Reconstruction of a hologram carved-up in fused silica:

Computer-generated binary holograms were also written in transparent materials (fused silica and polycarbonate) using direct laser writing by 120-fs pulses at 800 nm and 1kHz repetition rate.

It is well-known that tightly focused laser beam can cause an optical breakdown inside the bulk of transparent material by multi-photon absorption. This absorption takes place in confined space and causes permanent material modification. Writing a hologram into the bulk has advantage over surface holograms because the hologram once written beneath the surface is not prone to degradation.

We chose the pitch for our holograms to be 10 µm which gives 2540 DPI, and the size was 250 x 250 dots. The beam was focused beneath the surface using microscopic objective with NA = 0.42 (Mitutoyo 20x Plan Apo). Total power sent to the sample was 50 mW in the case of the fused silica sample and 0.66 mW for the polycarbonate sample.

[ A. L. Brkić, V. Cviljušac, H.Skenderović, and M. Rakić,

Unifying fast computer-generated hologram calculation and prepress for new and existing production techniques, 

Appl. Opt. 62, D119-D124 (2023).]