Interactions of atoms with high‐field single‐cycle infrared radiation

We have studied ionization and excitation of Rydberg atoms by intense sub‐picosecond electromagnetic field pulses of less than one cycle duration. The ionization threshold electric field scales as the binding energy (i.e., n∗−2, rather than the n∗−4 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 ‘‘in resonance’’ for dozens of transitions simultaneously.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87507/2/227_1.pd

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 J174540.7$-$290015 (T15) and SWIFT J174540.2$-$290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within $r\sim1$ 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 $8\times10^{36}$ and $3\times10^{37}$ 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 $L_X \lesssim 2 \times10^{31}$ 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 $\nu \sim 10$ 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 ($a_{*} \gtrsim 0.9$) and large disk density ($n_e\sim10^{21}$ cm$^{-3}$). 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 far‐infrared light pulses

We report the first interferometric characterization of freely propagating, subpicosecond, far‐infrared (FIR) light pulses. FIR light was generated via short pulse photoexcitation of a semi‐insulating InP wafer. The half width of the intensity interferogram was 230 fs. The FIR light contained frequency components from 3 to 150 cm−1.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