Focusing limits of intense ultrafast laser pulses in a high pressure gas : road to new spectroscopic source
As a result of focusing ultrafast Ti:Sapphire laser pulses in a gas, a plasma column is created. Its density does not exceed some limiting value as a result of the defocusing by the electrons resulting from multiphoton ionization and the involvement of high order nonlinearities. The spectrum of the radiation from the excited species in the column has little contribution from the plasma continuum and the line broadening is less than that of the spectra radiated from a plasma generated by a long laser pulse. Thus, the spectroscopic source obtained using short laser pulses is an excellent source for spectroscopy.
We present a detailed study of the optical trapping and manipulation of nanoparticles with complex permittivity using an apertureless near-field probe. We use a three-dimensional, self-consistent description of the electromagnetic scattering processes that accounts for retardation and the intricate many-body interaction between the substrate, the particle, and the probe. We analyze the influence of absorption on the optical force. For metals we describe how the optical force spectrum is influenced by the optical response of the metal, and in particular by plasmon resonances. We find that the optical force spectrum can provide an intrinsic signature of the particle composition which can be used to achieve a material-selective trapping and nanomanipulation.