Broadband Linear-to-Circular Polarization Conversion Enabled by Birefringent Off-Resonance Reflective Metasurfaces

Due to the scarcity of circular polarization light sources, linear-to-circular polarization conversion is required to generate circularly polarized light for a variety of applications. Despite significant past efforts, broadband linear-to-circular polarization conversion remains elusive particularly in the terahertz and midinfrared frequency ranges. Here we propose a novel mechanism based on coupled mode theory, and experimentally demonstrate at terahertz frequencies that highly efficient (power conversion efficiency approaching unity) and ultrabroadband (fractional bandwidth up to 80%) linear-to-circular polarization conversion can be accomplished by the judicious design of birefringent metasurfaces. The underlying mechanism operates in the frequency range between well separated resonances, and relies upon the phase response of these resonances away from the resonant frequencies, as well as the balance of the resonant and nonresonant channels. This mechanism is applicable for any operating frequencies from microwave to visible. The present Letter potentially opens a wide range of opportunities in wireless communications, spectroscopy, and emergent quantum materials research where circularly polarized light is desired.

Figure 1

(a) Amplitude reflectivity and (b) phase spectra along with the (wrapped) phase difference $\mathrm{\Delta }\varphi \equiv {\varphi }_{\parallel }-{\varphi }_{\perp }$ of a hypothetical reflective surface with birefringent properties following Eqs. (1) and (2) and using a set of parameters (based on the unit of THz for frequency): ${\tilde{r}}_{\mathrm{nr}}=0.98e^{i(0.95\pi )}$, ${\tilde{d}}_1=0.36+i0.025$, ${\tilde{d}}_2=0.36-i0.045$, ${\nu }_1=0.5$, ${\nu }_2=1.5$, ${\gamma }_1=0.1$, ${\gamma }_2=0.08$, $b_{\parallel }=0.8$, $b_{\perp }=2.2$, and ${\varphi }_0=\pi$.

Figure 2

(a) Schematic of the unit cell of a metasurface structure to realize broadband linear-to-circular polarization conversion. Inset: displacing the metallic structures from the dielectric pillar to reveal the structural details. (b) SEM micrograph of a fabricated metasurface sample, and (c) its false color close-up view. The normally incident THz radiation $E$ is linearly polarized at 45° with respect to the major principal axis. (d) Numerically simulated amplitude reflectivity and (e) phase spectra for the parallel ($r_{\parallel }$ and ${\varphi }_{\parallel }$) and perpendicular ($r_{\perp }$ and ${\varphi }_{\perp }$) field components, along with their phase difference $\mathrm{\Delta }\varphi$. (f) Experimentally measured amplitude reflectivity and (g) phase spectra and their difference.

Figure 3

(a) The retrieved circular polarization states from experimentally measured reflection data when the normally incident THz waves are linearly polarized at 45° with respect to the metasurface major principal axis. (b) Axial ratio of the reflected THz waves calculated from experimental measurements.

Figure 4

(a) Schematic of the unit cell of a metasurface structure for broadband linear-to-circular polarization conversion at 45° incidence angle. Inset: displacing the metallic structures to reveal the unit cell details. (b) Experimentally measured amplitude reflectivity and (c) phase spectra for the parallel ($r_{\parallel }$ and ${\varphi }_{\parallel }$) and perpendicular ($r_{\perp }$ and ${\varphi }_{\perp }$) field components, along with their phase difference $\mathrm{\Delta }\varphi$. (d) The retrieved circular polarization states when the incident THz waves are linearly polarized at 45° with respect to the plane of incidence. (e) Axial ratio of the reflected THz waves.

Figure 5

(a) Numerically simulated amplitude reflectivity and (b) phase spectra along with the phase difference $\mathrm{\Delta }\varphi$ of a reflective metasurface under 45° incidence angle and operating at near-infrared and visible wavelengths. (c) The retrieved circular polarization states when the incident light is linearly polarized at 45° with respect to the plane of incidence. (d) Axial ratio of the reflected light showing a 3 dB fractional bandwidth of 75%.