Tolk and his co-workers studied the "conduction band discontinuity" between such pairs of semiconductors as gallium arsenide and gallium aluminum arsenide
by irradiating the interface with infrared light from the Vanderbilt free-electron laser.
Using a relatively thick layer of gallium arsenide as a crystal template, they stack alternating layers of gallium arsenide and aluminum arsenide
molecules to form two mirror-like regions.
Next comes the thin laser assembly: A layer of semiconductive gallium arsenide underlies four more layers that alternate between less-conductive gallium aluminum arsenide
and gallium arsenide.
The California scientists build even more intricate structures, called quantum-well-wire arrays, by using molecular beam epitaxy to lay down vertically and sometimes horizontally alternating layers of gallium arsenide and less conductive materials, in this case aluminum arsenide
and aluminum gallium arsenide.