Daytime radiative cooling has attracted extensive research interest due to its potential impact for energy sustainability. To achieve subambient radiative cooling during the daytime, a white surface that strongly scatters incident solar light is normally desired. However, in many practical applications (e.g., roofing materials and car coatings), colored surfaces are more popular. Because of this, there is a strong desire to develop colorful surfaces for radiative cooling. We summarize the general design criteria of radiative cooling materials with different colors and discuss the limitations in cooling performance. Major efforts on this specific topic are reviewed with some suggested topics for future investigation.
Vapor condensation plays a crucial role in solar water-purification technologies. Conventional condensers in solar water-purification systems do not provide sufficient cooling power for vapor condensation, limiting the water production rate to 0.4 L m-2 hour-1. On the other hand, radiative dew condensation, a technique used by existing radiative dew condensers, only works at nighttime and is incompatible with solar water-purification technologies. Here, we develop daytime radiative condensers that reflect almost all solar radiation, and can thus create dew water even in direct sunlight. Compared to stateof- art condensers, our daytime radiative condenser doubles the production of purified water over a 24-hour period.
The Mid Infrared MIR wavelength range offers many advantages in different applications. Chemical and biological detection are one of these applications, as it contains the absorption fingerprints of many gases and molecules. In addition integrated plasmonics are suitable platform for high sensitivity on chip sensors. In this paper we propose plasmonic Mach-Zehnder Interferometer (MZI) working as a gas sensor near the absorption fingerprints of many gases in the mid-infrared region. The proposed MZI contains a vertically stacked metal-insulator-metal (MIM) and metalinsulator (MI) waveguide. The sensitivity of MI waveguide is lower at higher wavelengths and also lower for gaseous medium than for liquid medium. In addition the losses of the MIM waveguide with oxide layer as insulator are much larger than the losses of the MI waveguide with gas as insulator which will result in poor visibility interferometers. Using a high index layer above the metal of the MI waveguide the sensitivity of the waveguide to gaseous in the mid infrared has been significantly enhanced. This layer also balances the intrinsic losses of both MI and MIM waveguides. The thickness and the refractive index of this layer have been optimized using finite difference modal analysis. Using this layer high sensitivity and high figure of merit (FOM) have been achieved for our MZI. This structure offers simple fabrication and low cost sensor that is suitable for rapid, portable and high throughput optical detection using multiplexed array sensing technique.
One of the key issues limiting the efficiency of organic solar cells is the narrow absorption band of the polymer active layer. Thus, a huge amount of the incident sunlight is lost. Here, a new structure is theoretically proposed achieving wide band absorption in organic solar cells using multifunctional TiN nanowires. In addition to the plasmonic properties of TiN, it was reported that TiN has the capability to produce free carriers upon light absorption. Thus, the structure is based on the ability to collect these photo-generated carriers.
Using the combination of TiN and polymer significantly broadened the absorption band due to the ability of TiN to localize light inside P3HT:PC70BM in addition to its ability to absorb light at longer wavelengths. The optimized structure enhanced the absorbed power by 95% and the optimal short circuit current by 123% over the same structure without the TiN nanowires. Electric field distribution is studied at different wavelengths to gain further insight on the localization of light inside the structure.
A fiber based plasmonic sensor design is proposed. In principle, both the top surface insulator/metal interface and bottom surface can support SPP decoupled modes. The combination of sensitive interferometric techniques and the optimization process of the design and the material yields to enhanced sensitivities in range of 11000 nm/RIU.
We propose a novel structure with two input and output silicon waveguide ports separated by the Insulator-Metal- Insulator channel deposited on silicon nitride base. In principle, both the top surface insulator/metal interface and bottom surface can support SPP a decoupled modes. Once the SPP modes excited input silicon waveguide, the SPP signals from the two optical branches (the top and bottom interfaces) propagate to the output silicon waveguide. At the output waveguide both branches interfere with each other and modulate the far-field scattering. The top surface is considered as the sensing arm of this plasmonic Mach-Zehnder interferometer (MZI). The bottom surface is considered as the reference arm of the sensor. High sensitivity and small foot print is achieved using this integrated simple plasmonic design. The combination of sensitive interferometric techniques and the optimization process of the design and the material yields to enhanced sensitivities up to 3000 nm/RIU.
We demonstrate absorption improvement in organic solar cells due to the incorporation of TiN nanopatterned back electrode. Organic solar cells (OSC) have already reached 10% power conversion efficiency (PCE), which made them comparable to commercial solar cells. Localizing light using plasmonic nanostructures has the potential to overcome OSC absorption limitations and thus further improve their PCE. Using a C-MOS compatible, cheap and abundant material for light trapping could facilitate the commercialization of OSC. This work theoretically shows that the replacement of Ag nanopatterned back electrode with TiN in plasmonic OSC gives enhanced performance. In addition, the incorporation of TiN nanoparticles inside the active layer has been studied and analyzed.
Although Raman spectroscopy has been commercialized, low-cost and large-area surface enhanced Raman spectroscopy (SERS) substrates with localized enhanced field are heavily required. However, currently dominant manufacturing techniques are expensive and complicated for large-area fabrication. Furthermore, most SERS substrates can only be used for individual excitation wavelengths. In this work, we will report an ultra-broadband super absorbing metasurface to enhance SERS signals in a broadband region (i.e. from 450 nm to 1000 nm). The design consisting of an Ag ground plate, a SiO2 spacer, and a layer of Ag nanoparticles was fabricated using simple film deposition and thermal annealing techniques. A broadband absorption over 80% from 414 nm to 956 nm was obtained, resulting in localized field enhancement between adjacent nanoparticles. We employed this metasurface to test its broadband SERS signal by adsorbing 1,2-Bis(4-pyridyl)-ethylene (BPE) molecules on top of it. We employed 5 laser lines (i.e., 514, 532, 633, 671 and 785 nm) to excite the sample and observed fingerprint signature of BPE molecules under all 5 excitation wavelengths with the average enhancement factor up to 5.3×107. Therefore, the designed SERS substrate can work for almost “all” available excitation wavelengths over a broadband, which is particularly useful for sensing a broad spectrum of chemicals on the same chip.
Potential solar energy applications of metamaterial absorbers require spectrally tunable resonance to ensure the
overlap with intrinsic absorption profiles of active materials. Although those resonance peaks of metamaterial
absorbers can be tuned precisely by lithography-fabricated nanopatterns with different lateral dimensions, they are
too expensive for practical large-area applications. In this work, we will report another freedom to tune the spectral
position of the super absorbing resonance, i.e. the spacer thickness. The structure was fabricated by evaporating an
optically opaque metallic ground plate, a dielectric spacer layer, and a top metallic thin film followed by thermal
annealing processes to form discrete nanoparticles. As the spacer thickness increases from 10-90 nm, two distinct
shifts of the absorption peak can be observed [i.e. a blue-shift for thinner (10-30 nm) and a red-shift for thicker
spacer layers (30-90 nm)]. To understand the physical mechanism, we characterized effective optical constants of
top nanopattern layer and loaded them into numerical simulation models. A good agreement with experimental data
was only observed in the thick spacer region (i.e. 30-90 nm). The optical behavior for thinner spacers cannot be
explained by effective medium theory and interference mechanism. Therefore, a microscopic study has to be
performed to reveal strongly coupled modes under metallic nanopatterns, which can be interpreted as separate
antennas strongly coupled with the ground plate. Since the resonant position is sensitive to the spacer thickness, a
tunable super absorbing metasurface is realizable by introducing spatial tunable materials like stretchable chemical/
biomolecules.
Absorption enhancement by a double plasmonic nanostructure in molecular organic photovoltaics (OPVs) is theoretically investigated. The structure consists of a periodic array of metal nanodiscs on one side of the OPV active layers and a thin metal nanohole array on the other side. Excitation of coupled modes of localized surface plasmon polaritons at the nanodiscs and short-range surface plasmon polaritons at the nanohole array causes the electromagnetic field to be highly concentrated within the organic active layers, leading to a polarization-independent, broadband absorption enhancement in the visible and near-infrared portion of the solar spectrum. Calculations show that an optimized double plasmonic structure can enhance the total photon absorption by >125% for molecular OPVs based on a double heterojunction of an electron donor/hole transporter and an electron acceptor/transporter.
A high-power AlGaInP laser diode with current-injection-free region near the facet is successfully fabricated by metaorganic chemical vapor deposition (MOCVD) using the (100) direction n-GaAs substrates with a misorientation of 15 deg toward the (011) direction. The maximum continuous wave output power is about 90 mW for the traditional structure. In comparison, the maximum output power is enhanced by about 67%, and achieves 150 mW for LDs with current-infection-free regions. The fundamental transverse-mode operation is obtained up to 70 mW. Output characteristics at high temperatures are also improved greatly for an LD with a current-injection-free region, and the highest operation temperature is 70 °C at 50 mW without kink. The threshold current is about 33 mA, the operation current and the slope efficiency at 100 mW are 120 mA and 0.9 mW/mA, respectively. The lasing wavelength is 658.4 nm at room-temperature 50 mW.
A Very-Small-Aperture Laser with a 250X500 nm2 aperture has been created on a 650nm edge emitting LD. The highest far-field output power is 1.9mW and the power per unit emission area is about 15mW/μm2. The special fabrication process and high output power mechanism are demonstrated respectively. The near-field distribution properties are also analyzed theoretically and experimentally.
In AlGaInP/GaInP multi-quantum well (MQW) lasers, the electron leakage current is a much more serious problem than that in laser diodes with longer wavelength. To further improve the output performance, the leakage current should be analyzed. In this letter, the temperature dependence of electrical derivative characteristics in AlGaInP/GaInP multi-quantum well lasers was measured, and the potential barrier for electron leakage was obtained. With the help of secondary ion mass spectroscopy (SIMS) measurement, theoretical analysis of the potential barrier was presented and compared with the measurement result. The influence of p-cladding doping level and doping profile on the potential barrier was discussed, and this can be helpful in metalorganic chemical vapor deposition (MOCVD) growth.
Usually in the calculation of valence subband structure for III-V direct bandgap material, axial approximation had been used in the Luttinger-Kohn model to simplify the computational efforts. In this letter, the valence subband structure for the GaInP/AlGaInP strained and lattice-matched quantum wells was calculated without axial approximation, on the basis of 6×6 Luttinger-Kohn Hamiltonian including strain and spin-orbit splitting effects. The numerical simulation results were presented with help of the finite-difference methods. The calculation results with/without axial approximation were compared and the effect of axial approximation on the valence subband structure was discussed in detail. The results indicated that there was a strong warping in the GaInP valence band, and axial approximation can lead to an error when k was not equal to zero, especially for compressively strained and lattice-matched GaInP/AlGaInP quantum wells.
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