Enhancing coherent nonlinear-optical processes in nonmagnetic backward-wave materials

Novel concepts of nonlinear-optical (NLO) photonic metamaterials (MMs) are proposed. They concern with greatly enhanced coherent NLO energy exchange between ordinary and backward waves (BWs) through the frequency-conversion processes. Two different classes of materials which support BWs are considered: crystals that support optical phonons with negative group velocity and MMs with specially engineered spatial dispersion. The possibility to replace plasmonic NLO MMs enabling magnetic response at optical frequencies, which are very challenging to engineer, by the ordinary readily available crystals, are discussed. The possibility to mimic extraordinary NLO frequency-conversion propagation processes attributed to negative-index MMs (NIMs) is shown in some of such crystals, if optical phonons with negative group velocity and a proper phase-matching geometry are implemented. Here, optical phonons are used as one of the coupled counterparts instead of backward electromagnetic waves (BEMWs). The appearance of BEMWs in metaslabs made of carbon nanotubes, the possibilities and extraordinary properties of BW second harmonic generation in such MMs is another option of nonmagnetic NIMs, which is described too. Among the applications of the proposed photonic materials is the possibility of creation of a family of unique BW photonic devices such as frequency doubling metamirror and Raman amplifiers with greatly improved efficiency.