Read the full review for this Faculty Opinions recommended article: Soil respiration in perennial grass and shrub ecosystems: Linking environmental controls with plant and microbial sources on seasonal and diel timescales.
For maximal performance solar cells should resemble semiconductor lasers, i.e., they should be constructed in the form of a double heterostructure. We have found rather good performance in SIPOS-crystalline silicon-SIPOS double heterostructure solar cells, where SIPOS≡SiOx. The processing of these solar cells gives insights into the truly outstanding performance of the n+-SIPOS: p-Si heterojunction which has a forward saturation current coefficient J0=10−14 A/cm2, or equivalently an ‘‘emitter Gummel number’’ Ge=3.3×1015 s/cm4. This suggests that crystalline silicon solar cells can be much more efficient than had been suspected.
The objective of this program is to make infrared pigments whose emissivity can be controlled. The principal approach we have taken is to make 3-dimensionally microstructured metallodielectric materials by two-photon lithography in photoresist, and back-filling with metals. In this program we demonstrated that we can quickly and efficiency fabricate 3<1 microstructures in photoresist using a diode pumped Ti-sapphire modelocked laser in combination with a mechanical scanning technology. Metal back-filling of copper into porous membranes was accomplished using electrodeposition methods and complementary fundamental studies were able to establish some of the details of this metallization process. Initial results using self-assembly methods for metallization were also obtained. Another approach for producing 3<1 metallo-dielectric structures, based on the direct writing of silver structures by two-photon processes, was also demonstrated. In addition to experimental work, this program included computational activities. An analysis of the resolution limits of two-photon lithography showed that a doubling of resolution was possible. Other studies in this program included the design of frequency selective grid structures and an analysis showing that 1-d interference filters can be used to achieve a broad reflection spectrum.
The coincidence in excitation energy between surface plasmons on silver and the GaN band gap is exploited to couple the semiconductor spontaneous emission into the metal surface plasmons. A 3-nm InGaN/GaN quantum well (QW) is positioned 12 nm from an 8-nm silver layer, well within the surface plasmon fringing field depth. A spectrally sharp photoluminescence dip, by a factor \ensuremath{\approx}55, indicates that electron-hole energy is being rapidly transferred to plasmon excitation, due to the spatial overlap between the semiconductor QW and the surface plasmon electric field. Thus, spontaneous emission into surface plasmons is \ensuremath{\approx}55 times faster than normal spontaneous emission from InGaN quantum wells. If efficient antenna structures can be incorporated into the metal film, there could be a corresponding increase in external light emission efficiency.
We have found experimental conditions for the growth of n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -SIPOS:p-Si heterojunction emitters with forward saturation current J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> Amps/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or equivalently "emitter Gummel number" G <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> = 3.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> s/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> . This outstanding figure of merit seems to rely upon the presence of a thin interfacial oxide between the SIPOS and the crystalline silicon. We invoke a model in which majority-carrier (electron) contact is made by microcrystalline grains which protrude into the interfacial oxide but minority-carrier (hole) recombination is inhibited by the small fractional area coverage of such contacts. The result is an emitter structure which is robust and relatively insensitive to variations in processing conditions.
Read moreFor maximal performance, solar cells should resemble semiconductor lasers; i.e. they should be constructed in the form of a double heterostructure. This configuration is also sometimes called ''minority carrier mirrors''. We have found rather good performance in SIPOS-crystalline silicon-SIPOS double heterostructures as well as in a p-n homojunction made entirely of SIPOS. This sheds some light on the truly outstanding performance of the n/sup +/-SIPOS: Si heterojunction which has a J /sub o/ = 10/sup -14/ Amps/cm/sup 2/. It has been recognized for some time that the structure of an ideal solar cell should resemble that of a semiconductor laser. The solar cell should be built in the form of a double heterostructure. In this configuration, a narrow bandgap active layer is sandwiched between two wide bandgap layers of opposite doping. The wide bandgap materials may be called ''minority carrier mirrors'' although this term is more frequently applied to high-low homojunctions at the rear of solar cells.
Read moreA new type of metallic electromagnetic structure has been developed that is characterized by having high surface impedance. Although it is made of continuous metal, and conducts dc currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, this new surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The surface can be described using solid-state band theory concepts, even though the periodicity is much less than the free-space wavelength. This unique material is applicable to a variety of electromagnetic problems, including new kinds of low-profile antennas.
Read moreThe Internet has created a boom in long-distance optical communications. Web surfers click away and download ever-larger files, oblivious to their distance from a Web host. As a result the demand for capacity in undersea optical-fibre communications is escalating. A simple way to increase the capacity is to send many separate optical wavelengths through the same fibre, a technique known as wavelength division multiplexing. However, there is a limit to the optical power that can be used to send information along a fibre, and this – rather than the bandwidth of the fibre, which is prodigious – limits the capacity of optical fibres to carry information.
Read moreThe micrometeorological flux measurement technique known as relaxed eddy accumulation (REA) holds promise as a powerful new tool for ecologists. The more popular eddy covariance (eddy correlation) tech- nique requires the use of sensors that can respond at fast rates (10 Hz), and these are unavailable for many eco- logically relevant compounds. In contrast, the use of REA allows flux measurement with sensors that have much slower response time, such as gas chromatography and mass spectrometry. In this review, relevant micro- meteorological details underlying REA are presented, and critical analytical and system design details are discussed, with the goal of introducing the technique and its potential applications to ecologists. The validity of REA for measuring fluxes of isoprene, a photochemi- cally reactive hydrocarbon emitted by several plant species, was tested with measurements over an oak- hickory forest in the Walker Branch Watershed in eastern Tennessee. Concurrent eddy covariance mea- surements of isoprene flux were made using a newly available chemiluminesence instrument. Excellent agreement was obtained between the two techniques (r 2 a 0.974, n a 62), providing the first direct com- parison between REA and eddy covariance for mea- suring the flux rate of a reactive compound. The influ- ence of a bias in vertical wind velocity on the accuracy of REA was examined. This bias has been thought to be a source of significant error in the past. Measurements of normalized bias (w=rw) alone would lead us to think that a large potential error exists at this site. However, with our isoprene data and through simulations of REA with fast-response H2O and CO2 data, we conclude that accurate REA flux measurements can be made even in the presence of a bias in w.
Read morePhotonic crystals behave toward light waves as semiconductors do toward electron waves. Yablonovitch discusses a report by Noda et al., who have made a photonic crystal with unprecedented performance, using GaAs, the best material for integration into optoelectronic devices. According to Yablonovitch, the work thus represents a significant step toward photonic integrated circuits.
Read moreDirect collisionless multiphoton (MP) excitation of the triplet vibronic manifold of biacetyl is reported. Following a dye laser pulse which prepares some of the biacetyl molecules in the triplet metastable state, the system is irradiated by an intense 20 ns 9.6μ CO2 pulse. The CO2 radiation induces fast quenching of the phosphorescence emission from the 3Au excited molecules. It also induces an emission signal in the fluorescence spectral region of biacetyl. This signal is related to an inverse electronic relaxation (IER) from excited triplet vibronic levels into isoenergetic singlet 1Au vibronic levels. Analysis of the induced luminescence signals provides information on the collisionless MP prompted vibrational distribution. Excitation with 10.6μ CO2 pulses leads to the simultaneous MP pumping of both the ground and triplet manifolds. The generation of blue emission signals in this experiment bears a close resemblance to recent observations of prompt visible emission due to MP pumping of ground state molecules. General expressions for the emission intensities are derived with special emphasis on the specific features of MP vibrational distributions. The detectability of MP induced emission signals is discussed.
Read morePrinted antennas exemplified by the microstrip patch antenna offer an attractive solution to compact, conformal and low cost design of modem wireless communications equipment, RF sensors and radar systems. Recent applications have pushed the frequency well into the ram-wave region even in the commercial arena as evidenced by the worldwide race to develop advanced collision warning radar systems for automobiles at the 76 GHz band.[1] Microstrip-based planar antennas fabricated on a substrate with a high dielectric constant (Si, GaAs and InP) are strongly preferred for easy integration with the MMIC RF front-end circuitry. However, it is well known that patch antennas on high dielectric constant substrates are highly inefficient radiators due to surface wave losses and have very narrow frequency bandwidth (approximately one to two percent). This situation becomes extremely severe as applications move to higher frequencies, resulting in patch antennas with reduced gain and efficiency as well as an unacceptably high level of cross polarization and mutual coupling within an array environment. Therefore, much effort has been made recently to realize high efficiency patch antennas on high permittivity substrates, including using the latest micromachining technology.[2,3]
Read moreWe measure a maximum group delay of 22.6 ps for a light pulse propagating through a 1 cm long fiber Bragg grating when the frequency of the light is tuned near the band edge of the grating. Our measurements are performed in the time domain with single picosecond resolution using wavelength-tunable pulses of 0.5 nm bandwidth spectrally sliced from a mode-locked laser. Our experimental results are qualitatively confirmed by our numerical simulations. Promising applications include optical delay elements for phased-array radar and encoders/decoders in spread-spectrum code-division multiple-access systems. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 20: 17–21, 1999.
Read more
.webp){kind=small}