An apparatus and method for detecting motion of trapped particles in an optical trap

Abstract

The application discloses a device and method for detecting the motion of a particle captured in an optical trap. Transmission light is emitted from a transmission light source, and the transmission light is incident on an optical waveguide. The optical trap capturing light and the particle are arranged on the circumferential side of the optical waveguide, the particle is captured by the optical trap capturing light, the transmission light in the optical waveguide is coupled into the particle and then coupled back into the optical waveguide, and finally incident on a power detector. The transmission light, the particle and the waveguide form an acoustic-optic system. The waveguide is fixed, and at this time, the motion of the captured particle will change the relative distance between the two, and then change the transmittance of the transmission light in the waveguide, so as to calculate the motion information of the captured particle, and realize the detection of the motion of the particle captured in the optical trap. The application can greatly reduce the optical path complexity and volume of the particle motion detection module in the optical trap system, reduce the coupling of the motion signals between the axes of the particle, greatly reduce the light power intensity required for detecting the motion of the captured particle, improve the detection efficiency, and has practical application value.

Description

Technical Field

[0001] This invention relates to apparatus and methods for motion detection systems, and more particularly to an apparatus and method for detecting the motion of particles trapped in an optical trap. Background Technology

[0002] In 1971, American physicist Ashkin and others used a lens to weakly focus a vertically upward-shielding laser beam, achieving the first optical method to stabilize and suspend glass particles with a diameter of 20 μm. In 1986, he further discovered that by strongly focusing a single laser beam, particles could be stably captured without relying on gravity; this technique was named optical tweezers. In 2018, Ashkin was awarded the Nobel Prize in Physics for inventing optical tweezers. The principle of optical tweezers is that a strongly focused laser beam exerts a force on the medium particle that always points towards the focal point. The magnitude of this force is proportional to the electric field gradient, hence it is called the gradient force. This force traps the medium particle in three dimensions near the focal point. Optical tweezers provide an excellent non-contact, non-destructive method with high spatial and temporal resolution for controlling and measuring the properties of objects from the micrometer to submicrometer scale, and have broad applications and promising prospects in biology, high-sensitivity sensing, and quantum physics.

[0003] When Ashkin first achieved the suspension of particles in a vacuum, he predicted that "if the viscous damping (of air) is further reduced, (vacuum optical tweezers) could potentially be used to realize inertial sensors such as gyroscopes and accelerometers." In recent years, it has been discovered that placing the captured medium particles in a vacuum environment, thus isolating them from external thermodynamic noise, will result in measurement accuracy far exceeding that of current conventional methods. For example, David Moorex's group at Yale University achieved ng-level acceleration measurement sensitivity in 2017, three orders of magnitude higher than the detection sensitivity achievable by current room-temperature mechanical sensors. Vacuum optical tweezers have demonstrated significant application value and broad development prospects in areas such as precision mechanical quantity measurement, high-performance inertial sensors, exploration of non-Newtonian gravity, and preparation of macroscopic quantum states.

[0004] In sensing and detection systems based on optical traps, the particles trapped in the optical trap are often used as sensitive units, and the magnitude of the measured physical quantity is calculated by the particle motion caused by the measured physical quantity. Therefore, the detection of the motion of trapped particles in the optical trap is an important part of many optical trap systems.

[0005] Common detection schemes for the motion of captured particles in optical trap systems include four-quadrant detection and D-type mirror methods. These common detection schemes require the construction of a complex and bulky detection optical path outside the optical trap, which guides the capturing light or detection light passing through the captured particle into a corresponding optical power detector. Such systems require high optical power for detecting captured particles, typically in the milliwatt range. Furthermore, these systems are significantly affected by the coupling of motion signals between the axes of the captured particle, generally requiring specialized decoupling algorithms to eliminate the influence of motion signal coupling on detection.

[0006] Therefore, researching a device and method for detecting the motion of trapped particles in an optical trap, which has advantages such as simple optical path structure, low complexity, small size, low required optical power, high detection efficiency, and low coupling of motion signals between axes, is of great significance for improving the system complexity, detection efficiency, and sensing accuracy of the optical trap system. Summary of the Invention

[0007] In view of the current optical trap system, there is a lack of a detection method that is simple in structure, low in complexity, small in size, requires low optical power, has high detection efficiency, and has low coupling of motion signals between axes. This invention proposes a device and method for detecting the motion of trapped particles in an optical trap.

[0008] This invention places a waveguide close to a particle trapped by an optical trap and fixes it at an appropriate distance from the particle. Light of an appropriate wavelength is transmitted through the waveguide, exciting a whispering-gallery mode resonance within the trapped particle. The relative motion between the trapped particle and the waveguide couples with the excited light field, forming a whispering-gallery mode optomechanical system. In this optomechanical system, the transmittance of the light transmitted through the waveguide is related to the relative distance between the two and the frequency shift of the transmitted light relative to the whispering-gallery mode resonance frequency. By fixing the waveguide, the motion of the trapped particle changes the relative distance, thereby altering the transmittance of the transmitted light in the waveguide. Therefore, the motion information of the trapped particle can be calculated from the change in the transmittance of the transmitted light in the waveguide, enabling the detection of the motion of the trapped particle. This invention is the first to propose the detection of the motion of trapped particles in an optical trap based on a whispering-gallery mode optomechanical system. This invention can significantly reduce the optical path complexity and volume of the particle motion detection module in the optical trap system, reduce the coupling of motion signals between the axes of the particles, and greatly reduce the optical power intensity required to detect the motion of the captured particles, thereby improving detection efficiency and having practical application value.

[0009] The specific technical solution adopted in this invention is as follows:

[0010] I. A device for detecting the motion of particles trapped in an optical trap.

[0011] The device includes a transmission light source, a transmission light, an optical waveguide, and a power detector. The transmission light is emitted from the transmission light source and incident on the optical waveguide. The optical trap trapping light and the particles are disposed on the circumferential side of the optical waveguide, and the particles are trapped by the optical trap trapping light. The transmission light in the optical waveguide is coupled into the particles, and at the same time, the optical field in the particles is also coupled into the optical waveguide. Finally, the transmission light is emitted from the optical waveguide and incident on the power detector.

[0012] The transmitted optical source includes a laser source.

[0013] The optical waveguide includes tapered optical fiber or microneedle waveguide.

[0014] The power detector includes a photodetector and an avalanche diode.

[0015] The shape of the particles includes spherical, rod-shaped, and dumbbell-shaped, and the material of the particles includes fused silica and polyvinyl chloride.

[0016] The size of the particles in the three spatial dimensions is between 100 nanometers and 100 micrometers.

[0017] II. A method for detecting the motion of trapped particles in an optical trap

[0018] Step 1) Adjust the light trap to capture the particles so that they are captured by the light trap.

[0019] Step 2) Particles are placed on the circumferential side of the optical waveguide, with the particles and the optical waveguide arranged at intervals. Then, the distance g between the particles and the optical waveguide is adjusted so that the transmitted light incident on the optical waveguide is coupled into the particles and then coupled back into the optical waveguide. The optical waveguide is fixed, and then the power of the transmitted light finally emitted from the optical waveguide is detected.

[0020] Step 3) Detect the particle motion based on the power of the final emitted transmitted light.

[0021] Step 3) specifically refers to:

[0022] The transmittance of the transmitted light through the optical waveguide is determined by the ratio of the power of the final emitted transmitted light to the power of the transmitted light before incident. Then, the distance g between the particle and the optical waveguide is calculated based on the transmittance of the transmitted light through the optical waveguide, thereby realizing the detection of particle motion.

[0023] In step 3), the distance g between the particle and the optical waveguide is calculated based on the transmittance of the transmitted light through the optical waveguide. The specific calculation formula is as follows:

[0024] γ e (g)=γ e (0)exp(-ηg)

[0025]

[0026] γ0=ω0 / Q

[0027] Where, γ e (g) represents the coupling loss, γ e (0) is the normalized zero-gap coupling ratio, η is the coupling loss attenuation constant, T(g) is the transmittance of the transmitted light through the optical waveguide at a distance g, ω0 is the resonant frequency of the whispering-gallery mode, Q is the quality factor of the whispering-gallery mode formed in the optical waveguide, and γ0 is the intrinsic loss of the resonant cavity.

[0028] The beneficial effects of this invention are:

[0029] This invention proposes a device and method for detecting the motion of trapped particles in an optical trap. Based on a whispering-gallery mode optomechanical system, it achieves the detection of the motion of trapped particles. It also has advantages such as simple optical path structure, low complexity, small size, low required optical power, high detection efficiency, and low coupling of motion signals between axes.

[0030] Therefore, this invention has practical application value, can reduce the size of the optical trap system, improve detection efficiency, and reduce the coupling of signals that do not move along the axial direction. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the device of the present invention;

[0032] In the diagram: 1. Light source for transmission, 2. Transmission light, 3. Optical waveguide, 4. Power detector, 5. Light trap to capture light, 6. Particle.

[0033] Figure 2 This is a flowchart of the method of the present invention;

[0034] The dimensions of the components in the diagram do not represent the actual dimensions of the components. Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0036] like Figure 1 As shown, the device includes a transmission light source 1, a transmission light 2, an optical waveguide 3, and a power detector 4. Transmission light 2 is emitted from the transmission light source 1 and incident on the optical waveguide 3. An optical trapping light 5 and particles 6 are disposed on the circumferential side of the optical waveguide 3, with particles 6 being trapped by the optical trapping light 5. Particles 6 and the optical waveguide 3 are arranged at intervals. The transmission light 2 in the optical waveguide 3 couples into the particles 6, and simultaneously, the optical field in the particles 6 couples into the optical waveguide 3. The coupling strength is related to the distance between the particles 6 and the optical waveguide 3. Finally, the transmission light 2 exits from the optical waveguide 3 and is incident on the power detector 4. The detector detects the received optical power of the transmission light 2 and calculates the linear distance between the particles 6 and the optical waveguide 3 based on the received optical power.

[0037] The light source 1 for transmitting light includes a laser light source, and the light source 2 for transmitting light includes a laser.

[0038] Optical waveguide 3 includes tapered optical fiber or microneedle waveguide.

[0039] Power detector 4 includes, but is not limited to, photodetectors and avalanche diodes.

[0040] The light trap 5 captures light, including but not limited to laser light. The optical axis of the light trap 5 is directed, including but not limited to, the horizontal direction. The shape of the particles 6 includes, but is not limited to, spherical, rod-shaped, and dumbbell-shaped, and the material of the particles 6 includes, but is not limited to, fused silica and polyvinyl chloride.

[0041] Particle 6, with a size between 100 nanometers and 100 micrometers in all three spatial dimensions, is captured by light trap 5.

[0042] A method for detecting the motion of trapped particles in an optical trap includes the following steps:

[0043] Step 1) Adjust the light trap 5 to capture the particles 6 so that they are captured by the light trap 5.

[0044] Step 2) Particles 6 and the optical trap light 5 are positioned on the circumferential side of the optical waveguide 3, with a gap between them. The distance g between the particles 6 and the optical waveguide 3 is then adjusted so that the transmitted light 2 incident on the optical waveguide 3 is coupled into the particles 6 and then back into the optical waveguide 3. The optical waveguide 3 is then fixed, and the power of the transmitted light 2 ultimately emitted from the optical waveguide 3 is detected using a power detector 4. After the transmitted light 2 in the optical waveguide 3 is coupled into the particles 6, a whispering-gallery mode resonance is formed in the optical waveguide 3. The particles 6 act as the resonant cavity for this whispering-gallery mode. The particles 6, the transmitted light 2, and the optical waveguide 3 together form an opto-mechanical coupling system. The movement of the particles 6 causes a change in the distance between them and the optical waveguide 3, thereby altering the coupling rate of the transmitted light 2 into the optical waveguide 3, and consequently changing the transmittance of the transmitted light 2 through the optical waveguide 3. Therefore, the optical power detected by the power detector 4 will change with the movement of the particles 6.

[0045] Step 3) Detect the motion of particle 6 based on the power of the final emitted transmitted light 2. During analysis, appropriate signal processing operations will be performed on the power received by the power detector 4 to reduce interference from noise and other factors.

[0046] Step 3) specifically refers to:

[0047] The transmittance of the transmitted light 2 through the optical waveguide 3 is determined by the ratio of the power of the transmitted light 2 finally emitted from the optical waveguide 3 to the power of the transmitted light 2 before incident. The power ratio is equal to the transmittance. Then, the distance g between the particle 6 and the optical waveguide 3 is calculated based on the transmittance of the transmitted light 2 through the optical waveguide 3, thereby realizing the detection of the motion of the particle 6.

[0048] In step 3), the distance g between particle 6 and optical waveguide 3 is calculated based on the transmittance of transmitted light 2 through optical waveguide 3. The specific calculation formula is as follows:

[0049] γ e (g)=γ e (0)exp(-ηg)

[0050]

[0051] γ0=ω0 / Q

[0052] Where, γ e (g) represents the coupling loss, γ e (0) is the normalized zero-gap coupling ratio, η is the coupling loss attenuation constant, T(g) is the transmittance of the transmitted light 2 through the optical waveguide 3 at a distance g, ω0 is the resonant frequency of the whispering mode, Q is the quality factor of the whispering mode formed in the optical waveguide 3, and γ0 is the intrinsic loss of the resonant cavity.

[0053] Therefore, the motion of particle 6 perpendicular to the optical waveguide 3 has the greatest impact on the signal. This method can thus reduce the mutual coupling of signals moving in different directions.

[0054] The method only requires the optical waveguide 3 near the particle 6, the corresponding transmission light 2, and the power detector 4 to detect the motion of the particle 6. This avoids the drawbacks of complex optical paths and large size in many optical trap systems, reducing system complexity and size. The power of the transmission light 2 for detecting the motion of the particle 6 is much lower than the optical power required by common detection schemes in optical trap systems, significantly improving detection efficiency.

[0055] In practice, the captured particles are spherical in shape, made of fused silica, and have a diameter of 20 micrometers. The power P0 of the transmitted light 2 emitted from the transmitted light source 1 satisfies P0 = 1 μW. Since the particle movement range is small within the optical trap, theoretical calculations show that the standard deviation of the captured particle displacement is approximately 10 nanometers. Therefore, the change in transmittance relative to displacement can be considered linear. The sensitivity of the transmittance relative to displacement can be represented by dT / dg. In this example, we assume ω0 = 1.885 × 10⁻⁶. 15 rad·s -1 Q = 1 × 10 7 Through simulation calculations, γ e (0) = 2.5 × 109 rad·s -1 η = 2 × 10 7 m -1 At this point, when g = 1.15 μm, dT / dg reaches its maximum value of 7.7 × 10⁻⁶. 6 m -1 At this point, the corresponding transmittance is 0.33, and the corresponding light output power is 0.33 μW.

[0056] Assume the detection sensitivity of the optical power detector is With a sampling rate of 10kHz, the displacement detection accuracy of this method in this example is 1.3pm. Therefore, it can be seen that this invention can detect the motion of particles trapped in an optical trap.

[0057] In summary, this invention proposes a device and method for detecting the motion of trapped particles in an optical trap. Based on a whispering-gallery mode optomechanical system, it achieves the detection of the motion of trapped particles. It also boasts advantages such as simple optical path structure, low complexity, small size, low required optical power, high detection efficiency, and low coupling of motion signals between axes.

[0058] The above specific embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for detecting the motion of particles trapped in an optical trap, characterized in that, The method employs the following apparatus, which includes a transmission light source (1), a transmission light (2), an optical waveguide (3), and a power detector (4); the transmission light (2) is emitted from the transmission light source (1) and incident on the optical waveguide (3); an optical trapping light (5) and a particle (6) are disposed on the circumferential side of the optical waveguide (3), and the particle (6) is trapped by the optical trapping light (5); the transmission light (2) in the optical waveguide (3) is coupled into the particle (6), and at the same time, the light field in the particle (6) is also coupled into the optical waveguide (3); finally, the transmission light (2) is emitted from the optical waveguide (3) and incident on the power detector (4); the method includes the following steps: Step 1) Adjust the light trap (5) to capture the particles (6) with the light trap (5); Step 2) Particles (6) are placed on the circumferential side of the optical waveguide (3), with the particles (6) and the optical waveguide (3) spaced apart. Then the distance between the particles (6) and the optical waveguide (3) is adjusted. This causes the transmitted light (2) incident on the optical waveguide (3) to couple into the microparticle (6) and then couple back into the optical waveguide (3). The optical waveguide (3) is fixed, and the power of the transmitted light (2) that is finally emitted from the optical waveguide (3) is then detected. Step 3) Detect the motion of the particles (6) based on the power of the final emitted transmitted light (2); Step 3) specifically refers to: The transmittance of the transmitted light (2) through the optical waveguide (3) is determined by the ratio of the power of the final emitted transmitted light (2) to the power of the transmitted light (2) before incident. Then, the distance between the particle (6) and the optical waveguide (3) is calculated based on the transmittance of the transmitted light (2) through the optical waveguide (3). Thus, the motion of particles (6) can be detected.

2. The method for detecting the motion of trapped particles in an optical trap according to claim 1, characterized in that, In step 3), the distance between the particle (6) and the optical waveguide (3) is calculated based on the transmittance of the transmitted light (2) through the optical waveguide (3). The specific calculation formula is as follows: in, For coupling loss, The normalized zero-gap coupling rate, Let be the coupling loss attenuation constant. Distance The transmittance of the downward transmitted light (2) through the optical waveguide (3), Let Q be the resonant frequency of the whispering-gallery mode, and let Q be the quality factor of the whispering-gallery mode formed in the optical waveguide (3). This refers to the intrinsic loss of the resonant cavity.

3. The method for detecting the motion of particles trapped in an optical trap according to claim 1, characterized in that, The transmission optical source (1) includes a laser source.

4. The method for detecting the motion of trapped particles in an optical trap according to claim 1, characterized in that, The optical waveguide (3) includes a tapered optical fiber or a microneedle waveguide.

5. A method for detecting the motion of trapped particles in an optical trap according to claim 1, characterized in that, The power detector (4) includes a photodetector and an avalanche diode.

6. The method for detecting the motion of trapped particles in an optical trap according to claim 1, characterized in that, The shape of the particles (6) includes spherical, rod-shaped, and dumbbell-shaped, and the material of the particles (6) includes fused silica and polyvinyl chloride.

7. The method for detecting the motion of trapped particles in an optical trap according to claim 1, characterized in that, The particle (6) has a size between 100 nanometers and 100 micrometers in the three spatial dimensions.

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