Importance of the volume fraction and particle density of Ag nanoparticles to the coupling process of relief-type Ag/SiO2 nanocomposite grating coupler

600-nm-periodic relief-type SiO2 grating coupler and Ag/SiO2 nanocomposite grating coupler are fabricated by separately etching the SiO2 waveguide slabs before and after low-energy Ag ion implantation, and their vertical input coupling efficiencies (VICEs) as well as that of Ag/SiO2 nanocomposite grating coupler after 600 degrees C anneal in flowing nitrogen are investigated by means of the transmission spectroscopy. Our results clearly demonstrate that owing to the introduction of Ag nanoparticles (NPs), the Ag/SiO2 nanocomposite grating coupler is more efficient to couple the incident light beams in a wavelength range of 620-880 nm into the SiO2 waveguide layer than the SiO2 grating coupler. Especially, our results also demonstrate that 600 degrees C anneal can bring about a decrease in VICE of Ag/SiO2 nanocomposite grating coupler. Further discussions reveal that the anneal-induced decrease in VICE of Ag/SiO2 nanocomposite grating coupler is due to the decreased refractivity of Ag/SiO2 nanocomposite teeth in a wavelength range of 620-880 nm because of the reduction in volume fraction of Ag NPs and the lowered reflectivity of Ag/SiO2 nanocomposite teeth in the same wavelength range because of the decline in particle density of Ag NPs. The above findings substantially indicate the particular importance of the volume fraction and particle density of Ag NPs to the coupling process. In our opinion, the volume fraction and particle density of Ag NPs are probably improved by suppressing the sputtering loss of Ag implants, and thereby, we can have a chance to fabricate a thin but efficient Ag/SiO2 coupler in the future, which might be useful for the development of large-scale photonic integrated circuits.