Echo-wall mode microcavity magnetic sensor and detection method

Abstract

The application provides an echo wall mode microcavity magnetic sensor and a detection method. The echo wall mode microcavity magnetic sensor comprises an echo wall mode microcavity, a magnetostrictive layer and a base. The magnetostrictive layer is in contact with the echo wall mode microcavity, and the base is in contact with the echo wall mode microcavity and used for supporting the echo wall mode microcavity. The application combines the echo wall mode microcavity with the magnetostrictive layer, so that the magnetostrictive layer produces resonance under a magnetic field to produce a mechanical response, thereby changing the optical path length of the echo wall mode microcavity in contact with the magnetostrictive layer, further changing the resonance frequency of the echo wall mode microcavity, and the size of the magnetic field and the movement of the resonance frequency are in linear relationship, so that the echo wall mode microcavity magnetic sensor has high sensitivity and is suitable for different magnetic field detection, especially for biological magnetic detection with low magnetic field strength and low frequency.

Description

Technical Field

[0001] This invention belongs to the field of magnetic sensors and relates to a whispering-gallery mode microcavity magnetic sensor and detection method. Background Technology

[0002] Biomagnetism is the magnetic field generated by the bioelectrical activity of living organisms. It is characterized by low magnetic field strength (~pT or even ~100fT) and low frequency (a few Hz to hundreds of Hz). By capturing the magnetic field generated by organisms, the working mechanism of bioelectrical activity can be studied. Currently, biomagnetism has many applications, such as magnetoencephalography (MEG) and magnetocardiography (MCC).

[0003] Currently, the main types of biomagnetic sensors include: Superconducting Quantum Interference Device (SQUID), SERF (Spin-Exchange Relaxation-Free) atomic magnetometer, and Tunnel MagnetoResistance (TMR). Each has its own advantages and disadvantages. For example, SQUID has extremely high detection sensitivity, with commercial instruments reaching 1 fT / √Hz. However, this device requires operation in a low-temperature environment and is expensive, hindering its application in low-frequency biomagnetic fields. SERF atomic magnetometers are a new type of magnetometer, with commercial devices currently achieving a sensitivity of 15 fT / √Hz, but similarly, this device has very stringent environmental requirements. While TMR sensors are inexpensive, commercially available devices have relatively low sensitivity.

[0004] Therefore, it is necessary to provide a whispering-gallery mode microcavity magnetic sensor and detection method. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a whispering-gallery mode microcavity magnetic sensor and detection method to solve the series of detection problems faced by existing biomagnetic sensors when performing biomagnetic detection.

[0006] To achieve the above and other related objectives, the present invention provides a whispering-gallery mode microcavity magnetic sensor, the whispering-gallery mode microcavity magnetic sensor comprising:

[0007] Whispering-gallery mode microcavity;

[0008] A magnetostrictive layer is in contact with the whispering-gallery mode microcavity. The magnetostrictive layer receives magnetic signals to generate a mechanical response and changes the resonant frequency of the whispering-gallery mode microcavity.

[0009] A base is provided, which contacts the sound-gallery pattern microcavity to support it.

[0010] Optionally, the whispering-gallery pattern microcavity includes any one of the whispering-gallery pattern microsphere cavity, whispering-gallery pattern microdisc cavity, and whispering-gallery pattern microcore ring cavity.

[0011] Optionally, when the whispering-gallery pattern microcavity is the whispering-gallery pattern microdisk cavity or the whispering-gallery pattern microcore ring cavity, the morphology of the whispering-gallery pattern microcavity includes a racetrack-shaped whispering-gallery pattern microcavity or a ring-shaped whispering-gallery pattern microcavity.

[0012] Optionally, the magnetostriction coefficient of the magnetostrictive layer ranges from 10 ppm to 2000 ppm.

[0013] Optionally, the magnetostrictive layer includes a super magnetostrictive layer, which in turn includes a Tb-Dy-Fe alloy super magnetostrictive layer.

[0014] Optionally, the whispering-gallery microcavity is a silicon dioxide whispering-gallery microcavity, and the base is a silicon base.

[0015] Optionally, any of the above-described whispering-gallery mode microcavity magnetic sensors are suitable for detecting biomagnetic signals.

[0016] The present invention also provides a detection method for a whispering-gallery mode microcavity magnetic sensor, comprising the following steps:

[0017] Provide any of the above-described whispering-gallery mode microcavity magnetic sensors;

[0018] The whispering-gallery mode microcavity magnetic sensor is placed in a magnetic field, and receives magnetic signals through the magnetostrictive layer to generate a mechanical response, thereby changing the resonant frequency of the whispering-gallery mode microcavity.

[0019] Optionally, the sensitivity of the whispering-gallery mode microcavity magnetic sensor can be adjusted by changing the structural parameters of the sensor to obtain its characteristic frequency.

[0020] Optionally, the structural parameters of the sound-gallery pattern microcavity magnetic sensor include any one or a combination of the following: the area of ​​contact between the base and the sound-gallery pattern microcavity, the microcavity length of the sound-gallery pattern microcavity, the thickness of the sound-gallery pattern microcavity, the material of the magnetostrictive layer, and the material of the sound-gallery pattern microcavity.

[0021] As described above, the whispering-gallery pattern microcavity magnetic sensor and detection method of the present invention include a whispering-gallery pattern microcavity, a magnetostrictive layer and a base. The magnetostrictive layer is in contact with the whispering-gallery pattern microcavity. The magnetostrictive layer receives magnetic signals to generate a mechanical response and changes the resonant frequency of the whispering-gallery pattern microcavity. The base is in contact with the whispering-gallery pattern microcavity to support the whispering-gallery pattern microcavity.

[0022] This invention combines a whispering-gallery pattern microcavity with a magnetostrictive layer, causing the magnetostrictive layer to resonate under a magnetic field, thereby generating a mechanical response. This alters the optical path length of the whispering-gallery pattern microcavity in contact with it, and consequently changes the resonant frequency of the whispering-gallery pattern microcavity. The magnitude of the magnetic field is linearly related to the shift in the resonant frequency, thus giving the whispering-gallery pattern microcavity magnetic sensor high sensitivity, making it suitable for various magnetic field detection needs, especially for biomagnetic detection with low magnetic field strength and low frequency. Attached Figure Description

[0023] Figure 1 The diagram shown is a structural schematic of the whispering-gallery mode microcavity magnetic sensor in an embodiment of the present invention.

[0024] Figure 2 Displayed as Figure 1 A cross-sectional schematic diagram of a whispering-gallery mode microcavity magnetic sensor.

[0025] Figure 3 The diagram shows a schematic of the device structure for simulated detection using a microcavity magnetic sensor in whispering-gallery mode, as described in an embodiment of the present invention.

[0026] Figure 4 This diagram illustrates the operation flow for simulated detection using a microcavity magnetic sensor in whispering-gallery mode, as shown in an embodiment of the present invention.

[0027] Figures 5a-5d The diagram shows the characteristic modes of the whispering-gallery mode microcavity magnetic sensor at different frequencies in an embodiment of the present invention.

[0028] Figure 6 The diagram shows the characteristic frequency of the whispering-gallery mode microcavity magnetic sensor in Embodiment 2 of the present invention.

[0029] Figure 7 The diagram shows the characteristic frequency of the whispering-gallery mode microcavity magnetic sensor in Embodiment 3 of the present invention.

[0030] Figure 8 The diagram shows the magnetostriction coefficients of different magnetostrictive layers in embodiments of the present invention.

[0031] Figure 9The diagram shows the characteristic frequency of the whispering-gallery mode microcavity magnetic sensor in Embodiment 1 of the present invention.

[0032] Component designation explanation

[0033] 100-Sound-Galloper Mode Microcavity Magnetic Sensor

[0034] 110 sounding-gallery mode microcavity

[0035] 120 magnetostrictive layer

[0036] 130 base

[0037] 200 Magnetic field generating device Detailed Implementation

[0038] Whispering-gallery mode microcavities, with their high quality factor and small mode volume, have become an excellent platform for ultra-high sensitivity sensing and are widely used in various sensor fields, such as temperature sensors, nanoparticle sensors, and pressure sensors. Magnetostrictive materials have a large magnetostriction coefficient, high thrust, and high energy conversion efficiency, and as a new generation of smart materials, they are widely used in various fields.

[0039] Based on the current needs of magnetic detection, this invention combines a whispering-gallery-mode microcavity with a magnetostrictive material to apply the whispering-gallery-mode microcavity to magnetic sensing, thereby proposing a whispering-gallery-mode microcavity magnetic sensor and detection method. This method can reduce the resonant frequency of the magnetic sensor and improve the sensitivity of the device to magnetic signals, especially low-frequency biomagnetic signals. At the same time, compared with low-temperature superconducting quantum interference devices of the same application type, this device has higher temporal and spatial resolution, can operate at room temperature, and can be integrated and packaged. It can provide a new option for the field of magnetic detection, especially biomagnetic detection, and has good potential for widespread application.

[0040] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0041] In the detailed description of embodiments of the present invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged and not to scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0042] For ease of description, spatial relation terms such as “below,” “under,” “lower than,” “below,” “above,” and “upper” may be used herein to describe the relationship between one element or feature shown in the accompanying drawings and other elements or features. It will be understood that these spatial relation terms are intended to include directions other than those depicted in the drawings for devices in use or operation. Furthermore, when a layer is referred to as being “between” two layers, it may be the only layer between the two layers, or there may be one or more layers in between.

[0043] In the context of this application, the structure described above the first feature may include embodiments in which the first and second features are formed in direct contact, or embodiments in which additional features are formed between the first and second features, such that the first and second features may not be in direct contact.

[0044] It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the illustrations only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0045] See Figure 1 and Figure 2 This embodiment provides a sound-gallery mode microcavity magnetic sensor 100, which includes a sound-gallery mode microcavity 110, a magnetostrictive layer 120, and a base 130. The magnetostrictive layer 120 is in contact with the sound-gallery mode microcavity 110, receives magnetic signals to generate a mechanical response, and changes the resonant frequency of the sound-gallery mode microcavity 110. The base 130 is in contact with the sound-gallery mode microcavity 110 to support the sound-gallery mode microcavity 110.

[0046] Specifically, by combining the whispering-gallery microcavity 110 with the magnetostrictive layer 120, the magnetostrictive layer 120 can resonate under a magnetic field to generate a mechanical response, thereby changing the optical path length of the whispering-gallery microcavity 110 in contact with it, and thus changing the resonant frequency of the whispering-gallery microcavity 110. The magnitude of the magnetic field is linearly related to the shift in the resonant frequency, thereby giving the whispering-gallery microcavity magnetic sensor 100 high sensitivity to suit different magnetic field detection needs, especially for biomagnetic detection with low magnetic field strength and low frequency.

[0047] As an example, the whispering-gallery pattern microcavity 110 includes any one of the whispering-gallery pattern microsphere cavity, whispering-gallery pattern microdisc cavity, and whispering-gallery pattern microcore ring cavity.

[0048] Specifically, such as Figures 1-3 In this embodiment, the whispering-gallery pattern microcavity 110 adopts a whispering-gallery pattern micro-core ring cavity, but it is not limited to this. The whispering-gallery pattern microcavity 110 may also adopt a whispering-gallery pattern micro-sphere cavity or a whispering-gallery pattern micro-disc cavity.

[0049] The fabrication of the whispering-gallery mode microcavity magnetic sensor 100 may include the following steps:

[0050] S1: Provide or prepare the whispering-gallery pattern microcavity 110;

[0051] S2: Provide or prepare the magnetostrictive layer 120;

[0052] S3: Adhere the whispering-gallery pattern microcavity 110 to the magnetostrictive layer 120.

[0053] In step S1, when the whispering-gallery pattern microcavity 110 adopts a whispering-gallery pattern micro-core ring cavity or a whispering-gallery pattern micro-disk cavity, processes such as coating, exposure, development, and etching can be used to prepare the silicon base and silicon dioxide whispering-gallery pattern microcavity. When the whispering-gallery pattern microcavity 110 adopts a whispering-gallery pattern microsphere cavity, it can be prepared by drawing and melting. The preparation process, material, and size of the whispering-gallery pattern microcavity 110 and the base 130 can be selected as needed, and no excessive restrictions are imposed here. In step S3, the specific method of bonding the whispering-gallery pattern microcavity 110 to the magnetostrictive layer 120 is not excessively restricted here.

[0054] As an example, when the sound-gallery pattern microcavity 110 is the sound-gallery pattern microdisk cavity or the sound-gallery pattern microring cavity, the morphology of the sound-gallery pattern microcavity includes a racetrack-shaped sound-gallery pattern microcavity or a ring-shaped sound-gallery pattern microcavity.

[0055] Specifically, the racetrack-shaped whispering-gallery pattern microcavity possesses extremely high optical sensing sensitivity. Compared to the annular whispering-gallery pattern microcavity, the racetrack-shaped whispering-gallery pattern microcavity exhibits stronger deformation under the influence of the giant magnetostrictive material, resulting in a greater change in the optical path length. Therefore, in this embodiment, the whispering-gallery pattern microcavity 110 is preferably a racetrack-shaped whispering-gallery pattern microcavity. Figure 1 By increasing the length of the raceway cavity, the characteristic frequency of the device can be significantly reduced. However, the morphology of the whispering-gallery mode microcavity 110 is not limited to this. It can also be a ring-shaped whispering-gallery mode microcavity or other morphologies. No excessive restrictions are imposed here. The specific morphology of the whispering-gallery mode microcavity 110 can be selected as needed.

[0056] As an example, the magnetostriction coefficient of the magnetostrictive layer 120 ranges from 10ppm to 2000ppm.

[0057] For details, please refer to Figure 8 The magnetostrictive layer 120 can be a conventional magnetostrictive material, such as a metal and alloy magnetostrictive material or a ferrite magnetostrictive material, with a magnetostrictive coefficient of 10ppm to 800ppm, such as Ni. Alternatively, the magnetostrictive layer 120 can be a piezoelectric ceramic material PZT, with a magnetostrictive coefficient of 300ppm to 400ppm. Another option is a supermagnetostrictive material, such as a rare earth and transition metal intermetallic compound, like the Tb-Dy-Fe alloy supermagnetostrictive layer Terfeno1-D, with a magnetostrictive coefficient of 1500ppm to 2000ppm. In this embodiment, the magnetostrictive layer 120 is preferably a Terfeno1-D supermagnetostrictive material with good sensitivity to magnetic fields, but the type of magnetostrictive layer 120 is not limited to these.

[0058] This embodiment also provides a detection method for a whispering-gallery mode microcavity magnetic sensor, including the following steps:

[0059] Provide any of the above-mentioned whispering-gallery mode microcavity magnetic sensors;

[0060] The whispering-gallery mode microcavity magnetic sensor is placed in a magnetic field, and receives magnetic signals through the magnetostrictive layer to generate a mechanical response, thereby changing the resonant frequency of the whispering-gallery mode microcavity.

[0061] As an example, the whispering-gallery mode microcavity magnetic sensor is suitable for detecting biomagnetic signals.

[0062] Specifically, in this embodiment, the characteristic frequency of the whispering-gallery pattern microcavity magnetic sensor can be obtained by changing any one or a combination of the structural parameters of the whispering-gallery pattern microcavity magnetic sensor, such as the contact area between the base 130 and the whispering-gallery pattern microcavity 110, the microcavity length of the whispering-gallery pattern microcavity 110, the thickness of the whispering-gallery pattern microcavity 110, the material of the magnetostrictive layer 120, and the material of the whispering-gallery pattern microcavity 110. This allows the sensitivity of the whispering-gallery pattern microcavity magnetic sensor to be adjusted, especially making it suitable for detecting low-intensity, low-frequency biomagnetic signals with magnetic field strength on the order of pT to hundreds of fT and frequency on the order of Hz to hundreds of Hz. However, the detection range of the whispering-gallery pattern microcavity magnetic sensor is not limited to this.

[0063] See Figures 4-9 In this embodiment, a simulated detection of the whispering-gallery mode microcavity magnetic sensor was also performed.

[0064] See Figure 4First, a magnetic field generating device is provided to supply a magnetic signal to the whispering-gallery mode microcavity magnetic sensor. This magnetic field generating device may include a Helmholtz coil to construct a biomagnetic field, such as a low-frequency, variable-frequency, and variable-direction one. For example, in COMSOL software, a sinusoidal alternating current is applied to the Helmholtz coil. By changing the angle, current amplitude, and frequency of the Helmholtz coil, a magnetic field region is generated at the center of the coil to serve as the background biomagnetic field for the whispering-gallery mode microcavity magnetic sensor. The radius of the coil is much larger than the size of the whispering-gallery mode microcavity magnetic sensor to generate a suitable region at the center. Then, the whispering-gallery mode microcavity magnetic sensor is provided and placed in the central region of the magnetic field provided by the Helmholtz coil. By changing the structural parameters of the magnetic sensor, the characteristic frequency of the radial breathing pattern is obtained. The radial breathing pattern, such as... Figures 5a-5d .

[0065] The following specific embodiments further illustrate the detection capabilities of the aforementioned whispering-gallery mode microcavity magnetic sensor.

[0066] Example 1

[0067] According to such Figure 4 The steps are as follows: (1) Build a magnetic field generating device and control the current Asin(2πft) to generate the required magnetic field. (2) Rotate the angle of the coil to change the direction of the central magnetic field, and set it to 0 degrees. (3) Place the whispering-gallery mode microcavity magnetic sensor at the center of the Helmholtz coil. (4) Keep other quantities constant and gradually increase the radius of the whispering-gallery mode microcavity magnetic sensor from 50μm to 1000μm. (5) Obtain the change in the radius of the magnetic sensor at the characteristic frequency, according to the resonance condition 2πRN of the whispering-gallery mode microcavity. eff =mλ, R is the microcavity radius, N eff Where is the effective refractive index, m is a constant, and λ is the resonant wavelength. Changing the radius reflects a change in the resonant wavelength, which can be used to measure the sensitivity of different parameters to a magnetic field, thereby calculating the characteristic frequency change of the whispering-gallery mode microcavity magnetic sensor, such as... Figure 9 As shown, when the radius is 50 μm, the characteristic frequency of the whispering-gallery mode microcavity magnetic sensor is 2 × 10⁻⁶. 6 When the radius is 410 μm, the characteristic frequency of the whispering-gallery mode microcavity magnetic sensor is 26645 Hz. When the radius is 1000 μm, the characteristic frequency of the whispering-gallery mode microcavity magnetic sensor is 5403.2 Hz. By increasing the radius to 1000 μm, the characteristic frequency of the whispering-gallery mode microcavity magnetic sensor decreases by 99.73%, which is a significant effect.

[0068] Example 2

[0069] The thickness of the whispering-gallery mode microcavity was controlled to be 25 μm, the base contact area to be a circle with a radius of 5 μm, and the radius of the whispering-gallery mode microcavity to be 200 μm, 500 μm, and 800 μm respectively. With other conditions remaining the same, the characteristic frequencies of the annular microcavity and runway cavities of different lengths were compared. When the runway cavity length was 0, i.e., a straight runway did not exist, it was an annular microcavity. The results are as follows: Figure 6 As shown, by increasing the length of the raceway cavity, the characteristic frequency of the device can be significantly reduced.

[0070] Example 3

[0071] By controlling the thickness of the whispering-gallery mode microcavity to be 25 μm, the base contact area to be a circle with a radius of 5 μm, and the radius of the whispering-gallery mode microcavity to be 30 μm, 60 μm, 200 μm, and 500 μm respectively, while keeping other conditions the same, changing the material type of the magnetostrictive layer yields the characteristic frequencies of the device under three different magnetostrictive materials, such as... Figure 7 and Figure 8 Terfenol-D maintains the lowest characteristic frequency while ensuring a very high magnetostriction coefficient.

[0072] In summary, the whispering-gallery pattern microcavity magnetic sensor and detection method of the present invention include a whispering-gallery pattern microcavity, a magnetostrictive layer, and a base. The magnetostrictive layer is in contact with the whispering-gallery pattern microcavity, receives magnetic signals to generate a mechanical response, and changes the resonant frequency of the whispering-gallery pattern microcavity. The base is in contact with the whispering-gallery pattern microcavity to support it.

[0073] This invention combines a whispering-gallery pattern microcavity with a magnetostrictive layer, causing the magnetostrictive layer to resonate under a magnetic field, thereby generating a mechanical response. This alters the optical path length of the whispering-gallery pattern microcavity in contact with it, and consequently changes the resonant frequency of the whispering-gallery pattern microcavity. The magnitude of the magnetic field is linearly related to the shift in the resonant frequency, thus giving the whispering-gallery pattern microcavity magnetic sensor high sensitivity, making it suitable for various magnetic field detection needs, especially for biomagnetic detection with low magnetic field strength and low frequency.

[0074] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. An optical whispering gallery mode microcavity magnetic sensor, characterized in that The whispering gallery mode microcavity magnetic sensor includes: A whispering gallery mode microcavity, which is a whispering gallery mode microdisk cavity or a whispering gallery mode micro-core ring cavity, and the morphology of the whispering gallery mode microcavity is a racetrack-shaped whispering gallery mode microcavity; A magnetostrictive layer, which is in contact with the whispering gallery mode microcavity. The magnetostrictive layer receives a magnetic signal to generate a mechanical response and changes the resonance frequency of the whispering gallery mode microcavity; A base, which is in contact with the whispering gallery mode microcavity and is used to support the whispering gallery mode microcavity; By changing the structural parameters of the whispering gallery mode microcavity magnetic sensor, the characteristic frequency of the whispering gallery mode microcavity magnetic sensor is obtained to adjust the sensitivity of the whispering gallery mode microcavity magnetic sensor; The structural parameters of the whispering gallery mode microcavity magnetic sensor include the contact area between the base and the whispering gallery mode microcavity, the microcavity length of the whispering gallery mode microcavity, the thickness of the whispering gallery mode microcavity, the material of the magnetostrictive layer, and the material of the whispering gallery mode microcavity.

2. The whispering gallery mode microcavity magnetic sensor according to claim 1, characterized in that: The magnetostrictive coefficient range of the magnetostrictive layer is 10 ppm to 2000 ppm.

3. The whispering gallery mode microcavity magnetic sensor according to claim 2, wherein: The magnetostrictive layer includes a giant magnetostrictive layer, and the giant magnetostrictive layer includes a Tb-Dy-Fe-based alloy giant magnetostrictive layer.

4. The whispering gallery mode microcavity magnetic sensor according to claim 1, wherein: The whispering gallery mode microcavity is a silica whispering gallery mode microcavity, and the base is a silicon base.

5. The whispering gallery mode microcavity magnetic sensor according to any one of claims 1 to 4, wherein: The whispering gallery mode microcavity magnetic sensor is applicable to detecting biomagnetic signals.

6. A detection method for a whispering gallery mode microcavity magnetic sensor, characterized in that, It includes the following steps: Provide the whispering gallery mode microcavity magnetic sensor according to any one of claims 1 to 5; Place the whispering gallery mode microcavity magnetic sensor in a magnetic field. The magnetostrictive layer receives a magnetic signal to generate a mechanical response and changes the resonance frequency of the whispering gallery mode microcavity; By changing the structural parameters of the whispering gallery mode microcavity magnetic sensor, the characteristic frequency of the whispering gallery mode microcavity magnetic sensor is obtained to adjust the sensitivity of the whispering gallery mode microcavity magnetic sensor; The structural parameters of the whispering gallery mode microcavity magnetic sensor include the contact area between the base and the whispering gallery mode microcavity, the microcavity length of the whispering gallery mode microcavity, the thickness of the whispering gallery mode microcavity, the material of the magnetostrictive layer, and the material of the whispering gallery mode microcavity.

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