Intensity Of High-order Harmonics Research Articles

Quantum well-shape dependence of optical harmonic radiation

We theoretically investigate the spatial-non-homogeneous laser pulse propagation through the quantum well. It is shown that the odd-order and even-order harmonic generations can be found due to breaking the inversion symmetry of the system. Importantly, the spectral intensity of high-order harmonics can be enhanced by changing the propagation distances and spatial-non-homogeneity of incident laser field. Moreover, the quantum well’s shape has an important influence on the peak intensities of high-order harmonic generations, which may provide a new way to improve the spectral intensity of high-order harmonic generations.

Harmonic Extraction in Graphene: Monte Carlo Analysis of the Substrate Influence.

Graphene on different substrates, such as SiO, h-BN and AlO, has been subjected to oscillatory electric fields to analyse the response of the carriers in order to explore the generation of terahertz radiation by means of high-order harmonic extraction. The properties of the ensemble Monte Carlo simulator employed for such study have allowed us to evaluate the high-order harmonic intensity and the spectral density of velocity fluctuations under different amplitudes of the periodic electric field, proving that strong field conditions are preferable for the established goal. Furthermore, by comparison of both harmonic intensity and noise level, the threshold bandwidth for harmonic extraction has been determined. The results have shown that graphene on h-BN presents the best featuring of the cases under analysis and that in comparison to III–V semiconductors, it is a very good option for high-order harmonic extraction under AC electric fields with large amplitudes.

Comparison study on atomic and molecular ellipticity dependence of high-order harmonic generation

We systematically investigate ellipticity dependence of high-order harmonic generation of Ar and ${\mathrm{N}}_{2}$ in intense elliptically polarized laser fields. The experimental normalized ratios of low-order harmonic intensity to high-order harmonic intensity increase with ellipticity for both Ar and ${\mathrm{N}}_{2}$, and quantitatively depend on targets and trajectory paths. The experimental results are well reproduced by a nonadiabatic semiclassical simulation and explained by trajectory-based analysis. In addition, the influence of nuclear distance on the ratios is theoretically investigated. Our work reveals that the difference between atoms and molecules can be attributed to the influence of different ionic potentials, which depends on the molecular structure (internuclear distance) and alignment, on the evolution of the photoelectron.

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.

Two-color high-order-harmonic generation: Relativistic mirror effects and attosecond pulses

In this work, an analytical and numerical study of the relativistic interaction of a linearly polarized laser field of ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ frequencies with highly overdense plasma at normal and oblique incidence is presented. Combining harmonics of ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ frequencies causes the amplitude oscillation of relativistic mirrors to be increased. Therefore, the intensity of generated harmonics will be increased considerably. Also, in agreement with the fully relativistic one-dimensional particle-in-cell (PIC) simulation, the relativistic mirror model explains the enhancement of the harmonic intensity of the upper spectra in the interaction of the two-color mixing laser pulse with overdense plasma. Our results show that increasing the intensity of high-order harmonics by using the two-color pulse will enhance sharply the intensity of ultrashort attosecond pulses.

The control of the electron quantum paths for the generation of isolated attosecond pulse using a three-color laser scheme

We theoretically propose a three-color laser scheme to enhance the high-order harmonic intensity and generate an isolated attosecond pulse. By adding a 3 fs, 1600 nm laser pulse to a synthesized two-color laser field (5 fs, 800 nm and 10 fs, 1200 nm), the harmonic intensity is effectively enhanced and an isolated attosecond pulse with duration 41 as is generated. In this scheme, the short trajectory is suppressed, the selection of the long quantum path can be achieved. We also investigate emission time of harmonics in terms of the time-frequency analysis and the semi-classical three-step model to illustrate the physical mechanism of high-order harmonic generation.

Publisher's Note: Dependence of high-order harmonic intensity on the length of preformed plasma plumes [Phys. Rev. A79, 023807 (2009)

Publisher's Note: Dependence of high-order harmonic intensity on the length of preformed plasma plumes [Phys. Rev. A<b>79</b>, 023807 (2009)

Dependence of high-order harmonic intensity on the length of preformed plasma plumes

An experimental study has been carried out on high-order harmonic generation from the interaction of $45\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ Ti:sapphire laser pulses with low-excited silver plasma plumes. The dependence of high-order harmonic intensity $({I}_{H})$ on the plasma plume length $({L}_{\mathrm{med}})$ shows slower than quadratic scaling ${I}_{H}\ensuremath{\sim}{L}_{\mathrm{med}}^{p}$ with ``$p$''$\ensuremath{\approx}0.9$, 0.8, and 0.7 for 21st, 33rd, and 41st harmonics, respectively. The harmonic intensity is observed to decrease with increasing harmonic order, and this decrease is faster for longer plasma plumes. The results are explained from physical considerations of phase mismatch between the driving laser pulse and the harmonic radiation during propagation, and reabsorption of the harmonic radiation in the plasma plume.

Dependence of high order harmonics intensity on laser focal spot position in preformed plasma plumes

The dependence of the high-order harmonic intensity on the laser focal spot position in laser produced plasma plumes is experimentally studied. High order harmonics up to the 59th order (λ∼13.5 nm) were generated by focusing 48 fs laser pulses from a Ti:sapphire laser system in silver plasma plume produced using 300 ps uncompressed laser radiation as the prepulse. The intensity of harmonics nearly vanished when the best focus was located in the plume center, whereas it peaked on either side with unequal intensity. The focal spot position corresponding to the peak harmonic intensity moved away from the plume center for higher order harmonics. The results are explained in terms of the variation of phase mismatch between the driving laser beam and harmonics radiation produced, relativistic drift of electrons, and defocusing effect due to radial ionization gradient in the plasma for different focal spot positions.

Single-attosecond pulse generation with an intense multicycle driving pulse

Higher-order harmonic generation from strong laser-atom interaction in the multicycle regime is investigated using the Lewenstein model. While the peak intensity of the driving laser is oversaturated, the atom will be ionized completely during a few half optical cycles. The harmonic spectrum then reveals a continuous multiplateau structure in the cutoff region because of the ground state depletion. Each subplateau can be superposed to generate single attosecond pulse. Since the intensity of high-order harmonics from ion is comparable to that from atom if the peak intensity is super-intense, appropriate subplateau should be selected for single attosecond pulse generation. It is also shown that the nonadiabatic effect plays a crucial role in tuning the bandwidth of the subplateau.

Monte carlo simulation of threshold bandwidth for high-order harmonic extraction

The feasibility of bulk semiconductors subjected to strong periodic electric fields for terahertz radiation generation due to the high-order harmonic extraction is analyzed by using Monte Carlo simulations. The high-order harmonic intensity and the spectral density of velocity fluctuations are calculated for GaAs, InP, and InN. By comparing the harmonic intensity with the noise level the threshold bandwidth for high-order harmonic extraction determined by their ratio is introduced and evaluated for the above materials. The results show that semiconductor materials with a high value of the threshold field for the Gunn-effect are characterized by a high value of the threshold bandwidth under high-order harmonic generation and, hence, they are promising materials for microwave generation in the THz frequency range by high-order harmonic extraction.

Increasing the efficiency of high-order harmonic generation using counterpropagating laser pulses

We present results of computer simulations showing dramatic increases in the production and intensity of high-order harmonics (in the cutoff region) produced by 20-fs pulse with counterpropagating pulse in rare gases. Using two different models for the gas medium (i.e., classical anharmonic oscillators representing Ne, Ar, Kr and quantum oscillators representing ${\mathrm{Ne}}^{+},$ ${\mathrm{Ar}}^{+},$ ${\mathrm{Kr}}^{+}),$ we demonstrate an efficiency increase of four and six orders of magnitude for the respective models. This suggests a notable improvement over other methods.

Effect of multiphoton ionization on high-order harmonic generation and propagation in rare gases with subpicosecond laser pulses.

An experimental study was made on the effect of multiphoton ionization on high-order harmonic generation and propagation in rare gases with subpicosecond dye-laser (\ensuremath{\lambda}\ensuremath{\sim}616 nm) pulses at the intensity up to \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W/${\mathrm{cm}}^{2}$. Both high-order harmonic intensity and the ion signal were measured as a function of the position of the laser focus. The harmonic signal in Ar, Kr, and Xe shows a remarkable decrease near the peak position of the ion signal. We have observed the asymmetry in the shape of the harmonic signal and the change of the asymmetry with harmonic order. Furthermore, the dependence of harmonic signals on fundamental intensity were found to be quite different at the specific positions of the laser focus. These experimental results are consistently explained by the medium ionization and the resulting self-defocusing of the laser beam. The results obtained with the XeCl laser (\ensuremath{\lambda}\ensuremath{\sim}308 nm) pulses back up our interpretations for those obtained with the dye-laser pulses.