Dual-polarization pump rejection filter in silicon nitride technology.

On-chip pump rejection filters are key building blocks in a variety of applications exploiting nonlinear phenomena, including Raman spectroscopy and photon-pair generation. Ultrahigh rejection has been achieved in the silicon technology by non-coherent cascading of modal-engineered Bragg filters. However, this concept cannot be directly applied to silicon nitride waveguides as the comparatively lower index contrast hampers the suppression of residual light propagating in the orthogonal polarization, limiting the achievable rejection. Here, we propose and demonstrate a novel, to the best of our knowledge, strategy to overcome this limitation based on non-coherent cascading of the modal- and polarization-engineered Bragg filters. Based on this concept, we experimentally demonstrate a rejection exceeding 60 dB for both polarizations, with a bandwidth of 4.4 nm. This is the largest rejection reported for silicon nitride Bragg gratings supporting both polarizations.

Library of single-etch silicon nitride grating couplers for low-loss and fabrication-robust fiber-chip interconnection.

Silicon nitride (Si3N4) waveguides become an appealing choice to realize complex photonic integrated circuits for applications in telecom/datacom transceivers, sensing, and quantum information sciences. However, compared to high-index-contrast silicon-on-insulator platform, the index difference between the Si3N4 waveguide core and its claddings is more moderate, which adversely affects the development of vertical grating-coupled optical interfaces. Si3N4 grating couplers suffer from the reduced strength, therefore it is more challenging to radiate all the waveguide power out of the grating within a beam size that is comparable to the mode field diameter of standard optical fibers. In this work, we present, by design and experiments, a library of low-loss and fabrication-tolerant surface grating couplers, operating at 1.55 µm wavelength range and standard SMF-28 fiber. Our designs are fabricated on 400 nm Si3N4 platform using single-etch fabrication and foundry-compatible low-pressure chemical vapor deposition wafers. Experimentally, the peak coupling loss of - 4.4 dB and - 3.9 dB are measured for uniform couplers, while apodized grating couplers yield fiber-chip coupling loss of - 2.9 dB, without the use of bottom mirrors, additional overlays, and multi-layered grating arrangements. Beside the single-hero demonstrations, over 130 grating couplers were realized and tested, showing an excellent agreement with finite difference time domain designs and fabrication-robust performance. Demonstrated grating couplers are promising for Si3N4 photonic chip prototyping by using standard optical fibers, leveraging low-cost and foundry-compatible fabrication technologies, essential for stable and reproducible large-volume device development.

Wafer-level testing of inverse-designed and adjoint-inspired vertical grating coupler designs compatible with DUV lithography.

Perfectly vertical grating couplers have various applications in optical I/O such as connector design, coupling to multicore optical fibers and multilayer silicon photonics. However, it is challenging to achieve perfectly vertical coupling without simultaneously increasing reflection. In this paper, we use the adjoint method as well as an adjoint-inspired methodology to design devices that can be fabricated using only a single-etch step in a c-Si 193 nm DUV immersion lithography process, while maintaining good coupling and low reflection. Wafer-level testing of devices fabricated by a pilot line foundry confirms that both design paradigms result in state-of-the-art experimental insertion loss (<2 dB) and bandwidths (∼20 nm) while having only moderate in-band reflection (<-10 dB). Our best design has a (median) 1.82 dB insertion loss and 21.3 nm 1 dB-bandwidth.

Silicon nitride-on-silicon bi-layer grating couplers designed by a global optimization method.

Silicon nitride-on-silicon bi-layer grating couplers were designed for the O-band using an optimization-based procedure that accounted for design rules and fabricated on a 200 mm wafer. The designs were sufficiently robust to fabrication variations to function well across the wafer. A peak fiber-to-chip coupling efficiency to standard single mode fiber of -2.2 dB and a 1-dB bandwidth of 72.9 nm was achieved in the representative device. Over several chips across the wafer, we measured a median peak coupling efficiency of -2.1 dB and median 1-dB bandwidth of 70.8 nm. The measurements had good correspondence with simulation.

A graphene Zener-Klein transistor cooled by a hyperbolic substrate.

The engineering of cooling mechanisms is a bottleneck in nanoelectronics. Thermal exchanges in diffusive graphene are mostly driven by defect-assisted acoustic phonon scattering, but the case of high-mobility graphene on hexagonal boron nitride (hBN) is radically different, with a prominent contribution of remote phonons from the substrate. Bilayer graphene on a hBN transistor with a local gate is driven in a regime where almost perfect current saturation is achieved by compensation of the decrease in the carrier density and Zener-Klein tunnelling (ZKT) at high bias. Using noise thermometry, we show that the ZKT triggers a new cooling pathway due to the emission of hyperbolic phonon polaritons in hBN by out-of-equilibrium electron-hole pairs beyond the super-Planckian regime. The combination of ZKT transport and hyperbolic phonon polariton cooling renders graphene on BN transistors a valuable nanotechnology for power devices and RF electronics.