Highly Efficient Broadband Wave Plates Using Dispersion-Engineered High-Index-Contrast Subwavelength Gratings

A wave plate consisting of dielectric subwavelength gratings (SWGs) with a certain thickness is typically used for polarization control. Since the phase retardation strongly depends on the wavelength, the wave plates made from dielectric SWGs usually operate in narrow or limited bandwidths, which severely hinders their device applications. In this paper, we demonstrate theoretically and experimentally that broadband quarter- and half-wave plates may be achieved in a single layer of high-index-contrast SWG by tailoring the dispersion relations of two orthogonal waveguide-array modes supported by the structure. The broadband SWG-based wave plates demonstrated in this work could be an important step forward in the development of functional polarization conversion devices for practical applications.

Figure 1

The schematic configuration of the high-index-contrast SWG structure.

Figure 2

Dispersion curves of the waveguide-array modes with different structural parameters. The right column represents the magnified view of the parallel dispersion band in the left column. The parallel dispersion branches for the TE and TM polarizations are pivotal for achieving the broadband wave plates in high-index-contrast SWGs. The refractive indices of ridges and grooves are $n_g=3.48$ ($\mathrm{Si}$) and $n_a=1$ (air), respectively.

Figure 3

Amplitude and phase of the transmission coefficients of high-index-contrast SWGs with different grating thicknesses for the normal incident TE-polarized (blue solid lines) and TM-polarized waves (red dash lines). The transmitted fields for TE- and TM-polarization have uniform phase retardation of $\pi /2$ and $\pi$ over a broad bandwidth when the grating thicknesses satisfy $t_g=0.35$ and $t_g=0.7\mu \mathrm{m}$, respectively. The uniform phase retardation is adequate for producing broad wave plates. The remaining parameters are $w=0.19\mu \mathrm{m}$, $\mathrm{\Lambda }=0.3\mu \mathrm{m}$, $n_g=3.48$ ($\mathrm{Si}$), $n_a=1$ (air), and $n_s=1.445$ (${\mathrm{Si}\mathrm{O}}_2$).

Figure 4

Transmittance contour of high-index-contrast SWGs as a function of wavelength and grating thickness for TE polarization (a) and TM polarization (b) under normal incidence. The white dashed lines on the top of transmittance contour represent the folded dispersion curves of waveguide-array modes by applying the FP resonance conditions.

Figure 5

Calculated transmittance for broadband quarter-wave plate (a) and half-wave plate (b). The inserts show the schematic configuration of SWG-based wave plates, in which the normal incident lights have a polarization angle of $\theta =\pi /4$ with respect to the direction of the grating vector (i.e., x axis). The structural parameters in Figs. 5 and 5 are the same as in Figs. 3 and 3, respectively.

Figure 6

(a, b) SEM image of the fabricated high-index-contrast SWGs with different thickness. (c) Schematic of the phase retardation measurement setup.

Figure 7

Experimental and theoretical transmittance spectra of the quarter- (a, b) and half-wave plates (c, d). The grating thicknesses are $t_g=0.4\mu \mathrm{m}$ (a, b), and $t_g=0.77\mu \mathrm{m}$ (c, d). The remaining parameters are $w=0.21\mu \mathrm{m}$ and $\mathrm{\Lambda }=0.35\mu \mathrm{m}$.