Studies of the generation and propagation of light fields in the extreme ultraviolet (XUV) can provide insights into the fundamental interaction of atoms in highly excited levels and ionized atoms. In this paper, we present experimental results of nonlinear four-wave mixing (FWM) processes using a combination of XUV radiation and optical pulses in argon gas. The XUV pulses are produced by phase-matched high-order harmonic generation (HHG). Optimized phase-matching of collinear multiple-cycle laser pulses with incommensurate frequencies (800 nm, 1400 nm, and 560 nm) is used to indicate the different pathways of the third-order and fifth-order nonlinear responses in the mixing process in a single gas cell configuration. A perturbative nonlinear optics approach can be used to explain our cascaded wave-mixing patterns. Our results reveal that the time-dependent spectral features of the mixing fields are associated with auto-ionization processes. Overall, the intensity and frequency modulation of the wave-mixing fields provides a new technique to investigate the dynamical evolution of electron wave-packets in atomic and molecular gases.
Using two multiple-cycle optical pulses with incommensurate frequencies (e.g., at wavelengths 1400 nm and 800 nm) in a collinear configuration, a cascaded background-free four-wave mixing EUV field can be realized because the EUV pulse produced by phase-matched high order harmonic generation in combination with the other two optical pulses creates a third-order nonlinear polarisation which drives a phase-matched four-wave mixing process along the propagating direction. The four-wave mixing emission can be manipulated by varying the delay of the second optical pulse. The phase-matching of the four-wave mixing process together with the relatively long interaction path combine to produce a strong output signal which leads to an enhanced signal-noise ratio of the Fourier-transformed signal used to obtain the two-dimensional cross correlation spectrum. Some key features of this two-dimensional spectroscopy, such as the on-axis and off-axis peaks, can lead to the determination of interaction pathways of real dipole-allowed transitions and virtual transitions in the EUV.
We report here evidence of phase-matched optical wave mixing in the extreme ultraviolet (XUV) region. This process has been studied with a collinear two-colour high-order harmonic generation scheme. An 800 nm, 30 fs driving field is used to produce a small bandwidth comb of odd harmonic orders (wavelength around 30 nm) in a long cell filled with argon gas. Mixing frequencies in this spectral range are produced by applying a second weak control-field of 1,400 nm, 40 fs. Low order (third- and fifth-order) nonlinear optical wave mixing is observed to be a phase-matched process. The dependence of the intensity of the harmonic orders and the mixing frequencies on different control-field intensities, gas pressure, and interaction length is analysed to verify the phase matching process.
We study the use of a second driving beam to enhance the phase matching and also to create wave mixing and parametric amplification in extreme ultraviolet region. New methods for studying coherent processes in atoms and molecules and for imaging with high spatial resolution have been proposed and developed
We report the investigation of the wave-mixing and amplification process with two multiple-cycle pulses with incommensurate frequencies (at 1400 nm and 800 nm). With a non-collinear configuration of the two beams, a different extreme ultraviolet mixing field can be created at low intensity of the 800 nm field. When a very high intensity 800 nm pulse is applied we are able to amplify the coherent extreme ultraviolet radiation in the photon energy range around 80 eV.
An optical parametric amplifier system pumped by 1 kHz, 8 mJ femtosecond laser pulses at 800 nm has been constructed to generate high intensity, infrared pulses (~ 3 mJ, 1400 nm, 40 fs) with a total conversion efficiency of ~ 40%. The femtosecond high intensity laser pulses at 800 nm and 1400 nm have been used for high order harmonic generation in a semi-infinitive gas cell and have allowed the achievement of bright phase-matched harmonic radiation in the extremeultraviolet and soft X-ray region.
Using spectrally resolved femtosecond two- and three-pulse nonlinear spectroscopy we study the dynamics and
coherence properties of excited carriers in ZnO/ZnMgO quantum wells (QW) and silicon quantum dot (QD) structures
embedded in silicon nitride (SiN). The contribution of biexcitons in ZnO/ZnMgO quantum wells at room temperature is
identified. For Si quantum dots embedded in SiN a very short dephasing time of < 180 fs at room temperature is
observed.
The numerical calculations of the spectrum of nonlinear polarization for 3- and 4-level system in femtosecond pulses
have been made. The strong field interaction is responsible for coherence transform, red -, blue shifts and splitting on the
spectrum. The reverse Fourier transform for restore of time domain is proposed for dephasing rate determination. The
comparison with experimental results for complex molecules and semiconductors has been made.
Using time-resolved and time-integrated photoluminescence and spectrally resolved two-colour three-pulse photon
echo spectroscopy we study the quantum confinement and dephasing properties of near spherical Si QDs with an
average size of 4.3 nm. Filling of the low energy states and parabolic confinement of the quantum dot structures can be
inferred from the dependence of the photoluminescence intensity on the detection wavelengths. A dephasing time of 1 -
1.8 ps which is slightly dependent on the quantum dot energy states can be measured. We show that the dephasing time
of the electrons in the quantum dots is strongly influenced by the density of excited carriers.
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