Material optimization of Er3+:Y 2 SiO 5 at 1.5 μm for optical processing, memory, and laser frequency stabilization applications

Abstract

Spatial-spectral holography using spectral hole burning materials is a powerful technique for performing real-time, wide-bandwidth information storage and signal processing. For operation in the important 1.5 μm communication band, the material Er³⁺:Y₂SiO₅ enables applications such as laser frequency stabilization, all-optical correlators, analog signal processing, and data storage. Site-selective absorption and emission spectroscopy identified spectral hole burning transitions and excited state T₁ lifetimes in the 1.5 μm spectral region. The effects of crystal temperature, Er³⁺-dopant concentration, magnetic field strength, and crystal orientation on spectral diffusion were explored using stimulated photon echo spectroscopy, which is the "prototype" interaction mechanism for device applications. The performance of Er³⁺:Y₂SiO₅ and related Er³⁺ materials has been dramatically enhanced by reducing the effect of spectral diffusion on the coherence lifetime T₂ through fundamental material design coupled with the application of an external magnetic field oriented along specific directions. A preferred magnetic field orientation that maximized T₂ by minimizing the effects of spectral diffusion was determined using the results of angle-dependent Zeeman spectroscopy. The observed linewidth broadening due to spectral diffusion was successfully modeled by considering the effect of one-phonon (direct) processes on Er³⁺ - Er³⁺ interactions. The reported studies improved our understanding of Er³⁺ materials, explored the range of conditions and material parameters required to optimize performance for specific applications, and enabled measurement of the narrowest optical resonance ever observed in a solid-with a homogeneous linewidth of 73 Hz. With the optimized materials and operating conditions, photon echoes were observed up to temperatures of 5 K, enabling 0.5 GHz bandwidth optical signal processing at 4.2 K and providing the possibility for operation with a closed-cycle cryocooler.