Once upon a time I was working with a high power atmospheric laser. To check the beam alignment and where the focal point was I had to hold piece of cardboard in front of the device’s aperture and press a handheld button. A loud bang would occur and the cardboard would turn instantly black. If the mark was a perfect square it meant was in line and the louder the noise the closer to the desired distance I was. At one point I held the cardboard far past the focal point and pressed the button multiple times. It made me jump when the last exposure let out the same massive bang that the perfect one had. It turned out that the heat, gas and vapour given off lensed the laser back into a focal point even at that distance resulting in the shocking noise. I was reminded of this story while planning today’s post, for it is on the focusing and scattering of lasers in free space due to an induced plasma.
Scientists have found that with a strong enough laser the air it moves through becomes ionised. Passing through the produced ions can cause the originally near infra red laser to give off white light that can then scatter further and also diffract based in the plasma’s nature. Understanding this is important as it allows the structure of the plasma to be deduced based on the diffraction pattern produced and also means that this effect can be mitigated in actual experimental or industrial equipment. When the laser is allowed to focus a high pitch noise is heard similar to the bang in my experience. The patterns projected on a screen by this laser are intricate and multichromatic, it seems that the projections are very sensitive perhaps being chaotic in nature meaning the slightest change in parameters can lead to the biggest effect.