Michel Orrit#

Laudatio by Christoph Bräuchle#

Summary of International Impact of Scientific Work

M. Orrit's scientific field is optical spectroscopy of molecular materials (organic crystals, Langmuir-Blodgett films, dye solutions in polymers and molecular liquids). He has been consistently pursuing the detection of weak optical signals arising from less and less I molecules. Starting with surface excitons in a single layer of a molecular crystal, he moved to dye-doped Langmuir-Blodgett films during his post-doctoral stay in Gottingen (l985-86).

In 1990, M. Orrit and J. Bernard reached the ultimate detection limit of one molecule, a feat many at that time had deemed impossible. One year before them, L. Kador and W. E. Moerner had detected a single-molecule signal in an absorption spectrum, but the vastly better signal-to-noise ratio of the fluorescence excitation method allowed Orrit and Bernard to prove that their signals stemmed from individual molecules, starting the new field of single-molecule spectroscopy. Since then, single—molecule fluorescence as a technique has percolated towards biophysics, physical chemistry, and material science. It forms one of the two pillars of the current scientific revolution of superresolution in optical microscopy.

After the discovery of single-molecule signals, Orrit's group explored the new opportunities opened by single molecules for studies of structure and dynamics at nanometer scales, quantum optics, single spin and single photon manipulations, and later proposed to produce single photons on command. His recent interests include the photothermal detection of individual absorbers as alternatives to fluorescent labels, opto-mechanical probing of single gold nano-particles, probing of charge transport in organic solids, molecular aspects of the structure and dynamics of soft and complex matter.

Major Scientific Contributions

The main scientific achievement of M. Orrit is the first clear optical detection of a single immobilized molecule. This experiment was done in a molecular crystal at low temperature and was published in 1990 [1]. This work gave rise to a flurry of work in different directions, first at cryogenic conditions, but after l993 also at ambient conditions [2]. Some other important contributions of M. Orrit are listed hereafter:

l. Prior to single-molecule experiments, Orrit worked on the interaction of monolayers and thin films with light. He proposed a general theory for the quantitative evaluation of optical reflection and transmission such thin films, including spontaneous emission corrections, which are very important for molecular assemblies such as J-aggregates [3]. These results are used in particular to determine the orientations of dye molecules with respect to Langmuir films.

2. An original application of single molecules, at the border between physical chemistry and quantum optics, is their use as model quantum systems. Orrit's group has achieved a number of founding experiments in this area, including measurements of the ac-Stark effect at optical frequencies, or the delivery of single photons on command by a single organic molecule [4]. These experiments were later duplicated with inorganic systems such as self-assembled quantum dots or color centers in diamond [5].

3. Single fluorescent objects often present a characteristic intermittency (also called on-off blinking), even under steady excitation conditions. Verberk and Orrit have proposed a simple model of charge tunneling and trapping to explain the pecular statistical self-similarity of the blinking traces. First discovered characterized on semiconductor nanocrystals, this power-law blinking was later observed for single molecules and other fluorescent emitters, to which the same theory may apply [6].

4. Because fluorescent signals are often interrupted by blinking, it is very attractive to detect optical absorption of single nano—objects directly. Orrit's group has proposed the first photothermal detection of immobilized gold nanoparticles as an alternative to fluorescent labels [7]. Photothermal detection relies on the time-modulated thermal inhomogeneity around an absorbing particle, and provides a high signal-to noise ratio again non-absorbing scatterers. It holds great promise for single-biomolecule imaging and tracking in cell biology. Orrit's successor in Bordeaux, B. Lounis, has improved the method and made it more practical. Applying the photothermal principle with short laser pulses, Orrit’s group have studied the acoustic vibrations of individual gold nanoparticles (spheres, rods) and individual clusters (dumbbells). The selection of a single particle eliminates heterogeneity and gives access to the damping mechanisms of the vibration [8]. In the future, a wide panel of optical diagnostics (spectral, time-resolved, chemical, thermal) will be applied to a single gold nanoparticle used as a local probe.

5. Following the rotational diffusion of dye molecules in a molecular glass-former, super-cooled glycerol, Orrit's group has confirmed earlier observations on ortho-terphenyl of dynamical heterogeneity, They provided evidence for exceeding long exchange times [9].

These have been correlated with the onset of a weak solid-like behavior above the glass transition temperature, which had not been reported previously. This result illustrates the power of single molecules to reveal and study heterogeneity even in supposedly well-known systems. This discovery has been the center of a 5-year project supported by Orrit’s ERC Advanced Grant (2008). The broader idea of this project is to exploit molecular insight from chemical physics to substantiate the general ideas on soft matter developed by statistical physicists, the missing link toward a molecular control of the physical properties of soft materials.

6. Orrit's initial research field is the low-temperature, high-resolution spectroscopy of organic molecular crystals. He still pursues research in this area with high-resolution spectroscopy of single absorbing molecules in conducting crystals such as anthracene. His group has recently discovered local acoustic oscillators at very low frequencies [10], which appear to be localized around crystal defects. Only local reporters such as single molecules can identify these low—frequency oscillators, which had not been observed previously. These modes may be related to the local oscillators thought to be responsible for the boson peak in the light scattering of glasses and other disordered materials.

[1.] M. Orrit and .1. Bernard, Phys. Rev. Lett. 65 (1990) 2716.
[2.] W. E. Moemer and M. Orrit, Science 283 (1999) 1670.
[3.] M. Orrit et al. J. Chem. Phys. 85 (1986) 4966.
[4.] Ch. Brunel et al., Phys. Rev, Lett. 83 (1999) 2722.
[5.] B. Lounis, M. Orrit, Rep. Prog. Phys. 68 (2005) 1129
[6.] F. Cichos, C. von Borczyskowski, M. Orrit. Curr. Opin. Coll. Interf Sci. 12 (2007) 272.
[7.] D. Boyer et al., Science 297 (2002) 1160.
[8.] M. A. van Dijk, M. Lippitz, M. Orrit, Phys. Rev. Lett. 95 (2005) 267406
[9.] R. Zondervan et al., Proc. Natl. Acad. Sci. USA 104 (2007) 12628
[10.] M. Kol'chenko et al., New J. Phys. 11 (2009) 023037.

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