Fabrication and testing results of sine-top, high-efficiency, broadband gold-coated gratings (BGCG) for high-power laser pulse compression applications are reported. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by directly etching the quartz substrate. The groove depth and duty cycle of the photoresist mask was controlled by changing photoresist thickness and adjusting exposure and development times, respectively. The duty cycle of the photoresist mask was further corrected by oxygen plasma etching. Using this method, high efficiency, sine-top, BGCG with line densities of 1740 lines/mm was achieved. The average diffraction efficiency at the-1st order was 89.2% and the peak value was 90% for TM polarized light as the wavelength increases from 750 to 850 nm.
Broadband gold-coated grating (BGCG) is one of the key elements of large pulse compression systems. Compared with
other pulse compression grating (PCG), BGCG have the advantages of simple structure and low cost etc. More
importantly, this kind of grating can get high diffraction efficiency within a broadband range (usually 200 nm or more).
In this paper the authors report a process for fabrication of sine-top BGCG. When gratings are intended for use with
high-power lasers, their laser-damage threshold has an importance equal to that of the diffraction efficiency. These
gratings fabricated by this method differ from conventional metal-on-photoresist PCGs in that the gratings patterns are
generated by etching the fused silica substrate directly. This can improve the laser damage threshold. The groove depth
and duty cycle of the photoresist mask were controlled by changing photoresist thickness and adjusting exposure and
development time. The duty cycle of the fused silica grating was further corrected by oxygen plasma etching. Using this
method, high efficiency sine-top BGCGs with line densities of 1740 lines /mm have been achieved, this paper has a good
reference value to the further fabrication of larger aperture gold-coated PCG.
We proposed a technique for conducting on-the-fly fine adjustment of etch depths with sub-nanometer precision during the course of ion beam etching (IBE). Simulations were performed to evaluate the etch-depth control precision. The simulation prediction shows that the precision of fine control of etch depths is at the level of 0.1nm. The preliminary experiment was conducted. The early result and the simulation prediction are in agreement with each other, which indicates that this approach is feasible for finely controlling groove-depth variations of large-area diffraction gratings.
The authors report a new process combining interference lithography with potassium hydroxide (KOH) anisotropic etch
technique for fabrication of high aspect ratio silicon gratings on (110) oriented silicon wafers. This new process has the
ability in fabricating high aspect ratio silicon gratings with extremely smooth sidewalls over a large sample area. An
alignment method was developed to align interference fringes to the vertical (111) planes of (110) oriented wafers. In
addition, a room temperature etch process with 50 wt % KOH solution was chosen to finally get an etch anisotropy of 188.
Better etch uniformity was achieved by adding a surfactant to the aqueous KOH to promote the release of hydrogen bubbles.
To increase latitude in KOH etching process, deposition of aluminum under a sloped angle with respect to the grating
structures was utilized to obtain a high duty cycle nitride mask. To prevent the collapse of high aspect ratio grating
structures caused by surface tension, a liquid carbon dioxide supercritical point dryer was used in the drying process. The
authors successfully fabricated 320nm period gratings with aspect ratio up to 100 on 5-μm-thick silicon membranes on
(110) oriented silicon-on-insulator wafers. The sample area is about 50 mm × 60 mm. The roughness (root mean square)
of the sidewall is about 0.267 nm.
Fabrication-induced metal contaminations and subsurface damage are generally identified as the laser damage initiators
that are responsible for the laser induced damage in fused silica. In this paper, the removal of those two initiators are
realized by two methods: wet chemical surface cleaning and optimized HF-based etch process. Two kinds of chemical
leaching are used to removing the Ce and other metal impurities respectively. In order prevent the redeposition of the
reactive byproducts during HF etch process, we optimized the traditional HF etch process in two ways: absence of NH<sub>4</sub>F in etch solution and presence of megasonic and ultrasonic agitation during and after etch respectively. And laser damage tests show that these two treatments greatly improve the laser damage resistance of fused silica.