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Sensors play an important part in many aspects of daily life such as infrared sensors in home security systems, particle sensors for environmental monitoring and motion sensors in mobile phones. High-quality optical microcavities are prime candidates for sensing applications because of their ability to enhance light-matter interactions in a very confined volume. Examples of such devices include mechanical transducers, magnetometers, single-particle absorption spectrometers, and microcavity sensors for sizing single particles and detecting nanometre-scale objects such as single nanoparticles and atomic ions. Traditionally, a very small perturbation near an optical microcavity introduces either a change in the linewidth or a frequency shift or splitting of a resonance that is proportional to the strength of the perturbation. Here we demonstrate an alternative sensing scheme, by which the sensitivity of microcavities can be enhanced when operated at non-Hermitian spectral degeneracies known as exceptional points. In our experiments, we use two nanoscale scatterers to tune a whispering-gallery-mode micro-toroid cavity, in which light propagates along a concave surface by continuous total internal reflection, in a precise and controlled manner to exceptional points. A target nanoscale object that subsequently enters the evanescent field of the cavity perturbs the system from its exceptional point, leading to frequency splitting. Owing to the complex-square-root topology near an exceptional point, this frequency splitting scales as the square root of the perturbation strength and is therefore larger (for sufficiently small perturbations) than the splitting observed in traditional non-exceptional-point sensing schemes. Our demonstration of exceptional-point-enhanced sensitivity paves the way for sensors with unprecedented sensitivity.

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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...

The ability to detect and size individual nanoparticles with high resolution is crucial to understanding behaviours of single particles and effectively using their strong size-dependent properties to develop innovative products. We report real-time, insitu detection and sizing of single nanoparticles, down to 30 nm in radius, using mode-splitting in a monolithic ultra-high-Q whispering-gallery-mode (WGM) microtoroid resonator. Particle binding splits a WGM into two spectrally shifted resonance modes, forming a self-referenced detection scheme. This technique provides superior noise suppression and enables extracting accurate size information in a single-shot measurement. Our method requires neither labelling of the particles nor apriori information on their presence in the medium, providing an effective platform to study nanoparticles at single particle resolution.

Detection and characterization of individual nano-scale particles, virions, and pathogens are of paramount importance to human health, homeland security, diagnostic and environmental monitoring 1 .There is a strong demand for high-resolution, portable, and cost-effective systems to make label-free detection and measurement of individual nanoparticles, molecules, and viruses 2-6 . Here, we report an easily accessible, real-time and label-free detection method with single nanoparticle resolution that surpasses detection limit of existing micro-and nano-photonic devices. This is achieved by using an ultra-narrow linewidth whispering gallery microlaser, whose lasing line undergoes frequency splitting upon the binding of individual nano-objects. We demonstrate detection of polystyrene and gold nanoparticles as small as 15 nm and 10 nm in radius, respectively, and Influenza A virions by monitoring changes in self-heterodyning beat note of the split lasing modes. Experiments are performed in both air and aqueous environment. The built-in self-heterodyne interferometric method achieved in a microlaser provides a self-reference scheme with extraordinary sensitivity 7,8 , and paves the way for detection and spectroscopy of nano-scale objects using micro-and nano-lasers.Most of the biological agents and synthetic particles of interest have low polarizability due to their small size and low refractive index contrast with the surrounding medium, leading to weak light-particle interactions which make their label-free optical detection at single particle resolution difficult. Micro-and nano-photonic resonant devices have emerged as highly sensitive platforms for detection of individual virions and nanoparticles due to the significantly enhanced light-matter interactions originating from the high ratio of their quality-factor (Q) to mode volume (V) 7,9-15 . For example, detection of single Influenza virions and polystyrene nanospheres as small as 30 nm in radius have been demonstrated in whispering gallery mode (WGM) microspheres using reactive shift 12 and in microtoroids using mode splitting technique 7 , respectively. In both techniques, wavelength of a tunable laser is scanned to obtain the transmission spectrum of a resonant mode from which specific information, either the resonance shift and/or the mode splitting, is extracted to detect and measure nanoparticles. The ultimate detection limit strongly relies on Q/V which not only determines the light-matter interaction strength but also sets the smallest resolvable changes in the WGM spectrum 16 . Higher Q, limited by material absorption, implies narrower resonance linewidth and better resolution.Here we report the first microlaser-based detection scheme with single particle resolution surpassing the detection capabilities of existing micro-and nano-photonic devices. The ultimate detection limit is set by the laser linewidth, which can be as narrow as a few Hertz for a WGM microlaser, and is certainly much narrower than the resonance linewidth of any passive resonators 17 ...

Controlling the emission and the flow of light in micro-and nanostructures is crucial for on-chip information processing. Here we show how to impose a strong chirality and a switchable direction of light propagation in an optical system by steering it to an exceptional point (EP)-a degeneracy universally occurring in all open physical systems when two eigenvalues and the corresponding eigenstates coalesce. In our experiments with a fiber-coupled whispering-gallery-mode (WGM) resonator, we dynamically control the chirality of resonator modes and the emission direction of a WGM microlaser in the vicinity of an EP: Away from the EPs, the resonator modes are nonchiral and laser emission is bidirectional. As the system approaches an EP, the modes become chiral and allow unidirectional emission such that by transiting from one EP to another one the direction of emission can be completely reversed. Our results exemplify a very counterintuitive feature of nonHermitian physics that paves the way to chiral photonics on a chip.exceptional points | asymmetric backscattering | chiral modes | directional lasing | whispering-gallery-mode resonator C hirality lies at the heart of the most fascinating and fundamental phenomena in modern physics like the quantum Hall effect (1), Majorana fermions (2), and the surface conductance in topological insulators (3) as well as in p-wave superconductors (4). In all of these cases chiral edge states exist, which propagate along the surface of a sample in a specific direction. The chirality (or handedness) is secured by specific mechanisms, which prevent the same edge state from propagating in the opposite direction. For example, in topological insulators the backscattering of edge states is prevented by the strong spin-orbit coupling of the underlying material.Transferring such concepts to the optical domain is a challenging endeavor that has recently attracted considerable attention. Quite similar to their electronic counterparts, the electromagnetic realizations of chiral states typically require either a mechanism that breaks time-reversal symmetry (5) or one that gives rise to a spinorbit coupling of light (6-8). Because such mechanisms are often not available or difficult to realize, alternative concepts have recently been proposed, which require, however, a careful arrangement of many optical resonators in structured arrays (9,10).Here, we demonstrate explicitly that the above demanding requirements on the realization of chiral optical states propagating along the surface of a system can all be bypassed by using a single resonator with non-Hermitian scattering. The key insight in this respect is that a judiciously chosen non-Hermitian outcoupling of two near-degenerate resonator modes to the environment leads to an asymmetric backscattering between them [an effect that has been ignored in previous models based on coupled mode theory (11)] and thus to an effective breaking of the time-reversal symmetry that supports chiral behavior (12). More specifically, we show that a strong spatia...