Scientists at Berkeley have created a laser that's a fraction of the size of a hair. This ultra-tiny laser is so precise that its beam can detect individual molecules.
Lasers are used for everything from media storage, to medicine and manufacturing. Their precision and energy have allowed people to very accurately measure or convey information, but there's always a need for more. In the growing biomedical field, for example, scientists need lasers that can examine smaller and smaller substances, right down to single molecules. To do that, they have to shrink the lasers down to an incredibly small size. But how can a microsopic laser ever be accurate?
A standard laser is a tube where one particular substance is held. The atoms in that substance are excited, and give of one particular wavelength of light. At either end of the tube are mirrors that bounce the photons of light off each other, exciting other atoms as they do. When the photons encounter each other inside the tube, they vibrate with the same frequency and direction. One of the mirrors is half-silvered, allowing half the photons out into the world as monochromatic, directional light. The mirrored tube that the photons bounce around in could not be less than one half the wavelength that the light the laser produced. This was called the diffraction limit, and no one could deny that it was small. It just wasn't small enough.
Making the smallest lasers in the world
Then came plasmon lasers. Plasmons are little quantized packets of energy, like photons or phonons. Plasmons, however, were generated by the oscillation of plasma, and were generated by shining powerful light on certain kinds of metal. Interacting with plasmons 'localized' light - confined it to a much smaller area than it would usually take up. Plasmon lasers were able to shrink the necessary space for laser generation down to one twentieth of the wavelength of light. Plasmons looked like a great way to shrink down lasers, and apply them and the tiny beams they generated to delicate biomedical and optical situations.
The problem was, scientists noticed loss of of metal and heat in plasmon lasers. They either had to pump a lot of extra energy into the laser to keep it going, or put the laser in a specialized environment that would minimize the loss of heat and metal.
Doctor Renmin Ma, of Berkeley, describes the standard plasmon laser predicament like this:
Metallic plasmon laser cavities generally exhibit both high metal and radiation losses. To achieve lase, these losses must be compensated by gain. Semiconductor material which supplies gain in a plasmon lasers usually have a much higher gain at cryogenic temperature than at room temperature. To compensate the extremely high losses of plasmon laser, researchers put the materials into a deep freeze where the material's gain is higher. To be kept supercooled, in vacuum chambers, cannot stop losing energy, but can increase amplification of the remaining light energy to sustain the laser operation.
Putting the plasmon laser in this specialized environment made it impossible to use in most practical situations.
Out of the vacuum chamber and into action
Ma and others at Lawrence Berkeley, came up with a solution. They used a 45 nanometer thick layer of cadmium sulfide, a compound used in solar cells and light meters. It's a photoresister, something that resists the passage of photons, but has a resistance that decreases with the amount of light it's exposed to. The cadmium sulfide was placed next to a silver surface, with a gap of 5 nanometers between them. The gap was filled with magnesium fluoride, which allows easy passage for many different wavelengths of light.
The result has been likened to a 'whispering gallery' effect. Instead of leaking here, there and everywhere, the light is passed between the surfaces of these metals and keeps itself mostly in the little gap filled with magnesium flouride. This causes less shed heat, and keeps the laser running at room temperature.
If this isn't impressive to you, think of it this way. The square of cadmium sulfide was 1 micron long and 45 nanometers thick. One micron is 1000 nanometers. Twenty to a hundred microns is the thickness of a human hair. The five nanometer gap between the cadmium sulfide and the silver was the size of a protein molecule. This tiny machine was kept running while containing large amounts of energy at room temperature.
This kind of technological breakthrough - overcoming the diffraction limit and doing it under standard temperature conditons - could change a lot of things. I automatically thought of becoming a spy, attaching tiny little lasers to my eye lashes and taking out the enemy with a flutter of my eyes. Ma has other ideas, such as "The ability to generate laser light in such a small size means that these plasmon lasers could be used for single-molecule biochemical sensing, as well as integrated ultra-fast plasmonic circuits, data storage and optical communications." Computers with components too small to see. Biochemical sensors that can pick up on just one molecule.
Or eyelash lasers. To each their own.