Reducing the size of lasers to microscale dimensions enables new technologies¹ that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors² to live brain tissue³. However, reduced cavity sizes increase optical losses and require greater input powers to reach lasing thresholds. Multiphoton-pumped lasers^4,5,6,7 that have been miniaturized using nanomaterials such as lanthanide-doped upconverting nanoparticles (UCNPs)⁸ as lasing media require high pump intensities to achieve ultraviolet and visible emission and therefore operate under pulsed excitation schemes. Here, we make use of the recently described energy-looping excitation mechanism in Tm³⁺-doped UCNPs⁹ to achieve continuous-wave upconverted lasing action in stand-alone microcavities at excitation fluences as low as 14 kW cm⁻². Continuous-wave lasing is uninterrupted, maximizing signal and enabling modulation of optical interactions¹⁰. By coupling energy-looping nanoparticles to whispering-gallery modes of polystyrene microspheres, we induce stable lasing for more than 5 h at blue and near-infrared wavelengths simultaneously. These microcavities are excited in the biologically transmissive second near-infrared (NIR-II) window and are small enough to be embedded in organisms, tissues or devices. The ability to produce continuous-wave lasing in microcavities immersed in blood serum highlights practical applications of these microscale lasers for sensing and illumination in complex biological environments.

将激光器的尺寸减小到微米级可以实现专门为在从超高速微处理器²到活脑组织^3的狭窄空间中操作而量身定制的新技术¹。但是，减小的腔体尺寸会增加光学损耗，并需要更大的输入功率才能达到激光阈值。使用诸如镧系元素掺杂的上转换纳米粒子（UCNPs）⁸等纳米材料作为激光介质已被小型化的多光子泵浦激光器^4,5,6,7需要高的泵浦强度来实现紫外线和可见光发射，因此在脉冲激发方案下运行。在这里，我们利用最近描述的Tm ^3+中的能量循环激励机制掺杂的UCNP ⁹可以在低至14 kW cm ^-2的激发通量下在独立的微腔中实现连续波上转换的激光作用。连续波激光不间断，可最大化信号并实现光学相互作用的调制¹⁰。通过将能量循环纳米粒子与聚苯乙烯微球的耳语画廊模式耦合，我们可以在蓝色和近红外波长下同时诱导超过5小时的稳定激光发射。这些微腔在生物透射第二近红外（NIR-II）窗口中被激发，并且足够小以嵌入生物体，组织或设备中。在浸没在血清中的微腔中产生连续波激光的能力凸显了这些微型激光在实际生物环境中传感和照明的实际应用。