A gamma wavelength handheld laser would be cool; "and on this petri dish, we see a dot of cells instantaneously develop cancer"

At high energies I think you could point two at a spot in space and get antimatter where the beams cross (also matter, and then an explosion... see the Breit-Wheeler process).

We have a hard enough time building shipping-container sized devices that reflect extreme ultraviolet though... so I think a handheld gamma ray laser is off the table for this century.