Who am I? Not the philosophical way… but more, what do I do everyday? You can discover part of my days by following the very unregular update of my LEARN posts.

But I never answered the annoying question, what is my research about? Some hints here and there.. but not a great deal of details

Since younger me was more motivated than the empty shell currently writing this blog, I let her telling you… (there is also a more “mature” version of it.. less swearing, compensated by some sexual innuendo)

Still here? For those of you that have survived my bad humourism and still have not pressed that unsubscribe button, I can tell you a secret: I lied in that video!

Optical tweezers are not some sort of magic. They might look like that to the untrained eye, but all it takes to make them is a bit of physics (and several thousand pounds to buy the laser).

Let’s break it into pieces.

Stealing one of my work pal example (PHOQUS Newsletter, page 6), I am going to talk about pirates, space pirates. If sea-pirates have sails that wonder the sea by being propelled through wind, space-pirates would employ something called Lightsail – exactly what it sounds: a sail that can propels itself by being pushed by light, in space.

Pretty Sci-Fi, uh? And still very intuitive if you follow the right reasoning.

Every object moving with a certain speed possesses a physical quantity called momentum. A ball rolling on a pool table has a momentum equals to its mass multiplied by the speed it is moving at. If you pushed with in the right trajectory, it might hit a second ball, that will start moving upon the first one stopping. Whether you just won the game, or hit the wrong one, what you were exploiting is one of physicists’ besty: a conservation law. In this case, the momentum conservation law. Some fun fact: photons, the elementary particle associated with light, possess momentum too. They might not have a mass, but they move in a direction with a very famous speed, actually the highest speed anything can move.

Don’t you trust me? Well, if you have ever got a sunburn, you might then believe that light must have some energy to burn you that way. And Energy can be written as E=mc2, although this is a hermitic way to write a very famous formula that in reality is telling us much more. In fact, in its extended form, we would write it as


Interestingly, this expression is telling us that the energy of an entity is given by its rest energy (m0 . c2), but also by its momentum p. Photons are massless, m0=0, so the energy your skin cells are annoyingly exchanging must come from something else: their momentum p.

Back to the pool table and the conservation law: if photons have momentum and hit an object, than the object will be pushed. Why then we don’t feel to be pushed to the ground by sunlight when getting out the front door? Well, nothing to do with the fact that Scotland does not have a great deal of sunlight, but mostly with the fact that the momentum photons carry is very small! It is about an octillion (a billion billion billion) smaller than the typical momentum of a pool table ball.

This is why a lightsail needs to be big and works just in space, with almost no resistance. Or you can scale things down. Take a micrometric sphere, shine a laser on it and there you go: you can push it. This exchange of momentum is referred to as scattering force. These are still not optical tweezers, since you are just pushing the sphere around, but it is the first concept you need to grasp. The rest still comes from the fact that light can interact with matter and exchange energy with it. By focusing the laser light on a tight spot, we are generating a second force, a gradient force, that pull the sphere to the focal point.

I find it extremely interesting how physics apply to different scales and how we can make advantage of simple notions, like the one of momentum and its conservation, to produce new exciting tools. And if I am not travelling space with my tweezers, I like to think I am pioneering in a field they more applicable, like biology, where they can act as probe to exert and measure forces on the biological samples. But this is a story for another time.

For now, let me reply to my initial question:

Who am I? A Space Pirate!

Some interesting links:

  • Wikipedia page on lightsails, with the list of operations
  • StarShot: a Stephen Hawking’s Breakthrough Initiative about employing lightsails nanocraft for interstellar exploration
  • Optical Trapping: an excellent review on the topic
  • The first papers on optical trapping by Ashkin: the very first one, that was not even using the gradient force, but purely the scattering one by two counter-propagating laser beams; the first demonstration on how to use a single beam,  therefore employing the gradient force; finally, the first paper on using them for biology.. this is envisioning the future!
  • Brightclub: the video linked to this post comes from a gig I did for Brightclub, an alternative way to share science and to expand communication skills and grow confidence. Check for the Brightclub nearest to you and give it a shot, it is free and training is provided.


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