# Linear frequency conversion via sudden merging of resonances in time-variant metasurfaces

## TL;DR

This paper introduces a novel approach using rapidly time-variant metasurfaces to achieve efficient linear frequency conversion at THz frequencies by merging resonances through ultrafast optical excitation.

## Contribution

It demonstrates the experimental realization of frequency conversion via sudden resonance merging in time-variant metasurfaces, a new method for dynamic spectral control.

## Key findings

- Successful experimental demonstration at THz frequencies.
- Frequency conversion efficiency depends on temporal coherence.
- Metasurfaces enable on-demand frequency synthesis.

## Abstract

Energy conversion in a physical system requires time-translation invariance breaking according to Noether's theorem. Closely associated with this symmetry-conservation relation, the frequencies of electromagnetic waves are found to be converted as the waves propagate through a temporally varying medium. Thus, effective temporal control of the medium, be it artificial or natural, through which the waves are propagating, lies at the heart of linear optical frequency conversion. Here, we propose rapidly time-variant metasurfaces as a frequency-conversion platform and experimentally demonstrate their efficacy at THz frequencies. The proposed metasurface is designed for the sudden merging of two distinct resonances into a single resonance upon ultrafast optical excitation. From this spectrally-engineered temporal boundary onward, the merged-resonance frequency component is radiated. In addition, temporal coherence of the two original resonating modes with respect to the abrupt temporal boundary is found to be strongly related to the amount of frequency conversion as well as the phase of the converted wave. Due to their design flexibility, time-variant metasurfaces may become on-demand frequency synthesizers for various frequency ranges.

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Source: https://tomesphere.com/paper/1706.00664