Fluorescence, that is, spontaneous emission, is generally more sensitive than absorption measurement, and is widely used in optical imaging 1,2 . However, many chromophores, such as haemoglobin and cytochromes, absorb but have undetectable fluorescence because the spontaneous emission is dominated by their fast non-radiative decay The phenomenon of stimulated emission was first described by Einstein in 1917 (ref. 4). An atom or molecule in its excited state can be stimulated down to the ground state by an incident light field, resulting in the creation of a new coherent photon identical to those in the original incident field. This process only occurs when the frequency of the incident field matches the transition energy. Stimulated emission was later used as a fundamental principle for light amplification in the laser 5 . The depopulation aspect of stimulated emission has been used for population dumping from excited states 6 , super-resolution fluorescence microscopy 7 , and fluorescence lifetime imaging 8 . Here we use the light-amplification aspect of stimulated emission as a contrast mechanism for highly sensitive imaging of chromophores that have undetectable fluorescence. Such chromophores have very short-lived excited states with much faster non-radiative decay rates than their spontaneous emission rates. As a result, their feeble fluorescence is overwhelmed by backgrounds, such as stray light, solvent Raman scattering, and detector dark counts. Our solution to this problem is to conduct a dual-beam experiment to interrogate the short-lived excited state by stimulated emission, which can compete with the non-radiative decay under a strong enough stimulating field Considering the optical excitation at frequency v 01 where N 0 is the number of molecules in the ground state. For a single chromophore, DI E /I E is of the order of 10 27 . Such small attenuation cannot be detected by conventional absorption microscopy. We note that single-molecule absorption was previously achieved in cryogenic temperatures using a frequency-modulation method 10 , which is, however, difficult to implement at room temperature because of the broad molecular absorption linewidth. Moreover, absorption measurement is often complicated by scattering when imaging biological specimens. Instead of detecting direct absorption, here we detect the stimulated emission followed by the excitation of the molecule. According to Einstein 4 , the molecular cross-section s stim for stimulated emission is comparable to s abs , because of microscopic reversibility. Unlike the absorption that results in attenuation, the stimulation beam will experience an intensity gain after interacting with the molecules
