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Introduction
Introduction
If photons are impacted on a mirror made of metal and bounce off it, there will be an equivalent force on the mirror in the opposite direction. This is due to the law of conservation of momentum. However, the light may not appear to be moving the metal mirror, the small force provided by the light accelerates the metal mirror by an indistinguishable amount. If one takes that mirror and shrinks it down to a tiny particle made of glass and has photons hit it, then there will also be an equivalent force on the glass particle, creating an optical trap.
Who first discovered Optical Tweezers?
Who first discovered Optical Tweezers?
Optical trapping was discovered by Arthur Ashkin in 1969 when he experimented with two gaussian beams acting upon a transparent micrometer glass sphere. He discovered this phenomenon by being curious about the idea of light pressure. Arthur Ashkin measured several important quantities using optical tweezers such as the charge of the electron in the Millikan oil-drop experiment, the proportionality constant of DNA, and base-pair step size of RNA Polymerase, the efficiency of biological motors, and many other applications in the field of biology (Ashkin, 1997). Arthur Ashkin was awarded the 2018 Nobel Prize in Physics for the research of optical tweezers and their applications in biological systems in 1987.
How optical trapping works?
How optical trapping works?
Optical trapping occurs when a particle is suspended by focused light pressure from a laser that is focused by a high numerical aperture. High numerical lenses are used to focus a laser beam creating an intensity gradient. A dielectric particle that is latterly next to the focal point of the high numerical lens, where light intensity is maximum, will experience gradient forces. These forces are caused by the refracted light rays that are entering and exiting the dielectric particle, which changes the light rays' momentum. This causes the dielectric particle to exert an equal but opposite change in momentum, moving the particle to a distance that is just after the laser’s focus (Ashkin, 1997). The particle is held in place by balancing light forces, which are the scattering force and gradient force.
The scattering force is the amount of light that does not get refracted but gets reflected towards the high numerical aperture. The scattering force can be determined by:
The gradient force pushes the particle towards maximum light intensity at the center of the beam when the refractive index of a particle is higher than the surrounding medium. Stable traps have gradient forces that overpower all other forces. The longitudinal gradient force must overcome other forces such as buoyancy, gravity, and the bombardment of Brownian particles (Suarez, Neves, Gesualdi, 2020). The gradient force can be determined by:
Depending on the power, the scattering force is about 10x less than the gradient force while both are in the range of pico-newtons. There are more ways to explain the optical trapping phenomena other than just light pressure. Electromagnetic dipole approximation considers the Lorentz force pushes on the particle towards the center of beam focus. "Dielectric particles much smaller than the wavelength of light can be considered to be perfect dipoles (Shaevitz 2006).”
The trapped particle can be represented as a solid in a fluid medium. Properties of the surrounding medium will change as it interacts with focused, high-intensity light such as phase change and polarization change. Trap stability depends on several factors: particle size, power, wavelength, thermal effects, laser type, numerical aperture, Kerr nonlinearity effects. The escape potential of the optical trap is dependent on the particle size. Particles in the ranges of 10-30 nm to 60-75 nm form stable traps with the data collected (Bandyopadhyay, Gaur, Goswami, 2002). Increasing the power results in a more stable trap; however, thermal effects increase the escape potential by about ten times. Increasing the power increases the gradient force (Bandyopadhyay, Gaur, Goswami, 2002). Common wavelengths that are used for optical trapping are in the infrared range. The size of the object must be smaller than the wavelength so that the method for Rayleigh scattering can be applied. Optical trapping of a certain sample is dependent on the wavelength. Low light absorption prevents thermal effects. The surrounding medium will experience thermal nonlinearity effects causing the beam to defocus and increase its radius as the temperature of the surrounding medium increases. Optical traps can use different types of time-variant lasers such as continuous wave or femtosecond pulsed lasers. Continuous-wave lasers are standard lasers where intensity is a constant as a function of time. Femtosecond pulsed lasers are light pulses that are equal to the sum of multiple different wavelengths of light that are stretched, amplified, and then compressed. Continuous-wave thermal effects are more prominent than with femtosecond pulsed laser (Bandyopadhyay, Gaur, Goswami, 2002). This data was generated from "particle sizes ranging from 5 nm to 80 nm in 5 nm intervals for average powers ranging up to 100 mW" (Bandyopadhyay, Gaur, Goswami, 2002). A numerical aperture is used to create sharp intensity gradients. Some lenses that are being developed in optical trapping applications are 3D-printed Fresnel lenses and fiber-based total internal reflection lenses because they have a high numerical aperture. “The Kerr effect is a change Δn in the optical refractive index which is quadratic in the externally applied electric field” (Hoffmann, 2009). Including the Kerr effect increases the total force of a femtosecond by a factor of 10; however, thermal effects reduce the over force (Bandyopadhyay, Gar, Goswami, 2002). The stability of optical traps can be improved by incorporating multiple beams.
Operations of Optical Traps
Operations of Optical Traps
Many innovations have changed optical tweezers over time: holographic optical tweezers, spatial light modulators, fan-shaped optical vortex beams, optical fiber tweezers, graded-index fiber, and surface plasmon-based nano-optical traps. Holographic optical tweezers (HOTs) are cables for splitting a laser beam into more laser beams with the use of a computerized diffractive optical element (DOE) (Curtis, Koss, Greir, 2002). The spatial light modulator can sort out the light coming from the laser beam and send it in many different configurations towards the highly converging lens, creating several optical traps that can be dynamically controlled using a computer. Spatial light modulators modulate the intensity, polarization, and phase of light. Liquid crystal spatial light modulators can dynamically manipulate vortex beams. Optical fiber tweezers (OFTs) are optical fibers that have lenticular-shaped ends which produce an optical trap at that output. The use of the lenticular shape end produces a higher gradient force to create a stable trap. This technology is heavily implemented in the biological operations. OFT's utilize either photothermal effects or optical forces (Zhao et al., 2020). Photothermal effects are the result of the laser inducing thermal and acoustic gradients. In an optical fiber tweezer, the light intensity is highest near the tip of the lenticular shape end. The characteristic of the optical fiber tweezers is heavily dependent on the style of their lenticular shape ends because this determines the high numerical aperture. Multiple cells can be trapped in an array when the light is able to pass through other cells extending the gradient force. Such cells can be used as optical waveguides. The efficiencies of OFT and a standard optical trap is 0.227 and 0.12, respectively (Zhao et al., 2020). Graded index fibers have more robust trapping capabilities as in the focal points are adjustable based on power. Plasmonic nanostructure-based fiber tweezers are optical fiber tweezers that have the lenticular end of an optical fiber coated in gold and has a bowtie plasmonic aperture. These tweezers are able to trap and manipulate particles smaller than 100 nanometers by utilizing self-inductive back action, which reduces photothermal effects (Zhao et al., 2020). Plasmonic nanostructure-based fiber tweezers share the same mechanics as optical fiber tweezers when it comes to trapping an array of particles. These tweezers can be used as nano detectors and require little power compared to other forms of optical tweezers. Plasmonic tweezers use subwavelengths to trap nanoparticles. Fan-shaped optical vortex beams (FOVBs) are fan-shaped optical vortex beams that are like rotating blades of light that push microparticles. A donut or hollow shaped optical trap surrounded by fan-shaped optical vortex beams allows for the effective clearing and shielding of microparticles (Haiping et al. 2020)
Conclusion
Conclusion
There are many different ways to manipulate light to create optical traps, all having very unique characteristics, and abilities. Thanks to the curiosity of light pressure from Arthur Ashkin, people are able to utilize his findings and further the realm of optical tweezers. Further research into optical tweezers will provide many opportunities in the near future
References
References
Ashkin A. (1997). Optical trapping and manipulation of neutral particles using lasers. Proceedings of the National Academy of Sciences of the United States of America, 94(10), 4853–4860. https://doi.org/10.1073/pnas.94.10.4853
Bandyopadhyay S. N. Gaur, T. Goswami, D. (2002). Comparative study of the real-time optical trapping in the Rayleigh regime for continuous and pulsed lasers. Retrieved from https://arxiv.org/ftp/arxiv/papers/2002/2002.00227.pdf
Curtis, J. Koss, B. Greir, D. (2002). Dynamic Holographic Optical Tweezers. Retrieved from https://physics.nyu.edu/grierlab/dynamic4c/
Haiping, W. Tang,Liqin, T. Jina, M. Hao,Huiwen, H. Xiuyan, Z. Daohong, S. Hu,Yi, H. Yigang, L. (January 6, 2020). Optical clearing and shielding with fan-shaped vortex beams. Retrieved from https://aip.scitation.org/doi/10.1063/1.5133100
Hoffmann, M. (2009). Terahertz Kerr effect Retrieved from https://arxiv.org/ftp/arxiv/papers/0907/0907.5081.pdf
Shaevitz, J. W. (August 2022, 2006). A Practical Guide to Optical Trapp. Retrieved from http://genomics.princeton.edu/shaevitzlab/OT_Practicle_Guide.pdf
Suarez, R. Neves, A. Gesualdi, M. (January 14, 2020). Optimizing optical trap stiffness for Rayleigh particles with an Airy array beam. Retrieved from https://arxiv.org/pdf/1910.01985.pdf
Wen, J. Gao, B. Liu, D. Zhu, G. Wang, L. (February 18, 2020). Precisely position- and angular -controllable optical trapping and manipulation via a single vortex-pair beam. Retrieved from https://arxiv.org/pdf/2002.06860.pdf
[1] Zhao, X. Zhao, N. Shi, Y. Xin, H. Li, B. (January 16, 2020). Review Optical Fiber Tweezers: A Versatile Tool for Optical Trapping and Manipulation. Retrieved from https://www.mdpi.com/2072-666X/11/2/114
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