Sub-megahertz homogeneous linewidth for Er in Si via in situ single photon detection

Silicon is an attractive material for photonics applications due to its large dielectric constant that enables compact devices at telecom wavelengths and the synergies with CMOS fabrication. However, due to the indirect bandgap Si is not ideal for emitters. This can be mitigated by the use of sites that possess optical transitions that are not linked to the silicon band structure. Here, we present the optical properties of a resonantly excited Er ensemble in Si accessed via in situ single photon detection. A novel approach which avoids nano-fabrication on the sample is introduced, resulting in a highly efficient detection of 70 excitation frequencies, of which 63 resonances have not been observed in literature. We observed inhomogeneous broadening of less than 50 MHz and an upper bound on the homogeneous linewidth to below 1MHz, which is a reduction of more than an order of magnitude observed to date. In addition, two color excitation in magnetic field allowed us to assign crystal field levels and extract spin lifetimes. These narrow optical transition properties with long spin lifetimes show that Er in Si is an excellent candidate for future quantum information and communication applications.