A method and device for photothermal imaging tiny metal particles which are immersed in a given medium like a living cell. The given medium and immersed tiny metal particles are illuminated through separate phase reference laser beam and sensitive probe laser beam, with the sensitive probe laser beam undergoing through impingement on the given medium slight phase changes induced by photothermal effect due to a local heating thanks to a heating laser beam, in the absence of any substantial phase changes to the phase reference laser beam. Illuminating is performed by focusing the separate phase reference laser beam. The induced slight phase changes on the sensitive probe laser beam with reference to the phase reference laser beam are detected through differential phase interference contrast phenomenon so as to allow each of the tiny metal particles in the given medium to be imaged as an optical label.