Optically pumped Milliwatt Whispering-Gallery microcavity laser

Whispering-gallery-mode (WGM) microlasers offer high-Q factors and small mode volumes, enabling low lasing thresholds and sensitivity to environmental perturbations. Two main WGM laser categories exist: semiconductor (often electrically pumped) and solid-state (typically optically pumped). While semiconductor WGM lasers have reached milliwatt-level outputs and are amenable to integrated platforms, solid-state WGM lasers have generally been limited to only microwatt outputs and <1% optical conversion efficiencies, hampering practical deployment. A recent Ti:sapphire microring reached 0.5 mW output but with a high threshold (~6 mW) and low efficiency (~0.5%). A key performance bottleneck is the choice of gain medium: commonly used rare-earth-doped LiNbO3 and silica suffer from low emission/absorption cross-sections and thermal issues. Nd:YAG, by contrast, is a mature, robust solid-state laser medium supporting multiple wavelengths and, in its four-level Nd3+ configuration at ~1.06 µm, promises low thresholds suitable for on-chip lasers. Another challenge is efficient pumping. Co-integration of semiconductor pump lasers is desirable but difficult; conventional coupling via waveguides, fiber tapers, or prisms leads to inefficient energy injection into the WGM. This work addresses both challenges by fabricating an ultrathin crystalline Nd:YAG film from bulk via ion-implantation-enhanced etching, shaping it into a 30 µm WGM microcavity, and introducing an eccentric air hole to enable efficient free-space pump coupling and waveguide integration, thereby achieving milliwatt-level emission and markedly improved conversion efficiency.

Prior WGM microlasers have advanced rapidly, with semiconductor implementations achieving milliwatt outputs and strong on-chip integration leveraging high refractive indices and mature fabrication. Solid-state WGMs, though attractive for their three-/four-level systems and applications in non-Hermitian optics, communications, and biosensing, typically exhibit low output power (microwatts) and poor optical conversion efficiency (<1%). A notable exception is a Ti:sapphire microring with 0.5 mW output but high threshold (~6 mW) and low efficiency (~0.5%), underlining the need for better gain media and coupling strategies. Common solid-state WGM gain media like rare-earth-doped LiNbO3 and silica are limited by weak cross-sections and thermal stability. YAG crystals, widely successful in bulk solid-state lasers and dopable for emissions from ~1.03 to ~2.9 µm, are promising for on-chip WGMs; Nd:YAG, in particular, offers low thresholds around 1.06 µm. However, extracting membrane structures from doped YAG bulk crystals has hindered WGM applications. Pump coupling is another bottleneck: while electrical pumping via co-integrated semiconductor sources is ideal, it remains challenging; off-chip optical coupling via waveguides, tapers, or prisms is inefficient. These gaps motivate exploring Nd:YAG thin films and novel coupling schemes, such as eccentric microcavities for direct free-space pump injection.

Fabrication of Nd:YAG thin film and microcavity: A 10 × 10 × 1 mm3 Nd:YAG crystal (one optically polished facet) was implanted with C3+ ions at 6 MeV and fluence 2 × 10^15 ions cm^-2, with a 7° tilt to suppress channeling. The implanted facet was diced into 10 µm-deep grooves with 100 µm separation to expose the damaged layer. The sample was immersed in 80% phosphoric acid at 80 °C for 12 h to chemically etch the damaged regions and exfoliate a ~1 µm-thick crystalline film from the bulk. The exfoliated film was picked up via PDMS-based mechanical transfer. Surface roughness of the film was characterized by AFM: ~0.574 nm rms (top) and ~0.729 nm rms (bottom), ensuring minimal scattering losses. A 30 µm-diameter WGM microcavity was patterned into the free-standing ~1 µm film using focused ion beam (FIB) milling and then transferred to a pedestal using PDMS-assisted site-specific transfer. Ion-irradiation analysis: SRIM-2018 simulations quantified electronic (Se) and nuclear (Sn) energy loss versus depth, indicating a buried damage layer of ~900 nm thickness centered ~3.5 µm below the surface, with less damage in the intermediate region. Cross-sectional SAED/TEM confirmed amorphization near-surface and at the ion-stopping region, while the intermediate ~1 µm layer retained crystallinity and remained after acid etching. SAED/TEM of the exfoliated film verified preserved crystal structure. Optical characterization and mode analysis: Transmission spectra were measured in the Nd:YAG emission band (1060–1075 nm) with low probe power to avoid thermal effects, using a setup as in Fig. 3a. Resonant modes were identified via finite element simulations (COMSOL Multiphysics) to determine free spectral range (FSR) and mode families (TE/TM, labeled by azimuthal m and radial n). The lowest radial TE modes (TE147,01 and TE148,01) gave an FSR of 6.62 nm, matching the theoretical value (~6.626 nm). The TE147,01 mode profile and Q were extracted via Lorentzian fitting. To decouple Nd3+ absorption from intrinsic losses, telecom-band (1535–1558 nm) transmission was measured, and TE96,01 mode properties were simulated and fitted, yielding a higher Q reflective of intrinsic and scattering/radiative limits. Laser excitation and performance measurement: CW pumping at 810 nm was coupled via a fiber taper into the microcavity (taper–cavity coupling efficiency ~40%). Emission spectra near 1.06 µm were recorded versus coupled pump power Ppump. Thresholds, output powers, linewidths (FWHM), and thermal shifts were quantified for two lasing lines (λ1 = 1064.12 nm, TE147,01; λ2 = 1062.82 nm, TM132,03). Eccentric microcavity for free-space coupling: An air hole (~4 µm diameter) was introduced into the microcavity to form an eccentric cavity. The low-index hole acts as a concave lens, scattering/coupling incident pump light (810 nm) into higher-order radial WGMs (n > 1) while minimally perturbing the primary WGM confinement, enabling efficient free-space pump injection and facilitating coupling to an integrated waveguide.

• Successful fabrication of a crystalline Nd:YAG (~1 µm) membrane microcavity (30 µm diameter) via ion-implantation-assisted exfoliation and FIB shaping, preserving low surface roughness (~0.57/0.73 nm rms) and crystallinity (SAED/TEM confirmed).
• Resonant mode analysis: Measured FSR in the 1.06 µm emission band was 6.62 nm, consistent with theory (~6.626 nm). The TE147,01 mode exhibited Q ≈ 1.08 × 10^5. In the telecom band (outside Nd absorption), TE96,01 showed a higher Q ≈ 2.8 × 10^5, indicating that Nd-ion absorption limits Q near 1.06 µm.
• Pumping at 810 nm via fiber taper (≈40% coupling efficiency) produced dual-wavelength lasing at 1064.12 nm (λ1, TE147,01) and 1062.82 nm (λ2, TM132,03).
• Thresholds: λ1 threshold ≈ 5 µW; λ2 threshold ≈ 13 µW (coupled pump into cavity).
• Power scaling and saturation: Rapid output increase above threshold; onset of thermal effects around ~26 µW; gain saturation observed when Ppump > ~5.5 mW for both modes.
• Linewidth narrowing: FWHM reduced to ~0.08 nm near threshold, with fluctuations at higher pump due to thermal effects/gain saturation.
• Thermal redshift: λ1 and λ2 redshifted by ~0.70 nm and ~0.66 nm, respectively, as Ppump increased to 15 mW.
• Efficiencies and output powers: Maximum optical conversion efficiency up to 12.4% with maximum output power 1.12 mW (λ1); λ2 achieved 1.29% efficiency and 0.20 mW maximum output.
• Eccentric microcavity with a ~4 µm air hole enabled efficient free-space pump coupling and integration with a waveguide, achieving single-wavelength waveguide output of 0.5 mW with 6.18% optical conversion efficiency (as reported).

The study addresses two longstanding limitations of solid-state WGM microlasers: low output power/efficiency and inefficient pump coupling. By leveraging Nd:YAG—a high-gain, thermally robust crystal widely used in bulk lasers—and fabricating a high-quality ultrathin membrane cavity, the device achieves milliwatt-level output with markedly improved optical conversion efficiency (12.4%), surpassing typical solid-state WGM performance. The measured Q near 1.06 µm is limited by Nd3+ absorption, as evidenced by a higher Q at telecom wavelengths, indicating that intrinsic radiative/scattering losses are comparatively low thanks to the smooth sidewalls and low surface roughness. Dual-mode lasing behavior with low thresholds (5 µW and 13 µW) demonstrates effective confinement and gain. Thermal effects manifest as wavelength redshifts and performance saturation at higher pump powers, highlighting the importance of thermal management for further power scaling. The eccentric microcavity approach provides a practical route for direct free-space pump injection by using an internal low-index scatterer (air hole) to couple pump photons into higher-order radial WGMs, reducing dependence on delicate taper or prism coupling. This design also facilitates on-chip waveguide integration, demonstrated by single-wavelength emission with 0.5 mW output and 6.18% efficiency, underscoring the potential of Nd:YAG WGM microlasers as compact, high-power on-chip light sources for photonic integrated circuits and sensing applications.

This work demonstrates a solid-state Nd:YAG WGM microcavity laser with milliwatt-level output and high optical conversion efficiency by combining ion-implantation-assisted film exfoliation, precision microcavity fabrication, and an eccentric microcavity design for efficient pump coupling. Key achievements include 1.12 mW output at ~1.06 µm with 12.4% efficiency, low thresholds, and high Q factors; and waveguide-integrated, single-wavelength emission of 0.5 mW with 6.18% efficiency enabled by free-space pump coupling via an internal air hole. These results significantly advance the performance of solid-state WGM microlasers and point to their viability as compact on-chip photonic sources. Future work could focus on improved thermal management for higher power scaling, further optimization of coupling geometries (hole size/placement), integration with on-chip pump sources, and extension to other rare-earth dopants in YAG to access diverse wavelengths.

• Thermal effects induce wavelength redshift and contribute to gain saturation and linewidth fluctuations at higher pump powers, limiting power scaling beyond ~5.5 mW coupled pump.
• The Q factor in the 1.06 µm band is constrained by Nd3+ absorption; while intrinsic and scattering losses are low, material absorption limits ultimate cavity performance at the lasing wavelength.
• Initial demonstrations rely on fiber-taper coupling (≈40% efficiency) for pumping; although the eccentric microcavity enables free-space coupling, comprehensive quantitative comparison of coupling efficiencies across methods is not fully detailed.
• Detailed thermal management strategies and long-term stability/reliability data are not reported in the provided text.