Whispering Gallery Mode Cavities Research Articles

Droplet microfluidic generation of a million optical microparticle barcodes.

Micron-scale barcode particles enable labelling of small objects. Here, we demonstrate high-throughput barcode fabrication inside a microfluidic chip that can embed multiple, dye-doped high quality-factor whispering gallery mode cavities inside aqueous droplets at kilohertz rates. These droplets are then cured to form polyacrylamide hydrogel beads as small as 30 μm in diameter. Optical resonance spectra of the embedded cavities provide the hydrogels with unique barcodes with their diversity combinatorically scaled with the number of embedded cavities. Using 3 cavities per hydrogel, we obtain approximately one million uniquely identifiable, optically readable barcode microparticles.

Bandwidth Tunable Filter Based on Ideal Quasi-Critical Coupling State in WGM Cavity

A tunable bandwidth add-drop filter in ideal quasi-critical coupling (QCC) state was analyzed in theory in detail and demonstrated in experiment, with two vector modes HE $^{\rm X}_{11}$ and HE $^{\rm Y}_{11}$ coupled single whispering gallery mode (WGM) as its basic configuration. In the experiment, the bandwidth tuning range at the Drop port was from 54.0 MHz to 775.8 MHz with the conversion efficiency maintained above 95%, and bandwidth tuning range at the Through port was from 6.6 MHz to 720.6 MHz with the insertion loss fluctuation less than 5%. The demonstrated filter gains advantages of stable efficiency, narrow band, and broad bandwidth tuning range, which will be helpful for its application in microwave photonics, narrow linewidth lasers and nonlinear optics.

Tunable coupling of chip-scale photonic molecules via thermal actuation

Photonic molecules (PMs) are of great interest for, e.g., optical filters/sensors or topological and exceptional-point photonics. A key requirement for their versatile application is the tunability of the PM’s coupling strength. This important feature is realized in the here introduced widely and precisely tunable PM on an all-polymeric chip-scale platform. The PM consists of two disk-shaped whispering gallery mode cavities on a liquid crystal elastomer (LCE) substrate. The coupling strength of the PM is controlled via the contraction of the LCE under an external stimulus like local heating. We reveal the reversible (de)coupling via the analysis of laser supermodes emitted from a dye-doped PM. The tunability of the PM’s coupling strength is apparent from the pronounced mode splittings observed in single-fiber transmission spectra and is consistent with coupled-mode theory. Finally, we demonstrate the applicability of the PM as an add-drop filter with a highly controllable intensity transfer. In this light, our PM on an LCE substrate represents a novel platform system for tunably coupled photonic resonators.

Selectively Visualizing the Hidden Modes in Random Lasers for Secure Communication

AbstractToward a higher degree of secure communication, great efforts have been made to develop encryption and anticounterfeiting strategies. In particular, optical encryption with high complexity has become one of the most significant approaches. To strengthen optical information security, the concept of a spectral encryption approach is proposed based on selectively visualized spectral modes in random lasers. By coupling with external whispering gallery mode cavity, hidden localized lasing modes with special multiple‐peaks spectra are selected from phase‐correlated random lasers with smooth single‐peak spectra. The selectively visualized localized modes are utilized for spectral encryption of secure information. The encoded spectral modes that are hidden in phase‐correlated modes with identical spectra can only be selectively recognized by a special external coupler. This multiplex encryption strategy is super for the simple operation, low cost, and high security, supplying a new approach for further applications in secure communication and anticounterfeiting.

In-Fiber Butt-Coupled Spherical Microcavity With Whispering Gallery Mode and Fabry-Perot Resonances

An integrated in-fiber coupling configuration for simultaneously exciting whispering gallery mode (WGM) and Fabry-Perot (FP) resonances in one cavity is proposed and demonstrated experimentally. The WGMs traveling circularly in both clockwise and counter-clockwise directions are simultaneously excited by perpendicularly injecting the light from the single-mode-fiber (SMF) core into a polymeric microsphere embedded in the femtosecond laser micromachined coupling structure. While the standing wave between the two axial interfaces of the microsphere and its external media results in a spherical FP resonator, meaning that the WGM and the FP resonances coexist in the same spherical cavity. The superposition of WGM and FP resonances results in asymmetric Fano-like lineshape, which is of high importance for optical fiber sensors, filters, and switches due to its sharp slope around the resonant wavelength. Experimental results show that asymmetric spectral lineshape can be achieved even as the spatial coupling gap is larger than the wavelength, indicating that the proposed coupler has higher fabrication tolerance than the evanescent field-based coupling configuration. Such an integrated coupling configuration provides an effective method for the coupling between the optical fiber and WGM cavities made of any functional material by liberating them from the phase-matching condition and enriching the functionality by importing other resonance into the same cavity.

Packaged whispering gallery resonator device based on an optical nanoantenna coupler.

In this work, we present a packaged whispering gallery mode (WGM) device based on an optical nanoantenna as the coupler and a glass microsphere as the resonator. The microspheres were fabricated from either SiO2 fiber or Er3+-doped fiber, the latter creating a WGM laser with a threshold of 93 µW at 1531 nm. The coupler-resonator WGM device was packaged in a glass capillary. The performance of the packaged microlaser was characterized, with lasing emission both excited in and collected from the WGM cavity via the nanoantenna. The packaged system provides isolation from environmental contamination, a small size, and unidirectional coupling while maintaining a high quality (Q-) factor (∼108).

High quality factor, protein-based microlasers from self-assembled microcracks

In the last decade, microlasers with biological origin have shown their great potential in biosensing and bioimaging. Several micro-structures have been developed for high quality (Q) factor biolasers including Fabry–Pérot, distributed feedback and whispering gallery mode cavities. However, the fabrication of these lasers is generally complicated and their operation is strongly affected by cavity defects. In this work, we demonstrate random protein-based microlasers fabricated by a simple one-step self-assembled method. The lasing can be achieved from microcracks with random structures. The lasing threshold is around 14 mJ cm−2 and the quality factor of lasing modes can be up to which are comparable with other conventional biolasers. Our work opens a new possibility for the fabrication of high Q factor microlasers from biocompatible materials, with great potential for biosensing and biomedical applications.

Fano-Assisted Tunable X-Band Microwave Ring Resonator

Control of the resonance frequency of microwave cavities is of great interest in many applications such as sensing and wavelength filtering. However, this task normally relies on active control via functional materials (i.e., polymers and ferrites, or a combination of both) excited by an external electric or magnetic field. In this article, instead, we propose an innovative whispering gallery mode cavity resonator with tunable bandwidth for sensing and wavelength filtering applications. The proposed approach exploits Fano-type resonances (FTRs) to improve the cavity performance in the microwave regime. We have measured a ~74% increase in the loaded quality factor compared to regular ring cavities, with only 1.9-dB additional loss. In addition, by carefully choosing the size of the transmitting and receiving monopole antennas placed inside the cavity and their relative angular spacing, we can easily tune the cavity response. The extra degree-of-freedom introduced by such variable angular spacing, responsible for the onset of FTRs, allows the cavity quality factor Q to be continuously tuned from 385 to 670.

A Fiber-Attached Coupler for Transmission Bandpass Whispering Gallery Mode Resonator

A fiber attached coupler for transmission bandpass whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The coupler fabricated by femtosecond laser ablation consists of a pair of curved waveguides with a curvature radius of 50 μm, by which the WGM field is excited and collected through evanescent field coupling, respectively. As a proof of concept, a polymer microsphere functioning as the WGM cavity is embedded into a femtosecond laser ablated hole between the two curved waveguides. A passband with a full width of 0.15 nm at half maximum is observed when the microsphere is surrounded by air, verifying the feasibility of the proposed coupler for constructing a WGM cavity-based transmission narrow bandpass optical fiber filter. Benefitting from the relatively high thermo-optical coefficient, the passband of a packaged transmission bandpass WGM resonator can be thermo-optically tuned with a sensitivity of 446.6 pm/°C.

Ultrasensitive and high resolution mass sensor by photonic-molecule optomechanics with phonon pump

Whispering gallery mode cavities, due to their high quality factors, small mode volumes, and simple fabrications, have potential applications in photonic devices and ultrasensitive mass sensing. Cavity optomechanic systems based on whispering gallery mode cavities have draw tremendous attention in recent years due to the fact that they reveal and explore fundamental quantum physics and pave the way for potential applications of optomechanical devices. Unfortunately, whispering gallery mode based cavity optomechanics still lies in a single optical mode coupled to a single mechanical mode. Here, in this work, we propose an ultrasensitive and high resolution optical mass sensor by photonic-molecule optomechanics driven by two-tone field, where the optomechanical cavity driven by a weak coherent phonon pump is coupled to an auxiliary optical cavity. The auxiliary cavity plays the role of decreasing the mechanical damping rate while the weak coherent phonon pump to the mechanical mode will improve the intensity of the probe transmission spectrum, which together enhance the mass sensitivity. Compared with the optical mass sensor based on single-cavity optomechanics, the mass sensitivity presented here is improved significantly. We also investigate different parameter regimes which guide us to choose the optimal parameters. Finally, we illustrate the accurate measurement for the mass of nanoparticles, and the photonic-molecule optomechanics system will provide a new platform for exploring the on-chip applications in ultrahigh resolution sensing devices.

Tunable photonic devices by 3D laser printing of liquid crystal elastomers

Liquid crystal elastomers (LCEs) are highly suitable materials for the fabrication of flexible photonic elements due to their ability for directional actuation induced by external stimuli. 3D laser printing (3DLP) is a well-established method to realize complex photonic architectures. In this paper, we present the technological adaptations necessary to combine the actuation-controlled flexibility of LCE with the design options inherent to 3DLP to realize a platform for tunable photonics. The role of birefringence of the LCE in the 3DLP fabrication is addressed and theoretically modelled. We demonstrate how LCEs can be used both as a flexible substrate for arrays of rigid photonic elements and as a material for tunable photonic structures itself. Flexible coupling of two optical whispering gallery mode cavities and full spectral tunability of a single cavity are presented as exemplary applications.

Plasmonic-induced self-assembly of WGM cavities via laser cavitation.

We show how photoexcitation of a single plasmonic nanoparticle (NP) in solution can create a whispering-gallery-mode (WGM) droplet resonator. Small nano/microbubbles are initially formed by laser-induced heating that is localized by the plasmon resonance. Fast imaging shows that the bubbles collect and condense around the NP and form a droplet in the interior of the bubble. Droplets containing dye generated lasing modes with wavelengths that depend on the size of the droplet, refractive index of the solvent, and surrounding environment, matching the behavior of a WGM. We demonstrated this phenomenon with two kinds of Au NPs in addition to TiN NPs and observed cavity diameters as small as 4.8 µm with a free spectral range (FSR) of 12 nm. These results indicate that optical pumping of plasmonic NPs in a gain medium can generate lasing modes that are not directly associated with the plasmon cavity but can arise from its photophysical processes. This process may serve as a method to generate plasmonic/photonic optical microcavities in solution on demand at any location in a solvent using free-space coupling in/out of the cavity.

In-fiber wavelength-selective reflector based on Y-junction coupled whispering gallery mode resonator

An in-fiber wavelength-selective reflector based on a compact Y-junction coupled whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The Y-junction used to simultaneously excite and collect the WGM field through evanescent field coupling is fabricated by femtosecond laser ablation, where the common port of the Y-junction connects to the original single-mode fiber core. As a proof of concept, a microsphere functioning as a WGM cavity is embedded into a femtosecond laser ablated hole in the middle of the two branches of the Y-junction. A reflection passband with a full width at half maximum (FWHM) less than 0.1 nm is experimentally achieved, corresponding to a quality factor (Q-factor) on the order of 104. Such a highly integrated in-fiber WGM reflector holds the potential for remote sensing with high resolution or providing narrow band optical feedback to lasers.

Effect of Al concentration on optically pumped random lasing in Zn1−xAlxO (0 ≤ x ≤ 0.10) thin films fabricated via pulsed laser deposition technique

Effect of Al content on the lasing emission of pulsed laser deposited nanostructured Zn1−xAlxO (0 ≤ x ≤ 0.10) thin films is reported in the present paper. The thin films were grown onto fused silica substrate employing 2nd harmonic of a Q-switched Nd:YAG laser (532 nm, 10 ns, 10 Hz). The Random lasing emission was observed in these films by pumping with 3rd harmonic of the Q-switched Nd:YAG laser (355 nm, 10 ns, 10 Hz). The longitudinal modes of nearly equal separation of ~3 nm were observed having FWHM in the range of 0.38–0.56 nm. The optical cavity length in these was found to be in the range of 7.3–8.1 μm. The lasing mechanism in the films having spherical particulates was due to the pure Random Lasing action where as that of containing the nano rods and nano wire was due to the formation of Fabry-Perot and Whispering Gallery Mode cavity together. The quality factor was observed to be highly influenced with Al content in the films and was found to be in the range of 691–1004. The maximum quality factor of 1004 was obtained for the thin film having x = 0.05 containing nanorods. The RL threshold was estimated to be ~3.2 × 102 MW/cm2.

Optimal mode matching in cavity optomagnonics

Inelastic scattering of photons is a promising technique to manipulate magnons but it suffers from weak intrinsic coupling. We theoretically discuss an idea to increase optomagnonic coupling in optical whispering gallery mode cavities, by generalizing previous analysis to include the exchange interaction. We predict that the optomagnonic coupling constant to surface magnons in yttrium iron garnet (YIG) spheres with radius $300\,\mathrm{\mu}$m can be up to $40$ times larger than that to the macrospin Kittel mode. Whereas this enhancement falls short of the requirements for magnon manipulation in YIG, nanostructuring and/or materials with larger magneto-optical constants can bridge this gap.

Analysis of Quality Factor Enhancement in the Monolithic InGaN/GaN Nanorod Array

Whispering gallery mode (WGM) lasers and resonators are key building blocks for photonic integrated circuits. The quality factor and resonant wavelength are strong functions of the cavity size. Nanoscale WGM cavities suffer from a low-quality factor due to prominent scattering loss. However, the quality factor could be enhanced by forming an optically-coupled rod array or photonic molecules. Through simulations, we revealed how rod-to-rod optical coupling influenced the threshold pumping level and dominant mode selection, where the trend showed good agreement with the experimental observation. According to the simulation, the quality factor could be enhanced by up to eight times by forming a six-rod photonic molecule. The quality factor and effective mode were both superior to the single rods with the same wafer device footprint.

Anomalous blue-shift of terahertz whispering-gallery modes via dielectric and metallic tuning.

The vast majority of resonant systems show a red-shift for the resonance frequency when a perturbation, e.g.,losses, is introduced to the system. In contrast, here we report for the first time, to the best of our knowledge, the experimental demonstration of both red- and anomalous blue-shifting of whispering-gallery modes (WGMs) using dielectric and metallic substrates. The maximum blue-shift is more than three times as large as the expected red-shift, proving that the anomalous blue-shift is more than a peculiar curiosity. The experiments are performed in the terahertz frequency range with coherent continuous-wave spectroscopy. The results establish dielectric and metallic tuning as a novel and viable approach to tune high-quality WGMs and provide valuable insights into the anomalous blue-shift of WGM cavity systems. The tuning capabilities for these compact monolithic resonators are of significant interest for fundamental science and technological applications alike.

Field Evaluation of a Portable Whispering Gallery Mode Accelerometer.

An accelerometer utilising the optomechanical coupling between an optical whispering gallery mode (WGM) resonance and the motion of the WGM cavity itself was prototyped and field-tested on a vehicle. We describe the assembly of this portable, battery operated sensor and the field-programmable gate array automation. Pre-trial testing using an electrodynamic shaker demonstrated linear scale-factors with <0.3% standard deviation ( g range where g = 9.81 ms), and a strong normalised cross-correlation coefficient (NCCC) of r when compared with an integrated circuit piezoelectric (ICP) accelerometer. A noise density of 40 g Hz was obtained for frequencies of 2–7 kHz, increasing to 130 g Hz at 200 Hz, and 250 g Hz at 100 Hz. A reduction in the cross-correlation was found during the trial, r = 0.36, which we attribute to thermal fluctuations, mounting differences, and the noisy vehicle environment. The deployment of this hand-fabricated sensor, shown to operate and survive during ±60 g shocks, demonstrates important steps towards the development of a chip-scale device.

Time-resolved universal temperature measurements using NaYF4:Er3+,Yb3+ upconverting nanoparticles in an electrospray jet.

Hexagonal upconverting nanoparticles (UCNPs) of NaYF4:Er3+,Yb3+ (ca. 300 nm) have been widely used to measure the temperature at the nanoscale using luminescence ratio thermometry. However, several factors limit their applications. For example, changes in the peak shape, mainly is the S-band emission, hinders their ability to be used as a universal temperature sensor. Herein, we introduce a universal calibration protocol for NaYF4:Er3+,Yb3+ upconverting nanoparticles that is robust to environmental changes and gives a precise temperature measurement. We used this new procedure to calculate the temperature profile inside a Taylor cone generated with an electrospray jet. Inside the Taylor cone the fluid velocity increases toward the tip of the cone. A constant acquisition length leads to a decrease in excitation and acquisition time. This decrease in excitation time causes a peak shape change that corrupts the temperature measurement if the entire peak shape is integrated in the calibration. Our universal calibration circumvents this problem and can be used for time-resolved applications. The temperature at the end of the Taylor cone increases due to the creation of a whispering gallery mode cavity with 980 nm excitation. We use time-resolved energy balance equations to support our optical temperature measurements inside the Taylor cone. We believe that the findings of this paper provide a foundation for time-resolved temperature measurements using NaYF4:Er3+,Yb3+ upconverting nanoparticles and can be used to understand temperature-dependent reactions such as protein unfolding inside microjet/microdroplets and microfluidic systems.

Quasicritical coupling in a few-mode tapered-fiber coupled whispering-gallery-mode system

A quasicritical coupling state was demonstrated both in theory and experiment with a few-mode tapered-fiber (TF) coupled whispering-gallery-mode (WGM) cavity. In such a configuration, the coupling mode to activate the WGM and the drop mode are in the same TF and of almost the same decay rate with the variation of the gap between the TF and WGM cavity. Consequently, when the gap decreases to zero, the transmission of the coupling mode decreases monotonously to zero at the resonance wavelength, while the transmission of the drop mode increases monotonously to unit, which means that neither the coupling efficiency in the coupling mode nor the transmission of the drop mode will decrease in the traditional overcoupling region. Thus a steadily high coupling efficiency can be achieved accordingly, which we defined as quasicritical coupling. Such a coupling state will be helpful to develop WGM-cavity-based filters and benefit the fabrication of ultranarrow-linewidth lasers.