Characterization of nanometric thin films with far-field light

Abstract

The fabrication and characterisation of ultra-thin, transparent films is paramount for protective layers on semiconductors, solar cells, as well as for nano-composite materials and optical coatings. Similarly, the probe volume of nano-sensors, as well the calibration of axial distances in super-resolution microscopies, all require the metrology of axial fluorophore distances. However, the reliable production and precise characterisation of such nanometric thin layers are difficult and labor-intense and they require specialized equipment and trained personnel. In our present work, we describe a simple, non-invasive, all-optical technique for simultaneously measuring the refractive index, thickness, and homogeneity of such thin films. We assemble transparent layers from My-133-MC, a biomimetic transparent polymer with a refractive index of 1.33, amenable for applications in the life sciences. All parameters characterising the films are obtained in a single measurement from the analysis of supercritical angle fluorescence radiation patterns acquired on a minimally modified inverted microscope. Results compare favorably to those obtained through a combination of atomic force and electron microscopy, surface-plasmon resonance spectroscopy and ellipsometry. To illustrate the utility of our technique, we present two applications, one in metrology and one in bio-imaging; (i), the calibration of axial fluorophore distance in a total internal reflection fluorescence geometry; and, (ii), live-cell super-resolution imaging of organelle dynamics in cortical astrocytes, an important type of brain cell. Our approach is cheap, versatile and it has obvious applications in profilometry, biophotonics, photonic devices, and optical nano-metrology.