(Top left) An optical fiber consists of a high-refractive index core (red) surrounded by a “cladding” (light blue) with lower index of refraction. The fiber can pipe light because the gradient in the refractive index causes total internal reflection of the light. (Top right) If the core of the guide has a lower refractive index as in the plasma wake of an intense filament of light, an optical beam injected in the pipe will leak instead of being efficiently guided. (Bottom) To create a transient waveguide in air, Milchberg and his colleagues generated a quadruplet of femtosecond pulses that generated expanding hot air in their wakes. The superposition of these hot air waves in the center of the quadruplet (red) has a higher refractive index than the surrounding air and can create a waveguiding effect for subsequent pulses of light for up to several milliseconds.