Abstract: Use of ultrashort, focused pulses to alter a detectable optical property in a specific region in a structure allows lower energy to be used in fabrication of a three-dimensional, periodic array of altered regions in a material. These properties may be, for example, an index of refraction, absorption or scattering. The typical spacing between altered regions may be larger than a wavelength of interest, to create diffractive optical elements, or may be roughly the same as a wavelength of interest, to create photonic crystal elements. The photonic crystal may have a photonic band gap, i.e., a frequency range in which no modes may propagate, or may simply have altered dispersion properties but no gap, as in a photonic crystal superprism.