Photothermal optical microscopy

Photothermal optical microscopy / "photothermal single particle microscopy" is a technique that is based on detection of non-fluorescent labels. It relies on absorption properties of labels (gold nanoparticles, semiconductor nanocrystals, etc.), and can be realized on a conventional microscope using a resonant modulated heating beam, non-resonant probe beam and lock-in detection of photothermal signals from a single nanoparticle. It is the extension of the macroscopic photothermal spectroscopy to the nanoscopic domain. The high sensitivity and selectivity of photothermal microscopy allows even the detection of single molecules by their absorption. Similar to Fluorescence Correlation Spectroscopy (FCS), the photothermal signal may be recorded with respect to time to study the diffusion and advection characteristics of absorbing nanoparticles in a solution. This technique is called photothermal correlation spectroscopy (PhoCS).

Forward detection scheme

In this detection scheme a conventional scanning sample or laser-scanning transmission microscope is employed. Both, the heating and the probing laser beam are coaxially aligned and superimposed using a dichroic mirror. Both beams are focused onto a sample, typically via a high-NA illumination microscope objective, and recollected using a detection microscope objective. The thereby collimated transmitted beam is then imaged onto a photodiode after filtering out the heating beam. The photothermal signal is then the change $\Delta$ in the transmitted probe beam power $P_{d}$ due to the heating laser. To increase the signal-to-noise ratio a lock-in technique may be used. To this end, the heating laser beam is modulated at a high frequency of the order of MHz and the detected probe beam power is then demodulated on the same frequency. For quantitative measurements, the photothermal signal may be normalized to the background detected power $P_{d,0}$ (which is typically much larger than the change $\Delta P_{d}$), thereby defining the relative photothermal signal $\Phi$

$\Phi ={\frac {\Delta P_{d}}{P_{d,0}}}={\frac {P_{d}\left({\text{heating beam on}}\right)-P_{d}\left({\text{heating beam off}}\right)}{P_{d}\left({\text{background, no particle}}\right)}}$

Detection mechanism

The physical basis for the photothermal signal in the transmission detection scheme is the lensing action of the refractive index profile that is created upon the absorption of the heating laser power by the nanoparticle. The signal is homodyne in the sense that a steady state difference signal accounts for the mechanism and the forward scattered field's self-interference with the transmitted beam corresponds to an energy redistribution as expected for a simple lens. The lens is a Gadient Refractive INdex (GRIN) particle determined by the 1/r refractive index profile established due to the point-source temperature profile around the nanoparticle. For a nanoparticle of radius $R$ embedded in a homogeneous medium of refractive index $n_{0}$ with a thermorefractive coefficient $\mathrm {d} n/\mathrm {d} T$ the refractive index profile reads:

$n\left(\mathbf {r} \right)=n_{0}+{\frac {\mathrm {d} n}{\mathrm {d} T}}\Delta T\left(\mathbf {r} \right)=n_{0}+\Delta n{\frac {R}{r}}$

in which the contrast of the thermal lens is determined by the nanoparticle absorption cross-section $\sigma _{\rm {abs}}$ at the heating beam wavelength, the heating beam intensity $I_{h}$ at the point of the particle and the embedding medium's thermal conductivity $\kappa$ via $\Delta n=\left(\mathrm {d} n/\mathrm {d} T\right)\sigma _{\rm {abs}}I_{h}/4\pi \kappa R$. Although the signal can be well-explained in a scattering framework, the most intuitive description can be found by an intuitive analogy to the Coulomb scattering of wave packets in particle physics.

Backwards detection scheme

In this detection scheme a conventional scanning sample or laser-scanning transmission microscope is employed. Both, the heating and the probing laser beam are coaxially aligned and superimposed using a dichroic mirror. Both beams are focused onto a sample, typically via a high-NA illumination microscope objective. Alternatively, the probe-beam may be laterally displaced with respect to the heating beam. The retroreflected probe-beam power is then imaged onto a photodiode and the change as induced by the heating beam provides the photothermal signal

Detection mechanism

The detection is heterodyne in the sense that the scattered field of the probe beam by the thermal lens interferes in the backwards direction with a well-defined retroreflected part of the incidence probing beam.

References

• Boyer, D. (2002-08-16). "Photothermal Imaging of Nanometer-Sized Metal Particles Among Scatterers". Science. 297 (5584). American Association for the Advancement of Science (AAAS): 1160–1163. doi:10.1126/science.1073765. ISSN 0036-8075. PMID 12183624. S2CID 8758957.
• Cognet, L.; Tardin, C.; Boyer, D.; Choquet, D.; Tamarat, P.; Lounis, B. (2003-09-17). "Single metallic nanoparticle imaging for protein detection in cells". Proceedings of the National Academy of Sciences. 100 (20): 11350–11355. doi:10.1073/pnas.1534635100. ISSN 0027-8424. PMC 208760. PMID 13679586.
• Gaiduk, Alexander; Ruijgrok, Paul V.; Yorulmaz, Mustafa; Orrit, Michel (2010). "Detection limits in photothermal microscopy". Chemical Science. 1 (3). Royal Society of Chemistry (RSC): 343–350. doi:10.1039/c0sc00210k. ISSN 2041-6520.
• Selmke, Markus; Cichos, Frank (2013). "Photonic Rutherford scattering: A classical and quantum mechanical analogy in ray and wave optics". American Journal of Physics. 81 (6). American Association of Physics Teachers (AAPT): 405–413. arXiv:1208.5593. doi:10.1119/1.4798259. ISSN 0002-9505. S2CID 119276853.
• Selmke, Markus; Cichos, Frank (2013-03-06). "Photothermal Single Particle Rutherford Scattering Microscopy". Physical Review Letters. 110 (10). American Physical Society (APS): 103901. doi:10.1103/physrevlett.110.103901. ISSN 0031-9007. PMID 23521256.
• Selmke, Markus; Braun, Marco; Cichos, Frank (2012-02-28). "Photothermal Single-Particle Microscopy: Detection of a Nanolens". ACS Nano. 6 (3). American Chemical Society (ACS): 2741–2749. doi:10.1021/nn300181h. ISSN 1936-0851. PMID 22352758.
• Selmke, Markus; Braun, Marco; Cichos, Frank (2012-03-22). "Nano-lens diffraction around a single heated nano particle". Optics Express. 20 (7). The Optical Society: 8055–8070. arXiv:1109.2772. doi:10.1364/oe.20.008055. ISSN 1094-4087. PMID 22453477.
• Selmke, Markus; Braun, Marco; Cichos, Frank (2012-09-28). "Gaussian beam photothermal single particle microscopy". Journal of the Optical Society of America A. 29 (10). The Optical Society: 2237–41. arXiv:1105.3815. doi:10.1364/josaa.29.002237. ISSN 1084-7529. PMID 23201674.
• Selmke, Markus; Schachoff, Romy; Braun, Marco; Cichos, Frank (2013). "Twin-focus photothermal correlation spectroscopy". RSC Adv. 3 (2). Royal Society of Chemistry (RSC): 394–400. doi:10.1039/c2ra22061j. ISSN 2046-2069.
• Selmke, Markus; Braun, Marco; Schachoff, Romy; Cichos, Frank (2013). "Photothermal signal distribution analysis (PhoSDA)". Physical Chemistry Chemical Physics. 15 (12). Royal Society of Chemistry (RSC): 4250–7. doi:10.1039/c3cp44092c. ISSN 1463-9076. PMID 23385281.
• Bialkowski, Stephen (1996). Photothermal spectroscopy methods for chemical analysis. New York: Wiley. ISBN 978-0-471-57467-5. OCLC 32819267.
• "Molecular Nanophotonics Group: Photothermal Imaging". Retrieved 2020-03-19.