Photothermal microscopy (PTM) is an emerging label-free optical microscopy technique that can image the photothermal property of single nanoparticles and single molecules. The image contrast relied on the sensitive detection of local temperature gradients and refractive index distribution associated with the photothermal effect of individual molecules or nanoparticles when it was illuminated with particular incident light. Since non-radiative relaxation is usually the dominant process for an excited molecule to return back to the ground state, photothermal properties are of more generalized significance compared to fluorescence. The past decade has witnessed the great growth and development of photothermal microscopy, mainly because of its advantages of label-free detection, high sensitivity and stability. Furthermore, this technique has received increasing attention in nano-science and life science, ranging from fundamental studies on molecule-photon interactions to promising applications in single cell imaging and bio-sensing. The review mainly describes the imaging principle, optical apparatus, technical concerns for performance optimization of PTM, and subsequently enumerates the important applications in live-cell studies and bioscience. In the last section, we provide perspectives regarding the major strength and challenges for the further development and applications of photothermal microscopy technology.

Contents
1 Introduction
2 Principle and apparatus of PTM
2.1 Imaging principle
2.2 Photothermal interference contrast (PIC)
2.3 Photothermal heterodyne imaging (PHI)
2.4 Performance optimization
3 Imaging single molecules and single nanoparticles
3.1 Plasmonic nanoparticles
3.2 Semiconductor nanocrystals
3.3 Carbon nanotubes
3.4 Organic molecules
4 Application
4.1 Photothermal probes for live cell imaging
4.2 Label-free imaging of biomolecules
4.3 PTM combined with other techniques
5 Conclusion and outlook