Near-Field Optics Imaging in Silica Waveguide Using Near-Field Scanning Optical Microscope

Abstract

A dramatic drop of ≈5 orders of magnitude in the resistance (R) of La 0.175 Pr 0.45 Ca 0.375 MnO 3 epitaxial films upon exposure to optical photons derived from both continuous and pulsed lasers, as well as broad-band sources at temperatures (T ) < 30 K is reported. The strength of change is a sensitive function of both the incident photon flux and temperature. Under isothermal conditions the photo-generated low resistance state persists eternally after removal of light. This non-equilibrium state is metallic, as revealed by the positive dR/dT for T T p (≈120 K). This electrically conducting state is presumably ferromagnetic as T p coincides with the temperature where a weak ferromagnetism sets in on cooling the insulating film from room temperature. To rule out the possibility of photon-induced local heating of the sample as a mechanism of the observed effects, photo-illumination experiments were performed under identical conditions on thin films of two non-charge-ordered manganites deposited on substrates of similar thermal conductivity. Our model for the observed transition encompasses a global charge-ordered state in which ferromagnetic metallic clusters of fraction p much less than the critical fraction p c for percolation exists at low temperatures. Photo-induced melting of the charge-ordered state increases this fraction beyond p c in a cumulative manner as successive pulses of light fall on the sample.