Fig. 4. Experimental setup for amplitude modulation.

(A) An RF voltage with amplitude $V_{\mathrm{m}}$ at ${\mathrm{\nu }}_{\text{mod}}=50$ MHz is applied to the metasurface using an arbitrary waveform generator. The lock-in amplifier is referenced to the same frequency ${\mathrm{\nu }}_{\text{mod}}$. After coupling the output light into a fiber using coupler FC1 and directing it to photodiode PD1, the detected signal appears as $V_0+\mathrm{\Delta }V$, consisting of a dc component $V_0$ and an RF component $\mathrm{\Delta }V$ oscillating at ${\mathrm{\nu }}_{\text{mod}}$. The lock-in amplifier measures the modulated signal $\mathrm{\Delta }V$, while the dc component $V_0$ is recorded from the dc output of PD1 using a voltmeter. (B) The plots show the variation of the detected voltage $\mathrm{\Delta }V$ as a function of injection current $I_{\text{in}}$ and RF amplitude $V_{\mathrm{m}}$, measured at a modulation frequency of ${\mathrm{\nu }}_{\text{mod}}=50$ MHz. The bipolar colorbar indicates the magnitude and sign of $\mathrm{\Delta }V$. (C) Measured modulation efficiency $\mathrm{\Delta }I/I_0$ as a function of injection current $I_{\text{in}}$ for various RF amplitudes $V_{\mathrm{m}}$ at ${\mathrm{\nu }}_{\text{mod}}=50$ MHz. The efficiency peaks near the laser threshold, exceeding 100% for $V_{\mathrm{m}}>4 \mathrm{V}$. Inset: Zoomed-in view showing the absolute voltage variation $\mathrm{\Delta }V$ (orange), the dc component $V_0$ (gray), and the corresponding modulation efficiency $\mathrm{\Delta }I/I_0$ (blue, right axis) at $V_{\mathrm{m}}=5$ V. Mode-hopping events are also visible as discrete jumps in the traces. (D) The maximum modulation efficiency is plotted for two regimes: injection currents near threshold $I_1=5$ mA (gold) and well above threshold $I_3=20$ mA (gray). (E to G) Oscilloscope time-trace measurements showing $V_0+\mathrm{\Delta }V$ at fixed voltage ($V_{\mathrm{m}}=5$ V) and varying currents. Traces are shown near threshold at $I_{\text{in}}=13.8$ mA, slightly above threshold at $I_{\text{in}}=14.2$ mA, and well above threshold at $I_{\text{in}}=20$ mA.