Large Spectral Width Research Articles

Nd:glass diode-pumped regenerative amplifier, multimillijoule short-pulse chirped-pulse-amplifier laser.

We have built a diode-pumped Nd:glass regenerative amplifier that is able to produce energies up to 20 mJ within a 470-fs pulse duration at a 1-Hz repetition rate. We obtained this amplifier by using specific intracavity components such as a phase mirror and a birefringent filter to generate a large spatial mode and a large spectral width.

Optically preamplified receiver performance due to VSB filtering for 40-Gb/s optical signals modulated with various formats

Optically preamplified receiver performance according to the vestigial sideband (VSB) filtering has been numerically investigated for 40-Gb/s optical signals modulated with nonreturn-to-zero, duobinary nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed RZ, and duobinary carrier-suppressed RZ formats. The VSB filtering enables the spectral widths of NRZ, duobinary NRZ, and RZ signals to be reduced without severe power penalties at the receiver. On the other hand, carrier-suppressed RZ and duobinary carrier-suppressed RZ signals have no large advantages over VSB filtering because of the characteristics of their signals. Our results suggest that RZ signals are the most suitable modulation format for VSB filtering, without considering the filter loss, because of the tolerance of the intersymbol interference and a large spectral width. However, duobinary NRZ signals are the most suitable modulation format for VSB filtering, considering the filter loss, because of their narrow spectral width.

The role of Pc1‐2 waves in spectral broadening of SuperDARN echoes from high latitudes

Previous studies of Super Dual Auroral Radar Network (SuperDARN) echoes from high latitudes have shown large spectral widths (≥200 m/s), which cannot be interpreted in terms of ionospheric drifts. A recent model of André et al. [1999, 2000a, 2000b] invoked modulation of ionospheric plasma drift velocity by ultra low frequency (ULF) waves within the Pc1‐2 band (0.1–5 Hz). Their simulated signal was processed in the same way as real radar echoes, and the output spectral width was much larger than the input signal bandwidth. This was attributed to signal undersampling. After re‐examining their model we found that the spectral broadening was caused by the way the signal was simulated. This paper presents a revised model that gives spectral width estimates close to the input signal bandwidth. An alternative explanation of the experimental data involves spatio‐temporal non‐uniformity of the ionospheric irregularities due to particle precipitation.

High-Al-content crack-free AlGaN/GaN Bragg mirrors grown by molecular-beam epitaxy

We report on the growth by molecular-beam epitaxy on 2 in. sapphire substrates of crack-free AlxGa1−xN/GaN distributed Bragg reflectors (DBRs) with high-Al composition (x=0.5). This is achieved by introducing a thick AlN interlayer and strain mediating AlyGa1−yN layer between the substrate and DBR. The relatively larger refractive index ratio between Al0.5Ga0.5N and GaN permits one to obtain a quite large spectral stopband width (49 nm) and a high reflectance value (69%) for only eight mirror periods.

First Jicamarca radar observations of two-streamEregion irregularities under daytime counter equatorial electrojet conditions

We report the first Jicamarca Radio Observatory (JRO) observations of two‐stream irregularities under daytime counter equatorial electrojet (CEEJ) conditions, made with a VHF radar. This type of irregularity, although predicted by linear theory, has been observed only very few times at other sites (once in Africa and once in India). In the first 40 years of the existence of JRO, these irregularities were not observed. Since June 2000 and November 2001, we have observed them, surprisingly on seven different days. Our observations reveal that CEEJ irregularities appear only when large reversed zonal electric fields exist. We have used the horizontal component of the magnetic field due to the equatorial electrojet (ΔH) as a proxy of the zonal electric fields. Initially, the CEEJ echoes are weak, with a Doppler velocity comparable to the ion‐acoustic speed, and a relatively narrow spectral width and coming only from small elevation angles. As the ΔH value gets more negative (∼−200 nT around noon), very strong CEEJ echoes appear, still with ion‐acoustic velocities at low elevation angles but with large spectral widths. The Doppler shifts of the spectra follow a dependence on the cosine of elevation angle. Weak CEEJ echoes are again observed when the irregularities are about to disappear (i.e., when ΔH is just slightly negative of the threshold value). In order to get a better understanding of the scattering mechanism responsible for these echoes, we have devised a simple statistical model and compared its spectra to the spectral radar data. The CEEJ spectral characteristics can be explained on the basis of turbulent plasma breakdown of pure two‐stream instabilities. These echoes are thought to be generated by nonlinear mode coupling and, when sufficiently strong, by secondary wave instabilities driven by pure two‐stream waves at the density maximum and the topside of the E region.

Potentialities of GaN-Based Microcavities Grown on Silicon Substrates

A bulk GaN half-wave (λ/2) cavity is grown on Si(111) substrate by molecular beam epitaxy. It is embedded between twelve AlN/(Al,Ga)N quaterwave stacks and the silicon substrate, which acts as a metallic bottom mirror in the investigated wavelength range. A (4λ - λ/4) buffer layer has been inserted just above the silicon substrate in order to accommodate the strain, experienced by the GaN layer, and to satisfy the resonance condition of the cavity. Reflectivity experiments at low temperature (T = 5 K) reveal a cavity mode centered around 355 nm with a spectral width of 3.6 nm. The modeling of the reflectivity spectrum accounts well for the experiment. By recording both the photoluminescence and reflectivity signals at the same position, it is shown that the emission is controlled by the cavity mode. The splitting due to strong exciton-photon coupling is not observed, which could be due to the large spectral width of GaN emission.

Potentialities of GaN-Based Microcavities Grown on Silicon Substrates

A bulk GaN half-wave (λ/2) cavity is grown on Si(111) substrate by molecular beam epitaxy. It is embedded between twelve AlN/(Al,Ga)N quaterwave stacks and the silicon substrate, which acts as a metallic bottom mirror in the investigated wavelength range. A (4λ—λ/4) buffer layer has been inserted just above the silicon substrate in order to accommodate the strain, experienced by the GaN layer, and to satisfy the resonance condition of the cavity. Reflectivity experiments at low temperature (T = 5 K) reveal a cavity mode centered around 355 nm with a spectral width of 3.6 nm. The modeling of the reflectivity spectrum accounts well for the experiment. By recording both the photoluminescence and reflectivity signals at the same position, it is shown that the emission is controlled by the cavity mode. The splitting due to strong exciton–photon coupling is not observed, which could be due to the large spectral width of GaN emission.

A case study of HF radar spectra and 630.0 nm auroral emission in the pre-midnight sector

Abstract. A comparison of HF radar backscatter observed by the CUTLASS Finland radar, meridian scanning photometer data from Longyearbyen, magnetic field variations from IMAGE stations, and particle precipitation measured by the DMSP F12 spacecraft is presented. The interval under discussion occurred in the pre-midnight local time sector, during a period of weakly northward interplanetary magnetic field. A region of HF backscatter, typically 8 degrees wide, occurred in the field of view of the CUTLASS Finland radar. A well defined gradient in the spectral width parameter was present, with mainly low (< 200 m s - 1 ) spectral widths in the lower latitude part of the scatter and predominantly large (> 200 ms - 1 ) spectral widths in the higher latitude part. The relationship between the spectral width and the red line (630.0 nm) emission measured by the meridian scanning photometer is considered. The poleward border of the red line emission, which has, in the past, been proposed as being representative of the polar cap boundary, was co-located to within 1° of magnetic latitude with the gradient in spectral width for part of the interval. Statistically, large spectral widths occurred poleward of the red line emission, while small spectral widths occurred within or equatorward of the red line emission. Near simultaneous DMSP particle observations in the 20 eV to 20 keV range indicate that the poleward border of the red line emission and the gradient in spectral width occurred at the same latitude as the transition from auroral oval to polar rain particle energies. We conclude that the large spectral widths were not caused by particle precipitation associated with the auroral oval. There were two periods of special interest when the relationship between the red line and the spectral width broke down. The first of these happened during enhanced red line and green line (557.7 nm) emission, with a drop out of the radar scatter and an enhanced, narrow westward electrojet. We conclude that this event was a magnetospheric substorm occurring at much higher than usual latitudes. The second period of special interest happened when equatorward moving bands of large spectral width occurred within the region of scatter. Up to 4 of these bands were present during an interval of 100 minutes. Associated with these narrow bands of large spectral width were narrow channels of enhanced westward ion velocities. We conclude that these equatorward moving bands of large spectral width may be related to reconnection processes in the tail. The observations demonstrate that the tail continues to be active even under low solar wind energy input conditions. Furthermore, we conclude that the gradient in the spectral width may be used as a proxy for the polar cap boundary, but only with extreme caution.Key words. Ionosphere (ionosphere-magnetosphere inter-actions; polar ionosphere) – Magnetospheric physics (storms and substorms)

Identification of the low‐altitude cusp by Super Dual Auroral Radar Network radars: A physical explanation for the empirically derived signature

The Super Dual Auroral Radar Network (SuperDARN) radars are proving to be a very powerful experimental tool for exploring solar wind‐magnetosphere‐ionosphere interactions. They measure the autocorrelation function (ACF) of the signal backscattered from ionospheric irregularities, and they derive parameters such as the Doppler velocity and the spectral width. The associated spectra have a specific behavior inside the cusp, a strong temporal and spatial evolution of the velocity and spectral width, and a high value of the spectral width. Until now, no studies have explained these characteristics, but they are routinely used to detect the cusp in the radar data, for example, to estimate the location of the open/closed field line boundary. Both satellite and ground‐based magnetometer data from the cusp region show broadband wave activity in the Pc1 and Pc2 frequency band. In this study we evaluate how such wave activity modifies the radar's ACF, and we conclude that it explains the spectra seen in the cusp. More specifically, we find that (1) even a monochromatic electric field variation can cause apparently turbulent behavior, including wide spectral widths and apparent multiple components, (2) even low‐amplitude waves are capable of causing large spectral widths, if the frequency is sufficiently high, (3) for a fixed low‐amplitude electric field variation the measured spectral width increases with wave frequency, displaying a sharp transition from low to high spectral width above an onset frequency, and (4) the determination of the background velocity field is not strongly affected by such conditions. While the wave activity is shown to have a major impact on the spectral width, it is found that the radar does accurately represent the large‐scale plasma velocity.

Nonparametric estimation of mean Doppler and spectral width

This paper proposes a new nonparametric method for estimation of spectral moments of a zero-mean Gaussian process immersed in additive white Gaussian noise. Although the technique is valid for any order moment, particular attention is given to the mean Doppler (first moment) and to the spectral width (square root of the centered second spectral moment). By assuming that the power spectral density (PSD) of the underlying process is bandlimited, the maximum-likelihood estimates of its spectral moments are derived. A suboptimal estimate based on the sample covariance is also studied. Both methods are robust in the sense that they do not rely on any assumption concerning the PSD (besides being bandlimited). Under weak conditions, the set of estimates based on sample covariance is unbiased and strongly consistent. Compared with the classical pulse pair and the periodogram-based estimators, the proposed methods exhibit better statistical properties for asymmetric spectra and/or spectra with large spectral widths, while involving a computational burden of the same order.

Ripening suppression and large photoluminescence blueshift in aligned InGaAs quantum dots on a vicinal (100) GaAs substrate

We observe ripening suppression in aligned InGaAs quantum dots (QDs) along multiatomic steps on a vicinal (100) GaAs substrate. By varying the growth interruption times, a study of QD morphologies by atomic force microscope reveals a clear Ostwald ripening suppression effect on QD formation. On the other hand, we observe a regular ripening for InGaAs QDs on an exact (100) substrate. In addition, n-QD chains aligned along multiatomic steps are observed. An inhomogeneously broadened photoluminescence (PL) spectrum with larger spectral width is obtained for a QD ensemble on a vicinal (100) substrate as compared to those on an exact (100) substrate although statistical analyses of QD size distributions show similar size dispersions. The spectral broadening is attributed to the lateral quantum coupling of aligned n-QD chains. PL spectra for QDs on vicinal (100) substrates show a larger rate of blueshift with increasing interruption times, which is interpreted as a lack of the PL compensation effect by size enlargement of the QDs during growth interruption. Thus the result supports the ripening suppression effect which is observed in the morphology study.

Solid-State Dipolar INADEQUATE NMR Spectroscopy with a Large Double-Quantum Spectral Width

A technique for obtaining dipolar-mediated INADEQUATE NMR spectra with a large spectral window in the double-quantum dimension is presented. Using the dipolar recoupling sequence C7 to excite the double-quantum coherence under magic-angle spinning, the technique involves incrementing the evolution period in synchrony with the phase of the radiofrequency pulses in the C7 sequence. The technique is demonstrated on a uniformly13C-labeled amino acid and an extensively13C-labeled protein to identify13C connectivity patterns for spectral assignment.

Solar‐wind‐driven pulsed magnetic reconnection at the dayside magnetopause, Pc5 compressional oscillations, and field line resonances

A series of observations by satellites of the solar wind and magnetosphere, by HF radars of the F region, and by ground‐based magnetometers of E region currents are presented to show the correlation between quasiperiodic fluctuations in the solar wind, magnetosphere, and ionosphere on January 24, 1996. The Iceland Stokkseyri SuperDARN radar observed quasiperiodic convection flow bursts in or near the ionospheric footprint of the cusp, identified by large spectral widths. Sometimes called flow channel events (FCEs), these enhanced convection flows are generally believed to be the ionospheric signatures of flux transfer events (FTEs) in the cusp. Initially, the flows pulsed with a periodicity of ∼12 min. When the “radar” cusp shifted to lower latitudes, possibly as a result of a series of FTEs, FCEs occurred every 10 min or less. The Wind spacecraft observed linearly polarized interplanetary magnetic field (IMF) oscillations with dominant periodicities of about 12 and 20 min (∼1.3 and 0.8 mHz) that were superposed on long‐period IMF fluctuations on the scale of a few hours. The pure state power spectra of the IMF components also showed several subsidiary peaks at higher frequencies (e.g., ∼1.7, 2.0, 2.4, and 2.9 mHz) or shorter periods (∼10, 8.5, 7, and 6 min). Similar oscillations were observed in the solar wind dynamic pressure which was anticorrelated with the IMF magnitude and included a strong oscillation with a period of ∼15 min (1.1 mHz). About 20 min later, in the postnoon magnetosheath, Geotail's magnetic and electric field sensors observed an ULF wave packet showing similar discrete spectra. The compressional MHD waves in the solar wind applied oscillating magnetic/electric fields and dynamic pressure on the dayside magnetopause driving multifrequency compressional oscillations in the magnetosphere. Discrete frequencies similar to those found in the solar wind and magnetosheath data were identified in the fast Fourier transform spectra of the ground‐based and space‐borne (GOES 8 and 9) magnetograms. A series of solar wind dynamic pressure pulses caused compressions and rarefactions of the magnetosphere observed by GOES 8 and GOES 9 magnetometers at geostationary orbit. The dynamic pressure driven magnetic perturbations, including those observed on the ground, propagated antisunward. In the noon sector the compressional waves coupled to shear modes resulting in propagation delays of the ground/ionospheric response. The convection flow bursts in the ionospheric footprint of the cusp were closely correlated with the low‐frequency oscillations of the IMF BZ and magnetosheath duskward electric field suggesting that the ULF waves in the solar wind modulated the magnetic reconnection at the dayside magnetopause into pulses. The higher‐frequency components of the IMF oscillations and/or sudden IMF changes excited field line resonances (FLRs) on the magnetic shells adjacent to the noon magnetopause. As the relative power of these higher‐frequency oscillations in the solar wind IMF increased and the cusp shifted to lower latitudes, the FCE rate also increased. A possibility of a reverse coupling (feedback) from the resonating magnetic shells to the reconnection region at the dayside magnetopause is suggested.

On origin of outshifted plasma lines during HF modification experiments

Outshifted plasma lines, observed during injections of intense HF radiowaves into the ionosphere, are due to scattering on Langmuir waves with an unusually large spectral width shifted above the injected frequency. Incoherent scatter radar observations reveal that these waves sometimes are located at altitudes exceeding by several kilometers the altitude of the reflection layer of the ordinary HF wave, thus making an explanation in terms of the so‐called “free mode” problematic. We show that these observations may be consistently described by the creation, due to the ponderomotive force, of density depletions in the Airy pattern of the Z mode. The generation of Langmuir plasmons by accelerated suprathermal electrons inside the depletions causes the wide spectral width of these plasma line features and their mean frequency shift.

2D EPR data analysis using Linear Prediction singular value decomposition and zero-filling

A 2D plot of a spectrum with relatively large (±25 MHz) spectral width containing many narrow lines of widths (FWHM) below 200 kHz can be improved noticeably by invoking Linear Prediction (LP) for FID reconstruction and zero-filling with respect to spectral resolution and weak peak detection capability. This was demonstrated by a comparison of two 2D contour plots calculated with and without the help of linear prediction singular value decomposition and zero-filling.

Incident-photon energy-distribution effects on radiationless resonant Raman scattering.

In the experimental observation of radiationless resonant Raman scattering, features that differ from the idealized case can arise when the spectral width of the incident photon distribution is comparable to the width of pertinent atomic energy levels. These features include nonlinear dispersion of the peak maxima and changes in the symmetry and width of the ejected-electron lines with changes in the average incident photon energy. Under certain conditions, the electron line shapes can become highly asymmetric or even double peaked. The origin of these features in actual experiments is explored through a model calculation for large spectral widths. Some possible applications of these effects are pointed out.

Millimetric response of stressed gallium doped germanium photoconductor

Conventional Ge:Ga photoconductors presents a residual responsivity at millimetric wavelengths, well below the cut-off wavelength due to the gallium impurity ionization energy. At constant bias voltage, this millimetric responsivity is greatly enhanced when a strong uniaxial stress is applied. We suggest that this response is mainly due to a change in the free hole lifetime when the hole gas is heated by the absorbed microwave photons. This photoconductive process has a shorter response time than the InSb Putley mechanism. Thus optimization of this millimetric Ge:Ga detector could find an application for example for heterodyne detection with larger spectral width than InSb.

17] Two-dimensional nuclear magnetic resonance spectra of paramagnetic systems

The development of 2D Nuclear Magnetic Resonance (NMR) experiments produced an enormous increase in the application of NMR for structural and dynamics characterization of molecules in solution. In particular 2D NMR techniques have been applied with success to proteins, DNA fragments, and biological macromolecules. With 2D experiments, both scalar, through-bond, and dipolar, through-space, interactions may be detected. The potential of 2D experiments is because of their increased resolution, which allows assignment of signals even in crowded regions of the spectrum, and to their capability of detecting all or a large number of nucleus- nucleus couplings with a single experiment. Only relatively recently have 2D NMR experiments been applied to paramagnetic systems that are characterized by fast relaxation rates and large spectral widths. In paramagnetic systems long acquisition times, t 2 max , are not necessary, as the magnetization disappears from the xy plane with a time constant T 2 , usually very short: after the signal has decayed only noise is acquired. On the other hand, short dwell times are needed to permit the detection over the large spectral width necessary in these systems.

Local-Field Measurements on Powder Samples from Polarization Inversion of the Rare-Spin Magnetization

A simple two-dimensional procedure is used to obtain chemical-shift-dipolar-correlated spectra for static or magic-angle spinning powder samples. The polarization inversion of the S-spin magnetization during t 1 is used to introduce dipolar evolution as a modulation of the chemical-shift domain t 2. This permits a large spectral width in the unscaled dipolar dimension and does not require synchronization with the macroscopic rotation. For a static or slowly rotating sample of ferrocene, there is good agreement between experimental and calculated line-shapes which permits visualization of coherent energy transfer effects and the expected coincidence of the two tensors in the presence of fast internal rotation. A slowly spinning experiment as well as a spin-echo experiment with dipolar modulation shows that, in analogy to the particular chemical-shift position for a stationary sample, each particular spinning sideband in the slow-spinning regime or peak in the spin-echo spectrum represents respectively a narrow range of orientations or a particular orientation of crystallites with respect to the magnetic field. Other examples of dipolar local-field measurements for rapidly spin-fling samples are presented in more typical organic solids.

Multiple pulse NMR imaging of polymers and chemistry

Multiple pulse line narrowing techniques can be used to improve resolution and sensitivity in solid state NMR imaging. For example, pulse sequences which remove homonuclear dipolar broadening have been used to image proton-containing materials. Further enhancements in resolution and sensitivity are obtained by removing inhomogeneous interactions such as chemical shift, susceptibility, and heteronuclear dipolar broadening. Pulse sequences have been designed which provide efficient line narrowing over large spectral widths by taking into account the experimenter's control over the amplitude and time dependence of the gradient-induced resonance offset. These methods have been applied to centimeter sized samples to obtain images of polymers, composite materials, and gas-solid chemical reactions. T 1 and T 2 contrast allows differentiation between materials.