Precision Patterning of ZnO using Femtosecond Laser: Opening Doors to Nanophotonic Device Innovations

Patterned nanostructure exhibits interesting optoelectronic characteristics due to light-matter interaction at comparable dimensions. The optical response of the nanostructure-based devices is modulated by the size and the periodicity of the structure. Numerous techniques based on top-down and bottom-up approaches are used to obtain patterned nanostructure. However, top-down methods provide more precise control over the patterning of the surfaces. Recently patterned structures of metal oxide semiconductors are getting attention for various electronic devices like photodetectors, solar cells, etc. The femtosecond laser has shown the capability to precisely pattern the surfaces with regular and smaller structures. In this work, we have fabricated ZnO nanopillars by patterning the ZnO thin film using a femtosecond laser. ZnO film (thickness ~600 nm) is deposited on a glass slide using an organic salt solution of zinc in isopropanol and annealed at 400°C. The ultrafast mode-locked Ti: sapphire-based femtosecond pulsed laser (Coherent made, Chameleon Ultra I) with fixed pulse width (140 fs) and a repetition rate (80 MHz) is used for this patterning. The laser beam of 800 nm with a beam diameter of 1.2 mm at 1/e² tightly is focused onto the ZnO film for the selective patterning (ablation). The patterns are prominent after increasing the laser power beyond 1.5 W. ZnO is patterned in a square mesh with a periodicity of 1 um, which resulted in the formation of ZnO nanopillars with a side of ~500 nm and a height of 600 nm. The fabricated structure exhibits different colors at different angles due to the scattering of the light. These reflected colors are analyzed with the camera by varying the incident angle of the white light. It is observed that the photo emitted light colour changes from colourless to blue, orange, pink, green, red, and finally deep red. Due to the multiple scattering, the overall path length of the light inside the active material increases. Also, the total surface area of the active material in the patterned film is larger as compared to the plane film. The array of semiconducting ZnO nanopillars exhibits unique electronic and optical characteristics for optoelectronic and photonic applications. Similarly, the femtosecond laser can pattern different metal oxide layers and incorporate the fabricated structures in optoelectronic devices to improve their performances.