Abstract: A handpiece can treat biological tissue using electromagnetic radiation, which can be substantially fluorescent light. The handpiece includes a source of electromagnetic radiation and a nonlinear waveguide. The nonlinear waveguide is adjacent the source, receives electromagnetic radiation from the source, and delivers the electromagnetic radiation to the biological tissue. The handpiece also includes a system for circulating a fluorescent substance through the nonlinear waveguide. The fluorescent substance is capable of modulating at least one property of the electromagnetic radiation.