Mark J. Abel

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.

Circular phase mask for control and stabilization of single optical filaments

We experimentally demonstrate an efficient way to control and stabilize single optical filaments initiated by ultrashort laser pulses in a rare gas medium. This is done by the application of a stationary two-dimensional phase mask to the laser beam prior to focusing. Simple circular phase-step patterns of a given radius and relative phase are sufficient to stabilize the pointing of the filament output and to optimize the spectral bandwidth of the light without any resulting loss of input laser power.

Generating coherent broadband continuum soft-x-ray radiation by attosecond ionization gating

The current paradigm of isolated attosecond pulse production requires a few-cycle pulse as the driver for high-harmonic generation that has a cosine-like electric field stabilized with respect to the peak of the pulse envelope. Here, we present simulations and experimental evidence that the production of high-harmonic light can be restricted to one or a few cycles on the leading edge of a laser pulse by a gating mechanism that employs time-dependent ionization of the conversion medium. This scheme enables the generation of broadband and tunable attosecond pulses. Instead of fixing the carrier-envelope phase to produce a cosine driver pulse, the phase becomes a control parameter for the center frequency of the attosecond pulse. A method to assess the multiplicity of attosecond pulses in the pulse train is also presented. The results of our study suggest an avenue towards relaxing the requirement of few-cycle pulses for isolated attosecond pulse generation.

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.

Driving Rotational Transitions in Molecules on a Chip

Polar molecules in selected quantum states can be guided, decelerated, and trapped using electric fields created by microstructured electrodes on a chip. Herein we explore how transitions between two of these quantum states can be induced while the molecules are on the chip. We use CO (a(3) Π(1) , v=0) molecules, prepared in the J=1 rotational level, and induce the J=2←J=1 rotational transition with narrow-band sub-THz (mm-wave) radiation. First, the mm-wave source is characterized using CO molecules in a freely propagating molecular beam, and both Rabi cycling and rapid adiabatic passage are examined. Then we demonstrate that the mm-wave radiation can be coupled to CO molecules that are less than 50 μm above the chip. Finally, CO molecules are guided in the J=1 level to the center of the chip where they are pumped to the J=2 level, recaptured, and guided off the chip.

Imaging Cold Molecules on a Chip

We present the integrated imaging of cold molecules in a microchip environment. The on-chip detection is based on resonance-enhanced multiphoton ionization, which is quantum state selective and generally applicable. We demonstrate and characterize time-resolved spatial imaging and subsequently use it to analyze the effect of a phase-space manipulation sequence aimed at compressing the velocity distribution of a molecular ensemble with a view to future high-resolution spectroscopic studies. The realization of such on-chip measurements adds the final fundamental component to the molecule chip, offering a new and promising route for investigating cold molecules.

Vibrationally exciting molecules trapped on a microchip

Polar molecules in selected quantum states can be guided, decelerated and trapped using electric fields created by microstructured electrodes on a chip. Here we demonstrate that transitions between two vibrational quantum states can be induced while the molecules are trapped above the chip. We use CO molecules, prepared in the J = 1 rotational level of the a 3Π1, v = 0 state and induce the transition to either the J = 1 or the J = 2 level in the vibrationally excited a 3Π1, v = 1 state with pulsed, narrowband IR radiation. First, the vibrational excitation is studied using CO molecules in a freely propagating molecular beam, in a well-defined homogeneous electric field. Then, we demonstrate that the IR radiation can be coupled to CO molecules that are trapped less than 50 µm above the chip by guiding molecules in the v = 0 level to the center of the chip where they are pumped to the v = 1 level. The molecules remain trapped and are then guided off the chip and state-selectively detected.

Frequency-Controlled Isolated Attosecond Pulses Characterized by Both 750 and 400 nm Wavelength Streak Fields

Frequency tunability of isolated attosecond pulses provides options for the study of temporal dynamics and phases of electronic processes [1]. Techniques to generate frequency-controlled attosecond pulses (XUV and VUV) and wavelength selective streak pulses (NIR and UV) are discussed here. A novel Mach-Zehnder (MZ) interferometer is used to combine all optical fields before the high-harmonic generation region.