Fig. 1. (a) Schematics of atomic structures and bandgaps of graphene oxide (GO), semi-reduced GO (srGO), and highly reduced GO (hrGO). (b) Schematic illustration of a GO-coated silicon waveguide as an optical polarizer. Inset illustrates the layered GO film structure fabricated by self-assembly. (c) TE and TM mode profiles for the hybrid waveguide with 2 layers of GO. (d) Microscopic image of a GO-coated silicon-on-insulator (SOI) chip with opened windows. Inset shows a scanning electron microscopy (SEM) image of the layered GO film, where numbers (1‒4) refer to the number of layers for that part of the image. (e) Measured Raman spectra of the SOI chip in (d) without GO and with 2 layers of GO.

Fig. 2. (a) Measured insertion loss (IL) versus GO film length (L_GO) for the hybrid waveguides with a monolayer GO film (N =1) after the chip was heated at various temperatures T_R. (b) Measured IL versus T_R for the waveguides with 1−2 layers of GO (N =1, 2). In (a) and (b), (i) and (ii) show the corresponding results for TE and TM polarizations, respectively. (iii) shows the polarization dependent loss (PDL) calculated from (i) and (ii). (c) Polar diagrams for the measured IL of devices with (i) 1 and (ii) 2 layers of GO after the chip was heated at various temperatures T_R. In (a‒c), the input continuous-wave (CW) power and wavelength were ~0 dBm and ~1550 nm, respectively. In (b) and (c), L_GO = ~0.4 mm.

Fig. 3. (a) TE- and TM-polarized propagation loss (PL) versus T_R for the hybrid waveguides with 1 and 2 layers of GO (N = 1, 2). (b) Extinction coefficients (k's) of 2D GO films versus T_R obtained by fitting the results in (a) with optical mode simulations. (c) Anisotropy ratios of k values for TE and TM polarizations (k_TE / k_TM) extracted from (b). (d) Measured (Exp.) and simulated (Sim.) PDL versus T_R for the waveguides with 1−2 layers of GO (N = 1, 2). The simulated PDL values were obtained by using the same k value for both TE and TM polarizations. (e) Fractional contributions (η's) to the overall PDL from polarization-dependent mode overlap and material loss anisotropy, which were extracted from (d). (e-i) and (e-ii) show the results for N = 1 and 2, respectively.

Fig. 4. (a) Measured (i) TE- and TM-polarized IL and (ii) calculated PDL versus input power (P_in) for the hybrid waveguide with 1 layer of GO. (b, c) Measured (i) TE- and TM-polarized IL and (ii) calculated PDL versus P_in for the waveguide with 1 layer of rGO after heating at T_R = ~100 °C and ~200 °C, respectively. (d) Measured (i) TE- and TM-polarized IL and (ii) calculated PDL versus P_in for the waveguide with 2 layers of GO. In (a‒d), the red and blue shaded areas in (i) indicate the power ranges associated with reversible GO reduction for TE and TM polarizations, respectively. (e) Measured PDL versus input CW wavelength for the waveguide with 1 layer of unreduced GO, rGO at T_R = ~100 °C, and rGO at T_R = ~200 °C. In (a‒e), the GO film length was ~0.4 mm. In (a‒d), the input CW wavelength was ~1550 nm. In (e), the input CW power was P_in = ~0 dBm.

Fig. 5. (a) Schematic of a silicon microring resonator (MRR) coated with rGO films as an MRR polarizer. (b) Microscopic image of a silicon MRR coated with 1 layer of unreduced GO. (c) Measured (i) TE- and (ii) TM-polarized transmission spectra of the MRR with 1 layer of GO at different degrees of reduction. The same hybrid MRR underwent heating at temperatures T_R ranging from ~50 to 200 °C prior to the measurement. The corresponding results measured at room temperature before heating (initial) are also shown for comparison. (d) Extinction ratios (ERs) of the MRRs extracted from (c). (e) Polarization extinction ratios (PER's) extracted from (d). In (c‒e), the CW input power was P_in = ~−10 dBm.

Fig. 6. (a) Measured TE- and TM-polarized ER versus input CW pump power P_p for the hybrid MRR with 1 layer of GO at different degrees of reduction. (i‒iii) Show the results measured for the same hybrid MRR with 1 layer of GO, rGO after heating at T_R = ~100 °C, and rGO after heating at ~200 °C, respectively. (b) PER's extracted from (a). In (a, b), the red and blue shaded areas indicate the power ranges associated with reversible GO reduction for TE and TM polarizations, respectively.

Fig. 7. (a) Extinction coefficients (k's) of GO versus P_p obtained by fitting the results in Fig. 6(a) with optical mode simulations. (i ‒ iii) show the results for GO, rGO after heating at T_R = ~100 °C, and rGO after heating at ~200 °C, respectively. (b) Anisotropy ratios of k values for TE and TM polarizations (k_TE / k_TM) extracted from (a).