Sungmo Ahn

Plasmon Enhancement Mechanism for the Upconversion Processes in NaYF4:Yb3+,Er3+ Nanoparticles: Maxwell versus Förster

Rare-earth activated upconversion materials are receiving renewed attention for their potential applications in bioimaging and solar energy conversion. To enhance the upconversion efficiency, surface plasmon has been employed but the reported enhancements vary widely and the exact enhancement mechanisms are not clearly understood. In this study, we synthesized upconversion nanoparticles (UCNPs) coated with amphiphilic polymer which makes UCNPs water soluble and negatively charged. We then designed and fabricated a silver nanograting on which three monolayers of UCNPs were deposited by polyelectrolyte-mediated layer-by-layer deposition technique. The final structures exhibited surface plasmon resonance at the absorption wavelength of UCNP. The green and red photoluminescence intensity of UCNPs on nanograting was up to 16 and 39 times higher than the reference sample deposited on flat silver film, respectively. A thorough analysis of rate equations showed that the enhancement was due entirely to absorption enhancement in the strong excitation regime, while the enhancement of both absorption and Förster energy transfer contribute in the weak excitation regime. The Purcell factor was found to be small and unimportant because the fast nonradiative decay dominates the relaxation process. From the experimentally observed enhancements, we concluded 3.1× and 1.7× enhancements for absorption and Förster energy transfer, respectively. This study clearly shows the plasmon enhancement mechanism and its excitation power dependence. It provides the basis for comparison of the enhancements of various plasmonic UCNP systems in the literature. It also lays the foundation for rational design of optical plasmonic structures for upconversion enhancement.

Experimental demonstration of plasmon enhanced energy transfer rate in NaYF4:Yb3+,Er3+ upconversion nanoparticles

Energy transfer upconversion (ETU) is known to be the most efficient frequency upconversion mechanism. Surface plasmon can further enhance the upconversion process, opening doors to many applications. However, ETU is a complex process involving competing transitions between multiple energy levels and it has been difficult to precisely determine the enhancement mechanisms. In this paper, we report a systematic study on the dynamics of the ETU process in NaYF4:Yb3+,Er3+ nanoparticles deposited on plasmonic nanograting structure. From the transient near-infrared photoluminescence under various excitation power densities, we observed faster energy transfer rates under stronger excitation conditions until it reached saturation where the highest internal upconversion efficiency was achieved. The experimental data were analyzed using the complete set of rate equations. The internal upconversion efficiency was found to be 56% and 36%, respectively, with and without the plasmonic nanograting. We also analyzed the transient green emission and found that it is determined by the infrared transition rate. To our knowledge, this is the first report of experimentally measured internal upconversion efficiency in plasmon enhanced upconversion material. Our work decouples the internal upconversion efficiency from the overall upconverted luminescence efficiency, allowing more targeted engineering for efficiency improvement.

Plasmonic nanostructures for organic photovoltaic devices

Due to the increasing demand on high-efficiency organic photovoltaic (OPV) devices, light management technique has become an active research subject. Especially, plasmonic approach was proven to be suitable for application in OPV and has shown lots of successful results. In this review, we summarize recent studies on plasmonic nanostructures for OPV with their underlying enhancement mechanisms. Optical absorption enhancement by the resonant scattering and the strong plasmonic near field will be discussed for various implementation geometries including metal nanoparticles, patterned electrodes, and plasmonic metamaterials. In addition, we will also look into the electrical effects originating from plasmonic nanostructures, which inevitably affect the devices efficiency. Future research directions will be also discussed.

Nonlinear characterization of Ge 28 Sb 12 Se 60 bulk and waveguide devices

Single-mode Ge₂₈Sb₁₂Se₆₀ strip waveguides, fabricated with thermal evaporation and lift-off, were demonstrated at 1.03 µm. The linear and nonlinear optical properties of these waveguides were shown to be similar to bulk samples, with differences attributed to small variations in composition of ~4 atomic % or less. From z-scan measurements at 1.03 µm using circularly polarized, ~200 fs pulses at 374 kHz, Ge₂₈Sb₁₂Se₆₀ was found to have a nonlinear refractive index ~130 x fused silica and a two-photon absorption coefficient of 3.5 cm/GW. Given the large two-photon absorption coefficient, this material shows promise for optical limiting applications at 1 µm.

Optical Characterization of Chalcogenide Ge–Sb–Se Waveguides at Telecom Wavelengths

Nonlinear single-mode Ge 28 Sb 12 Se 60 strip waveguides were demonstrated at 1.53-1.55 μm. The waveguides were fabricated by photo-or e-beam lithography, followed by thermal evaporation and lift-off. The linear propagation loss, ranging from 4.0 to 6.1 dB/cm, is compared for waveguides under various fabrication conditions. Using measurements of the power-dependent transmission and spectral broadening, the nonlinear loss β and nonlinear refractive index n 2 of the waveguides fabricated with e-beam lithography are determined to be 0.014±0.003 cm/GW and 5±2×10 -19 m 2 /W, respectively, at 1.55 μm. Given the large measured figure of merit, n 2 /(βλ) = 2.3±0.9, this platform holds promise for nonlinear applications at telecom wavelengths.

Integrated optical and electrical modeling of plasmon-enhanced thin film photovoltaics: A case-study on organic devices

The nanoscale light control for absorption enhancement of organic photovoltaic (OPV) devices inevitably produces strongly non-uniform optical fields. These non-uniformities due to the localized optical modes are a primary route toward absorption enhancement in OPV devices. Therefore, a rigorous modeling tool taking into account the spatial distribution of optical field and carrier generation is necessary. Presented here is a comprehensive numerical model to describe the coupled optical and electrical behavior of plasmon-enhanced polymer:fullerene bulk heterojunction (BHJ) solar cells. In this model, a position-dependent electron-hole pair generation rate that could become highly non-uniform due to photonic nanostructures is directly calculated from the optical simulations. By considering the absorption and plasmonic properties of nanophotonic gratings included in two different popular device architectures, and applying the Poisson, current continuity, and drift/diffusion equations, the model predicts quantum efficiency, short-circuit current density, and desired carrier mobility ratios for bulk heterojunction devices incorporating nanostructures for light management. In particular, the model predicts a significant degradation of device performance when the carrier species with lower mobility are generated far from the collecting electrode. Consequently, an inverted device architecture is preferred for materials with low hole mobility. This is especially true for devices that include plasmonic nanostructures. Additionally, due to the incorporation of a plasmonic nanostructure, we use simulations to theoretically predict absorption band broadening of a BHJ into energies below the band gap, resulting in a 4.8% increase in generated photocurrent.

Linear and nonlinear optical properties of Ge-Sb-Se waveguides at telecom wavelengths

Sub-micron Ge-Sb-Se waveguides, fabricated by e-beam lithography, thermal evaporation, and lift-off, were characterized at 1.53-1.55 μm. Linear loss is compared under various fabrication conditions, and spectral broadening measurements reveal a large nonlinearity.

Surface plasmon enhanced infrared absorption in P3HT-based organic solar cells: the effect of infrared sensitizer (Presentation Recording)

We have theoretically and experimentally investigated the effects of Ag-grating electrode on the performance of polymer:fullerene based bulk heterojunction organic solar cells. First, an integrated numerical model has been developed, which is capable of describing both the optical and the electrical properties simultaneously. The Ag-grating patterned back electrode was then designed to enhance the absorption in sub-bandgap region of P3HT:PCBM binary devices. Laser interference lithography and metal lift-off technique were adopted to realize highly-uniform and large-area nanograting patterns. We measured almost 5 times enhancement of the external quantum efficiency at the surface plasmon resonance wavelength. However, the overall improvement in power conversion efficiency was not significant due to the low intrinsic absorption of active layer in this sub-bandgap region. We, then, investigated about the effect of surface plasmon on the ternary device of P3HT:Si-PCPDTBT:ICBA. It was demonstrated that the infrared absorption by the Si-PCPDTBT sensitizer can be substantially enhanced by matching the surface plasmon resonance to the sensitizer absorption band. Besides, we also observed an additional enhancement in the visible range which is due to the scattering effect of the gratings. An overall short-circuit current enhancement of up to 40% was predicted numerically. We have then fabricated the device by the lamination technique and observed a 30% increase in the short circuit current. Plasmon enhancement of sensitized organic solar cell presents a promising pathway to high-efficiency, broadband-absorbing polymer:fullerene bulk heterojunction organic solar cells.

Surface plasmon enhanced infrared absorption in the sensitized polymer solar cell

We have theoretically demonstrated an enhanced infrared absorption of the sensitizer in ternary polymer solar cell by introducing silver gratings at the back metal electrode. A combined model which incorporates the complex optical absorption profile and the electrical transport of the generated charge carriers was successfully developed. Using this model, we considered Si-PCPDTBT as an infrared sensitizer for P3HT:ICBA bulk heterojunction solar cells. A silver grating feature was optimized to produce a highly localized optical field inside the active polymer layer and enhance the infrared absorption of the sensitizer. Finally, an overall short-circuit current enhancement of about 40% is obtained theoretically.

Corrections to “Optical Characterization of Chalcogenide Ge–Sb–Se Waveguides at Telecom Wavelengths”

Nonlinear single-mode Ge₂₈Sb₁₂Se₆₀ strip waveguides were demonstrated at 1.53-1.55 μm. The waveguides were fabricated by photo-or e-beam lithography, followed by thermal evaporation and lift-off. The linear propagation loss, ranging from 4.0 to 6.1 dB/cm, is compared for waveguides under various fabrication conditions. Using measurements of the power-dependent transmission and spectral broadening, the nonlinear loss β and nonlinear refractive index n₂ of the waveguides fabricated with e-beam lithography are determined to be 0.014±0.003 cm/GW and 5±2×10^-19 m²/W, respectively, at 1.55 μm. Given the large measured figure of merit, n₂/(βλ) = 2.3±0.9, this platform holds promise for nonlinear applications at telecom wavelengths.