Results for “photoexcitation dynamics”

NASA NTRS1990

Applications of ultrashort laser pulses in science and technology; Proceedings of the Meeting, The Hague, Netherlands, Mar. 12, 13, 1990

Topics discussed are on the generation of high-intensity femtosecond lasers, the high-repetition and infrared femtosecond pulses, and physics of semiconductors and applications. Papers are presented on the femtosecond pulse generation at 193 nm; the generation of intense subpicosecond and femtosecond pulses; intense tunable subpicosecond and femtosecond pulses in the visible and infrared, generated by optical parametric oscillators; a high-efficiency high-energy optical amplifier for femtosecond pulses; and the generation of solitons, periodic pulsing, and nonlinearities in GaAs. Other papers are on ultrafast relaxation dynamics of photoexcited carriers in GaAs, high-order optical nonlinear susceptibilities of transparent glasses, subnanosecond risetime high-power pulse generation using photoconductive bulk GaAs devices, femtosecond studies of plasma formation in crystalline and amorphous silicon, and subpicosecond dynamics of hot carrier relaxation in InP and GaAs.

Antonetti, Andre↗

NASA NTRS1980

Modeling of a solar-pumped iodine laser

The direct conversion in space of solar radiation into laser radiation for power transmission to earth, satellites, or deep space probes shows promise as a reasonably simple technology and may have cost advantage in deployment and greater reliability compared to other methods of space power generation and transmission. The main candidates for solar pumping are the gas dynamic, photochemical, and direct photoexcited lasers. Here consideration is given to the photochemical reaction of alkyliodides which predominantly excite the I(2P1/2) state which then lases at 1.315 microns. The iodine ground state is eventually lost to reconstituting the gas or in the formation of molecular iodine. The rates at which the gas is required to be recycled through the laser system are modest. The side exposure at 100-fold solar concentration of a 100-m long tube with a 1 sq m cross section is estimated to provide 20 kW of continuous laser output. Scaling laws and optimum operating conditions of this system are discussed.

Wilson, J. W.↗

NASA NTRS2021

Fe xvii 2p–3s Line Ratio Diagnostic of Shock Formation Radius in O Stars

The 2p–3s lines of Fe xvii in the X-ray spectrum of the O-type star ζ Puppis exhibit an anomalous (3G + M2)/(3F) line ratio of ∼1.4, in comparison with ∼2.4 for almost all other collisionally excited astrophysical spectra. Based on the work of Mauche et al., we conjectured that the strong UV field of ζ Puppis produces the observed ratio by depopulation of metastable 3s excited states, and that the ratio can potentially be used as an independent diagnostic of plasma formation radius. We used the Flexible Atomic Code collisional-radiative model to model the effect of UV photoexcitation from O stars on the Fe xvii lines. We compared our model calculations to archival spectra of coronal and hot stars from the Chandra HETGS and XMM-Newton RGS to benchmark our calculations for various electron densities and UV field intensities. Our calculations show that UV photoexcitation does not produce a sufficiently large dynamic range in the 3F / (3F + 3G + M2) fraction to explain the difference in the observed ratio between coronal stars and ζ Pup. Thus, this effect likely cannot explain the observed line ratio of ζ Pup, and its origin is still unexplained.

Gabriel J. Grell↗

NASA NTRS2021

Fe XVII 2p–3s Line Ratio Diagnostic of Shock Formation Radius in O Stars

The 2p–3s lines of Fe xvii in the X-ray spectrum of the O-type star ζ Puppis exhibit an anomalous (3G + M2)/(3F) line ratio of ∼1.4, in comparison with ∼2.4 for almost all other collisionally excited astrophysical spectra. Based on the work of Mauche et al., we conjectured that the strong UV field of ζ Puppis produces the observed ratio by depopulation of metastable 3s excited states, and that the ratio can potentially be used as an independent diagnostic of plasma formation radius. We used the Flexible Atomic Code collisional-radiative model to model the effect of UV photoexcitation from O stars on the Fe xvii lines. We compared our model calculations to archival spectra of coronal and hot stars from the Chandra HETGS and XMM-Newton RGS to benchmark our calculations for various electron densities and UV field intensities. Our calculations show that UV photoexcitation does not produce a sufficiently large dynamic range in the 3F / (3F + 3G + M2) fraction to explain the difference in the observed ratio between coronal stars and ζ Pup. Thus, this effect likely cannot explain the observed line ratio of ζ Pup, and its origin is still unexplained.

Gabriel Grell↗

NASA NTRS1999

Carrier Dynamics in Quantum Dot Structures with Different Interdot Spacings

The time of carrier transfer into a dot decreases with increasing dot density as well as with photoexcited carrier denstiy and temperature. It is suggested that potential barriers at the barrier, wetting layer and quantum dot interfaces hinder the carrier capture in low-density quantum dot structures.

quantum↗

NASA NTRS1995

Investigation of the basic physics of high efficiency semiconductor hot carrier solar cell

The main purpose of this research program is to investigate potential semiconductor materials and their multi-band-gap MQW (multiple quantum wells) structures for high efficiency solar cells for aerospace and commercial applications. The absorption and PL (photoluminescence) spectra, the carrier dynamics, and band structures have been investigated for semiconductors of InP, GaP, GaInP, and InGaAsP/InP MQW structures, and for semiconductors of GaAs and AlGaAs by previous measurements. The barrier potential design criteria for achieving maximum energy conversion efficiency, and the resonant tunneling time as a function of barrier width in high efficiency MQW solar cell structures have also been investigated in the first two years. Based on previous carrier dynamics measurements and the time-dependent short circuit current density calculations, an InAs/InGaAs - InGaAs/GaAs - GaAs/AlGaAs MQW solar cell structure with 15 bandgaps has been designed. The absorption and PL spectra in InGaAsP/InP bulk and MQW structures were measured at room temperature and 77 K with different pump wavelength and intensity, to search for resonant states that may affect the solar cell activities. Time-resolved IR absorption for InGaAsP/InP bulk and MQW structures has been measured by femtosecond visible-pump and IR-probe absorption spectroscopy. This, with the absorption and PL measurements, will be helpful to understand the basic physics and device performance in multi-bandgap InAs/InGaAs - InGaAs/InP - InP/InGaP MQW solar cells. In particular, the lifetime of the photoexcited hot electrons is an important parameter for the device operation of InGaAsP/InP MQW solar cells working in the resonant tunneling conditions. Lastly, time evolution of the hot electron relaxation in GaAs has been measured in the temperature range of 4 K through 288 K using femtosecond pump-IR-probe absorption technique. The temperature dependence of the hot electron relaxation time in the X valley has been measured.

Alfano, R. R.↗