Enabling Ultra-Broadband High-Efficiency Gratings for Single-Cycle Infrared Lasers

To generate the shortest high power laser pulse, a single-cycle pulse, gratings inside the ultrafast laser's compressor and stretcher need to meet three requirements: high efficiency, ultra-broad bandwidth, and high damage threshold. These three requirements are especially difficult to meet for Infrared (IR) lasers. We have demonstrated for the first time that combination of E-beam lithography and anisotropic wet etching of silicon wafers can produce gratings meeting the three requirements. A global optimization approach is used to achieve diffraction efficiency > 90% for mid IR gratings centered at 5 um with a 65% relative bandwidth, which is 1.47x (2.1x) lager than required by Gaussian-shaped (sech(2)-shaped) single-cycle pulses. A similar high-efficiency mid-IR grating was recently fabricated and tested, demonstrating an efficiency of 92.0% at 4.3 mu m, which closely matches the designed efficiency of 94.5%. Conformal coatings in fabrication and large angle of incidence in design are employed to improve damage threshold. Further enhancement of the grating performance has been realized through a double-blazing feature, combining the classic triangular groove geometry with a "land-ridge" modulation. The design methodology has been applied to near IR and long IR successfully to prove that the new design approach and fabrication technology combined is a reliable way to make gratings for single-cycle high-power ultrafast laser for entire IR region (0.8 um - 20 um).