Laser oscillation from a distributed feedback structure with a diffraction grating directly fabricated on an organic crystal surface using a focused ion beam and plasma etching

Laser oscillation was achieved through optical excitation of a distributed feedback (DFB) laser with a diffraction grating directly fabricated on the surface of an organic crystal using focused ion beam (FIB) processing and subsequent plasma etching. The apparent core thickness of the crystal, which served as the core layer containing the corrugated structure, was determined by analyzing the dependence of the diffraction peak on the measurement direction from the DFB structure. Measurements on multiple samples revealed that the apparent core thickness is proportional to the crystal thickness in the unprocessed and etched regions, excluding the regions of the corrugated structure. For a specific crystal, the unprocessed and etched area thickness was predicted from the preprocessed crystal thickness, and the apparent core thickness was calculated accordingly. Using this value, the effective refractive index of the DFB structure was estimated, and the optimal diffraction grating period was designed based on the maximum fluorescence wavelength of the crystal corresponding to the Bragg wavelength. A DFB laser device with this optimized grating period was fabricated via FIB processing and plasma etching. When optically excited with increasing pulse laser intensity, the device exhibited a nonlinear rise in emission intensity and a marked narrowing of the emission spectrum. These findings demonstrated that laser oscillation was successfully achieved using a diffraction grating directly fabricated on the organic crystal surface through FIB processing and plasma etching. This work underscores the potential of direct surface fabrication for the development of high-performance organic laser devices.