Phase control of femtosecond pulses on the nanoscale using second harmonic nanoparticles

Investigations of ultrafast processes occurring on the nanoscale require a combination of femtosecond pulses and nanometer spatial resolution. However, controlling femtosecond pulses with nanometer accuracy is very challenging, as the limitations imposed both by dispersive optics on the time duration of a pulse and by the spatial diffraction limit on the focusing of light must be overcome simultaneously.

In this paper, we provide a universal method that allows full femtosecond pulse control in subdiffraction-limited areas. We achieve this aim by exploiting the intrinsic coherence of the second harmonic emission from a single nonlinear nanoparticle of deep subwavelength dimensions. The method is proven to be highly sensitive, easy to use, quick, robust and versatile.

This approach allows measurements of minimal phase distortions and the delivery of tunable higher harmonic light in a nanometric volume. Moreover, the method is shown to be compatible with a wide range of particle sizes, shapes and materials, allowing easy optimization for any given sample. This method will facilitate the investigation of light–matter interactions on the femtosecond–nanometer level in various areas of scientific study.

Scientists at ICFO, the Institute of Photonics Sciences in Barcelona, have demonstrated control of laser pulses with both femtosecond- and nanometre-scale precision. Nicolò Accanto and co-workers combined broad-band pulse shaping with second-harmonic detection from individual nanoparticles inside a microscope to gain control over ultra-short light pulses in a sub-diffraction area.

The universal approach allows to compensate for the phase distortions experienced by a laser pulse in-situ, and to assess Fourier limited pulses at the nanoscale. The scheme's precision will aid the study of ultrafast dynamics for a variety of nanoscale objects, such as single molecules, quantum dots, plasmonic nanostructures and diamond colour centres.

The technique could also prove useful for realizing nanometre-scale sources of tuneable coherent light.