Implications of Low-Refractive Mode Index on Nonlinear Optical Interactions

Despite ample experimental evidence that the so-called epsilon-near-zero (ENZ) materials enable enhancement of nonlinear interactions in limited volumes of materials, there is still a lot of controversy about theoretical interpretation of results in search of optimal materials and structural configurations. The nonlinear performance of materials depending on their refractive index is compared and direct simulations prove with the indium-tin oxide material that the regime with low refractive index is superior to the ENZ regime in terms of the third harmonic generation (THG) efficiency. It is shown that as a rule for low-index materials the THG scales inversely proportional to the refractive index squared. Another advantage of low-index modes is that they satisfy phase-matching conditions by default. As exemplified by the direct numerical simulations, silicon-based photonic crystals can effectively produce third harmonic waves, which can be manipulated to propagate in any desired direction. Such an approach helps to avoid unnecessary complexity in on-chip linear and nonlinear circuitry and reduces potential sources of losses by unifying the whole production chain with implementation of the same material, e.g., silicon.