Lasers come in many variations of light: Red, blue, infrared, ultraviolet and so on. Now there is a laser that produces non-light – the dark pulse laser. Developed by a joint project of the National Institute of Standards (NIST, USA) and Joint Institute for Laboratory Astrophysics (JILA, University of Colorado, USA), the dark pulse laser produces regular periods of non-light (a.k.a. dark) against a background stream of light. Two questions: Why and how?
The why question is rather easy to answer: The dark pulse laser uses infrared frequencies, which come in very short wavelengths. The short wavelength produces a pulse with a span of about 90 picoseconds, that is 90 trillionths of a second. This very short pulse has advantages in certain applications; communications, for example, where the ultra-short pulses tend to transmit (propagate) without distortion. It could also be used for ‘exposure’ as in a camera, to capture events happening in extremely short periods of time. Most of the applications will take advantage of the short duration of this laser’s pulse.
The how question is a little more involved, mainly because it involves quantum mechanics. Anything quantum tends to become somewhat murky, even though with laser technology it is a relatively standard phenomenon. In any case, the dark pulse laser uses quantum dots (or qdots). Quantum dots are a type of semiconductor (think of semiconductors in computer chips), which are crystals that ‘excited’ by an electric current store energy until, in a burst, they emit the energy at a specific wavelength (usually within the spectrum of light). The size and electrical properties of the quantum dot crystals can be controlled with great precision.
In the dark pulse laser millions of quantum dots (all about the same size of 10 nanometers, that is, billionths of a meter) in a ‘laser cavity’ are subjected to a small electric current until they begin to emit light. Since there are millions of them, all emitting at the same frequency and voila – laser light. Now comes the interesting part: After emitting light the qdots ‘rest’ (recover energy) for all of 1 picosecond, but recover more slowly from their energy loss as a group in the laser cavity (about 200 picoseconds). This sets up a cycle of energy gain and loss increasingly dominated by the loss period, until the laser reaches a steady state of repeating dips in intensity of about 70%, a very substantial drop or ‘darkness’ against a background of the emitted laser light. Hence, the name ‘dark pulse.’
The NIST/JILA dark pulse laser, using all ‘homegrown’ quantum dots, is the first to produce the pulses without needing post emission electrical or optical shaping – in short, a simpler, purer form of quantum dot crystal that produces more precise pulses.
The results, as described in the journal Optics Express represent just a step in developing the dark pulse laser technology. Like all laser technologies, the path leads down the trials of production – attempting to take instrumentation and procedures that work under the extreme control of a laboratory, and make them work in ‘other locations.’ Eventually, if all goes well, this leads to commercial production.