326 research outputs found
Field-cycle-resolved photoionization in solids
The Keldysh theory of photoionization in a solid dielectric is generalized to
the case of arbitrarily short driving pulses of arbitrary pulse shape. We
derive a closed-form solution for the nonadiabatic ionization rate in a
transparent solid with a periodic dispersion relation, which reveals ultrafast
ionization dynamics within the field cycle and recovers the key results of the
Keldysh theory in the appropriate limiting regimes.Comment: 4 pages, 2 figure
CEP-stable Tunable THz-Emission Originating from Laser-Waveform-Controlled Sub-Cycle Plasma-Electron Bursts
We study THz-emission from a plasma driven by an incommensurate-frequency
two-colour laser field. A semi-classical transient electron current model is
derived from a fully quantum-mechanical description of the emission process in
terms of sub-cycle field-ionization followed by continuum-continuum electron
transitions. For the experiment, a CEP-locked laser and a near-degenerate
optical parametric amplifier are used to produce two-colour pulses that consist
of the fundamental and its near-half frequency. By choosing two incommensurate
frequencies, the frequency of the CEP-stable THz-emission can be continuously
tuned into the mid-IR range. This measured frequency dependence of the
THz-emission is found to be consistent with the semi-classical transient
electron current model, similar to the Brunel mechanism of harmonic generation
Time-domain spectroscopy in the mid-infrared
When coupled to characteristic, fingerprint vibrational and rotational motions of molecules, an electromagnetic field with an appropriate frequency and waveform offers a highly sensitive, highly informative probe, enabling chemically specific studies on a broad class of systems in physics, chemistry, biology, geosciences, and medicine. The frequencies of these signature molecular modes, however, lie in a region where accurate spectroscopic measurements are extremely difficult because of the lack of efficient detectors and spectrometers. Here, we show that, with a combination of advanced ultrafast technologies and nonlinear-optical waveform characterization, time-domain techniques can be advantageously extended to the metrology of fundamental molecular motions in the mid-infrared. In our scheme, the spectral modulation of ultrashort mid-infrared pulses, induced by rovibrational motions of molecules, gives rise to interfering coherent dark waveforms in the time domain. These high-visibility interference patterns can be read out by cross-correlation frequency-resolved gating of the field in the visible generated through ultrabroadband four-wave mixing in a gas phase
Ultralow-power local laser control of the dimer density in alkali-metal vapors through photodesorption
Ultralow-power diode-laser radiation is employed to induce photodesorption of
cesium from a partially transparent thin-film cesium adsorbate on a solid
surface. Using resonant Raman spectroscopy, we demonstrate that this
photodesorption process enables an accurate local optical control of the
density of dimer molecules in alkali-metal vapors.Comment: 4 pages, 4 figure
A strong-field driver in the single-cycle regime based on self-compression in a kagome fibre
Over the past decade intense laser fields with a single-cycle duration and even shorter, subcycle multicolour field transients have been generated and applied to drive attosecond phenomena in strong-field physics. Because of their extensive bandwidth, single-cycle fields cannot be emitted or amplified by laser sources directly and, as a rule, are produced by external pulse compression—a combination of nonlinear optical spectral broadening followed up by dispersion compensation. Here we demonstrate a simple robust driver for high-field applications based on this Kagome fibre approach that ensures pulse self-compression down to the ultimate single-cycle limit and provides phase-controlled pulses with up to a 100 μJ energy level, depending on the filling gas, pressure and the waveguide length
Amplitude concentration in a phase-modulated spectrum due to femtosecond filamentation
We present a method by which the spectral intensity of an ultrafast laser pulse can be accumulated at selected frequencies by a controllable amount. Using a 4-f pulse shaper we modulate the phase of the frequency components of a femtosecond laser. By inducing femtosecond filamentation with the modulated pulse, we can concentrate the spectral amplitude of the pulse at various frequencies. The phase mask applied by the pulse shaper determines the frequencies for which accumulation occurs, as well as the intensity of the spectral concentration. This technique provides a way to obtain pulses with adjustable amplitude using only phase modulation and the nonlinear response of a medium. This provides a means whereby information which is encoded into spectral phase jumps may be decoded into measurable spectral intensity spikes
Mid-infrared laser filaments in the atmosphere
Filamentation of ultrashort laser pulses in the atmosphere offers unique
opportunities for long-range transmission of high-power laser radiation and
standoff detection. With the critical power of self-focusing scaling as the
laser wavelength squared, the quest for longer-wavelength drivers, which would
radically increase the peak power and, hence, the laser energy in a single
filament, has been ongoing over two decades, during which time the available
laser sources limited filamentation experiments in the atmosphere to the
near-infrared and visible ranges. Here, we demonstrate filamentation of
ultrashort mid-infrared pulses in the atmosphere for the first time. We show
that, with the spectrum of a femtosecond laser driver centered at 3.9 um, right
at the edge of the atmospheric transmission window, radiation energies above 20
mJ and peak powers in excess of 200 GW can be transmitted through the
atmosphere in a single filament. Our studies reveal unique properties of
mid-infrared filaments, where the generation of powerful mid-infrared
supercontinuum is accompanied by unusual scenarios of optical harmonic
generation, giving rise to remarkably broad radiation spectra, stretching from
the visible to the mid-infrared
The phase-controlled Raman effect
Unlike spontaneous Raman effect, nonlinear Raman scattering generates fields with a well-defined phase, allowing Raman signals from individual scatterers to add up into a highly directional, high-brightness coherent beam. Here, we show that the phase of coherent Raman scattering can be accurately controlled and finely tuned by using spectrally and temporally tailored optical driver fields. In our experiments, performed with spectrally optimized phase-tunable laser pulses, such a phase control is visualized through the interference of the coherent Raman signal with the field resulting from nonresonant four-wave mixing. This interference gives rise to Fano-type profiles in the overall nonlinear response measured as a function of the delay time between the laser pulses, featuring a well-resolved destructive-interference dip on the dark side of the Raman peak. This phase-control strategy is shown to radically enhance the coherent response from weak Raman modes, thus helping confront long-standing challenges in nonlinear Raman imaging and microspectroscopy
Axial spectral scans of polarization dependent third harmonic generation in a multimode photonic crystal fiber
We demonstrate a nondestructive axial scanning technique for the spectrally resolved analysis of femtosecond nonlinear-optical transformation in photonic crystal fibers. This technique is applied to map the generation of a polarization-switched third harmonic of femtosecond Cr:forsterite laser pulses in a multimode silica photonic crystal fiber. Obtained results confirmed the intermodal phase-matching to be responsible for the observed polarization dependent multipeak third-harmonic generation. The axial scans revealed, that it is necessary to distinguish between the low and high energy excitation regime of the fiber sample. The proposed technique allows to measure the spectra of nonlinear signals generated in a photonic crystal fiber as a function of the propagation distance without cutting the fiber
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