A monolithic microcavity laser with simultaneous upconversion and frequency-doubled lasing via crystal-in-glass engineering

This work demonstrates a novel crystal-in-glass composite structure for multifunctional micro-nano light sources in integrated photonics. Based on a Er³⁺/Yb³⁺-codoped glass-ceramic (GC) whispering gallery mode (WGM) microcavity incorporating Ba₂TiGe₂O₈ (BTG) crystals, this microcavity enables dual-mode responses combining upconversion (UC) and frequency-doubled lasing. Made from a low-phonon-energy germanate glass matrix codoped with Er³⁺/Yb³⁺ for UC gain, the microcavity is crystallized to form BTG microcrystals for second harmonic generation (SHG). By leveraging the high-quality factor (Q ≈ 5.7 × 10⁴) and small mode volume, we achieve green (550 nm) and red (660 nm) UC lasing in a 30-μm-diameter microcavity with low thresholds of 13.31 μW and 12.97 μW, respectively. Benefitted from the random quasi-phase-matching (RQPM) mechanism in BTG GC, the microcavity also demonstrates an ultrabroadband frequency-doubling response from 900 to 1200 nm. By combining tapered fiber near-field coupling and femtosecond free-space pumping, we achieve simultaneous output of green/red UC lasing and frequency-doubled lasing within a single microcavity. We believe this work offers insights into hybrid material design and cooperative optical field manipulation for tunable lasers and on-chip nonlinear photonic systems.