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

Tight focusing induces pulse delay and pulse compression of double-ring-shaped radially polarized ultrashort light pulses

“…This implies that at the focus of the high-NA objective, W of the longitudinal component, radial component and total light pulse are compressed by factors of 2.48, 1.55 and 1.55, respectively. The spatial compression of the ultrashort light pulse near the focus can be attributed to pulse delay near the focus [10,29]. This is because the velocity of the ultrashort light pulse decreases gradually prior to the focus, the front edge of the light pulse slows down first, and the lagging edge catches up with the front edge gradually.…”

“…[10], we introduced a slowdown factor (S in short), which is defined as the ratio of the phase velocity to the group velocity (S = v φ /v g ) of the light pulse. As the group velocity decreases, S increases near the focus.…”

“…As mentioned in Ref. [10,29], the intensity distribution in the propagation direction of the ultrashort light pulse can be expressed as: …”

“…Tight focusing of light beams with radially and azimuthally polarized fields by objectives with high numerical aperture (NA) has been studied extensively in recent years because of their unusual properties and potential applications [1][2][3][4][5][6][7][8][9][10][11][12], such as material processing, optical trapping, particle acceleration and high-resolution microscopy [13][14][15][16]. A double-ring-shaped radially polarized light beams as a single transverse mode was demonstrated by use of a polarization-selective grating mirror [17].…”

“…Study of double-ring-shaped radially polarized light beams in tight focusing has shown that near the focus of the high-NA objective, the longitudinal component varied with the degree of truncation of the incident beam by the pupil of the objective lens because of the interference between the longitudinal components of the inner and outer rings [18]. Meanwhile, ultrashort light pulses that have temporal resolution down to the femtosecond range have recently been used in several time-resolved optical measurements [10,[19][20][21]. In optical measurements with a femtosecond light pulse, the spatial and temporal properties of the focused ultrashort light pulse may influence the resolution of a produced image [22].…”