Light absorption is a common phenomenon in nature, but accurate and quantitative absorption measurement at the nanoscale remains challenging especially in the application of widefield imaging. Here, we demonstrated optical widefield interferometric photothermal microscopy that allowed us to visualize and quantify the heat generation of single nanoparticles. The working principle was to measure the scattering signal due to the refractive index change of the surrounding media induced by the dissipated heat (known as the thermal lens effect). The sensitivity of our local heat measurement was a few nanowatts—the high sensitivity made it possible to detect single gold nanoparticles, as small as 5 nm. By changing the particle sizes, we found that, for small metallic nanoparticles (gold and silver nanoparticles < 40 nm), the photothermal signal was determined by the amount of the dissipated heat, independent of the particle size. A model was established to explain our experimental results, indicating that the photothermal signal was essentially contributed by the interferometric detection of the scattered field of the thermal lens. Importantly, on the basis of this model, we further investigated the photothermal signal of large nanoparticles (40–100 nm for our setup) where the scattered light of the particle was considerable relative to the probe light. In this regime, the strong scattered field of the particle effectively served as the main reference beam that interfered with the scattered field of the thermal lens, resulting in an enhanced photothermal signal. Our work illustrates an important fact that the measured photothermal signal is fundamentally affected by the scattering property of the sample. This finding paves the way to accurate and sensitive absorption-based imaging in complex biological samples where the scattering is often spatially heterogeneous.

中文翻译：

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宽场干涉光热显微镜对单个吸光纳米颗粒的定量成像

光吸收是自然界中的常见现象，但是在纳米级进行准确和定量的吸收测量仍然具有挑战性，尤其是在宽视场成像的应用中。在这里，我们展示了光学宽视野干涉光热显微镜，使我们能够可视化和量化单个纳米颗粒的发热。工作原理是测量由于散热引起的周围介质的折射率变化（称为热透镜效应）而引起的散射信号。我们本地热量测量的灵敏度为几纳瓦-高灵敏度使得检测5 nm以下的单个金纳米颗粒成为可能。通过更改粒径，我们发现，对于小的金属纳米粒子（金和银纳米粒子<40 nm），光热信号由散发的热量决定，与颗粒大小无关。建立了一个模型来解释我们的实验结果，表明光热信号主要是通过干涉检测热透镜散射场而产生的。重要的是，在此模型的基础上，我们进一步研究了大型纳米粒子（对于我们的设置为40–100 nm）的光热信号，其中粒子的散射光相对于探测光而言相当大。在这种情况下，粒子的强散射场有效地充当了主要参考光束，干扰了热透镜的散射场，从而增强了光热信号。我们的工作表明了一个重要的事实，即所测量的光热信号从根本上受样品的散射特性影响。这一发现为复杂的生物样本中的散射通常在空间上是异质性的精确和敏感的基于吸收的成像铺平了道路。