Optically pumped Milliwatt Whispering-Gallery microcavity laser

Whispering-gallery-mode (WGM) microlasers, known for their high Q factors and small mode volumes, have shown significant advancements. While semiconductor WGM lasers readily achieve milliwatt outputs and compact designs, solid-state WGM lasers have been limited by low power (microwatts) and efficiency (<1%). This limitation stems from suboptimal gain media like rare-earth-doped lithium niobate and silica, which suffer from low emission/absorption cross-sections and poor thermal stability. Nd:YAG crystals, known for their excellent performance in bulk lasers, offer a promising alternative. However, the challenge lies in extracting a suitable membrane structure from the bulk crystal and achieving efficient pumping. Current methods, such as fiber-taper or prism coupling, result in low energy injection into the microcavity. This research aims to overcome these limitations by fabricating an ultrathin Nd:YAG film and employing a novel eccentric microcavity design for efficient free-space pump light coupling.

The paper reviews the existing literature on WGM microlasers, highlighting the advancements in semiconductor-based lasers, which have successfully achieved milliwatt-level output powers and compact integrated designs. In contrast, solid-state WGM lasers have lagged behind, typically producing only microwatts of power due to limitations in gain media and pumping techniques. The authors discuss the limitations of commonly used gain media, such as rare-earth-doped lithium niobate and silica, and introduce Nd:YAG as a superior alternative due to its well-established performance in bulk lasers. The challenges in pumping solid-state WGM lasers are addressed, emphasizing the limitations of current off-chip pumping methods.

The researchers fabricated a free-standing Nd:YAG thin film (1 µm thick) by using carbon ion implantation (6 MeV, 2 x 10¹⁵ ions cm^-2) at a 7° angle to enhance etching. Phosphoric acid etching (80%, 80°C, 12h) selectively removed the damaged layers, leaving behind a high-quality crystalline film. A mechanical transfer method using polydimethylsiloxane (PDMS) was employed to handle the delicate film. Focused ion beam (FIB) milling was used to create a 30 µm diameter microcavity. The transmission spectrum of the microcavity was measured to characterize its resonant modes. The Q factor was determined through Lorentzian fitting of the transmission peaks. Laser emission was achieved by continuous-wave (CW) pumping at 810 nm via fiber-taper coupling. The output power and spectral linewidth were measured as a function of pump power. An eccentric microcavity with a 4 µm air hole was designed to facilitate free-space pumping, and its performance was evaluated.

The fabricated Nd:YAG microcavity laser demonstrated a maximum output power of 1.12 mW at 1064.12 nm (λ₁) and 0.20 mW at 1062.82 nm (λ₂) with optical conversion efficiencies of 12.4% and 1.29%, respectively. The laser emission showed a redshift with increasing pump power, attributed to thermal effects. The Q factor of the microcavity was measured to be 1.08 × 10⁵ in the laser emission band and 2.8 × 10⁵ in the telecom wavelengths. The threshold pump power for λ₁ was 5 µW, and for λ₂ it was 13 µW. An eccentric microcavity design enabled efficient free-space pump light coupling, demonstrating an output power of 0.5 mW and an optical conversion efficiency of 6.18% when integrated with a waveguide. The linewidth of both lasers narrowed significantly from approximately 0.1 nm to 0.08 nm as the pump power was increased from 5µW to 13µW.

The results demonstrate a significant improvement in the performance of solid-state WGM lasers compared to previous reports. The achieved milliwatt-level output power and high optical conversion efficiency open up new possibilities for applications in compact photonic systems. The eccentric microcavity design offers a practical approach for efficient free-space pump light coupling, simplifying the integration of solid-state WGM lasers into on-chip photonic circuits. The observed redshift and linewidth variation with pump power highlight the importance of thermal management in optimizing the performance of these devices.

This work successfully demonstrated a milliwatt-level, optically pumped solid-state WGM microcavity laser using a novel Nd:YAG microcavity fabrication technique and an eccentric microcavity design for enhanced pumping efficiency. The high output power and optical conversion efficiency achieved represent a significant advance in the field, paving the way for compact and efficient on-chip light sources. Future research could focus on further optimizing the microcavity design, exploring different gain media, and investigating integrated electrical pumping schemes.

The study primarily focused on demonstrating the feasibility of high-power solid-state WGM lasers. Further research is needed to optimize the thermal management of the microcavity to improve stability at higher pump powers. The current pumping scheme uses fiber-taper coupling for high efficiency but might not be ideal for scalable integration. The long-term stability of the exfoliated Nd:YAG film under continuous operation also warrants further investigation.