利用寬場干涉光熱顯微術定量單一奈米粒子吸收影像

Quantitative Absorption Imaging of Single Nanoparticles by Widefield Interferometric Photothermal Microscopy

黃郁茜(Yu-Chien Huang)

指導教授 ： 朱士維

共同指導教授 ： 謝佳龍(Chia-Lung Hsieh)

國立臺灣大學/理學院/物理學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202101282

摘要

吸收是光和物質基本的交互作用之一，藉由量測物質的吸收特性，我們便能區分不同材料組成及結構。定量奈米物體的吸收通常是仰賴比爾-朗伯定律：量測光在通過大量奈米物體後的衰減量。雖然這個方法相當直接有效，但它量測的結果是眾多奈米物體吸收的平均，缺乏單一奈米粒子的解析度，也因此無法得知個別奈米粒子之間的不同。量測單一奈米粒子的吸收十分困難，因為奈米粒子的吸收截面積比可見光的繞射極限小了數個數量級，因此很難在奈米尺度下直接量測吸收。
光熱顯微術(photothermal microscopy)是一種能夠定量量測奈米材料吸收的技術，利用泵探針(pump-probe)的原理來量測因光吸收而造成的散射變化。當加熱泵束(pump beam)被樣品吸收後，樣品釋放出的熱改變周圍介質的折射率，即為熱透鏡效應(thermal lens effect)，再利用探測光束(probe beam)去偵測因熱透鏡生成而造成的光散射變化。然而，量測奈米材料的微小散射場變化通常必須利用對相位高度敏感的干涉技術來達成，這使得定量闡釋光熱訊號十分不容易。
在這篇論文中，我們利用寬場干涉光熱顯微術量測不同大小的金屬奈米粒子(金、銀奈米粒子)的光熱訊號。實驗數據顯示：對於很小的奈米粒子(在我們系統為小於40奈米的粒子)而言，光熱訊號是由熱透鏡的散射場所貢獻，也就是源自於奈米粒子逸散出的熱，且和加熱泵束的光強及樣品的吸收截面積成正比，與粒子的尺寸和材料無關。我們利用影像差頻鎖相偵測法(image-based difference-frequency lock-in detection)來提升信號雜訊比，能夠在寬場照明下量測到5奈米金粒子的吸收。此外，我們建立了一個模型來解釋實驗數據，證實光熱訊號的本質是熱透鏡所導致的散射場透過干涉影像偵測所造成的結果。我們更進一步研究大粒子(40奈米至100奈米)的光熱訊號，在這個區間，奈米粒子的散射場已經無法被忽視且與探測光束光強相當，此時，熱透鏡的散射場會與奈米粒子的散射場互相干涉，造成光熱訊號的放大。這個事實指出光熱訊號不僅只和樣品吸光後所逸散出的熱有關，樣品與環境的散射特性也會影響光熱訊號，所以若要定量量測奈米尺度的吸收，樣品本身的散射性質必須同時被考慮進光熱量測模型中。這個研究成果將有助於未來在不均勻環境的複雜樣品中，進行定量且準確的吸收量測研究。

關鍵字

光熱顯微術 ； 干涉散射顯微術 ； 同調式明場顯微術 ； 光散射 ； 光吸收 ； 奈米粒子

並列摘要

Absorption is one of the most fundamental light-matter interactions, serving as an important mechanism to characterize new materials and novel nanostructures. Quantification of light absorption of a nano-sized object is typically done with a bulk sample based on Beer-Lambert law, which measures the attenuation of the light by many nanoparticles. Although this method is straightforward and effective, it provides the ensemble results without the resolution of a single nanoparticle and thus it cannot detect heterogeneity between individual nanoparticles. Measuring the absorption directly at the nanoscale is challenging due to the mismatch between the absorption cross section of the nanomaterial and the diffraction limit of visible light.
Utilizing the pump-probe technique to measure the change of scattering due to light absorption, photothermal microscopy is a promising approach to quantifying absorption of nano-sized materials with the resolution of a single nanoparticle. When a heating pump beam is absorbed by a nanoparticle, it generates heat that changes the refractive index locally, known as the thermal lens effect. Then, a probe beam is delivered to the nanoparticle to detect the difference of the light scattering induced by the created thermal lens. As the photothermal measurement involves phase-sensitive interferometric detection of the superposition of complex scattering fields, it is nontrivial to interpret the photothermal signal for quantification of light absorption of the nanoparticle.
In this work, we demonstrate widefield interferometric photothermal microscopy and measure the photothermal signal of metallic nanoparticles (gold and silver) of various sizes. The data show that, for small particles (< 40 nm in our case), the photothermal signal is contributed by the scattered field of the thermal lens, originating from the dissipated heat of the nanoparticles. The signal-to-noise ratio (SNR) is improved by the image-based difference-frequency lock-in detection, allowing us to visualize the absorption of individual 5 nm gold nanoparticles under the widefield illumination. The photothermal contrast scales linearly with the heating beam intensity and the absorption cross section of the nanoparticles, independent of the physical size and materials. A model is established to explain our experimental results, showing that the photothermal signal essentially represents the interferometric detection of the scattered field of the thermal lens. We further examine the photothermal signal of relatively large nanoparticles (40-100 nm), where the scattered intensity of the nanoparticle cannot be neglected when compared to the probe beam intensity. In this regime, the thermal lens interferes with the strong scattered field of the nanoparticle, leading to an enhanced photothermal signal. We conclude that the photothermal signal is not only determined by the amount of heat dissipated from the absorbers, but also affected by the scattering property of the sample. This work paves the way to accurate quantification of the absorption of the complex sample in the heterogeneous environment through photothermal measurement.

Part of the work of this thesis has been published in:
Huang, Y.-C., Chen, T.-H., Juo, J.-Y., Chu, S.-W., Hsieh, C.-L. (2021). Quantitative imaging of single light-absorbing nanoparticles by widefield interferometric photothermal microscopy. ACS Photonics, 8(2), 592-602.

並列關鍵字

photothermal microscopy ； interferometric scattering microscopy ； coherent brightfield microscopy ； scattering ； absorption ； nanoparticles

參考文獻

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