Spatial photoselection of single molecules on the surface of spherical microcavities

White-light nanosource with directional emission

We report the first observation of white-light emission from femtosecond laser-induced plasma in a water droplet. Such emission is not observed with water in a cell. The microdroplet acts as a lens, focusing the incident light to nanosized regions within itself and directing the emission from these regions primarily back toward the laser source. This focusing increases the intensity so that multiphoton ionization generates plasma and causes it to reach the critical density during the initial part of the pulse, enabling the rest of the pulse to heat the plasma enough to emit in the visible.

Forbidden light detection from single molecules

We present a new concept for ultrasensitive detection of surface-generated fluorescence which is made possible by a new optical module. The detection method leads to an enhancement in fluorescence collection efficiency to more than 65% of the total of emitted light, whereas high-aperture microscope objectives are able to collect 44% at best. Moreover, by employing this new optical module, the detection volume can be restricted to approximately 10(-17) L. This allows for an exceptional discrimination of bulk-generated against surface-generated fluorescence, which may be of great value when surface-binding processes are monitored. We demonstrate the performance of the new detection system by detecting single fluorescent molecules and by determining antigen concentrations down to 5 fmol.