Formation of nanojets and nanodroplets by an ultrashort laser pulse at focusing in the diffraction limit

A new model has been developed and calculations have been performed for the formation of nanodroplets after action of an ultrashort laser pulse on a thin ($10$–$100$ nm) gold film deposited on a glass substrate. The action of a laser results in the melting of the film in the region of a laser spot and in its thermomechanical separation from the substrate. The separated film acquires a dome shape because of a decrease in the temperature in the direction from the center of the laser spot. This theoretical model provides the explanation of the formation of nanodroplets. It has been established that, first, the separation speed of a gold film from glass decreases sharply because the acoustic impedance of gold is much larger than that of glass. Second, nanodroplets are formed owing to the capillary focusing of the substance, which is manifested in the appearance of the drag component directed toward the axis of symmetry of the dome. The surface tension becomes dynamically significant because of the indicated sharp decrease in the separation speed from glass and of the smallness of the diameter of the focal spot ($D\sim 1\,\mu$m), which is determined by the diffraction limit of optical radiation.