Cornelis J. Uiterwaal

Creation of optical vortices in femtosecond pulses

We experimentally created a femtosecond optical vortex using a pair of computer-synthesized holographic gratings arranged in a 2f - 2f optical setup. We present measurements showing that the resulting donut mode is free of spatial chirp, and support this finding with an analysis of the optical wave propagation through our system based on the Kirchhoff-Fresnel diffraction integral. An interferogram confirms that our ultrashort vortex has topological charge 1, and a conservative experimental estimation of its duration is 280 fs. We used 25-fs radiation pulses (bandwidth approximately 40 nm) produced by a Ti:sapphire laser oscillator.

Laser-induced ultrafast electron emission from a field emission tip

We show that a field emission tip electron source that is triggered with a femtosecond laser pulse can generate electron pulses shorter than the laser pulse duration (100 fs). The emission process is sensitive to a power law of the laser intensity, which supports an emission mechanism based on multiphoton absorption followed by over-the-barrier emission. Observed continuous transitions between power laws of different orders are indicative of field emission processes. We show that the source can also be operated so that thermionic emission processes become significant. Understanding these different emission processes is relevant for the production of sub-cycle electron pulses.

Temporal lenses for attosecond and femtosecond electron pulses

Here, we describe the “temporal lens” concept that can be used for the focus and magnification of ultrashort electron packets in the time domain. The temporal lenses are created by appropriately synthesizing optical pulses that interact with electrons through the ponderomotive force. With such an arrangement, a temporal lens equation with a form identical to that of conventional light optics is derived. The analog of ray diagrams, but for electrons, are constructed to help the visualization of the process of compressing electron packets. It is shown that such temporal lenses not only compensate for electron pulse broadening due to velocity dispersion but also allow compression of the packets to durations much shorter than their initial widths. With these capabilities, ultrafast electron diffraction and microscopy can be extended to new domains,and, just as importantly, electron pulses can be delivered directly on an ultrafast techniques target specimen.

In Situ Measurement of Three-Dimensional Ion Densities in Focused Femtosecond Pulses

We image spatial distributions of Xeq+ ions in the focus of a laser beam of ultrashort, intense pulses in all three dimensions, with a resolution of approximately 3 microm and approximately 12 microm in the two transverse directions. This allows for studying ionization processes without spatially averaging ion yields. Our in situ ion imaging is also useful to analyze focal intensity profiles and to investigate the transverse modal purity of tightly focused beams of complex light. As an example, the intensity profile of a Hermite-Gaussian beam mode HG1,0 recorded with ions is found to be in good agreement with optical images.

Photoionization and photofragmentation of gaseous toluene using 80-fs, 800-nm laser pulses

is found to be effectively proportional to the sixth power of the peak intensity. This is shown to be in good agreement with a multiple lowest-order perturbation multiphoton ionization model which takes into account successive channel closing for increasing peak intensities and orders up to 11 inclusive. On the assumption that the excess energy acquired by the toluene cation as a result of the interaction with the electromagnetic field is of the order of the ponderomotive energy for the intensity prevailing at the moment of the ionization, the internal energy distribution of the toluene cations created that is brought about by this multiple-order multiphoton ionization model is calculated. This internal energy distribution is in perfect agreement with the measured C 7H 7 yield, if the rate-energy curve for the fragmentation of excited toluene cations as given by Golovin et al. @Sov. J. Chem. Phys. 2, 632 ~1985!# is moderately reduced by a factor of 4.5.

State‐selected ion‐molecule reactions: Charge transfer and atomic rearrangement processes in thermal energy collisions of H2+(X;v)+N2 and of N2+(X,A;v) + H2

Using the photo‐electron‐product‐ion‐coincidence method (PEPICO) we have measured state‐selective cross sections for the following processes: (A) N+2(X,A;v)+H2→N2H++H, (B) H+2(X;v)+N2→N2H++H, (C) N+2(X,A;v)+H2→H+2+N2, and (D) H2+(X;v)+N2→N2++H2. The measurements were performed at thermal velocities (Ec.m.≊40 meV). We have found that the charge transfer processes (C) and (D) have cross sections that are at least an order of magnitude smaller than the cross sections for the rearrangement processes (A) and (B). The cross section for reaction (A) with N2+(A;v) as reactant is found to be (50.2±2.4)% of the cross section for the same reaction with N2+(X;v) as reactant. The cross section for reaction (B) is found to be independent of the internal energy of the reactant ion. The measured variation of the cross sections as a function of the internal energy of the reacting ion is compared with calculations based on a RRKM type statistical model and an electronic correlation diagram of the (N2–H2)+ system. Excellent...

Ultrashort intense-field optical vortices produced with laser-etched mirrors.

We introduce a simple and practical method to create ultrashort intense optical vortices for applications involving high-intensity lasers. Our method utilizes femtosecond laser pulses to laser etch grating lines into laser-quality gold mirrors. These grating lines holographically encode an optical vortex. We derive mathematical equations for each individual grating line to be etched, for any desired (integer) topological charge. We investigate the smoothness of the etched grooves. We show that they are smooth enough to produce optical vortices with an intensity that is only a few percent lower than in the ideal case. We demonstrate that the etched gratings can be used in a folded version of our 2f-2f setup [Opt. Express 19, 7599 (2005)] to compensate angular dispersion. Finally, we show that the etched gratings withstand intensities of up to 10(12) W/cm(2).

Exploring temporal and rate limits of laser-induced electron emission

To achieve high temporal resolution for ultrafast electron diffraction, Zewail (Proc. Natl Acad. Sci. USA 102, 7069 (2005)) has proposed to use high repetition rate, ultrafast electron sources. Such electron sources emitting one electron per pulse eliminate Coulomb broadening. High repetition rates are necessary to achieve reasonable data acquisition times. We report laser-induced emission from a nanometre-sized tip at one electron per pulse with a 1 kHz repetition rate in the femtosecond regime. This source, combined with 1 MHz repetition rate lasers that are becoming available, will be a primary candidate for next generation ultrafast, high-coherence electron diffraction experiments. We also report that the measured energy bandwidth of our electron source does not support sub-cycle electron emission. This result addresses a current debate on ultrafast nanotip sources. Regardless of the limited bandwidth, this source may be used in conjunction with a recently proposed active dispersion compensation technique (Proc. Natl Acad. Sci. USA 104, 18409 (2007)) to deliver attosecond electron pulses on a target.

Efficient angular dispersion compensation in holographic generation of intense ultrashort paraxial beam modes

We experimentally demonstrate that small misalignments of the pulse stretcher or compressor of our chirped-pulse-amplification laser can precompensate for angular chirp when producing ultrashort paraxial beam modes with holographic gratings. Using this approach we can eliminate one of the two gratings needed in our 2f-2f setup [Mariyenko, Opt. Express 13, 7599 (2005)]. This allows for up to an order of magnitude more output power. We see our method as the next step in the production of intense exotic forms of ultrashort pulses, which can be used in the investigation of intense laser-matter interactions. In addition, we produce the first femtosecond (helical-)Ince-Gaussian beams.

Charge transfer and atomic rearrangement in collisions of Ar+(2P12,32) with H2 measured with a new PEPICO apparatus

Abstract A new apparatus designed to perform photoelectron product-ion coincidence (PEPICO) measurements is described. The collision system Ar + ( 2 P 1 2 , 3 2 ) + H 2 is studied as a first test case. Relative cross sections for ArH + and H 2 + formation by the two fine structure states of the ion are measured at three average center-of-mass collision energies, 0.05, 0.075 and 0.125 eV. The results are interpreted in the context of data available in the literature and confirm the importance of a vibronic resonance for the Ar + ( 2 P 1 2 )+ H 2 system at low collision energies.