  Over the last decade, single-molecule optical microscopy has become the
gold-standard approach to decipher complex molecular processes in cellular
environments. [1-3] Single-molecule fluorescence microscopy has several
advantages such as ease of application, high sensitivity, low invasiveness and
versatility due the large number of available fluorescent probes. It bears
however some drawbacks related to the poor photostability of organic dye
molecules [4] and auto-fluorescent proteins [5-7] and and to the relatively
large size of semiconductor nanoparticles in the context of live cell
applications. [4,8,9] The overall size of the functional biomarkers is a
general issue for any imaging approach because of steric hindrance effects in
confined cell regions. Small red-shifted nano-emitters that are highly
photostable are not currently available, while they would combine the best
physical and optical penetration properties in biological tissues. Although
single-molecule absorption microscopy was early used to detect single-molecules
[10] at cryogenic temperatures, it is only with the advent of photothermal
microscopy [11,12] that practical applications of absorption microscopy were
developed in single-molecule research. Photothermal imaging (PhI)
