This study theoretically investigates optical injection processes in few-layer black phosphorene (BP), including one-photon carrier injection, two-photon carrier injection, and two-color coherent current injection. Employing maximally localized Wannier functions (MLWF) constructed from first-principles calculations, we obtain the band structures and matrix elements of position operator, and then evaluate the frequency-dependent injection response tensors for monolayer (1L), bilayer (2L), and trilayer (3L) BP. All injection response tensors show pronounced layer dependence and strong in-plane anisotropy originated from the puckered lattice, which yields anisotropic band dispersion and optical transitions; and the degree of anisotropy can be well controlled by the layer number. Furthermore, such anisotropic injection is analyzed using the anisotropic effective masses along two in-plane directions at the conduction and valence band edges, the selection rules of the optical transitions, and an analytic expression for the spectra using a simple parabolic band structure approximation for transitions close to the band edge. As the layer number increases, interlayer coupling reduces the in-plane anisotropy of the band dispersion and consequently reduces the anisotropy of quantities such as the effective masses and injection response. Unlike one-photon injection response for light polarized along the armchair direction, which turns on at the band edge with a finite value, the two-photon carrier injection in few-layer BP starts from zero at $\hslash \omega =E_g/2$ and exhibits a peak induced by the competition between the increasing transition amplitude and the ${\omega }^{-4}$ dependence induced by the frequency dependence of the light vector potential. The magnitude and direction of the injected current can be coherently controlled by the polarization and the relative phase between the $\omega$ and $2\omega$ beams. Our results highlight the interplay of layer-tunable anisotropy, symmetry, and quantum interference in few-layer BP, providing a microscopic basis for polarization-dependent injection and coherent-control schemes in the midinfrared regime.

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• Band gap
• Coherent control
• Nonlinear optics
• Polarization of light
• 2-dimensional systems
• Phosphorene
• Tight-binding model
• Wannier function methods