Single crystal erbium compound nanowires as high gain material for on-chip light source applications

Integrated photonics requires high gain optical materials in the telecom wavelength range for optical amplifiers and coherent light sources. Erbium (Er) containing materials are ideal candidates due to the 1.5 μm emission from Er³⁺ ions. However, the Er density in typical Er-doped materials is less than 10²⁰ cm^–3, thus limiting the maximum optical gain to a few dB/cm, too small to be useful for integrated photonic applications. Er compounds could potentially solve this problem since they contain much higher Er³⁺ density. So far the existing Er compounds suffer from short lifetime and strong upconversion effects, mainly due to poor crystal qualities. Recently, we explore a new Er compound: erbium chloride silicate (ECS, Er₃(SiO₄)₂Cl) in the form of nanowire, which facilitates the growth of high quality single crystal with relatively large Er³⁺ density (1.62 × 10²² cm^–3). Previous optical results show that the high crystal quality of ECS material leads to a long lifetime up to 1 ms. The Er lifetime-density product was found to be the largest among all the Er containing materials. Pump-probe experiments demonstrated a 644 dB/cm signal enhancement and 30 dB/cm net gain per unit length from a single ECS wire. As a result, such high-gain ECS nanowires can be potentially fabricated into ultra-compact lasers. Even though a single ECS nanowire naturally serves as good waveguide, additional feedback mechanism is needed to form an ultra-compact laser. In this work, we demonstrate the direct fabrication of 1D photonic crystal (PhC) air hole array structure on a single ECS nanowire using focused ion beam (FIB). Transmission measurement shows polarization-dependent stop-band behavior. For transverse electric (TE) polarization, we observed stop-band suppression as much as 12 dB with a 9 μm long airholed structure. Through numerical simulation, we showed that Q-factor as high as 11000 can be achieved at 1.53 μm for a 1D PhC micro-cavity on an ECS nanowire. Such a high Q cavity combined with the high material gain of ECS nanowires provides an attractive solution for ultra-compact lasers, an important goal of this research.