Nonlinear femtosecond laser induced scanning tunneling microscopy.

We demonstrate ultrafast laser driven nonlinear scanning tunneling microscopy (STM), under ambient conditions. The design is an adaptation of the recently introduced cross-polarized double beat method, whereby z-polarized phase modulated fields are tightly focused at a tunneling junction consisting of a sharp tungsten tip and an optically transparent gold film as substrate. We demonstrate the prerequisites for ultrafast time-resolved STM through an operative mechanism of nonlinear laser field-driven tunneling. The spatial resolution of the nonlinear laser driven STM is determined by the local field intensity. Resolution of 0.3 nm-10 nm is demonstrated for the intensity dependent, exponential tunneling range. The demonstration is carried out on a junction consisting of tungsten tip and gold substrate. Nano-structured gold is used for imaging purposes, to highlight junction plasmon controlled tunneling in the conductivity limit.

The effect of propagation in air on the filament spectrum.

Filamentation studies traditionally start from letting a beam focus in air. We present filament studies with control over the preparation propagation, in air or vacuum, using an aerodynamic window. The spectral content of the filament strongly depends on its preparation medium.