Artifact free time resolved near-field spectroscopy

We report on the first implementation of ultrafast near field measurements carried out with the transient pseudoheterodyne detection method (Tr-pHD). This method is well suited for efficient and artifact free pump-probe scattering-type near-field optical microscopy with nanometer scale resolution. The Tr-pHD technique is critically compared to other data acquisition methods and found to offer significant advantages. Experimental evidence for the advantages of Tr-pHD is provided in the near-IR frequency range. Crucial factors involved in achieving proper performance of the Tr-pHD method with pulsed laser sources are analyzed and detailed in this work. We applied this novel method to femtosecond time-resolved and nanometer spatially resolved studies of the photo-induced effects in the insulator-to-metal transition system vanadium dioxide.publishe