The transition towards designs which co-package electronic and photonic die together in data cen ter switch packages has created a scaling path to Petabyte per second (Pbps) input/output (I/O) in such systems. In a co-packaged design, the scaling of bandwidth, cost, and energy will be governed by the number of optical I/O channels and the data rate per channel. While optical communica tion provide an opportunity to exploit wavelength division multiplexing to scale data rate, the lim ited 127µm pitch of V-groove based single mode fiber arrays and the use of active alignment and bonding for their packaging present challenges to scaling the number of optical channels. Flip-chip optical couplers which allow for low loss, broadband operation and automated passive assembly represent a solution for continued scaling. In this paper, we propose a novel scheme to vertically couple between silicon based waveguides on separate chips using graded index couplers in com bination with an evanescent coupler. Simulation results using a 3D finite-difference time-domain solver are presented, demonstrating coupling losses as low as 0.35dB for a chip-to-chip gap of 11µm; 1dBvertical and lateral alignment tolerances of approximately 2.45µm and ± 2.66µm, respectively; and a possible 1dB bandwidth of greater than 1500nm. These results demonstrate the potential of our coupler as a universal interface in future co-packaged optics systems.

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The co-packaging of optics and electronics provides a potential path forward to achieving beyond 50 Tbps top of rack switch packages. In a co-packaged design, the scaling of bandwidth, cost, and energy is governed by the number of optical transceivers (TxRx) per package as opposed to transistor shrink. Due to the large footprint of optical components relative to their electronic counterparts, the vertical stacking of optical TxRx chips in a co-packaged optics design will become a necessity. As a result, development of efficient, dense, and wide alignment tolerance chip-to-chip optical couplers will be an enabling technology for continued TxRx scaling. In this paper, we propose a novel scheme to vertically couple into standard 220 nm silicon on insulator waveguides from 220 nm silicon nitride on glass waveguides using overlapping, inverse double tapers. Simulation results using Lumerical’s 3D Finite Difference Time Domain solver are presented, demonstrating insertion losses below -0.13 dB for an inter-chip spacing of 1 µm; 1 dB vertical and lateral alignment tolerances of approximately 2.6 µm and ± 2.8 µm, respectively; a greater than 300 nm 1 dB bandwidth; and 1 dB twist and tilt tolerances of approximately ± 2.3 degrees and 0.4 degrees, respectively. These results demonstrate the potential of our coupler for use in co-packaged designs requiring high performance, high density, CMOS compatible out of plane optical connections.

Citation: Drew Weninger, Samuel Serna, Achint Jain, Lionel Kimerling, and Anuradha Agarwal, “High density vertical optical interconnects for passive assembly,” Opt. Express 31, 2816-2832 (2023)

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An experimentally demonstrated, vertical chip-to-chip evanescent coupler between silicon nitride (SiNx) and silicon (Si) is presented with the coupler loss measured to be 0.39 ± 1.06 dB at 1550 nm with a 1-dB bandwidth of 160 nm extending across the C-band, S-band, and L-band (1480–1640 nm). The average coupling loss is determined to be 0.73 dB for the 1480–1640 nm wavelength range with a ±2σ tolerance of ±0.92 dB. The 1-dB lateral alignment tolerance is 1.56 ± 0.14 μm at 1550 nm and the average tolerance is 1.38 ± 0.24 μm across the 1480–1640 nm wavelength regime. In addition, the average coupling loss varies by less than ±0.35 dB and the average 1-dB alignment tolerance varies by less than ±30 nm for temperatures varying from 23 to 60 °C. Finally, the average coupling loss range is less than 1.5 dB range across four sets of identically packaged die. This is the first experimental demonstration of an interchip, passively assembled evanescent coupler using standard complementary metal-oxide-semiconductor foundry processes for directly coupling between Si and SiNx, overcoming a waveguide refractive index difference of Δ⁢n = 1.32 without requiring taper tip widths of less than 100 nm.

Citation: Weninger, D., Serna, S., Ranno, L., Kimerling, L. and Agarwal, A. (2025), Low Loss Chip-to-Chip Couplers for High-Density Co-Packaged Optics. Adv. Eng. Mater., 27: 2402095. https://doi.org/10.1002/adem.202402095

Click to read more in Advanced Engineering Materials.

Perfluoroalkyls (PFAS) continue to emerge as a global health threat making their effective detection and capture extremely important. Though metal-organic frameworks (MOFs) have stood out as a promising class of porous materials for sensing PFAS, detection limits remain insufficient and a fundamental understanding of detection mechanisms warrants further investigation. Here, we show the use of a 2D conductive MOF film based on copper hexahydroxy triphenylene (Cu-HHTP) to fabricate chemiresistive sensing devices for detecting PFAS in drinking water. We further show ultrasensitive detection using electrochemical impedance spectroscopy. Owing to excellent electrostatic attractions and electrochemical interactions between the copper-based MOF and PFAS, confirmed by high-resolution spectroscopy and theoretical simulations, the MOF-based sensor reported herein exhibits excellent affinity and sensitivity toward perfluorinated acids at concentrations as low as 0.002 ng/L.

Citation: Roh H, Quill TJ, Chen G, Gong H, Cho Y, Kulik HJ, Bao Z, Salleo A, Gumyusenge A. Copper-Based Two-Dimensional Conductive Metal-Organic Framework Thin Films for Ultrasensitive Detection of Perfluoroalkyls in Drinking Water. ACS Nano. 2025 Feb 18;19(6):6332-6341. doi: 10.1021/acsnano.4c16212. Epub 2025 Feb 8. PMID: 39921641.

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