1) create a layer, fill it with green.
2) create another layer, fill it with black.
3) using a circular marquee select a round selection in the center on the black layer
3) on the bottom of the layers pallete select add vector mask.
4) click on the vector mask and hit ctrl+i to invert it.
5) with the mask still selected, hit filter->blur-.gausian blur and make it fade
6) with mask still selected, select filter->sketch->halftone pattern and adjust it.
7) realize I listed step 3 twice, giggle like a little schoolgirl
8) Profit.