  We report on the first implementation of ultrafast near field nanoscopy
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 time-resolved and spatially resolved studies of the
photo-induced effects in the insulator-to-metal transition system vanadium
dioxide with nanometer scale resolution.
