Mid-IR diffraction gratings fabricated via nanoimprint lithography for chirped pulse amplification

Lasers emitting in the 2 μm spectral region have attracted considerable attention in recent years owing to their wide-ranging applications in science and industry, including the detection of atmospheric constituents like H₂O, surgical procedures in medicine, and LIDAR technologies. In industry, they can be utilized in high-precision machining and materials engineering. For these latter purposes, a high-power laser is needed. This can be achieved by amplifying the laser pulse, typically using chirped pulse amplification (CPA). The crucial component of CPA, used for the compression and stretching steps, is the grating waveguide structure (GWS). In the present paper, we explore the fabrication of GWSs via nanoimprint lithography (NIL) for operation at a central wavelength of 2050 nm. The GWSs were integrated into the topmost hafnium oxide (HfO₂) layer of a multilayer HfO₂/SiO₂ dielectric mirror. The fabrication process comprised master wafer production via laser interference lithography, stamp fabrication, NIL patterning, and reactive-ion etching. The desired etch depth in HfO₂ was a key challenge, which was addressed by implementing a chromium hard mask and a Taguchi experimental design. Scanning electron microscopy and atomic force microscopy revealed slight deviations from the designed GWS dimensions, mainly in the duty cycle. Consequently, the maximum diffraction efficiency was slightly blue-shifted and measured to be 97% at 2030 nm, i.e., lower than the 99% predicted theoretically. Our study demonstrates the feasibility of using NIL for large-area GWS fabrication with high diffraction efficiency.