Mach-Zehnder (MZ) modulators in Silicon-on-Insulator (SOI) are key components for integrated silicon photonic devices. Reducing their energy dissipation is a crucial step for applications of silicon photonics, especially in large data centers. In this work, we combine band-edge slow light structures consisting of silicon waveguide gratings with a periodic (interleaved) p-n junction. The slow-light structures consist of a waveguide grating with wide/narrow sections realized in a 300-nm thick silicon layer, on top of an unetched silicon layer of 50 to 150 nm thickness, fully embedded in SiO2. The grating gives rise to a photonic stop band and to a slow-light region close to the lowest band edge. The profile of the p-n junction varies periodically along the waveguide with interleaved n and p regions. This structure maximizes the spatial overlap between the optical mode and the depletion regions, yielding a further improvement of modulator efficiency beyond the slow-light effect.
Under an applied reverse bias, the silicon refractive index is modified by the plasma dispersion effect, thus the waveguide grating acts as a phase shifter. The modulator efficiency VpiLpi is strongly improved in comparison with modulators without slow light or with a lateral p-n junction. Thanks to the optimized overlap between electric field and depletion regions, this improvement takes place over a spectral interval that is much larger than the slow-light bandwidth. Insertion losses due to free carriers are also lower than in conventional modulators. The advantage of combining slow-light grating waveguides with an interleaved p-n junction is especially pronounced at low driving voltage (of the order of 1V), where the dissipated energy can be as low as 0.4 pJ/bit over an optical bandwidth of several 10 nm. Thus, the present modulator structure is promising in view of realizing integrated MZ modulators with low power dissipation.
Raman spectroscopy (RS) is a non-destructive analytical technique, that provides a unique fingerprint of molecules with high accuracy. It proves to be a reliable and practical alternative to chemical analysis, allowing sample identification without the use of reagents. This label-free technique finds applications in quality control and in-line process monitoring, however, like any other technique RS also presents its challenges such as expensive and delicate instrumentation and complex design, which often confines the technique to the laboratory. In order to address these challenges, a 3D printed Lab-On-Chip (LOC) was fabricated and assembled with four channel optical fibres, which will collect the Raman scattering. The performance of our Raman Probe on Chip is evaluated using Isopropanol alcohol (IPA) as a validation sample.
Slow light photonic crystal waveguides (PCWs) have been the subject of intensive study due to their potential for on-chip applications such as optical buffers and the enhancement of nonlinear phenomenon. However, due to high group velocity mismatch between the strip waveguide and the slow light waveguide efficient coupling of light is challenging. The coupling efficiency is also very sensitive to the truncation at the interface between the two waveguides. This sensitivity can be removed and light can efficiently be coupled from the strip waveguide to the slow light waveguide by adding an intermediate photonic crystal waveguide (or coupler) that operates
at a group index of ∼ 5. Several designs have been proposed for couplers to obtain higher coupling efficiency
within the desired group index range. We have studied uniaxial stretched couplers in which the lattice constant
is stretched in the direction of propagation by 10-50 nm in the coupler region. Using a Finite Difference Time Domain (FDTD) Simulation Method that allows the extraction of the group index, we have observed 8.5 dB improvement in the coupling efficiency at the group index of 30. Efficient coupling is dominantly determined by the band edge position of the coupler region and maximum transmission efficiency is limited by the maximum transmission of the coupler PCW. If the band edge of coupler PCW is sufficiently red shifted relative to the band edge of the slow light PCW then higher coupling efficiency can be achieved.