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Extending the spectrum of fully integrated photonics to submicrometre wavelengths

Integrated photonics has profoundly affected a wide range of technologies underpinning modern society1–4. The ability to fabricate a complete optical system on a chip offers unrivalled scalability, weight, cost and power efficiency5,6. Over the last decade, the progression from pure III–V materials platforms to silicon photonics has significantly broadened the scope of integrated photonics, by combining integrated lasers with the high-volume, advanced fabrication capabilities of the commercial electronics industry7,8. Yet, despite remarkable manufacturing advantages, reliance on silicon-based waveguides currently limits the spectral window available to photonic integrated circuits (PICs). Here, we present a new generation of integrated photonics by directly uniting III–V materials with silicon nitride waveguides on Si wafers. Using this technology, we present a fully integrated PIC at photon energies greater than the bandgap of silicon, demonstrating essential photonic building blocks, including lasers, amplifiers, photodetectors, modulators and passives, all operating at submicrometre wavelengths. Using this platform, we achieve unprecedented coherence and tunability in an integrated laser at short wavelength. Furthermore, by making use of this higher photon energy, we demonstrate superb high-temperature performance and kHz-level fundamental linewidths at elevated temperatures. Given the many potential applications at short wavelengths, the success of this integration strategy unlocks a broad range of new integrated photonics applications.

Integrated photonics has made rapid progress in the last two decades, and the most crucial steps in its advance have been the emergence of novel integration platforms (Fig. 1a). The earliest photonic integration was based purely on III-V materials on native substrates 9 , in which active and passive photonic components were combined on a chip to form optical systems. This approach led to the first generation of commercially viable photonic technologies. Since then, integrated photonics has benefited from the expansion of the electronics industry, resulting in high-volume adoption of silicon photonics (SiPh). Whereas III-V manufacturing has not grown apace with silicon, it is possible to manufacture photonic integrated circuits (PICs) on large-scale silicon-on-insulator (SOI) wafers by heterogeneously bonding III-V epitaxy in a variety of different ways 10 . Leveraging mature complementary metal-oxide-semiconductor foundry infrastructures, the SOI integrated photonics platform significantly reduces the cost of photonic chips at scale.
Another key factor driving the evolution of integrated photonics is low propagation loss. As SOI waveguides exhibit propagation losses an order of magnitude lower than III-V waveguides 6 , SiPh PICs can accommodate more individual components and thus support more complex photonic systems. Moreover, lower loss boosts the performance of passive structures and coherent light sources. These advantages have driven explosive growth in SiPh, opening up a plethora of new applications, from data centres 6 , to neural networks 11 , to Lidar 12 and to quantum photonics 13 .
With this broadening of the application scope, however, the limitations of the SOI platform are beginning to surface. One comes from on a Si substrate, they do not require any expensive smart-cut process, suggesting an opportunity to further reduce the cost of foundry-manufactured PICs.
However, until recently, the integration of active components onto SiN PICs has been impeded by the large index mismatch between SiN (approximately 2) and III-V materials (>3). SiN and III-V structures have been integrated on the same substrate to form highly coherent lasers and microcombs at telecommunication wavelengths, but only with an intermediary Si layer for passive-active transitions, which still prohibits short-wavelength operation 25,26 .
This work presents a new generation of integrated photonics with active and passive elements united in a heterogeneously integrated III-V/SiN platform. This integration scheme offers a fully integrated submicrometre photonics platform with versatile building blocks, including lasers, semiconductor optical amplifiers (SOAs), modulators, photodetectors and various passive elements. The combination of a III-V gain section with SiN external cavities yields a heterogeneously integrated, narrow linewidth, widely tunable laser operating beyond the bandgap energy of Si, a device with tremendous implications for atomic physics, sensing and precise metrology. Moreover, the short-wavelength platform exhibits superior high-temperature performance among coherent light sources, which can be used to improve power efficiency in data centres and other hot environments. These results herald the mass production of PICs covering a much broader spectrum and open doors to many new applications.

Heterogeneously integrated III-V on SiN photonics platform
Heterogeneous III-V/SiN photonic devices consist of III-V-based epitaxial layer structures bonded on top of SiN waveguides. A simplified fabrication process flow for the III-V/SiN heterogeneous photonic devices is illustrated in Fig. 2a, with a detailed description provided in the Methods. Figure 2b shows a photograph of a completed wafer with hundreds of lasers fabricated on a 4 inch silicon substrate. Scanning electron microscope images ( Fig. 2c (I-IV)) show a single SiN waveguide, a coupler, a III-V waveguide with III-V/SiN coupler on one side and an array of lasers connecting with an array of photodiodes via SiN waveguides, respectively.
An essential feature of the platform is efficient light coupling between III-V and SiN waveguides. The large refractive index of III-V material compared to SiN leads to a highly localized optical mode in the III-V layer for a III-V/SiN heterogeneous waveguide. This is a fundamental distinction from a typical III-V/Si heterogeneous waveguide, in which the similar refractive indices of Si and III-V make it possible for the optical mode to hybridize in both materials 27 . As a result, the usual adiabatic coupling scheme based on evanescent fields, although well suited to III-V/Si photonics, does not serve well for III-V/SiN. Butt coupling, a non-adiabatic method widely used in conventional optics, is advantageous in this case. However, efficient butt coupling requires maximal spatial overlap between the waveguides being coupled, which is unobtainable in a wafer-scale heterogeneous integration platform because the bonded layers cannot be vertically aligned. The following III-V/SiN coupler structure addresses this challenge by combining both aforementioned coupling schemes: an intermediary waveguide is patterned in the dielectric cladding between the III-V and SiN waveguides; at the III-V end, the geometry of the intermediary waveguide is optimized for butt coupling; and on the SiN end, it is optimized for adiabatic evanescent coupling to the SiN waveguide. A coupling efficiency of up to 70% was demonstrated in the first generation, and 90% efficiency is achievable with optimal design 28 Figure 2d, showing a proposed integrated PIC for an integrated atomic clock system, illustrates the potential of a fully integrated, short-wavelength PIC ecosystem with direct III-V/SiN coupling. The essential components have been implemented and characterized around 980 nm, as shown in Fig. 2e. Fabry-Perot (FP) lasers, formed with near-100% loop mirrors on the back side and 10% mirrors on the front side, provide a light source. An 800 μm long FP laser exhibits a low threshold current of 12 mA, whereas the output power and slope efficiency exceed 25 mW and 0.38 W A −1 , respectively. Integrated SOAs are fabricated with more than 22 dB optical gain and 20 nm 3 dB bandwidth. For detection, III-V photodiodes (PDs) exhibit nA-level dark current and more than 0.6 A W −1 responsivity and 80% quantum efficiency at 980 nm. We also demonstrate a 2 mm long phase shifter using the same GaAs epitaxial material with a V π of only 2.4 V and Mach-Zehnder modulators with more than 22 dB extinction ratio, measured at a wavelength of 1,060 nm. Complementing the III-V active elements are SiN passive waveguides, with loss reaching below 0.5 dB cm −1 measured near 980 nm, which corresponds to a quality factor (Q) above 1.5 × 10 6 .

Article
It is also worth noting that recently developed ultra-low-loss SiN waveguides 24,29 can further reduce waveguide loss by two orders of magnitude. This thin SiN platform will have a greater effective index mismatch between passive and active waveguides, but efficient coupling can still be achieved with the same coupling strategy.

Integrated coherent laser beyond silicon bandgap
One key application of heterogeneous photonics is coherent lasing. At the telecommunication band, for example, low-loss silicon waveguides have been paired with InP-based optical gain material to produce integrated narrow linewidth lasers 30 . By uniting high-quality SiN passives with short-wavelength III-V gain, our platform offers a similar capability beyond the silicon bandgap limit.
An integrated laser operating at 980 nm, which consists of a GaAs gain region and a SiN external cavity, is presented as a proof of concept. Figure 3a,b shows the principle of Vernier rings and the schematic design of the laser, whose details are provided in the Methods. The output power from the laser is greater than 10 mW near the gain peak, as shown in the LI (light-current) curve in Fig. 3c, where the power is measured while wavelength is maintained around 976.5 nm. For a fixed gain current of 75 mA, the power output is measured to be higher than 6 mW across the whole wavelength range. A compact laser, with a footprint of less than 1 mm 2 , as shown in Fig. 3b, is valuable for a wide range of applications at short wavelengths 31 . One important example is in atomic physics. The III-V/SiN heterogeneous laser described here offers a performance comparable to a bulky external cavity-diode laser 32,33 , but with the form factor of a fully integrated device. Figure 3e shows the two-sided power spectral density of the laser noise at 980 nm wavelength, measured with a delayed self-heterodyne setup and cross-correlation technique (Methods). The spectrum is dominated by 1/f noise at low-offset frequency (f ) range, as commonly observed in semiconductor lasers. Between 100 kHz to 30

Fig. 3 | Integrated coherent, widely tunable lasers on silicon nitride. a,
The wavelength response of individual ring resonators and the resulting measured Vernier spectrum with two rings of different free spectral range. b, Schematic of a dual-ring tunable laser with a back mirror formed by two ring resonators in a 100% loop mirror, a 50% reflectivity front loop mirror and a GaAs-based SOA section in between. Thermal microheaters are fabricated on the rings and a part of the laser cavity to align the rings, select the wavelength and tune the round-trip phase accumulation. The photograph shows a tunable laser chip with a form factor of less than 3 × 0.3 mm 2 . c, LI characteristic of the laser at a fixed wavelength, showing 30.3 mA threshold current and more than 10 mW output power. Inset: single-mode lasing spectrum. d, Improved linewidth with low-loss SiN external cavity. e, Frequency noise spectrum, simulated thermorefractive noise and a white noise floor of 450 Hz 2 /Hz, corresponding to a 2.8 kHz Lorentzian linewidth (2π times the white noise floor). Inset: Lorentzian linewidth at 25 °C across the laser tuning range. f, Relative intensity noise (RIN), less than −155 dB Hz −1 outside the relaxation oscillation resonance. g, Wide tuning range enables access to many atomic resonances. h, Vernier wavelength tuning of more than 20 nm wavelength with high SMSRs across the whole range. i, A 'UCSB' logo created by stepping the wavelength of the laser over time. The colour of each dot indicates the measured SMSR at that time step. j, Mechanism of locking a resonance with a single continuous tuning parameter, crucial for locking to atomic transitions. k, Mode-hop-free, continuous tuning of the III-V/SiN laser frequency obtained over more than 8 GHz by sweeping the phase-tuning section alone. l, A 'Nexus' logo created by tuning the laser frequency without mode hop, showing great stability and precise control over time.
Article noise floor of 450 Hz 2 /Hz is reached, corresponding to a Lorentzian linewidth of 2.8 kHz, with a 10 kHz-level linewidth across the whole tuning range (Fig. 3e). Unlike previous integrated pure III-V lasers, whose fundamental linewidths (typically above 100 kHz (ref. 6 )) are broader than many atomic transition lines 34,35 , the III-V/SiN heterogeneous laser presented here derives significant noise reduction from its low-loss SiN ring-resonator-based mirror, opening access to those narrow-line atomic transitions. The III-V/SiN heterogeneous laser also shows good amplitude noise performance, with relative intensity noise lower than −155 dB Hz −1 (noise floor of the measurement tool) outside the relaxation oscillation resonance near the 2 GHz offset frequency, as shown in Fig. 3f.
Another key feature of the Vernier laser design is its wide tunability. With only narrow tuning capability, producing specific wavelengths (for example, targeting atomic transitions) demands tight fabrication tolerances. Using microheaters placed on top of the ring resonators, one can make use of the thermo-optic effect to tune each ring comb, shifting the Vernier location to the desired wavelengths. This simple Vernier comb principle provides a mechanism to obtain a reconfigurable optical filter on-chip, which is key to a widely tunable laser. Figure 3h shows the lasing spectra measured by coarsely stepping the wavelength in 1 nm increments, characterized at 25 °C. The tuning range is about 20 nm (equivalent to approximately 6 THz), primarily limited by the gain bandwidth from the 980 quantum wells. The lasing side-mode-suppression ratio (SMSR) is greater than 35 dB across the entire tuning range, and approaches 50 dB when the lasing wavelength is located near the gain peak, as shown in the figure inset. The wavelength of the laser can be stepped repeatably over a wide range without sacrificing SMSR, as shown in Fig. 3i, in which the y axis shows the lasing wavelength as a function of time and the dot colour indicates the SMSR of the lasing mode.
In addition to broad tuning, when locking a laser to a high-Q cavity or atomic transitions, continuous fine tuning is often required over a smaller range. As shown in Fig. 3k, by simply sweeping the phase tuner, our laser supports a mode-hop-free tuning range of 8 GHz. Note that a much larger mode-hop-free tuning range can be achieved by simultaneously tuning the rings and the phase section 36 . As shown in Fig. 3l, frequency can also be repeatably and precisely controlled over several GHz.

High-temperature advantage of short-wavelength PICs
A major challenge for integrated photonics is the requirement of active cooling. As the performance of diode lasers degrades at elevated temperatures, it is necessary to cool the PICs to maintain performance. Thermal degradation of lasers is caused by gain reduction due to the wider spreading of the Fermi distribution of carriers at increased temperature 37 and by the loss of radiative carriers via various mechanisms, The temperature dependence of carrier recombination processes for fully integrated long-wavelength lasers and the short-wavelength lasers in this work. Non-radiative recombination increases exponentially with temperature, but the effect is reduced in a short-wavelength GaAs platform due to the increased energy bandgap and quantum well depth. The allowed working temperature is represented by the length of the solid bar, and a cooling process is necessary if the free-running temperature of the device goes beyond the working temperature range. notably including carrier leakage over hetero-barriers 38 , Auger recombination 38,39 and intervalence band absorption 40,41 (Fig. 4a), all of which exponentially increase with temperature. Of these three carrier-loss mechanisms, Auger recombination and intervalence band recombination both decrease exponentially with material bandgap 38,41 . Hence, shorter wavelength lasers are inherently more resilient to these non-radiative loss processes. In addition, the material systems grown on GaAs substrates used for near-IR-to visible-wavelength lasers have a favourably larger conduction band offset than that of the InP system of longer wavelengths, and thus higher quantum well barriers and better carrier confinement at elevated temperature 38 . Together, the above effects give the short-wavelength GaAs platform superior high-temperature performance (Fig. 4b), which could significantly reduce power consumption by operating with only passive cooling.
To study thermal performance, our heterogeneous III-V/SiN FP lasers were characterized by LI measurements at stage temperatures from 25 °C up to 185 °C, as shown in Fig. 4c. Continuous-wave lasing was achieved up to 185 °C, which is the highest operation temperature among all lasers integrated on a silicon chip so far, and significantly higher than the previous record (150 °C) 1 . Threshold currents up to 90 °C are well described by an exponential model with a characteristic T 0 of 148 K (Fig. 4d), which is on a par with the best thermal performance among diode lasers on native substrate 42 . Additionally, spectral measurements indicate red-shifting of the lasing wavelength window at a rate of 0.33 nm K −1 , with a maximum lasing wavelength of 1,044.5 nm at 185 °C, which is more than 50 nm redder than at room temperature, as shown in Fig. 4e,f.
Beyond simply lasing, the III-V/SiN heterogeneous platform also demonstrates integrated narrow linewidth lasers at elevated temperature, showing great promise for applications, including coherent communications in data centres, remote sensing or metrology in harsh environments. Ring-resonator-based tunable lasers (similar to those in the previous section) were characterized. Phase noise measurements were carried out at temperatures from 35 °C up to 145 °C (Methods). The best overall fundamental linewidth measured was lower than 7 kHz, and a linewidth of lower than 10 kHz was measured at 145 °C. Only minimal linewidth degradation was observed (Fig. 4e). Note that integrating both III-V and SiN on the same substrate ensures a robust coupling between the gain and the external cavity over a broad temperature range, whereas other linewidth-narrowing methods, such as hybrid integration with chip-to-chip butt coupling 23,29,43 , face positional misalignment challenges due to thermal expansion mismatch between different elements.

Discussion
Using the integration strategy demonstrated in this work, the wavelength range of silicon photonics can be extended down to green wavelengths with GaAs-based material (GaP, InGaP, AlGaAs), and down into blue, violet and UV ranges by incorporating GaN-based material. With the ultra-low-loss SiN waveguide recently characterized at blue and violet wavelengths 24 , it will be possible to produce scalable PICs throughout the entire visible wavelength range. By using high-Q SiN cavities, fully integrated non-linear systems may also be realized on this platform, such as microcombs [44][45][46] , stimulated Brillouin lasers 47 and strong frequency conversion systems 48 . The same integration strategy is well suited to different thicknesses of SiN waveguide, including thinner ones (<100 nm) for ultra-high-Q or thick SiN (>700 nm) for microcomb generation in the anomalous dispersion regime. Other materials, such as LiNbO 3 , AlN, SiC, AlGaAs and chalcogenide glass, can also be used intermittently as the media for passive waveguides, further enriching the toolbox of integrated photonics and extending the spectrum of PICs towards longer wavelengths (>10 μm) not supported by current PICs.
Short-wavelength PICs have the potential to rewrite the map of photonics applications. In atomic physics, short-wavelength PICs will support on-chip atomic clocks and quantum computing with trapped ion qubits 14 . With a platform spanning the vast wavelength range from visible to telecommunication, coherent links can be designed to support octave-spanning self-reference systems for time-frequency metrology 49 and visible-telecommunication entanglement in quantum communication 50 . In the consumer market, improved high-temperature performance will relax the cooling requirements of photonic devices, providing an energy-efficient solution for data centres and photonic computation. By combining highly coherent light sources at visible ranges with low-loss optical phase arrays 51 , the III-V/SiN heterogeneous photonics platform can potentially remove bulky lens imaging systems from augmented reality/virtual reality equipment, making it lighter and more power efficient.
Finally, because the fabrication of this platform is compatible with existing photonic foundries producing heterogeneous III-V/Si photonics, we expect that this technology will soon be adopted for larger scale high-volume production. As the material cost of SiN-on-insulator is lower than that of SOI, this development will make III-V/SiN economically preferable to the now-ubiquitous III-V/Si, reducing costs throughout the industry and truly revolutionizing integrated photonics.

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Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-022-05119-9.