Looking at optical Fano resonances under a new light

Credit: SPIE

In 1961, physicist Ugo Fano offered the very first theoretical description to an anomalous asymmetry observed in the spectral profiles of honorable gases. He presented an impactful analysis of this phenomenon, now called ‘Fano resonance,’ mentioning that if a discrete fired up state of a system falls within the energy series of a continuum of other possible states, these 2 can disrupt each other and generate irregular peaks and dips in the system’s frequency action.

Though Fano resonance can take place in numerous physical systems, current development in metasurfaces and nanotechnology has actually accentuated this phenomenon as a possibly effective tool in optics. The standard understanding of optical Fano resonances is that they are selective in the momentum-frequency domain; to put it simply, they can just be delighted by planar light waves with particular frequencies and occurrence angles, hence restricting their applicability. But could this photo in fact be insufficient?

In a current research study released in Advanced Photonics, researchers Adam Overvig and Andrea Alù from the Advanced Research Center, City University of New York, USA, examined Fano-resonant metasurfaces and found new homes that might open their real capacity. Overvig and Alù exceeded the regular metasurfaces traditionally utilized for generating Fano resonances, showing that stringent periodicity is not in fact needed to allow this phenomenon, and as a result existing metasurfaces just represent a particular subset of the Fano resonances that can emerge in optical systems.

A basic example works to get the total essence of the research study. A traditional, regular Fano-resonant metasurface uses strong polarization, and both spectral and angular selectivity. This suggests that the system hardly shows light of any provided frequency, occurrence angle, and polarization unless they particularly match those of its Fano resonance (in which case, ideal reflection takes place). As mentioned previously, another essential element of such regular metasurfaces is that they can just go through Fano resonances if the occurrence light waves have a planar wavefront. In plain contrast with these restrictions, the scientists showed that it is possible to craft a nonperiodic metasurface that attains ideal reflection, strangely enough accompanied by stage conjugation of the inbound fields, for light waves with an arbitrarily customized wavefront shape and type.

Overvig and Alù mathematically showed that these metasurfaces can be developed by tactically presenting nonperiodic perturbations in otherwise extremely regular photonic crystal pieces. Their work sheds light on yet-unexplored elements of optical Fano resonance, extending the idea beyond standard understanding.

The proposed technique has several pertinent applications, as summed up by Alù: “Our finding generalizes the concept of a Fano resonance, showing that it is not necessarily associated with a planar wavefront. In practice, this enables a new class of optical devices that are transparent and weakly interacting with the incoming light for most excitations but are somehow triggered by a specific wavefront form, frequency, and polarization, which can be selected by design. Only under this specific excitation condition, the device becomes highly reflective and sends back a time-reversed version of the specific input.”

He elaborates on the performance of such gadgets: “An example can be a transparent surface that can be illuminated from any angle and any frequency and polarization, and it is always transparent. However, if you illuminate it with a localized point source placed at a specific location only, with the precise frequency and polarization, all the input energy is reflected and focused back at the location of the source.”

The presented idea of generalized Fano resonances might lead the way for advanced metamaterials that control light in unique methods, with amazing applications in a diverse variety of situations not restricted to optics, however likewise extendable to acoustics and other wave phenomena.

An optical covering like no other

More info:
Adam Overvig et al, Wavefront-selective Fano resonant metasurfaces, Advanced Photonics (2021). DOI: 10.1117/1.AP.3.2.026002

Looking at optical Fano resonances under a new light (2021, March 19)
recovered 21 March 2021
from https://phys.org/news/2021-03-optical-fano-resonances.html

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