Wavefront Deformation / Thermal Lenses

Absorption of intense laser radiation causes a thermal load in optical materials leading to local heating of the sample. Subsequentially there are two effects observable:

• Change of local refractive index (dn/dT)
• Surface deformation due to volume exapansion

Each effect originates a reversible change in the optical properties, i.e. in particular a modification of the transmitted wavefront ("thermal lens") either enhancing or compensating the overall distortion.

Furthermore in case of amorphous optical materials (e.g. irradiation of Quartz samples with ArF excimer laser) there are irrversible changes like compaction leading to changes in density and thus in refrcative index.

Both thermal lenses and compaction can be detected in real time with the help of a Hartmann-Shack wavefront sensor (cf. page Wavefront Detection). The image below shows a corresponding measurement of a thermal lens within a ArF laser irradiated (193 nm) Quartz plate. Thermal induced deformation of the wavefront develops inside the material within a few seconds. The optical power of the effect (w_pv~2nm) corresponds to a converging lens with a focal length of about 10 km.

Further reading:

 Press release
 Flyer: Wavefront Sensor