"The ever-increasing demand for bandwidth density and internet protocol (IP) traffic causes a bottleneck in inter and intra-datacenter communication systems. This imposes a challenge in bandwidth density, communication speed, and power consumption. As a promising solution, complementary metal oxide semiconductor (CMOS) compatible silicon-on-insulator (SOI) technology offers low absorption loss, high index contrast between core and surrounding oxide, and high bandwidth data transmission capability which improves the performance of data center interconnects in terms of energy consumption per bit transfer. The bandwidth density can be further increased by employing multiplexing techniques such as polarization division multiplexing (PDM), wavelength division multiplexing (WDM), and mode division multiplexing (MDM). Recently, MDM has received significant attention as an enabling technology to send multiple data streams over a channel using a single wavelength. On the other hand, subwavelength grating (SWG) structures in SOI technology are opening numerous opportunities to contribute to the design of more complex photonic integrated circuits. SWG structures enable us to control the refractive index, birefringence, and dispersion of optical materials such as silicon in SOI technology. This helps us to design and optimize silicon photonics (SiPh) structures for specific requirements of applications. In some fabrication processes, the SiO2 cladding may not fully fill the small gap between the fine features of SWG structures which results in buried air gaps (voids) in the cladding of the structure. We investigate the voids' impact on several common SiPh structures including SWGs by simulation and experiment. Considering Applied NanoTools (ANT) center as the accessible fabrication facility, we characterize the ANT process voids using the focused ion beam-helium ion microscopy (FIB-HIM) methodology. We find out that the voids' cross-section area in this process is less than 500 nm2 which is very low compared to other reported studies. Investigating the impact of such small voids verified by both simulation and experiment, demonstrate that ANT voids do not have a significant impact on the performance of our designs. We then proceed with fabricating our SWG designs at the ANT center. As an essential part of multimode switches, we design and characterize a multimode compact and broadband SWG power splitter. The use of SWG structures is not only reducing the overall length of the power splitter by 66% but also increases the bandwidth from 40 nm to 100 nm compared to conventional structures. The total experimental insertion loss in the experiment is less than 0.1 dB and 0.65 dB for TE0 and TE1, respectively, and the intermodal crosstalk is measured to be less than -17 dB for both input modes. Next, we utilize the designed SWG-based power splitter to design a dual-mode broadband compact switch (DBcS) capable of simultaneously transferring the first two transverse electric (TE) modes. The switch worst-case insertion loss is measured to be less than -1.3 dB. In addition, the crosstalk within the wavelength range from 1500 nm to 1600 nm is below -13 dB for both TE0 and TE1input modes. Open eye diagrams for 10 Gb/s non-return-to-zero (NRZ) data transmission for single-mode and multimode transmission demonstrates the capability of the switch in handling multiple data streams simultaneously using a single wavelength laser. The bit error rate (BER) power penalty tests are performed and an error-free BER of 10-12 for both single-mode and multimode transmission is demonstrated.We believe that such experimentally demonstrated multimode switch benefiting from a compact and broadband design combined with WDM techniques may have potential applications in high data rate MDM-WDM switches for optical interconnects"--