1. Science, 2010, 330,
Room-Temperature Detection of a Single Molecule’s Absorption by Photothermal Contrast
A. Gaiduk, M. Yorulmaz, P. V. Ruijgrok, M. Orrit*
Institute of Physics, Leiden University, Netherlands

2. Single-molecule Optical Detection
☆ Fluorescence
E. Betzig et al.,
Science, 262, (1993)
Intensity, Spectrum, Lifetime, Real-Time Imaging, ……
Physics
Chemistry
Biology
environmental
intersystem crossing
fluctuation
spectral diffusion
fluorescence switching
enzymatic states
quantum optics…
photochemistry….
folding….
Very convenient technique
Problems: Necessity of fluorescent label, blinking, fast photobleaching

3. Single-molecule Optical Detection
☆ Absorption
Q: Is it impossible to probe the single molecule’s absorption at room-temperature ?
A: No!, but challenging…..
because…
Very low absorption cross section of single molecules at r.t. (~ cm2)
(2) High background noise in a direct extinction measurement (based on the number of missing photons)
⇒It is very difficult to discriminate true absorption signal from scattering
W. E. Moerner et al.,
Phys. Rev. Lett., 62, (1989)
Limited to the cryogenic temperature experiments (~ 1.4 K)

4. Label-free Optical Microscopy
☆ Stimulated Emission
X. S. Xie et al., Nature, 461, (2009)
☆ Thermal Lens Microscopy
T. Kitamori et al., Anal. Chem., 73, (2001)

5. Photothermal Lens Effect

6. Photothermal Imaging
B. Louis, M. Orrit et al., Science, 297, (2002)
B. Louis et al., Phys. Rev. Lett., 93, (2004)
B. Louis et al., Phys. Rev. B, (2006)
M. Orrit et al., Chem. Sci., 1, 343. (2010)

7. Experimental Setup

8. Photothermal Images of Gold Nanospheres

9. Photothermal Images of Gold Nanospheres
Key points to achieve single-molecule sensitivity
Fast modulation of the pomp (heating) beam (1 MHz)
　　　⇒Elimination of mechanical, electronic, and laser noise
The change of surrounding medium from to water to glycerol
The use of relatively high probe power
　　 ⇒Increase of Photothermal signals and reduction of photon noise
Detectable dissipated powers can be decreased as low as 3 nW with SNR (signal to noise ratio) of 8 in a 10 ms integration time

10. Calculation of the Dissipated Power

11. Samples

12. Photothermal Images of BHQ1-10T-BHQ1