Whispering Gallery Mode Microcavities Research Articles

Extended Surface Bands Enabled Lasing Emission and Wavelength Switch from Sulfur Quantum Dots.

The development of a lasing wavelength switch, particularly from a single inorganic gain material, is challenging but highly demanded for advanced photonics. Nonetheless, all current lasing emission of inorganic gain materials arises from band-edge states, and the inherent fixed bandgap limitation of the band-edge system leads to the inaccessibility of lasing wavelength switching from a single inorganic gain material. Here the realization of a single inorganic gain material-based lasing wavelength switch is reported by proposing an alternative lasing emission strategy, that is, lasing emission from surface gain. Previous efforts to achieve surface-gain-enabled lasing emission have been hindered by the limited gain volume provided by surface states due to the broad emission bandwidth and/or low emission efficiency. This challenge is overcome by introducing extended surface bands onto the surface of sulfur quantum dots. The extended surface bands contribute to a high photoluminescence quantum yield and narrow emission bandwidth, thereby providing sufficient gain volume and facilitating stimulated emission. When combined with whispering gallery mode microcavity, surface gain enabled lasing emission manifests an ultralow threshold of 8.3 µJcm-2. Remarkably, the reconfigurable perturbation to surface gain, facilitated by molecular affinity, allows for the realization of the lasing wavelength switch from a single inorganic gain material.

Single Molecule Thermodynamic Penalties Applied to Enzymes by Whispering Gallery Mode Biosensors.

Optical microcavities, particularly whispering gallery mode (WGM) microcavities enhanced by plasmonic nanorods, are emerging as powerful platforms for single-molecule sensing. However, the impact of optical forces from the plasmonic near field on analyte molecules is inadequately understood. Using a standard optoplasmonic WGM single-molecule sensor to monitor two enzymes, both of which undergo an open-to-closed-to-open conformational transition, the work done on an enzyme by the WGM sensor as atoms of the enzyme movethrough the electric field gradient of the plasmonic hotspot during conformational change has been quantified. As the work done by the sensor on analyte enzymes can be modulated by varying WGM intensity, the WGM microcavity system can be used to apply free energy penalties to regulate enzyme activity at the single-molecule level. The findings advance the understanding of optical forces in WGM single-molecule sensing, potentially leading to the capability to precisely manipulate enzyme activity at the single-molecule level through tailored optical modulation.

High-Q WGM microcavity-based optofluidic sensor technologies for biological analysis.

High-quality-factor (Q) optical microcavities have attracted extensive interest due to their unique ability to confine light for resonant circulation at the micrometer scale. Particular attention has been paid to optical whispering-gallery mode (WGM) microcavities to harness their strong light-matter interactions for biological applications. Remarkably, the combination of high-Q optical WGM microcavities with microfluidic technologies can achieve a synergistic effect in the development of high-sensitivity optofluidic sensors for many emerging biological analysis applications, such as the detection of proteins, nucleic acids, viruses, and exosomes. They can also be utilized to investigate the behavior of living cells in human organisms, which may provide new technical solutions for studies in cell biology and biophysics. In this paper, we briefly review recent progress in high-Q microcavity-based optofluidic sensor technologies and their applications in biological analysis.

Origin of Optical Gain in Narrow ZnO Microrods with Whispering Gallery Modes

Due to sufficiently high lasing thresholds, stimulated emission in relatively small ZnO microcrystal lasers is often considered to be fed by an inverted electron–hole plasma (EHP). In this study, the nature of optical gain in such emitters is investigated using ZnO microrods 1–6 µm in diameter synthesized by a modified thermal evaporation method and exhibiting whispering-gallery mode (WGM) lasing in the near ultraviolet range. It is demonstrated that optical gain in these objects is not a consequence of population inversion of the EHP at either low or room temperatures. Instead, the primary gain mechanism is the process of scattering of electron–hole pairs by free electrons. Unlike the case of large ZnO WGM microcavities, in small-diameter microrods this process turns out to be dominant over a wide temperature range.

Whispering-Gallery Mode Optoplasmonic Microcavities: From Advanced Single-Molecule Sensors and Microlasers to Applications in Synthetic Biology.

Optical microcavities, specifically, whispering-gallery mode (WGM) microcavities, with their remarkable sensitivity to environmental changes, have been extensively employed as biosensors, enabling the detection of a wide range of biomolecules and nanoparticles. To push the limits of detection down to the most sensitive single-molecule level, plasmonic nanorods are strategically introduced to enhance the evanescent fields of WGM microcavities. This advancement of optoplasmonic WGM sensors allows for the detection of single molecules of a protein, conformational changes, and even atomic ions, marking significant contributions in single-molecule sensing. This Perspective discusses the exciting research prospects in optoplasmonic WGM sensing of single molecules, including the study of enzyme thermodynamics and kinetics, the emergence of thermo-optoplasmonic sensing, the ultrasensitive single-molecule sensing on WGM microlasers, and applications in synthetic biology.

Large mode volume microresonator with a gradient refractive index

Whispering gallery mode microcavities have been widely explored because of high Q factors and small mode volumes. Although small mode volumes are beneficial in sensors and nonlinear applications, recent studies suggest that large mode volumes are required for noise reduction in advanced applications. To enhance the mode volume, one possible method is to utilize the material inside the cavity. Herein, a radial gradient refractive index (GRIN) microresonator has an internal potential well and allows the mode field to shift inward. The GRIN resonator is formed by changing the radial refractive index n(r), and can be produced by the mature ion-exchange process. By adjusting the process parameters, the internal potential well can be widened to generate large mode volumes. However, it takes a lot of time to optimize the process parameters of wide potential well resonators using commercial software. Therefore, in this paper, we propose a fast algorithm for radial GRIN microcavities. Using this algorithm, we optimize the diffusion and annealing times of the GRIN resonator to increase the mode volume to more than four times that of the WGM microcavity. COMSOL simulations also support the conclusion with no additional loss in the Q factor.

Ultrahigh-sensitivity temperature sensor based on an elastic TPU capillary whispering gallery resonator.

An ultrahigh sensitivity temperature sensor based on an elastic thermoplastic urethane (TPU) capillary whispering-gallery mode (WGM) microcavity is proposed. The temperature sensor comprises a dye-doped TPU capillary and two sealed fused silica capillaries covered at both ends and is fabricated via a thin film assembly and wet etching. The fused silica capillaries limit the thermal volume expansion of the air within it. The volume of the exposed part of the elastic TPU capillary, which has an ultrahigh sensitivity to temperature compared with the thermal volume expansion of material, is increased; the designed elastic TPU capillary WGM microcavity exhibited an ultrahigh sensitivity of 11.28 nm/°C.

Asymmetric transmission of cascaded acousto-optic device and whispering gallery mode microcavity

In this paper, an asymmetric transmission scheme is proposed by cascading an acousto-optic device and a coupled whispering gallery mode (WGM) microsphere cavity, and it is demonstrated theoretically and experimentally. With the acousto-optic interaction in a fiber, the vector modes of the fundamental core mode can be converted into the different vector modes of a cladding (linear polarization, LP) mode, and because of the optical path difference between the cladding vector modes, the polarization of the cladding mode will be changed. The cladding mode can be converted back into the core fundamental mode by coupling a WGM microcavity. By calculating the overlapping of the mode fields in the tapered fiber and the microcavity at the resonance wavelength, the coupling coefficients between different LP modes and WGM will be solved. And, the transmitivities and conversion coefficients of the two fiber modes can be obtained on condition that the polarization of the incident light does not coincide with the polarization orientation of the WGM. The transmission spectra of the coupled WGM microcavity are calculated by using Matlab program for eight states, including the states at different incident directions, different incident polarizations of input, whether the acoustic wave is on or off. The results show that the conversion coefficient from the cladding mode to the core mode is completely different from that of the contrary process when the acoustic wave is working. And the forward incident light and backward incident light have completely different transmission characteristics, thus resulting in the asymmetric transmission. The transmittances of forward incidence and reverse incidence at different polarizations are also studied, both of them change periodically with the polarization angle, and their phase difference is equal to the polarization change caused by acousto-optic interaction in the fiber. In the experiment, a two-stage tapered fiber is used to realize the acousto-optic interaction and the coupling of whispering gallery mode at the same time. By controlling the working states of the system, the same 8 states as in the calculation are studied experimentally. The results show that due to the polarization-selection effect of the WGM, the light energy incident from the opposite directions will show different transmission characteristics. While the forward transmittance reaches a maximum value of about 0.505, the reverse transmittance reaches a minimum value of about 0.010, and the transmission isolation reaches about 17 dB. The transmittances in two directions are measured at different incident polarization angles, the transmission isolation is analyzed, and the polarization change of cladding mode in the fiber is verified to be about <inline-formula><tex-math id="M1">\begin{document}${80^ \circ }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20230653_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20230653_M1.png"/></alternatives></inline-formula>. The measured results coincide with the calculations from the developed theory well. Finally, the shortcomings and optimization method of the scheme are discussed. The asymmetric transmission scheme in this paper inherits the advantages of rapid response and good tuning of acousto-optic device, and has an all-fiber structure, which has important application potential in optical switch and isolator.

Electrical single mode switching in coupled microcavity

Flexible lasing mode regulation is significant for optoelectronic applications, such as on-chip communication and optoelectronic integration. However, reversible and dynamic single mode lasing output is still a challenge. Herein, reversible lasing modes tuning is demonstrated in whispering gallery mode microcavity controlled by bias voltages. Especially in coupled microcavity consisting of two microrods, applied electrical fields cause lasing modes shifting of one microrod, and the optical field coupling process with the other microrod occurs simultaneously to select distinct single mode lasing. Hence, the switchable single mode lasing can be achieved. The mechanisms of Vernier effect, electro-optic effect, and their synergy on lasing mode modulation are analyzed via experimental results and theoretical analysis. Synergistically running the two effects, the high quality factor of the single mode indicates an effective coupling process, providing an effective way for controllable lasers.

Fano resonance in whispering gallery mode microcavities and its sensing applications

With the advantages of high quality factor (Q) and small mode volume, whispering gallery mode (WGM) microcavities obtain lower detection limits and have been increasingly applied to the detection in physics, chemistry, and biology. The Fano resonance phenomenon, which could further improve the sensing performance in the sensitivity and detection limits because of its sharp change in phase and amplitude, has attracted wide attention and demonstrated that it could be achieved in whispering gallery mode resonators (WGMRs). Fano effect is caused by the interference between the discrete mode and the continuous mode and then shows it in the spectrum. In this paper, first, the relevant knowledge of the WGM resonator and the resonance phenomenon are introduced in detail. Second, the theory of Fano resonance is analyzed. Third, the generation methods and sensing applications of Fano resonance in WGMR in current are summarized. Finally, the existing problems and development prospects of this sensor are discussed.

Fabrication of optical WGM microcavities using high-resistance wire

This paper describes a method for fabricating whispering gallery mode (WGM) optical microcavities based on optical fiber using a nickel-chromium wire. The above method makes it possible to manufacture optical WGM microcavities with high reproducibility and low cost compared to other methods.

High-Speed Directly Modulated Widely Tunable Hybrid Square/Rhombus-Rectangular Coupled Cavity Lasers

We report the latest development in high-speed directly modulated tunable laser based on simple and compact hybrid square/rhombus-rectangular laser (HSRRL), aiming for low-cost deployment in next generation high-throughput optical interconnects, intra-/inter-data center networks, and optical fiber communication system. The HSRRL is composed of a deformed square whispering gallery mode (WGM) microcavity and a Fabry–Pérot (FP) cavity, and the WGM microcavity is used as an end face of the FP cavity with mode selection. A deformable WGM microcavity is designed to obtain a high reflectivity. The HSRRL, fabricated with deep-etching processing steps similar to FP laser without grating or epitaxial regrowth, has achieved 7-mA threshold current, 0.4-mW/mA slope efficiency, above 52-dB side mode suppression ratio (SMSR), and 16.2-GHz modulation bandwidth with injection currents of 34.3 mA and 60 mA for WGM microcavity and FP cavity, respectively. A wavelength tuning from 1537.40 nm to 1570.42 nm is realized by adjusting the injection currents simultaneously. In the tuning range, the laser has an SMSR greater than 40 dB, a linewidth less than 4 MHz, and a 3-dB modulation bandwidth greater than 14 GHz.

Micropascal-sensitivity ultrasound sensors based on optical microcavities

Whispering gallery mode (WGM) microcavities have been widely used for high-sensitivity ultrasound detection, owing to their optical and mechanical dual-resonance enhanced sensitivity. The ultrasound sensitivity of the cavity optomechanical system is fundamentally limited by thermal noise. In this work, we theoretically and experimentally investigate the thermal-noise-limited sensitivity of a WGM microdisk ultrasound sensor and optimize the sensitivity by varying the radius and a thickness of the microdisk, as well as using a trench structure around the disk. Utilizing a microdisk with a radius of 300 μm and thickness of 2 μm, we achieve a peak sensitivity of 1.18 μPa Hz−1/2 at 82.6 kHz. To the best of our knowledge, this represents the record sensitivity among cavity optomechanical ultrasound sensors. Such high sensitivity has the potential to improve the detection range of air-coupled ultrasound sensing technology.

The nonlinear effects and applications of gain doped whispering-gallery mode cavities

Whispering-gallery mode (WGM) cavities formed by dielectric structures have attracted intensive interest in various fields. The high-quality factor and smaller mode volume associated with the optical modes have inspired experiments in nonlinear optics, nanophotonics, and quantum information science. Moreover, they are also used in optical biosensors and other significant applications. To further reduce the material loss of the resonator, optical gain materials, such as erbium and ytterbium, are doped into the dielectric structure to increase the nonlinear effect and enhance the interaction between light and matter. Here in this review, we outline the most recent advancements in gain-doped optical WGM microcavities. Moreover, we introduce the dynamics of the gain in WGM resonators, the integration of gain media into WGM microcavities with various shapes, and the fabrication and applications of the gain microcavities. Also, the applications of the gain cavity based on the whispering-gallery mode have been introduced, e.g., ultra-sensitive sensors, low-threshold lasers, and high-performance optical systems.

Optical Supercontinuum Generation via Rotational and Vibrational Molecular Cavity Optomechanics

AbstractThe solid high quality (Q) factor whispering gallery mode microcavities are demonstrated as an excellent experimental platform for enhanced light–matter interactions, and are applied in the studies of nonlinear optics, quantum optics, and sensing. However, the interaction between the microcavity and liquid anisotropic molecule, which is unique for abundant vibrational and rotational degrees of freedom, is rarely studied. Here, the gap is bridged by filling an optofluidic microbubble resonator with carbon disulfide, and the related anisotropic molecular cavity optomechanics effects are investigated. Due to the enhanced coupling between the optical mode and the rotation/vibration of molecules, stimulated Rayleigh–Kerr/Raman–Kerr scattering effects are observed. Molecular cavity optomechanics provides a broadband gain for generating Stokes photons in high‐Q optical modes, and eventually allows quasi‐supercontinuum generation with a bandwidth of 436 nm under a low continuous laser pump power of about 10 mW. This work opens a new avenue for strong light–matter interactions by exploring rotational and vibrational molecular cavity optomechanics, which provides a promising platform for nonlinear optics, cavity‐enhanced molecular reorientation dynamics, and novel optical sources.

High-precision whispering gallery microsensors with ergodic spectra empowered by machine learning

Whispering gallery mode (WGM) microcavities provide increasing opportunities for precision measurement due to their ultrahigh sensitivity, compact size, and fast response. However, the conventional WGM sensors rely on monitoring the changes of a single mode, and the abundant sensing information in WGM transmission spectra has not been fully utilized. Here, empowered by machine learning (ML), we propose and demonstrate an ergodic spectra sensing method in an optofluidic microcavity for high-precision pressure measurement. The developed ML method realizes the analysis of the full features of optical spectra. The prediction accuracy of 99.97% is obtained with the average error as low as 0.32 kPa in the pressure range of 100 kPa via the training and testing stages. We further achieve the real-time readout of arbitrary unknown pressure within the range of measurement, and a prediction accuracy of 99.51% is obtained. Moreover, we demonstrate that the ergodic spectra sensing accuracy is∼11.5%higher than that of simply extracting resonating modes’ wavelength. With the high sensitivity and prediction accuracy, this work opens up a new avenue for integrated intelligent optical sensing.

High-Sensitivity Air-Coupled Megahertz-Frequency Ultrasound Detection Using On-Chip Microcavities

Owing to their dual-resonance enhanced sensitivity, cavity optomechanical systems provide an ideal platform for ultrasound sensing. In this work, we realize high-sensitivity air-coupled ultrasound sensing from kilohertz to megahertz frequency range based on whispering gallery mode microcavities. Using a $57$-$\ensuremath{\mu}\mathrm{m}$-diameter microtoroid with high optical Q factor (approximately ${10}^{7}$) and mechanical Q factor (approximately $700$), we achieve sensitivities of $46\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{Pa}$ ${\mathrm{Hz}}^{\ensuremath{-}1/2}$--10 mPa ${\mathrm{Hz}}^{\ensuremath{-}1/2}$ in a frequency range of 0.25--3.2 MHz. Thermal-noise-limited sensitivity is realized around a mechanical resonance at 2.56 MHz, in a frequency range of 0.6 MHz. We also observe the second- and third-order mechanical sidebands, and quantitatively study the intensities of each mechanical sideband as a function of the mechanical displacement. Measuring the combination of signal-to-noise ratios at all sidebands has the potential to extend the dynamic range of ultrasound sensing.

Liquid Crystal Biosensors: Principles, Structure and Applications.

Liquid crystals (LCs) have been widely used as sensitive elements to construct LC biosensors based on the principle that specific bonding events between biomolecules can affect the orientation of LC molecules. On the basis of the sensing interface of LC molecules, LC biosensors can be classified into three types: LC–solid interface sensing platforms, LC–aqueous interface sensing platforms, and LC–droplet interface sensing platforms. In addition, as a signal amplification method, the combination of LCs and whispering gallery mode (WGM) optical microcavities can provide higher detection sensitivity due to the extremely high quality factor and the small mode volume of the WGM optical microcavity, which enhances the interaction between the light field and biotargets. In this review, we present an overview of the basic principles, the structure, and the applications of LC biosensors. We discuss the important properties of LC and the principle of LC biosensors. The different geometries of LCs in the biosensing systems as well as their applications in the biological detection are then described. The fabrication and the application of the LC-based WGM microcavity optofluidic sensor in the biological detection are also introduced. Finally, challenges and potential research opportunities in the development of LC-based biosensors are discussed.

III–V Semiconductor Whispering-Gallery Mode Micro-Cavity Lasers: Advances and Prospects

III-V semiconductor lasers are ubiquitous in modern optoelectronic devices, with applications ranging from telecommunication to general lighting. Among the different kinds of laser cavity designs, whispering-gallery mode (WGM) micro-cavity lasers boast outstanding optical performance due to advantages such as ultra-high Q-factor, compact mode volume, and narrow emission linewidth. Over the past decades, research in III-V WGM micro-cavity lasers has progressed rapidly in various aspects, including the fabrication techniques, emission outcoupling methods, and practical applications. In this paper, a comprehensive review is performed on the advances in these aspects. Although III-V WGM lasers are conventionally fabricated with top-down approaches, recent reports have demonstrated the potential of highly-scalable bottom-up methods in fabricating low-loss WGM lasers. Despite the strong optical confinement in III-V WGM cavities, various techniques have been developed to either outcouple WGM emission into waveguides or direct it into free-space with small beam divergence. Finally, recent developments in different applications of III-V WGM micro-cavity lasers are discussed. Other than serving as integrated photonic components, III-V WGM lasers have also displayed exciting potential in other applications such as label-free sensing and the study of cavity quantum electrodynamics (cQED).

Scattering-Assisted and Logic-Controllable WGM Laser in Liquid Crystal Micropillar

Whispering gallery mode (WGM) microcavities can efficiently store and manipulate light with strong light confinement and long photon lifetime, while coupling light into and from WGMs is intrinsically hindered by their unique feature of rotational symmetry. Here, a scattering-assisted liquid crystal (LC) micropillar WGM laser is proposed. WGM lasing at the surface of the micropillar is obviously enhanced by fluorescence scattering in the core of the micropillar. Besides, weak scattering of LC molecules also builds efficient coupling channels between the laser modes and the axial transmission modes of the micropillar-based waveguide, providing an all-in-one liquid WGM laser with functions of self-seeding and self-guiding. Furthermore, based on the hysteresis characteristics of the electrically anchored LC molecules under the interaction of thermal force, an erasable read-write liquid memory device is proposed, paving the way for the application of logic-controllable WGM lasers in optical storage and optical control.