The Molecular Foundry, Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States.
School of Chemistry and Chemical Engineering , Harbin Institute of Technology , Harbin 150001 , China.
ACS Nano. 2020 Feb 25;14(2):1508-1519. doi: 10.1021/acsnano.9b06102. Epub 2020 Feb 13.
Micron-sized lasers fabricated from upconverting nanoparticles (UCNP) coupled to whispering gallery mode (WGM) microresonators can exhibit continuous-wave anti-Stokes lasing useful for tracking cells, environmental sensing, and coherent stimulation of biological activity. The integration of these microlasers into organisms and microelectronics requires even smaller diameters, however, which raises threshold pump powers beyond practical limits for biological applications. To meet the need for low lasing thresholds and high fidelity fabrication methods, we use correlative optical and electron microscopy to uncover the nanoparticle assembly process and structural factors that determine efficient upconverted lasing. We show that 5 μm microspheres with controlled submonolayer UCNP coatings exhibit, on average, 25-fold lower laser thresholds (1.7 ± 0.7 kW/cm) compared to the mean values of the lowest threshold UCNP lasers, and variability is reduced 30-fold. WGMs are observed in the upconversion spectra for TiO-coated microspheres as small as 3 μm, a size at which optical losses had previously prevented such observations. Finally, we demonstrate that the WGM signatures of these upconverting microlasers can be imaged and distinguished through tissue-mimicking phantoms. These advances will enable the fabrication of more efficient upconverting lasers for imaging, sensing, and actuation in optically complex environments.
基于上转换纳米粒子（UCNP）与回音壁模式（WGM）微谐振器结合制成的微尺寸激光器可以实现连续波反斯托克斯激光，这种激光在细胞追踪、环境传感和生物活性相干刺激等方面具有广泛的应用。然而，为了将这些微激光器集成到生物体和微电子系统中，需要更小的直径，这就导致了泵浦功率的阈值升高，超出了生物应用的实际限制。为了满足低激光阈值和高保真度制造方法的需求，我们使用相关的光学和电子显微镜来揭示纳米粒子组装过程以及决定高效上转换激光的结构因素。我们发现，与最低阈值上转换激光的平均值相比，具有受控亚单层 UCNP 涂层的 5μm 微球的激光阈值平均降低了 25 倍（1.7±0.7kW/cm），并且可变性降低了 30 倍。在 TiO 涂层的微球中可以观察到上转换光谱中的 WGM，这些微球的尺寸小至 3μm，而在之前，由于光学损耗，这种尺寸无法观察到 WGM。最后，我们证明了这些上转换微激光器的 WGM 特征可以通过组织模拟体模进行成像和区分。这些进展将使更高效的上转换激光器的制造成为可能，从而在光学复杂环境中实现成像、传感和驱动功能。
