Niek F. van Hulst – Institut de Ciències Fotòniques (ICFO)

Molecules are composed of atoms and electrons; their arrangement determines the energy states and spectroscopy, i.e. the response of a molecule to incident colours of light. A specific molecule has a corresponding specific response, which underlies the essence of optical spectroscopy. Yet in the real world, in a liquid or solid host, molecules do change their conformation depending on the local “nano”-environment. As a result, even for chemically identical molecules, each molecule has a shifted absorption spectrum, and in a typical single molecule experiments an unknown fraction is easily missed. Capturing all molecules is challenging as they fluctuate, blink and bleach: one needs to keep track of all colours at the same time.

To this end we have developed a novel approach based on interferometric white light excitation combined with confocal fluorescence detection, revealing the specific colour of each molecule individually and bringing single molecule excitation spectroscopy side-by-side to single molecule emission spectroscopy. The interferometric approach, exploiting broad-band femtosecond pulses and rapid delay line scanning, is resilient against blinking and bleaching as the entire excitation spectrum is probed at once. Unprecedented spectral heterogeneities of single molecules, with individual excitation spectra shifted in wavelength by more than 100 nm are revealed. Conventional narrow-band excitation techniques would be incapable to capture the whole extent of the spectral distribution and would meagerly miss out on molecules detected by the broad-band scheme.

The new femtosecond single molecule excitation spectroscopy addresses the ultrafast dynamics in the electronic excited state, giving access to the interaction between individual molecules and their environment as well as biomolecules in more complex systems. Equally, the technique will prove useful to follow slow-occurring chemical reactions in time through changes in the molecular excitation spectrum both in solution and on the single-molecule level.


Piatkowski L, Gellings E & van Hulst N F 2016, ‘Broadband Single Molecule Excitation Spectroscopy’, Nature Communications, 7:10411.