## 2 Device  
Figure 1a shows the schematic of the Si LSPCW MZ modulator with the interleaved p-n junction. The details of the structure, theoretical modal behaviors, and fundamental modulation characteristics can be seen in Reference [15]. Third-row-shifted LSPCWs with a central p-n junction and SiO\({}_{2}\) cladding as phase shifters were employed; these exhibit typical LD slow light. The phase shifter length was fixed at \(L=200\) um and parameters of the LSPCW were \(a\) (lattice constant) = 400 nm, \(2r\) (hole diameter) = 220 nm, and \(s\) (lattice shift) = 95 nm. An interleaved p-n junction with a comb period of \(w_{\rm x}\) = 300 nm and a comb depth of \(w_{\rm y}\) = 600 nm was used. The p- and n-type doping concentrations were given as \(N_{\rm A}\) = 9.5 \(\times\) 10\({}^{17}\) cm\({}^{-3}\) and \(N_{\rm D}\) = 5.7 \(\times\) 10\({}^{17}\) cm\({}^{-3}\), respectively, and the p\({}^{+}\) and n\({}^{+}\) doping concentrations for electrical contacts were \(N_{\rm A}\)\({}^{+}\) = 1.0 \(\times\) 10\({}^{19}\) cm\({}^{-3}\) and \(N_{\rm D}\)\({}^{+}\) = 1.0 \(\times\) 10\({}^{19}\) cm\({}^{-3}\), respectively. To avoid the strong free carrier absorption in heavily doped regions, a distance of \(\sim\)4 um across the p-n junction was set. As radio frequency (RF) electrodes of the phase shifters, coplanar waveguides consisting of an Al electrode for signal (S) and two Al electrodes for ground (G) were employed. No termination resistors were used, to simplify the device layout. In addition, 36.5 um TiN heaters were integrated inside the SiO\({}_{2}\) cladding above the same LSPCWs, and were used for setting the initial phase of each MZ arm. The total footprint of the device excluding the input and output waveguides was \(\sim\)400 \(\times\) 500 um\({}^{2}\), but they could be reduced to \(\sim\)50 \(\times\) 250 um\({}^{2}\) if we shrink the electrodes and neglect the RF pads. The devices were constructed on a 200 mm silicon-on-insulator (SOI) wafer, using a 180 nm CMOS process at the Institute of Micro-Electronics foundry service with a KrF excimer laser exposure at \(\lambda\) = 248 nm. The original thickness of the Si slab in the SOI wafer was 220 nm, but was subsequently thinned to 210 nm by thermal oxidation. The thickness of the SiO\({}_{2}\) BOX layer was 2 \(\upmu\)m. Fourteen mask layers were used: two layers for the waveguides, four layers for the p-n junctions, two layers for the heaters and thermal isolation trench, four layers for RF and DC electrodes, contact holes, and contact pads, and two layers for the formation of waveguide end facets with spot size converters (SSC). For the waveguide layers, phase-shift masks were applied to improve the resolution to approximately 100 nm. Figure 1b shows the microscope image of the fabricated device. First, we measured the fundamental slow-light characteristics of the device. Using objective lenses or compact lens modules, we input light from and output light to single mode fibers with the SSC consisting of SiO\({}_{2}\) waveguides and Si inverse tapers located at the chip facets. Figure 2a shows the transmission spectrum normalized by the transmission in a Si wire waveguide of the same length. The device exhibits a flat transmission band at \(\lambda\) = 1550-1570 nm with a maximum transmission of \(-\)5 dB. This excess loss came from the two 1 \(\times\) 2 couplers in the MZ circuit (<0.5 dB in total), the propagation loss in the LSPCW (~3 dB), and the coupling loss between the LSPCW and Si wire (~1.5 dB in total). More than 60% of the LSPCW loss is due to the free-carrier absorption, which is expected to be reduced by optimizing the doping. Therefore, the excess loss is expected to fall by ~3 dB in future studies. Since the Si wire for the optical wiring had a 4 dB/cm propagation loss and a <0.1 dB loss at 90\({}^{\circ}\) bends, which corresponds to a 2 dB loss for ~4.5-mm-long wiring with six bends, the total on-chip loss of the device was 7 dB. Figure 2b shows the \(n_{\text{g}}\) spectrum measured by the modulation phase shift method.