Enhancement of Circular Dichroism in Chiral Dielectric Metasurfaces by Ion Beam Irradiation

Abstract

Resonant chiral dielectric metasurfaces can support circular dichroism exceeding that of natural materials, but their small dissipative losses simultaneously limit the maximization of circular dichroism, which inherently relies on absorption. Importantly, while the condition for optimal circular dichroism in resonant structures can be rigorously formulated based on the concept of critical coupling, controlling the amount of absorption experimentally, and ideally tuning it to the optimal value post-fabrication, remains elusive. Here, we experimentally tailor the dissipative losses of chiral bilayer dielectric metasurfaces post-fabrication using energetic ion beam irradiation. Specifically, we study the transmission characteristics of C4-symmetric chiral metasurface consisting of silicon nanocuboid arrays embedded in silica glass using polarization-resolved spectroscopy. We enhance the circular dichroism from 0.70 in the pristine, unirradiated metasurface to 0.85 after irradiation. Our experimental results are complemented by numerical simulations allowing us to retrieve the refractive index changes induced by the ion beam irradiation in the constituent materials of the metasurface. Our work offers a new approach to globally maximize optical chirality in engineered nanostructures, paving the way towards chiral emission and advanced polarization control applications

Figures (5)

• Figure 1: (a) Schematic illustration of the ion beam irradiated $C$4-symmetric chiral dielectric metasurface. Cross sectional images of the unit cell for the (b) side and (c) top view perspectives.
• Figure 2: Calculated co-polarized transmission spectra of the $C$4 chiral dielectric metasurface for different absorption scaling factor $\alpha^\mathrm{u}_s$ under (a) LCP (solid) and RCP (dashed) illumination. (b) Calculated circular dichroism spectra (c) CD peak as function of fluence, a maximum is shown at $\alpha^\mathrm{u}_s=6$. (d ) Vacancy profiles in the multilayer target simulated using a Stopping and Ranges of Ions in Matter (SRIM) simulation for 200 keV incident Ne.
• Figure 3: Measured polarization-resolved transmittance and circular dichroism for the $C$4-symmetric chiral metasurface under normal incidence illumination at varying ion fluences. (a) Co-polarized transmittance for LCP light, (b) co-polarized transmittance for RCP light. The resonance positions are marked with labels 1, 2, and 3. (c) CD spectra calculated as $\mathrm{T}_{LCP\rightarrow LCP}$-$\mathrm{T}_{RCP\rightarrow RCP}$. (d) Evolution of CD peak magnitude and wavelength as functions of ion fluence.
• Figure 4: (a) Unit cell of the chiral metasurface used for retrieval of ion irradiation induced refractive index changes in the different layers and materials. (b) Experimentally measured (solid) and numerically calculated (dashed) results of the zeroth order transmission spectra for the pristine metasurfaces under LCP and RCP illuminations, with the three dominant features labeled as resonances 1, 2, and 3. (c) Experimentally measured (solid) and numerically calculated (dashed) zeroth order transmission spectra for the irradiated metasurface under LCP and RCP illuminations and for six different ion fluences (F1-F6). (d) Retrieved scaling factors for the real (blue) and imaginary (red) parts of the refractive indices for silicon and glass.
• Figure 5: Normalized field intensity $\left|E/E_0\right|$ calculated in the upper and lower cuboids shown on out-of-plane (top) and in-plane (bottom) cross sections. Panels i,v,ix,xiii show $\left|E/E_0\right|$ under LCP excitation calculated for optimized parameters at F2 fluence and $\lambda$ = 1.538 µ m. Panels ii--iv, vi--viii, x--xii, and xiv--xvi show $\left|E/E_0\right|$ under RCP excitation evaluated in the pristine metasurface (F0) at 1.527 µ m and in the irradiated metasurface (F2 and F6) at 1.538 µ m, respectively.