## 5 III-V-on-Silicon Devices for Optical Sensing Applications
### III-V-on-Silicon Broadband Light Sources for Optical Sensing Applications
#### 5.1.3 Supercontinuum Generation in a III-V on Silicon Waveguide Structure  
On-chip supercontinuum generation (SCG) has attracted a lot of attention from researchers because of a number of exciting potential applications such as sensing [62, 63], spectroscopy [64], telecommunications [65], optical clocks and frequency metrology [66]. On the SOI platform, by taking advantage of the large Kerr index of silicon (n\(\approx\) 6 \(\times\)10\({}^{-18}\) m\({}^{2}\)/W) and the tight confinement of light due to its large index contrast, SCG has been successfully demonstrated in different pump pulse width regimes and wavelength ranges [67, 68, 69, 70, 71]. However, silicon as a nonlinear material for SCG has certain drawbacks: its small bandgap of 1.1 eV means that it suffers from large nonlinear losses due to two-photon absorption (TPA) and the associated free-carrier absorption (FCA) when pumping at wavelengths below 2.2 \(\upmu\)m. While this effect is somewhat reduced by the use of hydrogenated amorphous silicon [68, 70], certain III-V materials like InGaP offer not only a similarly large Kerr index and linear refractive index as silicon but also have a large bandgap (E\({}_{g}\) = 1.9 eV) thereby eliminating TPA-based nonlinear losses for pumping in the telecom wavelength range. Unlike silicon, III-V materials also possess a strong second-order nonlinearity, which can be further exploited alongside the third-order nonlinearity. In order to generate a supercontinuum in the 1-2 \(\upmu\)m wavelength range we consider the use of InGaP membrane waveguides integrated on top of a SOI or SiN waveguide circuit. As a first step, the generation of a supercontinuum on an InGaP membrane waveguide not interfaced with a silicon-based PIC is demonstrated. In this case, the InGaP membrane--which is grown lattice matched on a GaAs substrate--is adhesively bonded to a silicon substrate with 3 \(\upmu\)m thick thermal oxide on top. The GaAs substrate is then removed by wet etching. Using e-beam lithography and ICP etching in a two-step process--in collaboration with the Laboratoire de Photonique et Nanostructure, Paris and Thales Research and Technology--photonic wire waveguides coupled to shallow etched fiber-to-chip grating couplers are fabricated. Details about the fabrication and the linear and nonlinear properties of the fabricated waveguides can be found in [72]. The large index contrast between the III-V membrane and the oxide or DVS-BCB layer beneath allows for strong nonlinear interactions through the tight confinement of light. For SCG, we pump dispersion engineered InGaP membrane waveguides with 170 fs pump pulses centered at 1550 nm [73]. In this pulse duration regime, SCG is known to occur via soliton fission and dispersive wave generation both of which depend critically on the dispersion properties of the waveguide [74, 75]. As such, the waveguide is designed to exhibit anomalous dispersion at the pump wavelength and have two zero-dispersion wavelengths (ZDWs) on either side of the pump. Figure 23a shows the supercontinuum spectrum at the output of InGaP membrane waveguides of different widths. Dispersive waves can be observed flanking the spectra on both ends of the spectrum and their positions vary for the different waveguides. The locations of the ZDWs relative to the pump wavelength (see Figure 23b) are critical in determining the position of these dispersive waves and thus the overall bandwidth of the supercontinuum. For the case of the 700 nm wide waveguide, the dispersive waves are located at 1005 nmand 2097 nm as shown in Figure 23c. The supercontinuum is thus octave-spanning which is important for certain frequency metrology applications. This clearly demonstrates the importance of dispersion engineering for achieving the widest possible supercontinuum. The octave-spanning spectrum was achieved with a pulse energy of only 2 pJ which constitutes an order of magnitude improvement over the state-of-the-art SCG achieved in SOI at these wavelengths [71].