An increase in the optical absorption coefficient during illumination [photodarkening (PD)] is known to be described by a stretched exponential function (SEF) $1-\text{exp}\left[-{\left(t/\tau \right)}^{\beta }\right]$ , where $\tau$ and $\beta$ , respectively, are the effective response time and the dispersion parameter. $\tau$ and $\beta$ deduced experimentally depend on the thickness of films. A model calculation, assuming a series sequence of PD along the thickness direction from the illuminated surface to the back surface, produces the SEF and replicates well the experimental results.

It is known that deviation from the Drude law for free carriers is dramatic in most electronically conductive nanomaterials. We review recent studies of the conductivity of nanoscale materials at terahertz (THz) frequencies. We suggest that among a variety of theoretical formalisms, a model of series sequence of transport involving grains and grain boundaries provides a reasonable explanation of Lorentz-type resonance (non-Drude behavior) in nanomaterials. Of particular interest is why do free carriers exhibit a Lorentz-type resonance.

The time-dependent change in photocurrent during illumination has been studied in amorphous arsenic tritelluride (a-As Te ), which is an amorphous 2 3 chalcogenide material with a narrow optical bandgap. No photodegradation in the photocurrent is observed in this material, although photodegradation occurs in most wide-bandgap amorphous chalcogenides.

Although a proposal of resonance bonds in crystalline phase-change materials based on the GeSbTe system has been provided, we do not find any clear evidence in favor of the proposal. The ellipsometric study demonstrates that a change in the high frequency dielectric constant ε_∞ between the amorphous and crystalline phases is only scaled by the average bandgap (the Penn gap rule). Even for a pure antimony film, regarded as a prototype resonance bonding material, ε_∞ was found to follow the Penn gap rule. Experimentally, we did not find any evidence of a significant change in the optical transition matrix element during the phase change, which is necessary to support the idea of resonance bonds.