Low emittance electron beam generation from a laser wakefield accelerator using two laser pulses with different wavelengths

Ionization injection triggered by short wavelength laser pulses inside a nonlinear wakefield driven by a longer wavelength laser is examined via multidimensional particle-in-cell simulations. We find that very bright electron beams can be generated through this two-color scheme in either collinear propagating or transverse colliding geometry. For a fixed laser intensity $I$, lasers with longer/shorter wavelength $\ensuremath{\lambda}$ have larger/smaller ponderomotive potential ($\ensuremath{\propto}I{\ensuremath{\lambda}}^{2}$). The two-color scheme utilizes this property to separate the injection process from the wakefield excitation process. Very strong wakes can be generated at relatively low laser intensities by using a longer wavelength laser driver (e.g., a $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ ${\mathrm{CO}}_{2}$ laser) due to its very large ponderomotive potential. On the other hand, a short wavelength laser can produce electrons with very small residual momenta (${p}_{\ensuremath{\perp}}\ensuremath{\sim}{a}_{0}\ensuremath{\sim}\sqrt{I}\ensuremath{\lambda}$) inside the wake, leading to electron beams with very small normalized emittances (tens of nm). Using particle-in-cell simulations we show that a $\ensuremath{\sim}10\text{ }\text{ }\mathrm{fs}$ electron beam with $\ensuremath{\sim}4\text{ }\text{ }\mathrm{pC}$ of charge and a normalized emittance of $\ensuremath{\sim}50\text{ }\text{ }\mathrm{nm}$ can be generated by combining a $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ driving laser with a 400 nm injection laser, which is an improvement of more than 1 order of magnitude compared to the typical results obtained when a single wavelength laser is used for both the wake formation and ionization injection. With the transverse colliding geometry, simulations show that similarly low emittance and much lower slice energy spread ($\ensuremath{\sim}30\text{ }\text{ }\mathrm{keV}$, comparing with the typical value of few MeV in the longitudinal injection scheme) can be simultaneously obtained for electron beams with a few pC charge. Such low slice energy spread may have significant advantages in applications relevant to future coherent light sources driven by plasma accelerators.