Diffraction grating enhanced photoluminescence from etching-free erbium thin films

Micro-structuration by etching is commonly used in integrated optics, adding complex and costly processing steps that can also potentially damage the device performance, owing to degradation of the etched sidewalls. For diffraction grating fabrication, different strategies have been developed to avoid etching, such as layer deposition on a structured surface or grating deposition on top of active layers. However, etching remains one of the best processes for making high aspect ratio diffraction gratings. In this work, we have developed fully structured diffraction gratings (i.e., like fully etched gratings) using lift-off based processing performed in pulsed laser deposited layers, since the combination of both techniques is of great interest for making micro-structures without etching. We have first studied the influence of the lithography doses in the lift-off process, showing that (1) micrometric spatial resolution can be achieved and (2) the sidewall angle can be controlled from 50° to 150° in 0.5 µm thick layers. Using such optimizations, we have then fabricated Er-doped Y₂O₃ uniaxial diffraction gratings with different periods ranging from 3 to 8 µm. The fabricated devices exhibit emission and reflectivity properties as a function of the collection angle in good agreement with the modeling, with a maximum luminescence enhancement of ×15 compared with an unstructured layer at a wavelength of 1.54 µm. This work thus highlights lift-off based processing combined with pulsed laser deposition as a promising technique for etch-free practical applications, such as luminescence enhancement in Er-doped layers.