Interference rings formation inside cellulose from a back-reflected femtosecond laser pulse

We report on the formation of concentric rings within the bulk of cellulose films after laser machining with high energy femtosecond laser pulses. The rings originate from an interference effect, where the front of the pulse is reflected from the back surface of the sample and interferes with its tail. Ray tracing simulations confirmed the interference and rings formation. Modification at the back surface of materials may go unnoticed and lead to undesired properties. These findings are therefore important for the ultrafast laser community interested in machining of transparent materials.