Relativistic ponderomotive force in the regime of extreme focusing

When charged particles interact with laser fields, they are usually thought to be pushed out from the regions of higher laser intensity via the mechanism known as the ponderomotive force (PMF). In contrast to the existing theories, we show that there exist several regimes in which charged particles are drawn into the regions of strongly focused laser fields. We derive a simple, covariant, and relativistically correct expression for the ponderomotive laser force that holds for arbitrary strength of field focusing and for all particle velocities. We predict three new, to the best of our knowledge, physical effects: (1) focusing-dependent reversal of PMF, (2) non-relativistic reversal of PMF, and (3) non-reciprocity of PMF for weakly relativistic particle velocities.