Phillip M. Nagel

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.

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.

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.

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).

Coherent control using the carrier-envelope phase

Octave-spanning laser pulses enable a new coherent control scheme based on the carrier-envelope phase (CEP). Multiphoton ionization of Xe atoms demonstrates CEP control over the phases of photoelectron quantum states and therefore the photoelectron direction.

Ionization gating for tunable isolated attosecond pulse generation

Ionization gating confines high-harmonic generation to the leading edge of the driver pulse. Experimentally produced soft-X-ray continuous radiation is spectrally broad and tunable. The method suggests isolated attosecond-pulse production with long driver pulses.

Multi-Cycle Driven Isolated Attosecond Pulse Generation

Conversion of 1% of the fundamental pulse intensity to another wavelength before high-harmonic generation is shown theoretically to quadruple the period of the attosecond pulse train in the cutoff photon-energy region. In simulations, a 24-fs pulse can produce an isolated attosecond pulse.