Broadband on-chip SiN lasers

Broadband active materials are pivotal for advancing emerging technologies spanning on-chip optical interconnects, artificial intelligence, quantum systems and precision metrology. Current semiconductor gain media face bandwidth limitations; and Ttitanium-doped sapphire (Ti:sapphire), the most widely used broadband light-emitting material, covering the red to short-wave near-infrared (SW-NIR) spectrum, lacking emission in the entire visible range. Here, a mechanism for generating ultra-broadband gain is revealed, which utilizes defect and band-tail states in the bandgap, and balances cavity enhanced reabsorption and radiation. By leveraging this mechanism, the gain of on-chip integrated silicon nitride (SiN) is greatly enhanced at longer wavelengths, thereby achieving broadband emission, from blue light to SW-NIR (approximately 450 nm to 1000 nm), and mode-hop-free tuning about 1.6 nm at about 738 nm and amplification at about 532.3 nm was achieved. By leveraging the maturity, cost-effectiveness, and CMOS compatibility of SiN photonics, this work transitions SiN from conventional passive photonic material to ultrawide-band active medium, establishing a disruptive foundation for next generation visible and SW-NIR integrated photonic platforms.

💡 Research Summary

This paper demonstrates a breakthrough approach to achieving ultra‑broadband gain on silicon nitride (SiN) platforms, enabling on‑chip lasers that emit continuously from the blue (≈450 nm) to the short‑wave near‑infrared (≈1000 nm). The authors identify defect states and band‑tail states within SiN’s wide bandgap (~4.6 eV) as the source of gain. By carefully tuning PECVD/LPCVD growth parameters—silicon‑to‑nitrogen ratio, temperature, and NH₃ flow—they control the density and energy distribution of these states. Time‑resolved photoluminescence reveals two decay components (≈1–2.5 ns and ≈8–15 ns), indicating coexistence of radiative and non‑radiative recombination pathways. Samples with enhanced non‑radiative recombination exhibit strong amplification of a 532 nm probe when pumped at 405 nm, achieving up to 3.6× gain at 2 mW pump power.

The work proceeds to fabricate SiN nanostructures using electron‑beam lithography, electron‑beam lithography (EBL) patterning, and inductively coupled plasma (ICP) etching. Waveguides of 300 nm height with widths of 1 µm and 2 µm are studied; the wider waveguide improves confinement for both 450 nm and 1000 nm TE₀ modes and also supports higher‑order TE₁ modes, maintaining quality factors (Q) between 600 and 1300 across the spectrum.

Ring resonators of 12 µm and 16 µm diameter are integrated with waveguides of varying widths (300 nm–1 µm). The 12 µm ring (1 µm waveguide) provides continuous emission from 600 nm to 750 nm, while the 16 µm ring (300 nm waveguide) extends emission down to 450 nm and up to 650 nm. Mode‑hop‑free tuning of 1.6 nm is demonstrated at 738 nm, and amplification at 532.3 nm is achieved.

Compared with conventional Ti:sapphire (gain 650–1100 nm) and semiconductor quantum‑well gain media, SiN offers a broader gain bandwidth (>360 THz, ≈450 nm) and a higher refractive index (2.0–2.2 vs. 1.76), which enhances waveguide confinement. Importantly, SiN is fully CMOS‑compatible, allowing low‑cost, large‑scale integration without the harsh processing required for sapphire.

In summary, the authors establish a defect‑mediated ultra‑broadband gain mechanism in SiN, optimize growth and device geometry to preserve high Q while spanning the visible to SW‑NIR range, and demonstrate functional on‑chip lasers with mode‑hop‑free tuning and amplification. This work converts SiN from a passive waveguide material into an active laser medium, providing a disruptive foundation for next‑generation integrated photonic systems in communications, AI, quantum technologies, and precision metrology.