Absorption-induced transmission in plasma microphotonics.

Ionised gas, i.e., plasma, is a medium where electrons-ions dynamics are electrically and magnetically altered. Electric and magnetic fields can modify plasma's optical loss, refraction, and gain. Still, plasma's low pressure and large electrical fields have presented as challenges to introducing it to micro-cavities. Here we demonstrate optical microresonators, with walls thinner than an optical wavelength, that contain plasma inside them. By having an optical mode partially overlapping with plasma, we demonstrate resonantly enhanced light-plasma interactions. In detail, we measure plasma refraction going below one and plasma absorption that turns the resonator transparent. Furthermore, we photograph the plasma's micro-striations, with 35 µm wavelength, indicating magnetic fields interacting with plasma. The synergy between micro-photonics and plasma might transform micro-cavities, and electro-optical interconnects by adding additional knobs for electro-optically controlling light using currents, electric-, and magnetic-fields. Plasma might impact microphotonics by enabling new types of microlasers and electro-optical devices.

A Liquid Mirror Resonator.

We present the first experimental demonstration of a Fabry‒Perot resonator that utilizes total internal reflection from a liquid-gas interface. Our hybrid resonator hosts both optical and capillary waves that mutually interact. Except for the almost perfect reflection by the oil-air interface at incident angles smaller than the critical angle, reflections from the liquid-phase boundary permit optically examining thermal fluctuations and capillary waves at the oil surface. Characterizing our optocapillary Fabry‒Perot reveals optical modes with transverse cross-sectional areas of various shapes and longitudinal modes that are separated by the free spectral range. The optical finesse of our hybrid optocapillary resonator is Fo = 60, the optical quality factor is Qo = 20 million, and the capillary quality factor is Qc = 6. By adjusting the wavelength of our laser near the optical resonance wavelength, we measure the liquid's Brownian fluctuations. As expected, the low-viscosity liquid exhibits a distinct frequency of capillary oscillation, indicating operation in the underdamped regime. Conversely, going to the overdamped regime reveals no such distinct capillary frequency. Our optocapillary resonator might impact fundamental studies and applications in surface science by enabling optical interrogation, excitation, and cooling of capillary waves residing in a plane. Moreover, our optocapillary Fabry‒Perot might permit photographing thermal capillary oscillation, which the current state-of-the-art techniques do not support.

Spectrally tunable liquid resonator based on electrowetting.

We present a tunable on-chip liquid resonator in conjunction with a tapered fiber coupling scheme. The resonator consists of a glycerol droplet submerged within an immiscible liquid bath, which mitigates the effects of environmental fluctuations. The platform is fabricated using standard semiconductor techniques, which enable the future integration of photonic components for an on-chip liquid resonator device. The liquid resonator maintains its high Q-factor on chip (105) due to surface tension forming an atomically smooth liquid-liquid interface. Higher Q-factor resonance modes experienced linewidth broadening due to the random excitation of thermal capillary vibrations. Spectral tuning is demonstrated using the electrowetting effect, increasing the surface's wettability and an expansion in the droplet diameter. A maximum spectral tuning of 1.44 nm ± 5 pm is observed by applying 35 V. The tuning range is twice the free spectral range (FSR) of 0.679 nm measured at a pumping wavelength range of 770-775 nm. A 2D axisymmetric finite-element simulation shows resonance modes in good agreement with experimentally measured spectra and with predicted tuning speeds of 20 nm/s.

Positive rumination can (also) interfere with sleep: A study in a non-clinical sample.

It is postulated that negative ruminations perpetuate insomnia symptoms by increasing arousal. Less is known about the role of positive rumination. In this study, we set out to test the association between positive and negative ruminations and insomnia symptoms in a non-clinical sample, asking whether reappraisal and suppression moderate the relationship between rumination types and symptoms of insomnia. Methods: A convenience sample of 354 participants (59% women), ages 18-50, responded to online questionnaires regarding symptoms of insomnia (Insomnia Severity Index [ISI]), Emotion Regulation Questionnaire that provides separate scales for Reappraisal and Suppression, Negative Rumination (Ruminative Response Scale), Positive Rumination and Dampening (Responses to Positive Affect questionnaire), and general health and demographics. Results: About 30% of respondents had moderate to severe symptoms of insomnia according to the ISI. The primary hypothesis was tested using three moderation models, where rumination type, emotion regulation styles, and interaction terms were predictors, and ISI scores were the outcome variable. Negative rumination positively predicted ISI (ß = 0.56, p < 0.001), while the interaction terms with Reappraisal (ß = 0.02, p = 0.575) and Suppression (ß = 0.07, p = 0.092) were not significant. Dampening also positively predicted ISI (ß = 0.56, p < 0.001), with the interaction term with Reappraisal nearly significant (ß = -0.09, p = 0.060), but not with Suppression (ß = 0.08, p =0.098). Positive rumination negatively predicted ISI (ß = -0.12, p = 0.021), this relationship was reversed with emotion regulation factors in the model (ß = 0.11, p = 0.094), where the interaction with Reappraisal (ß = 0.13, p = 0.020) and Suppression (ß = -0.13, p = 0.024) were both significant. Discussion: Positive Rumination weakly and negatively correlated with ISI, but the combination with Reappraisal was associated with more insomnia symptoms. By contrast, Dampening was associated with more insomnia symptoms, with minimal to no moderating effects. These observations are interpreted in the context of the role of emotion regulation strategies and sleep, and their potential clinical implications.

A Monolithic 3D Printed Axisymmetric Co-Flow Single and Compound Emulsion Generator.

We report a microfluidic droplet generator which can produce single and compound droplets using a 3D axisymmetric co-flow structure. The design considered for the fabrication of the device integrated a user-friendly and cost-effective 3D printing process. To verify the performance of the device, single and compound emulsions of deionized water and mineral oil were generated and their features such as size, generation frequency, and emulsion structures were successfully characterized. In addition, the generation of bio emulsions such as alginate and collagen aqueous droplets in mineral oil was demonstrated in this study. Overall, the monolithic 3D printed axisymmetric droplet generator could offer any user an accessible and easy-to-utilize device for the generation of single and compound emulsions.

Design and Fabrication of an Optical Fiber Made of Water.

In this report, an optical fiber of which the core is made solely of water, while the cladding is air, is designed and manufactured. In contrast with solid-cladding devices, capillary oscillations are not restricted, allowing the fiber walls to move and vibrate. The fiber is constructed by a high direct current (DC) voltage of several thousand volts (kV) between two water reservoirs that creates a floating water thread, known as a water bridge. Through the choice of micropipettes, it is possible to control the maximal diameter and length of the fiber. Optical fiber couplers, at both sides of the bridge, activate it as an optical waveguide, allowing researchers to monitor the water fiber capillary body waves through transmission modulation and, therefore, deducing changes in surface tension. Co-confining two important wave types, capillary and electromagnetic, opens a new path of research in the interactions between light and liquid-wall devices. Water-walled microdevices are a million times softer than their solid counterparts, accordingly improving the response to minute forces.

Cavity optofluidics: a µdroplet's whispering-gallery mode makes a µvortex.

We experimentally demonstrate light-flow interaction, in which the angular momentum of circulating light excites micro-vortices. In contrast with the solid-phase of matter, where one has to overcome static friction in order to start motion, liquids have no "static drag." Relevant to almost all optofluidic micro-systems hence, µWatt optical power is sufficient to start flows, even in liquids 50 times more viscous than water. We map the flows to be three-dimensional (3D) by using a technique based on fluorescent nano-emitters; to reveal, as expected, flow speeds proportional to power divided by viscosity.

Flying couplers above spinning resonators generate irreversible refraction.

Creating optical components that allow light to propagate in only one direction-that is, that allow non-reciprocal propagation or 'isolation' of light-is important for a range of applications. Non-reciprocal propagation of sound can be achieved simply by using mechanical components that spin1,2. Spinning also affects de Broglie waves 3 , so a similar idea could be applied in optics. However, the extreme rotation rates that would be required, owing to light travelling much faster than sound, lead to unwanted wobbling. This wobbling makes it difficult to maintain the separation between the spinning devices and the couplers to within tolerance ranges of several nanometres, which is essential for critical coupling4,5. Consequently, previous applications of optical6-17 and optomechanical10,17-20 isolation have used alternative methods. In hard-drive technology, the magnetic read heads of a hard-disk drive fly aerodynamically above the rapidly rotating disk with nanometre precision, separated by a thin film of air with near-zero drag that acts as a lubrication layer 21 . Inspired by this, here we report the fabrication of photonic couplers (tapered fibres that couple light into the resonators) that similarly fly above spherical resonators with a separation of only a few nanometres. The resonators spin fast enough to split their counter-circulating optical modes, making the fibre coupler transparent from one side while simultaneously opaque from the other-that is, generating irreversible transmission. Our setup provides 99.6 per cent isolation of light in standard telecommunication fibres, of the type used for fibre-based quantum interconnects 22 . Unlike flat geometries, such as between a magnetic head and spinning disk, the saddle-like, convex geometry of the fibre and sphere in our setup makes it relatively easy to bring the two closer together, which could enable surface-science studies at nanometre-scale separations.

Light and Capillary Waves Propagation in Water Fibers.

The confinement of light and sound, while they are traveling in fibers, enables a variety of light-matter interactions. Therefore, it is natural to ask if fibers can also host capillary waves. Capillary waves are similar to those we see when throwing a stone into a puddle. Such capillary waves are prohibited in microfluidic devices where the liquid is bounded by solid walls. In contrast, we have fabricated fibers, which are made entirely from water and are suspended in air. The water fiber can therefore move, e.g. in a resonant mode that reassembles the motion of a guitar string. In our experiment, light guided through the water fiber allows optical interrogation of is capillary oscillations. Co-confining two important oscillations in nature: capillary and electromagnetic, might allow a new type of devices called Micro-Electro-Capillary-Systems [MECS]. The softness of MECS is a million times higher when compared to what the current solid-based technology permits, which accordingly improves MECS response to minute forces such as small changes in acceleration. Additionally, MECS might allow new ways to optically interrogate viscosity and surface tension, as well as their changes caused by introducing an analyte into the system.

Liquid whispering-gallery-mode resonator as a humidity sensor.

We experimentally demonstrate the high sensitivity of a novel liquid state, whispering-gallery-mode optical resonator to humidity changes. The optical resonator used consists of a droplet made of glycerol, a transparent liquid that enables high optical quality factor, doped with fluorescent material. As glycerol is highly hygroscopic, the refractive index and radius of the droplet change with ambient humidity. This produces a shift on the whispering gallery mode's wavelengths, which modulates the emission of the fluorescent material. This device shows an unpreceded sensitivity of 10-3 per relative humidity percent.

Level-crossing and modal structure in microdroplet resonators.

We fabricate a liquid-core liquid-clad microcavity that is coupled to a standard tapered fiber, and then experimentally map the whispering-gallery modes of this droplet resonator. The shape of our resonator is similar to a thin prolate spheroid, which makes space for many high-order transverse modes, suggesting that some of them will share the same resonance frequency. Indeed, we experimentally observe that more than half of the droplet's modes have a sibling having the same frequency (to within linewidth) and therefore exhibiting a standing interference-pattern.

Regular oscillations and random motion of glass microspheres levitated by a single optical beam in air.

We experimentally report on optical binding of many glass particles in air that levitate in a single optical beam. A diversity of particle sizes and shapes interact at long range in a single Gaussian beam. Our system dynamics span from oscillatory to random and dimensionality ranges from 1 to 3D. The low loss for the center of mass motion of the beads could allow this system to serve as a standard many body testbed, similar to what is done today with atoms, but at the mesoscopic scale.

Regular oscillations and random motion of glass microspheres levitated by a single optical beam in air: publisher's note.

This publisher's note amends a recent publication [Opt. Express24(3), 2850-2857 (2016)] to include Acknowledgments.

Water-walled microfluidics for high-optical finesse cavities.

In submerged microcavities there is a tradeoff between resonant enhancement for spatial water and light overlap. Why not transform the continuously resonating optical mode to be fully contained in a water microdroplet per se? Here we demonstrate a sustainable 30-µm-pure water device, bounded almost completely by free surfaces, enabling >1,000,000 re-circulations of light. The droplets survive for >16 h using a technique that is based on a nano-water bridge from the droplet to a distant reservoir to compensate for evaporation. More than enabling a nearly-perfect optical overlap with water, atomic-level surface smoothness that minimizes scattering loss, and ∼99% coupling efficiency from a standard fibre. Surface tension in our droplet is 8,000 times stronger than gravity, suggesting a new class of devices with water-made walls, for new fields of study including opto-capillaries.

Tweezers controlled resonator.

We experimentally demonstrate trapping a microdroplet by using an optical tweezer and then activating it as a microresonator by bringing it close to a tapered-fiber coupler. Our tweezers facilitated the tuning of the coupling from the under-coupled to the critically-coupled regime while the quality-factor [Q] is 12 million and the resonator's size is at the 80 µm scale.

Optical binding in white light.

We experimentally demonstrate, for the first time, binding of aerosols of various sizes and shapes in white light. The optomechancial interaction between particles is long range and is in the underdamped regime. Incoherency allows mitigation of interference fringes to enable monotonically changing the distance between particles from 60 µm to contact, constituting a parametrically controlled testbed for transition studies at new scales.

Interfacing whispering-gallery microresonators and free space light with cavity enhanced Rayleigh scattering.

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 impede 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 issues 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.

Fabrication and testing of microfluidic optomechanical oscillators.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Finite element simulation of a perturbed axial-symmetric whispering-gallery mode and its use for intensity enhancement with a nanoparticle coupled to a microtoroid.

We present an optical mode solver for a whispering gallery resonator coupled to an adjacent arbitrary shaped nano-particle that breaks the axial symmetry of the resonator. Such a hybrid resonator-nanoparticle is similar to what was recently used for bio-detection and for field enhancement. We demonstrate our solver by parametrically studying a toroid-nanoplasmonic device and get the optimal nano-plasmonic size for maximal enhancement. We investigate cases near a plasmonic resonance as well as far from a plasmonic resonance. Unlike common plasmons that typically benefit from working near their resonance, here working far from plasmonic resonance provides comparable performance. This is because the plasmonic resonance enhancement is accompanied by cavity quality degradation through plasmonic absorption.

Brillouin cavity optomechanics with microfluidic devices.

Cavity optomechanics allows the parametric coupling of phonon- and photon-modes in microresonators and is presently investigated in a broad variety of solid-state systems. Optomechanics with superfluids has been proposed as a path towards ultra-low optical- and mechanical-dissipation. However, there have been no optomechanics experiments reported with non-solid phases of matter. Direct liquid immersion of optomechanics experiments is challenging, as the acoustic energy simply leaks out to the higher-impedance liquid surrounding the device. Here we confine liquids within hollow resonators to circumvent this issue and to enable optical excitation of mechanical whispering-gallery modes at frequencies ranging from 2 to 11,000 MHz. Our device enables optomechanical investigation with liquids, while light is conventionally coupled from the outer dry side of the capillary, and liquids are provided by means of a standard microfluidic inlet.