used to control the laser power reaching the sample to ensure non-damaging measurement conditions. The microscope is based on objective lenses with high numerical aperture. The sample, or object of study, is placed on a 3D, micrometre resolution motorized translation stage that selects the position of the focal spot of the laser on the sample. The lateral and axial resolution of measurements are 1 and 2 micrometres, respectively. The non-linear optical signal is collected in the reflexion mode and measured with a photomultiplier and the adequate combination of optical filters. Signal discrimination is enhanced with the use of a lock-in amplifier.
The combined use of the different NLOM modalities derives information, in a totally non-invasive way, on the presence of layers of different chemical nature, their thickness or their crystalline or hierarchical internal organization. It is possible to obtain highly contrasted 3D images at the micrometer scale, without any preparation or sampling, of the artifacts.