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 similar to 10(3) times. The photoluminescence band is redshifted by 2-3 nm and 40% narrower ( full width at half-maximum), while at the same time the E-2 (439 cm(-1)) Raman band intensity increases up to similar to 50 times. The process is found irreversible with the threshold fluence of 11 mJ/cm(2), which is considerably lower than the ablation threshold 115 mJ/cm(2). Fine surface nanostructuring on the scale of similar to 10 nm may be responsible for the observed effect. (C) 2008 Optical Society of America (C) 2010 Optical Society of America