Active near-field probe based on ultrabroadband hot luminescence of Si/Au nanoparticle

Efficient white-light sources at the nanoscale are of great importance for probing near-field properties of optical nanoantennas and nanophotonics devices operating in a broad spectral range. Tunnelling and scattering... [ view full abstract ]

Efficient white-light sources at the nanoscale are of great importance for probing near-field properties of optical nanoantennas and nanophotonics devices operating in a broad spectral range. Tunnelling and scattering near-field scanning optical microscopes (NSOMs) allow to retrieve information on electromagnetic field components in the vicinity of the sample, but the possibility to map the local density of states (LDOS) is crucially important.

Here we demonstrate highly-efficient optical NSOM with an active probe operating in unprecedentedly broad UV-VIS-NIR spectral band. The probe is fabricated in two steps. A hybrid nanoparticle with size ranging of 100-200 nm is printed from thin double-layer Si/Au film by means of the laser printing technique. This particle is then transferred to a tip of an atomic force microscopy cantilever by using nanoscale manipulation under an electron beam.

In experiment, the fabricated probe is irradiated by a near-IR femtosecond laser (1053 nm), which results in intense three-photon hot white-light luminescence from the nanoparticle. While the sample is scanned with respect to the tip, the emission spectra are recorded from the substrate side in confocal arrangement.

The measured NSOM signal varies depending on a relative sample-probe position, and the results are in a good agreement with partial LDOS maps simulated numerically as the emission of three incoherent orthogonal electric dipole sources placed in the vicinity of the structure.

The hot luminescence of Si/Au nanoparticle originates from a balance in electron relaxation and recombination times. The latter is strongly increased as compared to purely phonon mechanism by contributing defects and plasmons in silicon. The efficient excitation of plasmons becomes possible due to high nearly metallic-type density of free carriers (of more than 10^20 1/cm^3) generated by intense femtosecond laser pulses, while gold enables an efficient three-photon absorption and injection of hot electrons.

Thus, we have employed hybrid Si/Au nanoparticles to fabricate an active near-field probe operating in an ultra-broad spectral band. The intense broadband luminescence originates from dense electron-hole plasma generated in silicon by femtosecond radiation. The demonstrated device would be a powerful tool for the study of LDOS of nanoscale systems.