Holographic aperture principle and application overview

Introduction: The complex aperture formed by holographic technology has important application value in different occasions such as capturing and manipulating microscopic particles or atoms, because it is more adaptable than the usual apertures that only control the amplitude of the optical field. For example, a holographic diaphragm created with a special phase plate can transmit, sort, or control the aggregation of tiny particles.

A new generation of optical tweezers--holographic aperture

    Optical tweezers technology plays an extremely important role in molecular biology, colloid science, experimental atom physics and other fields. The optical tweezers themselves have also been continuously developed and many derivative tweezers have been produced. The holographic aperture formed by the Spatial Light Modulator (SLM) has advantages in multi-particle manipulation, which has opened up a new phase for the practical application of optical tweezers and large-scale industrial production. It is currently a very dynamic member of the Kwangmai family. This paper briefly introduces the principle and application of holographic aperture, and the only commercially available holographic aperture system on the market. It is the holographic aperture system CUBE of American Meadowlark (BNS).

introduction

    Optical tweezers, also known as single-beam particle traps, were invented in 1986 by A. Ashkin on the basis of the experiment of light-particle interactions since 1969. The single-beam particle trap is essentially an optical radiation pressure gradient force trap, and is a potential well formed by the interaction of the scattering force and the radial pressure gradient force capable of meshing the entire Mie and Rayleigh scattering range particles. It is formed by a highly converged single beam laser, which can elastically capture organisms or other macromolecules (balls), organelles, etc. from several nm to several tens of μm, and capture the material without substantially affecting the surrounding environment. Sub-contact, non-destructive live operation.

    Since its invention in 1986, Guanghan has played an extremely important role in the experimental research of laser cooling, colloid chemistry, and molecular biology with its advantages of non-contact and low damage. With the continuous expansion of the field of application of optical tweezers, in order to meet more research needs, optical tweezers technology itself is constantly improving in terms of real-time and controllable complex optical traps. At present, researchers can improve the accuracy of displacement measurements on the time scale of seconds by constantly improving the experimental method and controlling the Brownian motion of the sample. At the same time, particles as small as 25 nm can be captured and observed, and are expected to capture smaller nanoparticles. In the past few decades, the development of optical tweezers has made it possible for people to understand in more detail the molecular mechanisms of motion in complex biological systems. In the form of expression, the aperture device gradually evolved many types of optical potential traps from the original single beam gradient force optical trap. Such as double light, three light, four light, scanning light, femtosecond light and so on. This series of optical derivatization technologies has not only enriched the family of light families, but also provided a very ingenious tool for micro-nanoscale research in different fields such as biological sciences, such as the measurement of the unwinding process of double-stranded DNA and the study of molecular motors. Movement mechanism, separation of rice chromosomes, etc. Multi-optical trap control technology has become increasingly important in numerous experimental studies. Optical tweezers in the continuous development of a simple, single-beam gradient force optical trap to control multiple apertures and trap positions, holographic tweezers as a method for generating multi-optical traps or novel optical potential traps stand out. It can not only constitute light traps of various functions, but also realize three-dimensional optical trap arrays, and it has led to a series of research and development. Scientist Grir predicted that the holographic aperture will lead to a technological revolution in optical manipulation.

The principle of holographic aperture

    The hologram element is a key element constituting the hologram diaphragm. It is an interference pattern formed by the film recording object light and the reference light. When the object light field is reproduced, only the original reference light is irradiated on the hologram element to obtain the reconstructed object. Light field. A holographic aperture is a diaphragm formed by using a holographic element to construct a light field with a specific function. Due to the different nature of the light field formed, holographic apertures will achieve different functions, such as rotation of single particles, manipulation and sorting of multiple particles, and the like. The earliest holographic apertures were implemented by Eric R. Dufresne of the University of Chicago in 1998. They used a diffractive optical element (DOE) to split a collimated laser beam into multiple independent beams, which were focused by a strong converging lens to form a multiple aperture. The key to building a holographic aperture is to select the appropriate holographic element based on actual needs. The traditional method of generating holograms is the use of coherent light interferometry. The disadvantage is that the holograms they take are low in diffraction efficiency, time-consuming and poorly versatile. Therefore, they are not widely used in holographic apertures. At present, holographic holograms are mostly formed by spatial light modulators (SLMs). Common spatial light modulators include liquid crystal spatial light modulators, magneto-optical spatial light modulators, digital micromirror arrays (DMD), multiple quantum well spatial light modulators, and acousto-optic modulators. It is also possible to use UV lithography to make specific diffractive optical elements to modulate the light field. Most of what is used now is a computer-addressed liquid crystal spatial light modulator to implement a hologram element, and the formed light trap can be dynamically changed by changing the hologram element.

Prior to the advent of computers, laser holograms were needed to form holograms of limited shape. With the help of computers, holograms of any shape can be realized. However, every time a newly designed light trap is implemented, the corresponding hologram needs to be recalculated. With the constant refresh of computer speed and the emergence of new algorithms, holographic apertures of any shape can be easily realized in general scientific research laboratories. In principle, holographic apertures can produce light traps of any shape, size, and number. By changing the phase distribution of the captured light, the trapped particles can be moved in the light trap according to a set course, thereby providing a more convenient tool for implementing the iris sorting particles. With the continuous progress of laser capture technology and the constant change of capture objects, traditional single-beam gradient force optical traps can no longer meet the new demand for microscopic particle capture. As an emerging optical technology, the addition of holographic apertures has enabled the Krypton family to be full of vitality. Holographic apertures have shown great promise in the fields of capturing and manipulating multiple particles and achieving surface plasmon resonance capture particles. Fully recognizing the advantages and disadvantages of holographic apertures helps people to take full advantage of their advantages in designing holographic apertures and overcome deficiencies by designing holographic apertures with superior performance that meet actual needs, enabling them to be used in molecular biology and biology. Chemicals, nano-manufacturing and other fields exert their unique advantages and provide more valuable information for interdisciplinary research.

The typical application of holographic aperture

    Due to the exchange of momentum or angular momentum between light and particles, the light field becomes a traditional non-contact tool for capturing, moving, stretching, or rotating microscopic particles. The traditional method utilizes a wave plate and a polarization device to obtain a beam with a determined spin angular momentum, and a certain hologram can be used to obtain a beam with orbital angular momentum, such as a vortex beam. This expands the application of holographic apertures and shows its unique advantages in terms of photo-rotation of particles, manipulation of multiple particles, and complex motion.

1 New Hollow Light Field Captures and Rotates Fine Particles

    Photons have linear momentum and angular momentum, and angular momentum includes orbital angular momentum and spin angular momentum. Among them, the spin angular momentum depends on the polarization state of the beam, which can be changed by prisms, wave plates, and the like. In 2007, the Wang group used nano-manufacturing technology to prepare cylindrical nano-quartz particles. The particles rotate in the aperture and measure the torsional forces and moments of the dsDNA. This technique uses the spin angular momentum of the photons to make the birefringent particles rotate.

    In 1991, Sato et al. first realized the photo-induced rotation of particles in an aperture, and the beam used was a rotating high-order Hermite-Gaussian light. A series of new light traps have been used to study the photo-induced rotation of particles, such as hollow Gaussian, Laguerre-Gaussian, higher-order Bessel, hollow hollow bagel, and hollow LP01 output hollow beams. The advantage of this method is that the thermal effect generated when the particles are trapped is small, and there is no new characteristic of the single-beam gradient force trap formed by a common Gaussian beam. Traditional holographic technology has promoted the application of these new types of beams in photo-induced rotation. The orbital angular momentum is related to the specific spatial distribution of the light field.

    The beam with orbital angular momentum can be generated by a rotating Dove prism, but this requires a very precise arrangement of the prisms in the optical wavelength range, which is difficult to achieve and cannot dynamically change the beam characteristics. The use of holographic technology overcomes these shortcomings. It allows one to easily obtain light beams with orbital angular momentum or specific diffraction characteristics using suitable holograms, such as Laguerre-Gaussian (LG) beams, Bessel Bessel Beam, Hermite-Gaussian beam, etc.

    In addition, new light traps created by holographic techniques, such as vortex light traps, create near-field apertures at the interface that can be used to capture and rotate metal particles. In 2008, Maria Dienerowitz et al. of St. Andrews University in Scotland used LG light to capture nanogold particles. They used a beam near surface plasmon resonance to confine gold particles to the dark field region of LG light, and to use the orbital angle of photons. Momentum transfer, realizing the rotation of two 100 nm gold nanoparticles trapped in the optical trap at the same time.

2 Multi-particle complex movement

    The force generated by the use of optical wavefront correction technology can achieve rapid control in many areas of science and technology and engineering applications. For example, holographic apertures can dynamically capture and manipulate multiple particles in real time. Jesacher et al. of the Innsbruck School of Medicine in Austria conducted extensive research on the utility of liquid crystal spatial light modulators to generate complex light waves. They capture and manipulate microscopic dielectric pellets in pre-configured optical traps by separately controlling the amplitude and phase of the optical field. Changing the amplitude and phase of the light field can not only realize light traps of special shapes such as cross, rectangle, and circle, but also control the movement of particles along specific paths. In principle, it is possible to realize particle trapping in light traps of arbitrary shapes. Control.

3 Other applications of hologram

    The complex aperture formed by the holographic technique has important application value in different occasions such as capturing and manipulating microscopic particles or atoms, because it is more adaptable than the usual apertures that can only control the amplitude of the optical field. For example, a holographic diaphragm created with a special phase plate can transmit, sort, or control the aggregation of tiny particles.

    At present, holographic array apertures with up to 400 optical traps can be obtained by using holographic technology. In combination with computer technology, the characteristics of individual optical traps can also be dynamically changed. The real-time light traps thus generated can capture moving and highly dispersed objects such as viruses, small colloids, and swimming bacteria. In addition, linear, Bessel optical traps, and optical vortex traps with angular momentum can also be generated. These unusual light traps make it possible to adjust, rotate, rotate, and create circular objects in the image plane or optical axis, as well as other atypical manipulations. These studies have further expanded the scope of application of holographic apertures, making it a wonderful work in the cross-science research hall.

    Holographic apertures are characterized by the freedom to control multiple particles, making the fusion, adsorption, and interactions between particles or surfaces easier. For example, the virus is implanted into cells or the sperm is implanted into the egg cells, and the intermolecular binding force is detected using multifunctional balls and surfaces. By observing the behavior of the object in the light trap, it is also possible to accurately measure the characteristics of the object or the surrounding environment. A multi-well trap can stretch or bend material at the interfaces between single molecules and cell membranes and fluids. Such experiments can obtain information on elastic modulus, surface energy, and adsorption force in many systems, and at the same time simplify the study of micro-scale mechanical properties. Holographic apertures can be used to assemble specific structures. With fluorescent or reflected light illumination, a specific material can be viewed and positioned on a transparent substrate or electrode. Holographic apertures can organize many materials with new physical or optical properties in three dimensions. Potential applications are the construction of photonic crystal bandgap materials, the fabrication of biological or nanoscale electronic components, and the deposition of different materials on electrodes to measure their electrical properties. In 2007, scientists in the United States used infrared light to control the movement of particles on a silicon wafer. They selected a silicon wafer of the appropriate thickness and doping concentration so that it could penetrate infrared light and could be detected by the CCD. This technology breaks through the bottleneck of the traditional capture of particles in the liquid phase. If you combine holographic diaphragm technology, you can assemble some meaningful structures on specific solid surfaces.

In particular, before the invention of the holographic aperture, the aperture diaphragm technology focuses mainly on the basic research of single particles, and the advantage of holographic aperture in the manipulation of multiple particles opens up the practical application of optical tweezers to large-scale industrial production. A new situation.

Product examples

    Currently, commercially available aperture systems mostly use AOD, and Meadowlark (BNS)'s holographic aperture system CUBE is the only commercial holographic aperture system. Its structure is as follows:

Compared to other apertures using AOD, Meadowlark (BNS)'s holographic aperture system CUBE has the following features:

1. Currently, commercially available multi-aperture systems all use AOD (high-speed modulation) lasers to form different light traps, and can only handle two-dimensional (x, y-direction) planes such as translation. The liquid crystal spatial light modulator can modulate the intensity and phase of the light and is a true 3D manipulation. Not only translation, but also three-dimensional rotation and other manipulation of particles and cells.

2. Since the light can be phase-adjusted, the liquid crystal spatial light modulator can correct the phase difference, adjust the laser spot, and make the light trap distribution more ideal.

(a) Phase diagrams used to correct phase differences loaded on the SLM (b) Pre-correction light spot (c) Corrected light spot

3. The liquid crystal spatial light modulator (SLM) has a diffraction efficiency of greater than 90%, higher than AOD, and has a higher utilization of the laser.

4. Unlike the AOD which forms multiple light traps through high-speed switching, the Liquid Crystal Spatial Light Modulator (SLM) can generate multiple focused beams at the same time, each beam forming a separate light trap, so the light The stable shape of the trap is better.