Fig. 1. Working principle of the metasurface-assisted tunable external cavity.

(A) Conventional Littrow configuration with EO modulation: The injection current $I_{\text{in}}$ supplies carriers to the gain medium of the laser diode. The back facet has reflection $r_1$, while the front facet is antireflection coated with $r_2\ll r_1$. For EO wavelength control, an EO crystal is placed in the external cavity and modulates the optical path length via the Pockels effect. An applied electric field induces a refractive index change in the crystal, which results in a modulation of the optical field $E_i$ inside the cavity as $e^{2i\mathrm{\phi }}{E}_{\mathrm{i}}$, where $\mathrm{\phi }$ is the EO phase shift induced by the modulator. Afterward, the beam is diffracted by the grating. The first-order diffraction provides coherent feedback to the cavity via $r_{\text{ext}}$, while the zeroth-order serves as the laser output. (B) Metasurface-controlled external cavity laser: The metasurface condenses the reflection grating and the EO control. The reflection from the metasurface is frequency-selective, exhibiting a resonance around ${\mathrm{\nu }}_{\text{res}}$, with a maximum value $r_{\text{max}}\gg r_1$. An electric field applied to the metasurface modifies its refractive index and hence resonance, yielding control over the lasing frequency. (C) The simulation illustrates the resonant behavior of the EO metasurface (top graph shows the reflection amplitude, and bottom graph the corresponding phase response). Applying a voltage to the metasurface modifies both the amplitude and phase of the light circulating in the cavity, as illustrated by semitransparent curves. (D) For the solitary diode, the lasing threshold is determined solely by its length and gain bandwidth. When the metasurface provides frequency-selective feedback, the effective gain threshold decreases near the resonance. (E) A reduced gain threshold results in a lower required injection current for lasing, from $I_{\text{th},\mathrm{i}}$ to $I_{\text{th}}$. This simultaneously decreases the output power $P_{\text{out}}$ through decreased slope efficiency.