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14 General background According to Richard Feynman, the single most important insight science brought us is that “matter is made of atoms”. Throughout the 20th century, we have grown accustomed to more and more direct atomic and molecular manifestations, starting with Brownian motion and X-ray diffraction, and eventually ending with electron micrographs showing single atoms and molecules, or scanning tunneling microscopy images, which routinely allow us to ‘feel’ individual atoms and molecules on solid surfaces in an immediate way. But what about ‘seeing’ atoms and molecules with light? This century-old dream was born with fluorescence experiments by Jean Perrin [1] around 1910, but only came true in the 1970s with attenuated atomic beams and later with single trapped ions in ultrahigh vacuum. For molecules in condensed matter, the limit of optical single-molecule detection was reached only in the late 1980s, essentially because of two technical problems: background emission from the molecule’s surroundings and limited total number of fluorescence photons for each molecule because of bleaching, i.e., photo-induced degradation of the emitter. Both problems were solved by cryogenic experiments at liquid-helium temperatures, through resonant absorption cross sections and thanks to the excellent photostability of molecules held in a rigid cage. These cryogenic experiments gave rise to many striking observations in nanophysics and quantum optics. However, single-molecule optics only turned into a real revolution with ambient conditions. The loss of a factor 100,000 in cross section upon warming from liquid-helium to room temperature is compensated by an equivalent improvement in optical quality and selectivity by the diffraction-limited spots of modern microscopes. Thanks to objective lenses with large numerical apertures, single-molecule techniques became readily applicable in material science and biology. A typical single-molecule fluorescence experiment at room temperature requires a confocal microscope, laser excitation of a small, diffraction-limited volume of some hundreds of nm in size, and a photon-counting detection of the Stokes-shifted fluorescence emitted by the excited spot, often collected by the excitation objective lens. The Stokes shift allows for easy and efficient spectral filtering of the signal. Critical points for a high signal-to-background ratio are a tiny (diffraction-limited) excitation volume, clean samples and optics, and a high stability of setup, laser, and sample. With this basic setup, a large number of timeand spaceresolved experiments can be conducted on nanometer scales. In this article, we would like to share our enthusiasm for the novel physics that the signals of single molecules have opened and illustrate them with some work from our group.

Single molecules and single nanoparticles as windows to the nanoscale