Resolving the topology of encircling multiple exceptional points.

Non-Hermiticity has emerged as a new paradigm for controlling coupled-mode systems in ways that cannot be achieved with conventional techniques. One aspect of this control that has received considerable attention recently is the encircling of exceptional points (EPs). To date, most work has focused on systems consisting of two modes that are tuned by two control parameters and have isolated EPs.

Controllable oscillatory lateral coupling in a waveguide-microdisk-resonator system.

We report a theoretical and experimental study of coupling between a whispering-gallery-mode (WGM) microdisk resonator and a fiber taper which exchange energies at two distinct regions. We observe an oscillatory behavior in the coupling strength as a function of the distance between the two coupling regions when a fiber taper is moved laterally above the resonator at fixed vertical distance. This oscillation is clearly seen in the linewidth of the resonance as well as in the on-resonance transmission.

Exceptional points enhance sensing in an optical microcavity.

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.

Raman lasing and Fano lineshapes in a packaged fiber-coupled whispering-gallery-mode microresonator.

We report Raman lasing and the optical analog of electromagnetically-induced-transparency (EIT) in a whispering-gallery-mode (WGM) microtoroid resonator embedded in a low refractive index polymer matrix together with a tapered fiber coupler. The microtoroid resonator supports both single mode and multimode Raman lasing with low power thresholds. Observations of Fano and EIT-like phenomena in a packaged microresonator will enable high resolution sensors and can be used in networks where slow-light effect is needed.

Ultrafast laser-probing spectroscopy for studying molecular structure of protein aggregates.

We report the development of a new technique to screen protein aggregation based on laser-probing spectroscopy with sub-picosecond resolution. Protein aggregation is an important topic for materials science, fundamental biology as well as clinical studies in neurodegenerative diseases and translation studies in biomaterials engineering. However, techniques to study protein aggregation and assembly are limited to infrared spectroscopy, fluorescent assays, immunostaining, or functional assays among others.

Metrology with PT-Symmetric Cavities: Enhanced Sensitivity near the PT-Phase Transition.

We propose and analyze a new approach based on parity-time (PT) symmetric microcavities with balanced gain and loss to enhance the performance of cavity-assisted metrology. We identify the conditions under which PT-symmetric microcavities allow us to improve sensitivity beyond what is achievable in loss-only systems. We discuss the application of PT-symmetric microcavities to the detection of mechanical motion, and show that the sensitivity is significantly enhanced near the transition point from unbroken- to broken-PT regimes.

Stimulated Brillouin scattering and Brillouin-coupled four-wave-mixing in a silica microbottle resonator.

We report the first observation of stimulated Brillouin scattering (SBS) with Brillouin lasing, and Brillouin-coupled four-wave-mixing (FWM) in an ultra-high-Q silica microbottle resonator. The Brillouin lasing was observed at the frequency of ΩB = 2π × 10.4 GHz with a threshold power of 0.

Chiral modes and directional lasing at exceptional points.

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.

Gain competition induced mode evolution and resonance control in erbium-doped whispering-gallery microresonators.

Precise control of resonance features in microcavities is of significant importance both for researches and applications. By exploiting gain provided by the doped rare earth ions or Raman gain, this can be achieved through changing the pump. Here we propose and experimentally show that by using gain competition, one can also control the evolution of resonance for the probe signal while the pump is kept unchanged.

Vertically coupled microresonators and oscillatory mode splitting in photonic molecules.

We report the formation of a photonic molecule by vertically coupling two microdisk resonators. We show that mode splitting monotonously increases when one resonator vertically approaches the other. However, when the vertical distance is kept fixed and one resonator is moved horizontally with respect to the other, the strength of coupling and hence the mode splitting demonstrate an oscillatory behavior.

Transient microcavity sensor.

A transient and high sensitivity sensor based on high-Q microcavity is proposed and studied theoretically. There are two ways to realize the transient sensor: monitor the spectrum by fast scanning of probe laser frequency or monitor the transmitted light with fixed laser frequency. For both methods, the non-equilibrium response not only tells the ultrafast environment variance, but also enable higher sensitivity.

Raman gain induced mode evolution and on-demand coupling control in whispering-gallery-mode microcavities.

Waveguide-coupled optical resonators have played an important role in a wide range of applications including optical communication, sensing, nonlinear optics, slow/fast light, and cavity QED. In such a system, the coupling regimes strongly affect the resonance feature in the light transmission spectra, and hence the performance and outcomes of the applications. Therefore it is crucial to control the coupling between the waveguide and the microresonator.

Distillation of photon entanglement using a plasmonic metamaterial.

Plasmonics is a rapidly emerging platform for quantum state engineering with the potential for building ultra-compact and hybrid optoelectronic devices. Recent experiments have shown that despite the presence of decoherence and loss, photon statistics and entanglement can be preserved in single plasmonic systems. This preserving ability should carry over to plasmonic metamaterials, whose properties are the result of many individual plasmonic systems acting collectively, and can be used to engineer optical states of light.

Lithium-Niobate-Silica Hybrid Whispering-Gallery-Mode Resonators.

Lithium-niobate-silica hybrid resonators with quality factors higher than 10(5) are fabricated by depositing a layer of polycrystalline lithium niobate on the flat top surfaces of inverted-wedge silica microdisk resonators. All-optical modulation with improved performance over silica-only resonators and electro-optic modulation not achievable in silica-only resonators are realized in the hybrid resonators.

What is and what is not electromagnetically induced transparency in whispering-gallery microcavities.

There has been an increasing interest in all-optical analogues of electromagnetically induced transparency and Autler-Townes splitting. Despite the differences in their underlying physics, both electromagnetically induced transparency and Autler-Townes splitting are quantified by a transparency window in the absorption or transmission spectrum, which often leads to a confusion about its origin. While the transparency window in electromagnetically induced transparency is a result of Fano interference among different transition pathways, in Autler-Townes splitting it is the result of strong field-driven interactions leading to the splitting of energy levels.

Highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman microlaser.

Optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms for label-free detection of nano-objects. The ultimate sensitivity of WGMRs is determined by the strength of the light-matter interaction quantified by quality factor/mode volume, Q/V, and the resolution is determined by Q. To date, to improve sensitivity and precision of detection either WGMRs have been doped with rare-earth ions to compensate losses and increase Q or plasmonic resonances have been exploited for their superior field confinement and lower V.

Inverted-wedge silica resonators for controlled and stable coupling.

Silica microresonators with an inverted-wedge shape were fabricated using conventional semiconductor fabrication methods. The measured quality factors of the resonators were greater than 10(6) in 1550 nm band. Controllable coupling from undercoupling to the overcoupling regime through the critical coupling point was demonstrated by horizontally moving a fiber taper while in touch with the top surface of the resonator.

Quantum internet using code division multiple access.

A crucial open problem inS large-scale quantum networks is how to efficiently transmit quantum data among many pairs of users via a common data-transmission medium. We propose a solution by developing a quantum code division multiple access (q-CDMA) approach in which quantum information is chaotically encoded to spread its spectral content, and then decoded via chaos synchronization to separate different sender-receiver pairs. In comparison to other existing approaches, such as frequency division multiple access (FDMA), the proposed q-CDMA can greatly increase the information rates per channel used, especially for very noisy quantum channels.

Detection and size measurement of individual hemozoin nanocrystals in aquatic environment using a whispering gallery mode resonator.

We, for the first time, report the detection and the size measurement of single nanoparticles (i.e. polystyrene) in aquatic environment using mode splitting in a whispering gallery mode (WGM) optical resonator, namely a microtoroid resonator.

Photonic molecules formed by coupled hybrid resonators.

We describe a method that enables free-standing whispering-gallery-mode microresonators, and report spectral tuning of photonic molecules formed by coupled free and on-chip resonators with different geometries and materials. We study direct coupling via evanescent fields of free silica microtoroids and microspheres with on-chip polymer coated silica microtoroids. We demonstrate thermal tuning of resonance modes to achieve maximal spectral overlap, mode splitting induced by direct coupling, and the effects of distance between the resonators on the splitting spectra.

Single virus and nanoparticle size spectrometry by whispering-gallery-mode microcavities.

Detecting and characterizing single nanoparticles and airborne viruses are of paramount importance for disease control and diagnosis, for environmental monitoring, and for understanding size dependent properties of nanoparticles for developing innovative products. Although single particle and virus detection have been demonstrated in various platforms, single-shot size measurement of each detected particle has remained a significant challenge. Here, we present a nanoparticle size spectrometry scheme for label-free, real-time and continuous detection and sizing of single Influenza A virions, polystyrene and gold nanoparticles using split whispering-gallery-modes (WGMs) in an ultra-high-Q resonator.

Detecting single viruses and nanoparticles using whispering gallery microlasers.

There is a strong demand for portable systems that can detect and characterize individual pathogens and other nanoscale objects without the use of labels, for applications in human health, homeland security, environmental monitoring and diagnostics. However, most nanoscale objects of interest have low polarizabilities due to their small size and low refractive index contrast with the surrounding medium. This leads to weak light-matter interactions, and thus makes the label-free detection of single nanoparticles very difficult.

Demonstration of local expansion toward large-scale entangled webs.

We demonstrate an optical gate that increases the size of polarization-based W states by accessing only one of the qubits. Using this gate, we have generated three-photon and four-photon W states with fidelities 0.836 ± 0.

Controlled manipulation of mode splitting in an optical microcavity by two Rayleigh scatterers.

We report controlled manipulation of mode splitting in an optical microresonator coupled to two nanoprobes. It is demonstrated that, by controlling the positions of the nanoprobes, the split modes can be tuned simultaneously or individually and experience crossing or anti-crossing in frequency and linewidth. A tunable transition between standing wave mode and travelling wave mode is also observed.

Self-pulsation in fiber-coupled, on-chip microcavity lasers.

We report self-pulsation in an erbium-doped silica toroidal microcavity laser coupled to a tapered fiber and investigate the effects of pump power and taper-cavity coupling condition on the dynamic behaviors of the pulse train. The microcavity is pumped at 1444.8 nm, and lasing occurs at 1560.