Narrow linewidth visible light lasers are critical for atomic, molecular and optical (AMO) applications including atomic clocks, quantum computing, atomic and molecular spectroscopy, and sensing. Historically, such lasers are implemented at the tabletop scale, using semiconductor lasers stabilized to large optical reference cavities. Photonic integration of high spectral-purity visible light sources will enable experiments to increase in complexity and scale. Stimulated Brillouin scattering (SBS) is a promising approach to realize highly coherent on-chip visible light laser emission. While progress has been made on integrated SBS lasers at telecommunications wavelengths, barriers have existed to translate this performance to the visible, namely the realization of Brillouin-active waveguides in ultra-low optical loss photonics. We have overcome this barrier, demonstrating the first visible light photonic integrated SBS laser, which operates at 674 nm to address the 88Sr+ optical clock transition. To guide the laser design, we use a combination of multi-physics simulation and Brillouin spectroscopy in a 2 meter spiral waveguide to identify the 25.110 GHz first order Stokes frequency shift and 290 MHz gain bandwidth. The laser is implemented in an 8.9 mm radius silicon nitride all-waveguide resonator with 1.09 dB per meter loss and Q of 55.4 Million. Lasing is demonstrated, with an on-chip 14.7 mW threshold, a 45% slope efficiency, and linewidth narrowing as the pump is increased from below threshold to 269 Hz. To illustrate the wavelength flexibility of this design, we also demonstrate lasing at 698 nm, the wavelength for the optical clock transition in neutral strontium. This demonstration of a waveguide-based, photonic integrated SBS laser that operates in the visible, and the reduced size and sensitivity to environmental disturbances, shows promise for diverse AMO applications.

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Visible light photonic integrated Brillouin laser

Integrated photonics is an emerging technology for many existing and future tele- and data communication applications. One platform of particular interest is Silicon Nitride (SiN) thanks to - amongst others - its very low-loss waveguides. However, it lacks active devices, such as lasers, amplifiers and photodiodes. For this, hybrid or heterogeneous integration is needed. Here, we bring high-speed uni-travelling-carrier photodiodes (UTC PDs) to a low-loss SiN-platform by means of micro-transfer-printing. This versatile technology for heterogeneous integration not only allows very dense and material-efficient III-V integration, it also eases fabrication yielding high-performance detectors. The waveguide-coupled photodiodes feature a responsivity of 0.3 A/W at 1550 nm, a dark current of 10 nA and a bandwidth of 155 GHz at a low bias. At zero bias, a record bandwidth of 135 GHz is achieved. We further demonstrate that this integrated detector can be used for direct photomixing at terahertz frequencies. A back-to-back communication link with a carrier frequency around 300 GHz is set up, and data rates up to 160 Gbit/s with low error vector magnitude (EVM) are shown, showcasing near-identical performance at zero bias.

High-Speed Uni-Travelling-Carrier Photodiodes on Silicon Nitride.

By the pulsed, heterodyned, and modulated laser illuminations, the impulse and noise responses of O-band germanium (Ge) p-i-n waveguide photodetector (WPD) with different transmission-line designs are characterized for receiving the NRZ-OOK data beyond 60 Gbit/s. The Ge lateral p-i-n WPD reveals an optical responsivity of 0.63-0.74 A/W at 1327.5 nm under −1.8 V bias.The noise equivalent power (NEP) is increased from 0.11 to 0.2 pW/√Hz by shrinking the transmission-line contact electrode length from 280 to 80 μm. After optimizing the transmission-line designs with shorter length and less right corner number (C⊥#) between PD and bonding pad, the Ge lateral p-i-n WPD improves its RLC response to suppress its switching time with broadened receiving bandwidth. Shrinking the connecting transmission-line length from 280 μm to 80 μm extends the flattened S11 spectral response from 51 to 57 GHz by up-shifting the inductor inductor-induced frequency notch from 58 to 61 GHz, as confirmed by the shrank spiral curve in the Smith chart with the decreased inductance by shortening the transmission-line electrode. The dual-wavelength optical heterodyne analysis demonstrates that the Ge p-i-n WPD significantly improves its analog receiving bandwidth from 29.5 to 44.3 GHz measured at 6-dB decay. The sub-ps laser pulse illumination observes that the Ge p-i-n WPD response shortens its impulse response from 23.14 to 19.92 ps by reducing the contact inductor length (by 200 μm) and right corner number (C⊥#) from 4 to 2) of its electrode. When receiving the optical NRZ-OOK data stream, the long-electrode (L = 280 μm, C⊥# = 4) device reveals only 53-Gbit/s receiving performance. The short-electrode (L = 80 μm, C⊥# = 2) Ge lateral p-i-n WPD with significantly reduced inductance successfully receives the NRZ-OOK data beyond 56 Gbit/s and 60 Gbit/s under error-free communication criterion of BER<10−12 and BER<10−9, respectively.

Ge p-i-n Photodiode as 60-Gbit/s Optical NRZ-OOK Data Receiver

We demonstrate modified uni-traveling carrier (MUTC) waveguide photodiodes on a silicon nitride/silicon (Si3N4/Si) photonic platform using micro-transfer printing. The photodiodes exhibit high bandwidth of 54 GHz and low dark current of 30 nA at −3 V. The MUTC photodiode (PD) with short InGaAsP waveguide is directly (butt-) coupled to the Si3N4 waveguide and was designed to facilitate efficient light coupling while preserving the favorable high-power capability of InGaAsP/InP evanescently-coupled waveguide PDs. A responsivity of 0.42 A/W at 1310 nm wavelength and an output power as high as +7 dBm at 50 GHz and 22 mA photocurrent were measured.

High-Power High-Speed MUTC Waveguide Photodiodes Integrated on Si3N4/Si Platform Using Micro-Transfer Printing

We report local nitride-stressor engineering in combination with quantum-size tunability in Ge quantum dots (QDs) for tailoring photoluminescence (PL) wavelength and exciton lifetime. Spherical-shaped Ge QDs with tunable diameters ranging from 25–90 nm embedded within layers of thermally-grown SiO2 and chemical-vapor-deposited Si3N4 were fabricated by thermal oxidation of lithographically-patterned poly-Si0.85Ge0.15 pillars on top of buffer layers of SiO2 and Si3N4, respectively, in a self-organization approach. The effects of local stressors and quantum confinement on the strain and optical properties of Ge QDs were systematically investigated using Raman and PL measurements. We observed that when Ge QD diameter gets smaller than 60 nm, quantum confinement sets in and has a predominant influence on PL wavelength and exciton lifetime. Our self-organized Ge QD/Si3N4 system provides a generic building block for the fabrication of active photonic devices on Si3N4 platform.

Nitride-stressor and quantum-size engineering in ge quantum-dot photoluminescence wavelength and exciton lifetime

A Ge photodiode, directly coupled to a silicon nitride waveguide, showing more than 67 GHz bandwidth is demonstrated for the first time, which paves the way for utterly new SiN waveguide platform based applications. By light feeding through SiN waveguides, the new photodiode can also be a key enabler for a bulk-Si based, monolithically integrated electronic-photonic integrated circuit platform. We show that the new devices, fabricated on bulk-Si, provide the same bandwidths as Si waveguide coupled SOI based reference Ge photodiodes. However, their O-band responsivity is 0.3 A/W, which is about three times lower compared to the SOI waveguide coupled devices. We attribute this effect to substrate losses and few specific layout features but see some potential for improvement by design and technological optimizations. We demonstrate that the diodes can be fabricated with high yield and low metrics tolerances.

Silicon nitride waveguide coupled 67+ GHz Ge photodiode for non-SOI PIC and ePIC platforms

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Silicon nitride waveguide coupled 67+ GHz Ge photodiode for non-SOI PIC and ePIC platforms