Laser in a droplet



      A typical laser – Terminator rules. (Image by US Air Force, via Wikimedia Commons)

When you hear about a laser, you imagine a medium-size apparatus with a light beam coming out of it. You don’t imagine bacteria in a drop of liquid. Well, Turkish and British scientists went beyond ordinary imagination and published their findings in Lab on Chip


If you don’t own a laser in a form of a laser pointer, you certainly use one when you scan your purchases. To construct a laser, you need three things – a source of energy to get it going, a material capable of amplifying it and a feedback mechanism allowing to amplify the initial energy even more.

In a run of the mill lasers, the initial electromagnetic waves are trapped by mirrors, which bounce and enhance them. One of the mirrors is semi-transparent, allowing some of the amplified energy to escape. If the escape is a narrow slit and the energy in the visible light part of the spectrum, it creates a laser beam, the weapon of choice of SF battles.

(A nice  animation of laser)

In a typical laser, you have a gas-filled cylinder or a glass rod with ions for the energy amplification. However, the internal paraphernalia can be replaced by an illuminated liquid droplet suspended in mid-air via a standing sonic wave (Whovians, rejoice!) or optical tweezers.


Water droplet

A suspended water droplet – unlikely laser. (Image by Thomas Bresson [CC BY 2.0 (, via Wikimedia Commons

A  suspended droplet is a perfect sphere, which allows the initial light to go inside and bounce from the internal edges of the sphere. A photon from an excitation laser can make tens of thousands of bounces.  The liquid inside the drop serves as the amplification medium and the droplet edge as mirrors.


Some of the light leak from the droplet-cavity in all directions. Because the leakage is omnidirectional, there is no beam, but the droplets size – from nano to micrometers – and variable droplet composition allow using the droplet lasers for various applications.



Bacteria expressing differently coloured fluorescent proteins glow in the dark.  (Image  by Nathan Shaner [GFDL  or CC-BY-SA-3.0 ] via Wikimedia Commons)

Once you’ve done your initial setup of an external light source and supply of droplets, you can vary the droplet composition. You can put in fluorescent dyes for a signal amplification.  Or you can use a fluorescent protein, which emits light.


The Turkish scientists went further and instead of a purified protein, used a live E.coli cell, which synthesized a fluorescent protein. One bacterium containing lots of fluorescent molecules was enough to create a tiny laser.



Bacteria E.coli seen via tunneling electron microscope – even unlikelier laser.  (Image by    NIAID [CC BY 2.0] via Wikimedia Commons)

The droplet-based lasers, containing biomolecules and even live cells can be used in biosensors, environmental analysis and lab-on-chip.