341 research outputs found
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Evidence for frequency comb emission from a Fabry-PĂ©rot terahertz quantum-cascade laser
We report on a broad-band terahertz quantum-cascade laser (QCL) with a long Fabry-PĂ©rot ridge cavity, for which the tuning range of the individual laser modes exceeds the mode spacing. While a spectral range of approximately 60 GHz (2 cmâ1) is continuously covered by current and temperature tuning, the total emission range spans more than 270 GHz (9 cmâ1). Within certain operating ranges, we found evidence for stable frequency comb operation of the QCL. An experimental technique is presented to characterize frequency comb operation, which is based on the self-mixing effect
Comparison of luminance based metrics in different lighting conditions
In this study, we evaluate established and newly developed metrics for predicting glare using data from three different research studies. The evaluation covers two different targets: 1. How well the userâs perception of glare magnitude correlates to the prediction of the glare metrics? 2. How well do the glare metrics describe the subjectsâ disturbance by glare? We applied Spearman correlations, logistic regressions and an accuracy evaluation, based on an ROC- analysis. The results show that five of the twelve investigated metrics are failing at least one of the statistical tests. The other seven metrics CGI, modified DGI, DGP, Ev, average Luminance of the image Lavg, UGP and UGR are passing all statistical tests. DGP, CGI, DGI mod and UGP have largest AUC and might be slightly more robust. The accuracy of the predictions of afore mentioned seven metrics for the disturbance by glare lies in the range of 75-83% and does not confirm findings from other studies stating a poor performance of existing glare metrics
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High-temperature, continuous-wave operation of terahertz quantum-cascade lasers with metal-metal waveguides and third-order distributed feedback
Currently, different competing waveguide and resonator concepts exist for terahertz quantum-cascade lasers (THz QCLs). We examine the continuous-wave (cw) performance of THz QCLs with single-plasmon (SP) and metal-metal (MM) waveguides fabricated from the same wafer. While SP QCLs are superior in terms of output power, the maximum operating temperature for MM QCLs is typically much higher. For SP QCLs, we observed cw operation up to 73 K as compared to 129 K for narrow (†15 Όm) MM QCLs. In the latter case, single-mode operation and a narrow beam profile were achieved by applying third-order distributed-feedback gratings and contact pads which are optically insulated from the intended resonators. We present a quantitative analytic model for the beam profile, which is based on experimentally accessible parameters
Proton stopping in C+C, d+C, C+Ta and d+Ta collisions at 4.2A GeV/c
The shape of proton rapidity distributions is analysed in terms of their
Gaussian components, and the average rapidity loss is determined in order to
estimate the amount of stopping in C+C, d+C, C+Ta and d+Ta collisions at 4.2A
GeV/c. Three Gaussians correspond to the nuclear transparency and describe well
all peripheral and also C+C central collisions. Two-component shape is obtained
in case of d+C and C+Ta central collisions. Finally one Gaussian, found in d+Ta
central collisions, corresponds to the full stopping. The calculated values of
the average rapidity loss support the qualitative relationship between the
number of Gaussian components and the corresponding stopping power. It is also
observed, in central collisions, that the average rapidity loss increases with
the ratio of the number of target and the number of projectile participants.Comment: 9 pages REVTeX, 1 PS figure replaced, to be published in Phys.Rev.
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Frequency modulation spectroscopy with a THz quantum-cascade laser
We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening
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Lateral distributed-feedback gratings for single-mode, high-power terahertz quantum-cascade lasers
We report on terahertz quantum-cascade lasers (THz QCLs) based on first-order lateral distributed-feedback (lDFB) gratings, which exhibit continuous-wave operation, high output powers (>8 mW), and single-mode emission at 3.3â3.4 THz. A general method is presented to determine the coupling coefficients of lateral gratings in terms of the coupled-mode theory, which demonstrates that large coupling strengths are obtained in the presence of corrugated metal layers. The experimental spectra are in agreement with simulations of the lDFB cavities, which take into account the reflective end facets
Elliptical flow -- a signature for early pressure in ultrarelativistic nucleus-nucleus collisions
Elliptical energy flow patterns in non-central Au(11.7AGeV) on Au reactions
have been studied employing the RQMD model. The strength of these azimuthal
asymmetries is calculated comparing the results in two different modes of RQMD
(mean field and cascade). It is found that the elliptical flow which is readily
observable with current experimental detectors may help to distinguish
different reasonable expansion scenarios for baryon-dense matter. The final
asymmetries are very sensitive to the pressure at maximum compression, because
they involve a partial cancelation between early squeeze-out and subsequent
flow in the reaction plane. This cancelation can be expected to occur in a
broad energy region covered by the current heavy ion fixed-target programs at
BNL and at CERN.Comment: 14 pages LaTeX including 3 postscript figure
Bremsstrahlung from a Microscopic Model of Relativistic Heavy Ion Collisions
We compute bremsstrahlung arising from the acceleration of individual charged
baryons and mesons during the time evolution of high-energy Au+Au collisions at
the Relativistic Heavy Ion Collider using a microscopic transport model. We
elucidate the connection between bremsstrahlung and charge stopping by
colliding artificial pure proton on pure neutron nuclei. From the intensity of
low energy bremsstrahlung, the time scale and the degree of stopping could be
accurately extracted without measuring any hadronic observables.Comment: 25 pages using revtex with 9 embedded EPS figures, modified somewhat
the discussion on the method in sect. II B, to appear in Phys. Rev.
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