We present experimental realization of Cerenkov difference frequency generation (CDFG) in a 2-dimentional rectangular periodically poled LiTaO 3. The backward reciprocal vector of the nonlinear photonic crystal accelerates the phase velocity of the nonlinear polarization wave so as to exceed the phase speed limit of achieving Cerenkov down-conversions in this normal dispersion medium. Some characteristics of CDFG, such as radiation angles, pumping power, and pulse temporal overlapping dependences were discussed. The experimental results matched well with theoretical predictions. V

We observe multiple simultaneous cascaded-v (2) Č erenkov conical radiations in radially poled nonlinear photonic crystals. By using two incident fundamental waves x 1 and x 2 , a variety of cascaded nonlinear up-conversion processes occur which result in high-frequency Č erenkov radiations at 2x i þ x j ði; j ¼ 1; 2Þ exhibiting as multicolor conical beams. Two types of phase-matching geometries with different emission angles are demonstrated for each kind of cascaded-v (2) Č erenkov radiation. The external angle of the Č erenkov radiation exhibits strong dependence on the fundamental wavelengths. The experimental results agree well with the theoretical calculations.