The objective of this research is to develop ultrafast all-optical switches, which are key components for the next generation optical communication networks. The approach is to utilize ultrafast carrier relaxation in intersubband transitions in wide band gap II-VI semiconductor quantum wells. Intellectual Merit: The proposed material systems have the potential to overcome many of the disadvantages associated with the materials currently used for ultrafast all-optical switches. The knowledge gained and the technology developed through the proposed research will not only enrich the spectrum of available materials for photonic devices used in telecommunication networks, but may also lead to new important applications of the proposed materials. In addition to producing devices with better performance, the outcome of the research advances the understanding of material properties of II-VI low-dimensional systems, which may lead to the discovery of new scientific phenomena. Broader Impact: The devices based on the proposed materials, if commercialized and implemented, will greatly increase the data transmission rate of telecommunication networks. The proposed research will have a substantial societal impact because multimedia and data communication are now an integral part of daily lives. The proposed research program is also poised to have extensive educational and outreach impact. Particular effort will be devoted to increase the number of students from underrepresented and disadvantaged groups in science and engineering. The students involved in the research will acquire training and knowledge in a competitive field and thus will be better prepared to enter the workforce with highly valued skills.
The project is aimed at developing wide band gap II-VI semiconductor quantum structures with intersuband absorption at the optical communication wavelength of 1.55 micrometers. During the project period we have invented a new metastable CdSe/MgSe quantum well system. The new material is based on wide band gap II-VI semiconductors and is prepared by molecular beam epitaxy (MBE - an advanced crystal growth technique) on InP substrate. To make the MgSe (which naturally favors rock-salt structure) structurally compatible with the other materials in the structure, a ZnCdSe layer that is lattice-matched to InP was inserted as a spacer. To eliminate the interband optical absorption of the InGaAs buffer layer (which has been proven to be essential for the growth of high quality II-VI semiconductors on InP), we have replaced the conventional InGaAs buffer with the InAlAs buffer, which has a wider band gap and does not absorb the optical signal at our interested wavelength of 1.55 micrometers. We have found out that high quality II-VI semiconductors can be grown on InP with an InAlAs buffer. Since the CdSe well layers have a relatively large mismatch with InP, we have inserted ZnSe layers in the structure to partially compensate the strain. We have found out that with the ZnSe strain-compensation layers, the overall material quality is significantly improved. To achieve the intersubband absorption at 1.55 micrometers, we have employed band structure engineering and fabricated coupled quantum wells. Intersubband absorption at 1.55 micrometers has been experimentally observed, which demonstrate the potential to use the material system for the fabrication of ultrafast all-optical switches for the next generation of optical communication systems. We have also studied the growth of ZnO/ZnMgO quantum well structures by plasma-assisted MBE. By optimizing the crystal growth conditions (buffer layers, growth temperature, etc.), high quality ZnO/ZnMgO quantum well samples have been prepared on c-plane sapphire substrate. We have observed, for the first time, mid infrared intersubband absorption in ZnO/ZnMgO quantum wells, suggesting that the ZnO-based quantum structures have the potential to be used for the fabrication of various intersubband photonic devices, such as emitters, detectors and switches. We have also invented II-VI semiconductor based quantum well infrared photodetectors (QWIPs) that operate in the long-wave IR and medium-wave IR spectral regions. The high performance MWIR QWIPs can operate up to room temperature with a record high responsivity of 30 A/W. These high performance detectors have high probability to be commercialized and find applications in imaging and sensing where compact room-temperature IR detectors are not currently available. The research outcomes have been disseminated to the research community through the publication of peer-reviewed papers in scientific journals (11 papers) and giving presentations at international conferences (18 presentations, including 2 invited talks). One patent has also been filed. In addition, three graduate students (2 Ph. D. students) and 7 undergraduate students (including 2 summer REU) have been trained through participating in the proposed research.
