Adhikari, SubhasisSubhasisAdhikariSpaeth, PatrickPatrickSpaethKar, AshishAshishKarBaaske, Martin DieterMartin DieterBaaskeKhatua, SaumyakantiSaumyakantiKhatuaOrrit, MichelMichelOrrit2025-08-312025-08-312020-12-2210.1021/acsnano.0c076382-s2.0-85097877062http://repository.iitgn.ac.in/handle/IITG2025/2381233216527The photothermal (PT) signal arises from slight changes of the index of refraction in a sample due to absorption of a heating light beam. Refractive index changes are measured with a second probing beam, usually of a different color. In the past two decades, this all-optical detection method has reached the sensitivity of single particles and single molecules, which gave birth to original applications in material science and biology. PT microscopy enables shot-noise-limited detection of individual nanoabsorbers among strong scatterers and circumvents many of the limitations of fluorescence-based detection. This review describes the theoretical basis of PT microscopy, the methodological developments that improved its sensitivity toward single-nanoparticle and single-molecule imaging, and a vast number of applications to single-nanoparticle imaging and tracking in material science and in cellular biology.truelabel-free imaging | live-cell imaging | nano-optics | nanoparticles | nonlinear spectroscopy | photothermal microscopy | single-molecule imaging | single-particle absorption spectroscopy | thermal lens microscopy | thermoplasmonicsArticlehttps://pubs.acs.org/doi/pdf/10.1021/acsnano.0c076381936086X16414-1644522 December 2020144157WOS:000603308800013