Downconverting UVC from a merucry lamp (255nm) to UVB broadband? I would like to take light from a UVC lamp (~255nm) and downconvert it to a broadband from 200nm to 400nm. How could I do this?
If I wanted to develop my own materials or source them, what elements would be a good place to start looking?
 A: As you say the source is a UV lamp I’m going to rule out spontaneous parametric down conversion  and suggest that you consider using down-shifting instead. 
Down-shifting uses a luminophore to absorb light of high energy and emit it at a lower energy: fluorescence.
You could use inexpensive plastic sheets doped with a fluorescent dye. 

You will need to find an dye which has the correct characteristics for your purpose. 
You didn’t mention anything about performance or details of your setup, so I’ve listed some potential problems,


*

*At the wavelength of 255nm it’s going to be hard to cut out parasitic absorption from glass and plastics. I believe the absorption peak at ~250nm in the plot below is from the acetone rather than the dye.

*Your beam will be lost and replaced with Lambertian emission from the sheet edges


I have some experience with Lumogen F Violet 570, this will convert high energy photons to emission in the range 350nm to 550nm.

It does not seem ideal of your purpose. But it is readily available in inexpensive fluorescent sheets you can buy online.
Another option is to use a nanocrystals but that’s becoming a bit specialist. Start-ups are using this technology to make “power from glass”.
