Harmonic Generation In Argon Research Articles

Optimization of few-cycle laser pulse pumped high-order harmonic generation at 60–70 eV by changing the gas pressure

We have experimentally investigated the gas cell pressure effect of high-order harmonic generation in argon and neon. The selective enhancement of high-order harmonic intensity in the energy range of 60–70 eV in the spectrum has been obtained in argon. The experimental results are simulated theoretically, and the main characterizations of the experimental observations are reproduced. By studying the ionization term effect in the simulation, we conclude that it is the plasma effect improves the phase-matching condition for this specific spectrum range and finally brings the enhancement.

Pressure optimization of high harmonic generation in a differentially pumped Ar or H₂ gas jet.

We experimentally studied the dependence of high harmonic generation in argon and molecular hydrogen on pressure changes in a gas jet that cause variations of the phase matching conditions and absorption. The study was performed at a peak laser intensity of ∼1.5 × 10(14) W/cm(2). To enable measurements over a wide range of pressures, we employed differential pumping with an additional cell (∼20 cm(3) volume) enclosing the gas jet. By increasing the pressure in the gas jet up to a maximum of 1.5 bars with argon or 0.5 bars with hydrogen, we observed an increase in the high harmonic (HH) yield until an optimum pressure of 0.2 bars was reached for Ar, beyond which the output began decreasing. For H2, we observed an increase of the HH output up to the maximum pressure of 0.5 bars. This pressure-dependence study allowed us to achieve a tenfold enhancement in the high harmonic yield at the optimum pressure.

Enhancing the macroscopic yield of narrow-band high-order harmonic generation by Fano resonances.

Resonances in the photoabsorption spectrum of the generating medium can modify the spectrum of high-order harmonics. In particular, window-type Fano resonances can reduce photoabsorption within a narrow spectral region and, consequently, lead to an enhanced emission of high-order harmonics in absorption-limited generation conditions. For high harmonic generation in argon it is shown that the 3s3p(6)np(1)P(1) window resonances (n = 4, 5, 6) give rise to enhanced photon yield. In particular, the 3s3p(6)4p(1)P(1) resonance at 26.6 eV allows a relative enhancement up to a factor of 30 in a 100 meV bandwidth compared to the characteristic photon emission of the neighboring harmonic order. This enhanced, spectrally isolated, and coherent photon emission line has a relative energy bandwidth of only ΔE/E = 3 × 10(-3). Therefore, it might be very useful for applications such as precision spectroscopy or coherent diffractive imaging. The presented mechanism can be employed for tailoring and controlling the high harmonic emission of manifold target materials.

Atomic line emission and high-order harmonic generation in argon driven by 4-MHz sub-μJ laser pulses

We report on the coexistence of atomic line emission (ALE) and high-order harmonic generation (HHG) from argon with experimental conditions bridging the multiphoton and tunnel ionization regimes. Driven by sub-$\ensuremath{\mu}$J femtosecond laser pulses in tight-focusing geometry, characteristic spectra of ALE from highly excited neutral argon as well as from singly ionized argon are detected in the presence of the harmonics. The results are discussed with respect to the electronic structure of argon and the phase-matching condition of the HHG process.

From higher-order Kerr nonlinearities to quantitative modeling of third and fifth harmonic generation in argon

The recent measurement of negative higher-order Kerr effect (HOKE) terms in gases has given rise to a controversial debate, fed by its impact on short laser pulse propagation. By comparing the experimentally measured yield of the third and fifth harmonics, with both an analytical and a full comprehensive numerical propagation model, we confirm the absolute and relative values of the reported HOKE indices.

Interference effects in two-color high-order harmonic generation

We study high-order harmonic generation in argon driven by an intense 800 nm laser field and a small fraction of its second harmonic. The intensity and divergence of the emitted even and odd harmonics are strongly modulated as a function of the relative delay between the two fields. We provide a detailed analysis of the underlying interference effects. The interference changes drastically when approaching the cutoff region due to a switch of the dominant trajectory responsible for harmonic generation.

Multi-quantum-path interference in high harmonic generation driven by a chirped laser pulse

We theoretically investigate the quantum-path interference during the high harmonic generation in argon and observe the multi-quantum- path-interference (MQPI) effect related to the chirp of driving laser pulse. We successfully identify the interference originated from four significantly contributing quantum paths in the interaction of "isolated" atom with the driving laser field. Moreover, the MQPI effect is further demonstrated beyond the single atom response by spatial filtering which is used for angle-selective detection of transmitted light when three-dimensional propagation is considered. It implies that the role of high-order electron returning trajectories during high harmonic generation can be observed in the experiment.

Extended phase matching of high harmonics driven by mid-infrared light

We demonstrate that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths. Experimentally, we show that the phase-matching cutoff for harmonic generation in argon increases from 45 to 100 eV when the driving laser wavelength is increased from 0.8 to 1.3 microm. Phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response. Theoretical calculations suggest that phase-matched high harmonic frequency upconversion driven by mid-infrared pulses could be extended to extremely high photon energies.

Quantum Path Interferences in High-Order Harmonic Generation

We have investigated the intensity dependence of high-order harmonic generation in argon when the two shortest quantum paths contribute to the harmonic emission. For the first time to our knowledge, experimental conditions were found to clearly observe interference between these two quantum paths that are in excellent agreement with theoretical predictions. This result is a first step towards the direct experimental characterization of the full single-atom dipole moment and demonstrates an unprecedented accuracy of quantum path control on an attosecond time scale.

Temporal confinement and enhancement of high-order harmonic emission with a synthesized laser field

We demonstrated that a synthesized laser field consisting of an intense long ($45\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, multi-optical-cycle) laser pulse and a weak short ($7\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, few-optical-cycle) laser pulse can control the electron dynamics and high-order harmonic generation in argon, and generate extreme ultraviolet supercontinuum towards the production of a single strong attosecond pulse. The long pulse offers a large amplitude field, and the short pulse creates a temporally narrow enhancement of the laser field and a gate for the highest energy harmonic emission. This scheme paves the way to generate intense isolated attosecond pulses with strong multi-optical-cycle laser pulses.

Single attosecond pulses from high harmonics driven by self-compressed filaments

We show that isolated subfemtosecond, extreme ultraviolet (XUV) pulses can be generated via harmonic generation in argon by few-cycle infrared pulses formed through filamentation-induced self-compression in neon. Our calculations show that the time structure of the XUV pulses depends sensitively on both the amplitude and the phase modulation that are induced in the driving pulse during the self-compression process.

Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field

We have proposed and demonstrated a novel approach for generating high-energy extreme-ultraviolet (XUV) continuum radiation. When a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ. This broadband emission indicates the possibility of generating intense single attosecond pulses in the XUV region.

Cluster effects in high-order harmonics generated by ultrashort light pulses

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield.

High-order harmonic generation in argon by elliptically polarized picosecond dye laser pulses

High-order harmonic generation is observed in an argon jet using elliptically polarized picosecond dye laser pulses. The experimental data appear to be in near agreement with the perturbation theory. However, there is a general trend of small deviation from the theory. We speculate that such a deviation could be due to the laser induced coupling of upper states in or near the continuum which relaxes the selection rules leading to an enhancement of harmonic emission.