High-gain optical amplification and lasing from erbium-doped single-crystal films epitaxially grown on silicon

Abstract

On-chip erbium-doped optical amplifiers and lasers are essential for realizing fully integrated active silicon photonic circuits, but their performance has been limited by the low gain of amorphous host materials and the difficulty of direct integration on silicon. Here, we demonstrate optical amplification and lasing from erbium-doped single-crystal gadolinium oxide (Er:Gd$_2$O$_3$) thin films epitaxially grown on silicon. Optical gain measurements on waveguides fabricated on this platform exhibit a giant material gain of $78.3\pm2.1$ dB/cm and an on-chip net gain exceeding 13 dB in a 6-mm-long waveguide at 2.3 K, while a measurable gain is maintained up to room temperature. Continuous-wave lasing with low threshold, narrow linewidth, and large side-mode suppression ratio is also demonstrated in Er:Gd$_2$O$_3$ microring resonators. These results establish Er:Gd$_2$O$_3$ as the first monolithic crystalline gain medium directly integrated on silicon, providing a scalable route toward high-performance cryogenic and quantum photonic integrated circuits.

Figures (4)

• Figure 1: Design and characterization of Er:Gd$_2$O$_3$ waveguides. (a) Mode profile of the fundamental TM mode of the SiN/Er:Gd$_2$O$_3$/SOI strip-loaded waveguide. The parameters of the waveguide are: Si layer thickness $t_{\mathrm{Si}} = 90$ nm, Er:Gd$_2$O$_3$ layer thickness $t_{\mathrm{Gd2O3}} = 130$ nm, SiN layer thickness $t_{\mathrm{SiN}} = 300$ nm, and waveguide width $w = 1120$ nm. (b) Cross-sectional SEM and (c) microscope images of a fabricated waveguide with input and output grating couplers. (d) Propagation-loss spectrum of the waveguide at room temperature. The light blue shaded region represents the error bars of the propagation loss for each wavelength. (e) Transmission spectrum of a 1.5-mm-long waveguide at $T = 2.3$ K, with the inset showing a zoomed view of the absorption dip around 1536 nm and the Voigt fit (solid line) to the experimental data (dots).
• Figure 2: Characterization of Er:Gd$_2$O$_3$ microring resonators. (a) Optical microscope image of a fabricated microring resonator with ring waveguide width $w_{\mathrm{ring}} = 1080$ nm, bus waveguide width $w_{\mathrm{bus}} = 1080$ nm, gap between the bus and ring waveguides $g = 300$ nm, and CDC arc length $\theta = 15^{\circ}$. (b) Zoomed-in microscope image of the CDC between bus and ring waveguides. (c) Transmission spectrum of the waveguide-coupled microring resonator at $T = 2.3$ K. (d) Zoomed-in view of a resonance near 1538.3 nm with a loaded $Q$-factor of $4.23\times10^4$ extracted through Lorentzian fitting.
• Figure 3: Measurement results of optical gain in Er:Gd$_2$O$_3$ waveguides. (a) Comparison of transmission spectra of a 1.5-mm-long waveguide with and without optical pumping at $T = 2.3$ K. The pump power was 41.6 mW. (b) Dependence of optical signal enhancement on pump power. The orange dashed line represents the absorption loss of the waveguide caused by Er ions ($\alpha_{\mathrm{wg,abs}} = 63.6\pm0.8$ dB/cm), and the purple dashed line represents the background passive loss ($\alpha_{\mathrm{wg,passive}} = 3.37\pm0.03$ dB/cm). (c) Dependence of total internal optical gain on waveguide length. (d) Temperature dependence of optical absorption, gain, and relative population inversion factor of Er:Gd$_2$O$_3$ thin films.
• Figure 4: Measurement results of lasing in Er:Gd$_2$O$_3$ microring resonators. (a) Normalized emission spectra acquired under different pump powers. The pump powers corresponding to each spectrum are indicated as red squares in (b). (b) Dependence of on-chip emission power and linewidth of the emission peak on on-chip pump power. (c) Laser emission spectrum at an on-chip pump power of 22.2 mW. The inset shows a zoomed-in view of the laser peak near 1536 nm. (d) Dependence of normalized emission power on off-chip pump power at different temperatures.