Fig. 2. Metasurface-controlled laser diode feedback.

(A) To characterize the two lasing configurations, the optical spectrum is sent directly to an optical spectrum analyzer (OSA), and the output power is monitored with a power meter. The EO metasurface is then placed in the external cavity and aligned such that the electric field inside the cavity is oriented perpendicular to the gold stripes of the metasurface. (B) The reflectance spectrum of the metasurface is plotted. The maximum reflectance, $R_{\text{max}}=0.28$, occurs at the resonance wavelength, with a linewidth of ${\mathrm{\delta \lambda }}_{\text{res}}=7$ nm. (C) The solitary diode shows a lasing threshold of $I_{\text{th},\text{sd}}=22$ mA (gray curve). Once optical feedback is established, the lasing threshold is reduced to $I_{\text{th}}=13.4$ mA. (D) Spectral response with and without feedback. Top panel shows the emission spectrum of the solitary laser diode, denoted $S_{\text{sd}}$. Below threshold (light gray), at $I_{\text{in}}=5$ mA, and above threshold, at $I_{\text{in}}=25$ mA, the laser exhibits spontaneous and stimulated emission, respectively. The gain spectrum is ∼80 nm wide, with a noticeable blue shift of $\sim 32$ nm as the current increases. Bottom panel shows the emission spectra in the presence of feedback, denoted $S_{\text{out}}$, shown both below threshold at $I_{\text{in}}=5$ mA (light blue) and above threshold at $I_{\text{in}}=14$ mA (dark blue). Below threshold, the resonance of the metasurface appears as a dip in transmission. Above threshold, lasing occurs at the resonance, and off-resonant modes are suppressed. The internal and external cavity mode spacings are marked as ${\text{FSR}}_{\mathrm{i}}$ and ${\text{FSR}}_{\mathrm{e}}$, respectively. The differences in intensity between the main lasing peak and the adjacent internal and external sidebands are quantified as ${\text{SMSR}}_{\mathrm{i}}$ and ${\text{SMSR}}_{\mathrm{e}}$, respectively. The zoomed-in view of the dominant lasing peak reveals the external cavity mode structure.