For the original paper see ibid., vol. 47, no. 11, p. 2059-2074 (1999). In the aforementioned paper the authors used a jar of water as a substitute for a human's head and then made a statement that the dielectric constant of water is similar to most human tissues. The commenter disagrees with this statement. In reply the authors also disagree with the comments, pointing out that while a jug of water and a human head may differ significantly in function, they differ little in form from the perspective of an electromagnetic field, and the substitution of one for the other offers simple insight into antenna performance near a high-dielectric lossy mass. They concede that the dielectric constant of water is roughly 1.5 times that of average human tissue. However, they believe that this neither invalidates their observation that antenna performance is significantly affected by the presence of a watery mass, nor their statement that one may expect similar degradation of performance near a human head.

We introduce the concept of a photovoltaic band gap Epvg for amorphous solar cells. This is the minimum photon energy thermodynamically required for the generation, of two free carriers in an operating solar cell. For hydrogenated amorphous silicon the photovoltaic band gap is 1.57 eV at 1-sun illumination.

Direct 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.

The 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.

Photonic 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.

Printed 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]

We 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.

In recent years, with the advent of femtosecond pulse technology, two-photon absorption has commenced to be used for exposing photo-resists. It is natural to ask then, what is the spatial resolution of two-photon lithography? There has already been some discussion of resolution limits in two-photon, scanning confocal fluorescence microscopy. We will find that ordinary two-photon exposure of photo-resist merely enhances the photographic contrast, or gamma. While this improves the spatial resolution somewhat, it does so at the expense of a requirement for tighter control over the incident light intensity. Instead, we introduce a new type of exposure system employing a multiplicity of 2-photon excitation frequencies which interfere with one another to produce a super-resolution stationary image, exhibiting a true doubling of the spatial resolution.

We demonstrate a technique that coherently frequency shifts a direct sequence encoded optical channel by mixing it with an identically encoded RF local oscillator in a dual-electrode Mach Zehnder modulator (DE-MZM). The spectral separation would reduce the effects of cumulative shot noise and speckle, and full code orthogonality is possible since the bipolar nature of the optical field is recognized by the frequency shifting process.

Three independent methods were used to evaluate transpiration of a boreal forest: the branch bag, sap flow and eddy covariance methods. The branch bag method encloses several thousand needles and gives a continuous record of branch transpiration. The sap flow method provides a continuous record of sap velocity and an estimate of tree transpiration. The eddy covariance method typically measures evaporation rates between a forest and the atmosphere. We deployed an extra eddy covariance system below the forest to estimate canopy transpiration by difference. The three systems detected small water vapor fluxes despite a plentiful supply of energy to drive evaporation. We also observed that transpiration rates were low even when the soil was well supplied with water. Low rates of transpiration were attributed to the canopy's low leaf area index and the marked reduction in stomatal conductance as vapor pressure deficits increased. Water vapor fluxes, derived from the sap flow method, lagged behind those derived by the branch bag method by 1 to 2 h. The sap flow method also suffered from sampling errors caused by the non-uniformity of flow across the sapwood and the spatial variability of sapwood cross section throughout the forest. Despite technical difficulties associated with hourly measurements, daily totals of transpiration agreed well with values derived from micrometeorological systems.

We have achieved long-term stability in producing high-power single-axial-mode CO2 laser pulses with a hybrid oscillator (TEA laser + low-pressure section) by using a novel digital feedback system that electronically adjusts cavity length.