Lasers and Optical Tweezers, in specific, have forever changed our view of the the microscopic world. For those who have never heard of this extraordinary tool, created by physicists and used by all, all that you need to know is with laser light you can move objects such as cells and colloids, you can rotate them, squeeze them, heck you can make them into another laser if you wanted or at the least make them fluorescent! In this article, we will review the fundamentals of this optical tweezers, the key technological advancements in optical tweezers and their application within the field of biological sciences and medicine. We select a few examples to illustrate the properties that can be studied and further discuss the possible future uses of optical tweezers in other areas.1. Introduction
Imagine holding a tennis ball in your hand. Stretch your hand out and you can feel the force of gravity pulling it and thus you obtain a “feel” for its weight, relative to of course other objects you have carried in your hands before. The touch of the tennis ball upon your hands can also tell you about the texture of its surface, the material its made of and if you attempt to squeeze (or stretch) the tennis ball, you learn further about the nature of the material, its elasticity to be precise, but you could even try and figure out what is inside the tennis ball in this manner. What about cells? Or bacteria? Can we get the same “feel” for such microscopic entities found within us? How elastic is a cell? Of course in our science, we are a lot more quantitative and would like to be able to say how much force is required to, say, stretch an object or permanently deform it. This also tells us a lot about the object of our study. Consider a tennis ball waterlogged, such that its bounce is damped. What if a cell, such as the red blood cell, does not stretch the same way a healthy red blood cell would. What then? What does this tell us about the red blood cell? Is it healthy or unhealthy?
With the advent of Optical tweezers (also known as laser tweezers) we have had the opportunity to answer some of these questions and many more, about the biological world. Questions like how elastic is a cell, is not a silly question but rather one for which we have answers for. In this review we will attempt to outline some of the basic physical principles governing optical tweezers, after which we look at the various set ups and the related properties of trapped particles that can be determined with optical tweezers. Optical Tweezers are not confined to biological material, though this forms the topic of our article, optical tweezers can be found used in engineering, physics and material sciences abundantly. Due to the rapid progression of this field it has become unfeasible to study all the applications of the optical tweezers in one review, let alone one book. Even the application in biological sciences has become so vast that will we have to curb our enthusiasm to certain studies that highlight the manner in which optical tweezers are being used.
2. Historical Overview
The curiosity of light and its effects on matter has fascinated man since time immemorial. Examples can be dated back to Antiquity. However, one can start as far as Kepler (in 1619) who seems to be first person to have suggested that light might be able to exert a mechanical force on particles, which he gathered after his observation of comet tails . It was around 1700s that John Michell tried to measure optical radiation pressure , where he showed sunlight can be concentrated, though to his dismay destroying his experimental apparatus. But for all this investigation to be put onto a proper theoretical and scientific platform it required the genius of James Clerk Maxwell and his seminal work on electromagnetic theory predicting the existence of radiation pressure of light and electromagnetic waves  . The first real experimental evidence of radiation pressure came with the work of P.Ledebew in 1902  and from the work of E.F.Nichols and G.F.Hull through 1901 to 1903 , where they investigated pressure due to radiation. However, no real technological application was sought, as the radiation pressure was small compared to the power required to generate and sustain the radiation. Scientist had to wait till the advent of lasers in 1960  with which everything changed.
In 1969 Arthur Ashkin (who is considered to be the “father” of optical tweezers) showed that radiation can be used to trap and accelerate particles  but the experiment was a delicate balance between the optical levitation forces and gravity. Nonetheless, we see a split of the field into two branches, the trapping and cooling of atoms and the optical tweezers (optical cooling is beyond the scope of this review, interested readers should look at ). However, it wasn’t till 1986 that optical tweezers were fully realized with the trapping of microparticles using the gradient force of a single-beam  by Ashkin, Dziedzie, Bjorkholm and Chu. We will see through the various advancements made in optical tweezers and its applications, that Ashkin is a key recurring figure in this story.
Creating non-invasive traps and manipulating particles whilst also measure forces on the order of piconewtons naturally led to uses in biology-related fields, as early as 1989. This opened up possibilities of measuring the dynamics of such biological materials as DNA  and eventually led to intercellular studies using optical tweezers.
Parallel development occurred in rotational manipulations of optical tweezers. Which could eventually spin particles continuously, achieved by using such beam profiles as the Laguerre – Gaussian mode beams (don’t ask they look rather funky), which would transfer angular momentum to absorbing particles . These advancements in optical tweezers allowed progress towards optically driven micromachines.
3. The Fundamental
When a light beam is incident upon a particle, there are multiple interactions occurring from absorption, reflection, refraction, scattering etc. We can narrow this down to two components of the total force experienced on the small particle(s). One is the scattering force, and is simply the results of photon pressure. The other is a gradient force and is the result of the electromagnetic potential a dielectric particle would “feel”. Best way to imagine this is a moth’s attraction to light. A dielectric particle is naturally attracted to high intense concentration of electromagnetic fields, characterized as potential wells. Upon acting on this attraction the sphere becomes trapped. Therefore, the only thing that makes it loose its much-desired place in the potential well is the scattering force it experiences due the forward propagating motion of the wave.
Ultimately, the trapping results from transfer of momentum from the beam to the particle. If we recall that force = time x rate of change of momentum, then the optical forced must result from a change in momentum flux of the beam. Where momentum flux of a ray of light is p = nP/c, where p is momentum flux, P is the power, or energy flux, n is the refractive index of the medium ray moves in and c is speed of light. We can also consider the focused beam to be bundle of converging rays. One finds that convergence or divergence of the rays will decrease the momentum flux.
The above description of laser and particle interaction is just a simple picture of what really goes on. The reality is much more complex and simple geometric optics cannot be used to calculate forces or tell the full story. Alternatively we can adopt an approximation using the Rayleigh Limit, that is to say, particles are considered to be relatively small to the incident wavelength. The effect of the electromagnetic field is to induce a dielectric polarization in the particle. The complete derivation is well explained in , but here we simply present the gradient force and the scattering force to further discuss the regime required for optical tweezers work under.To understand the forces and the result they have on our particle consider the particle as a weak positive converging lens as shown in figure 1. When the particle is at the focus, the light rays will pass right through them (fig.1a). When the particle is before the focus (fig.1b), then the convergence is increased for the incident beam, this results in decrease of momentum flux; consequently particle feels a force in the direction of propagation. If the particle is after the focus (fig.1c), then the beam divergence is decreased, resulting in an increase of momentum flux, which produces a restoring force towards the focus. If the particle is displaced sideways (fig.1d), then the beam is deflected towards the centerline of the particle, resulting in a lateral momentum and naturally a lateral reaction force, which acts towards the beam axis.
The gradient force is proportional to r3 implying that for small r, the field can be approximated as uniform, thus this forms a useful approximation for the gradient force.
The scattering force is proportional to square of the volume, that is to say it is proportional to (r3)2. Which implies that for a small sphere the gradient force (proportional to the volume) will be much greater than the scattering force on the sphere.
In the trap, the particle will also be influenced by thermal fluctuations. To overcome this one has to make sure that the trapping potential achieved from the gradient force is greater than the thermal kinetic energy otherwise the particle will escape due to Brownian motion.
4.Types of Optical Tweezers
Over the past few decades, new of forms of optical tweezers have been developed, many for specific applications and as result there are many types of optical tweezers now in use.
4.1 The Conventional Setup
The conventional set up for an optical trap is achieved by focusing a laser beam onto some substrate or medium containing the particles prepared for study. This single-beam set up, as detailed above and also in , it is used in many ways to control and manipulate the trapped particles and provides a novel upgrade from the first optical trap of an optical fountain . The optical fountain, required a balanced between gravitation forces and the upward optical levitation forces. A more interesting set up was introduced, in the early 90s of a dual beam trap, or a trap made of counter-propagating beams. This set-up, however, is very difficult at times to align and steer. The excellent advantage of a single beam trap  is its control. Other than the obvious trapping feature, the ability to rotate provides another degree of freedom to the object trapped.
4.2 Optical stretcher“]
Until now the forms of optical tweezers we have considered are focused beams that will control a trapped particle. An optical stretcher is similar to a dual-beam optical trap, however; in this case the optical fibers are used to guide the beam into accomplishing a trap, without focusing the beam. An example is shown in Fig 2. When a photon enters an optical fiber it is normally moving from a low refractive index to a higher one and therefore it experiences an increase in momentum, its wavelength will decrease (momentum p = hn/c, where h planks constant, n is the refractive index and c is the speed of light in vacuum). We find that this results in a “pulling” force from both the counter propagating beams resulting in a trap. The “pulling” can also result in the stretching of the trapped objects, specifically cells, such that we can measure various properties relating to them, like their elasticity. Alongside, this one may also have the flexibility to rotate the trapped object thus adding another level of control.
4.3 Other setups“]
Other interesting set-ups include the use of evanescent fields* produced at the boundary of nano-fibers, as a laser beam propagates within it, to trap and guide particles along the surface of the nano-fiber. As shown in the figure 3, the laser beam is guided through an optical fiber, completely contained within the fiber cable, and as a result the particles are unaffected. However, once the beam reaches the nano-diameter (better known as the tapered region) of the fiber, an evanescent field is created along the boundary, which attracts the micro-particles to the surface of the tapered optical fiber.
Another interesting configuration, which we mention here though will not go into detail with, is of an array of potentials created using diffraction optics. This creates 2D or a 3D array of optical traps that are then used to trap a flow of cells depending on their refractive indices , thus sorting by via the potential wells.
Applications presented here are only a few of the dozens upon dozens of applications that are currently using optical tweezers. The examples chosen are so that only a few of the key ways are mentioned where optical tweezers are illuminating the biological universe for us.
5.1 Measuring Biological Forces and mechanics
The measurements of optical forces open another dimension of knowledge when it comes to biological materials. With the introduction of control through optical tweezers, the level of detail and accuracy with which we can understand these mechanics and biological mechanisms is unprecedented. One example is of kinesine molecules that use ATP Hydrolyses (a form of reaction that releases chemical energy) to move along the microtubules on cells, providing support in cellular functions such as mitosis, meiosis, and cargo transport. Researcher in the Block Group at Stanford University, have gone on to show how much ATP is consumed for one step that a kinesin molecule takes  and each step is normally 8nm. How wonderfully detailed is this understanding, that we can begin to properly understand qualitatively the true nature of kinesin and other such molecules.
5.2 Diagnosis Tool – Malaria in red blood cells
Another beautiful application of optical tweezers comes in form of a diagnostic tool. Though there are many examples, one should suffice to show its importance. Healthy red blood cells (RBC) when optically trapped rotate freely . As the trap-beam power is increased, the rotational speed also increases linearly. Repeat the same experiment this time for a RBC containing a malaria parasite and the cell does not rotate. Increasing the trap-beam power also results in the increase in rotation of the infected RBC however that increase is very slow in comparison to the healthy cells. As a result optical tweezers can be utilized in diagnosing malaria.
6. Future Prospects
The last few decades have been geared towards perfecting optical tweezers as a tool. Though, application in biological sciences of these novel devices started right at its birth, the last decade gone, has seen them mature into being utilized most biological/Physical sciences departments and have become as common as the laser labs! The future could see the realization of real-time surgery at the micro and nano scales using nano-lasers controlled optical tweezers . Further developments are being made in imaging and sensing; a finer control is sought at the nano-Newton scale and applications spring up from bioscience to quantum optics. It’s almost seems like that the field of optical tweezers is just starting.
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Here are some videos of optical tweezers at work: