Sensor based on high-order singular points and preparation method thereof

Abstract

The application relates to the technical field of photoelectric materials and devices, in particular to a sensor based on high-order singular points and a preparation method thereof. The sensor comprises optical microcavities, a dielectric waveguide and a photodetector. The number of the optical microcavities is several and the structures are the same. The distance from each optical microcavity to the dielectric waveguide is equal, and the distance between adjacent optical microcavities is equal. After the optical microcavities are excited by circularly polarized light emitted by a laser, the same chiral mode is generated, the one-way coupling between the optical microcavities occurs through the dielectric waveguide, the high-order singular point for feedback on the disturbance is generated in the system, and the order of the singular point is determined only by the number of the same optical microcavities. The photodetector is arranged at the output end of the dielectric waveguide and used for receiving the spectrum. The application effectively feeds back the slight disturbance based on the high-order singular point, and increases the sensitivity of the sensor.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optoelectronic materials and devices, and particularly relates to a sensor based on high-order singular points and a preparation method thereof. BACKGROUND

[0002] A sensor is a kind of detection device capable of sensing measured information and converting the collected information into an electrical signal or other form of information output according to a certain rule. Sensors play an important role in many aspects of people's daily life, such as infrared sensors in home security systems, particle sensors in environmental monitoring, and motion sensors in mobile phones.

[0003] In the field of optics, high-Q optical microcavities are the main carriers of sensing applications because they can enhance the interaction between light and matter in a very limited volume, which is called resonance enhancement effect. A very small perturbation near the optical microcavity will cause a change in the line width or a shift or splitting of the resonance, which is proportional to the strength of the perturbation. The introduction of the resonance effect can greatly reduce the size of the device while improving the sensitivity. Examples of such sensors mainly include mechanical transducers, magnetometers, single-particle absorption spectrometers, and microcavity sensors for measuring the size of single particles and detecting nanoscale objects.

[0004] In recent years, people are constantly exploring new sensor implementation schemes and methods for improving sensor sensitivity. One particular scheme is to use a highly sensitive optical structure generated by a high-order singular point in the system to construct a sensor. Related research has shown that high-order singular points can effectively feedback to small perturbations (Nature, 548(7666): 192-196(2017), Physical Review Letters, 112(20): 203901(2014)). If the Hamiltonian of a system is non-Hermitian, the eigenvalues of the system are generally complex numbers. By effectively modulating the non-Hermiticity of the system, two eigenvalues can be merged at a location, which is called a singular point. Singular points usually occur in non-Hermitian systems with parity-time symmetry, and the symmetry of the system can be broken by simultaneously increasing the gain and loss, which is manifested as the eigenvalue spectrum of the system changing from purely real numbers to complex numbers. Moreover, singular points can also exist in passive systems without parity-time symmetry, which is the result of the imbalance between the loss difference of the two states and the coupling strength. High-order singular points are the result of the merging of multiple system eigenvalues, so for a system containing many physical parameters, it is necessary to carefully optimize the system parameters in a large parameter space to achieve high-order singular points. Moreover, in actual systems, the effects of parameters can be interrelated, making parameter adjustment more complex, so high-order singular points in the system are usually difficult to obtain, which limits the application of high-order singular points in sensing. SUMMARY

[0005] The purpose of the present application is to provide a high-order singular point-based sensor and a preparation method thereof, which increases the sensitivity of the sensor based on the effective feedback of high-order singular points to small perturbations.

[0006] To solve the above technical problems, the technical scheme of the present application is as follows:

[0007] In a first aspect, the present application provides a high-order singular point-based sensor, which comprises optical microcavities, a dielectric waveguide, and a photodetector. The number of optical microcavities is several and their structures are the same. The distance from each optical microcavity to the dielectric waveguide is equal, and the distance between adjacent optical microcavities is equal. The optical microcavities produce the same chiral mode after being excited by circularly polarized light emitted by a laser, and the optical microcavities are unidirectionally coupled through the dielectric waveguide to generate a high-order singular point in the system for feedback to perturbations. The order of the singular point is determined only by the number of identical optical microcavities. The photodetector is arranged at the output end of the dielectric waveguide to receive the optical spectrum.

[0008] Further, the shape of the optical microcavity should be a ring cavity, a cylindrical cavity, a spherical cavity, a disc cavity and a series of optical resonant cavities that can exist whispering gallery modes. In such optical microcavities, modes appear in pairs, divided into forward transmission of the traveling wave mode, namely the CW mode, and the reverse transmission of the traveling wave mode, namely the CCW mode, both of which are the chiral modes required in the present application.

[0009] Further, the materials of the optical microcavity and the dielectric waveguide include silicon, silicon dioxide or III-V group materials.

[0010] Further, the spacing of the optical microcavity to the edge of the dielectric waveguide is in the range of 1 nanometer to 1 micrometer, and the distance between the optical microcavities is in the range of 1 nanometer to 1 millimeter. Smaller coupling distance can be used to increase the coupling strength.

[0011] Further, the sensor can also add a quarter wave plate between the laser and the optical microcavity; the quarter wave plate between the laser and the optical microcavity is to ensure that the light reaching the optical microcavity is as circularly polarized light as possible, rather than elliptically polarized light, so as to excite 100% of the chiral modes in the optical microcavity, thereby enhancing the strength of the one-way coupling between the microcavities and improving the sensitivity of the sensor.

[0012] Further, the chiral modes required in the sensor can be obtained by the sagnac effect generated by the rotation of the microcavity to achieve the frequency splitting between the CW mode and the CCW mode, in addition to the selective excitation of the circularly polarized light of the laser.

[0013] Further, the working frequency range of the photodetector matches the frequency of the chiral mode used, and the working bandwidth of the photodetector is greater than the frequency spread of the chiral mode.

[0014] Further, the on-chip photodetector can be replaced by other photoelectric conversion devices, including other on-chip devices and external devices, which can facilitate different sensing sensitivities in different use environments.

[0015] Further, the opposite end of the output end of the dielectric waveguide is provided with a light-absorbing surface; the light-absorbing surface is formed by coating a light-absorbing material or ion-doped high-loss waveguide, and the purpose is to prevent the influence of reflected light in the waveguide. The main source of reflected light is the incomplete one-way coupling between the optical microcavities.

[0016] Further, the number of optical microcavities used in the sensor can be reasonably selected according to the required detection sensitivity and other factors in actual situations.

[0017] In a second aspect, a preparation method of a sensor based on high-order singular points is provided, which is used to prepare the sensor based on high-order singular points described above, and the method comprises:

[0018] Step S1: first, several identical optical microcavities are placed near the dielectric waveguide, the distance from each optical microcavity to the dielectric waveguide is equal, and the distance between adjacent optical microcavities is equal;

[0019] Step S2: the same chiral mode in the optical microcavity is excited by outputting circularly polarized light from the laser above the optical microcavity, one-way coupling between the chiral modes is generated through the dielectric waveguide, a high-order singular point for feedback to the disturbance is generated in the system, and the order of the singular point is only determined by the number of identical optical microcavities; the optical microcavity and the dielectric waveguide form a one-way coupling system;

[0020] Step S3: an on-chip photodetector is placed at the output end of the dielectric waveguide to receive the optical spectrum; when the substance is close to the optical microcavity or the waveguide, a disturbance is generated in the system, the high-order singular point feeds back to the external disturbance, the frequency spectrum near the high-order singular point changes, and the on-chip integrated photodetector receives the change, so that the detection is realized.

[0021] The present application has the following beneficial effects:

[0022] First, the basic structure of the present application includes several identical optical microcavities, waveguide coupling structures and an on-chip integrated photodetector; the chiral mode in the optical microcavity is excited by circularly polarized laser, and one-way coupling between the chiral modes in the same optical microcavity is generated through the dielectric waveguide placed at a short distance, which is ensured by the spin-orbit interaction of light and the locking between the orbital angular momentum of the mode; the sensing mechanism of such a sensor is derived from the effective feedback of high-order singular points to external disturbances, one-way coupling between the same microcavities can generate singular points of any order in the system, and the order of the singular point is only determined by the number of identical optical microcavities; the present application is based on the effective feedback of high-order singular points in the system to small disturbances, and in principle, it can be extended to singular points of any order by increasing the number of identical optical microcavities accordingly, so that the sensitivity of the sensor can be greatly increased, and thus it can be used to detect smaller substances in various complex environments;

[0023] Second, the sensor construction method proposed by the present application does not require complex design, special materials or experimental conditions, the optimization direction is clear, the adjustment is very easy, and it can be directly used for on-chip integration, and it is suitable for integrated waveguide systems and optical fiber systems, thereby providing a new design idea for the device and miniaturization of the sensor. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a schematic diagram of the overall structure of the sensor based on high-order singular points of the present application;

[0025] Figure 2A schematic diagram of the working principle of the sensor based on high-order singular points of the application;

[0026] Figure 3 The amplitude distribution curve of each optical microcavity when the number of optical microcavities is 5 and there is no disturbance in the embodiment of the application;

[0027] Figure 4 The amplitude distribution curve of each optical microcavity when the number of optical microcavities is 5 and there is no disturbance in the embodiment of the application; The normalized spectral amplitude distribution curve received by the photodetector.

[0028] The drawing label: 1, dielectric waveguide; 2, substrate material; 3, optical microcavity; 4, laser; 5, light absorption surface; 6, photodetector; 7, external disturbance material. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the application more clear, the application will be further described in detail below with reference to the drawings and specific embodiments.

[0030] Please refer to Figure 1 The application is a sensor based on high-order singular points, which comprises: optical microcavities 3, dielectric waveguides 1 and photodetectors 6; the number of optical microcavities 3 is several and the structures are the same; the distance from each optical microcavity 3 to the dielectric waveguide 1 is equal, and the distance between adjacent optical microcavities 3 is equal; the photodetector 6 is arranged at the output end of the dielectric waveguide 1; the laser 4 emits circularly polarized light to excite the same chiral mode in the optical microcavities 3, and the one-way coupling between the optical microcavities 3 occurs through the dielectric waveguide 1, thereby generating high-order singular points in the system for feedback to the disturbance, and the order of the singular points is only determined by the number of the same optical microcavities 3; an on-chip photodetector 6 is integrated at the output end of the waveguide to receive the energy spectrum; as a whole, it constitutes a high-sensitivity, miniaturized and integrable sensor based on high-order singular points.

[0031] The application also provides a preparation method for preparing the above-mentioned sensor based on high-order singular points, and the steps comprise:

[0032] Step S1: first, several completely same optical microcavities 3 are placed near the dielectric waveguide 1, and the distance from each microcavity to the waveguide is equal, and the distance between adjacent microcavities is equal;

[0033] Step S2: circularly polarized light is output on the optical microcavities 3 through the laser 4 to excite the same chiral mode in the optical microcavities 3, and one-way coupling occurs between the chiral modes through the dielectric waveguide 1, thereby generating high-order singular points in the system for feedback to the disturbance, and the order of the singular points is only determined by the number of the same optical microcavities 3; the optical microcavities 3 and the dielectric waveguide 1 constitute a one-way coupling system;

[0034] Step S3: An on-chip photodetector 6 is placed at the output end of the dielectric waveguide 1 to receive the optical spectrum; when the external perturbation material 7 approaches the optical microcavity 3 or the dielectric waveguide 1, a perturbation is generated in the system, the high-order singular point feeds back the external perturbation, and the frequency spectrum near the high-order singular point changes, which is received by the on-chip integrated photodetector 6 to realize detection.

[0035] Specifically, the sensor of the present application is arranged on the substrate material 2, and the shape of the optical microcavity 3 is a series of optical resonant cavities capable of existing whispering gallery modes, such as ring cavities, cylindrical cavities, spherical cavities and disc cavities. In such an optical microcavity 3, modes appear in pairs, which are divided into forward transmission traveling wave modes, i.e. CW modes, and reverse transmission traveling wave modes, i.e. CCW modes, which are the chiral modes required in the present application.

[0036] In this embodiment, referring to Figure 1 , a plurality of identical optical microcavities 3 are placed near the dielectric waveguide 1, and the distance from each microcavity to the waveguide is equal, and the distance between adjacent microcavities is equal. In this embodiment, the number of optical microcavities 3 is selected to be 5.

[0037] The materials of the optical microcavity 3 and the dielectric waveguide 1 can be, but are not limited to, silicon, silicon dioxide, III-V group materials, etc. Considering cost and feasibility, the optical microcavity 3 is made of silicon material, and the optical microcavity 3 is a disc cavity with a diameter of 1 um and a height of 220 nm. In this embodiment, on the same chip, in order to facilitate the experiment, the materials of the dielectric waveguide 1 and the optical microcavity 3 are the same, and the material of the dielectric waveguide 1 is also silicon, the width of the waveguide is 500 nm, and the height is 220 nm. The spacing of the optical microcavity to the edge of the dielectric waveguide ranges from 1 nanometer to 1 micrometer, and the distance between the optical microcavities ranges from 1 nanometer to 1 millimeter. In this embodiment, the spacing of the optical microcavities 3 to the edge of the dielectric waveguide 1 is 200 nm, and the distance between the optical microcavities 3 is 5 um.

[0038] A circularly polarized light is outputted by the laser 4 from above to excite the same chiral modes in the optical microcavities 3, and unidirectional coupling is generated between the chiral modes through the dielectric waveguide 1. In this embodiment, the center wavelength of the selected laser 4 is selected to be 1550 nm, and the power is 20 mW.

[0039] An optical detector 6 is placed at the output end of the dielectric waveguide 1 to receive the energy spectrum in the frequency range, and an optical absorption surface 5 is coated on the other end of the dielectric waveguide 1 or a high-loss waveguide is formed by ion doping to prevent the influence of reflected light. In this embodiment, the optical detector 6 selects an on-chip integrated pn junction photodiode design, and the working wavelength is near 1550 nm.

[0040] The embodiment sensor, when in practical application, when different diameter external disturbance material 7 such as material particles, dust or various microorganisms approaches optical microcavity 3 or dielectric waveguide 1, the frequency spectrum near the high-order singular point will obviously change when the system is disturbed, and the normalized amplitude spectrum curve received by the on-chip integrated photoelectric detector 6 is as shown in Figure 4 .

[0041] In the embodiment, the laser 4 wavelength and power, the material and diameter of the optical microcavity 3, the waveguide material, the wavelength of the chiral mode and other design parameters can be adjusted accordingly.

[0042] In the application, the formation principle of the high-order singular point in the system is:

[0043] Consider a typical system of two optical microcavities coupled, and represent the coupling coefficients from 1 column to 2 column and from 2 column to 1 column respectively, and represent the material loss and radiation loss of the two microcavities, when the coupling coefficients (* represents transpose), even without loss, the Hamiltonian of the system is non-Hermitian, at this time the non-Hermitian property of the system comes from the asymmetric coupling between modes. In the limit case, the loss difference of two identical microcavities, a singular point will be generated in the system, which means that in the system composed of two microcavities with the same loss, the 2 column to 1 column coupling is completely suppressed, and only 1 column to 2 column unidirectional coupling occurs. In this case, the Hamiltonian H of the system can be simplified as the Jordan block form: which has a 2-fold degenerate single eigenvalue about , where the center frequency of the chiral mode is , . Then, the amplitude distribution formula in the two microcavities can be obtained as: , , is the frequency of the mode, is the amplitude of the input light, and i is the imaginary unit. From the amplitude distribution formula, it can be seen that different order poles are generated in the two microcavities, and the second-order pole in p2 is the symbol of the appearance of the second-order singular point. Similarly, for a system of N identical optical microcavities with unidirectional coupling between chiral modes, all coupling coefficients will disappear, at this time the Hamiltonian of the system can be written as: The above Hamiltonian matrix represents that the system generates an N-order singular point, which has a 2-fold degenerate single eigenvalue about N degenerate single eigenvalue. This means that in the system of unidirectional coupling between chiral modes, higher order singular points can be easily realized without complex design and multi-parameter optimization adjustment. The amplitude distribution curves in each microcavity in the system of unidirectional coupling between five optical microcavities without perturbation are shown in Fig. 1. Figure 3 As can be seen from the extreme value positions of each curve, singular points of each order are generated near the center frequency.

[0044] The working principle of the sensor based on high-order singular points is that in the system with singular points, perturbation in the system near the singular point frequency can produce effective feedback, which is usually manifested as frequency splitting and mode Q value change, and the higher the order of the singular point, the stronger the feedback ability to the perturbation. In the case of a small perturbation, the splitting of the eigenfrequency near the N-order singular point has the order of , wherein represents the strength of the small perturbation. In the system of unidirectional coupling between microcavities to generate singular points, the order of the singular point only depends on the number of the same microcavities, so the sensitivity of such a sensor can be easily controlled. When different diameter substances such as material particles, dust or various microorganisms approach the optical microcavities or waveguides, the frequency spectrum near the high-order singular point will change obviously when the perturbation in the system, which is the basic working principle of such a sensor. In the system of unidirectional coupling between five optical microcavities, when different perturbation strengths are generated near the microcavities or waveguides, the normalized amplitude distribution curves received by the on-chip photodetector are shown in Fig. 2. Figure 4 As can be seen from the figure, different strengths of the perturbation can cause the amplitude spectrum to split the frequency to different degrees, and the stronger the perturbation, the more obvious the frequency splitting.

[0045] In the present application, the unidirectional coupling strength between the optical microcavities is also an important factor affecting the sensitivity of the sensor, and the unidirectional coupling strength between the microcavities can be improved from the following aspects: first, a quarter wave plate can be added between the light source and the microcavity. This ensures that the light reaching the optical microcavity is as circularly polarized light as possible, rather than elliptically polarized light, so as to excite 100% chiral modes in the optical microcavity; second, higher refractive index materials can be used to make optical microcavities; third, the optical microcavities are as close as possible; fourth, the optical microcavities are as close as possible to the dielectric waveguide; high refractive index materials and smaller coupling distance are effective methods to increase the coupling strength; fifth, lower refractive index materials can be used to make dielectric waveguides; lower refractive index materials have weaker binding to light, which can increase the evanescent wave coupling strength between the microcavity chiral modes and the waveguide modes.

[0046] In the present application, the high-order singular points are generated in the system by unidirectional coupling of the waveguide through the chiral mode in the optical microcavity, which is ensured by the locking between the spin-orbit interaction of light and the orbital angular momentum of the mode; the order of the singular point is determined only by the number of optical microcavities above the waveguide. By using the characteristic of the high-order singular point sensitive to the feedback of the disturbance, an on-chip photodetector is integrated at the output end to receive the optical spectrum, and a sensor is further constructed. The performance of the sensor based on the high-order singular point can be effectively improved by increasing the number of the same optical microcavities and enhancing the strength of the unidirectional coupling, because the higher-order singular point is more sensitive to the feedback of the disturbance.

[0047] In other embodiments, the present application is also applicable to the coupling structure of the optical fiber and the optical microcavity, so the present application can be extended to the optical fiber system.

[0048] The parts not involved in the present application are the same as or realized by using the prior art.

[0049] The above is a further detailed description of the present application in combination with specific embodiments, and the specific implementation of the present application cannot be limited to these descriptions. For ordinary skilled persons in the technical field to which the present application belongs, some simple deductions or substitutions can be made without departing from the concept of the present application, and all of them should be regarded as falling within the protection scope of the present application.

Claims

1. A sensor based on higher-order singularities, characterized in that: The sensor is mounted on a substrate material (2) and includes an optical microcavity, a dielectric waveguide, and a photodetector; The system comprises several identical optical microcavities; each microcavity is equidistant from the dielectric waveguide, and adjacent microcavities are equidistant from each other. When excited by circularly polarized light emitted from a laser, each microcavity generates the same chiral mode. Unidirectional coupling occurs between the microcavities via the dielectric waveguide, creating a high-order singularity within the system for feedback of disturbances. Under small disturbances, the splitting of the eigenfrequency near the Nth-order singularity exhibits… On the order of magnitude, of which It represents the intensity of a small perturbation; the order of a singularity is determined only by the number of identical optical microcavities. A photodetector is placed at the output end of the dielectric waveguide to receive the spectrum; the optical microcavity is made of a material with a higher refractive index, the dielectric waveguide is made of a material with a lower refractive index, and a quarter-wave plate is placed between the laser and the optical microcavity.

2. The sensor based on higher-order singularities according to claim 1, characterized in that: Optical microcavities are whispering-gallery mode optical resonators. In these optical microcavities, chiral modes appear in pairs, namely the forward-propagating traveling wave mode (CW mode) and the reverse-propagating traveling wave mode (CCW mode).

3. The sensor based on higher-order singularities according to claim 2, characterized in that: The optical microcavity can be a ring cavity, cylindrical cavity, spherical cavity, or disk cavity.

4. The sensor based on higher-order singularities according to claim 1, characterized in that: Materials used in optical microcavities and dielectric waveguides include silicon, silicon dioxide, or group III-V materials.

5. The sensor based on higher-order singularities according to claim 1, characterized in that: The spacing between the optical microcavity and the edge of the dielectric waveguide ranges from 1 nanometer to 1 micrometer, and the distance between optical microcavities ranges from 1 nanometer to 1 millimeter.

6. The sensor based on higher-order singularities according to claim 1, characterized in that: The operating frequency range of the photodetector is matched with the frequency of the chiral mode used, and the operating bandwidth of the photodetector is greater than the frequency broadening of the chiral mode.

7. The sensor based on higher-order singularities according to claim 1, characterized in that: The photodetector is replaced by other photoelectric conversion devices, including other on-chip devices and external equipment.

8. The sensor based on higher-order singularities according to claim 1, characterized in that: The opposite end of the output terminal of the dielectric waveguide is provided with a light-absorbing surface; the light-absorbing surface is formed by coating with a light-absorbing material or by forming a high-loss waveguide by ion doping.

9. A method for fabricating a sensor based on higher-order singularities, used to fabricate the sensor based on higher-order singularities as described in any one of claims 1-8, characterized in that, The methods include: Step S1: First, place several identical optical microcavities near the dielectric waveguide. Each optical microcavity is equidistant from the dielectric waveguide, and the distance between adjacent optical microcavities is equidistant. Step S2: Circularly polarized light is output from a laser above the optical microcavity to excite the same chiral modes in the optical microcavity. The chiral modes are unidirectionally coupled through a dielectric waveguide, generating higher-order singularities within the system for feedback of disturbances. The order of the singularities is determined only by the number of identical optical microcavities. The optical microcavities and dielectric waveguides constitute a unidirectionally coupled system. Step S3: Place an on-chip photodetector at the output end of the dielectric waveguide to receive the spectrum; when the material approaches the optical microcavity or dielectric waveguide, a disturbance is generated in the system. The higher-order singularity feeds back to the external disturbance, and the frequency spectrum near the higher-order singularity changes, which is received by the on-chip integrated photodetector to realize the detection function.

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