Stimulated emission (StE) remains relatively unused as an image-forming signal despite having potential advantages over fluorescence in speed, coherence, and ultimately resolution. Several ideas for the radiation pattern and directionality of StE remain prevalent, namely, whether a single molecule would radiate StE itself in a pure dipole pattern, or whether its emission direction depends on the driving field. Previous StE imaging has been carried out in transmission, which would collect signal either way. Here, we introduce the StE driving field (the probe) at an angle, using total internal reflection to avoid incident probe light and its specular reflections in our detection path. In this non-collinear detection configuration that also collects some fluorescence from the sample, we observe fluorescence depletion even in the spectral window where an increase in detected signal from StE would be expected if StE radiated like a simple classical dipole. Because simultaneous direct measurement of the fluorescence represents a calibration of the potential size of StE were it spatially patterned like a classical dipole emitter, our study clarifies a critical characteristic of StE for optimal microscope design, optical cooling, and applications using small arrays of emitters.

中文翻译：

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受激发射不会以纯偶极子模式辐射

尽管受激发射 (StE) 在速度、相干性和最终分辨率方面比荧光具有潜在优势，但它作为成像信号仍然相对未使用。关于 StE 的辐射模式和方向性的几个想法仍然很流行，即单个分子是否会以纯偶极模式辐射 StE 本身，或者其发射方向是否取决于驱动场。以前的 StE 成像是在传输中进行的，无论哪种方式都会收集信号。在这里，我们以一定角度引入 StE 驱动场（探头），使用全内反射来避免入射探头光及其在检测路径中的镜面反射。在这种还从样品中收集一些荧光的非共线检测配置中，我们甚至在光谱窗口中也观察到荧光损耗，如果 StE 像简单的经典偶极子一样辐射，则预计来自 StE 的检测到的信号会增加。因为同时直接测量荧光代表了对 StE 的潜在尺寸的校准，如果它的空间图案像经典的偶极发射器一样，我们的研究阐明了 StE 的关键特性，以实现最佳显微镜设计、光学冷却和使用小型发射器阵列的应用。