Femtosecond UV laser non-ablative surface structuring of ZnO crystal: impact on exciton photoluminescence

The ultraviolet (UV) laser irradiation (248 nm) of monocrystalline wurtzite ZnO with 450 fs pulses results in surface modification. A formation of two orthogonal ripple structures with a period of 400-500 nm was observed oriented parallel and perpendicular to the laser beam polarization. The UV exciton emission obtained on the irradiated domains is found greatly enhanced locally up to ~103 times. The photoluminescence band is redshifted by 2-3 nm and 40% narrower (full width at half-maximum), while at the same time the E2(439 cm−1) Raman band intensity increases up to ~50 times. The process is found irreversible with the threshold fluence of 11 mJ/cm2, which is considerably lower than the ablation threshold 115 mJ/cm2. Fine surface nanostructuring on the scale of ~10 nm may be responsible for the observed effect.