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†These authors contributed equally to this work.Controlling and reversing the effects of loss are major challenges in optical systems. For lasers losses need to be overcome by a sufficient amount of gain to reach the lasing threshold. We show how to turn losses into gain by steering the parameters of a system to the vicinity of an exceptional point (EP), which occurs when the eigenvalues and the corresponding eigenstates of a system coalesce. In our system of coupled microresonators, EPs are manifested as the lossinduced suppression and revival of lasing. Below a critical value, adding loss annihilates an existing Raman laser. Beyond this critical threshold, lasing recovers despite the increasing loss, in stark contrast to what would be expected from conventional laser theory. Our results exemplify the counterintuitive features of EPs and present an innovative method for reversing the effect of loss.2 Dissipation is ubiquitous in nature; the states of essentially all physical systems thus have a finite decay time. A proper description of this situation requires a departure from conventional Hermitian models with real eigenvalues and orthogonal eigenstates to non-Hermitian models featuring complex eigenvalues and nonorthogonal eigenstates (1,2,3). When tuning the parameters of such a dissipative system, its complex eigenvalues and the corresponding eigenstates may coalesce, giving rise to a non-Hermitian degeneracy, also called an Exceptional Point (EP) (4). The presence of such an EP has a dramatic effect on the system, leading to nontrivial physics with interesting counterintuitive features such as "resonance trapping" (5), a mode exchange when encircling an EP (6), and a singular topology in the parameter landscape (7). These characteristics can control the flow of light in optical devices with both loss and gain.In particular, waveguides having parity-time symmetry (8), where loss and gain are balanced, have attracted enormous attention (9,10), with effects such as loss-induced transparency (11), unidirectional invisibility (12), and reflectionless scattering (13,14) having been already observed.Theoretical work indicates that EPs give rise to many more intriguing effects when they occur near the lasing regime; for example, enhancement of the laser linewidth (15,16), fast selfpulsations (15), and a pump-induced lasing death (17). Realizing such anomalous phenomena, however, requires moving from waveguides to resonators, which can trap and amplify light resonantly beyond the lasing threshold. With the availability of such devices (18,19), we discuss here the most counterintuitive aspect that close to an EP lasing should be inducible solely by adding loss to a resonator.Our experimental system (20) consists of two directly-coupled silica whispering-gallery-mode resonators (WGMRs) μR 1 and μR 2 , each coupled to a different fiber-taper WG1 and WG2 ( Fig. 1A and sec. S1). The resonance frequencies of the WGMRs were tuned to be the same via thermo-optic effect, and a controllable coupling strength κ was achieve...

Physical systems with loss or gain feature resonant modes that are decaying or growing exponentially with time. Whenever two such modes coalesce both in their resonant frequency and their rate of decay or growth, a so-called "exceptional point" occurs, around which many fascinating phenomena have recently been reported to arise [1][2][3][4][5][6] . Particularly intriguing behavior is predicted to appear when encircling an exceptional point sufficiently slowly 7,8 , like a state-flip or the accumulation of a geometric phase 9,10 . Experiments dedicated to this issue could already successfully explore the topological structure of exceptional points [11][12][13] , but a full dynamical encircling and the breakdown of adiabaticity inevitably associated with it 14-21 remained out of reach of any measurement so far. Here we

We demonstrate that the above-threshold behavior of a laser can be strongly affected by exceptional points which are induced by pumping the laser nonuniformly. At these singularities, the eigenstates of the non-Hermitian operator which describes the lasing modes coalesce. In their vicinity, the laser may turn off even when the overall pump power deposited in the system is increased. Such signatures of a pump-induced exceptional point can be experimentally probed with coupled ridge or microdisk lasers.