P. Maurer, J. I. Cirac, O. Romero-Isart Ultrashort Pulses for Far-Field Nanoscopy,
Phys. Rev. Lett. 117 103602 (2016-08-29),
http://dx.doi.org/10.1103/PhysRevLett.117.103602 doi:10.1103/PhysRevLett.117.103602 (ID: 719476)
The Abbe diffraction limit prevents focusing monochromatic light in the far-field beyond a spot size half its wavelength. For microscopy purposes at the nanoscale, namely nanoscopy, such limit can be circumvented by either using near-fields, which are not diffraction-limited, or, in fluorescence nanoscopy, by manipulating bright and dark states of the fluorescent markers. Here we propose and analyze an alternative approach for far-field nanoscopy based on using coherent polychromatic light, that is, ultrashort pulses. Such pulses have spectral bandwidths comparable, and even larger in the attosecond regime, than a carrier optical frequency. We show that a train of ultrashort pulses can be used to excite markers with nanoscale resolution. In particular, we show that they can be focused to a spot size given by the wavelength associated to its spectral bandwidth and that they can excite a two-level marker with an optical transition. The excitation mechanism is non-conventional for two-level systems, as it relies on the existence of processes where an excitation is created together with the emission of a photon. The detection of the light emitted after fluorescence, or any other method used to detect the excitation, would thus lead to far-field nanoscopy. In this sense, our results open the door to design fluorescence nanoscopes that circumvent the Abbe's barrier without manipulating the states of the markers but using coherent polychromatic light.