Fig. 1. Realization of PMFs in PhCs. (a) Schematic of the pristine PhC with a honeycomb lattice (left) and band diagrams of the PhCs with C6v symmetry (middle) and C3v symmetry (right). The rhombic unit cell is encircled by the red-dashed line. The first Brillouin zone is shown in the inset of the band diagram. (b) and (c) Schematics of the PhCs (top), band diagrams of the supercells encircled by the red-dashed line (middle/bottom left), and mode profiles corresponding to the blue star points in the band diagrams (bottom/bottom right) with inhomogeneous effective mass terms (b) m(x)=ax and (c) m(y)=by. The propagation paths of light are shaded in purple. The rhombic unit cells in panels (b) and (c) are encircled by the black and orange dashed lines, respectively. (d) Schematics of the S-bend (left) and the power splitter (right) based on PMFs, with the propagation paths of light shaded in purple.

Fig. 2. Demonstration of a straight waveguide and an S-bend based on PMFs. (a) Schematic of the S-bend implemented on an SOI platform. The purple-shaded area indicates the propagation path of light. Light is coupled into and out of the chip by grating couplers. (b) Simulated propagation profiles for the straight waveguide with m(x)=ax (top) and the S-bend with a nonuniform PMF (bottom) at a central wavelength of 1550 nm. (c) SEM images of the fabricated devices. The purple-shaded areas indicate the propagation paths of light. The areas encircled by the red, blue, and orange dashed boxes are zoomed in and displayed at the bottom of the figure. (d) Simulated and measured transmission spectra of the straight waveguide and the S-bend.

Fig. 3. Demonstration of a 50:50 power splitter based on a PMF. (a) Schematic illustration of the operation principle of the power splitter. (b) Schematic of the PhC structure of the splitter. The propagation paths of light are shaded in purple. (c) Simulated propagation profile for the PMF-based power splitter at a central wavelength of 1550 nm. (d) SEM images of the fabricated device. The purple-shaded areas indicate the propagation paths of light. The areas encircled by the red, blue, and orange dashed boxes are zoomed in and displayed at the bottom of the figure. (e) Simulated and measured transmission spectra of the power splitter.

Fig. 4. High-speed data transmission experiment based on the S-bend and the power splitter. (a) Experimental setup for the data transmission of 140-Gb/s PAM-4 signals. The black and yellow lines represent optical and electrical links, respectively. PC, polarization controller; DAC, digital-to-analog converter; EA, electrical amplifier; MZM, Mach–Zehnder modulator; EDFA, erbium-doped fiber amplifier; PD, photodiode; DSO, digital storage oscilloscope. (b) DSP flow charts for the transceiver. (c) and (d) Measured optical spectra of the PAM-4 signals before and after passing the (c) S-bend and (d) power splitter. BERs (e) and recovered eye diagrams (f) of the PAM-4 signals for different channels of the S-bend and the splitter.