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Not only do short pulses of light change the materials with which the encounter, but these changes in the material (index of refraction, &c.) then change the light! Initially, we see a beam of short-pulsed light which is essentially monochromatic (althought there is a bandwidht of approximately 15 nm near 800 nm which looks white because the Infra-red light saturates the camera!). When the light is focused through the air, two things happen. First, the light breaks down the air so that a plasma is formed. This can be seen as a tiny, bright light half way between the lens and the grating. Second, since the light so-drastically changes the index of refraction of the air in this region that the light is also then changed. Since the light is thus changing itself, the process is called "self-phase modulation". Whereas before the focus the light wavelengths are concentrated near the Infra-red, a full spectrum emerges after the focus, which we can then look at after the light diffracts off of the grating!

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