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The phosphors Ca3(PO4)2:Tm3+ by co-doping Ce3+ have been synthesized by conventional high-temperature solid-state reaction method. Their spectroscopic properties in the UV-VIS-NIR range have been investigated. The first 5d crystal field level location and stokes shift have been determined from the UV excitation and emission spectra of Ca3(PO4)2:Ce3+. The three-photon NIR quantum-cutting luminescence of Tm3+ assigned to the electronic transitions of 1G4→3H4, 3H4→3F4 and 3F4→3H6 is observed, whether in Ca3(PO4)2:Tm3+ or Ca3(PO4)2:Tm3+,Ce3+. The energy transfer from Ce3+ to Tm3+ takes place with energy-transfer efficiency up to 34.5% for the Ca3(PO4)2:Tm3+,Ce3+. A cross relaxation scheme using the 5d states of Ce3+ and f-f transition of Tm3+ is proposed. The mechanism is revealed from energy level and decay measurements. The results show that the broadband absorption of Ce3+ sensitizer not only extends the spectrum conversion in UV region but also greatly enhances the photoluminescence intensities of the three-photon quantum cutting luminescence of Tm3+ doped Ca3(PO4)2. [ABSTRACT FROM AUTHOR]

Diamond thin films were deposited on WC–Co substrates by hot filament chemical vapor deposition to improve the tribological performance. The influence of the substrate surface topography was found to play an important role during the nucleation stage and the later growth rate as well. In this study, we systematically investigated the relation between substrate surface irregularity, which was evaluated by fractal dimension as well as statistical roughness parameters and the quality of the later deposited diamond film. Preseeding processes, in diamond acetone suspensions with two particle diameters, by supersonic vibrator were also implemented to investigate the effect of particular size on diamond nucleation. The original surfaces were measured with a stylus profiler and contact-mode atomic force microscopy. The diamond deposited substrates were examined by scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, and Rockwell-C indentation to study substrate topography, crystalline structure of the coating, the composition of diamond films, and adhesion between deposited layers and substrates, respectively. The synergetic influence of the substrate’s fractal dimension and the particular size of pre-seeding diamond suspension were studied and addressed. The deposited film of a WC–Co substrate with higher surface fractal dimension (>2.50), preseeded by fine diamond suspension (4–12 nm particle size) in advance, has a high diamond-rich composition and adhesion strength. [ABSTRACT FROM AUTHOR]

High-resolution transmission electron microscopy in conjunction with autocorrelation function analysis have been applied to investigate the evolution of structural order in germanium ion-implanted amorphous silicon (a-Si) layers. A high density of Si nanocrystallites as small as 1 nm in size was detected in as-implanted a-Si layers. The density of embedded nanocrystalline Si was found to diminish in a-Si layers with annealing temperature first then increase. The results are discussed in the context of free energy change with annealing temperature. [ABSTRACT FROM AUTHOR]

Presents a study that examined photoreflectance temperature dependence of graded heterojunction bipolar transistor layers. Method of the study; Results and discussion; Conclusion.

An efficient three-photon near-infrared (NIR) quantum cutting (QC) is reported in Tm3+ singly doped YVO4 polycrystalline phosphors, where an optimized content of Tm3+ is determined to be 1.0 mol. %. Upon the absorption of a visible photon around 473 nm, three NIR photons emitting at 1180, 1479, and 1800 nm can be obtained efficiently by the sequential three-step radiative transitions of Tm3+. The underlying mechanisms are analyzed in terms of the steady and dynamic fluorescence spectra measurements. Internal quantum yield is calculated to be 161.8% as a theoretical value when luminescence quenching due to defect species can be overcome. In addition, the broadband ultraviolet (UV)-excited [VO4]3- can strongly sensitize the 1G4 level of Tm3+ in the wavelength range likely from 250 to 360 nm, greatly increasing the UV photo-response and NIR fluorescent intensity of Tm3+. The further development of this broadband three-photon NIR QC material would explore the new route to improve the photo-response of novel photoelectronic devices, particularly in 250-360 nm. [ABSTRACT FROM AUTHOR]

We present photoreflectance (PR) and surface photovoltage spectroscopy (SPS) studies of GaAs1-xSbx/GaAs multiple quantum well structures with fundamental transition close to 1.3 μm. These two optical diagnostic tools are powerful supplementary techniques in the investigations of complicated quantum systems. PR gives detailed information about the heavy-holes related transitions, while SPS contains additional information concerning light-holes related ones. The comparison of experimental data and theoretical analysis based on the envelope function approximation, including strain and exciton binding energy, allows us to identify the observed PR and SPS features. The results point to the existence of weak type-I band alignment in samples being studied, and the Sb content dependent of conduction band offset is found to agree well with that reported by Wang et al. [Phys. Rev. B 70, 195339 (2004)]. [ABSTRACT FROM AUTHOR]

This work systematically investigates the influence of InAs growth conditions and superlattice parameters on the optical properties of InAs/GaAs quantum dot (QD) superlattice structures grown by molecular beam epitaxy. Using surface photovoltage spectroscopy, one directly obtains the absorption spectra up to the highest confined QD levels at room temperature. Based on photoluminescence measurements at different excitation wavelengths, a feature below the fundamental transition is attributed to the transition from uncoupled dots in the bottom layers. The QD transition energy shift was found to be correlated with material intermixing, driven by enhanced strain strength with the increase of layer number and the decrease of spacer thickness, and the growth rate of InAs. A blueshift observed in low growth rate samples is indicative of greatly enhanced intermixing. For QD superlattices grown at a relatively high deposition rate of InAs, the material intermixing effect is gradually enhanced with increasing layer numbers, which will compensate the electronically coupled effect and will prevent further energy shift toward the lower energy side. The absorption wavelengths in 10- and 30-period InAs/GaAs QD superlattices with higher growth rates are near 1.32 μm. The results suggest that these QD superlattice structures can be used as promising active media for long-wavelength QD lasers operating at room temperature. [ABSTRACT FROM AUTHOR]

We report a detailed investigation on the optical transitions of strained type II GaAs[SUB0.7]Sb[SUB0.3]/GaAs (100) multiple quantum wells. For the theoretical calculations, both of the elastic deformational potential of intrinsic compressive biaxial strain, and quantum confinement effects are included. The asymmetric photoluminescence spectra reveal the features of excited state transition and quantum confinement Stark effect at high and low temperatures, respectively. The asymmetry features have also been investigated and confirmed by low-temperature photoluminescence experiments under different excitation power. From polarized photoluminescence excitation and photoconductivity spectra, both of the type I and type II optical transitions can also be clearly identified. [ABSTRACT FROM AUTHOR]

Investigates the thermal relaxation of the photoluminescence (PL) in InAs/GaAs quantum dot superlattices having two to more than three times the critical thickness for spontaneous island formation. Role of repopulation in the PL quenching; Attribution of the temperature dependence of PL energy to the dilation of lattice and electron-lattice interaction.

Describes techniques of the photoreflectance and electroreflectance used to study the built-in electric fields and the surface Fermi levels of InP surface-intrinsic-n+ (SIN+) structures. Calculation of the Franz-Keldysh oscillations of the photoreflectance spectra; Measurement of photoreflectance spectra and electroreflectance spectra.