Shu-Man Liu

Surface states induced photoluminescence from Mn2+ doped CdS nanoparticles

Manganese doped CdS nanoparticles were synthesized in basic aqueous solution by using mercapto acetate as capping reagents. The nanoparticles were characterized by HRTEM, EPR, photoluminescence and optical absorption measurements. Out of our expectation, doping of Mn2+ ions altered the recombination paths in CdS nanoparticles markedly. The surface stares facilitated PL from Mn2+ doped CdS nanoparticles is reported. A complete suppression of the emission from surface states at room temperature when doping with Mn2+ ions has been observed for the first time

Characterization of CdSe and CdSe/CdS core/shell nanoclusters synthesized in aqueous solution

CdSe nanoclusters overcoated with CdS shell were prepared with macapoacetic acid as stabilizer. The optical properties of CdSe nanoclusters and the influence of CdS shell on the electronic structures of CdSe cores were studied by optical absorption, photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Based on PL and PLE results and the theoretical calculation on fine structure of bandedge exciton, a model of formation of excimer within the small clusters was proposed to explain the large Stokes shift of luminescence from absorption edge observed in PL results

Relaxation of carriers in terbium-doped ZnO nanoparticles

Terbium ions were successfully incorporated in nano-sized zinc oxide particles with a doping concentration up to 3% by using a wet chemical route. Four narrow emission peaks of Tb3+ ions and a broad emission band of the surface states on ZnO nano-hosts were observed for all Tb-doped nanoparticles. Relaxation of carriers from excited states of ZnO hosts to rare earth (RE) dopants is disclosed by the fact that the emission intensity of Tb3+ centers increases with increased Tb content at the expense of the emission from surface defect states in ZnO matrix

Correlated structural and optical investigation of terbium-doped zinc oxide nanocrystals

Tb3+-doped zinc oxide nanocrystals with a hexagonal wurzite structure were successfully prepared by reaction between Zn-O-Tb precursors and LiOH in ethanol. Good incorporation of Tb3+ in ZnO nanocrystals is proved by XRD, FTIR, PL and PLE measurements. The presence of acetate complexes to zinc atoms on particle surfaces is disclosed by FTIR results. Emission from both Tb3+ ions and surface states in ZnO matrix, as well as their correlation were observed. The luminescence mechanism is discussed

Quantum dot quantum cascade infrared photodetector

We demonstrate an InAs quantum dot quantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3 μm. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantum dots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantum dots for the infrared absorption were formed by making use of self-assembled quantum dots in the Stranski–Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64 × 1011 and 4.83 × 106 Jones at zero bias were obtained at 80 K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.

Impact of the misfit dislocations on two-dimensional electron gas mobility in semi-polar AlGaN/GaN heterostructures

The mobility of two-dimensional electron gas as influenced by the charged misfit dislocations located at the interface of the semi-polar AlGaN/GaN heterostructure is quantitatively analyzed. The results indicate that the strength of the scattering due to the misfit dislocations in the semi-polar AlGaN/GaN heterointerface is comparable to the well-known scattering associated with the threading dislocations in the well-known polar c-plane AlGaN/GaN heterostructures.

Temperature independent infrared responsivity of a quantum dot quantum cascade photodetector

We demonstrate a quantum dot quantum cascade photodetector with a hybrid active region of InAs quantum dots and an InGaAs quantum well, which exhibited a temperature independent response at 4.5 μm. The normal incident responsivity reached 10.3 mA/W at 120 K and maintained a value of 9 mA/W up to 260 K. It exhibited a specific detectivity above 1011 cm Hz1/2 W−1 at 77 K, which remained at 108 cm Hz1/2 W−1 at 260 K. We ascribe the devices good thermal stability of infrared response to the three-dimensional quantum confinement of the InAs quantum dots incorporated in the active region.

Luminescence of La0.2Y1.8O3 nanostructured scintillators

For the first time, transparent La0.2Y1.8O3 nanostructured polycrystalline scintillators were fabricated by sintering nanoparticle powders at high temperatures and their scintillation properties are reported. La0.2Y1.8O3 is a host material that has never been investigated as scintillators for radiation detection. Our observations found that La0.2Y1.8O3 has an intense scintillation luminescence, a detection efficiency higher than that of YAG:Ce and a comparable energy resolution to NaI and CsI scintillators. In addition, La0.2Y1.8O3 is stable and has luminescence decay lifetime in the picosecond range which is favorable for radiation detection. The luminescence of La0.2Y1.8O3 has a large Stokes-shift and a large emission bandwidth, and the luminescence is highly temperature dependent. Different from most doped scintillators, the luminescence of La0.2Y1.8O3 is most likely from the self-trapped excitons. The discovery of La0.2Y1.8O3 scintillators opens a new door for the research of new materials for radiation detection.

High efficiency, single-lobe surface-emitting DFB/DBR quantum cascade lasers.

We demonstrate a surface-emitting quantum cascade laser (QCL) based on second-order buried distributed feedback/distributed Bragg reflector (DFB/DBR) gratings for feedback and outcoupling. The grating fabricated beneath the waveguide was found to fundamentally favor lasing in symmetric mode either through analysis or experiment. Single-lobe far-field radiation pattern with full width at half maximum (FWHM) of 0.18° was obtained along the cavity-length direction. Besides, the buried DFB/DBR grating structure successfully provided an efficient vertical outcoupling mechanism with low optical losses, which manages to achieve a high surface outcouping efficiency of 46% in continuous-wave (CW) operation and 60% in pulsed operation at room temperature. Single-mode emission with a side-mode suppression ratio (SMSR) about 25 dB was continuously tunable by heat sink temperature or injection current. Our work contributes to the realization of high efficiency surface-emitting devices with high far-field beam quality that are significantly needed in many application fields.

Phase-locked array of quantum cascade lasers with an intracavity spatial filter

We show a phase-locked array of quantum cascade lasers with an intracavity spatial filter based on the Talbot effect. All the laser arrays show in-phase operation from the threshold current to full power current with a near-diffraction-limited divergence angle. The maximum power is just about 5 times that of a single-ridge laser for an eleven-laser array device and 3 times for a seven-laser array device. The structure was analyzed by using the multi-slit Fraunhofer diffraction theory, showing very good agreement with the experimental results. Considering the great modal selection ability, simple fabricating process, and potential for achieving continuous wave operation, this phase-locked array may be a hopeful solution to obtain higher coherent power.