Extending the spectrum of fully integrated photonics to submicrometre wavelengths

The study addresses the spectral limitations of silicon-on-insulator (SOI) photonic platforms, which become highly absorptive below the ~1.1 µm silicon bandgap, restricting access to ultraviolet, visible, and parts of the near-infrared for fully integrated photonics. Integrated photonics has evolved from pure III–V platforms to silicon photonics, leveraging CMOS foundries for scale, cost, and low-loss waveguides. However, many emerging applications (atomic physics, AR/VR, biosensing, quantum communications) require operation at submicrometre wavelengths that SOI cannot support. Silicon nitride (SiN) waveguides offer extremely low loss across a broad spectral range, including down to below 460 nm, and can be deposited directly on silicon substrates, potentially lowering cost. The research question is whether heterogeneous integration of III–V active materials directly with SiN passives can create a fully integrated platform operating at submicrometre wavelengths, enabling coherent sources and essential active/passive functions with high performance, thereby extending integrated photonics beyond the silicon bandgap.

The paper situates the work within two decades of progress in integrated photonics: early III–V-on-native-substrate integration enabled the first commercial PICs; later, heterogeneous III–V-on-SOI leveraged CMOS-scale manufacturing to reduce cost and loss, spurring applications in data centers, neural networks, LIDAR, and quantum photonics. The SOI platform is limited spectrally by silicon’s bandgap (~1.1 µm), preventing UV/visible/near-IR access. SiN is highlighted as a passive platform with ultra-low propagation loss (<0.1 dB/m at telecom) and scattering-limited loss down to <460 nm, enabling ultra-high-Q cavities, narrow-linewidth lasers, microcombs, and on-chip frequency conversion. Prior III–V/SiN integrations required an intermediary Si layer for passive-active transitions, restricting short-wavelength operation. The work builds on heterogeneous integration methods and prior demonstrations of narrow-linewidth lasers and microcomb devices, aiming to remove the silicon intermediary to unlock submicrometre operation.

Platform and fabrication: Heterogeneous III–V/SiN devices are realized by bonding III–V epitaxial stacks directly onto patterned SiN waveguides on silicon substrates. The simplified wafer-scale process flow includes: (1) SiN deposition on thermally oxidized silicon; (2) SiN waveguide patterning; (3) bonding of multiple III–V epitaxial structures; (4) removal of III–V native substrates; (5) III–V device processing (dry/wet etches to form p–n junctions for active devices); (6) dielectric cladding deposition, via openings, and metallization to complete contacts. The process yields wafers with hundreds to thousands of devices. Coupling strategy: To overcome the large refractive index mismatch (nIII–V >3 vs. nSiN ~2) which localizes optical modes in the III–V layer and makes standard adiabatic evanescent coupling inefficient, a hybrid coupler is used. An intermediary waveguide is patterned within the dielectric cladding between III–V and SiN. At the III–V side, geometry is optimized for non-adiabatic butt-coupling; at the SiN side, it supports adiabatic evanescent transfer into SiN. First-generation coupling efficiency up to 70% is demonstrated, with simulations indicating up to 90% achievable with optimized design. Device set and characterization: The platform integrates active and passive building blocks at ~900–1,060 nm: Fabry–Perot (FP) lasers with integrated loop mirrors; semiconductor optical amplifiers (SOAs); Mach–Zehnder interferometer (MZI) modulators using GaAs phase shifters; III–V photodiodes (PDs); and low-loss SiN waveguides/resonators. Key device characterizations include LI curves, output power and threshold currents, SOA gain spectra and 3 dB bandwidths, PD responsivity and dark current, modulator Vπ and extinction ratio, and SiN waveguide propagation losses via cutback. Integrated tunable laser: A GaAs-based gain section is combined with a SiN external cavity featuring a dual-ring Vernier back mirror (two rings of different FSRs with thermal microheaters for alignment and wavelength selection), a 100% loop mirror, and a 50% front loop mirror. A phase-tuning section enables continuous tuning. The laser footprint is <3 × 0.3 mm² (active cavity ~1 mm total, compact version <1 mm²). Frequency noise is measured via delayed self-heterodyne with cross-correlation; RIN is measured over GHz offsets. Thermal performance is assessed using a temperature-controlled stage (25–185 °C for FP lasers; 35–145 °C for tunable lasers), extracting characteristic temperature T0 from threshold vs. temperature, lasing wavelength shift with temperature, and linewidth vs. temperature. High-temperature analysis: Thermal degradation mechanisms (carrier leakage, Auger recombination, intervalence band absorption) and their dependence on bandgap are discussed to motivate the GaAs short-wavelength advantage. Experimental LI curves, spectra, and noise metrics are collected across elevated temperatures to validate resilience without active cooling.

• Full heterogeneous III–V/SiN platform at submicrometre wavelengths integrating lasers, SOAs, modulators, PDs, and low-loss passives on Si wafers.
• Passive–active coupling: Novel intermediary-waveguide coupler achieves up to 70% coupling efficiency in first-generation devices; designs indicate up to 90% is achievable.
• Passive waveguides: SiN propagation loss <0.5 dB/cm near 980 nm (Q > 1.5 × 10^6); ultra-low-loss SiN (from literature) offers two orders of magnitude lower loss potential.
• FP lasers (≈980 nm): Threshold current <12 mA (for 800 µm cavity); output power >25 mW into SiN waveguide; slope efficiency >0.38 W/A; operation maintained from 25 °C to 185 °C.
• SOAs: On-chip gain >22 dB at 980 nm with 100 mA bias; 3 dB gain bandwidth >20 nm.
• Photodiodes: Responsivity >0.6 A/W at 980 nm (≈80% QE); dark current at nA level.
• Modulators: 2 mm GaAs phase shifter with Vπ ≈ 2.4 V; MZI extinction ratio >20–22 dB (measured at 1,060 nm).
• Integrated tunable laser (GaAs gain + SiN dual-ring Vernier cavity): • LI at fixed wavelength shows threshold ≈30.3 mA; output >10 mW near 976.5 nm; >6 mW across entire tuning range at 75 mA. • Frequency noise white floor ≈450 Hz^2/Hz, implying Lorentzian (fundamental) linewidth ≈2.8 kHz; 10 kHz-level linewidth across full tuning range at 25 °C. • RIN < −155 dB/Hz outside relaxation oscillation (~2 GHz offset). • Wide wavelength tuning ≈20 nm (~6 THz) with SMSR >35 dB across the range and approaching 50 dB near gain peak; repeatable step tuning demonstrated. • Mode-hop-free continuous tuning >8 GHz by sweeping the phase section; larger ranges possible with coordinated ring and phase tuning.
• High-temperature performance: • Continuous-wave lasing up to 185 °C (record high for a heterogeneously integrated laser on silicon). Threshold current vs. temperature yields characteristic temperatures T0 ≈148 K (20–90 °C) and ≈110 K (90–150 °C); lasing maintained beyond 150 °C. • Lasing wavelength red-shifts ≈0.33 nm/K; maximum lasing wavelength ≈1,044.5 nm at 185 °C (>50 nm red shift from room temperature). • Tunable laser fundamental linewidth remains <10 kHz at 145 °C (best measured <7 kHz), showing minimal degradation with temperature.
• Application relevance: Performance (linewidth, tunability, power) comparable to bulk external-cavity diode lasers but in a fully integrated form factor, enabling access to narrow atomic transitions and robust operation at elevated temperatures.

The work demonstrates that directly integrating III–V gain with SiN passives on silicon extends photonic integration below the silicon bandgap, addressing the core limitation of SOI-based platforms. By employing a tailored intermediary-waveguide coupler, efficient passive–active transitions are achieved despite large index mismatch, enabling a comprehensive device set (lasers, SOAs, modulators, PDs) around 980 nm. The low-loss SiN external cavity significantly suppresses frequency noise, yielding few-kHz fundamental linewidths and high SMSR across a wide tuning range, satisfying requirements for atomic physics and precision metrology. The platform exhibits strong high-temperature resilience, attributable to larger bandgaps and better carrier confinement in GaAs-based materials; this enables continuous-wave lasing up to 185 °C and kHz-level linewidths at 145 °C, reducing or eliminating active cooling needs. These findings confirm the feasibility and advantages of III–V/SiN heterogeneous integration for short-wavelength coherent photonics, offering compact, scalable solutions for atomic clocks, sensing, quantum systems, and energy-efficient data center components.

This paper introduces a fully integrated heterogeneous III–V/SiN photonics platform that operates at submicrometre wavelengths, overcoming the spectral limitations of SOI. It demonstrates efficient III–V–to–SiN coupling, low-loss passives, and high-performance active devices, including integrated narrow-linewidth, widely tunable lasers with excellent high-temperature operation. The approach maintains compact footprints while achieving performance comparable to bulk external-cavity systems. Future work can extend the platform to green, blue, violet, and UV wavelengths using GaAs- and GaN-based materials; leverage ultra-low-loss SiN for ultra-high-Q devices; and incorporate other passive materials (LiNbO3, AlN, SiC, AlGaAs, chalcogenide glass) to cover wavelengths up to and beyond 10 µm. Anticipated large-scale foundry adoption promises reduced costs and broad impact across atomic physics, quantum communication, AR/VR, metrology, and energy-efficient photonic computation.

• Current coupling efficiency demonstrated up to 70% in first-generation devices; while designs indicate 90% is achievable, further process and design optimization are needed to consistently reach this target.
• The laser’s coarse tuning range (~20 nm) is primarily limited by the gain bandwidth of the employed ~980 nm quantum wells; broader ranges would require different gain media or cavity designs.
• Ultra-low-loss thin SiN (<100 nm) increases effective index mismatch, potentially complicating coupling; though the proposed coupler strategy remains applicable, practical implementations may require additional optimization.
• Results are centered around ~980 nm operation; while the integration strategy is general, performance at other visible/UV wavelengths will depend on specific material systems and device designs not experimentally detailed here.