A band gap discontinuity is propagated across a Photonic Crystal (PC) to capture thermal energy in a region near the primary emission wavelength of the Planck spectral distribution and transfer that energy to a different spectral region where it is emitted. To extend the range of frequency shifting beyond the width of a single band gap, the intrinsic control parameters (e.g., lattice geometry factors, scattering element geometric factors, and variations in the index of refraction) are spatially varied across the PC to form a band gap gradient. Propagation of the band gap discontinuity, starting in the infrared wavelength region where the thermally generated electromagnetic energy is concentrated and propagating towards the long wavelength region, locally captures the thermal electromagnetic radiation, shifts it downwards in frequency, and pushes the lower-frequency thermal electromagnetic radiation on to the next region. The same principles apply to shift the frequency to shorter wavelengths. A PC-based power combining and waveguide structure can be constructed in the same or a surrounding structure to combine and guide the shifted radiation to an antenna or exit aperture.