sentences:  However, if the chromophore has an energy level corresponding to the (weak) absorption of one photon then this may be used as a stepping stone, allowing more freedom in the arrival time of photons and therefore a much higher sensitivity.However, this approach results in a loss of nonlinearity compared to nonresonant two–photon absorbance (since each two-photon absorption step is essentially linear), and therefore risks compromising the 3D resolution of the system.In microholography, focused beams of light are used to record submicrometre-sized holograms in a photorefractive material, usually by the use of collinear beams.