The present application discloses optical microscopy systems and related method that use modulation techniques and contrast agents to enable the systems to detect nonlinear photoacoustic signals with high spectrum sensitivity and frequency selectivity for imaging. A laser beam is amplitude modulated for pure sinusoidal modulation using either the loss modulation technique or the single light amplitude modulation technique. The sample used in the invention is an endogenous contrast agent by itself or is treated by at least one exogenous contrast agent to produce or enhance photoacoustic effect induced by multi-photon absorption. The modulated laser beam is focused via a focusing device onto a sample which absorbs multiple photons simultaneously and generates ultrasonic (acoustic) waves via nonlinear photoacoustic effect. The ultrasonic waves are received and transformed into electrical signals and the frequency signals within the electrical signals are detected and recorded to create images.