# Multiphoton lithography

Two-photon absorption is a third-order with respect to the third-order optical susceptibility ${\displaystyle \chi ^{(3)}}$ and a second-order process with respect to light intensity. For this reason it is a non-linear process several orders of magnitude weaker than linear absorption, thus very high light intensities are required to increase the number of such rare events. For example, tightly-focused laser beams provide the needed intensities. Here, pulsed laser sources are preferred as they deliver high-intensity pulses while depositing a relatively low average energy. To enable 3D structuring, the light source must be adequately adapted to the photoresist in that single-photon absorption is highly suppressed while two-photon absorption is favoured. This condition is met if and only if the resist is highly transparent for the laser light's output wavelength λ and, simultaneously, absorbing at λ/2. As a result, a given sample relative to the focused laser beam can be scanned while changing the resist's solubility only in a confined volume. The geometry of the latter mainly depends on the iso-intensity surfaces of the focus. Concretely, those regions of the laser beam which exceed a given exposure threshold of the photosensitive medium define the basic building block, the so-called voxel. Other parameters which influence the actual shape of the voxel are the laser mode and the refractive-index mismatch between the resist and the immersion system leading to spherical aberration.