The origin of second harmonic generation hotspots in chiral optical metamaterials

Recently, a great amount of research has been triggered by the prediction, formulated by Pendry and co-authors, that novel and enhanced nonlinear optical phenomena could be observed in metamaterials. This prediction is based on the fact that, in metamaterials, local field enhancements can have a dramatic influence over the optical properties of the material. One of the largest contributions to such local field enhancements is attributable to surface plasmon resonances. Plasmons are collective oscillations of the electrons under the influence of light's electromagnetic field. Plasmons occur naturally on the surfaces of homogeneous metal films, where they usually dissipate quickly and cancel each other's influence. However, upon patterning the metal surface at the nanoscale, plasmons can be manipulated in a manner similar to classical waveguiding, whereby propagation or standing wave patterns can be achieved. In other words, artificial structuring allows for nanoengineering the position and intensity of the local fields. Incidentally, nonlinear optical effects, such as second-, third-, or forth-harmonic generation, scale as the second, third or fourth power of the electromagnetic intensity, respectively. Therefore, it is perfectly reasonable to assume that the large electromagnetic local field enhancements in metamaterials should yield large or previously unobserved nonlinear optical effects.