  We experimentally demonstrate a very large dynamic optical reflection
modulation from a simple unpatterned layered stack of phase-change materials
ultrathin films. Specifically, we theoretically and experimentally demonstrate
that properly designed deeply subwavelength GeSbTe (GST) films on a metallic
mirror produce a dynamic modulation of light in the near-infrared from very
strong reflection (R>80%) to perfect absorption (A > 99,97%) by simply
switching the crystalline state of the phase-change material. While the
amplitude of modulation can lead to an optical contrast up to 10^6, we can also
actively "write" intermediate levels of reflection in between extreme values,
corresponding to partial crystallization of the GST layer. We further explore
several layered system designs and provide guidelines to tailor the wavelength
efficiency range, the angle of operation and the degree of crystallization
leading to perfect absorption.
