Marcel W. Pruessner

Silicon‐photonics platforms operating at visible and near‐infrared wavelengths are essential for a growing range of applications, including biosensing, chemical detection, Raman spectroscopy, quantum, optogenetics, and displays. However, unlike their infrared counterparts, visible‐wavelength platforms have lacked comprehensive component libraries with pre‐characterized devices, limiting the ability to rapidly design photonic integrated circuits for these applications. In this work, we present the design, fabrication, and broadband experimental characterization of a comprehensive visible‐to‐near‐infrared‐spanning component library for the AIM‐Photonics 300‐mm‐wafer silicon‐nitride‐based platform, which features two silicon‐nitride waveguiding layers, a sensing trench, thermo‐optic phase shifters for modulation, and a dicing trench for fiber‐to‐chip edge coupling. We design and characterize components for both the fundamental quasi‐transverse‐electric and quasi‐transverse‐magnetic modes for wavelengths spanning from 450 to 1000 nm. To enable this characterization, we utilize a spectrum‐spanning broadband measurement setup and methodology. The component library includes passive waveguides, edge couplers, grating couplers, multimode‐interferometer splitters, directional couplers with varying splitting ratios, waveguide crossings, interlayer‐transition escalators, polarization rotators, polarization splitters, sensing‐trench components, and thermo‐optic phase shifters. This work provides a foundry‐compatible pre‐characterized library of photonic building blocks that enables the rapid design and deployment of visible‐to‐near‐infrared photonic integrated circuits for sensing, quantum, biomedical, display, and other emerging applications.

We present a suite of non-destructive optical methods to accurately extract key design parameters using waveguides that experience the complete foundry fabrication flow. These techniques are broadband and are being automated for inline verification and validation at a state-of-the-art photonic foundry (AIM Photonics). Broadband, polarization-dependent loss is measured with an accuracy of better than 0.1 dB/cm. The effective indices of the TE and TM waveguide modes are measured using Mach-Zehnder interferometers (MZIs) of several widths, thicknesses, and length imbalances. An iterative least-squares-fit algorithm is then used to extract the waveguide geometry and the Sellmeier coefficients of the core and cladding materials. The wavelength-dependent thermo-optic coefficient of the waveguide core is also measured using MZIs. We describe how background emission from the waveguide core and cladding is measured and quantified. Finally, we show how the use of our extracted waveguide parameters improves the accuracy of designs of silicon-nitride-based photonic components.

To enable the use of coherent detectors in free-space optical communication (FSOC) systems, a method must be devised to compensate for strong spacial variations in the beam front caused by atmospheric turbulence. One method that has seen development in recent years is that of a integrated optical coherent combiner. In this work we present our efforts to measure and characterize a packaged optical coherent combiner designed using AIM Photonics’ active multiproject wafer (MPW) technology.

We describe a method to experimentally determine the refractive index and thermo-optic coefficient of waveguides fabricated from low-pressure chemical vapor deposition silicon nitride over a broad wavelength range (500–1600 nm) using Mach–Zehnder interferometers. Silicon nitride refractive index Sellmeier coefficients are extracted with a weighted root mean squared deviation that is typically better than 0.04% from the visible to the near-infrared. The thermo-optic coefficient of silicon nitride is measured over a similar wavelength range and shows a significant increase at visible wavelengths compared to the value in the near-infrared. Accurate knowledge of the waveguide core and cladding refractive indices and thermo-optic coefficients in foundry-based photonic integrated circuits allows for successful first-time design of passive and active components. This method can be used with any transparent waveguide material and can be automated for process monitoring and wafer-scale verification.

The thermo-optic effect allows for the modulation of phase with very low induced optical losses in otherwise passive materials used for integrated optical phased arrays. A key drawback of this effect is the required electrical power, typically requiring tens or even hundreds of milliwatts to induce a sufficient phase shift in traditional waveguide materials such as silicon. In this work we describe a foundry-implemented method to increase the efficiency of silicon waveguide-integrated thermo-optic phase shifters by more than an order of magnitude compared to traditional devices. By reducing paths of conductive heat loss through the use of a thermal isolation trench, we experimentally demonstrate efficiency increases of 19x in Si waveguide thermo-optic phase shifting efficiency. We extend this application to OPAs and observe a 15x increase in beam steering efficiency in an 8 channel integrated OPA as well as an effective field of view extension in a wide-pitch 32 channel OPA. We also measure the modulation bandwidth and find a correlation between steering efficiency gains and device speed. Detailed finite-element simulations show excellent agreement with measurements. These simulations provide unique insights into the thermal behavior of these devices and inform tradeoffs between phase shift efficiency and temporal response in future device design.

Microwave photonics is an area of research centered on encoding radio frequency (RF) signals onto optical signals via amplitude, phase, or polarization modulation.¹ The primary application of this field was optical communication, with the first breakthroughs in optical modulation dating back to the 1960s.^2,3 As the field moved to take advantage of low loss and dispersion in silica fiber, components such as fiber delay lines, couplers, and arrayed waveguide gratings emerged to support new applications such as radar and RF beam-steering. Fiber-based components, including delay lines,^4,5 beamformers,^6,7 and modulators,^{8 – 10} have historically been successful for microwave photonics links,¹¹ but they face challenges in phase stability, bulkiness, and increased cost when integrated into full systems.

Light sources monolithically integrated with optical filters, modulators, and detectors are necessary components for photonic systems on a chip. For broadband applications such as chemical or biological sensing using absorption spectroscopy, white light sources are preferred over lasers or amplified spontaneous emission sources. In particular, thermal sources offer a straightforward means for broadband optical emission. However, to date, there have been few reports of waveguide-coupled thermal sources. In this work, we demonstrate a suspended nanophotonic waveguide-coupled broadband thermal source. It is heated by an adjacent resistive heater that permits temperatures in excess of 1000 °C at electrical powers of tens of milliwatts. We measure the waveguide-coupled emission, confirming broadband operation from 875 to 1600 nm (instrumentation limited). Thermal simulations show good agreement with measurements, and optical modeling accurately describes the heater–waveguide coupling and polarization.

Silicon photonic integrated circuit foundries enable wafer-level fabrication of entire electro-optic systems-on-a-chip for applications ranging from datacommunication to lidar to chemical sensing. However, silicon’s indirect bandgap has so far prevented its use as an on-chip optical source for these systems. Here, we describe a fullyintegrated broadband silicon waveguide light source fabricated in a state-of-the-art 300-mm foundry. A reverse-biased p-i-n diode in a silicon waveguide emits broadband near-infrared optical radiation directly into the waveguide mode, resulting in nanowatts of guided optical power from a few milliamps of electrical current. We develop a one-dimensional Planck radiation model for intraband emission from hot carriers to theoretically describe the emission. The brightness of this radiation is demonstrated by using it for broadband characterization of photonic components including Mach-Zehnder interferometers and lattice filters, and for waveguide infrared absorption spectroscopy of liquid-phase analytes. This broadband silicon-based source can be directly integrated with waveguides and photodetectors with no change to existing foundry processes and is expected to find immediate application in optical systems-on-a-chip for metrology, spectroscopy, and sensing.

Polarization management, and in particular polarization rotation, is becoming increasingly important for photonic integrated circuits (PICs). While fiber-optic networks are generally polarization insensitive, the large aspect ratio of high-index-contrast PIC waveguides leads to a large polarization-dependent response of integrated components such as waveguides, optical cavities, couplers, etc. Although foundry-processed polarization rotators operating at telecom and datacom wavelengths (C- and O-band) have been demonstrated, to date, there have been few reports of devices operating at shorter wavelengths. This work demonstrates silicon nitride (SiN) polarization rotators operating from λ=700-1000 nm (the I/Z-band) that take advantage of optical coupling between two waveguiding layers in a standard foundry process. We demonstrate a broadband white-light polarization measurement setup that enables precise characterization of the polarization-dependent transmission of photonic waveguide devices. Measurements on foundry-processed devices confirm full TE-to-TM rotation exhibiting a maximum polarization extinction ratio (PER) approaching 20 dB (limited by our measurement setup), and an exceptionally large bandwidth of up to 160 nm with an insertion loss less than 0.2 dB. Beam propagation method (3D-BPM) simulations show good agreement with experimental data and enable the device parameters to be adjusted to accommodate different operating wavelengths and geometries with no changes to the existing foundry process. This work opens up opportunities for applications in quantum information and bio-sensing where operation at λ<1000nm is needed.

Waveguide-enhanced Raman spectroscopy (WERS) is an analytical technique frequently employed for chemical and biological sensing. Operation at visible wavelengths to harness the inverse fourth power with excitation wavelength signal scaling of Raman scattering intensity is desirable, to combat the inherent inefficiency of Raman spectroscopy. Until now, WERS demonstrations in the visible have required custom materials and fabrication, resulting in high losses and low yields. In this work, we demonstrate a silicon nitride (SIN) visible WERS platform fabricated in a 300 mm complementary metal-oxide semiconductor (CMOS) foundry. We measure the propagation loss, coupling loss, WERS signal, and background for WERS spirals designed for 532 nm and 633 nm pump wavelengths. We compare these results to the state-of-the-art near-infrared WERS platform at 785 nm. Further, we theoretically validate the relative performance of each of these WERS configurations, and we discuss the optimal WERS configuration at visible wavelengths. We conclude that a configuration optimized for 785 nm pumping provides the greatest signal-to-background ratio in the fingerprint region of the spectrum, and pumping at 633 nm maximizes Stokes signal out to 3000 cm⁻¹.