19,275 research outputs found
Novel mid-infrared dispersive wave generation in gas-filled PCF by transient ionization-driven changes in dispersion
Gas-filled hollow-core photonic crystal fibre (PCF) is being used to generate
ever wider supercontinuum spectra, in particular via dispersive wave (DW)
emission in the deep and vacuum ultraviolet, with a multitude of applications.
DWs are the result of the resonant transfer of energy from a self-compressed
soliton, a process which relies crucially on phase matching. It was recently
predicted that, in the strong-field regime, the additional transient anomalous
dispersion introduced by gas ionization would allow phase-matched DW generation
in the mid-infrared (MIR)-something that is forbidden in the absence of free
electrons. Here we report for the first time the experimental observation of
such MIR DWs, embedded in a 4.7-octave-wide supercontinuum that uniquely
reaches simultaneously to the vacuum ultraviolet, with up to 1.7 W of total
average power
Single-shot fluctuations in waveguided high-harmonic generation
For exploring the application potential of coherent soft x-ray (SXR) and
extreme ultraviolet radiation (XUV) provided by high-harmonic generation, it is
important to characterize the central output parameters. Of specific importance
are pulse-to-pulse (shot-to-shot) fluctuations of the high-harmonic output
energy, fluctuations of the direction of the emission (pointing instabilities),
and fluctuations of the beam divergence and shape that reduce the spatial
coherence. We present the first single-shot measurements of waveguided
high-harmonic generation in a waveguided (capillary-based) geometry. Using a
capillary waveguide filled with Argon gas as the nonlinear medium, we provide
the first characterization of shot-to-shot fluctuations of the pulse energy, of
the divergence and of the beam pointing. We record the strength of these
fluctuations vs. two basic input parameters, which are the drive laser pulse
energy and the gas pressure in the capillary waveguide. In correlation
measurements between single-shot drive laser beam profiles and single-shot
high-harmonic beam profiles we prove the absence of drive laser
beam-pointing-induced fluctuations in the high-harmonic output. We attribute
the main source of high-harmonic fluctuations to ionization-induced nonlinear
mode mixing during propagation of the drive laser pulse inside the capillary
waveguide
Focusing and Compression of Ultrashort Pulses through Scattering Media
Light scattering in inhomogeneous media induces wavefront distortions which
pose an inherent limitation in many optical applications. Examples range from
microscopy and nanosurgery to astronomy. In recent years, ongoing efforts have
made the correction of spatial distortions possible by wavefront shaping
techniques. However, when ultrashort pulses are employed scattering induces
temporal distortions which hinder their use in nonlinear processes such as in
multiphoton microscopy and quantum control experiments. Here we show that
correction of both spatial and temporal distortions can be attained by
manipulating only the spatial degrees of freedom of the incident wavefront.
Moreover, by optimizing a nonlinear signal the refocused pulse can be shorter
than the input pulse. We demonstrate focusing of 100fs pulses through a 1mm
thick brain tissue, and 1000-fold enhancement of a localized two-photon
fluorescence signal. Our results open up new possibilities for optical
manipulation and nonlinear imaging in scattering media
Nonlinear beam self-imaging and self-focusing dynamics in a GRIN multimode optical fiber: theory and experiments
Beam self-imaging in nonlinear graded-index multimode optical fibers is of
interest for many applications, such as implementing a fast saturable absorber
mechanism in fiber lasers via multimode interference. We obtain an exact
solution for the nonlinear evolution of first and second order moments of a
laser beam carried by a graded-index multimode fiber, predicting that the
spatial self-imaging period does not vary with power. Whereas the amplitude of
the oscillation of the beam width is power-dependent. We have experimentally
studied the longitudinal evolution of beam self-imaging by means of femtosecond
laser pulse propagation in both the anomalous and the normal dispersion regime
of a standard telecom graded-index multimode optical fiber. Light scattering
out of the fiber core via visible fluorescence emission and harmonic wave
generation permits us to directly confirm that the self-imaging period is
invariant with power. Spatial shift and splitting of the self-imaging process
under the action of self-focusing are also emphasized
Self-phase modulation of a single-cycle terahertz pulse by nonlinear free-carrier response in a semiconductor
We demonstrate the self-phase modulation (SPM) of a single-cycle THz pulse in
a semiconductor, using bulk n-GaAs as a model system. The SPM arises from the
heating of free electrons in the electric field of the THz pulse, leading to an
ultrafast reduction of the plasma frequency, and hence to a strong modification
of the THz-range dielectric function of the material. THz SPM is observed
directly in the time domain. In the frequency domain it corresponds to a strong
frequency-dependent refractive index nonlinearity of n-GaAs, found to be both
positive and negative within the broad THz pulse spectrum, with the
zero-crossing point defined by the electron momentum relaxation rate. We also
observed the nonlinear spectral broadening and compression of the THz pulse.Comment: 5 pages, 6 figure
Ultra-compact optical auto-correlator based on slow-light enhanced third harmonic generation in a silicon photonic crystal waveguide
The ability to use coherent light for material science and applications is
directly linked to our ability to measure short optical pulses. While
free-space optical methods are well-established, achieving this on a chip would
offer the greatest benefit in footprint, performance, flexibility and cost, and
allow the integration with complementary signal processing devices. A key goal
is to achieve operation at sub-Watt peak power levels and on sub-picosecond
timescales. Previous integrated demonstrations require either a temporally
synchronized reference pulse, an off-chip spectrometer, or long tunable delay
lines. We report the first device capable of achieving single-shot time-domain
measurements of near-infrared picosecond pulses based on an ultra-compact
integrated CMOS compatible device, with the potential to be fully integrated
without any external instrumentation. It relies on optical third-harmonic
generation in a slow-light silicon waveguide. Our method can also serve as a
powerful in-situ diagnostic tool to directly map, at visible wavelengths, the
propagation dynamics of near-infrared pulses in photonic crystals.Comment: 20 pages, 6 figures, 38 reference
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