Conversion Efficiency Of High-order Harmonic Generation Research Articles

Quasi-phase-matched high-harmonic generation in gas-filled hollow-core photonic crystal fiber

High-order harmonic generation (HHG) provides a promising tabletop source of coherent short wavelength radiation. However, the low generation efficiency limits the mean photon flux of HHG sources when driven with low average power, kHz repetition rate lasers. The HHG flux could be increased by employing MHz repetition rate driving lasers. However, the low pulse energy necessitates tight focusing, which limits the effective interaction volume. Generating harmonics in a gas-filled photonic crystal fiber (PCF) mitigates this problem, but for both free focus and PCF targets, the HHG conversion efficiency is optimized at technologically challenging multibar gas pressures. Here, we perform HHG with μJ-level driving pulses in a hollow-core PCF. We observe dramatic 60-fold enhancement of the HHG flux through a time-multiplexed multimodal quasi-phase-matching technique at low gas pressures. We also observe high harmonic photon energies up to 61.6 eV with continuous spectral tunability made possible by controlled ionization-induced blue shifting of the driving laser.

Efficiency of cavity-enhanced high harmonic generation with geometric output coupling

Cavity-enhanced high-order harmonic generation (HHG) affords broadband, coherent extreme-ultraviolet (XUV) pulse trains with repetition rates of several tens of MHz. Geometrically coupling out the intracavity generated XUV beam through a small on-axis hole in the cavity mirror following the HHG focus has enabled scaling the photon energies attainable with this technology to 100 eV and more, promising new applications of XUV frequency-comb spectroscopy and attosecond-temporal-resolution, multidimensional photoelectron spectroscopy and nanoscopy. So far, in this approach the features of the macroscopic response of the gas target are neither accessible directly nor indirectly via the out-coupled XUV beam due to the loss of spatial information caused by the truncation at the hole. Here, we derive a simple analytical model for the divergence of the intracavity harmonic beam as a function of experimental design parameters such as gas target position, cavity geometry and driving pulse intensity, thereby establishing a connection between the measured XUV spectra and the macroscopic response of the intracavity nonlinear medium. We verify this model by comparison to numerical simulations as well as to systematic measurements, and apply it to elucidate a trade-off between the efficiency of geometric output coupling and that of the HHG process, and the underlying physical mechanisms. These findings illuminate the share of the output coupling efficiency to the overall HHG conversion efficiency and provide—together with previously studied plasma-related enhancement limitations—a holistic means of optimizing the overall efficiency with this architecture that uniquely combines high repetition rates with high photon energies. Furthermore, quantitatively connecting the output coupled, observable XUV radiation to the nonlinear conversion at the cavity focus allows for a better insight into the dynamics of intracavity HHG and might benefit other applications of femtosecond enhancement cavities, such as high-repetition-rate HHG spectroscopy.

Simultaneous control of harmonic yield and energy cutoff of high-order harmonic generation using seeded plasmonically enhanced fields

We study high-order harmonic generation (HHG) driven by seeded plasmonic-enhanced fields. On one hand, plasmonic-enhanced fields have shown a great potential to extend the HHG cutoff, an instrumental pre-requisite for the generation of attosecond pulses. On another hand, the use of XUV seeds appears to have a considerable potential to improve the HHG conversion efficiency, which is typically modest when a unique fundamental laser pulse is employed. By mixing these two sources, we show it is possible to, simultaneously, boost the HHG cutoff and to increase the harmonic photon flux. The combination of these features potentially enables to generate intense and spectrally broad attosecond pulse trains.

Efficiency control of high-order harmonic generation in gases using driving pulse spectral features

The low conversion efficiency of high-order harmonic generation (HHG) in gases is an insurmountable limitation for many applications, where a considerable photon flux is an instrumental prerequisite. We present a study of the HHG conversion efficiency dependence on the driving laser intensity and analyze the conditions for optimal phase-matching in a loose focusing configuration and long generation medium using a Ti:Sapphire laser. Moreover, by determination of the influence of plasma effects on the driving laser pulse, we observe a correlation between the HHG conversion efficiency and the blueshift of the fundamental pulse. The maximal HHG conversion efficiency is achieved just before the driving laser spectrum is considerably affected by the plasma. Similar behavior is observed in HHG for different noble gases. In this respect, the appearance of a plasma-induced spectral shift in the driving laser might serve as an indication of a substantial loss of HHG conversion efficiency. Consequently, our findings can be exploited to obtain essential information about the laser-plasma interaction process during HHG and can pave the way for a more convenient control of optimal HHG conditions.

Single attosecond pulse generation for a dielectric driven by mid-infrared laser field

We theoretically investigate the high-order-harmonic generation (HHG) from solids by simulating the dynamics of a single active electron in a dielectric. Interband tunnelling is investigated, by which the harmonic generation can be modeled based on the classical trajectory analysis of electron-hole pairs. The supercontinuum region of the fundamental HHG plateau, whose intensity is about seven-order magnitude enhancement as compared with that of the second plateau, is used in the generation of isolated attosecond pulses. Thus, it provides us a practical way to increase the conversion efficiency of HHG with laser intensity below the damage threshold.

Efficient high-order harmonic generation boosted by below-threshold harmonics

High-order harmonic generation (HHG) in gases has been established as an important technique for the generation of coherent extreme ultraviolet (XUV) pulses at ultrashort time scales. Its main drawback, however, is the low conversion efficiency, setting limits for many applications, such as ultrafast coherent imaging, nonlinear processes in the XUV range, or seeded free electron lasers. Here we introduce a novel scheme based on using below-threshold harmonics, generated in a “seeding cell”, to boost the HHG process in a “generation cell”, placed further downstream in the focused laser beam. By modifying the fundamental driving field, these low-order harmonics alter the ionization step of the nonlinear HHG process. Our dual-cell scheme enhances the conversion efficiency of HHG, opening the path for the realization of robust intense attosecond XUV sources.

Frequency-selected enhancement of high-order-harmonic generation by interference of degenerate Rydberg states in a few-cycle laser pulse

We demonstrate that the frequency-selected enhancement of high-order-harmonic generation (HHG) can be achieved by a few-cycle laser pulse interacting with a coherent superposition state, which is prepared by the ground state and two degenerate Rydberg states. The degenerate states have the same orbital radius and hence have a large overlap in the electronic density distribution. By controlling the relative phase between the two degenerate states, the constructive or destructive interference of them can markedly change the initial density distribution of the Rydberg electron, thereby we can manipulate the characteristics and the conversion efficiency of HHG. Specifically, a significant enhancement in the continuous harmonics near HHG cutoff can be obtained, hence an intense isolated pulse with a duration less than 100 attoseconds is straightforwardly generated. On the other hand, since there exists a specific dependence of the harmonic efficiency on the relative phase of the two degenerate states, one can expect that the relative phase may be probed by examining the corresponding harmonic intensity. In practice, we may apply a weak static electric field in the whole dynamic process to obtain an asymmetry electron density distribution at a large radius; hence similar HHG results can be obtained.

Use of carbon-containing materials for efficient high-order harmonic generation of laser radiation

A systematic study of the high-order harmonic generation (HHG) of laser radiation in various carbon-containing plasma plumes (CCPPs) is presented. The materials studied are: graphite, boron carbide, C60, polytetrafluoroethylene, polyethylene, carbon nanotubes, and soot. These studies show that CCPPs present the best plasma media for efficient lower order (from 9th to 19th) harmonic generation, while the harmonic cutoff restricted to the 29th order. The advantages of CCPPs for the harmonic generation are confirmed by comparison of the HHG conversion efficiency with that in Ag and In plasma plumes, where the highest conversion efficiency was reported. Use of two-color pump scheme allows further enhancement of harmonic yield from the CCPPs.

Systematic studies of two-color pump-induced high-order harmonic generation in plasma plumes

High-order harmonic generation (HHG) has been studied in various laser-produced plasma plumes using a two-color orthogonally polarized beam with a 12:1 energy ratio between the fundamental and second-harmonic (SH) components. The influence of the relative phase between the fundamental and SH waves on the HHG efficiency has been investigated. Odd and even harmonic generation in plasma plumes containing nanoparticles, fullerenes, carbon nanotubes, and other samples was optimized. The effect of the variation in the SH intensity on the HHG conversion efficiency in carbon aerogel and silver plasma plumes has also been studied. It is shown that by increasing the SH intensity, one can generate only even harmonics by suppressing the odd harmonics.

Numerical study of the enhancement of efficiency of high-order harmonic generation in two-color laser field

The time-dependent Schrdinger equations （TDSE） of the interaction of two-color laser pulse with Pschl-Teller potential and He+ are solved by using the asymptotic boundary condition and symplectic algorithm. The population of ionization, average distance, high-order harmonic generation （HHG） and population of transition are calculated. The numerical results show that the conversion efficiency of HHG is enhanced in the two-color laser fields, then the qualitative and quantitative analysis for the enhancement of conversion efficiency of HHG are given.

Maximizing the yield and cutoff of high-order harmonic generation from plasma plume

We study high-order harmonic generation (HHG) from various lowly ionized laser plasmas. We study harmonic generation from targets of Al (Z=13) to Bi (Z=83). Varying the wavelength, chirp, and pulse duration of the femtosecond pump laser resulted in the change in the harmonic distribution, cutoff, and conversion efficiency of HHG. We also study the use of doubly charged ions, and resonances for some materials. We were able to obtain high HHG conversion efficiency and harmonic cutoff by implementing the above approaches and by observing the time-resolved spectra of the laser plasma.

The quantitative analysis of enhancement of high-order harmonics in two-color intense laser fields

The time-dependent Schrodinger equation of the interaction of laser pulse with He+ is solved by using the asymptotic boundary condition and symplectic algorithm in fundamental laser-field and two-color laser fields. We find that the conversion efficiency of high-order harmonic generation (HHG) is higher in the two-color laser fields than in the fundamental laser field, especially for the combination of ω 0 − 19ω 0. To explain these phenomena, the ionization, the average distance, the probability of first excited sate, and the transition probability are calculated. We give the qualitative and quantitative analysis for the enhancement of conversion efficiency of HHG.

The enhancement of efficiency of high-order harmonic generation in two-color laser field

We solve the time-dependent Schr?dinger equation in two-color laser fields by using asymptotic boundary condition and symplectic algorithm. The P?schl-Teller potential model with three bound states was considered in the numerical calculation. The population of ionization, average distance, HHG and the population of transition were calculated. We show that the efficiency of high-order harmonic generation (HHG) in two-color laser field can be enhanced, and analyze the mechanism of the enhancement of HHG. We demonstrate qualitatively the enhancement of conversion efficiency of HHG by the transition probability.

The Enhancement of Efficiency of High-order Harmonic in Intense Laser Field Based on Asymptotic Boundary Conditions and Symplectic Algorithm

Asymptotic boundary condition (ABC) of laser-atom interaction presented recently is applied to transform the initial value problem of the time-dependent Schrodinger equation (TDSE) in infinite space into the initial and boundary value problem in the finite space, and then the TDSE is discretized into linear canonical equations by substituting the symmetry difference quotient for the 2-order partial derivative. The canonical equation is solved by symplectic algorithm. The ground state and the equal weight coherent superposition of the ground state and the first excited state have been taken as the initial conditions, respectively, while we calculate the population of bound states, the evolution of average distance and the high-order harmonic generation (HHG). The conversion efficiency of HHG can be enhanced by initial coherent superposition state and moderate laser intensities

Transient enhancement of high-order harmonic generation in expanding molecules

High-order harmonic generation (HHG) is limited by the probability of recombination of the returning electron with the parent ion. We show here that the internuclear distance of simplest diatomic molecules, determining differently delocalized initial electronic states, can be used to enhance the HHG conversion efficiency exceeding the atomic one. We propose an experiment based on a double-pulse pump-drive scheme where a strong transient enhancement of the HHG yield is predicted.