Attosecond spectroscopy with laser-generated photons can in principle resolve electronic processes in real time, but a movie-like space–time imaging is impeded by the wavelength, which is ~100 times longer than atomic distances. Here we advance attosecond science to sub-atomic spatial resolution by using sub-relativistic electron beams instead of the high-harmonic photons. A beam of 70-keV electrons at 4.5-pm de Broglie wavelength is temporally modulated by the electric field of laser cycles into a train of attosecond pulses with the help of a dielectric modulation element. The pulses in the train have 820-as duration and maintain the degree of coherence of the original electron beam. We demonstrate the feasibility of analytic attosecond–Angstrom imaging by recording time-resolved Bragg diffraction from a singlecrystalline silicon. Real-space electron microscopy with the attosecond electron pulses visualizes the propagation of optical waves at a dielectric membrane with sub-wavelength and sub-optical-cycle resolution. This unification of attosecond science with electron diffraction/microscopy will enable the direct visualization of fundamental and complex light-matter interaction in space and time.
We demonstrate a simple method for spectral broadening and compression of laser pulses at megahertz repetition rates
by self-phase modulation in a large mode area (LMA) fiber. In order to avoid the currently limiting factor of damage by
self-focusing, we positively chirp the input pulse, which allows coupling of significantly more energy into the fiber,
while maintaining the same spectral bandwidth and compression as compared to the Fourier-limited case at lower
energy. Using a commercial chirped pulse Ti:Sa oscillator (Femtolasers, Femtosource XL) with 55fs, 400nJ pulses at
5MHz and an LMA fiber with 25μm core diameter, we generate 16fs, 350nJ pulses, which is a factor of 4 more energy
than possible with unchirped input pulses. Good stability has been measured over at least 1 hour for the chirped case and
unchirped case. Furthermore, with a 5μm core diameter LMA fiber we generated compressed pulses with 6fs and 18nJ
output energy. This would allow a carrier-to-envelope phase stabilization of the laser system by external selfstabilization
via acoustic difference-frequency modulation. The compact size and its simplicity makes the combination
of a chirped pulse oscillator with chirped-fiber-broadening an attractive option for ultrafast spectroscopy at MHz
repetition rate.
We present a novel detection scheme for coherent anti-Stokes Raman scattering (CARS) which is capable of substantially suppressing coherent background signals which are a basic problem associated with CARS. It exploits the fixed phase relationship between pump, Stokes and CARS fields together with the strong phase coherence in supercontinua generated by femtosecond pulses. Three phase-locked noncollinearly phase-matched optical parametric amplifiers (NOPAs) seeded by a common white-light continuum are used for the realization of a heterodyne signal detection. In combination with proper pulse timing, i.e. appropriately time-delayed probing and heterodyning, a gating mechanism is provided to significantly suppress resonant and nonresonant solvent background signals. Therefore the demonstrated technique is suitable for high-contrast vibrational imaging.
Current and planned helmet system require precision metrology technique in order to provide accurate pointing information with low latency times. Existing systems using magnetic sensors to determined helmet position and orientation can provide adequate information to meet most requirements, however, the amount of time required for mapping the magnetic field within an aircraft cockpit is often seen to be excessive. While highly accurate optical based metrology systems can be designed to overcome the time consuming cockpit mapping problem, they have also been criticized as being overly complex and unsuitable for aircraft use. Visidyne, Inc. has developed an optical system that uses a proprietary measurement technique to measure the phase of low power, eyesafe, intensity modulated light beams, which, when properly installed within the cockpit can measure the x, y, z position and roll, pitch, yaw of the helmet, providing pointing precision that is within 1 milliradian, at an update rate of at least 100 Hz over a large motion box. The technique uses state-of-the-art electronics and optics that are both robust and reliable and add minimally to the helmet mass. This paper describes two approaches for applying this technology to the helmet tracking problem, each using precise measurements of distance between points on the helmet and known locations within the cockpit.
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