Although it is known that the excitation of light emitters can be enhanced by surface plasmon polaritons, the corresponding
rate has not yet been determined quantitatively. Here, we combine coupled mode theory and rate equation model to
formulate the excitation rate in terms of measurable quantities and measure them systematically by using angle- and
polarization-resolved reflectivity and photoluminescence spectroscopy. For CdSeTe quantum dots deposited on 2D Au
nanohole array, we find the excitation increases by six times and the enhanced excitation rate is determined to be 8.52 meV.
Our experimental results are consistent with finite-difference time-domain simulations.
We propose to integrate the surface-enhanced Raman spectroscopy (SERS) detection capability with a surface plasmon resonance (SPR) biosensor platform. As a demonstration setup, the experimental scheme is built from a Total Internal Reflection Fluorescence (TIRF) microscope. The sample surface is a gold-coated plasmonic crystal substrate. Two oligonucleotide (ODN) probes that have been labeled with two different Raman active dyes are used to achieve a sandwich assay of target ODNs or polynucleotide. Upon complementary hybridizations between the target and probe ODNs, the target can be identified by detecting the narrow-band spectroscopic fingerprints of the Raman tags. This concept has high potential for achieving multiplexed detection of ODN targets because a very large number of probes can be incorporated to the plasmonic crystal substrate, which may find applications in gene based diseases diagnostics. We also explored the detection of single molecules and achieved some preliminary results.
In this study, we have attempted to enhance the forward emission from metal-capped
ZnO mediated by surface plasmon (SP) cross coupling. By using metal alloys and
dielectric grating, it is proposed that energy from ZnO can be resonantly coupled to
metal/ZnO SPs, transferred to metal/grating SPs and then Bragg scattered to the free
space. Although the experimental conditions are not yet been optimized, preliminary
results from Al (30 nm) capped ZnO thin film show the forward/backward emission
intensity ratio can be increased from 0.1 to 0.6 after the introduction of dielectric
grating with periodicity of ~ 800nm on metal side. The ratio is compared favorably
with the bare ZnO emission ratio of 0.5. It is thus believed SP cross coupling can be
used for fabricating high brightness top-emitting light emitting diodes (LEDs).
The thermal stability of AlOx and MgOx on ZnO films has been studied by using
photoluminescence, cathodoluminescence and current-voltage measurements. It is
found that the interfaces degrade significantly upon thermal annealing, which are
evident by the reduction of the band-edge emission as well as the increase of
conductance with annealing temperature and duration. By using secondary ion mass
spectroscopy and diffusion model, the dependence of luminescence on thermal
treatment can be well simulated and the degradation of oxide/ZnO can be attributed to
the out-diffusion of Zn into the oxide layer from ZnO. Our studies point out the
importance of developing appropriate diffusion barrier for the fabrication of low
temperature processed ZnO transistors.
Although metal/semiconductor and oxide/semiconductor junctions have long been studied in the areas of microelectronics, new phenomena and interests arise from time to time. In particular, in the realm of nanotechnology where materials are shrunk at a length scale of nanometers, the role of heterojunctions in controlling the overall characteristics of the system will become more and more important. In this paper, we will show our recent results on the light emission and charge transport properties of metal/ZnO and oxide/ZnO system at different dimensionalities. On one hand, it is found that by capping metal on ZnO, it is possible to excite the surface plasmon polaritorn at the metal/ZnO interface and resonantly couple it with the spontaneous recombination of ZnO. This results in a significant enhancement of emission efficiency of ZnO. On the other hand, providing an oxidic overlayer (AlOx) is present on ZnO, a focused electron beam can be used to locally modify optical and electrical properties of ZnO. Under electron bombardment, we find the emission profile of ZnO gradually changes from green-yellow emitting into ultra-violet emitting while the conductivity decreases by more than two orders of magnitude at the same time. Well-defined sub-micron patterns with tunable optical and electrical properties can be fabricated on 2-D ZnO films and 1-D nanoribbons by carefully controlling the dose and energy density of the electron beam. Since ZnO is a versatile material, we believe our studies will shed light on the further use of ZnO in frontier technologies such as gas sensing, display technology, catalysis, spintronics, etc.
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