Nonlinear optics Second harmonic generation Centrosymmetric media Electromagnetic waves Electronic density of states Energy conservation Energy conserving permutation symmetry Energy exchange Imaging techniques Light Light absorption Light propagation Manley-Rowe relations Mathematical models Nanoscope Near field optical microscope Near field scanning optical microscopy Nonlinear optical nanoscopes Nonlinear susceptibilities Optical fibers Optical resolving power Optical waveguides Photons Polarization Poynting vector Scanning near-field optical microscopes (SNOM) Scanning tunneling microscopy Second harmonic (SH) Sensitivity analysis Sum frequency generation