Silicon waveguide asymmetric Y junction mode multiplexers and demultiplexers are demonstrated for applications in on-chip mode-division multiplexing (MDM). We measure demultiplexed crosstalk as low as -30 dB, <-9 dB over the C band, and insertion loss <1.5 dB for multimode links up to 1.2 mm in length. The frequency response of these devices is shown to depend upon Y junction angle and multimode interconnect length. Interference effects are shown to be advantageous for low-crosstalk MDM, even while using compact Y junctions designed to be outside the mode-sorting regime.

CMOS-compatible Si⁺-implanted Si-waveguide p-i-n photodetectors operating at room temperature and at mid-infrared wavelengths from 2.2 to 2.3 µm are demonstrated. Responsivities of 9.9 ± 2.0 mA/W are measured at a 5 V reverse bias with an estimated internal quantum efficiency of 2.7 - 4.5%. The dark current is found to vary from a few microamps down to less than a nanoamp after a post-implantation annealing of 350°C. The measured photocurrent dependence on input power shows a linear correspondence over more than three decades, and the frequency response of a 250 µm-length p-i-n device is measured to be ~1.7 GHz for a wavelength of λ = 2.2 µm, thus potentially opening up new communication bands for photonic integrated circuits.

While a number of Si waveguide (SiWG) integrated optoelectronic devices have been demonstrated in this wavelength range 8,9 , efficient photodetection remains an important and challenging task. Thus, spectral translation of mid-infrared signals to the telecom regime via four-wave mixing in SiWGs has been proposed for on-chip detection 10 , which makes use of the sensitive integrated photodiodes (PDs) in the telecommunications wavelength range 11 . However, this method requires a high-powered pump laser and long, on-chip-waveguide lengths to achieve efficient wavelength conversion. In addition, heterogeneous integration of both narrow-bandgap semiconductors 9,12-14 and graphene 15,16 with SiWGs has been demonstrated for on-chip mid-infrared detection. Though viable PDs have been demonstrated, 2 heterogeneous integration schemes present an inherent difficulty by imposing constraints on material quality and process integration. Extrinsic detectors, which utilize absorption transitions from dopant-induced trap states within the bandgap of a host material, present a simple solution for high-performance integrated mid-infrared PDs and alleviate the need for heterogeneous integration. These PDs can potentially be integrated into a standard CMOS process flow by adding an ion implantation and annealing step after activation of the source and drain implants, and prior to the deposition of back-end dielectrics and interconnect metallization.Alternatively, this additional fabrication step can be performed as a post-process, as is done here (see Methods). The Si dopants used in bulk photoconductors can potentially be integrated for detection from a range of wavelengths from 1.5 μm to greater than 25 µm (Fig. 1a) ). Subsequent to implantation, the PDs were annealed in atmosphere for a series of increasing temperatures, reaching a maximum of 350°C, and the responsivity was found to increase with annealing temperature.The detection mechanism of our p-Si:Zn-n PDs is due to substitutional Zn atoms in the Si lattice, which act as a double-acceptors and result in the two defect levels ( Fig. 2a) with energy levels of Ed1 ≈ Ev + 0.3 eV and Ed2 ≈ Ev + 0.58 eV 5,6 . While the position of the Fermi level in the Si:Zn region is not known, the excess carrier concentration is below that of the p and n regions, ensuring that the Si:Zn region will be fully depleted with the application of a reverse bias voltage. Photon-induced transitions occurs between Ed2 and the conduction band, which corresponds to a transition energy of Eg -Ed2 = 1.12eV -0.58eV = 0.54eV with a peak absorption wavelength of ≈ 2.3 µm. Photocurrent generation due to this transition is shown to 4 be a single-photon process by the linearity measurements in Fig. 2b. The presence of Ed1 does not contribute to photocurrent generation in the wavelength range of interest; however, its presence should impact the thermally assisted re-population rate of Ed2 and thus the internal quantum efficiency, ηi, of the PD. The responsivity, defined as R = Iph/Pin, where Iph is the photocu...

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Mode-division-multiplexing (MDM) and wavelength-division-multiplexing (WDM) are employed simultaneously in a multimode silicon waveguide to realize on-chip MDM and MDM-WDM transmission. Asymmetric Y-junction MDM multiplexers and demultiplexers are utilized for low coherently suppressed demultiplexed crosstalk at the receiver. We demonstrate aggregate bandwidths of 20 Gb/s and 60 Gb/s for MDM and MDM-WDM on-chip links, respectively, with measured 10(-9) BER power penalties between 0.1 dB and 0.7 dB per channel.