Bottle Microresonators Research Articles

Tunable SNAP microresonators via internal ohmic heating

We demonstrate a thermally tunable surface nanoscale axial photonics (SNAP) platform. Stable tuning is achieved by heating a SNAP structure fabricated on the surface of a silica capillary with a metal wire positioned inside. Heating a SNAP microresonator with a uniform wire introduces uniform variation of its effective radius which results in constant shift of its resonance wavelengths. Heating with a nonuniform wire allows local nanoscale variation of the capillary effective radius, which enables differential tuning of the spectrum of SNAP structures, as well as the creation of temporary SNAP microresonators that exist only when current is applied. As an example, we fabricate two bottle microresonators coupled to each other and demonstrate differential tuning of their resonance wavelengths into and out of degeneracy with precision better than 0.2pm. The developed approach is beneficial for ultra-precise fabrication of tunable ultralow loss parity-time symmetric, optomechanical, and cavity quantum electrodynamics (QED) devices.

Self-assembling and packaging of microbottle resonators for all-polymer lab-on-chip platform

An integrated full polymeric sensing platform composed by a self-assembled bottle microresonators packaged with planar waveguide in a stable structure is presented. Microbottles made of SU-8 resist and NOA resins have been fabricated and characterized. The interrogation system is constituted by evanescently coupled planar waveguide fabricated in SU-8 on polymethyl methacrylate substrate. A simple and effective method for the packaging of the microbottle has been adopted. It provides high mechanical stability to avoid surrounding disturbance able to affect the coupling efficiency between the resonator and the waveguide. The choice of polymers as fabrication material enables the use of this method for low-cost lab-on-chip production. The viability of the platform has been demonstrated via refractometric sensing establishing a bulk sensitivity of 120 nm per refractive index unit.

Electrospun polymer bottle microresonators for stretchable single-mode lasing devices.

We report a simple electrospinning method to fabricate polymer bottle microresonators, which are doped with a lasing gain material and supported by electrospun polymer micro/nanofibers on a flexible grooved polymer substrate. The fabricated bottle microresonators have smooth outer surfaces and high quality. By using an interference light pump approach, single whispering gallery mode lasing is obtained, with a side-mode suppression factor over 20dB. By mechanically stretching the grooved substrate, tunability of the lasing peaks is demonstrated. Our method has the advantages of saving time and being low in cost and may have promising applications in stretchable lasing and sensing devices.

Two-dimensional nonlinear modes and frequency combs in bottle microresonators.

We theoretically investigate frequency comb generation in a bottle microresonator accounting for the azimuthal and axial degrees of freedom. We first identify a discrete set of the axial nonlinear modes of a bottle microresonator that appear as tilted resonances bifurcating from the spectrum of linear axial modes. We then study azimuthal modulational instability of these modes and show that families of two-dimensional (2D) soliton states localized both azimuthally and axially bifurcate from them at critical pump frequencies. Depending on detuning, 2D solitons can be stable, form persistent breathers or chaotic spatio-temporal patterns, or exhibit collapse-like evolution.

Tunable Fano-Like Lineshape in an Adiabatic Tapered Fiber Coupled to a Hollow Bottle Microresonator

Fano-like resonances and electromagnetic induced transparency (EIT)-like resonances in whispering gallery-mode (WGM) microcavities have important applications in high-sensitivity biosensing, optical switches, and slow light. Hollow microbottle resonators (HMBRs) provide special opportunities for optofluidic devices. Here, we theoretically and experimentally demonstrate a simple system to produce Fano resonance lineshapes in which a HMBR is coupled to the transition region of an adiabatic tapered optical fiber. The Fano resonance results from the interference between the fundamental mode and higher order modes of the taper that are coupled to the WGM of the HMBR. The degree of asymmetry can be tuned by scanning the coupling position along the taper and controlling the input light polarization. It is shown that by tuning polarization, the Fano resonance line converts from a Lorentzian shape to an EIT-like line. This system potentially can be used as an optical filter with the advantage of easy alignment.

Single-mode lasing via loss engineering in fiber-taper-coupled polymer bottle microresonators

Due to the lack of mode selection capability, single whispering-gallery-mode (WGM) lasing is a challenge to achieve. In bottle microresonators, the highly nondegenerated WGMs are spatially well-separated along the long-axis direction and provide mode selection according to their axial mode numbers. In this work, we use a loss-engineering approach to suppress the higher-order WGMs and demonstrate single-mode lasing emission in small polymer bottle microresonators. The fiber tapers are not only used to couple pump light into the bottle microresonators to excite the WGMs but also to bring optical losses that are induced from the diameter mismatch between fiber tapers and microresonators. By adjusting the coupling positions, the diameters of fiber tapers, and the coupling angles, single fundamental-mode lasing is efficiently generated with side-mode suppression factors over 15 dB. Our loss-engineering approach is convenient just by moving the fiber taper and may find promising applications in miniature tunable single-mode lasers and sensors.

Optical frequency combs generated mechanically.

An elongated bottle microresonator with nanoscale parabolic effective radius variation can possess a series of dense equally spaced optical eigenfrequencies whose separation can match the eigenfrequency of its axially symmetric acoustic mode. It is shown that this acoustic mode can parametrically excite optical modes and give rise to a highly equidistant and moderately broadband optical frequency comb with the teeth spacing independent of the input laser power and the amplitude of mechanical vibrations.

Frequency comb generation in SNAP bottle resonators.

We develop a theory of optical frequency comb generation in ultra-compact surface nanoscale axial photonic (SNAP) bottle microresonators, employing the nonlinear interaction of whispering gallery modes which are confined along an optical fiber with nanoscale radius variation. We predict that a SNAP microresonator with a radius of a few micrometers can generate a frequency comb with an ultra-fine sub-gigahertz spectral spacing, which would require traditional ring resonators of centimeter radius. We identify regimes of stable or quasi-periodic comb dynamics due to soliton excitation, and show that special engineering of the SNAP radius profile can be used to compensate for nonlinearity-induced dispersion.

Multiple nonlinear resonances and frequency combs in bottle microresonators.

We introduce the generalized Lugiato-Lefever equation describing nonlinear effects in the bottle microresonators. We demonstrate that the nonlinear modes of these resonators can form multiple coexisting and overlapping nonlinear resonances and that their instabilities lead to the generation of the low repetition rate frequency combs.

Single whispering-gallery mode lasing in polymer bottle microresonators via spatial pump engineering

Single-mode lasing in whispering-gallery mode (WGM) microresonators is challenging to achieve. In bottle microresonators, the highly non-degenerated WGMs are spatially well-separated along the long-axis direction and provide mode-selection capability. In this work, by engineering the pump intensity to modify the spatial gain profiles of bottle microresonators, we demonstrate a simple and general approach to realizing single-mode WGM lasing in polymer bottle microresonators. The pump intensity is engineered into an interference distribution on the bottle microresonator surface. By tuning the spacing between axial positions of the interference pump patterns, the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the interference stripes, intrinsically enabling single-mode lasing and selection. Attractive advantages of the system, including high side-mode suppression factors >20 dB, large spectral tunability >8 nm, low-lasing threshold and reversible control, are presented. Our demonstrated approach may have a variety of promising applications, ranging from tunable single-mode lasing and sensing to nonlinear optics.

Chalcogenide microresonators tailored to distinct morphologies by the shaping of glasses on silica tapers.

Production of chalcogenide (As2Se3) microresonators in sphere, loop, and bottle morphologies by the shaping of glasses at appropriate temperatures between cleaved silica tapers is reported. The quality factors exceed QS=6.2×105, QB=6.7×105, and QL=1.6×104 for the sphere, bottle, and loop microresonators, respectively. All-optical thermally assisted tuning with a rate of 0.61 nm/mW is demonstrated for a bottle microcavity pumped via a silica taper at a wavelength of 670 nm. This technique enables practical and robust in situ production of chalcogenide microresonators thermally spliced to silica fibers in several morphologies with a wide tuning range of size.

Bottle microresonator broadband and low-repetition-rate frequency comb generator.

We propose a new type of broadband and low repetition rate (RR) frequency comb generator that has the shape of an elongated and nanoscale-shallow optical bottle microresonator created at the surface of an optical fiber. The free spectral range (FSR) of the broadband azimuthal eigenfrequency series of this resonator is the exact multiple of the FSR of the dense and narrowband axial series. The effective radius variation of the microresonator is close to a parabola with a nanoscale height that is greater or equal to λ/2πn0. (Here λ is the characteristic radiation wavelength and n0 is the refractive index of the microresonator material.) Overall, the microresonator possesses a broadband, small FSR and accurately equidistant spectrum convenient for the generation of a broadband and low RR optical frequency comb. It is shown that this comb can be generated by pumping with a cw laser, with a radiation frequency that matches a single axial eigenfrequency of the microresonator or, alternatively, by pumping with a mode-locked laser, which generates a narrowband low RR comb matching a series of equidistant axial eigenfrequencies situated between adjacent azimuthal eigenfrequencies.

Permanent matching of coupled optical bottle resonators with better than 0.16 GHz precision.

The fabrication precision is one of the most critical challenges to the creation of practical photonic circuits composed of coupled high Q-factor microresonators. While very accurate transient tuning of microresonators based on local heating has been reported, the record precision of permanent resonance positioning achieved by post-processing is still within 1 and 5 GHz. Here we demonstrate two coupled bottle microresonators fabricated at the fiber surface with resonances that are matched with a better than 0.16 GHz precision. This corresponds to a better than 0.17 Å precision in the effective fiber radius variation. The achieved fabrication precision is only limited by the resolution of our optical spectrum analyzer and can be potentially improved by an order of magnitude.

Nanoparticle detection by mode splitting in hollow bottle microresonators

Mode splitting (MS) in whispering gallery microresonators provides excellent noise suppression in sensing signal compared to mode shifting. Here, we theoretically studied the ability of hollow bottle microresonators for detection of a single nanoparticle in air and water medium by MS phenomenon. To find out the optimum condition of sensor for nanoparticle (NP) detection, the effects of bottle geometry parameters, mode orders, and mode polarization state was investigated. The first radial transverse electric mode demonstrated the best sensitivity when the resonator radius and wall thickness were 10 and 0.3 μm, respectively. However, transverse magnetic modes manifested slightly better detection limit. In the air core hollow microbottle resonator (HMBR), the best detection limit of 3.1 nm radius for polystyrene NPs was achieved at an optimum condition of 30-μm resonator radius and 0.8-μm wall thickness. While MS could not be resolved in deionized water filled HMBRs for all of the investigated conditions at 1550 nm, changing the wavelength to 780 nm provided a detection limit of 15.1 nm in water. Furthermore, it is found that the sensitivity of HMBR is increased by at least two times in comparison with a microtoroid sensor. HMBRs are optofluidic platforms, so employing them could drastically enhance the applicability of microresonator-based systems for label-free NP detection.

Ringing phenomenon in a high-Q fiber bottle microresonator

It is demonstrated that high-Q (Q∼108) bottle microresonators can be fabricated by using a CO2 laser to heat a vertical single-mode fiber with a small weight attached to its lower end. A tunable continuous-wave laser is used to excite whispering-gallery modes in a bottle microresonator through a fiber taper, and a ringing phenomenon is observed. The observed ringing phenomenon is well explained through the numerical solution of a dynamic equation. In addition, an explicit function is given to describe the light field in the resonator, and the theoretical transmission based on the function also agrees very well with the experimental ringing phenomenon.

Microscopic optical buffering in a harmonic potential.

In the early days of quantum mechanics, Schrödinger noticed that oscillations of a wave packet in a one-dimensional harmonic potential well are periodic and, in contrast to those in anharmonic potential wells, do not experience distortion over time. This original idea did not find applications up to now since an exact one-dimensional harmonic resonator does not exist in nature and has not been created artificially. However, an optical pulse propagating in a bottle microresonator (a dielectric cylinder with a nanoscale-high bump of the effective radius) can exactly imitate a quantum wave packet in the harmonic potential. Here, we propose a tuneable microresonator that can trap an optical pulse completely, hold it as long as the material losses permit, and release it without distortion. This result suggests the solution of the long standing problem of creating a microscopic optical buffer, the key element of the future optical signal processing devices.

Microgrooved plasmonic bottle microresonator

In this paper, we demonstrate an enhancement to SPW cavity through the incorporation of high-Q WGM bottle microresonator (BMR) with surface microgrooves. A standard BMR fabricated through the “soften-and-compress” technique with initial length of 280 μm, bottle diameter of 187 μm and stem diameter of 125 μm was utilized in the experiment for supporting WGMs. Thin gold film was deposited on top of the BMR for generating SPWs. 21 microgrooves was then inscribed on the metal surface of the BMR along the azimuthal direction with 10 μm length, 485 nm width, 6 μm depth and pitch of 1.5 μm. Due to surface curvature, the gold film only covered half of the BMR with a characteristic meniscus shape and maximum thickness of 30 nm. The meniscus provides appropriately tapered metal edges that facilitate the adiabatic transformation of BMR WGMs to SPWs and vice-versa. Lorentzian shape-line fit performed on the TM excited resonances show that plasmonic Q values in excess of 4000 could be achieved from such structure with ∼ 25% coupling efficiency.

Polymer based planar coupling of self-assembled bottle microresonators

The investigation of a simple and self-assembling method for realizing polymeric micro-bottle resonators is reported. By dispensing precise amounts of SU-8 onto a cleaved optical fiber, employed as mechanical support, bottle microcavities with different shapes and diameters are fabricated. The balancing of surface energy between glass fiber and polymeric microresonator with surface tension of SU-8 confers different shape to these microstructures. Planar single-mode SU-8 based waveguide, realized on polymethylmethacrylate, is chosen for exciting the micro-bottle resonators by evanescent wave. The reliability of the fabrication process and the shape of the bottle microcavities are investigated through optical analysis. We observe whispering gallery modes in these resonant microstructures by a robust coupling with single mode planar waveguides around 1.5 μm wavelength. The resonance spectra of micro-bottle resonators and the spectral characteristics, such as Quality-factor (Q factor) and free spectral range, are evaluated for all the realized microstructures. SU-8 micro-bottle resonators show high Q-factors up to 3.8 × 104 and present a good mechanical stability. These features make these microcavities attractive for sensing and/or lasing applications in a planar platform.

Fiber-Optical Switch Controlled by a Single Atom

We demonstrate highly efficient switching of optical signals between two optical fibers controlled by a single atom. The key element of our experiment is a whispering-gallery-mode bottle microresonator, which is coupled to a single atom and interfaced by two tapered fiber couplers. This system reaches the strong coupling regime of cavity quantum electrodynamics, leading to a vacuum Rabi splitting in the excitation spectrum. We systematically investigate the switching efficiency of our system, i.e., the probability that the fiber-optical switch redirects the light into the desired output. We obtain a large redirection efficiency reaching a raw fidelity of more than 60% without postselection. Moreover, by measuring the second-order correlation functions of the output fields, we show that our switch exhibits a photon-number-dependent routing capability.

Theory of SNAP devices: basic equations and comparison with the experiment

A SNAP (Surface Nanoscale Axial Photonics) device consists of an optical fiber with introduced nanoscale effective radius variation, which is coupled to transverse input/output waveguides. The input waveguides excite whispering gallery modes circulating near the fiber surface and slowly propagating along the fiber axis. In this paper, the theory of SNAP devices is developed and applied to the analysis of transmission amplitudes of simplest SNAP models exhibiting a variety of asymmetric Fano resonances and also to the experimental characterization of a SNAP bottle microresonator and to a chain of 10 coupled microresonators. Excellent agreement between the theory and the experiment is demonstrated.