Relativistic nonlinear Thomson backscattering of excited electrons in ultra-tightly focused circularly polarized laser pulses with beam waist radius dependence

In our study, we delve into the effect of laser beam waist radius variations on the interaction of an ultra-tightly focused laser with off-axis electrons. We find that off-axis collisions are the most common case in our experiments, which highlights the relevance of our study. Under ultra-tight focusing conditions (<i>b</i>₀=<i>&Lambda;</i>₀), the electrons are subjected to a qualitative force that is asymmetric in the x+ axis directions and x -axis directions, leading to trajectory deviations and accelerated oscillations. At the same time, the electron radiated power and spectrum exhibit a clear asymmetry, which gradually diminishes as the beam waist radius increases, with increasing peak radiated power and decreasing full width half maximum. These findings are crucial for the generation of ultrashort pulses, especially in the field of ultrashort optics, and are important for applications utilizing nonlinear inverse Thomson scattering radiation.