Sulfur-doped zinc oxide (ZnO) nanowires grown on gold-coated silicon substrates inside a horizontal tube furnace exhibit remarkably strong visible wavelength emission with a quantum efficiency of 30%, an integrated intensity 1600 times stronger than band edge ultraviolet emission, and a spectral distribution that closely matches the dark-adapted human eye response. By comparatively studying sulfur-doped and undoped ZnO micropowders, we clarify how sulfur doping and nanostructuring affect the visible luminescence and the underlying energy transfer mechanisms.

We theoretically predict and experimentally demonstrate inhibition of linear absorption for phase and group velocity mismatched second and third harmonic generation in strongly absorbing materials, GaAs in particular, at frequencies above the absorption edge. A 100-fs pump pulse tuned to 1300nm generates 650nm and 435nm second and third harmonic pulses that propagate across a 450μm-thick GaAs substrate without being absorbed. We attribute this to a phaselocking mechanism that causes the pump to trap the harmonics and to impress them with its dispersive properties.

Surface-enhanced Raman Scattering (SERS) is studied in sub-wavelength metallic gratings on a substrate using a rigorous electromagnetic approach. In the ultraviolet SERS is limited by the metallic dampening, yet enhancements as large as 10(5) are predicted. It is shown that these enhancements are directly linked to the spectral position of the plasmonic band edge of the metal/substrate surface plasmon. A simple methodology is presented for selecting the grating pitch to produce optimal enhancement for a given laser frequency.

Significant improvement of structural and optical qualities of GaN thin films on sapphire substrates was achieved by metal organic chemical vapor deposition with in situ SiN x nanonetwork. Transmission electron microscope ͑TEM͒ studies revealed that screw-and edge-type dislocations were reduced to 4.4ϫ 10 7 and 1.7ϫ 10 7 cm −2 , respectively, for a ϳ5.5-m-thick layer. Furthermore, room temperature carrier lifetimes of 2.22 and 2.49 ns were measured by time-resolved photoluminescence ͑TRPL͒ for samples containing single and double SiN x network layers, respectively, representing a significant improvement over the previous studies. The consistent trends among the TEM, x-ray diffraction, and TRPL measurements suggest that in situ SiN x network reduces line defects effectively as well as the point-defect-related nonradiative centers.

We demonstrate second and third harmonic generation from a GaP substrate 500 m thick. The second harmonic field is tuned at the absorption resonance at 335 nm, and the third harmonic signal is tuned at 223 nm, in a range where the dielectric function is negative. These results show that a phase locking mechanism that triggers transparency at the harmonic wavelengths persists regardless of the dispersive properties of the medium, and that the fields propagate hundreds of microns without being absorbed even when the harmonics are tuned to portions of the spectrum that display metallic behavior. , that the inhomogeneous component of the second harmonic ͑INH-SH͒ signal travels at the group velocity of the pump pulse. In a recent study phase and group velocity matching were demonstrated in lithium niobate in the range of transparency for all fields. The INH-SH was shown to refract at the same angle, and travel at the same velocity, as the pump.5 At the same time, the homogeneous SH component refracts and travels according to the values one expects from material dispersion at that frequency.The inhomogeneous component is generally difficult to observe because it travels locked under the pump pulse, with relatively low conversion efficiencies. In a recent study the results were generalized to include third harmonic generation ͑THG͒ in both positive and negative index media.6 When a pump signal traverses an interface into a nonlinear medium it generates SH and/or TH fields. Each harmonic has two parts: ͑i͒ a homogeneous portion that walk-off from the pump field; ͑ii͒ an inhomogeneous component phase-and velocitylocked to the pump, with no energy transfer between the fields except at interface crossings. The key observation here is that the INH-SH has a k-vector always double that of the pump, even for large phase mismatches. Theoretical findings thus suggest that the INH-SH signal experiences an effective complex index of refraction given by 2k c / 2 = k c / = n , i.e., the same index of refraction as the pump pulse.In Ref. 7, a pump pulse tuned at 1300 nm was launched into a slab of GaAs 500 m thick. Transmitted SH and TH signals were detected at 650 nm and 433 nm, respectively, far below the absorption band edge ͑ϳ900 nm͒. Simulations showed that only the inhomogeneous components propagate. Further experimental evidence 8 shows that the INH-SH is enhanced by several orders of magnitude compared to bulk 7 in an opaque GaAs cavity environment thanks to pump and INH-SH field localization and overlap. 9 The results of more experiments carried out in a high-Q GaAs cavity 10 hint that the INH-SH signal ͑612 nm͒ can achieve conversion efficiencies of order 10 −3 with pumping intensities as low as 0.15 MW/ cm 2 inside the cavity. We now ask the following question: does this phenomenon hold for harmonic fields tuned at frequencies in the metallic range? In other words, will a harmonic field propagate if it happens to be tuned in a region where sign͑͒ sign͑͒, where one expects no propagating solutions? The short answer to this ...

Ultrafast time-resolved photoluminescence spectroscopy following one-and two-photon excitations of ZnO powder is used to gain unprecedented insight into the surprisingly high external quantum efficiency of its "green" defect emission band. The role of exciton diffusion, the effects of reabsorption, and the spatial distributions of radiative and nonradiative traps are comparatively elucidated for the ultraviolet excitonic and "green" defect emission bands in both unannealed nanometer-sized ZnO powders and annealed micrometersized ZnO:Zn powders. We find that the primary mechanism limiting quantum efficiency is surface recombination because of the high density of nonradiative surface traps in these powders. It is found that unannealed ZnO has a high density of bulk nonradiative traps as well, but the annealing process reduces the density of these bulk traps while simultaneously creating a high density of green-emitting defects near the particle surface. The data are discussed in the context of a simple rate equation model that accounts for the quantum efficiencies of both emission bands. The results indicate how defect engineering could improve the efficiency of ultraviolet-excited ZnO:Zn-based white light phosphors.

A series of continuous-wave spectroscopic measurements elucidates the mechanism responsible for the technologically important green emission from deep-level traps in ZnO:Zn powders. Analysis of low-temperature photoluminescence (PL) and PL excitation spectra for bound excitons compared to the temperature-dependent behavior of the green emission reveals a deep correlation between green PL and specific donor-bound excitons. Direct excitation of these bound excitons produces highly efficient green emission from near-surface defects. When normalized by the measured external quantum efficiency, the integrated PL for both excitonic and green emission features grows identically with excitation intensity, confirming the strong connection between green emission and excitons. The implications of these findings are used to circumscribe operational characteristics of doped ZnO-based white light phosphors whose quantum efficiency is almost twice as large when the bound excitons are directly excited.

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