Spherical Microcavities Research Articles

Research on controllable processing technology of microsphere cavity inside silicon substrates utilizing thermoelectric coupling effect

Micro cavity structures are extensively utilized in semiconductor micro- and nanosensor devices, especially spherical microcavity, whose high Q value not only significantly improves the sensitivity of the sensors, but also enhances their reliability in complex environments. The integration of this structure not only optimizes the performance of the sensor, but also provides the possibility for high-precision detection. In this study, a thermoelectric coupling method for controllable migration of microsphere cavity inside silicon materials is proposed in order to achieve stable formation of the internal microsphere cavity structure. The directional migration mechanism of atoms on the surface of microsphere cavities in silicon substrates under an electric field is explored using a phase field model. The model indicates that changes in the total free energy density induce a solid-gas phase transition on the surface of the microsphere cavity. It is shown that the migration velocity of the microsphere cavity increases proportionally with the electric field strength, and the migration distance increases by approximately 9% for every 10% increase in electric field strength. The migration direction aligns with the direction of the electric field. Simulation results validate the theoretical accuracy, the feasibility of controllable migration by thermoelectric coupling effect in conductive materials through experimental studies. This study provides novel methods and insights for fabricating high-quality spherical cavity in silicon materials and preparing highly sensitive micro- and nanosensor devices.

Ultraviolet photodetector improvement based on microspheres

Owing to the ideal optical cavity structure, the microsphere has always played a significant role in micro/nano optoelectronic devices for its high light-confined ability and large surface-volume ratio, which have been applied in lots of areas including intracellular lasers, tunable lasers, and so on. Furthermore, the microsphere cavity will be a useful and efficient means to improve the photodetector (PD) performance by enhancing the light absorption efficiency and light energy in the arrays. Herein, the wide-bandgap semiconductor microspheres are introduced in the PD, which are synthesized by a simple and universal hydrothermal method. The sensitive ultraviolet PD is constructed and obtained with a metal-semiconductor-metal type device structure. The oxide defect concentration in the ZnO microspheres is modulated through thermal treatment to improve the performance, including higher photo-to-dark current ratio near 600, responsivity of 7.6✕10−5 A/W, detectivity of 4.6✕107 Jones, and response speed. Moreover, the microspheres fabricated flexible device presents great detecting performance with different bending degrees. The results display the excellent prospect of semiconductor microspheres to flexible ultraviolet high-performance PDs, which possess the broad and potential development prospect in flexible even on-chip optoelectronic devices.

Manipulable multipurpose nanothermometers based on a fluorescent hybrid glass fiber microsphere cavity

Manipulable multipurpose nanothermometers based on a fluorescent hybrid glass fiber microsphere cavity

Controllable Fabrication of Highly Ordered Spherical Microcavity Arrays by Replica Molding of In Situ Self-Emulsified Droplets.

Ordered spherical hollow micro- and nanostructures hold great appeal in the fields of cell biology and optics. However, it is extremely challenging for standard lithography techniques to achieve spherical micro-/nanocavities. In this paper, we describe a simple, cost-effective, and scalable approach to fabricate highly ordered spherical microcavity arrays by replica molding of in situ self-emulsified droplets. The in situ self-emulsion involves a two-step process: discontinuous dewetting-induced liquid partition and interfacial tension-driven liquid spherical transformation. Subsequent replica molding of the droplets creates spherical microcavity arrays. The shapes and sizes of the microcavities can be easily modulated by varying the compositions of the droplet templates or utilizing an osmotically driven water permeation. To demonstrate the utility of this method, we employed it to create a spherical microwell array for the mass production of embryoid bodies with high viability and minimal loss. In addition, we also demonstrated the optical functions of the generated spherical microcavities by using them as microlenses. We believe that our proposed method will open exciting avenues in fields ranging from regenerative medicine and microchemistry to optical applications.

Lasing emission from ZnO hierarchical spherical microcavity

We investigated lasing characteristics of ZnO hierarchical micro-spherical particles with diameters of 1–5 μm . The lasing emission consists of a small number of discrete laser peaks unlike conventional random lasing from ZnO nanopowder-assembly. Theoretical calculations based on a many-body theory reveal that the optical gain is achieved at the observed lowest lasing threshold value, 4 mJ⋅cm−2⋅pulse−1 , which corresponds to the excited carrier density ∼2.4×1025 m−3 . Because the carrier density is much higher than the Mott density, the gain origin for lasing is electron-hole plasma recombination. The lasing frequency mode shift ( ∼1.2 meV ) is due to the refractive index change induced by exciting high carrier density up to 6.1×1025 m−3 . By changing hierarchical structures via controlling growth condition and performing the annealing treatment and performing the calculation of scattering efficiency of ZnO particles, we found that the hierarchy of the micro-spherical particle plays a crucial role of the lasing feedback: the strong light scattering at the interface between outer nanoparticles with the sizes of 100–200 nm and smaller ones with the sizes of 10–60 nm consisting in the micro-spherical particle results in a light confinement. Furthermore, it has been confirmed that each discrete lasing mode shows strong gain competition with each other probably due to spatial overlap between the modes. These results suggest that both random scattering and microcavity modes contribute to the lasing oscillation.

Boosted Second Harmonic Generation and Cascaded Sum Frequency Generation from a Surface Crystallized Glass Ceramic Microcavity.

The development of novel materials and structures for efficient second-order nonlinear micro/nano devices remains a significant challenge. In this study, the remarkable enhancement of second-harmonic generation (SHG) and cascaded sum frequency generation in whispering gallery mode microspheres made of surface-crystallized glass with a 6-µm Ba2TiSi2O8 crystal layer are demonstrated. Attributed to the core-shell design, the Ba2TiSi2O8 located on the surface can be efficiently coupled with whispering gallery modes, resulting in a highly efficient micron-scale cavity-enhanced second-order optical nonlinearity. Greatly enhanced SHG of the microcavity is observed, which is up to 80 times stronger than that of a non-resonant sample. Furthermore, owing to the wavelength non-selectivity of random quasi-phase matching, ultra-wideband SHG with a strong response ranging from 860 to 1600nm and high-contrast polarization characteristics is demonstrated. The glass-ceramic-based microsphere cavity also boosts the cascading optical nonlinearity, manifested by a two-magnitude enhancement of cascaded sum frequency generation. This work delineates an efficient strategy for boosting nonlinear optical response in glass ceramics, which will open up new opportunities for applications in photonics and optical communications.

Humidity sensing characteristics of compact coupled optical fiber structure by microsphere and tapered capillary tube

In this paper, an optical fiber humidity sensor has been proposed based on the coupling structure using BaTiO3 microsphere and conical cavity of capillary pigtail. The sensitive material polyvinyl alcohol (PVA) was coated near the coupling region of microsphere and conical cavity to improve its sensing performance. The humidity sensitivity has been demonstrated to be 0.09 dBm/%RH based on intensity demodulation technique. The morphological parameters of the coupling structure are further optimized to obtain the maximum sensitivity of −0.297 dBm/%RH. Compared with other microsphere based optical fiber sensors, the sensor has the advantages of compact structure, simple manufacture, low production cost, high sensitivity and good stability.

Strong‐Coupling‐Enhanced Raman Spectroscopy (SCERS) in a Cascade Microsphere‐Cavity

AbstractSurface‐enhanced Raman spectroscopy (SERS) is a powerful optical technique with high sensitivity to identify analytes down to a single molecule by fingerprinting vibrational information. The strong coupling in microcavities for supermodes with promoted quality factors demonstrates the capability to boost the interaction between photons and molecules for nonlinear effects. Here a strong‐coupling cascade microsphere‐cavity (MC) achieving spontaneous Raman enhancement is reported, for the first time. The cascade MC is composed of two size‐mismatched microspheres, both of which support optical whispering‐gallery modes (WGMs) with significantly different free‐space ranges. The strong coupling between the supported WGMs in the microspheres can be achieved when the quasi‐vernier effect of resonant modes occurs, by which the energy splitting is up to 4 meV. The fast energy transfer between the size‐mismatched MCs therefore increases the quality factor up to 2.1 × 103 for the upper supermode branch. The strong‐coupling cascade MC structure provides an extra enhancement channel for the Au‐based SERS substrate, demonstrating an enhancement factor of Raman intensity (EFRI) up to 2.6 × 1010 for the limit of detection (LoD) down to 10−12 M of 4‐aminothiophenol (4‐ATP). The strong‐coupling cascade MC substrates also exhibit the versatility for identification of multiple analytes for high‐performance Raman trace‐detection applications.

High-Q Tamm plasmon-like resonance in spherical Bragg microcavity resonators.

This work proposes what we believe to be a novel Tamm plasmon-like resonance supporting structure consisting of an Au/SiO2 core-shell metal nanosphere structure surrounded by a TiO2/SiO2 spherical Bragg resonator (SBR). The cavity formed between the core metal particle and the SBR supports a localized mode similar to Tamm plasmons in planar dielectric multilayers. Theoretical simulations reveal a sharp absorption peak in the SBR bandgap region, associated with this mode, together with strong local field enhancement. We studied the modification of a dipolar electric emitter's radiative and non-radiative decay rates in this resonant structure, resulting in a quantum efficiency of ∼90% for a dipole at a distance of r=60n m from the Au nanosphere surface. A 30-layer metal-SBR Tamm plasmon-like resonant supporting structure results in a Q up to ∼103. The Tamm plasmon-like mode is affected by the Bragg wavelength and the number of layers of the SBR, and the thickness of the spacer cavity layer. These results will open a new avenue for generating high-Q Tamm plasmon-like modes for switches, optical logic computing devices, and nonlinear applications.

Inversion ring in chromonic twisted hedgehogs: theory and experiment

ABSTRACT Twisted hedgehogs are defects in spherical cavities with homeotropic anchoring for the nematic director that arise when twist distortions are sufficiently less energetic than splay (and bend) distortions. They bear a characteristic inversion ring, where the director texture changes the sense it spirals about the centre of the cavity. This paper applies a quartic twist theory recently proposed to describe the elasticity of chromonics to explain a series of inversion rings observed in spherical microcavities containing aqueous solutions of SSY at two different concentrations. The theory features a phenomenological length a, whose measure is extracted from the data and shown to be fairly independent of the cavity radius, as expected for a material constant.

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.

Surface Scattering Loss of a Spherical Microcavity

Surface Scattering Loss of a Spherical Microcavity

Magnetic whispering-gallery super-resonance spoiling in a Drude-Kerr optical cavity

High-quality optical Fano resonances, which exist in a spherical dielectric microparticle in the spatial configurations of the whispering-gallery like modes (WGMs) and recently are referred to as the “super-resonances”, possess extremal sensitivity not only to the micro-cavity size and shape but also to the optical properties of the particulate material. The increase in the power of light radiation coupling into a WGMs can lead to the manifestation of strong optical nonlinearity of the resonator material due to the cubic nonlinearity (optical Kerr effect) and generation of free electron plasma in certain regions of the spherical microcavity. In fact, an optically linear medium of the resonance cavity transforms into a Drude-Kerr-type medium with a strong dependence on input laser intensity. With the help of the finite-elements numerical simulations, we theoretically analyze the transformations of the resonant contour for a number of high-quality WGMs-like excited in a lanthanum-glass microsphere exposed to an optical radiation of different intensity. We show that in a certain input intensity range, the competition of Kerr and plasma optical nonlinearities can stop and even reverse the Stokes shift of the super-resonance observed in a pure Kerr medium. Additionally, optical ionization and plasma generation within the mode volume significantly reduce the strength of the magnetic super-resonance and lead to its spoiling and frequency splitting. Our findings pav the way for an all-optical approach for the obtaining the strong magnetic fields of the order of tens Tesla at relatively moderate laser intensities without the use of structured laser beam.

Preparation and controllable tuning of a high-quality factor phonon-laser in the optomechanical microsphere cavity

Low-threshold, narrow linewidth phonon lasers can greatly improve the detection resolution of sensors and have enormous potential for development in classical and quantum sensing fields, as well as information processing. However, its development is limited due to unfavorable factors such as the complex process, low quality factor (Q-factor), difficult tuning, and harsh environments requirements. Here, we report an easy-to-excite phonon laser with an ultra-narrow linewidth in silica whispering gallery mode microsphere optomechanical resonators at room temperature and ambient pressure. The microsphere cavity is fabricated by high-temperature melting with a CO2 laser and designed by controlling the proportion of the sphere to the stem (sphere-to-stem ratio) to reduce mechanical damping. By using a single-frequency laser as the pump source, the microsphere optomechanical resonator exhibited multiple breathing mode phonon lasers with ultra-high optical Q-factor (1.78 × 109), mechanical Q-factor (3.1 × 107), and low threshold (2.4 μW). It is the first time to achieve such a high mechanical Q-factor in the microsphere cavity system of this kind to the best of our knowledge, which opens up an avenue to develop highly sensitive sensors.

Formation of a Strong Quasi-Static Electric Field under Irradiation of a Target with a Spherical Microcavity by Intense Laser Pulses

Formation of a Strong Quasi-Static Electric Field under Irradiation of a Target with a Spherical Microcavity by Intense Laser Pulses

Methane Sensor Based on High-Q Microsphere Whispering Gallery Mode

In this paper, the principle of the methane gas enhanced the laser absorption by the WGM of high-Q microsphere was studied. A taper optical fiber was used as a coupler to couple 1653.72 nm laser into a microsphere with a optical fiber handle to form whispering gallery mode(WGM). The resonant wavelength of the microsphere cavity was changed to 1653.72 nm by injecting heating light along the handle, so that one resonant wavelength of the microsphere is just 1653.72 nm. The 1653.72 nm laser forms WGM continuously to surround the microsphere, and the effective optical path is greatly increased, which increases the interaction distance with methane, and thus the sensitivity of methane detection is improved. The results show that the microsphere with WGM can be used as a gas sensor.

Comment to the paper “Quantum mechanical effects for a hydrogen atom confined in a dielectric spherical microcavity”

The article “Quantum mechanical effects for a hydrogen atom confined in a dielectric spherical microcavity” (Wang et al., 2022) does not describe correctly the Shannon entropy for the states 3p and 4p for the hydrogen atom confined by a dielectric spherical microcavity. In this comment, we show the correct behavior for these states and conclude that there was an error in the data found by the authors of that article. Thus, looking for a physical meaning for the behavior observed in that paper makes no sense.

All-Optical Valley Polarization Switch via Controlling Spin-Triplet and Spin-Singlet Interlayer Exciton Emission in WS2/WSe2 Heterostructure.

Although selective singlet and triplet interlayer exciton (IX) emission of transition metal dichalcogenides (TMD) heterostructures can be achieved by applying an electric or magnetic field, the device structure is complex and a low temperature is usually required. Here, we demonstrate a simple all-optical approach to selectively enhance the emission of singlet and triplet IX by selectively coupling singlet or triplet IX of a WS2/WSe2 heterostructure to a SiO2 microsphere cavity. Angle-resolved photoluminescene reveals that the transition dipole of triplet IX is almost along the out-of-plane direction, while singlet IX only has 69% out-of-plane dipole moment contribution. Since the out-of-plane dipole presents a higher Purcell factor within the cavity, we can simultaneously enhance the emission intensity of IX and control the emissive IX species at room temperature in an all-optical route. Importantly, we demonstrate an all-optical valley polarization switch with a record high on/off ratio of 35.

Photonic reservoir computing with a silica microsphere cavity.

We experimentally demonstrate a photonic reservoir computing (RC) system using a passive silica microsphere cavity. The microsphere cavity exhibits a consistent nonlinear response to the non-return-to-zero signal and the multiple-level signal due to strong interference between numerous whispering gallery modes in the "over-coupling" state. Benefiting from the fact that the long photon lifetime inside the microsphere cavity provides a memory of past inputs, this photonic reservoir does not require a delayed feedback loop. We evaluate the generalization property of the RC system and obtain a correlation coefficient of 0.923. In addition, we obtain a NMSE of 0.06 for the Santa-Fe chaotic time series prediction task and a SER of 0.02 at a SNR of 12 dB for the nonlinear channel equalization task. Moreover, a microsphere cavity with a higher quality factor can provide a larger memory capacity. The application of the silica microsphere cavity as a small-volume passive device in a reservoir furnishes a new avenue for achieving a low-consumption and integrated RC system.

Large measurement range temperature sensor based on WGM and MMI in an offset structure fiber coupler

In this paper, a coupler based on a single-mode fiber (SMF)- no-core fiber (NCF)- SMF offset structure was proposed and demonstrated, which can generate the evanescent field to excite whispering gallery mode (WGM) of a microsphere cavity. By reasonably controlling the length and offset distance of NCF, the multimode interference (MMI) and WGMs can be generated at the same time. The experimental results show that the temperature measurement with a large measurement range and low detection limit can be realized simultaneously with the assistance of the interference envelope, which solves the problem of large range measurement difficulty caused by the dense dips of WGM to some extent. In the range of 22 ℃-80 ℃, the sensitivity of WGM resonant dips is ∼10.5 pm/℃. On the other hand, symmetric Lorenz lines with a quality factor of 8.2 × 103 and asymmetric Fano lines with a slope of -21.94 dB/nm were obtained under the modulation of interference. The coupler proposed in this paper has the advantages of simple preparation and strong structural robustness, and shows good potential in applications such as sensors and filters.