Quantitative Absorption Imaging of Single Nanoparticles by Widefield Interferometric Photothermal Microscopy

Yu-Chien Huang^1,2*, Te-Hsin Chen¹, Jz-Yuan Juo¹, Shi-Wei Chu², Chia-Lung Hsieh¹

¹Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, Taipei, Taiwan
²Department of Physics, National Taiwan University, Taipei, Taiwan

* Presenter:Yu-Chien Huang, email:r08222030@ntu.edu.tw

Light absorption and scattering are fundamental light-matter interactions providing the intrinsic contrast mechanisms for microscope imaging without labeling. The absorption and scattering of light are fundamentally connected, and thus it is challenging to measure individual of them at high accuracy especially for the absorption measurements at the nanoscales. Photothermal microscopy is a powerful technique to map the absorption property of a sample at high sensitivity [1]. However, the photothermal measurement involves phase-sensitive detection of the complex scattering field, making it nontrivial to interpret the photothermal signal in a quantitative manner. Here, we demonstrate widefield interferometric photothermal microscopy to quantify the local heat generation from a nanoparticle by measuring the difference of scattering due to a local change of the refractive index, known as the thermal lens effect [2]. The sensitivity of our system is sufficient for visualizing very small single gold nanoparticles, as small as 5 nm. Our data shows that, for small particles (< 40 nm), the photothermal signal is contributed by the scattered field of the thermal lens, originating from the dissipated heat of the nanoparticles. We further study the photothermal signal of the large particles (40-100 nm), where the scattered field of the nanoparticle cannot be neglected when compared to the probe reference beam. We find that, in this regime, the photothermal signal is enhanced via the interference between the strong scattered field of the nanoparticle and that of the thermal lens. A model is established to explain the enhanced photothermal signal of large particles. Our work shows that the photothermal signal is closely related to scattering, and therefore accurate quantification of light absorption by the photothermal signal requires the scattering data of the sample.

Reference:
[1] A. Gaiduk, M. Yorulmaz, P. V. Ruijgrok, M. Orrit, Room-Temperature Detection of a Single Molecule’s Absorption by Photothermal Contrast, Science, 330, 353-356 (2010).
[2] Markus Selmke, Marco Braun, and Frank Cichos, Photothermal Single-Particle Microscopy: Detection of a Nanolens, ACS Nano, 6, 2741−2749 (2012).

Keywords: Photothermal Microscopy, Coherent Bright Field Microscopy (COBRI), Scattering, Absorption, Nanoparticles