Low-loss Silicon Nitride Waveguides Research Articles

Photon-Pair Generation in a Heterogeneous Nanophotonic Chip

Integrated silicon photonics has played an important role in advancing the applications of quantum information and quantum science. However, it is challenging to integrate all components with state-of-the-art performance using only a homogeneous platform. Here, by combining high nonlinearity and low losses in a heterogeneous silicon platform, we efficiently generate high-quality photon pairs through spontaneous four-wave mixing in a hydrogenated amorphous silicon waveguide and route them off-chip through a low-loss silicon nitride waveguide. A record high coincidence-to-accidental ratio value of 1632.6 (±260.4) is achieved in this heterogeneous design with a photon pair generation rate of 1.94 MHz. We also showcase a wide range of multichannel photon sources with a coincidence-to-accidental ratio consistently around 200. Lastly, we measure heralded single-photons with the lowest gH(2)(0) of 0.1085 ± 0.0014. Our results demonstrate the heterogeneous silicon platform as an ideal platform for the efficient generation of photon pairs and off-chip routing with low losses. It also paves the way for a future hybrid photonic integrated circuit by collecting distinct features from different materials.

Towards monolithic low-loss silicon nitride waveguides on a mature 200 mm CMOS platform

Silicon nitride photonics has enabled a broad class of low-loss integrated devices and chip-scale solutions that benefit from transparency over a wide wavelength range and compatibility with complementary metal-oxide-semiconductor process, allowing applications of tele/datacom, artificial intelligence, sensing and bio-photonics. As a complimentary platform to silicon-on-insulator photonics, low-loss silicon nitride waveguide technology was present to extend the capabilities of silicon photonics platform. Silicon nitride films prepared by low pressure chemical vapor deposition have low intrinsic loss and good repeatability, but silicon nitride film is prone to crack when the thickness is no less than 300 nm. In order to reduce high stress, the wafer-level silicon nitride waveguides with 400 nm thickness were fabricated by two-step deposition method on 200-mm complementary metal-oxide-semiconductor pilot-line. Due to the different linewidth change between single-stripe and multiple-stripe structure after the same layer etch process, the layout compensation is needed to correct this deviation. But the linewidth sensitivity for silicon nitride waveguides is relatively low, propagation losses in the C-band were slightly reduced from 0.66 dB/cm for 0.9 µm width down to 0.62 dB/cm for 1 µm width.

Spectrometric analysis of silicon nitride films deposited by low-temperature liquid-source CVD

Silicon nitride (SiN) films formed by liquid-source chemical vapor deposition (LSCVD) were analyzed for photonic applications. While this deposition technique for SiN has strengths in its low reaction temperature (>80 °C) and fast deposition rate (>50 nm/min), the material properties, such as its composition, chemical bond, and optical absorption bands in waveguides, have not been studied quantitatively. Hence, we probed SiN films to understand the material characteristics by ultraviolet-to-midinfrared ellipsometry, on-chip waveguide absorption spectrometry, Rutherford backscattering/hydrogen forward-scattering spectrometry, and Fourier-transform infrared spectrometry. As interpreted by a combination of the series of analyses, the N–H bond concentration of higher than 1021 cm−3 leaves an absorption notch (>6.5 dB/cm) at a wavelength of 1550 nm, which cannot be avoided even by systematically varying the deposition conditions. However, except for that absorption range, a low-loss SiN waveguide (0.5–1.0 dB/cm) with a practically applicable high refractive index (∼1.85) can be formed at the deposition temperature of 100 °C. This basic materials research, which combines multiple spectrometric analyses, will help to improve our understanding of a LSCVD SiN film for photonics circuit integration.

Chip-integrated metasurface for versatile and multi-wavelength control of light couplings with independent phase and arbitrary polarization

While metasurfaces are now widely considered in free-space optics, their potential for coupling and tailoring guided waves is not fully explored. Here we transfer the Jones matrix method to target versatile on-chip coupling using metasurface-patterned photonic waveguides around the telecommunication wavelength of 1.55 μm, which can accommodate both propagation and Pancharatnam-Berry phase metasurfaces for guided waves. One can either encode two arbitrary and independent phase profiles to any pair of orthogonal polarizations or deploy complete control over both the phase and polarization of coupled modes. A set of design scenarios synergizing silicon nanoantennas and low-loss silicon-nitride waveguides are proposed, including directional couplers with mode-selectivity and polarization splitters with directionality ranging from 10 to 20 dB. Furthermore, our optimization method can be further extended to cover multiple working wavelengths. Exemplary on-chip color routers are also numerically demonstrated. This chip-integrated metasurface platform further translates the concept of a metasurface into photonic integrated circuits, serving as a positive paradigm for versatile and complete control over waveguide optical signals and motivating chip-scale applications such as polarization/wavelength demultiplexers, optical switches, and multifunctional mode converters.

WDM Source Based on High-Power, Efficient 1280-nm DFB Lasers for Terabit Interconnect Technologies

We present a proof-of-concept eight-channel wavelength-division-multiplexed (WDM) source for future terabit interconnects based on a highly efficient laser array. The array is composed of novel, high-power DFB lasers with record electro-optical efficiency, operating at 1280 nm, with >250-mW laser output power and laser efficiencies of up to 36%. The eight-laser array, with ~100-GHz channel spacing is optically butt-coupled to a planar lightwave circuit that consists of low-loss silicon nitride waveguides clad with silicon oxide for appropriate optical routing. Two types of optical routing are explored: 1) all 8 laser wavelengths are mixed and output into 10 channels via a star coupler and 2) the laser wavelengths are combined into a single output via an arrayed waveguide grating router. The mixed/combined light is then butt-coupled to a fiber array to output the WDM source signal into polarization preserving single-mode fibers. Improvements in insertion loss, particularly from optical butt coupling, will make this approach a viable option for efficient WDM light sources.

Battery-operated integrated frequency comb generator.

Optical frequency combs are broadband sources that offer mutually coherent, equidistant spectral lines with unprecedented precision in frequency and timing for an array of applications1. Frequency combs generated in microresonators through the Kerr nonlinearity require a single-frequency pump laser and have the potential to provide highly compact, scalable and power-efficient devices2,3. Here we demonstrate a device-a laser-integrated Kerr frequency comb generator-that fulfils this potential through use of extremely low-loss silicon nitride waveguides that form both the microresonator and an integrated laser cavity. Our device generates low-noise soliton-mode-locked combs with a repetition rate of 194gigahertz at wavelengths near 1,550nanometres using only 98milliwatts of electrical pump power. The dual-cavity configuration that we use combines the laser and microresonator, demonstrating the flexibility afforded by close integration of these components, and together with the ultra low power consumption should enable production of highly portable and robust frequency and timing references, sensors and signal sources. This chip-based integration of microresonators and lasers should also provide tools with which to investigate the dynamics of comb and soliton generation.

Compact narrow-linewidth integrated laser based on a low-loss silicon nitride ring resonator.

We design and demonstrate a compact, narrow-linewidth integrated laser based on low-loss silicon nitride waveguides coupled to a III-V gain chip. By using a highly confined optical mode, we simultaneously achieve compact bends and ultra-low loss. We leverage the narrowband backreflection of a high-Q microring resonator to act as a cavity output mirror, a single-mode filter, and a propagation delay all in one. This configuration allows the ring to provide feedback and obtain a laser linewidth of 13kHz with 1.7mW output power around 1550nm. This demonstration realizes a compact sub-millimeter silicon nitride laser cavity with a narrow linewidth.

Low-loss silicon nitride waveguide hybridly integrated with colloidal quantum dots.

Silicon nitride waveguides with a monolayer of colloidal quantum dots embedded inside were fabricated using a low-temperature deposition process and an optimized dry etching step for the composite layers. We experimentally demonstrated the luminescence of the embedded quantum dots is preserved and the loss of these hybrid waveguide wires is as low as 2.69dB/cm at 900nm wavelength. This hybrid integration of low loss silicon nitride photonics with active emitters offers opportunities for optical sources operating over a very broad wavelength range.

Continuous wave-pumped wavelength conversion in low-loss silicon nitride waveguides.

In this Letter we introduce a complementary metal-oxide semiconductor (CMOS)-compatible low-loss Si3N4 waveguide platform for nonlinear integrated optics. The waveguide has a moderate nonlinear coefficient of 285 W/km, but the achieved propagation loss of only 0.06 dB/cm and the ability to handle high optical power facilitate an optimal waveguide length for wavelength conversion. We observe a constant quadratic dependence of the four-wave mixing (FWM) process on the continuous-wave (CW) pump when operating in the C-band, which indicates that the waveguide has negligible high-power constraints owing to nonlinear losses. We achieve a conversion efficiency of -26.1 dB and idler power generation of -19.6 dBm. With these characteristics, we present for the first time, to the best of our knowledge, CW-pumped data conversion in a non-resonant Si3N4 waveguide.

Multilayer Silicon Nitride-on-Silicon Integrated Photonic Platforms and Devices

We review and present additional results from our work on multilayer silicon nitride (SiN) on silicon-on-insulator (SOI) integrated photonic platforms over the telecommunication wavelength bands near 1550 and 1310 nm. SiN-on-SOI platforms open the possibility for passive optical functionalities implemented in the SiN layer to be combined with active functionalities in the SOI. SiN layers can be integrated onto SOI using a front-end or back-end of line integration process flow. These photonic platforms support low-loss SiN waveguides, low-loss and low-crosstalk waveguide crossings, and low-loss interlayer transitions using adiabatic tapers. Novel ultra-broadband and efficient grating couplers as well as polarization management devices are enabled by the close coupling between the silicon and SiN layers.

Design of integrated hybrid silicon waveguide optical gyroscope.

We propose and analyze a novel highly integrated optical gyroscope using low loss silicon nitride waveguides. By integrating the active optical components on chip, we show the possibility of reaching a detection limit on the order of 19°/hr/√Hz in an area smaller than 10 cm(2). This study examines a number of parameters, including the dependence of sensitivity on sensor area.

Integrated High Sensitivity Hybrid Silicon Magnetometer

We propose and analyze a novel highly sensitive optical magnetometer, using low-loss silicon nitride waveguides. With the recent advances in Ce:YIG pulsed laser deposition on silicon nitride, we show the possibility of reaching sensitivities on the order of 20 fT/ \(\surd \) Hz at room temperature in an area \(^{2}\) . This letter examines a number of parameters, including the dependence of sensitivity on sensor area.

Integrated TE and TM optical circulators on ultra-low-loss silicon nitride platform

In this paper, we present two four-port optical circulators for TE and TM modes, respectively. Exploiting the recent technological development concerning Ce:YIG pulse laser deposition on silicon nitride platform, we design two integrated circulators, which can be used to implement several functions in integrated optics, such as de-interleavers, input/output amplifier isolators and output laser isolators. The proposed devices combine the benefit of low loss silicon nitride waveguides with the non-reciprocal properties of magneto-optical materials. The ring cross-section has been optimized in order to maximize the non-reciprocal phase shift and finally the scattering coefficients have been computed using the transfer matrix method. The material stability and refractive index regularity of silicon nitride, the small micro-ring footprint, and the high wavelength selectivity make these devices particularly attractive.

Optically pumped Si nanocrystal emitter integrated with low loss silicon nitride waveguides

We describe the integration of optically pumped silicon nanocrystals (Si-ncs) embedded in SiO(2) with low loss silicon nitride slab waveguides. An emission waveguide containing Si-ncs with a broad band emission centered at 850 nm, together with a low loss transmission silicon nitride waveguide forms a two section device. The waveguides are fabricated via the deposition of SiO(x) and silicon nitride using ECR-PECVD. Incorporation of hydrogen through annealing, while beneficial to emission from the Si-ncs, is found to increase material absorption in silicon nitride. This is reconciled by annealing at low temperature. This work shows clearly the potential for this material system as a means for the integration of optical emission and waveguiding using a wholly VLSI compatible processing technology. We further suggest that immediate applications exist in particular in the field of evanescent sensing.