A 10x10 grid of 100μm squares drawn using the #repRapMicron #3Dprinter.

Etched into a layer of marker on a glass slide.

@vik

Tried a diffraction grating?

@zl2tod
No, that might need something more opaque than marker. I have made tiny little lenses though.

@vik

Closely scribed lines on glass or plastic should work. Opacity is undesirable.

@zl2tod
Scribing something tough might be a problem. I'd need a special point - those probe tips are nichrome or stainless steel wire and somewhat flexible. If I used a stout hypodermic I might manage to scratch up clear polystyrene but it would need to be flat. If I use something with a removable opaque surface layer, I can create diffraction slits.