37 research outputs found
Interactions of atoms with highâfield singleâcycle infrared radiation
We have studied ionization and excitation of Rydberg atoms by intense subâpicosecond electromagnetic field pulses of less than one cycle duration. The ionization threshold electric field scales as the binding energy (i.e., nââ2, rather than the nââ4 threshold scaling characteristic of static field ionization and high order multiphoton ionization. This altered behavior is due to the short duration and the subcycle nature of the field. We also find a wide distribution of final states produced by these pulses, which are effectively ââin resonanceââ for dozens of transitions simultaneously.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87507/2/227_1.pd
Attenuation of picosecond electrical pulses by twoâdimensional electron gases integrated in coplanar striplines
Ultrafast-ultrafine probing of high-speed electrical waveforms using a scanning force microscope with photoconductive gating
Picosecond photoconductivity in low-temperature-grown GaAs (LT GaAs) has been used to provide temporal resolution both in rigid probes and in scanning force microscope probes. This article reviews the fabrication and use of such probes. 2.5 ps temporal resolution and few microvolts sensitivity are obtained at arbitrary points on circuits with a spatial definition of 100 nm. Rigid probes are tested in application to analogue and digital circuits. As an alternative to electron beam testing, scanning force probes are applied to in situ imaging and waveform measurement. Finally, the use of time-resolved waveform analysis with scanning-force microscopy probes with semiconductor laser sources is demonstrated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43337/1/11082_2004_Article_BF00820152.pd
NuSTAR and Chandra observations of new X-ray transients in the central parsec of the Galaxy
We report NuSTAR and Chandra observations of two X-ray transients, SWIFT
J174540.7290015 (T15) and SWIFT J174540.2290037 (T37), which were
discovered by the Neil Gehrels Swift Observatory in 2016 within pc of
Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the
soft and hard states, with 3-79~keV luminosities of and
erg/s, respectively. No X-ray outbursts have previously been
detected from the two transients and our Chandra ACIS analysis puts an upper
limit of erg/s on their quiescent 2-8 keV
luminosities. No pulsations, significant QPOs, or type I X-ray bursts were
detected in the NuSTAR data. While T15 exhibited no significant red noise, the
T37 power density spectra are well characterized by three Lorentzian
components. The declining variability of T37 above Hz is typical
of black hole (BH) transients in the hard state. NuSTAR spectra of both
transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe
atomic features, and a continuum component well described by Comptonization
models. Their X-ray reflection spectra are most consistent with high BH spin
() and large disk density ( cm).
Based on the best-fit ionization parameters and disk densities, we found that
X-ray reflection occurred near the inner disk radius, which was derived from
the relativistic broadening and thermal disk component. These X-ray
characteristics suggest the outbursting BH-LMXB scenario for both transients
and yield the first BH spin measurements from X-ray transients in the central
100 pc region.Comment: 15 pages, 7 figures, accepted for publication in Ap
Interferometric characterization of 160 fs farâinfrared light pulses
We report the first interferometric characterization of freely propagating, subpicosecond, farâinfrared (FIR) light pulses. FIR light was generated via short pulse photoexcitation of a semiâinsulating InP wafer. The half width of the intensity interferogram was 230 fs. The FIR light contained frequency components from 3 to 150 cmâ1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70362/2/APPLAB-59-8-893-1.pd
THz spectroscopy and source characterization by optoelectronic interferometry
We demonstrate a new type of THz optoelectronic interferometer, by fully characterizing a recently developed THz source to beyond 6 THz, and by measuring the absorption coefficient of high-resistivity GaAs from 1 to 5 THz. The two source THz interferometer is driven with two 4 mW beams of 60 fs dye-laser pulses and produces interferograms with exceptional signal-to-noise ratios.Peer reviewedElectrical and Computer Engineerin