Measurement of the refractive index modulation induced by Quantum Confined Stark Effect for active metasurfaces realisation.

Overview & Synopsis

Achieving an active optical index modulation in materials is an enabling asset for the realisation of active metasurfaces.Ideally the modulation should be large and ultra-fast and without adding losses to the system.Multiple simulation works already showcase how high reflectivity and fine phase tuning could be obtained with semiconductor nano-resonators [1,2].It is thus interesting to concentrate our efforts of refractive index modulation for this kind of materials.The Quantum Confined Stark Effect (QCSE) is an ultrafast electro-optic effect which has been studied for some times already [3].It is known to induce changes in the absorption of a structure near the gap by tuning transition energies.At wavelengths larger than the bandgap, it is possible to have an ideally null absorption but still achieve a finite refractive index modulation.Inserting those active components in highly resonant structures, one can fabricates highly reflective active metasurfaces with ultra-fast tunability.An active metasurface exploiting such an electrooptic index modulation has been published in 2019 by Atwater teams [4].In this work, a PIN-type structure was used to apply an electric field in the quantum wells. An index modulation of 0.01 was expected but not directly measured.In this study, we present the design of a NIN-type, GaAs/AlGaAs structure and its refractive index electro-modulation at wavelengths below the band-gap.Ellipsometric measurements of the structure are used to precisely determine the different optical properties of the structure.These parameters are used to model the reflectivity modulation that can be induced by a refractive index modulation.These predictions are then compared to the experimentally measured reflectivity modulation induced by an external electric bias.A refractive index variation as high as 0.02 was deduced from measurements at wavelengths in the QW transparency range. This value is in good agreement with theoretical predictions of the Stark effect (Nextnano simulations).This refractive index variation measurement in electroreflection is an important milestone for the conception and development ofactive full dielectric metasurfaces exploiting the intrinsic speed of QCSE optical index modulation.[1]: M. Elsawy et al., Laser & Photonics Review, 2023, 10.1002/lpor.202200880[2]: B. Yang et al., Optics Express, 2025, 10.1364/OE.563702[3]: Y. Kan et al., IEEE Journal of Quantum Electronics, 1987, 10.1109/jqe.1987.1073288[4]: P. C. Wu et al., Nature Com., 2019, 10.1038/s41467-019-11598-8