Research projects:

• Terahertz and mid-infrared quantum cascade lasers, design, fabrication, characterization and applications
• Nanowire based energetic harvesting technology
• Terahertz and mid-infrared quantum well photodetector
• Nanofabrication and Nanotechnology
• Organic/inorganic hybrid optical upconversion, infrared imaging, biophotonics
• Scanning probe microscopy, scanning voltage microscopy, scanning spreading resistance microscopy, scanning differential spreading resistance microscopy

Significant contributions to research:

2016-2020 Developed a series of energy harvesting nanogenerators, which convert mechanical energy to electrical energy. The results were published in journals: Nano Energy, Journal of Material Chemistry A (JMCA), Nanoscale. Demonstrated a large-area, broadband and sensitive carbon nanotube (CNT)-based photo-thermoelectric infrared detector, the results were published in ACS Nano. Developed novel perovskite-based piezoelectric nanogenerator. Developed terahertz quantum cascade lasers with dual injection channels.

2014-2015 Nanoscopic imaging of the beating heart of lasing quantum devices: Developed and applied novel cryogenic-temperature scanning probe microscopy (SVM) to terahertz quantum cascade lasers and interband cascade lasers. For the first time, we visualized directly, and quantitatively measured, the electric field domains (EFDs) in a lasing THz QCL, and identified the exact location of the EFD boundary. The electrostatic potential, electric field and dynamic electric charges have been measured at nanometer scales with high accuracy. This represents a significant milestone in revealing the quantum dynamics in semiconductor quantum structures that hold many important applications. One paper was published in Laser & Photonics Review, one paper was published in Nature family journal - Scientific Reports.

2010-2013 Demonstrated a record-breaking THz QCL: the device sets a new world record of operation performance, its maximum lasing temperature was measured to be 199.5 K. Proposed and demonstrated a new type of THz QCLs that are based on so called "phonon-photon-phonon" relaxation scheme. Directly observed the formation and evolution of electric field domains in lasing THz QCLs, identified the exact location of the boundary of electric field domains and discovered discrete hopping of the boundary. Developed the first organic/inorganic optical amplifier. Developed the first organic/inorganic optical upconversion imaging device.

2007-2009 Improved the device efficiency of the hybrid organic/inorganic devices by more than one order of magnitude, experimentally measured the active-region temperature of lasing terahertz quantum cascade lasers, directly observed thermal lasing quenching of terahertz quantum cascade lasers, applied time-domain terahertz spectroscopy to study the device physics of terahertz quantum cascade lasers.

2006-2007 Successfully fabricated prototype hybrid organic/inorganic devices by direct tandem integration and studied the effects of interfacial states on device performance. The devices convert near-infrared light directly to visible light (green) at room temperature.

2004-2005 Demonstrated terahertz (THz) quantum-well-cascade lasers with lasing wavelength at 104 micrometer (~2.9 THz in frequency); experimentally studied the doping effect in THz quantum cascade lasers - pinpointing the balance between free carrier absorption loss and optical gain. The experimental finding provides a guideline in optimizing THz quantum cascade laser design.

2002-2005 Developed and prototyped the first wafer-fused optical up-converter for converting light at 1.5 to 0.87 micrometer and the first pixelless 1.5 mirometer optical up-conversion imaging device, successfully developed a wafer-fused InSb-GaAs mid-infrared to near-infrared optical up-converter. This provides an alternative approach in achieving near infrared and middle infrared imaging devices with low-cost and enhanced performance and flexibility, which would potentially be used for biophotonics and bio-imaging research.

2001-2002 Pioneered the development of new methods in scanning probe microscopy to observe, with nanometric spatial resolution, two-dimensional profiles of conductivity and potential inside actively-driven lasers; resolved directly the nanoscopic reason for anomalously high series resistance encountered in ridge waveguide lasers; reported the first direct experimental observation of electron overbarrier leakage in operating buried heterostructure multi-quantum-well lasers. Together, these work have provided the first experimental visualization of the inner workings of operating semiconductor lasers and provided a platform of enabling tools for quantum semiconductor device and nanotechnology research.

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