Fig. 3. Ultrafast frequency modulation of a laser diode with an EO metasurface.

(A) An RF source applies a triangular voltage waveform with peak amplitude $V_{\mathrm{m}}$ and frequency ${\mathrm{\nu }}_{\text{mod}}$ to the metasurface, dynamically tuning its reflection. This modulation alters the lasing frequency of the diode laser. The output beam is combined with a frequency-stable reference laser in a heterodyne detection setup, enabling extraction of the instantaneous frequency ${\mathrm{\nu }}_{\text{inst}}\left(t\right)$ through RF beatnote analysis. The modulation induces a frequency excursion $\mathrm{\Delta }\mathrm{\nu }$ centered around a carrier frequency ${\mathrm{\nu }}_0$. (B) Spectrograms of the heterodyne beatnote signal under triangular RF modulation at three different frequencies: 50 kHz, 250 kHz, and 1 MHz. All measurements were performed at a fixed RF amplitude of $V_{\mathrm{m}}=10$ V and injection current $I_{\text{in}}=16$ mA. The color scale indicates optical power, calibrated to the output power from the laser’s light-current (LI) curve. (C) Heterodyne spectra measured at a fixed modulation frequency ${\mathrm{\nu }}_{\text{mod}}=1$ MHz for injection currents $I_{\text{in}}=16$, 20, and 24 mA. The traces illustrate how the frequency excursion and heterodyne signal amplitude evolve with injection current. As the current increases, the frequency excursion gradually narrows. The amplitude of the beatnote is calibrated using the laser’s LI curve. (D) The left vertical axis (purple) shows the peak-to-peak frequency excursion $\mathrm{\Delta }{\mathrm{\nu }}_{\text{pp}}$, with the shaded region representing the standard deviation across modulation cycles. The right axis (black) shows the RMS deviation from an ideal triangular waveform. As ${\mathrm{\nu }}_{\text{mod}}$ increases, the frequency excursion decreases while the RMS deviation improves, illustrating the trade-off between modulation speed and dynamic range. (E) Frequency excursion as a function of injection current $I_{\text{in}}$ at fixed modulation frequency ${\mathrm{\nu }}_{\text{mod}}=1$ MHz. The shaded area indicates the standard deviation of the measured excursion.