Phase-matched High Harmonic Generation Research Articles

Investigation of High Harmonic Generation in Ar-Ne Gas Mixture

"In this study, we experimentally investigate the variation of the phase matching condition of the high harmonic generation (HHG) process with pure argon gas and an argon-neon gas mixture. Phase-matched HHG is generated around the absorption edge of argon gas and then neon gas is added to the original argon gas. The pressure-dependent intensity of the harmonics produced by pure argon gas and the gas mixture is examined. We show that as more neon gas is added to the mixture, the phase matching of the higher order harmonics is less favourable than that of the lower order harmonics. Finally, the total phase mismatch at various gas mixture pressures is discussed. Our experimental results are in agreement with the theoretical calculation."

Phase-Matched High-Harmonic Generation under Nonadiabatic Conditions: Model and Experiment

Nonadiabatic phase matching of high-harmonic generation (HHG) driven by few-cycle laser pulses is essential for extending harmonic energy and generating isolated attosecond pulses. However, understanding nonadiabatic HHG is challenging due to the complex interplay of various optical phases driven by temporally and spatially varying laser fields. Theoretical calculations typically rely on computationally demanding 3-dimensional simulations, which can make it difficult to extract the essential features of nonadiabatic HHG. In this work, we develop a computationally efficient 2-dimensional model that directly considers various phase contributions of HHG. Our model can well explain the experimentally observed pressure- and intensity-dependent behaviors of different harmonic orders. By appropriately parameterizing the single-atom response, our model can also estimate the variation of HHG spectra under different driving conditions. Our model can provide an efficient tool for the design and optimization of HHG-based applications.

Optimizing high harmonic generation in hollow-core gas cell considering variation of gas density

We have developed a coherent and compact extreme-ultraviolet light source based on phase-matched high harmonic generation by using a hollow-core gas cell that can be easily aligned and maintain long stability. The gas cell consists of three bodies to minimize gas leakage and maintain stable vacuum. The photon flux was calculated considering the dependence of the gas number density and the pressure on the radii of the hollow core and the hole in the Al plates on both sides of the gas cell, which showed good coincidence with the experimental values.The photon flux is 2 × 1010 photons s−1 near 13.5 nm (91.5 eV) within 10 nm bandwidth. Also, the compact extreme-ultraviolet light source is found to operate stably for hours, which is suitable for practical applications such as metrology and inspection for extreme-ultraviolet mask, spectroscopy, holography, microscopy, etc.

Spectrum modification of XUV radiation in the presence of a delayed weak control field

ABSTRACTWe use a delayed weak laser beam to control the spectral features of extreme ultraviolet (XUV) pulses generated by a strong femtosecond laser beam through phase-matched high harmonic generation (HHG) in an atomic medium, e.g. krypton. The variation of the HHG spectrum reveals the influence of the free electrons on the propagation of the XUV field in the medium. In addition, a signature of the autoionization process is visible. Our findings provide a promising technique to study ultrafast dynamics of atomic and molecular gases.

Phase-Matched High-Order Harmonics from N2 Using kHz Femtosecond Laser Pulses

We report the phase-matched high-harmonic generation from N2 driven by kHz femtosecond laser pulses. By varying the position of the laser focus in the gas tube, we observe the radiation of different harmonics. Our results provide a potential source for studying ultrafast molecular dynamics.

Absorption-limited and phase-matched high harmonic generation in the tight focusing regime

High harmonic generation (HHG) at a high repetition rate requires tight focusing of the moderate peak power driving pulses. So far the conversion efficiencies that have been achieved in this regime are orders of magnitude behind the values that have been demonstrated with loose focusing of high energy (high peak power) lasers. In this contribution, we discuss the scaling laws for the main physical quantities of HHG and in particular analyze the limiting effects: dephasing, absorption and plasma defocusing. It turns out that phase-matched and absorption-limited HHG can be achieved even for very small focal spot sizes using a target gas provided with an adequately high density. Experimentally, we investigate HHG in a gas jet of argon, krypton and xenon. By analyzing the pressure dependence we are able to disentangle the dephasing and absorption effects and prove that the generated high order harmonics are phase-matched and absorption-limited. The obtained conversion efficiency is as high as 8 × 10−6 for the 17th harmonic generated in xenon and 1.4 × 10−6 for the 27th harmonic generated in argon. Our findings pave the way for highly efficient harmonic generation at megahertz repetition rates.

Phase-matched high harmonic generation for the study of rotational coherence molecular dynamics

We report the observation of periodic modulations in the high order harmonic radiation from diatomic molecules in a semi-infinitive gas cell when a preceding femtosecond laser beam, which is off-axis to the high-harmonic generation beam, modifies the phase-matching condition through the field-free alignment of the molecules. The observed modulations of the high harmonic radiation versus the time delay between the pulses result from periodic changes in the nonlinear refractive index associated with rotational Raman coherence. This opens up a new potential technique for studying rotational coherence dynamics in the ground states of molecules.

Plasma Bragg density gratings produced by optical-field ionization

Plasma Bragg density gratings produced by optical-field ionization in underdense gas under the irradiation of two counterpropagating laser pulses at laser intensities around 1013 W/cm2 are investigated by particle-in-cell simulation. The grating is composed of interlacing layers of neutral gas and plasma (or partially ionized gas) with its density and period controlled by the initial gas density, laser wavelengths, and intersecting angles of the two laser pulses. The study shows that such gratings have a longer lifetime, about nanoseconds, as compared with those driven by the laser ponderomotive force at higher laser intensities around 1015 W/cm2. They may be used for phase-matched high-harmonic generation, laser self-guiding, laser pulse compression and stretching, etc.

Off-axially phase-matched high-order harmonic generation in an extended medium

We investigate theoretically high-order harmonic generation in gases and show that, if the target thickness is high, off-axial phase matching is of importance. Results of a numerical study of harmonic generation that takes into account the self-action of the laser pulse are presented. We show that self-guiding of the laser pulse in a noble gas makes possible off-axially phase-matched high harmonic generation if some easily ionizable gas is added to the main generating gas. In calculations for such mixtures we obtained conversion efficiencies as great as approximately 10-3–10-2 for the 33rd harmonic of a Ti:sapphire laser and approximately 10-4 for the 121st harmonic.

Highly Efficient, Phase-Matched High-Harmonic Generation by a Self-Guided Laser Beam

We demonstrate a phase-matched high-harmonic generation of 100 fs Ti:sapphire laser pulses by a self-guided beam in 30 Torr of neon gas. Phase matching of high harmonics is experimentally examined by changing the propagation length up to 7 mm. Phase-matched propagation over 7 mm magnifies the conversion efficiency around the 49th harmonic (in the cutoff region) by 40 times. On the other hand, harmonics around the 25th order (in the plateau region) peak at 4 mm, resulting in an enhancement of 4. This scheme enhances the conversion efficiencies in the cutoff region up to ${10}^{\ensuremath{-}6}$, corresponding to a few nJ per high-harmonic pulse in the 76 eV region.