Richard Zeltner

High-Q MgF 2 whispering gallery mode resonators for refractometric sensing in aqueous environment

We present our experiments on refractometric sensing with ultrahigh-Q, crystalline, birefringent magnesium fluoride (MgF₂) whispering gallery mode resonators. The difference to fused silica which is most commonly used for sensing experiments is the small refractive index of MgF₂ which is very close to that of water. Compared to fused silica this leads to more than 50% longer evanescent fields and a 4.25 times larger sensitivity. Moreover the birefringence amplifies the sensitivity difference between TM and TE type modes which will enhance sensing experiments based on difference frequency measurements. We estimate the performance of our resonators and compare them with fused silica theoretically and present experimental data showing the interferometrically measured evanescent field decay and the sensitivity of mm-sized MgF₂ whispering gallery mode resonators immersed in water. These data show reasonable agreement with the developed theory. Furthermore, we observe stable Q factors in water well above 1 × 10⁸.

Polarization-Selective Out-Coupling of Whispering-Gallery Modes

Whispering gallery mode (WGM) resonators are an important building block for linear, nonlinear and quantum optical experiments. In such experiments, independent control of coupling rates to different modes can lead to improved conversion efficiencies and greater flexibility in generation of non-classical states based on parametric down conversion. In this work, we introduce a scheme which enables selective out-coupling of WGMs belonging to a specific polarization family, while the orthogonally polarized modes remain largely unperturbed. Our technique utilizes material birefringence in both the resonator and coupler such that a negative (positive) birefringence allows selective coupling to TE (TM) polarized WGMs. We formulate a new coupling condition suitable for describing the case where the refractive indices of the resonator and the coupler are almost the same, from which we derive the criterion for polarization-selective coupling. We experimentally demonstrate our proposed method using a lithium niobate disk resonator coupled to a lithium niobate prism, where we show a 22dB suppression of coupling to TM modes relative to TE modes.

Fluorescence-based remote irradiation sensor in liquid-filled hollow-core photonic crystal fiber

We report an irradiation sensor based on a fluorescent “flying particle” that is optically trapped and propelled inside the core of a water-filled hollow-core photonic crystal fiber. When the moving particle passes through an irradiated region, its emitted fluorescence is captured by guided modes of the fiber core and so can be monitored using a filtered photodiode placed at the fiber end. The particle speed and position can be precisely monitored using in-fiber Doppler velocimetry, allowing the irradiation profile to be measured to a spatial resolution of ∼10 μm. The spectral response can be readily adjusted by appropriate choice of particle material. Using dye-doped polystyrene particles, we demonstrate detection of green (532 nm) and ultraviolet (340 nm) light.

Flying particle microlaser and temperature sensor in hollow-core photonic crystal fiber

Whispering-gallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dye-doped microparticle within a liquid-filled hollow-core photonic crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into core-guided modes that can be detected at the endfaces of the HC-PCF. Using radiation forces, the microlaser can be freely propelled along the HC-PCF over multi-centimeter distances-orders of magnitude farther than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperature-dependent frequency shift of the lasing modes, and may also permit precise delivery of light to remote locations.

Broadband, Lensless, and Optomechanically Stabilized Coupling into Microfluidic Hollow-Core Photonic Crystal Fiber Using Glass Nanospike

We report a novel technique for launching broadband laser light into liquid-filled hollow-core photonic crystal fiber (HC-PCF). It uniquely offers self-alignment and self-stabilization via optomechanical trapping of a fused silica nanospike, fabricated by thermally tapering and chemically etching a single mode fiber into a tip diameter of 350 nm. We show that a trapping laser, delivering ∼300 mW at 1064 nm, can be used to optically align and stably maintain the nanospike at the core center. Once this is done, a weak broadband supercontinuum signal (∼575–1064 nm) can be efficiently and close to achromatically launched in the HC-PCF. The system is robust against liquid-flow in either direction inside the HC-PCF, and the Fresnel back-reflections are reduced to negligible levels compared to free-space launching or butt-coupling. The results are of potential relevance for any application where the efficient delivery of broadband light into liquid-core waveguides is desired.

Long-range optical binding in a hollow-core photonic crystal fiber using higher order modes

We report long-range optical binding of multiple polystyrene nanoparticles (100-600 nm in diameter) at fixed interparticle distances that match multiples of the half-beat-lengths between the lowest order modes of a hollow-core photonic crystal fiber. Analysis suggests that each nanoparticle converts the incoming optical mode into a superposition of co-propagating modes, within the beat pattern of which further particles can become trapped. Strikingly, the entire particle arrangement can be moved over a distance of several cm, without changing the inter-particle spacing, by altering the ratio of backward-to-forward optical power. Potential applications are in multi-dimensional nanoparticle-based quantum optomechanical systems.

Long-range optical trapping and binding of microparticles in hollow-core photonic crystal fibre

Optically levitated micro- and nanoparticles offer an ideal playground for investigating photon–phonon interactions over macroscopic distances. Here we report the observation of long-range optical binding of multiple levitated microparticles, mediated by intermodal scattering and interference inside the evacuated core of a hollow-core photonic crystal fibre (HC-PCF). Three polystyrene particles with a diameter of 1 µm are stably bound together with an inter-particle distance of ~40 μm, or 50 times longer than the wavelength of the trapping laser. The levitated bound-particle array can be translated to-and-fro over centimetre distances along the fibre. When evacuated to a gas pressure of 6 mbar, the collective mechanical modes of the bound-particle array are able to be observed. The measured inter-particle distance at equilibrium and mechanical eigenfrequencies are supported by a novel analytical formalism modelling the dynamics of the binding process. The HC-PCF system offers a unique platform for investigating the rich optomechanical dynamics of arrays of levitated particles in a well-isolated and protected environment.Optical trapping: multiple particle bindingMicroparticles placed inside the evacuated hollow core of a photonic crystal fibre can be bound together by optical forces and moved as a single entity along the fibre by light. Dmitry Bykov and coworkers from the Max Planck Institute for the Science of Light in Erlangen, Germany performed the experiments with three, 1 µm-diameter polystyrene particles. When light pulses from a Ti:Sa laser were injected into both ends of the photonic crystal fibre, intermodal interference was found to be able to bind the three particles together, with a particle spacing of ~40 µm. It is thought that by adjusting parameters such as the fibre design and the laser pulse duration, chirp and power it should be possible to increase both the inter-particle binding distance and the number of bound particles.

High resolution position measurement of “flying particles” inside hollow-core photonic crystal fiber

Optically trapped “flying particles” inside hollow core photonic crystal fiber (HC-PCF) can be used as multiparameter sensors of, for example, temperature, radiation levels or external electric fields. They represent a new type of optical fiber sensor, offering a spatial resolution that is only limited by the particle size, while being functionally reconfigurable. Here we demonstrate accurate measurement of the axial position of flying particles using incoherent optical frequency domain reflectometry in combination with model-based estimation processing. The approach allows to measure the particle position inside the HC-PCF with a precision of ∼140 pm.

Dissipative optomechanical cooling of a glass-fiber nanospike coupled to a bottle resonator

Optical cooling of mechanical degrees of freedom is one of the biggest achievements of cavity optomechanics. Although it has mostly been demonstrated in the dispersive coupling regime, where the mechanical motion modulates the cavity frequency, in the dissipative coupling regime, i.e., when the mechanical motion changes the decay rate of the cavity, cooling can be achieved outside the stringent “good cavity” limit. In the most common experimental configurations of cavity optomechanics, however, where free-standing waveguides are evanescently coupled to an optical micro-cavity, low mechanical Q-factors have so far prohibited observation of dissipative cooling. Recently we reported that glass-fiber nanospikes, fashioned by tapering single-mode fibers, support high-Q flexural resonances (Q > 105) in the few kHz range, at the same time providing low loss, adiabatic guidance of light. Here we report the use of a silica nanospike to demonstrate dissipative cooling and amplification, by coupling it to an ultra-high-quality bottle resonator. In particular an effective temperature of 1.8 K can be inferred from the measurement of the mechanical power spectrum for a launched optical power of only ~200 µW. We believe this system could open the door to optomechanical cooling of low frequency mechanical resonators beyond the sideband-resolved regime.Optical cooling of mechanical degrees of freedom is one of the biggest achievements of cavity optomechanics. Although it has mostly been demonstrated in the dispersive coupling regime, where the mechanical motion modulates the cavity frequency, in the dissipative coupling regime, i.e., when the mechanical motion changes the decay rate of the cavity, cooling can be achieved outside the stringent “good cavity” limit. In the most common experimental configurations of cavity optomechanics, however, where free-standing waveguides are evanescently coupled to an optical micro-cavity, low mechanical Q-factors have so far prohibited observation of dissipative cooling. Recently we reported that glass-fiber nanospikes, fashioned by tapering single-mode fibers, support high-Q flexural resonances (Q > 105) in the few kHz range, at the same time providing low loss, adiabatic guidance of light. Here we report the use of a silica nanospike to demonstrate dissipative cooling and amplification, by coupling it to an ultra-h...

Fluorescence-based flying-particle sensor in liquid-filled hollow-core photonic crystal fiber

We present a novel irradiation sensor based on a fluorescent microparticle that is optically guided inside the core of a liquid-filled photonic crystal fiber. We demonstrate irradiance measurements with spatial resolution of ~10 μm.