Even relatively short exposure of glass to strong bases (e.g., NaOH, KOH) will fuse a ground glass stopper to a ground glass neck of a bottle.
This is because the base actually dissolves the glass.
I suspect that the action of the base may cause the formation of deeper, and more, peaks and valleys on the surface of the glass.
Your laser beam may then actually sample a surface area that is 10, 100 , 1000 etc times larger than on untreated glass.
What order of magnitude increase in signal do your expts give???