Smallest diameter collimated laser/optical beam?

[metalplastic]

Most of the applications requiring small diameter laser spots are generated by focusing of laser light and that has a theoretical diffraction limited minimum diameter equal to the wavelength.

Does anyone know what is the smallest fully collimated beam (small divergence say single digit milli or micro rads are ok) optical (particularly laser) beams?

I know they can generate 1um diameter synchrotron x-ray beams for diffraction experiments. What about for lasers? What kind of lens assembly do they need to accomplish this? I assume they don't use classical lens and start using fresnel lenses to achieve this?

 

[mfb]

I would try to use a large mirror for small divergence. The VLT telescopes could give something of the order of a milliarcsecond, if you could manage to use it in reverse direction.

However, small divergence corresponds to a large initial beam size. If you want to reduce the product of beam size and divergence, use a single atom as light source and measure directly next to the atom ;).