rus
Issue #3/2020
Y. V. Pichugina, A. S. Machikhin
Development of Acousto-Optic Device for Manipulating Micro-Objects
Development of Acousto-Optic Device for Manipulating Micro-Objects
DOI: 10.22184/1993-7296.FRos.2020.14.3.254.259
We discuss acousto-optic scanning characterized by high precision and repetition rate for manipulating micro-objects using optical tweezers. Bragg diffraction of light via ultrasonic waves allows creating robust solid-state devices for precise and fast laser beam deflection. We describe a scheme of the optical tweezers with PC‑driven two-dimensional scanning implemented by two sequential acousto-optical cells.
We discuss acousto-optic scanning characterized by high precision and repetition rate for manipulating micro-objects using optical tweezers. Bragg diffraction of light via ultrasonic waves allows creating robust solid-state devices for precise and fast laser beam deflection. We describe a scheme of the optical tweezers with PC‑driven two-dimensional scanning implemented by two sequential acousto-optical cells.
Теги: acousto-optic deflector bragg diffraction optical trap optical tweezers paratellurite. акустооптический дефлектор брэгговская дифракция оптическая ловушка оптический пинцет парателлурит
Development of Acousto-Optic Device for Manipulating Micro-Objects
Y. V. Pichugina 1, 2, A. S. Machikhin 1
Scientific and Technological Center of Unique Instrumentation, Russian Academy of Sciences, Butlerova street, 15, Moscow, Russia, 117342
Prokhorov General Physics Institute of the Russian Academy of Sciences, Vavilov street, 38, Moscow, Russia, 119991
e-mail: pichuginaa@yandex.ru
We discuss acousto-optic scanning characterized by high precision and repetition rate for manipulating micro-objects using optical tweezers. Bragg diffraction of light via ultrasonic waves allows creating robust solid-state devices for precise and fast laser beam deflection. We describe a scheme of the optical tweezers with PC‑driven two-dimensional scanning implemented by two sequential acousto-optical cells.
Keywords: acousto-optic deflector, optical tweezers, optical trap, Bragg diffraction, paratellurite.
Received on: 22.02.2020
Accepted on: 18.03.2020
Introduction
Currently, the development, study and use of optical tweezers are of great scientific and practical interest. Optical tweezers is an optical tool that allows you to manipulate microscopic objects using laser light. This method is based on the possibility of tight focusing of laser radiation, in which a spatially inhomogeneous optical field near the waist of a highly focused laser beam forms an effective spatial potential well.
A key element of optical tweezers is a scanning system designed to control the beam and its parameters [1–2]. Most existing optical tweezers use mirror and mirror-lens systems to control the position of the light trap. Such systems do not allow for quick movement of the trap from one arbitrary point of view to another due to the high inertia of the systems for moving mirrors and lenses. Moreover, there are high demands on the adjustment of circuits based on such systems, which leads to the need to use expensive and complex drive mechanisms and makes impossible to create multiple light traps at the same time. Control systems, which are based on liquid crystal (non-mechanical) optical radiation modulators, are devoid of some of the drawbacks inherent in mirror and mirror-lens systems, but they have low speed. In comparison with the well-known manipulation systems, the acousto-optical deflector is characterized by an order of magnitude higher speed, the ability to independently control several optical traps, instantaneous, high-precision, addressed (jump-like) movement of the trap within the field of view.
In this research, we consider a two-coordinate acousto-optic (AO) scanning system for non-mechanical manipulation of micro-objects using optical tweezers. In the AO system, the position of the trap is determined only by the frequency of the acoustic waves excited in the crystals. The tuning speed is limited mainly by the propagation time of the acoustic wave in the crystal, which is usually several microseconds [3].
The results show the advantages and prospects of AO non-mechanical manipulation of micro-objects using optical tweezers.
Circuit SCHEmE
A scheme of optical tweezers for manipulating micro-objects using two-dimensional AO scanning is shown in Fig. 1. The diameter of the laser beam is increased using a beam expander, and then directed to a two-coordinate acousto-optical deflector (AOD), which is two identical orthogonal AO cells. The first AO cell deflects the laser beam in the meridional plane, the second in the sagittal plane, so that the diameter of the beam does not change. A lens system is necessary for pairing AO cells and a micro lens, which focuses laser radiation on the sample under study located in the cuvette. Moving the last lens, you can move the hauling of laser radiation along the axis, i. e. focus the beam. A digital camera with a microscopic imaging system is located on the opposite side of the sample, and allows real-time monitoring and control of the position of the light trap.
Acousto-optical deflector
AO cell is a TeO2 crystal, to one of the edges of which a piezoelectric transducer is attached. When voltage is applied to the piezoelectric transducer, an acoustic wave propagates in the crystal, which creates a dynamic diffraction grating for the laser beam passing through the crystal [4]. By changing and modulating the voltage at the AOD, the first diffraction maximum of the laser beam is deflected by controlled angles. The excitation of sound waves occurs when the signals from the electronic driver to the electrodes. The driver consists of a generator and a broadband amplifier. To implement the traveling sound wave regime, an acoustic absorber is attached to the opposite face of the crystal. Fast modulation leads to the fact that the optical trap switches between different positions, i. e., it creates several traps. The schematic design and composition of a single-axis acousto-optical scanning system in the Bragg diffraction mode is shown in Fig. 2.
For two-dimensional spatial scanning, the deflector consists of two sequential AO cells rotated by 90°. Fig. 3 shows a scheme of a two-coordinate deflector with a two-channel electronic driver.
For this study, we developed a two-coordinate scanning AO system consisting of two identical cells from TeO2 crystals. Each of them operates in an anisotropic Bragg diffraction mode. The developed deflector has a typical configuration: the angle of incidence of light in the crystal is θ0 = 5.56° and the length of the acousto-optical interaction is L = 2 mm. A slow shear acoustic wave propagates in the crystal in the (001) plane at an angle α = 7.5° to the [110] direction (Fig. 4a). The sound wave vector is directed at an angle γ = 91.5° to the [110] axis and tangentially to the surface of the refractive indices of diffraction light. A vector diagram of this type of AO diffraction is shown in Fig. 4b.
The maximum deviation angles Δϕx × Δϕy can be calculated as:
(1)
(2)
where Δx × Δy are the dimensions of the sample, fMO is the focal length of the micro-lens, ΓRS is the increase in the relay system.
The angular resolution of the AOD is limited by diffraction and cannot exceed 1.22 (λ / D). The number of Nx × Ny positions allowed by the deflector is determined by the ratio of the angular scanning range Δϕx × Δϕy and the angular resolution:
(3)
(4)
where D0 is the diameter of the laser beam, λ is the laser wavelength.
The diameter of the entrance pupil AO of the cell D should be larger than the diameter of the laser beam D0:
(5)
where ΓBE is the increase in the beam expander, d is the initial diameter of the laser beam.
The frequency range of the ultrasound applied to the AO cells is found as:
(6)
Using these formulas, we can calculate the parameters of AO cells. For example, it is necessary to capture particles with a diameter of δ = 1 μm in the range Δx × Δy = 100 μm × 100 μm using a micro lens with fMO = 3.6 mm and a He-Ne laser (λ = 632.8 nm) with a beam diameter of d = 1.2 mm. Using formulas, we established the parameters of our setup: ГBE = 5, ГRS = 1, D0 = 6 mm, Δϕx × Δϕy ≈ 1,5° × 1,5°, Nx × Ny ≈ 250 × 250, Δf = 32 MHz. We are currently assembling the installation shown in Fig. 1 with the given parameters.
Conclusion
In this research, we discuss the AO two-dimensional deflection system for non-mechanical manipulation of microobjects using optical tweezers. We have developed and manufactured AO cells, which can become the basis of such a scanning system. Correct assignment of AOD parameters and parameters of other components allows constructing an optical capture system [5–7].
ABOUT AUTHORS
Pichugina Julia Vladimirovna, e-mail: pichuginaa@yandex.ru, Junior Researcher, Laboratory of Nano-Gradient Optics, Magnetic Materials and Structures, Scientific and Technological Center of Unique Instrumentation of the Russian Academy of Sciences; graduate student, Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia
ORCID: 0000-0002-0095-2066
Machikhin Alexander Sergeevich, Doctor of Technical Sciences, Senior Researcher, Laboratory of Acousto-Optic Spectroscopy,, Scientific and Technological Center of Unique Instrumentation of the Russian Academy of Sciences, Moscow, Russia.
ORCID: 0000-0002-2864-3214
Y. V. Pichugina 1, 2, A. S. Machikhin 1
Scientific and Technological Center of Unique Instrumentation, Russian Academy of Sciences, Butlerova street, 15, Moscow, Russia, 117342
Prokhorov General Physics Institute of the Russian Academy of Sciences, Vavilov street, 38, Moscow, Russia, 119991
e-mail: pichuginaa@yandex.ru
We discuss acousto-optic scanning characterized by high precision and repetition rate for manipulating micro-objects using optical tweezers. Bragg diffraction of light via ultrasonic waves allows creating robust solid-state devices for precise and fast laser beam deflection. We describe a scheme of the optical tweezers with PC‑driven two-dimensional scanning implemented by two sequential acousto-optical cells.
Keywords: acousto-optic deflector, optical tweezers, optical trap, Bragg diffraction, paratellurite.
Received on: 22.02.2020
Accepted on: 18.03.2020
Introduction
Currently, the development, study and use of optical tweezers are of great scientific and practical interest. Optical tweezers is an optical tool that allows you to manipulate microscopic objects using laser light. This method is based on the possibility of tight focusing of laser radiation, in which a spatially inhomogeneous optical field near the waist of a highly focused laser beam forms an effective spatial potential well.
A key element of optical tweezers is a scanning system designed to control the beam and its parameters [1–2]. Most existing optical tweezers use mirror and mirror-lens systems to control the position of the light trap. Such systems do not allow for quick movement of the trap from one arbitrary point of view to another due to the high inertia of the systems for moving mirrors and lenses. Moreover, there are high demands on the adjustment of circuits based on such systems, which leads to the need to use expensive and complex drive mechanisms and makes impossible to create multiple light traps at the same time. Control systems, which are based on liquid crystal (non-mechanical) optical radiation modulators, are devoid of some of the drawbacks inherent in mirror and mirror-lens systems, but they have low speed. In comparison with the well-known manipulation systems, the acousto-optical deflector is characterized by an order of magnitude higher speed, the ability to independently control several optical traps, instantaneous, high-precision, addressed (jump-like) movement of the trap within the field of view.
In this research, we consider a two-coordinate acousto-optic (AO) scanning system for non-mechanical manipulation of micro-objects using optical tweezers. In the AO system, the position of the trap is determined only by the frequency of the acoustic waves excited in the crystals. The tuning speed is limited mainly by the propagation time of the acoustic wave in the crystal, which is usually several microseconds [3].
The results show the advantages and prospects of AO non-mechanical manipulation of micro-objects using optical tweezers.
Circuit SCHEmE
A scheme of optical tweezers for manipulating micro-objects using two-dimensional AO scanning is shown in Fig. 1. The diameter of the laser beam is increased using a beam expander, and then directed to a two-coordinate acousto-optical deflector (AOD), which is two identical orthogonal AO cells. The first AO cell deflects the laser beam in the meridional plane, the second in the sagittal plane, so that the diameter of the beam does not change. A lens system is necessary for pairing AO cells and a micro lens, which focuses laser radiation on the sample under study located in the cuvette. Moving the last lens, you can move the hauling of laser radiation along the axis, i. e. focus the beam. A digital camera with a microscopic imaging system is located on the opposite side of the sample, and allows real-time monitoring and control of the position of the light trap.
Acousto-optical deflector
AO cell is a TeO2 crystal, to one of the edges of which a piezoelectric transducer is attached. When voltage is applied to the piezoelectric transducer, an acoustic wave propagates in the crystal, which creates a dynamic diffraction grating for the laser beam passing through the crystal [4]. By changing and modulating the voltage at the AOD, the first diffraction maximum of the laser beam is deflected by controlled angles. The excitation of sound waves occurs when the signals from the electronic driver to the electrodes. The driver consists of a generator and a broadband amplifier. To implement the traveling sound wave regime, an acoustic absorber is attached to the opposite face of the crystal. Fast modulation leads to the fact that the optical trap switches between different positions, i. e., it creates several traps. The schematic design and composition of a single-axis acousto-optical scanning system in the Bragg diffraction mode is shown in Fig. 2.
For two-dimensional spatial scanning, the deflector consists of two sequential AO cells rotated by 90°. Fig. 3 shows a scheme of a two-coordinate deflector with a two-channel electronic driver.
For this study, we developed a two-coordinate scanning AO system consisting of two identical cells from TeO2 crystals. Each of them operates in an anisotropic Bragg diffraction mode. The developed deflector has a typical configuration: the angle of incidence of light in the crystal is θ0 = 5.56° and the length of the acousto-optical interaction is L = 2 mm. A slow shear acoustic wave propagates in the crystal in the (001) plane at an angle α = 7.5° to the [110] direction (Fig. 4a). The sound wave vector is directed at an angle γ = 91.5° to the [110] axis and tangentially to the surface of the refractive indices of diffraction light. A vector diagram of this type of AO diffraction is shown in Fig. 4b.
The maximum deviation angles Δϕx × Δϕy can be calculated as:
(1)
(2)
where Δx × Δy are the dimensions of the sample, fMO is the focal length of the micro-lens, ΓRS is the increase in the relay system.
The angular resolution of the AOD is limited by diffraction and cannot exceed 1.22 (λ / D). The number of Nx × Ny positions allowed by the deflector is determined by the ratio of the angular scanning range Δϕx × Δϕy and the angular resolution:
(3)
(4)
where D0 is the diameter of the laser beam, λ is the laser wavelength.
The diameter of the entrance pupil AO of the cell D should be larger than the diameter of the laser beam D0:
(5)
where ΓBE is the increase in the beam expander, d is the initial diameter of the laser beam.
The frequency range of the ultrasound applied to the AO cells is found as:
(6)
Using these formulas, we can calculate the parameters of AO cells. For example, it is necessary to capture particles with a diameter of δ = 1 μm in the range Δx × Δy = 100 μm × 100 μm using a micro lens with fMO = 3.6 mm and a He-Ne laser (λ = 632.8 nm) with a beam diameter of d = 1.2 mm. Using formulas, we established the parameters of our setup: ГBE = 5, ГRS = 1, D0 = 6 mm, Δϕx × Δϕy ≈ 1,5° × 1,5°, Nx × Ny ≈ 250 × 250, Δf = 32 MHz. We are currently assembling the installation shown in Fig. 1 with the given parameters.
Conclusion
In this research, we discuss the AO two-dimensional deflection system for non-mechanical manipulation of microobjects using optical tweezers. We have developed and manufactured AO cells, which can become the basis of such a scanning system. Correct assignment of AOD parameters and parameters of other components allows constructing an optical capture system [5–7].
ABOUT AUTHORS
Pichugina Julia Vladimirovna, e-mail: pichuginaa@yandex.ru, Junior Researcher, Laboratory of Nano-Gradient Optics, Magnetic Materials and Structures, Scientific and Technological Center of Unique Instrumentation of the Russian Academy of Sciences; graduate student, Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia
ORCID: 0000-0002-0095-2066
Machikhin Alexander Sergeevich, Doctor of Technical Sciences, Senior Researcher, Laboratory of Acousto-Optic Spectroscopy,, Scientific and Technological Center of Unique Instrumentation of the Russian Academy of Sciences, Moscow, Russia.
ORCID: 0000-0002-2864-3214
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