The Droplet Laser
A laser is a fairly simple thing at heart. You need a couple of mirrors a chunk of material with the appropriate atomic characteristics and an energy source to get it all started. When teaching lasers to my undergraduates I often flippantly remark that “if you hit it hard enough, pretty much anything will lase,”  meaning that if you can get enough pump energy in the material requirement of the laser material don’t matter too much. While this isn’t quite true, it didn’t stop some of the original laser pioneers trying to make an ‘edible‘ laser out of jelly (or Jello-o for American viewers). In this they didn’t quite succeed, but they were able to get gelatin doped with fluorescein dye to lase and this could then be eaten, as the dye “was almost non-toxic”.
In some recent work (Optics Letters 38 1669 (2013), behind a paywall, sorry) work between my group and Alper Kiraz‘s at Koc University in Istanbul we have had a shot at making both slightly unusual and potentially edible substances lase, namely a microscopic droplet of water. This too is based on using the water droplet as a host medium for the lasing material (Rhodamine-B – which is likely a carcinogen, so you might not want to digest it) and a bit of glycerol for stability. Our work is based on using optical tweezers to trap the droplet in mid-air using an infra-red laser and then we illuminate it with a second pulsed, high energy, (green) laser. The droplets (water aerosols) are around 10 microns, so 10 millionths of a meter, in diameter. Pretty small! The nice thing about water droplets is that they tend to form very nice spheres, and this gives us a very simple way to form an optical cavity. Normally we would use mirrors to form the cavity, for example in a Helium-Neon laser, but here like that gets into the droplet can undergo total internal reflection and can get trapped inside. This enables a large optical field to build up and gets us above the energy threshold needed to see any laser action. This effect is called a whispering gallery resonance, and is the same effect as seen (or heard) in cathedral domes, like the ‘Whispering Gallery‘ in St. Paul’s Cathedral in London. Here, if you stand on one side of the gallery and whisper into the wall, the sound is able to creep around so that your friend on the opposite side of the dome can hear you clearly.
In the figure below you can see the output from the laser – these are in the form of cavity ‘modes’, which are the little spikes in the diagrams. The top two figures show the outputs below the laser threshold, while the third shows a higher pump energy and laser action. The inset shows the trapped laser droplet.
Our laser is not the first to make use of water droplets as the lasing host – there has been work on bigger droplets trapped using ultrasound and on surfaces but ours is the first to make use of optical tweezers to hold the laser. This should enable us to look at very small droplets, explore tuning of the laser through controlled heating, and it gives us significant control over the movement and placement of the droplets.
So what could you do with a droplet laser? Well there is quite a lot of work on using whispering gallery modes in solid spheres as sensors, and one could imagine extending this to liquid spheres. As we can easily place things within the droplets we could also use them as more general probes – the idea would be that perturbation of the laser in some way would allow us to probe the contents of the droplet. It might also allow us to sensitively probe the shape and dynamics of the droplet, which is very hard to do otherwise due to the very strong surface tension. We are only just starting to think about the possibilities.
On a personal note, this is an experiment that I thought up many years ago, and which we started to do when I worked in St. Andrews. We got some preliminary results showing droplet fluorescence but then the PhD student working on it had to write up and finish and we never quite got back to it. So it’s very satisfying to have finally done it, with a little help from my friends, and also that no-one else has done it in the meantime!
 Turns out this was a phrase used by Art Schawlow (see here, well worth a read), which means either great minds think alike, or I pinched it from him. I’ll stick with the former.