Using multiphoton absorption with high-intensity lasers to heat transparent liquids

Most pure, organic liquids are remarkably transparent materials, even when exposed to high laser intensities. At very high laser intensities, however, ∼10⁹-10¹⁰ W/cm² and under the appropriate conditions, liquids can absorb extremely strongly due to the mechanism of multiphoton absorption, which results in ionization and dissociation of the liquid molecules. Strong absorption results in significant heating and temperature increases. Multiphoton absorption thus provides a new mechanism for heating otherwise transparent liquids. Furthermore, the thermal energy is deposited in a very thin layer near the surface of the liquid, which is much thinner than the classical penetration depth. A thermal model based on multiphoton absorption is developed, and results are presented for heating liquid water with laser pulses from the popular Nd: YAG laser at a wavelength of 266 nm.