Extreme Light Concentration and High Absorption of the Double Cylindrical Microcavities

We numerically study the enhancement factor of energy density and absorption efficiency inside the double cylindrical microcavities based on a triple-band metamaterial absorber. The compact single unit cell consists of concentric gold rings with a gold disk in the center and a metallic ground plane separated by a dielectric layer. We demonstrate that the multilayer structure with subwavelength electromagnetic confinement allows 104-105-fold enhancement of the electromagnetic energy density inside the double cavities and contains the most energy of the incoming light. Particularly, the enhancement factor of energy density G shows strong ability of localizing light and some regularity as the change of the thickness of the dielectric slab and dielectric constant. At the normal incidence of electromagnetic radiation, the obtained reflection spectra show that the resonance frequencies of the double microcavities operate in the range of 10-30μm. We also calculate the absorption efficiency C, which can reach 95%, 97% and 95% at corresponding frequency by optimizing the structure＇s geometry parameters. Moreover, the proposed structure will be insensitive to the polarization of the incident wave due to the symmetry of the double cylindrical microcavities. The proposed optical metamaterial is a promising candidate as absorbing elements in scientific and technical applications due to its extreme confinement, multiband absorption and polarization insensitivity.

Photonic-plasmonic hybrid microcavities: Physics and applications

Photonic-plasmonic hybrid microcavities,which possess a higher figure of merit Q/V(the ratio of quality factor to mode volume)than that of pure photonic microcavities or pure plasmonic nano-antennas,play key roles in enhancing light–matter interaction.In this review,we summarize the typical photonic-plasmonic hybrid microcavities,such as photonic crystal microcavities combined with plasmonic nano-antenna,whispering gallery mode microcavities combined with plasmonic nano-antenna,and Fabry–Perot microcavities with plasmonic nano-antenna.The physics and applications of each hybrid photonic-plasmonic system are illustrated.The recent developments of topological photonic crystal microcavities and topological hybrid nano-cavities are also introduced,which demonstrates that topological microcavities can provide a robust platform for the realization of nanophotonic devices.This review can bring comprehensive physical insights of the hybrid system,and reveal that the hybrid system is a good platform for realizing strong light–matter interaction.

Application of Fourier Series Expansion Method with PMLs to the Microcavities on Two-Dimensional Photonic Crystals

By using a Fourier series expansion method combined with Chew＇s perfectly matched layers （PMLs）, we analyze the frequency and quality factor of a micro-cavity on a two-dimensional photonic crystal is analyzed. Compared with the results by the method without PML and finite-difference time-domain （FDTD） based on supercell approximation, it can be shown that by the present method with PMLs, the resonant frequency and the quality factor values can be calculated satisfyingly and the characteristics of the micro-cavity can be obtained by changing the size and permittivity of the point defect in the micro-cavity.

Near field enhancement and absorption properties of the double cylindrical microcavities based on triple-band metamaterial absorber

We numerically study the near field enhancement and absorption properties inside the double cylindrical microcavities based on triple-band metamaterial absorber. The compact single unit cell consists of concentric gold rings each with a gold disk in the center, and a metallic ground plane separated by a dielectric layer. At the normal incidence of electromagnetic radiation, the obtained reflection spectra show that the resonance frequencies of the double microcavities are 16.65 THz, 20.65 THz, and 25.65THz, respectively. We also calculate the values of contrast C （C = 1 - Rmin）, which can reach 95%, 97%, and 95% at the corresponding frequencies by optimizing the geometry parameters of structure. Moreover, we demon- strate that the multilayer structure with subwavelength electromagnetic confinement allows 104 -105-fold enhancement of the electromagnetic energy density inside the double cavities, which contains the most energy of the incoming electro- magnetic radiation. Moreover, the proposed structure will be insensitive to the polarization of the incident wave due to the symmetry of the double cylindrical microcavities. The proposed optical metamaterial is a promising candidate as an absorbing element in scientific and technical applications because of its extreme confinement, multiband absorptions, and polarization insensitivity.

EDESR and ODMR of Impurity Centers in Nanostructures Inserted in Silicon Microcavities

We present the first findings of the new electrically- and optically-detected magnetic resonance technique [ED electron spin resonance (EDESR) and (ODMR)] which reveal single point defects in the ultra-narrow silicon quantum wells (Si-QW) confined by the superconductor δ-barriers. This technique allows the ESR identification without the application of the external cavity as well as a high frequency source and recorder, with measuring the only magnetoresistance (EDESR) and transmission (ODMR) spectra within frameworks of the excitonic normal-mode coupling (NMC) caused by the microcavities embedded in the Si-QW plane. The new resonant positive magnetoresistance data are interpreted here in terms of the interference transition in the diffusive transport of free holes respectively between the weak antilocalization regime in the region far from the ESR of a paramagnetic point defect located inside or near the conductive channel and the weak localization regime in the nearest region of the ESR of that defect.

Solid-state quantum nodes based on color centers and rare-earth ions coupled with fiber Fabrye-Pérot microcavities

High-performance optical quantum memories serving as quantum nodes are crucial for the distribution of remote entanglement and the construction of large-scale quantum networks.Notably,quantum systems based on single emitters can achieve deterministic spin–photon entanglement,which greatly simplifies the difficulty of constructing quantum network nodes.Among them,optically interfaced spins embedded in solid-state systems,as atomic-like emitters,are important candidate systems for implementing long-lived quantum memory due to their stable physical properties and robustness to decoherence in scalable and compact hardware.To enhance the strength of light-matter interactions,optical microcavities can be exploited as an important tool to generate high-quality spin–photon entanglement for scalable quantum networks.They can enhance the photon collection probability and photon generation rate of specific optical transitions and improve the coherence and spectral purity of emitted photons.For solid-state systems,open Fabry–Pérot cavities can couple single emitters that are not in proximity to the surface,avoiding significant spectral diffusion induced by the interfaces while maintaining the wide tunability,which enables addressing of multiple single emitters in the frequency and spatial domain within a single device.This review described the characteristics of single emitters as quantum memories with a comparison to atomic ensembles,the cavity-enhancement effect for single emitters and the advantages of different cavities,especially fiber Fabry–Pérot microcavities.Finally,recent experimental progress on solid-state single emitters coupled with fiber Fabry–Pérot microcavities was also reviewed,with a focus on color centers in diamond and silicon carbide,as well as rare-earth dopants.

Emerging opportunities for ultra-high Q whispering gallery mode microcavities

Whispering gallery modes (WGMs) were first discovered for sound waves in the whispering gallery of St Paul’s Cathedral and explained by Rayleigh [1] in 1878. In 1961, Garrett et al.[2] applied the concept of WGMs to optical systems and realized stimulated emissions in Sm2+-doped CaF2 spheres.Since then, WGMs have been widely and intensively studied in a range of micro-sized systems, including microdroplets,microspheres, microtoroids, microdisks, and microtubes.

Periodically poled lithium niobate whispering gallery mode microcavities on a chip

In this study, we investigate the fabrication of periodically poled lithium niobate(PPLN) microdisk cavities on a chip. These resonators are fabricated from a PPLN film with a 16 μm poling period on insulator using conventional microfabrication techniques.The quality factor of the PPLN microdisk resonators with a 40-μm radius and a 700-nm thickness is 6.7×10~5. Second harmonic generation(SHG) with an efficiency of 2.2×10^(-6) mW(-1) is demonstrated in the fabricated PPLN microdisks. The nonlinear conversion efficiency could be considerably enhanced by optimizing the period and pattern of the poled structure and by improving the cavity quality factors.

Optomechanical sensing with on-chip microcavities

The coupling between optical and mechanical degrees of freedom has been of broad interest for a long time. However, it is only until recently, with the rapid development of optical mierocavity research, that we are able to manipulate and utilize this coupling process. When a high Q microeavity couples to a mechanical resonator, they can consolidate into an optomeehanieal system. Benefitting from the unique characteristics offered by optomeehanical coupling, this hybrid system has become a promising platform for ultrasensitive sensors to detect displacement, mass, force and acceleration. In this review, we introduce the basic physical concepts of cavity optomechanies, and describe some of the most typical experimental cavity optomechanical systems for sensing applications. Finally, we discuss the noise arising from various sources and show the potentiality of optomechanical sensing towards quantum-noise-limited detection.

Plasmon enhancement for Vernier coupled single-mode lasing from ZnO/Pt hybrid microcavities

It is essential to develop a single mode operation and improve the performance of lasing in order to ensure practical applicability of microlasers and nanolasers. In this paper, two hexagonal microteeth with varied nanoscaled air-gaps of a ZnO microcomb are used to construct coupled whispering-gallery cavities. This is done to achieve a stable single mode lasing based on Vernier effect without requiring any complicated or sophisticated manipulation to achieve positioning with nanoscale precision. Optical gain and the corresponding ultraviolet lasing performance were improved greatly through coupling with localized surface plasmons of Pt nanoparticles. The ZnO/Pt hybrid microcavities achieved a seven-fold enhancement of intensity of single mode lasing with higher side- mode suppression ratio and lower threshold. The mechanism that led to this enhancement has been described in detail.

Optical microcavities: new understandings and developments

Optical microcavities have attracted tremendous interest in both fundamental and applied research in the past few decades, thanks to their small footprint, easy integrability, and high quality factors. Using total internal reflection from a dielectric interface or a photonic band gap in a periodic system, these photonic structures do not rely on conventional metal-coated mirrors to confine light in small volumes, which have brought forth new developments in both classical and quantum optics. This focus issue showcases several such developments and related findings, which may pave the way for the next generation of on-chip photonic devices based on microcavities.

Frequency-tuning-induced state transfer in optical microcavities

Quantum state transfer in optical microcavities plays an important role in quantum information processing and is essential in many optical devices such as optical frequency converters and diodes.Existing schemes are effective and realized by tuning the coupling strengths between modes.However,such approaches are severely restricted due to the small amount of strength that can be tuned and the difficulty performing the tuning in some situations,such as in an on-chip microcavity system.Here we propose a novel approach that realizes the state transfer between different modes in optical microcavities by tuning the frequency of an intermediate mode.We show that for typical functions of frequency tuning,such as linear and periodic functions,the state transfer can be realized successfully with different features.To optimize the process,we use the gradient descent technique to find an optimal tuning function for a fast and perfect state transfer.We also showed that our approach has significant nonreciprocity with appropriate tuning variables,where one can unidirectionally transfer a state from one mode to another,but the inverse direction transfer is forbidden.This work provides an effective method for controlling the multimode interactions in on-chip optical microcavities via simple operations,and it has practical applications in all-optical devices.

Purcell effect of GaAs quantum dots by photonic crystal microcavities

We fabricate photonic crystal slab microcavities embedded with GaAs quantum dots by electron beam lithography and droplet epitaxy. The Purcell effect of exciton emission of the quantum dots is confirmed by the micro photoluminescence measurement. The resonance wavelengths, widths, and polarization are consistent with numerical simulation results.

Kerr frequency combs in large-size,ultra-high-Q toroid microcavities with low repetition rates [Invited]

By overcoming fabrication limitations, we have successfully fabricated silica toroid microcavities with both large diameter(of 1.88 mm) and ultra-high-Q factor(of 3.3 × 10~8) for the first time, to the best of our knowledge. By employing these resonators, we have further demonstrated low-threshold Kerr frequency combs on a silicon chip,which allow us to obtain a repetition rate as low as 36 GHz. Such a low repetition rate frequency comb can now bedirectly measured through a commercialized optical-electronic detector.

Impact of nanoparticle-induced scattering of an azimuthally propagating mode on the resonance of whispering gallery microcavities

Optical whispering gallery microcavities with high-quality factors have shown great potential toward achieveing ultrahigh-sensitivity sensing up to a single molecule or nanoparticle, which raises a huge demand on a deep theoretical insight into the crucial phenomena such as the mode shift, mode splitting, and mode broadening in sensing experiments. Here we propose an intuitive model to analyze these phenomena from the viewpoint of the nanoparticle-induced multiple scattering of the azimuthally propagating mode(APM). The model unveils explicit relations between these phenomena and the phase change and energy loss of the APM when scattered at the nanoparticle; the model also explains the observed polarization-dependent preservation of one resonance and the particle-dependent redshift or blueshift. The model indicates that the particle-induced coupling between the pair of unperturbed degenerate whispering gallery modes(WGMs) and the coupling between the WGMs and the free-space radiation modes, which are widely adopted in current theoretical formalisms, are realized via the reflection and scattering-induced free-space radiation of the APM, respectively, and additionally exhibits the contribution of cross coupling between the unperturbed WGMs and other different WGMs to forming the splittingresonant modes, especially for large particles.

Temperature-insensitive refractive index sensor based on Mach–Zehnder interferometer with two microcavities

We propose a temperature-insensitive refractive index（RI） fiber sensor based on a Mach–Zehnder interferometer. The sensor with high sensitivity and a robust structure is fabricated by splicing a short photonic crystal fiber（PCF） between two single-mode fibers, where two microcavities are formed at both junctions because of the collapse of the PCF air holes. The microcavity with a larger equatorial dimension can excite higher-order cladding modes, so the sensor presents a high RI sensitivity, which can reach 244.16 nm/RIU in the RI range of1.333–1.3778. Meanwhile it has a low temperature sensitivity of 0.005 nm/°C in the range of 33°C–360°C.

Enhanced photoluminescence from porous silicon microcavities by rare earth doping

The photoluminescence(PL) properties of porous silicon microcavities(PSMs) in the visible range at room temperature are improved by doping the rare earth ytterbium(Yb) into PSMs prepared by the electrochemical etching method.It is observed that PSMs doped with the rare earth have an emission band around 630 nm.Compared with the single-layer porous silicon(PS) film,the PSMs doped with Yb have narrower and stronger PL spectrum.

Femtosecond laser 3D fabrication of whispering-gallerymode microcavities

Whispering-gallery-mode(WGM) microcavities with high-quality factors and small volumes have attracted intense interests in the past decades because of their potential applications in various research fields such as quantum information, sensing, and optoelectronics. This leads to rapid advance in a variety of processing technologies that can create high-quality WGM micro-cavities. Due to the unique characteristics of femtosecond laser pulses with high peak intensity and ultrashort pulse duration, femtosecond laser shows the ability to carry out ultrahigh precision micromachining of a variety of transparent materials through nonlinear multiphoton absorption and tunneling ionization. This review paper describes the basic principle of femtosecond laser direct writing, and presents an overview of recent progress concerning femtosecond laser three-dimensional(3D) fabrications of optical WGM microcavities, which include the advances in the fabrications of passive and active WGMs microcavities in a variety of materials such as polymer, glass and crystals, as well as in processing the integrated WGM-microcavity device. Lastly, a summary of this dynamic field with a future perspective is given.

Strong light–matter interaction in ZnO microcavities

The strong light–matter interaction in ZnO-embedded microcavities has received great attention in recent years,due to its ability to generate the robust bosonic quasiparticles,exciton-polaritons,at or above room temperature.This review introduces the strong coupling effect in ZnO-based microcavities and describes the recent progress in this field.In addition,the report contains a systematic analysis of the room-temperature strong-coupling effects from relaxation to polariton lasing.The stable room temperature operation of polaritonic effects in a ZnO microcavity promises a wide range of practical applications in the future,such as ultra-low power consumption coherent light emitters in the ultraviolet region,polaritonic transport,and other fundamental of quantum optics in solid-state systems.

Multimode sensing based on optical microcavities

Optical microcavities have the ability to confne photons in small mode volumes for long periods of time,greatly enhancing light-matter interactions,and have become one of the research hotspots in international academia.In recent years,sensing applications in complex environments have inspired the development of multimode optical microcavity sensors.These multimode sensors can be used not only for multi-parameter detection but also to improve measurement precision.In this review,we introduce multimode sensing methods based on optical microcavities and present an overview of the multimode single/multi-parameter optical microcavities sensors.Expected further research activities are also put forward.