This document provides supplementary information to "Silicon photonic transmitter for polarization-encoded quantum key distribution," http://dx.doi.org/10.1364/optica.3.001274. Details on the refractive index and absorption change as well as the electroluminescence in the forward-biased silicon (Si) PIN diodes are described. © 2016 Optical Society of America http://dx.doi.org/10.1364/optica.3.001274.s001 REAL AND IMAGINARY REFRACTIVE INDEX CHANGE IN SI WAVEGUIDE PIN DIODESThe modulation of the refractive index in silicon (Si) is typically through the plasma dispersion effect. The carrier density changes both the real and imaginary parts of the refractive index, which can cause, for example, a reduced extinction ratio in an optical attenuator or modulator, and polarization dependent loss in a polarization controller. According to [1], the changes in the refractive index Δn and absorption Δα of Si near a wavelength of 1550 nm arewhere ΔN e and ΔN h are changes in the free electron density and free hole density measured in cm −3 . By incorporating both Δn and Δα into a mode-solver, the coupled changes in the real and imaginary parts of the effective index as a function of carrier density can be modelled. Experimentally, for the Si waveguide PIN diode with the cross-section illustrated in Fig. S1(a), which is similar to the one used in the present work, the measured phase-shift and attenuation as function of the applied forward bias voltage are shown in Fig. S1(b) and Fig. S1(c), respectively [2]. The length of the Si PIN diode used for these measurements was 500 μm. The measured differential phase-shift was about −7.3π/(mm · V) and the corresponding differential absorption change was about 20 dB /(mm · V). These figures have been reproduced from [2]. The Si PIN diodes we have used in the current work had P++ and N++ regions that were 700 nm away from the waveguide core compared to 800 nm in Fig. S1. The reduced separation would lead to a slightly lower series resistance. ELECTROLUMINESCENCE FROM SI PIN DIODESWe observed that the Si waveguide PIN diodes in forward bias could generate weak electroluminescence. Fig. S2(a) shows the electroluminescence spectrum of a 1000 μm-long PIN diode at several forward bias voltages. The electroluminescence is broadband and centered near a wavelength of 1150 nm, close to the bandgap energy of Si (1.1 eV = 1130 nm). This electroluminescence is not power efficient due to the indirect bandgap of Si. Fig. S2(b) shows the current vs. voltage relationship of the diode. Fig. S2(c) shows the total optical power collected

This paper discusses multilayer platforms using silicon nitride and silicon waveguides. This technology allows 3-D photonic circuits to be created.

We present a three-layer silicon nitride on silicon platform for constructing very large photonic integrated circuits. Efficient interlayer transitions are enabled by the close spacing between adjacent layers, while ultra-low-loss crossings are enabled by the large spacing between the topmost and bottommost layers. We demonstrate interlayer taper transitions with losses < 0.15 dB for wavelengths spanning from 1480 nm to 1620 nm. Our overpass waveguide crossings exhibit insertion loss < 2.1 mdB and crosstalk below -56 dB in the wavelength range between 1480 nm and 1620 nm with losses as low as 0.28 mdB. Our platform architecture is suited to meet the demands of large-scale photonic circuits which contain hundreds of crossings.

We demonstrate novel polarization management devices in a custom-designed silicon nitride (Si(3)N(4)) on silicon-on-insulator (SOI) integrated photonics platform. In the platform, Si(3)N(4) waveguides are defined atop silicon waveguides. A broadband polarization rotator-splitter using a TM0-TE1 mode converter in a composite Si(3)N(4)-silicon waveguide is demonstrated. The polarization crosstalk, insertion loss, and polarization dependent loss are less than -19 dB, 1.5 dB, and 1.0 dB, respectively, over a bandwidth of 80 nm. A polarization controller composed of polarization rotator-splitters, multimode interference couplers, and thin film heaters is also demonstrated.

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