Archive | Optics RSS feed for this section

A laser that likes a good shake

13 Jan

In general, lasers are not like old television sets: a good whack does not make a laser miraculously work better. Lasers (at least big lasers) take a lot of careful alignment to get working, so whacking, shaking, and leaning on are all out of the question. If you are being trained in a new lab, you will probably hear a lot of “no-touchy” around the laser mountings.

Some scientists in Russia, however, have created a laser that likes a good shake. The laser works by using mirrors to direct light at quantum dots, causing them to emit. The problem is that the quantum dots aren’t always in the state you need them to be in to emit light of the correct wavelength. This is where the shaking comes in. Scientists found that if they exposed quantum dots to sound waves, the properties of the dots shifted in a way that caused them to spend more time in the state that emitted at the correct wavelength. Quantum dots are perfect for this, as their electrons are willing to hang out in an excited state long enough for the sound waves to affect their properties before emitting a photon. The laser with the sound waves had a power 200 times better than without, which is pretty huge. Cool.

Article here.

Still curious? Here’s a bit more info about quantum dots: Quantum dots are tiny bits of matter with special properties. First you need to know about excitons. Excitons are basically an electron and the hole where that electron wants to go, together in a bound state. They are electrically neutral quasi-particles that exist in insulators and semiconductors. Quantum dots’ excitons are confined in all spatial dimensions. The result is a tiny bit of matter that acts (electrically) a bit like a bulk semiconductor and a bit like a single particle. Quantum dots are being researched all over the scientific world nowadays, from lasers and solar cells to quantum computing.

Advertisements

Breaking the diffusion limit in optical sensing

4 Oct

It’s pretty amazing that we can “see” single molecules nowadays. The problem is that we don’t always know where to look. For instance, if you have a highly-diluted liquid sample in which there is a low quantity of some molecule you are searching for (like a blood sample in which there is a molecule indicative of some disease), the chances of you happening upon the molecule you are interested in are pretty much nonexistent. You have to wait for the molecule to find it’s way to your sensor surface and it would take WAY too long to wait for diffusion to do its thing.

These people have found a clever way to get around this. They created a surface with tiny towers all over it of an appropriate radius and distance from one another to make them extremely hydrophobic (and it might be chemically hydrophobic too–i’m not sure). If a surface is hydrophobic, that means that it doesn’t want to touch water, so water droplets on the surface remain small and do not spread out or combine. The water droplet then evaporates rather quickly, leaving the non-water stuff on top of the towers. Now that we have a way to put the molecules we want in the place we want them, we need a way to detect them there. The researchers covered the top of the pillars with metal nano structures that cause a large magnetic field  to be created when exposed to light. When there’s a large enough electromagnetic field, you can see fluorescence from single molecules, so voilá.This technique can be combined with a variety of other techniques, including Raman Spectroscopy, to determine the chemical signature of the molecules. Pretty handy, and relatively easy to make.