# Fano Enhancement of Unlocalized Nonlinear Optical Processes

## TL;DR

This paper introduces Fano resonance schemes to enhance unlocalized nonlinear optical processes, achieving significant amplification and tunability, with potential applications across various nonlinear optical conversions.

## Contribution

It develops a novel Fano enhancement approach for unlocalized nonlinear processes, surpassing traditional field trapping techniques and enabling voltage-controlled tunability.

## Key findings

- Achieved up to 1000-fold Fano enhancement of nonlinear processes.
- Validated results through analytical and numerical Maxwell's equations solutions.
- Demonstrated continuous control of enhancement via applied voltage.

## Abstract

Field localization boosts nonlinear optical processes at the hot spots of metal nanostructures. Fano resonances can further enhance these "local" processes taking place at the hot spots. However, in conventional nonlinear materials, the frequency conversion takes place along the entire crystal body. That is, the conversion process is "unlocalized". The path interference (Fano resonance) schemes developed for localized processes become useless in such materials. Here, we develop Fano enhancement schemes for unlocalized nonlinear optical processes. We show that 3 orders of magnitude Fano enhancement multiply the enhancements achieved via field trapping techniques, e.g., in epsilon-near-zero~(ENZ) materials. We demonstrate the phenomenon both analytically and by numerical solutions of Maxwell's equations. The match between the two solutions is impressive. We observe that the interference scheme for unlocalized processes is richer than the one for the local processes. The method can be employed for any kind of nonlinear optical conversion. Moreover, the Fano enhancement can be continuously controlled by an applied voltage.