as possible. For given laser parameters, $\cal E$ can be maximized by focusing the laser beam down to the diffraction limit, which will be assumed to be the case when providing experimental estimates for the effect below. The beam diameter of a Gaussian beam of wavelength $\lambda$ focused down to the diffraction limit is given by $w=2\lambda f^\#$ and its Rayleigh range by $z_{\rm R}=\pi\lambda(f^\#)^2$, with $f^\#$, the so-called $f$-number, defined as the ratio of the focal length and the diameter of the focusing aperture [@Siegman]; $f$-numbers as low as $f^\#=1$ can be realized experimentally. Recall that in our approximation the length scale $\zeta$ mimics the Rayleigh range $z_{\rm R}$. Correspondingly, aiming at experimental estimates, we identify $\zeta=z_{\rm R}$.

Hence, and perhaps most strikingly, in the diffraction limit the combinations $\zeta^2\Omega^2=(2\pi)^2\pi^2(f^\#)^4$, $(\frac{w}{2})^2\Omega^2=(2\pi)^2(f^\#)^2$ become generic numbers. In turn, the functions $h_{(p)}^\omega(u,\varphi)$ and $h^\omega(u,\varphi)$ defined in Eqs. - become independent of any explicit laser parameters apart from $f^\#$. The entire dependence on the laser parameters in Eqs. , , and is encoded in the overall prefactor $$\left(\frac{e{\cal E}}{m^2}\right)^6 \left(\frac{w}{2}m\right)^4\frac{\Omega\tau}{2\pi} \ \ \xrightarrow{\text{diffraction\ limit}} \ \
 (f^\#)^4\left(\frac{e{\cal E}}{m^2}\right)^6 \left(\frac{\lambda}{\lambda_c}\right)^3\frac{\tau}{\tau_c}=(f^\#)^4\,2^3\left(\frac{I}{I_{\rm cr}}\frac{\lambda}{\lambda_c}\right)^3\frac{\tau}{\tau_c}\,,$$ where $I=\frac{1}{2}{\cal E}^2$ denotes the mean intensity per laser beam and $I_{\rm cr}\equiv(\frac{m^2}{e})^2=4.68\cdot10^{29}\frac{\rm 