Fig. 1. Design of the proposed metasurface-based reconfigurable (1×2) integrated switch working around λ=800  nm. (a) 3D illustrations of the device structure and its functionality when the Sb2S3 metasurface structure is in a crystalline or amorphous state. The top-left inset shows the top view of the device with dimensions of the metasurface consisting of a set of passive TiO2 nanorods and another set of PCM (Sb2S3) nanorods. The top middle inset shows the conditions required for the Sb2S3 to undergo a reversible structural transition from amorphous to crystalline states. (b) Cross section of the device. (c) and (d) Wavelength dependence of the complex optical constants (n, k) of amorphous Sb2S3, crystalline Sb2S3, amorphous titanium dioxide, and silicon nitride. Highlighted parts in red indicate the spectral region of interest where Sb2S3 exhibits low loss and high switching contrast.

Fig. 2. Metasurface parametric sweep. (a)–(d) Device cross talk as a function of metasurface footprint (Lx), nanorod length (L), separated gap width (g), and the nanorods’ center-to-center distance (Λ), respectively, for a-Sb2S3 and c-Sb2S3. The dotted gray lines show the dimensions used in our metasurface.

Fig. 3. Characterization of nanoantenna structure. (a) Profile (zy plane) of the normalized near field of the Ez component showing scattering Mie modes in the TiO2 and Sb2S3 nanoantennas for a-Sb2S3 (left panel) and c-Sb2S3 (right panel) at λ=800  nm. (b) Normalized electric field intensity |E|2 distribution (zy plane) in the same nanorods for a-Sb2S3 and c-Sb2S3. The boundaries of the SiN waveguide and nanoantennas are indicated in solid black lines. (c) Effective refractive index of the TE00 mode as a function of TiO2 nanoantenna length and Sb2S3 nanoantenna length for the a-Sb2S3 and c-Sb2S3 phases. The dashed gray line indicates the length of the nanorods (L) used in our simulation. The inset figure shows the FDE simulation setup.

Fig. 4. Simulated device performance for a-Sb2S3 (upper panel) and c-Sb2S3 (lower panel). (a) and (d) Full-wave simulation showing the optical field intensity |E|2 in the switch for the fundamental TE mode in the xy plane at λ=800  nm. The boundaries of the SiN waveguide and metasurface structure are indicated by dashed lines and rectangles, respectively. Inset: enlarged view of the field profile in the metasurface. (b) and (e) Transmission spectra at two output ports Port2 and Port3. (c) and (f) Total transmission at output ports (Port2+Port3), reflection, and scattering of the device.

Fig. 5. Multimode waveguide characterization. The dependence of waveguide width on the (a) neff and (b) dispersion D of different SiN waveguide modes at λ=800  nm. The shaded region indicates the waveguide widths that support asymmetric multimode. The dashed black lines show the maximal modal index (neff) in the waveguide and the waveguide width that support a single mode. The gray dotted line indicates the width considered in the stem SiN waveguide. (c) neff and D of the fundamental TE00 mode as a function of simulated wavelengths. The inset figure shows the Ey distribution of the TE00 mode at λ=800  nm.

Fig. 6. Parametric sweep of TiO2 nanorods for the TE00 mode at λ=800  nm. (a) and (b) Simulated device cross talk considering variations of the height and width of TiO2 nanorods when Sb2S3 is in the crystalline and amorphous state, respectively. (c) and (d) Calculated transmission at the desired output as a function of height and width of TiO2 nanorods for the crystalline and amorphous state, respectively. The black dashed lines indicate the dimensions used in our metasurface.

Fig. 7. Full-wave simulation showing the optical field intensity |E|2 in the switch for the fundamental TE mode in the xy plane at λ=800  nm and for different TiO₂ dimensions for (a)–(c) c-Sb2S3 and (d)–(f) a-Sb2S3, The boundaries of the SiN waveguide and metasurface structure are indicated by dashed lines and rectangles, respectively. Inset: enlarged view of the field profile in the metasurface.

Fig. 8. Suggested fabrication method employing three steps of electron beam lithography: (a) two positive resists followed by LPCVD to transfer the desired patterns of the nanorod arrays onto the developed gaps and (b) a negative resist followed by reactive ion etching (RIE) to define the SiN waveguide.

Fig. 9. Device fabrication tolerance. (a) and (b) Simulated device cross talk for the fundamental mode operating at λ=800  nm, considering possible parallel misalignment (offset) and axial misalignment (gap width), respectively. The gray dotted lines in the figures indicate the dimensions used in our device.

Fig. 10. Schematic illustration of the experimental setup suggested for characterizing the performance of the proposed reconfigurable switch. Here, PPG is a programmable pulse generator, PC is a power controller, and M is a mirror.

Fig. 11. Sketches showing x-periodic Sb2S3 and TiO2 nanoantennas patterned on a SiN waveguide in (a) xy plane and (b), (c) zy plane, where na/c are the effective indices of a-Sb2S3 and c-Sb2S3 nanorods, n2 is the effective index of the TiO2 nanorods, and nbare is the effective index of bare SiN waveguide. (d) Effective refractive indices for the waveguide cross section calculated using FDE. The inset shows the FDE simulation setup. (e) Effective indices for nanorod arrays calculated by Rytov’s approximation.

Fig. 12. Modes in the input and output ports of SiN waveguides. Simulated E_y component at λ=800  nm for the TE00 mode in the (a) input stem SiN waveguide before interacting with the metasurface, (b) output Port3 for a-Sb2S3 and (c) output Port2 for c-Sb2S3. The arrows indicate the vector diagrams of the electric field component of the modes. The dashed black lines show the boundaries of the SiN waveguides.