Modeling ultrashort field dynamics in surface emitting lasers by using finite-difference time-domain method

An approach based on the finite-difference time-domain (FDTD) method is developed for simulating the dynamics of vertical-cavity surface-emitting lasers (VCSELs). The material response is incorporated in our FDTD algorithm by the effective semiconductor Bloch equations, and its effects are accounted for through a resonant polarization term in the Maxwell's equations. Moreover, nonlinear gain saturation is incorporated through a gain suppression factor in the equation governing the dynamics of the resonant polarization. This approach is verified by modeling a λ-cavity VCSEL, with a multiple quantum-well (MQW) gain region; the corresponding continuous-wave operation is obtained at the expected wavelength. The dynamics of ultrashort pulses generated by a monolithic passively mode-locked one-dimensional VCSEL with a MQW gain region and a single QW saturable absorber are studied and it is demonstrated that a stable mode-locked pulse train can be generated. It is also demonstrated that with our FDTD approach subcycle temporal precision can be achieved. The need for this fine temporal resolution is established by investigating pulse propagation through the semiconductor saturable absorber. Fine features of the spatial profile of the mode-locked pulses are also examined within this approach. This knowledge of the fine spatial features is then used for lowering the current threshold through gain structure optimization. Various approaches for the reduction of the total simulation time are also discussed.