Pulse delay and pulse compression of ultrashort light pulses in tight focusing

The tight focusing of radially polarized ultrashort pulsed laser beam is investigated, based on the Richards–Wolf vector diffraction theory. It is found that pulse delay phenomenon occurs near the focus. This is, near the focus, the ultrashort light pulse slows down. Meanwhile, with the decrease of the velocity the ultrashort pulsed laser beam is compressed in propagation direction. The space compression in propagation direction indicates that, though the pulse duration of the ultrashort light pulse does not change, the spatial pulse length of the ultrashort light pulse is reduced due to the pulse delay phenomenon. It is also shown that the different numerical aperture of the objective exhibits different velocity of the pulse light.