In one aspect, the disclosure features an article, including a fiber waveguide extending along a waveguide axis, the fiber waveguide including a core extending along the waveguide axis and a confinement region surrounding the core. The confinement region is configured to guide radiation at a first wavelength, λ1, along the waveguide axis and is configured to transmit at least some of the radiation at a second wavelength, λ2, incident on the confinement region along a path, where λ1 and λ2 are different. The core includes a core material selected to interact with radiation at λ1 to produce radiation at λ2.