Martin Centurion Publications

Picosecond electron diffraction from molecules aligned by dissociation

Martin Centurion et al. — Wed, 03 Oct 2012 09:39:22 PDT

In gas electron diffraction an averaging over statistical directions of the molecules takes place. This results in diffraction patterns in the form of isotropic rings which yield information only on the radial distribution function displaying the inter-atomic distances. We demonstrate that by dissociating molecules with linearly polarized light the pattern becomes anisotropic. In the experiments the iodide C₂F₄I₂ is dissociated and molecular difference intensities and difference radial distribution curves are measured for directions parallel and perpendicular to the direction of polarization. With picosecond temporal resolution the curves clearly demonstrate transient anisotropy and its decay by molecular rotation. This experiment is a first step towards the determination of structure and of ultrafast structural changes by electron diffraction from aligned molecules.

Imaging of Isolated Molecules with Ultrafast Electron Pulses

Christopher J. Hensley et al. — Mon, 01 Oct 2012 13:03:37 PDT

Imaging isolated molecules in three dimensions with atomic resolution is important for elucidating complex molecular structures and intermediate states in molecular dynamics. This goal has so far remained elusive due to the random orientation of molecules in the gas phase. We show that threedimensional structural information can be retrieved from multiple electron diffraction patterns of aligned molecules. The molecules are aligned impulsively with a femtosecond laser pulse and probed with a femtosecond electron pulse two picoseconds later, when the degree of alignment reaches a maximum.

Dispersion compensation for attosecond electron pulses

Peter Hansen et al. — Mon, 01 Oct 2012 10:50:21 PDT

We propose a device to compensate for the dispersion of attosecond electron pulses. The device uses only static electric and magnetic fields and therefore does not require synchronization to the pulsed electron source. Analogous to the well-known optical dispersion compensator, an electron dispersion compensator separates paths by energy in space. Magnetic fields are used as the dispersing element, while a Wien filter is used for compensation of the electron arrival times. We analyze a device with a size of centimeters, which can be applied to ultrafast electron diffraction and microscopy, and fundamental studies.

Ultrashort pulse electron gun with a MHz repetition rate

D. Wytrykus et al. — Mon, 01 Mar 2010 09:58:02 PST

We report the construction of an electron gun emitting ultrashort pulses with a repetition rate of 2.7 MHz. The gun works at an acceleration voltage of 20 kV and is operated with a laser oscillator having an ultralong cavity. A low number of electrons per pulse eliminates space charge broadening. Electron yield and beam profiles are measured for operation with laser wavelengths of 800, 400, and 266 nm. The initial energy spread of the electrons is determined for these three wavelengths, and pulse durations of 600, 390, and 270 fs are inferred from the data.

Erratum: Dynamics of filament formation in a Kerr medium [Phys Rev. A 71, 063811 (2005)]

Martin Centurion et al. — Tue, 23 Feb 2010 10:32:40 PST

We have found an error in this paper -- [Phys Rev. A 71, 063811 (2005)]. In our calculation of the intensity threshold for breakdown in CS₂ we erroneously used the ionization energy of molecular CS₂ (10.08 eV). The correct value should be 3.3 eV, the band gap energy of liquid CS₂ . Using the band gap energy, nonlinear absorption will be a three-photon process, and the calculated breakdown threshold becomes 8.5 × 10¹¹ W/cm² (as opposed to the value of 2×10¹³ W/cm² reported previously). Multiphoton absorption and plasma defocusing could then become significant for the intensity of the filaments, I_fil = (5.9 ± 2.4) × 10¹¹ W/cm², that was measured experimentally. ...
In conclusion, our numerical results show that the effect of plasma alone cannot reproduce the experimental results, while including a fifth order nonlinearity (with or without plasma) generates good agreement with the experiments. The main conclusion of the numerical section of the original paper then remains unchanged in that the fifth order nonlinearity provides good agreement with the experiment while plasma effects alone do not.

Dynamics of filament formation in a Kerr medium (with Erratum)

Martin Centurion et al. — Tue, 23 Feb 2010 10:23:09 PST

We have studied the large-scale beam breakup and filamentation of femtosecond pulses in a Kerr medium. We have experimentally monitored the formation of stable light filaments, conical emission, and interactions between filaments. Three major stages lead to the formation of stable light filaments: First the beam breaks up into a pattern of connected lines (constellation), then filaments form on the constellations, and finally the filaments release a fraction of their energy through conical emission. We observed a phase transition to a faster filamentation rate at the onset of conical emission. We attribute this to the interaction of conical emissions with the constellation which creates additional filaments. Numerical simulations show good agreement with the experimental results.

Includes Erratum from PHYSICAL REVIEW A 74, 069902(E) (2006): We have found an error in this paper. In our calculation of the intensity threshold for breakdown in CS₂ we erroneously used the ionization energy of molecular CS₂ (10.08 eV). ...

Nonlinearity Management in Optics: Experiment, Theory, and Simulation

Martin Centurion et al. — Tue, 23 Feb 2010 10:16:14 PST

We conduct an experimental investigation of nonlinearity management in optics using femtosecond pulses and layered Kerr media consisting of glass and air. By examining the propagation properties over several diffraction lengths, we show that wave collapse can be prevented. We corroborate these experimental results with numerical simulations of the (2 + 1)-dimensional focusing cubic nonlinear Schrödinger equation with piecewise constant coefficients and a theoretical analysis of this setting using a moment method.

Modulational Instability in a Layered Kerr Medium: Theory and Experiment

Martin Centurion et al. — Tue, 23 Feb 2010 10:12:59 PST

We present the first experimental investigation of modulational instability in a layered Kerr medium. The particularly interesting and appealing feature of our configuration, consisting of alternating glass-air layers, is the piecewise-constant nature of the material properties, which allows a theoretical linear stability analysis leading to a Kronig-Penney equation whose forbidden bands correspond to the modulationally unstable regimes. We find very good quantitative agreement between theoretical, numerical, and experimental diagnostics of the modulational instability. Because of the periodicity in the evolution variable arising from the layered medium, there are multiple instability regions rather than just one as in a uniform medium.

Modulational instability in nonlinearity-managed optical media

Martin Centurion et al. — Tue, 23 Feb 2010 10:09:04 PST

We investigate analytically, numerically, and experimentally the modulational instability in a layered, cubically nonlinear (Kerr) optical medium that consists of alternating layers of glass and air. We model this setting using a nonlinear Schrödinger (NLS) equation with a piecewise constant nonlinearity coefficient and conduct a theoretical analysis of its linear stability, obtaining a Kronig-Penney equation whose forbidden bands correspond to the modulationally unstable regimes. We find very good quantitative agreement between the theoretical analysis of the Kronig-Penney equation, numerical simulations of the NLS equation, and the experimental results for the modulational instability. Because of the periodicity in the evolution variable arising from the layered medium, we find multiple instability regions rather than just the one that would occur in uniform media.

Proposed method for measuring the duration of electron pulses by attosecond streaking

Peter Reckenthaeler et al. — Tue, 23 Feb 2010 10:04:42 PST

We propose a method to measure the duration of ultrashort electron pulses. The electron pulse to be measured impinges on a solid target, causing the emission of Auger electrons through impact ionization. The energy spectrum of the Auger electrons is altered in the presence of an intense femtosecond laser field. Due to the extremely short lifetime of the Auger effect, this effect can be used to generate cross correlation between a laser and an electron pulse. The method is applicable to electron pulses ranging from hundreds of attoseconds to hundreds of femtoseconds in duration, and for a few hundreds of electron volts to relativistic energies.

Time-Resolved Electron Diffraction from Selectively Aligned Molecules

Peter Reckenthaeler et al. — Tue, 23 Feb 2010 10:01:10 PST

We experimentally demonstrate ultrafast electron diffraction from transiently aligned molecules in the absence of external (aligning) fields. A sample of aligned molecules is generated through photodissociation with femtosecond laser pulses, and the diffraction pattern is captured by probing the sample with picosecond electron pulses shortly after dissociation—before molecular rotation causes the alignment to vanish. In our experiments the alignment decays with a time constant of 2.6 ± 1.2 ps.

Holographic recording of laser-induced plasma

Martin Centurion et al. — Tue, 23 Feb 2010 09:56:48 PST

We report on a holographic probing technique that allows for measurement of free-electron distribution with fine spatial detail. Plasma is generated by focusing a femtosecond pulse in air. We also demonstrate the capability of the holographic technique of capturing the time evolution of the plasma-generation process.

Picosecond imaging of low-density plasmas by electron deflectometry

Martin Centurion et al. — Tue, 23 Feb 2010 09:42:01 PST

We have imaged optical-field ionized plasmas with electron densities as low as 10¹³ cm⁻³ on a picosecond timescale using ultrashort electron pulses. Electric fields generated by the separation of charges are imprinted on a 20 keV probe electron pulse and reveal a cloud of electrons expanding away from a positively charged plasma core. Our method allows for a direct measurement of the electron energy required to escape the plasma and the total charge. Simulations reproduce the main features of the experiment and allow determination of the energy of the electrons.

Holographic capture of femtosecond pulse propagation

Martin Centurion et al. — Tue, 23 Feb 2010 09:38:27 PST

We have implemented a holographic system to study the propagation of femtosecond laser pulses with high temporal (150 fs) and spatial resolutions (4 μm). The phase information in the holograms allows us to reconstruct both positive and negative index changes due to the Kerr nonlinearity (positive) and plasma formation (negative), and to reconstruct three-dimensional structure. Dramatic differences were observed in the interaction of focused femtosecond pulses with air, water, and carbon disulfide. The air becomes ionized in the focal region, while in water long plasma filaments appear before the light reaches a tight focus. In contrast, in carbon disulfide the optical beam breaks up into multiple filaments but no plasma is measured. We explain these different propagation regimes in terms of the different nonlinear material properties.

Harmonic holography: a new holographic principle

Ye Pu et al. — Tue, 23 Feb 2010 09:32:36 PST

The process of second harmonic generation (SHG) has a unique property of forming a sharp optical contrast between noncentrosymmetric crystalline materials and other types of material, which is a highly valuable asset for contrast microscopy. The coherent signal obtained through SHG also allows for the recording of holograms at high spatial and temporal resolution, enabling whole-field four-dimensional microscopy for highly dynamic microsystems and nanosystems. Here we describe a new holographic principle, harmonic holography (H²), which records holograms between independently generated second harmonic signals and reference. We experimentally demonstrate this technique with digital holographic recording of second harmonic signals upconverted from an ensemble of second harmonic generating nanocrystal clusters under femtosecond laser excitation. Our results show that harmonic holography is uniquely suited for ultrafast four-dimensional contrast microscopy.

Self-organization of spatial solitons

Martin Centurion et al. — Tue, 23 Feb 2010 09:27:56 PST

We present experimental results on the transverse modulation instability of an elliptical beam propagating in a bulk nonlinear Kerr medium, and the formation and self-organization of spatial solitons. We have observed the emergence of order, self organization and a transition to an unstable state. Order emerges through the formation of spatial solitons in a periodic array. If the initial period of the array is unstable the solitons will tend to self-organize into a larger (more stable) period. Finally the system transitions to a disordered state where most of the solitons disappear and the beam profile becomes unstable to small changes in the input energy.

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Holographic recording of fast events on a CCD camera

Zhiwen Liu et al. — Tue, 23 Feb 2010 09:24:24 PST

We report on holographic recording of nanosecond events on a conventional CCD camera. Three frames of an air-discharge event, with resolution of 5.9 ns and frame interval of 12 ns, are recorded in a single CCD frame. Each individual frame is reconstructed by digital filtering of the CCD frame, since successively recorded holograms are centered at different carrier frequencies in the spatial frequency domain.