Full Control of Fano Spectral Profile with GST-Based Metamaterial

Asymmetric line shape in a spectral profile, quantified by the Fano parameter q, is a hallmark feature of Fano resonances. In photonics, this asymmetry captures the Fano physics of coupling localized resonances to the radiative continuum and nourishes many photonic Fano devices. Recent advances have suggested full control of q from negative infinity to positive infinity; here we realize the negative-to-positive q transition in an active and successive manner using Ge2Sb2Te5 (GST)-based Fano resonator in the Mid-infrared. The structure consists of two resonating components: an array of dielectric rectangle resonators on dielectric substrate (for localized resonance) and a GST layer (for radiative continuum), coupled by a conductive spacer. With phase transition of GST, the q parameter can be continuously tuned from 5.2 to 0 to −5.6, which directly maps to the wavelength shifts in the GST resonating layer, that is, the modulation in the continuum. Incorporating geometrical optimization, an arbitrary q value is further achieved. Our Fano resonator also demonstrates a unique narrowband switching property based on the q tuning effect at a wavelength of 5 μm. It performs as a strong reflector of reflection intensity R = 0.77 (with Q = 135) for q = −0.2 and switches to a strong absorber with absorption intensity A = 0.94 (with Q = 190) for q = −5.6. This dynamic interplay between radiative continuum and Fano spectral profile in a vast range of q paves the way for new classes of active photonic devices.