A new scheme for ultrasensitive laser gyroscopes that utilizes the physics of exceptional points will be presented. By exploiting the properties of such non-Hermitian degeneracies, we show that the rotation-induced frequency splitting becomes proportional to the square root of the gyration speed (√𝛀)- thus enhancing the sensitivity to low angular rotations by orders of magnitudes. In addition, at its maximum sensitivity limit, the measurable spectral splitting is independent of the radius of the rings involved. Our work paves the way towards a new class of ultrasensitive miniature ring laser gyroscopes on chip.
Student contribution: In recent years, non-Hermitian degeneracies, also known as exceptional points (EPs), have emerged as a new paradigm for engineering the response of optical systems. This class of degeneracies represents points in parameter space where the eigenvalues and their corresponding eigenvectors simultaneously coalesce [1,2]. Among the large set of non-conservative photonic systems, parity-time (PT) symmetric arrangements are of particular interest since they provide an excellent platform to study the physics and properties of non-Hermitian degeneracies [3,4]. So far, the abrupt nature of the phase transitions at EPs has led to a number of new functionalities such as loss-induced transparency , unidirectional invisibility [6,7], and single mode lasing [8-11]. In addition, it has been suggested that the bifurcation properties associated with second-order exceptional points can be utilized to achieve enhanced sensitivity in micro-resonator arrangements . Of interest is to use even higher-order exceptional points that in principle could further amplify the effect of perturbations. While such higher-order singularities have been theoretically studied in a number of recent works [13,14], their experimental realization in the optical domain has so far remained out of reach. In this paper, for the first time, we show the emergence of third order exceptional points in ternary parity-time-symmetric coupled resonator lasers by judiciously designing the gain/loss distribution and coupling strengths following a recursive bosonic quantization procedure. Subsequently, the nature of the third order exceptional point is confirmed through the cubic root response of this ternary system to external perturbations. Our work may pave the way towards the utilization of higher order exceptional points in designing ultrasensitive photonic arrangements.
 W. D. Heiss, J. of Phys. A: Mathematical and Theoretical 45, 444016 (2012).
 N. Moiseyev, Non-Hermitian Quantum Mechanics. (Cambridge University Press, 2011).
 K. G. Makris, R. El-Ganainy, and D. N. Christodoulides, Phys. Rev. Lett. 100, 103904 (2008).
 C. E. Rüter, K. G. Makris, R. El-Ganainy, D. N. Christodoulides, M. Segev, and D. Kip, Nat. Phys. 6, 192 (2010).
 A. Guo et al., Phys. Rev. Lett. 103, 093902 (2009).
 B. Peng et al., Nat. Phys. 10, 394 (2015).
 A. Regensburger, C. Bersch, M.-A. Miri, G. Onishchukov, D. N. Christodoulides, and U. Peschel, Nature 488, 167 (2012).
 M. Miri, P. Likamwa, D. N. Christodoulides, Opt. Lett. 37, 764 (2012)
 H. Hodaei, M.-A. Miri, M. Heinrich, D. N. Christodoulides, and M. Khajavikhan, Science 346, 975 (2014).
 L. Feng, Z. J. Wong, R.-M. Ma, Y. Wang, X. Zhang, Science 346, 972 (2014).
 H. Hodaei et al., Laser & Photon. Rev. 10, 494 (2016).
 J. Wiersig, Phys. Rev. Lett. 112, 203901 (2014).
 G. Demange, and E.-M. Graefe, J. Phys. Math. Theor. 45, 25303 (2012).
 M.H. Teimourpour, R. El-Ganainy, A. Eisfeld, A .Szameit, D.N. Christodoulides, Phys. Rev. A 90, 053817 (2014).
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.