  The optical properties of phase-change materials (PCM) can be tuned to
multiple levels by controlling the transition between their amorphous and
crystalline phases. In multi-material PCM structures, the number of discrete
reflectance levels can be increased according to the number of PCM layers.
However, the effect of increasing number of layers on quenching and
reversibility has not been thoroughly studied. In this work, the phase-change
physics and thermal conditions required for reversible switching of single and
multi-material PCM switches are discussed based on thermo-optical phase-change
models and laser switching experiments. By using nanosecond laser pulses, 16
different reflectance levels in Ge2Sb2Te5 are demonstrated via amorphization.
Furthermore, a multi-material switch based on Ge2Sb2Te5 and GeTe with four
discrete reflectance levels is experimentally proven with a reversible
multi-level response. The results and design principles presented herein will
impact active photonics applications that rely on dynamic multi-level
operation, such as optical computing, beam steering, and next-generation
display technologies.
