We report parity-time (PT) symmetry breaking in electrically injected, coherently coupled, vertical cavity surface emitting laser arrays. We predict beam steering, mode evolution, and mode hopping as consequences of the non-Hermiticity of the array, analyzed by the temporal coupled-mode theory with both an asymmetric gain distribution and local frequency detuning. We present experimental confirmations of the predicted mode evolution, mode hopping, and PT symmetry breaking with quantitative agreement with the theory.

An approach for creating two-dimensional arrays of coherently coupled vertically emitting laser cavities is demonstrated. This is achieved by creating a 2×2 array of defect cavities within the top distributed Bragg reflector of a photonic crystal vertical cavity surface-emitting laser. The optical coupling occurs laterally through coupling regions defined between the defect cavities. Modifying the index within the coupling regions, accomplished by varying the hole parameters of the photonic crystal in those regions, leads to out-of-phase coherent coupling observed in the far field. Agreement is found between the simulated and observed out-of-phase far fields.

We experimentally demonstrated enhanced digital modulation of coherently coupled vertical-cavity surface-emitting laser arrays. Mutually coherent behavior of 2 × 1 proton-implanted index-confined photonic crystal vertical-cavity phased laser arrays is characterized using a novel digital modulation scheme. Far-field interference fringe visibility is used to estimate the degree of coherence between two coherently coupled laser cavities, and a visibility map is created to visualize the dependence of laser array coherence on the injection current bias. The visibility map indicates that a dual-channel in-phase modulation scheme can preserve the laser array in mutual coherence while under digital modulation. Experimentally, the high-speed digital modulation signal up to 36 Gb/s is supplied to both laser elements simultaneously. The resulting eye diagram is shown to be improved under coherent-coupling conditions.

Optically coupled semiconductor laser arrays are described by coupled rate equations. The coupled mode equations and carrier densities are included in the analysis, which inherently incorporate the carrier-induced nonlinearities including gain saturation and amplitude-phase coupling. We solve the steady-state coupled rate equations and consider the cavity frequency detuning and the individual laser pump rates as the experimentally controlled variables. We show that the carrier-induced nonlinearities play a critical role in the mode control, and we identify gain contrast induced by cavity frequency detuning as a unique mechanism for mode control. Photon-mediated energy transfer between cavities is also discussed. Parity-time symmetry and exceptional points in this system are studied. Unbroken parity-time symmetry can be achieved by judiciously combining cavity detuning and unequal pump rates, while broken symmetry lies on the boundary of the optical locking region. Exceptional points are identified at the intersection between broken symmetry and unbroken parity-time symmetry.

Coherently coupled laser arrays can be described by the temporal coupled mode theory in which the array modal behavior can be classified according to the coupling matrix, $\overline{\overline{\mathbf{M}}}$ . Accounting for a nonuniform gain/loss distribution in a laser array makes $\overline{\overline{\mathbf{M}}}$ a non-Hermitian matrix, and experimentally we find phase-front tuning (beam steering) of the coherent supermode as a result of the non-Hermiticity. We report the experimental characterization of the supermodes in coherently coupled vertical cavity surface emitting laser diode arrays and demonstrate control of non-Hermiticity by spatially varying injection currents. Exceptional points are identified in these electrically injected microcavity diode arrays.

Evanescent coupling can be achieved by patterning the top facet of a vertical-cavity surface-emitting laser (VCSEL) with a multiple defect photonic crystal pattern. We show that for 2×1 coupled defect arrays, the phase between these emission sites changes as the current is varied. The phase variation is manifest in the far field pattern and may be determined using antenna array theory. It is found that the phase varies 81 deg between transversely coupled cavities as the current to the VCSEL is varied by 24 mA. This corresponds to a far field angular change of approximately 1.5 deg.