Jiangang Zhu

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. Micro- and nano-photonic devices have emerged as highly sensitive platforms for such applications, because the combination of high quality factor Q and small mode volume V leads to significantly enhanced light-matter interactions. For example, whispering gallery mode microresonators have been used to detect and characterize single influenza virions and polystyrene nanoparticles with a radius of 30 nm (ref. 12) by measuring in the transmission spectrum either the resonance shift or mode splitting induced by the nanoscale objects. Increasing Q leads to a narrower resonance linewidth, which makes it possible to resolve smaller changes in the transmission spectrum, and thus leads to improved performance. Here, we report a whispering gallery mode microlaser-based real-time and label-free detection method that can detect individual 15-nm-radius polystyrene nanoparticles, 10-nm gold nanoparticles and influenza A virions in air, and 30 nm polystyrene nanoparticles in water. Our approach relies on measuring changes in the beat note that is produced when an ultra-narrow emission line from a whispering gallery mode microlaser is split into two modes by a nanoscale object, and these two modes then interfere. The ultimate detection limit is set by the laser linewidth, which can be made much narrower than the resonance linewidth of any passive resonator. This means that microlaser sensors have the potential to detect objects that are too small to be detected by passive resonator sensors.

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

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.Significance To date, loss compensation in optical microresonators has been done using rare-earth ions, which requires additional processing steps and costs and raises biocompatibility concerns. An alternative to integrating rare-earth ions for loss compensation is the use of intrinsic gain mechanisms such as Raman and parametric gain present in the materials from which resonators are fabricated. Here, we report the first implementation to our knowledge of Raman gain-induced loss compensation in silica whispering-gallery-mode (WGM) resonators for improved detection and the first demonstration to our knowledge of mode splitting in a WGM Raman microlaser for detecting and counting single nanoparticles down to 10 nm. This intrinsically self-referenced, self-heterodyned, and biocompatible scheme has enabled achieving record-high polarizability sensitivity (down to 3.82 × 10−6 μm3) without using plasmonic effects, passive or active stabilization, or frequency locking. 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. Here, we demonstrate, for the first time to our knowledge, enhanced detection of single-nanoparticle-induced mode splitting in a silica WGMR via Raman gain-assisted loss compensation and WGM Raman microlaser. In particular, the use of the Raman microlaser provides a dopant-free, self-referenced, and self-heterodyned scheme with a detection limit ultimately determined by the thermorefractive noise. Notably, we detected and counted individual nanoparticles with polarizabilities down to 3.82 × 10−6 μm3 by monitoring a heterodyne beatnote signal. This level of sensitivity is achieved without exploiting plasmonic effects, external references, or active stabilization and frequency locking. Single nanoparticles are detected one at a time; however, their characterization by size or polarizability requires ensemble measurements and statistical averaging. This dopant-free scheme retains the inherited biocompatibility of silica and could find widespread use for sensing in biological media. The Raman laser and operation band of the sensor can be tailored for the specific sensing environment and the properties of the targeted materials by changing the pump laser wavelength. This scheme also opens the possibility of using intrinsic Raman or parametric gain for loss compensation in other systems where dissipation hinders progress and limits applications.

Compensation of thermal refraction effect in high-Q toroidal microresonator by polydimethylsiloxane coating

We experimentally and theoretically characterize the thermal refraction effect in a silica microtoroid and demonstrate that such effect can be reduced or even eliminated by applying a thin layer of polydimethylsiloxane (PDMS) to the surface of the silica resonator. By increasing the coating thickness, the whispering gallery modes (WGMs) experience a transition from redshift to blueshift induced by thermal absorption. Experiment results demonstrate that at the thickness of 0.52 μm, the fundamental WGM with observed Q factor of 1.5×106 shows no shift with the input optical power since the thermal refraction of the silica for this mode is compensated completely by the PDMS layer, which has an opposite thermal refraction effect. This work shows that the PDMS layer could be used to reduce thermal noise in high-Q silica microcavities for applications in sensing, lasing, and nonlinear optics.

Electromagnetically induced transparency-like effect in a single polydimethylsiloxane-coated silica microtoroid

We study both experimentally and theoretically the coupling between a fiber taper and two whispering-gallery modes, which are simultaneously excited in a single pilydimethyl-siloxane-coated silica microtoroid system. The transmission spectrum of the fiber-coupled two-mode microresonator shows a sharp electromagnetically induced transparency-like window within the resonant absorption region. This line shape results from destructive interference between two optical pathways associated with two distinct coexisting modes in a single resonator. The sharp transparency peak has great potential applications in light modulation and highly sensitive biochemical sensors.

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. Underlying physics is discussed by developing a two-scatterer model which can be extended to multiple sscatterers. Observed rich dynamics and tunability of split modes in a single microresonator will find immediate applications in optical sensing, opto-mechanics, filters and will provide a platform to study strong light-matter interactions in two-mode cavities.

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. We show that the size of each particle and virion can be measured as they continuously bind to the resonator one-by-one, eliminating the need for ensemble measurements, stochastic analysis or imaging techniques employed in previous works. Moreover, we show that our scheme has the ability to identify the components of particle mixtures.

Oscillatory thermal dynamics in high-Q PDMS-coated silica toroidal microresonators

We study the oscillatory thermal dynamics of a high-Q PDMS-coated silica microtoroid both experimentally and theoretically. We demonstrate that the competing thermo-optic effects in silica and PDMS lead to thermally-induced self-modulation in the transmission spectra. A dynamical model is built using thermal dynamics and coupled-mode theory to analyze the oscillation behaviors. Effects of input power, taper-cavity air gap and wavelength scanning speed on the oscillation behaviors are investigated with a detailed comparison between theory and experiments.

Demonstration of mode splitting in an optical microcavity in aqueous environment

Scatterer induced modal coupling and the consequent mode splitting in a whispering gallery mode resonator is demonstrated in aqueous environment. The rate of change in splitting as particles enter the resonator mode volume strongly depends on the concentration of particle solution. The higher is the concentration, the higher is the rate of change. Polystyrene nanoparticles of radius 50 nm with concentration as low as 5×10−6 wt % have been detected using the mode splitting spectra. Observation of mode splitting in water paves the way for constructing advanced resonator based sensors for measuring nanoparticles and biomolecules in various environments.

Observation and characterization of mode splitting in microsphere resonators in aquatic environment

Whispering gallery mode (WGM) optical resonators utilizing resonance shift (RS) and mode splitting (MS) techniques have emerged as highly sensitive platforms for label-free detection of nanoscale objects. RS method has been demonstrated in various resonators in air and liquid. MS in microsphere resonators has not been achieved in aqueous environment up to date, despite its demonstration in microtoroid resonators. Here, we demonstrate scatterer-induced MS of WGMs in microsphere resonators in water. We determine the size range of particles that induces MS in a microsphere in water as a function of resonator mode volume and quality factor. The results are confirmed by the experimental observations.

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.