M. Justine Bell

Tunable frequency-controlled isolated attosecond pulses characterized by either 750 nm or 400 nm wavelength streak fields

A compact and robust Mach-Zehnder type interferometer coupled with the double optical gating technique provides tunable isolated attosecond pulses and streak field detection with fields centered at either 750 nm or 400 nm. Isolated attosecond pulses produced at 45 eV (with measured pulse duration of 114 ± 4 as) and 20 eV (with measured pulse duration of 395 ± 6 as) are temporally characterized with a 750 nm wavelength streak field. In addition, an isolated 118 ± 10 as pulse at 45 eV is measured with a 400 nm wavelength streak field. The interferometer design used herein provides broad flexibility for attosecond streak experiments, allowing pump and probe pulses to be specified independently. This capability is important for studying selected electronic transitions in atoms and molecules.

Carrier-envelope phase-dependent quantum interferences in multiphoton ionization

The angular distribution of photoelectrons created by multiphoton ionization of xenon atoms by a few-cycle laser pulse shows a carrier-envelope phase (CEP) dependent asymmetry. A simple perturbative model based on a sum over indistinguishable quantum paths describes the observed asymmetry as a function of photoelectron energy and CEP. Although the individual multiphoton transition rates depend on the intensity profile of the pulse, the experimentally measured photoelectron angular distributions are sensitive to the absolute spectral phase of the pulse, including both CEP and chirp. We discuss retrieval of the CEP and chirp from the asymmetry pattern, as well as the potential to extract the scattering phase shift.

Measurement and optimization of isolated attosecond pulse contrast

An experimental method is presented to experimentally measure and control the carrier-envelope-phase (CEP)-dependent pulse-energy contrast of isolated attosecond pulses. By scanning the CEP and measuring the photoelectron spectrum produced by the combined action of the attosecond pulses and the high-harmonic driving laser pulses at zero relative time delay, one can extract the pulse-energy ratio between the main attosecond pulse and its neighboring satellite pulses arriving in preceding or subsequent half-cycles of the driver pulse. Moreover, this method allows fast and efficient in situ retrieval of the optimal CEP for high-contrast isolated attosecond pulse generation.

Intensity dependence of light-induced states in transient absorption of laser-dressed helium measured with isolated attosecond pulses

Light-induced states in He atoms were characterized using attosecond transient absorption spectroscopy. A 400 as pulse covering the 20–24 eV spectral range serves as the probe pulse, and the effect of a few-cycle near infrared pulse (12 fs, 780 nm) on the absorption spectrum is measured as a function of time delay and near-infrared intensities varying from (5.0 ± 2) × 1010 to (1 ± 0.4) × 1013 W/cm2. Light-induced states resulting from near-infrared coupling of 1s2p to 1s2s, 1s3d, and 1s3s states are observed. Absorption features that likely result from coupling of 1s3p to 1s4s, 1s4d, 1s5s, and 1s5d states are also observed. The light-induced states with the smallest detunings (1s3d and 1s3s) from the dressing frequency may shift to higher frequencies as the dressing intensity is increased.

Investigation of coupling mechanisms in attosecond transient absorption of autoionizing states: comparison of theory and experiment in xenon

© 2015 IOP Publishing Ltd. Attosecond transient absorption spectra near the energies of autoionizing states are analyzed in terms of the photon coupling mechanisms to other states. In a recent experiment, the autoionization lifetimes of highly excited states of xenon were determined and compared to a simple expression based on a model of how quantum coherence determines the decay of a metastable state in the transient absorption spectrum. Here it is shown that this procedure for extracting lifetimes is more general and can be used in cases involving either resonant or nonresonant coupling of the attosecond-probed autoionizing state to either continua or discrete states by a time-delayed near infrared (NIR) pulse. The fits of theoretically simulated absorption signals for the 6p resonance in xenon (lifetime = 21.1 fs) to this expression yield the correct decay constant for all the coupling mechanisms considered, properly recovering the time signature of twice the autoionization lifetime due to the coherent nature of the transient absorption experiment. To distinguish between these two coupling cases, the characteristic dependencies of the transient absorption signals on both the photon energy and time delay are investigated. Additional oscillations versus delay-time in the measured spectrum are shown and quantum beat analysis is used to pinpoint the major photon-coupling mechanism induced by the NIR pulse in the current xenon experiment: the NIR pulse resonantly couples the attosecond-probed state, 6p, to an intermediate 8s (at 22.563 eV), and this 8s state is also coupled to a neighboring state (at 20.808 eV).

Frequency Tunable Attosecond Apparatus

The development of attosecond technology is one of the most significant recent achievements in the field of ultrafast optics; it opens up new frontiers in atomic and molecular spectroscopy and dynamics. A unique attosecond pump-probe apparatus using a compact Mach-Zehnder interferometer is developed. The interferometer system is compact (∼290 cm2) and completely located outside of the vacuum chamber. The location reduces the mechanical vibration from vacuum components such as turbopumps and roughing pumps. The stability of the interferometer is ∼50 as RMS over 24 hours, stabilized with an active feedback loop. The pump and probe fields can be easily altered to incorporate multiple colors. In the interferometer, double optical gating optics are arranged to generate isolated attosecond pulses with a supercontinuum spectrum. The frequencies of the attosecond pulses can be selected to be in the extreme ultraviolet (XUV) region (25–55 eV, 140 as) or the vacuum ultraviolet (VUV) region (15–24 eV, ∼400 as) by metal filters. Furthermore, the near infrared probe field (1.65 eV) can be upconverted to the ultraviolet (3.1 eV). The frequency tunability in the XUV and VUV is critical for selecting excited states of target atoms and molecules.

Isolated attosecond pulse contrast measurement and CEP optimization for optical streaking in molecules

A method to access and control isolated attosecond pulse contrast is presented, based on scanning the Carrier-Envelope Phase (CEP). The optimized pulses produced time-resolved streak-field photoelectron data in molecules (SF6, N2).