Intersubband transition based on a novel II-VI quantum well structure for ultrafast all-optical switching

We have achieved intersubband absorption as short as 1.58 mum in (CdS/ZnSe)/BeTe quantum wells (QWs) by employing relatively thicker BeTe barriers of 10 monolayers, covering an optical communication wavelength of 1.55 mum within its absorption band. The ISB carrier relaxation was investigated by means of femtosecond one-color pump and probe measurement at the ISB absorption peak position for samples with various well widths. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 mum. The slow decay component with a time constant of 2-4 ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Gamma(ZnSe)-X(BeTe) electron transfer is suppressed as expected from the band alignment. For applications toward ultrafast all-optical switching devices, a ridge-waveguide structure composed of (CdS/ZnSe)/BeTe QWs active and ZnMgBeSe cladding layers is proposed. Refractive indices of these layers have been examined and these data are fed to design ridge-waveguide structures. As a first step, a slab waveguide structure has been successfully grown.