In recent years, the concept of parity-time (PT) symmetry has received considerable attention in the field of optics and photonics. In PT-symmetric arrangements, the interaction between gain/loss-contrast and coupling leads to the formation of exceptional points in parameter space. At these junctures, not only the eigenvalues but also the eigenvectors tend to merge, resulting in a sudden reduction of the dimensionality of the eigen-space. Consequently, in the vicinity of such points, the eigenfrequencies are strongly affected by external perturbationsas the system regains its original dimensionality. This unique behavior can be utilized to fundamentally enhance the sensitivity of micro-resonators. Here, we experimentally investigate this effect in integrated semiconductor PT-symmetric microring lasers that are biased at exceptional points. Using this arrangement, we demonstrate >10- fold enhancement in sensitivity. Our results also show that unlike standard microcavities, the parity-time symmetric system responds to the square-root of the perturbation. Our work provides a new avenue for enhancing the sensitivity of optical integrated sensors.
Parity time (PT) symmetric systems are known to exhibit two distinct phases: those associated with an unbroken and broken symmetry. In the domain of optics, PT-symmetry can be established by incorporating a balanced distribution of gain and loss in a system. Under linear conditions, in a coupled dimer, composed of two cavities or waveguides, if the gain-loss contrast increases beyond a critical value with respect to the coupling constant, a transition is expected from the unbroken symmetry to the broken symmetry regime. However, in the presence of nonlinearity, this transition behavior can be drastically modified. We here study a system of two coupled semiconductor-based resonators that are lasing around an exceptional point. The quantum wells in such structures not only provide gain but also lead to strong levels of saturable loss in the absence of any optical pumping. Interestingly, in sharp contrast with linear PT-symmetric configurations, such nonlinear processes are capable of reversing the order in which the symmetry breaking occurs. If the ratio of the net loss to coupling is less than unity in one of the cavities, as the pumping level in the other resonator is increased, the nonlinear eigenmodes move from an unbroken symmetric state to a broken one. Moreover, in this nonlinear domain, the structural form of the resulting solutions are isomorphic to the corresponding linear eigenvectors expected above and below the phase transition point. Experimental results are in good agreement with these predictions.
We experimentally demonstrate single longitudinal mode operation in microring laser using the concept of PT symmetry.
A PT-symmetric coupled resonator arrangement can considerably enhance the maximum achievable gain of single mode
microring cavity. The method is broadband thus work well for inhomogenously broadened gain mediums. It doesn’t rely
on any additional component to ensure its mode selective performance, and it is robust with respect to fabrication
inaccuracies. This result may pave the way for a novel way of designing integrated laser sources based on PT symmetry.