Manufacturing

We have produced a variety of one- and two-dimensional reflective Fourier gratings by directly milling them from a block of brass. The grating structure has been produced with a 3 mm radius ball endmill on a numerically controlled milling machine. The typical surface error is on the order of $\pm$2 $\mu$m, which corresponds to $\pm\lambda/300$ at the design wavelength of 610 $\mu$m. With this surface accuracy, high quality gratings for frequencies as high as 2 THz and beyond should be feasible.

The minimum radius of curvature of the grating structure plays a key role in the machinability of the gratings. Since the height of the structure depends only on the wavelength but the lateral dimensions scale with the magnification of the optics, we have some freedom in choosing the minimum curvature. However, the time to machine the grating and the overall surface flatness suffer, if the grating is too large. We chose a grating size of 60$\times$60 mm$^2$ as a compromise between grating size and minimum curvature. For the 13 coefficient gratings, which we usually manufacture, the minimum radius of curvature is typically about 7 mm, comfortably larger than the mill radius.