高Q回音壁模式微泡光学谐振腔的光学特性研究.docx

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Abstract
Optical resonant cavities, in the form of high-Q whispering-gallery-mode microbubble resonators, have attracted attention and interest in various fields, from fundamental physics to biophotonics. Here, we review the optical properties of microbubble resonators, focusing on their potential applications in both basic research and practical settings.
Introduction
Optical resonant cavities, which support confined electromagnetic modes, are widely used in scientific research and technological applications, ranging from sensing and spectroscopy to optical communication and information processing. Among various types of optical resonators, whispering-gallery-mode (WGM) resonators have distinct advantages due to their exceptional optical properties. For example, high-Q WGM resonators have been used for ultrasensitive sensing of chemical and biological analytes, as well as for nonlinear optical phenomena and photonic devices.
Recently, microbubble resonators (MBRs) have attracted particular attention due to their high Q factor and unique morphology. Unlike conventional WGM resonators that are based on solid materials, MBRs are fabricated by thermal deformation of a glass capillary, which produces an air-filled spherical cavity with a diameter ranging from several tens of microns to a few hundred microns. The MBRs are optically transparent and exhibit high-Q factors due to total internal reflection at the curved interface between the air and glass.
In this paper, we review the optical properties of MBRs, focusing on the high-Q whispering gallery modes that arise from the spherical shape of the resonator. We first describe the fabrication methods for MBRs and analyze their optical properties, including their mode spectra, Q factors, and mode volumes. Then, we discuss the applications of MBRs in sensing, nonlinear optics, and optomechanics. Finally, we outline the challenges and opportunities for future research on MBRs.
Fabrication and Optical Properties of MBRs
MBRs are typically fabricated by melting and stretching a glass capillary using a flame-heating technique. The fabrication process involves heating the capillary in a flame until it becomes soft, and then applying a controlled gas flow and pressure to inflate the softened capillary into a spherical shape. The resulting MBR has a thin glass shell that serves as a resonant cavity, with an air-filled interior that supports WGMs.
The optical properties of MBRs are determined by both the geometry and material properties of the resonator. The spherical shape of the MBR provides a natural spatial confinement for WGMs, leading to high-Q factors. The Q factor is defined as the ratio of the stored energy in the resonator to the energy loss per cycle. The Q factor of typical MBRs can exceed 10^8, which is several orders of magnitude higher than that of other resonator types.
The mode spectra of MBRs are determined by the boundary conditions at the glass-air interface. In particular, the tangential magnetic field component must be zero at the interface, which leads to a discrete set of resonant modes. The mode frequencies can be calculated using the Mie theory, which describes the scattering of electromagnetic waves by spherical particles. The Mie theory predicts the existence of various families of modes, such as whispering gallery (WG), hybrid (HB), and radial (R) modes.
The WG modes are the most prominent modes in MBRs, and form a dense spectrum near the equator of the sphere. The WG modes are characterized by a high azimuthal mode number m, which determines the number of times the mode circulates around the equator of the sphere before returning to its origin. The WG modes can have a wide range of frequencies, covering the visible to infrared wavelength range. The HB modes are hybrid modes that arise from the coupling between the WG modes and the radial modes. The HB modes have a lower Q factor than the WG modes, but they can still exhibit high sensitivity to perturbations at the resonator surface. The R modes are purely radial modes that have a low Q factor and are easily perturbed by environmental factors.
The Q factor and mode volume of MBRs can be estimated using the finite element method (FEM) simulation. The Q factor depends on the intrinsic material loss and the scattering loss due to surface roughness and imperfections. The mode volume is determined by calculating the integration of the electric field intensity over the volume of the resonator. The small mode volume of MBRs, on the order of cubic wavelength, makes them particularly useful for enhancing light-matter interaction in small volumes.
Applications of MBRs
MBRs have emerged as versatile platforms for various applications due to their high-Q modes and small mode volumes. One of the most promising applications is sensing, where MBRs can be used to detect changes in refractive index or surface adsorption. For example, Liu et al. demonstrated the detection of nanoscale changes in dielectric properties of a single molecule using a MBR. The high sensitivity of MBRs to surface adsorption and conformational changes of biomolecules makes them attractive for label-free biosensing. Hsiao et al. reported the functionalization of MBRs with a DNA probe for the detection of single-nucleotide polymorphisms (SNPs) in a specific DNA sequence.
MBRs also offer unique opportunities for nonlinear optics and photonics. The high-Q WG modes of MBRs can enhance nonlinear optical effects, such as second harmonic generation (SHG) and four-wave mixing (FWM). For example, Wang et al. demonstrated SHG in a MBR made of lead silicate glass, achieving a conversion efficiency of 10^-9. MBRs have also been used for frequency comb generation and mode-locked laser operation. Jung et al. developed a mode-locked erbium-doped fiber laser using a MBR as the cavity, which exhibited a narrow linewidth and high repetition rate.
MBRs can also be used in optomechanics, where the mechanical motion of the resonator is coupled to the optical field. The high Q factor of MBRs makes them sensitive to tiny mechanical displacements, allowing them to be used as mechanical sensors. For example, Ward et al. used a MBR to detect the Brownian motion of a suspended particle, achieving a sensitivity of 5×10^-15 m Hz^-1/2. MBRs have also been used in optomechanical cooling and trapping of nanoparticles, demonstrating a cooling rate of K/s and a trapping force of 47 pN.
Challenges and Opportunities for Future Research
Despite the recent progress in MBR research, there are still challenges and opportunities for future exploration. The fabrication process of MBRs is still not optimized, and the quality of the MBRs can be affected by various factors, such as the thermal and mechanical stability of the glass capillary and the surface tension of the melted glass. The development of new glass materials with low intrinsic loss and high thermal and mechanical stability can enhance the performance of MBRs.
Another challenge is the environmental stability of MBRs. MBRs can be perturbed by various factors, such as temperature changes, mechanical vibrations, and external pressure fluctuations. The development of robust MBRs that can withstand harsh environments is essential for practical applications.
Finally, the integration of MBRs with other photonic components and systems can enhance their functionality and versatility. For example, the integration of MBRs with microfluidic channels can create a lab-on-a-chip platform for biological sensing. The integration of MBRs with semiconductor or plasmonic structures can enhance their nonlinear optical response and enable electrically driven optomechanical devices.
Conclusion
In conclusion, MBRs offer unique optical properties and applications due to their high-Q WGMs and small mode volumes. MBRs can be used in sensing, nonlinear optics, and optomechanics, and offer opportunities for integration with other photonic components and systems. The fabrication and environmental stability of MBRs are still challenges that need to be addressed. Future research on MBRs can lead to new discoveries and applications in various fields, and can further enhance the performance and versatility of these remarkable optical resonators.