My initial answer was simplistic. We have to consider the Wave/Particle duality of light. When thinking about light as a particle its speed is constant. It only appears to slow down through a medium due to its interaction with the atoms within that medium. Light can be absorbed and re-emitted from atoms in the medium which takes time, but its speed remains constant. Also, light can be refracted within the medium increasing its travel distance. The ratio of the speed of light in a material to the speed of light in vacuum is defined as n, the refractive index. For example, water has a refractive index of 1.33 so light will travel at c/1.33 in water. When it comes to stopping a photon we have to switch to thinking in waves. Waves traveling at different velocities add up to form a group, which is also known as a pulse. We are able to slow down the group velocity of this pulse by using a laser beam to create a region in the material with large dispersion for certain wavelengths centered on the pulse wavelength, causing the pulse of light to travel slower.