Harmonic Intensity Research Articles

Intensity of Resonant Harmonic Generated in the Multiphoton Ionization Regime

The relative intensities of the resonant and nonresonant high-order harmonics, generated by singly charged gallium and indium ions in the field of a laser pulse with a duration of 20 fs and intensity ranging from 5 × 1013 to 2 × 1014 W/cm2, are calculated. The harmonic intensity is found via the matrix element of multiphoton transition within the perturbation theory including the resonant contribution. The latter increases the harmonic intensity at the transition from the ground state to the autoionizing state. We consider laser wavelength when this transition corresponds to 7 and 15 photons.

Bifunctional Nd:CaSrNb2O7 crystal: A potential self-frequency-doubling laser material candidate

One of the convenient methods to obtain green laser is by self-frequency-doubling (SFD) technique, the key is the SFD laser crystal. In this article, a bulk single crystal of 3.34 at.% Nd:CaSrNb2O7 was successfully grown by Czochralski method at about 1481 °C. Its spectral and thermal properties were investigated. There is a broad and strong absorption band at around 804 nm, which is very suitable for AlGaAs laser diode pumping. The polarized emission spectra show large emission cross-sections and full width at half maximum at around 1068 nm, and the fluorescence lifetime is 202 μs? Moreover, the fluorescence quantum efficiency reaches to 98.6%. The long lifetime, large emission cross-section, and high fluorescence quantum efficiency of Nd:CaSrNb2O7 crystal indicate that it is easy to achieve laser oscillation. The second harmonic generation intensity of 3.34 at.% Nd:CaSrNb2O7 crystal is about 3 times that of KH2PO4, and phase matching can be realized in this crystal. All these results show that Nd:CaSrNb2O7 is a good bifunctional crystal, which possesses laser and nonlinear optical functions simultaneously, and also demonstrate that it is a promising SFD material.

Generation of the high-intensity single harmonic energy peak and attosecond pulse by using resonance ionization schemes from atoms and molecules

The lower harmonic intensity is still the reason that limits the application of attosecond pulse. Thus, in this paper, on the basis of resonance ionization schemes, we propose the effective methods to improve the harmonic intensity of atomic and molecular systems. For atomic system (i.e. He atom), with the combination of chirped-UV pulse, not only the harmonic cutoff can be extended, but also the harmonic intensity can be enhanced, caused by the UV resonance ionization between ground state and excited state. As a result, the single harmonic energy peak (HEP) with the higher intensity and higher emitted photon energy can be obtained, which can support the attosecond pulses shorter than 40 as. For the molecular system (i.e. [Formula: see text] and [Formula: see text]), by properly choosing the pulse duration of the fundamental pulse, the time region of charge resonance enhanced ionization (CREI) can be well controlled. Therefore, the maximum HEP with the higher intensity can be obtained. Second, by properly adding a half-cycle unipolar pulse, the selected maximum HEP in the CREI region can be further extended. Thus, an intense and broad spectral region can be produced, which can generate the attosecond pulses with durations of 36[Formula: see text]as.

Control of the half-cycle harmonic emission process for generating the intense and ultrashort single attosecond pulses (SAPs)

Abstract In this paper, the half-cycle harmonic generation process has been controlled by using the asymmetric inhomogeneous chirped pulse combined with the ultraviolet (UV) pulse. It is found that by properly optimizing the chirps and chirp delays of the fundamental two-color pulse, the optimal negative and positive half-cycle laser profiles for the harmonic cutoff extension can be obtained. Further, with the introduction of the negative and positive inhomogeneous effect, respectively, the harmonic cutoff from the negative and positive half-cycle laser profiles can be further improved. Next, with the assistance of the UV pulse, the harmonic intensity can be enhanced due to the UV resonance ionization. Moreover, the single and double UV photon resonance ionizations are much better for generating the higher harmonic intensity. As a result, the stronger and broader harmonic plateaus with the larger harmonic cutoff can be obtained, which can support the generation of the high-intensity ultrashort attosecond pulses with the pulse durations of sub-45 as.

High-order harmonic generation by bi-elliptical orthogonally polarized two-color fields

Based on the strong-field approximation, we report results for high-order harmonic generation by bi-elliptical orthogonally polarized two-color (BEOTC) fields with frequency ratios of 2:1 and 3:1 and fundamental wavelengths of 800 and 1800 nm. A BEOTC field denotes the superposition of two copropagating counter-rotating elliptically polarized fields with different wavelengths and orthogonal semimajor axes. Its two limiting cases are the bicircular field and the linearly polarized orthogonal two-color field [D. B. Milo\ifmmode \check{s}\else \v{s}\fi{}evi\ifmmode \acute{c}\else \'{c}\fi{} and W. Becker, Phys. Rev. A 100, 031401(R) (2019)]. A detailed analysis of the high-order harmonic intensities and ellipticities as functions of the harmonic order, the ellipticity, and the relative phase between the two driving-field components is presented. Regions of the parameter space are identified where the harmonic ellipticities are very high. Surprisingly, this can be the case already for very small ellipticity (as small as $\ensuremath{\varepsilon}=0.01$) of the driving field. This can be important for practical applications. In the opposite limit where the BEOTC field is close to bicircular, the selection rules that govern the latter case can also be very quickly invalidated. For the 2:1 case, this can result in an apparent shift of the selection rules by one harmonic order.

Mapping Electrochemical Heterogeneity at Gold Surfaces: A Second Harmonic Imaging Study.

Designing efficient catalysts requires correlating surface structure and local chemical composition with reactivity on length scales from nanometers to tens of microns. While much work has been done on this structure/function correlation on single crystals, comparatively little has been done for catalysts of relevance in applications. Such materials are typically highly heterogeneous and thus require methods that allow mapping of the structure/function relationship during electrochemical conversion. Here, we use optical second harmonic imaging combined with cyclic voltammetry to map the surface of gold nanocrystalline and polycrystalline electrodes during electrooxidation and to quantify the spatial extent of surface reconstruction during potential cycling. The wide-field configuration of our microscope allows for real-time imaging of an area ∼100 μm in diameter with submicron resolution. By analyzing the voltage dependence of each pixel, we uncover the heterogeneity of the second harmonic signal and quantify the fraction of domains where it follows a positive quadratic dependence with increasing bias. There, the second harmonic intensity is mainly ascribed to electronic polarization contributions at the metal/electrolyte interface. Additionally, we locate areas where the second harmonic signal follows a negative quadratic dependence with increasing bias, which also show the largest changes during successive cyclic voltammetry sweeps as determined by an additional correlation coefficient analysis. We assign these areas to domains of higher roughness that are prone to potential-induced surface restructuring and where anion adsorption occurs at lower potentials than expected based on the cyclic voltammetry.

Analytical account for the off-axis effects and undulator field harmonics in FELs

We give analytical description of the spontaneous and stimulated free electron laser (FEL) harmonic generation in multiperiodic fields, accounting for the off-axis effects and field harmonics in undulators. Exact analytical expressions for the Bessel coefficients involve generalized forms of Bessel- and Airy-type functions and describe also the effect of the non-periodic magnetic components in various elliptic and planar undulators with field harmonics. They allow clear identification of proper contributions in the UR harmonic intensities and polarizations. Advanced phenomenological model of the FELs harmonic powers provides analytical description of relevant effects in FEL harmonic radiation, verified with FEL experiments. The analysis of the off-axis effects for the odd and even harmonic generation is performed for LEUTL and LCLS FEL experiments. The modelled harmonic powers perfectly fit the experimentally measured data. High levels of the 2nd harmonic are explained by the wide beam in LEUTL experiment and by the off-axis beam deviation in LCLS experiment.

High harmonic generation in two-color elliptical laser fields of different helicity

This paper presents the results from the generation of elliptical high harmonics in a process of two-color elliptical laser field interaction with a single atom. Numerical simulations are performed for different values of ellipticities of the two-color driving laser field (the fundamental and the second harmonics of the Ti:Sapphire laser), together with their relative helicity. The dependency of the intensity and ellipticity of harmonics as functions of the ellipticity of the driving field components and their relative helicity is discussed. The presence of the sequence of two strong and one weak harmonics in the generated spectra in the case of a counter rotating bicircular laser field is demonstrated in this paper. The origin of weak harmonics generation is also discussed.

Ultrafast 1 MHz vacuum-ultraviolet source via highly cascaded harmonic generation in negative-curvature hollow-core fibers

Vacuum-ultraviolet (VUV) light is critical for the study of molecules and materials, but the generation of femtosecond pulses in the VUV region at high repetition rates has proven difficult. Here we demonstrate the efficient generation of VUV light at megahertz repetition rates using highly cascaded four-wave mixing processes in a negative-curvature hollow-core fiber. Both even- and odd-order harmonics are generated up to the 15th harmonic (69 nm, 18.0 eV), with high energy resolution of∼40meV. In contrast to direct high harmonic generation, this highly cascaded harmonic generation process requires lower peak intensity and therefore can operate at higher repetition rates, driven by a robust∼10Wfiber-laser system in a compact setup. Additionally, we present numerical simulations that explore the fundamental capabilities and spatiotemporal dynamics of highly cascaded harmonic generation. This VUV source can enhance the capabilities of spectroscopies of molecular and quantum materials, such as photoionization mass spectrometry and time-, angle-, and spin-resolved photoemission.

Polarization-resolved second harmonic generation from LiNbO3 powders

We report a polarization-resolved analysis of the second harmonic intensity collected in retro-reflection from a Lithium Niobate microcrystals powder. Depth intensity profiles exhibit first an increase due to the beam focus entering the powder then a decrease due to multiple scattering. With a polarization analysis performed at selected depths, we show that there is a competition between ballistic and multiply scattered photons. The contribution from the multiply scattered photons dominates at all depths whereas, at the maximum of the intensity depth profiles, the contribution of the ballistic photons is at its maximum and enhanced due to collection efficiency. This latter contribution from the ballistic photons is clearly observed through distortions in the polarization-resolved plots.

Pulse duration dependence of harmonic yield of H2+ and its isotopic molecule

We investigate the pulse duration dependence of harmonic yield in isotopic molecules of H2+ and T2+ driven by different laser intensities. Generally, the harmonic yield can be improved as pulse duration increases and then a maximum value can be obtained at the specific duration. Particularly, (i) driven by low laser intensity, the optimal pulse duration for H2+ is 20 cycles, and the harmonic intensity of H2+ is much higher than that of T2+. (ii) Driven by middle laser intensity, the optimal pulse durations for H2+ and T2+ are 15 cycles and 22 cycles, respectively. Moreover, the harmonic intensities of H2+ and T2+ from optimal pulse durations are almost the same. (iii) Driven by high laser intensity, the optimal pulse duration for T2+ is 15 cycles. For H2+, the enhancement of harmonic yield is very slow as pulse duration increases. Furthermore, the similar harmonic yields of H2+ and T2+ can be found before 12 cycles; while, when the pulse duration is larger than 12 cycles, the harmonic yield of H2+ is smaller than that of T2+. Taking advantage of harmonic yield changing law and with assistance of controlling pulse, the intense and broad spectral plateau can be obtained, which can support the generation of isolated attosecond pulses with durations of sub-40 as.

2D Inorganic Bimolecular Crystals with Strong In-Plane Anisotropy for Second-Order Nonlinear Optics.

2D inorganic bimolecular crystals, consisting of two different inorganic molecules, are expected to possess novel physical and chemical properties due to the synergistic effect of the individual components. However, 2D inorganic bimolecular crystals remain unexploited because of the difficulties in preparation arising from non-typical layered structures and intricate intermolecular interactions. Here, the synthesis of 2D inorganic bimolecular crystal SbI3 ·3S8 nanobelts via a facile vertical microspacing sublimation strategy is reported. The as-synthesized SbI3 ·3S8 nanobelts exhibit strong in-plane anisotropy of phonon vibrations and intramolecular vibrations as well as show anisotropic light absorption with a high dichroism ratio of 3.9. Furthermore, it is revealed that the second harmonic generation intensity of SbI3 ·3S8 nanobelts is highly dependent on the excitation wavelength and crystallographic orientation. This work can inspire the growth of more 2D inorganic bimolecular crystals and excite potential applications for bimolecular optoelectronic devices.

Harmonic generation in a nematic liquid crystal

The nonlinear optical properties of a nematic liquid crystal (NLC 1289) were experimentally studied according to the intensity and polarization of the pump radiation at a wavelength of 1.56 μm. It is shown that the maximum generation efficiencies of the second and third harmonics correspond to different polarization characteristics of the main radiance. In the investigated range (106 … 2 ∙ 107 W cm−2) the nonmonotonic dependence of the second harmonic intensity on the pump radiance power was demonstrated for the first time. In addition to this, explanation of this effect is also proposed.

Quantum Ring in a Magnetic Field: High Harmonic Generation and NOT Logic Gate

AbstractThe effect of a static magnetic field on the high harmonic generation (HHG) from a quantum ring driven by one laser polarized along the x‐axis is studied. The spin polarization and the temporal emission of the harmonics are studied by varying the intensity of the magnetic field and it is shown how these results have a significant technological impact in computer technology; in fact a boolean algebra can be implemented by assigning 0 and 1 values to low and high pulse intensities of the emitted harmonics and logic gates like the NOT can be created.

ABa2Cl][Ga4S8] (A = Rb, Cs): Wide-Spectrum Nonlinear Optical Materials Obtained by Polycation-Substitution-Induced Nonlinear Optical (NLO)-Functional Motif Ordering.

Noncentrosymmetry (NCS) is a prerequisite for second-order nonlinear optical (NLO) materials. In this work, a new polycation-substitution-induced centrosymmetry (CS)-to-NCS transformation strategy was applied in CS RbGaS2, affording two novel NCS salt-inclusion chalcogenides: [ABa2Cl][Ga4S8] (A = Rb, 1; Cs, 2). Remarkably, they exhibit the key merits of both Vis and IR NLO candidates, including strong phase-matchable second harmonic generation intensities (10.4-15.3 × KH2PO4 at 1064 nm; 0.9-1.0 × AgGaS2 at 1910 nm), high laser-induced damage thresholds (11-12 × AgGaS2), and a wide transparent window. Their prominent NLO performances originate from the orderly packing of T2-supertetrahedral Ga4S10 motifs resulting from the template effect of acentric polycation [ClA2Ba3]7+. The CS-to-NCS transformation first realized by polycation-substitution-induced NLO-functional motif ordering provides an effective approach for designing new NLO materials.

Role of potential on high-order harmonic generation from atoms irradiated by bichromatic counter-rotating circularly polarized laser fields**Project supported by the National Key Research and Development Program of China (Grant Nos. 2019YFA0307700 and 2017YFA0403300), the National Natural Science Foundation of China (Grant Nos. 11627807, 11774175, 11534004, 11774129, 11975012, and 11604119), the Fundamental Research Funds for the Central Universities of China (Grant

We investigate high-order harmonic generation from atoms irradiated by bichromatic counter-rotating circularly polarized laser pulses by numerically solving the time-dependent Schrödinger equation. It is found that the minimum energy position of the harmonic spectrum and the non-integer order optical radiation are greatly discrepant for different atomic potentials. By analyzing the quantum trajectory of the harmonic emission, discrepancies among the harmonic spectra from different potentials can be attributed to the action of the potential on the ionized electrons. In addition, based on the influence of the driving light intensity on the overall intensity and ellipticity of higher order harmonics, the physical conditions for generating a high-intensity circularly polarized harmonic can be obtained.

A theoretical model of high-harmonic generation from two-color relativistic circularly polarized laser pulse interacting with over-dense plasmas

In this article, we set up a theoretical model to investigate the physical mechanism of circularly polarized (CP) high-harmonic generation (HHG) by two-color relativistic driving lasers (with one at fundamental and the other at second harmonic). The compression effect of the electron density profile and the boundary oscillating are responsible for the harmonic emission. Based on this model, the scaling law between the intensity of the 4th harmonic and that of the fundamental driving laser can be successfully acquired. Our theoretical model holds only when the second-harmonic laser is much weaker than the fundamental laser. For more general cases, particle-in-cell (PIC) simulations are performed to demonstrate that the HHG efficiency. The intensity of higher order harmonics can be effectively tuned by gradually enhancing the intensity of the second-harmonic driving laser.

Effect of interband polarization on a solid's high-order-harmonic generation just belowthe band gap.

A series of theoretical and experimental results has proved that harmonics below/above the band gap are produced mainly by the intraband current/interband polarization for solids in strong mid-infrared laser pulses. However, which mechanism dominates the harmonic process is still debated. In this work, based on simulating high-order-harmonic generation from an MgO crystal in a linearly polarized mid-infrared laser by solving semiconductor Bloch equations, we demonstrate that harmonics just below the band gap originate from the interference between intraband and interband currents. Furthermore, it is found that intensities of harmonics just below the band gap are apparently enhanced with an increase in the incident laser's strength. By analyzing the band dispersion and the transition dipole moment of the 001-cut MgO crystal, this can be attributed to the interband polarization between two conduction bands.

Trace gas detect based on spectral analysis and harmonic ratio

A novel method of gas concentration calibration for tunable diode laser absorption spectroscopy, in which spectral analysis and harmonic signal intensity ratio measurement are combined, is proposed in this study. A harmonic signal is extracted from a transmitted light intensity by spectral analysis. The ratio of the second harmonic to the first harmonic is proportional to the gas concentration. The noise caused by electronics and environment is filtered out via frequency-domain filtering, and the harmonic ratio measurement eliminates the effects of light intensity fluctuations. The accuracy and effectiveness of the proposed method are verified by simulation and experiment. Experimental results show that the relative error of measurement is less than 3% and the correlation coefficient between measurement and calibration line is 0.9991.

A study of H2CO•••HF Complex by Advanced Quantum Mechanical Methods

Our research is focused on the ab initio calculations of the equilibrium structures, binding energies, harmonic and anharmonic vibrational frequencies of a hydrogen-bonded complex, which is formed between formaldehyde H2CO and hydrogen fluoride HF, using the Gaussian 09 package of programs with full 6311++G(3df, 3pd) basis sets in the MP2 second-order perturbation theory and CCSD(T) methods. Harmonic and anharmonic vibrational frequencies and intensities of the H2CO···HF complex were calculated by the Gaussian 16 package programs within the same approximation. Geometric changes and frequency shifts at the complex formation were evaluated. The H2CO···HF complex formation energy and the dipole moment were calculated in the CCSD(T)6311++G(3df, 3pd) approximation to be equal, respectively, to 7.78 kcal/mol and 4.2 D. Changes of the geometric, spectral, and energetic parameters of the complex proved the existence of a stable hydrogen bond F–H···O=CH2 between the components.