Femtosecond laser meets field emission tip - a sensor for the carrier envelope phase?

We investigate the possibility of a simple stand-alone sensor for the carrier envelope phase of ultrashort laser pulses. We focus the laser beam onto a field emission tip and observe electron emission in the optical field emission regime. In this regime the emission is quasi-static and highly non-linear in the laser electric field. We observe both experimentally and numerically that for three cycle pulses the modulation of the emission current due to the carrier envelope phase is below 0.1 % and thus too small to serve as a means to self-reference the laser. However, for shorter pulses our simulation yields a steep increase in modulation depth. We also show measurements of interferometric autocorrelation traces with the tip as non-linear detector and compare these results to two different theoretical models, thereby backing the choice of our numerical models.