Determine the refractive index profile

To find out the horizontal refractive index distribution and verify our theory, we repeated the experiment in Figure 3 of the main text. The He-Ne probe beam is weakly focused in to solution using a 250 mm lens to get a finer probe beam (diameter <0.5 mm). Thus the measured horizontal deflection can be more accurate. Then, we changed the distance between the focused probe beam and the boundary of the pump beam, and the deflection of the probe beam was recorded. Finally, after a serial procedure of data processing, we get the refractive index profile along the x direction from the center of the pumped area to 4 mm outside the pumped area.