Time-resolved Optical Transmission Measurements Research Articles

Hot electron injection driven phase transitions

We report on a general mechanism for photo-induced phase transitions. The process relies on the photo-injection of hot electrons from an adjacent metallic layer to trigger the structural dynamics of the materials of interest. This mechanism is demonstrated for the semiconductor-to-metal phase transition of VO${}_{2}$ using a 20 nm Au injection layer. The nature of the phase transition is demonstrated by time-resolved optical transmission measurements, as well as a well defined bias dependence that illustrates that the Au film is the source of nonequilibrium electrons driving the phase transition.

The Franz–Keldysh effect in shocked GaN:Mg

The band gap of uniaxially strained semi-insulating GaN:Mg has been examined using time-resolved optical transmission measurements in shock-wave experiments. For longitudinal stresses between 2.9 and 4.6 GPa, the absorption edge broadened significantly. Such a broadening is consistent with the presence of large piezoelectric fields (Franz–Keldysh effect) generated due to shock compression. For stresses greater than 4.6 GPa, however, the absorption edge remained relatively sharp. The sharp absorption onset suggests the presence of free charge carriers that screen the piezoelectric field, thereby suppressing the Franz–Keldysh effect. These observations indicate that electrical breakdown occurs at a field of 1 MV/cm. One possible mechanism that could result in free carriers is the dissociation of Mg–H complexes under the very high piezoelectric fields.

Band gap changes of GaN shocked to 13 GPa

The band gap of GaN under uniaxial-strain compression was determined using time-resolved optical transmission measurements in shock-wave experiments. Shock waves were generated by impacting the GaN samples with c-cut sapphire impactors mounted on projectiles fired by a gas gun. Impact velocities were varied to provide longitudinal stresses ranging from 4.5 to 13 GPa. An abrupt increase of the band gap is observed upon shock-wave compression, followed by a slower increase. By measuring the absorption threshold before and during shock compression of the GaN layer, the band-gap shift for a particular longitudinal stress was obtained. A linear fit to the data yields a band-gap shift of 0.02 eV/GPa. Comparison with ab initio calculations show that this slope lies between the calculated bounds for isotropic and uniaxial compression. Potential reasons for the differences are indicated.