Polymer films can be deposited onto a semiconductor/insulator substrate.
In the presence of a dc bias voltage, the semiconductor develops a space charge region adjacent to the insulator. A laser pulse causes a localised production of charge carriers which separate in the space charge region and can be detected as an electrical current. By modulating the laser beam intensity an ac current is induced which is related to the impedance of the polymer at that location. Scanning the laser over the substrate produces a map of the impedance of the polymer film.
Steffi Krause, Werner Moritz, Habib Talabani, Ming Xu, Andrea Sabot, Graham Ensell, Scanning Photo-induced Impedance Microscopy Resolution studies and polymer characterization, Electrochimica Acta, 51 (2006) 1423-1430