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Unseeded velocimetry in nitrogen for high-pressure, cryogenic wind tunnels: part III. Resonant femtosecond-laser tagging
摘要： Selective two-photon absorptive resonance femtosecond laser electronic excitation tagging (STARFLEET) velocimetry is characterized for the first time at high-pressure, low-temperature conditions. Studies were carried out in the NASA Langley Research Center’s 0.3 meter transonic, cryogenic wind tunnel, with flow conditions spanning the entire operational envelope of the facility; total pressures ranging from 100 kPa to 517 kPa, total temperatures from 80 K to 327 K, and Mach numbers from 0.2 to 0.85. STARFLEET signal intensity and lifetime measurements are examined for their thermodynamic dependencies since both intensity and lifetime have implications for measurement precision. Signal intensity is found to be inversely proportional to density, while lifetime scales nearly linearly with density until approaching the liquid-vapor saturation point of nitrogen. The velocity measurement accuracy and precision are assessed over the full domain of conditions, and standard error was determined to be 1.6%, while precision ranged from roughly 1.5% to 10% of the freestream velocity. The precision was also observed to have a temperature dependence, likely a result of the longer lifetimes experienced at higher densities.
关键词： femtosecond laser,resonant excitation,molecular tagging velocimetry
更新于2025-09-23 15:21:01
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Magnetosonic wave-aided terahertz emission by nonlinear mixing of lasers in plasmas
摘要： A scheme of phase-matched terahertz generation by beating two co-propagating lasers in magnetized plasma, in the presence of a magnetosonic wave (MSW), is developed. The beat frequency ponderomotive force of the lasers imparts an oscillatory drift to electrons. The electron drift velocity couples with the electron density perturbation associated with the MSW to produce an irrotational nonlinear current ? × (cid:1)J NL = 0. The beat current density resonantly excites a THz (Terahertz) radiation when the phase-matching conditions are satisfied. The MSW mediates the phase matching. At 9.6 and 10.6 μm wavelengths, and background magnetic field of 285 kG, one may achieve normalized THz wave amplitude of the order of 10?6 and one obtains the ratio of THz power to pump power of the order of 10?3.
关键词： terahertz radiation,phase-matching condition,Magnetized plasma,ponderomotive nonlinearity,resonant excitation
更新于2025-09-12 10:27:22
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Correlation between composition, microstructure, and emission properties in Nd-doped Si-rich Si oxynitride films: investigation into the nature of the sensitizer
摘要： Rare earth (RE) ions doped in Si-based materials, compatible with Si technology, are promising compounds with regards to optical communication and energy conversion. In this article, we show the emission properties of Nd-doped Si-rich Si oxynitride (Nd-SRSON) ?lms, and their dependence on the dangling bond density and the nature of the sensitizer. These ?lms were prepared by reactive magnetron sputtering and post-annealing. The ?lm composition, microstructure, and emission properties were investigated as a function of deposition parameters and annealing temperatures. Both Fourier transform infrared (FTIR) and ellipsometry spectroscopy measurements have con?rmed that the sample composition (Si/N ratio) can be carefully tuned by varying the ratio of reactive nitrogen to argon in the sputtering plasma. Moreover, FTIR and x-ray photoelectron spectroscopy measurements demonstrate the existence of both nitrogen and oxygen dangling bonds (N? and O?) in as-deposited samples. These dangling bonds were passivated during annealing. Under non-resonant excitation at 488 nm, the ?lms exhibit a signi?cant photoluminescence (PL) signal from Nd3+ ions demonstrating the occurrence of an effective sensitization of Nd3+ ions in the host matrix. Both PL excitation and ellipsometry results (the energy band gap from new amorphous model) exclude the sensitization by an exciton with energy over the band gap, whereas the presence of Si agglomerates, at the atomic scale, have been identi?ed as effective sensitizers towards Nd3+ ions. This work not only provides knowledge to optimize Si-based materials for favorable emission properties, but also, presents a universal methodology to investigate the nature of sensitizers for RE emitters. This allows one to ?nd correlations between composition, microstructure, and emission properties.
关键词： photoluminescence,rare earth,neodymium,non-resonant excitation,thin ?lm,Si-rich Si oxynitride
更新于2025-09-10 09:29:36
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Single-beam resonant spin amplification of electrons interacting with nuclei in a GaAs/(Al,Ga)As quantum well
摘要： The dynamic polarization of nuclear spins interacting with resident electrons under resonant excitation of trions is studied in a nominally undoped GaAs/(Al,Ga)As quantum well. Unlike in common time-resolved pump-probe techniques, we used a single-beam approach in which the excitation light is modulated between the circular and linear polarization states. The time-integrated intensity of the excitation laser re?ected from the sample surface, proportional to the optical generation rate and changing due to the pumping of the resident electrons, is detected. Polarized electrons, on the other hand, transfer their spin to the lattice nuclei via the hyper?ne interaction. Exciting the sample with a train of pulses in an external magnetic ?eld leads to resonant spin ampli?cation observed when the Larmor precession frequency is synchronized with the laser pulse repetition rate. A buildup of the nuclear spin polarization causes a shifting of the resonant spin ampli?cation peaks since the resulting nuclear ?eld alters the strength of the external magnetic ?eld experienced by the electrons. It was established that the nuclear spin polarization time T1 is temperature dependent and, owing to the electron localization at lower temperatures, becomes shorter. “Locking” of the nuclear (Overhauser) ?eld in the oblique external magnetic ?eld, related to the anisotropy of the electron g factor, was observed. The g factor ratio between the in-plane (g(cid:2)) and out-of-plane (g⊥) components was estimated to be g⊥/g(cid:2) = 1.3.
关键词： hyper?ne interaction,resonant spin ampli?cation,resonant excitation,electron g factor,quantum well,dynamic polarization,single-beam approach,nuclear spins
更新于2025-09-09 09:28:46
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Self-shaping of cavity-polariton systems created by resonant broadband excitation
摘要： The dynamics of a pure low polariton (LP) system created by resonant broadband excitation in a wide range of wave vectors was investigated in a high-Q GaAs-based microcavity. The LP system is shown to inherit the high spatial coherence from the laser pulse and does not lose it during decay. As a result, its dynamics is well controlled by the spatial and momentum distributions of photons in the exciting pulse and described by the Gross-Pitaevskii equations. In particular, the purely dynamic formation of the highly populated coherent LP state was implemented at the LP band bottom in the cavity excited in a large spot by converging ps-long Gaussian laser pulses when the active region of the cavity is in front of its waist. The formed state is found to persist for several picoseconds until the LP-LP repulsion leads to the creation of high-energy LPs dissipating from the ground state with high velocities.
关键词： resonant excitation,polariton,microcavity,coherent state,Gross-Pitaevskii equations
更新于2025-09-09 09:28:46