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 loss-induced 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.
Electromagnetically-induced-transparency (EIT) and Autler-Townes splitting (ATS) are two prominent examples of coherent interactions between optical fields and multilevel atoms. They have been observed in various physical systems involving atoms, mol
ecules, meta-structures and plasmons. In recent years, there has been an increasing interest in the implementations of all-optical analogues of EIT and ATS via the interacting resonant modes of one or more optical microcavities. Despite the differences in their underlying physics, both EIT and ATS are quantified by the appearance of a transparency window in the absorption or transmission spectrum, which often leads to a confusion about its origin. While in EIT the transparency window is a result of Fano interference among different transition pathways, in ATS it is the result of strong field-driven interactions leading to the splitting of energy levels. Being able to tell objectively whether a transparency window observed in the spectrum is due to EIT or ATS is crucial for clarifying the physics involved and for practical applications. Here we report a systematic study of the pathways leading to EIT, Fano, and ATS, in systems of two coupled whispering-gallery-mode (WGM) microtoroidal resonators. Moreover, we report for the first time the application of the Akaike Information Criterion discerning between all-optical analogues of EIT and ATS, and clarifying the transition between them.
Whispering gallery mode resonators (WGMRs) take advantage of strong light confinement and long photon lifetime for applications in sensing, optomechanics, microlasers and quantum optics. However, their rotational symmetry and low radiation loss imped
e energy exchange between WGMs and the surrounding. As a result, free-space coupling of light into and from WGMRs is very challenging. In previous schemes, resonators are intentionally deformed to break circular symmetry to enable free-space coupling of carefully aligned focused light, which comes with bulky size and alignment issue that hinder the realization of compact WGMR applications. Here, we report a new class of nanocouplers based on cavity enhanced Rayleigh scattering from nano-scatterer(s) on resonator surface, and demonstrate whispering gallery microlaser by free-space optical pumping of an Ytterbium doped silica microtoroid via the scatterers. This new scheme will not only expand the range of applications enabled by WGMRs, but also provide a possible route to integrate them into solar powered green photonics.
We demonstrate a thermal infrared (IR) detector based on an ultra-high-quality-factor (Q) whispering-gallery-mode (WGM) microtoroidal silica resonator, and investigate its performance to detect IR radiation at 10 micron wavelength. The bandwidth and
the sensitivity of the detector are dependent on the power of a probe laser and the detuning between the probe laser and the resonance frequency of the resonator. The microtoroid IR sensor achieved a noise-equivalent-power (NEP) of 7.46 nW, corresponding to IR intensity of 0.095 mW/cm^2
Recently optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms to achieve label-free detection of nanoscale objects and to reach single molecule sensitivity. The ultimate detection performance of WGMRs are limited by
energy dissipation in the material they are fabricated from. Up to date, to improve detection limit, either rare-earth ions are doped into the WGMR to compensate losses or plasmonic resonances are exploited for their superior field confinement. Here, we demonstrate, for the first time, enhanced detection of single-nanoparticle induced mode-splitting in a silica WGMR via Raman-gain assisted loss-compensation and WGM Raman lasing. Notably, we detected and counted individual dielectric nanoparticles down to a record low radius of 10 nm by monitoring a beatnote signal generated when split Raman lasing lines are heterodyne-mixed at a photodetector. This dopant-free scheme retains the inherited biocompatibility of silica, and could find widespread use for sensing in biological media. It also opens the possibility of using intrinsic Raman or parametric gain in other systems, where dissipation hinders the progress of the field and limits applications.
An add-drop filter (ADF) fabricated using a whispering gallery mode resonator has different crosstalks for add and drop functions due to non-zero intrinsic losses of the resonator. Here, we show that introducing gain medium in the resonator and optic
ally pumping it below the lasing threshold not only allows loss compensation to achieve similar and lower crosstalks but also tunability in bandwidth and add-drop efficiency. For an active ADF fabricated using an erbium-ytterbium co-doped microsphere, we achieved 24-fold enhancement in the intrinsic quality factor, 3.5-fold increase in drop efficiency, bandwidth tunability of 35 MHz and a crosstalk of only 2%.
Optical systems combining balanced loss and gain profiles provide a unique platform to implement classical analogues of quantum systems described by non-Hermitian parity-time- (PT-) symmetric Hamiltonians and to originate new synthetic materials with
novel properties. To date, experimental works on PT-symmetric optical systems have been limited to waveguides in which resonances do not play a role. Here we report the first demonstration of PT-symmetry breaking in optical resonator systems by using two directly coupled on-chip optical whispering-gallery-mode (WGM) microtoroid silica resonators. Gain in one of the resonators is provided by optically pumping Erbium (Er3+) ions embedded in the silica matrix; the other resonator exhibits passive loss. The coupling strength between the resonators is adjusted by using nanopositioning stages to tune their distance. We have observed reciprocal behavior of the PT-symmetric system in the linear regime, as well as a transition to nonreciprocity in the PT symmetry-breaking phase transition due to the significant enhancement of nonlinearity in the broken-symmetry phase. Our results represent a significant advance towards a new generation of synthetic optical systems enabling on-chip manipulation and control of light propagation.
A nanoparticle detection scheme with single particle resolution is presented. The sensor contains only a taper fiber thus offering the advantages of compactness and installation flexibility. Sensing method is based on monitoring the transmitted light
power which shows abrupt jumps with each particle binding to the taper surface. The experimental validation of the sensor is demonstrated with polystyrene nanoparticles of radii 120 nm and 175 nm in the 1550 nm wavelength band.
We investigate numerically and experimentally the statistics of the changes in the amount of frequency splitting upon the adsorption of particles one-by-one into the mode volume of whispering gallery mode (WGM) microresonator and microlasers. This mu
ltiple-particle induced frequency splitting (MPIFS) statistics carries information on the size and the number of adsorbed particles into the mode volume, and it is strongly affected by two experimental parameters, namely the WGM field distribution and the positions of the particles within the mode volume. We show that the standard deviation and maximum value of the MPIFS are proportional to the polarizability of the particles, and propose a method to estimate particle size from the MPIFS if the only available data from experiments is frequency splitting.
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, portab
le, 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.
