Characterization of photon combination pathways for harmonic generation from indium tin oxide thin films.

The harmonic generation in an indium tin oxide (ITO) thin film induced by a ω + 2ω two-color field (ω is the frequency of a fundamental laser field) is investigated based on the numerical solution of the full-wave Maxwell-paradigmatic-Kerr equations. By changing the topological charge number and the amplitude ratio of the ω and 2ω field components, different photon combination pathways in support of each harmonic generation are distinguished, which are manifested as characteristic tempo-spatial field distributions, doughnut-shaped intensity distributions with different diameters, and spiral phase diagrams with different topological charge numbers. The results here provide a new, to the best of our knowledge characterization method to distinguish the photon combination pathways for each order harmonic generation.

Vortex harmonic generation in indium tin oxide thin film irradiated by a two-color field.

When a two-color Laguerre-Gaussian laser beam propagates through an indium tin oxide (ITO) material, the spatial distributions of odd- and even-order vortex harmonics carrying orbital angular momentum (OAM) are studied. The origin of vortex harmonics can be directly clarified by investigating their dependence on the incident laser field amplitude and frequency. In addition, it is shown that the spectral intensities of vortex harmonics are sensitive to the epsilon-near-zero nonlinear enhancing effects and the thickness of ITO materials.

Enhancing the robustness of ocean sound speed profile representation via interpretable deep matrix decomposition.

Developing an effective and robust representation model for ocean sound speed profiles (SSPs) is crucial for numerous ocean acoustic applications. However, the performance of existing sound speed profile (SSP) representation methods, such as empirical orthogonal function and K-singular value decomposition, heavily relies on the number of selected basis functions. This could lead to overfitting of noise, as these methods are unable to distinguish between signals and noise during the basis function learning process.

Copper and cuproptosis-related genes in hepatocellular carcinoma: therapeutic biomarkers targeting tumor immune microenvironment and immune checkpoints.

Background: Hepatocellular carcinoma (HCC), one of the most common cancers worldwide, exhibits high immune heterogeneity and mortality. Emerging studies suggest that copper (Cu) plays a key role in cell survival. However, the relationship between Cu and tumor development remains unclear.

Enhancement and redshift of vortex harmonic radiation in epsilon-near-zero materials.

The harmonic radiation from a vortex laser field interacting with an epsilon-near-zero (ENZ) material is numerically investigated via solving the Maxwell-paradigmatic-Kerr equations. For a laser field of long duration, the harmonics up to the seventh-order can be generated with a low laser intensity (∼10 W/cm). Moreover, the intensities of high order vortex harmonics at the ENZ frequency are higher than at other frequency points due to the ENZ field enhancement effects.

Origin of unusual even-order harmonic generation by a vortex laser.

When a spatially-inhomogeneous few-cycle vortex laser interacting with quantum wells, besides the general odd-order harmonics occur, unusual "even-order" ones can be found, which are clarified to possess even orders, however their topological charge numbers are not consistent with those predicted from the vortex transformation criterion (i.e., topological charge number should be directly proportional to its harmonic order for any a harmonic).

Propagation effects in the generation process of high-order vortex harmonics.

We numerically study the propagation of a Laguerre-Gaussian beam through polar molecular media via the exact solution of full-wave Maxwell-Bloch equations where the rotating-wave and slowly-varying-envelope approximations are not included. It is found that beyond the coexistence of odd-order and even-order vortex harmonics due to inversion asymmetry of the system, the light propagation effect results in the intensity enhancement of a high-order vortex harmonics. Moreover, the orbital momentum successfully transfers from the fundamental laser driver to the vortex harmonics which topological charger number is directly proportional to its order.

Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses.

We experimentally demonstrate ultrafast dynamic of generation of the 337-nm nitrogen laser by injecting an external seed pulse into a femtosecond laser filament pumped by a circularly polarized laser pulse. In the pump-probe scheme, it is revealed that the population inversion between the C(3)Π(u) and B(3)Π(g) states of N(2) for the free-space 337-nm laser is firstly built up on the timescale of several picoseconds, followed by a relatively slow decay on the timescale of tens of picoseconds, depending on the nitrogen gas pressure. By measuring the intensities of 337-nm signal from nitrogen gas mixed with different concentrations of oxygen gas, it is also found that oxygen molecules have a significant quenching effect on the nitrogen laser signal.

Quantum path selection in high-order harmonic generation from aligned molecules.

We theoretically investigate high-order harmonic generation (HHG) from aligned N(2) molecules with a driving field composed of two-color circularly polarized laser pulses. It is shown that the combination of N(2) molecules and the waveform-controlled laser field allows us to select either long or short quantum path, depending on molecular alignment angles, while in atom Ar, two paths show comparable contribution to HHG. The selection of single quantum path in aligned N(2) molecules leads to an ultrabroad and smooth XUV supercontinuum, giving rise to isolated attosecond pulses generation.

Alignment-dependent fluorescence emission induced by tunnel ionization of carbon dioxide from lower-lying orbitals.

The study of the ionization process of molecules in an intense infrared laser field is of paramount interest in strong-field physics and constitutes the foundation of imaging of molecular valence orbitals and attosecond science. We show measurement of alignment-dependent ionization probabilities of the lower-lying orbitals of the molecules by experimentally detecting the alignment dependence of fluorescence emission from tunnel ionized carbon dioxide molecules. The experimental measurements are compared with the theoretical calculations of the strong field approximation and molecular Ammosov-Delone-Krainov models.

Identification of the physical mechanism of generation of coherent N2(+) emissions in air by femtosecond laser excitation.

Recently, amplification of harmonic-seeded radiation generated through femtosecond laser filamentation in air has been observed, giving rise to coherent emissions at wavelengths corresponding to transitions between different vibrational levels of the electronic B(2)Σ(u)(+) and X(2)Σ(g)(+) states of molecular nitrogen ions [Phys. Rev. A.

Enhanced narrow-bandwidth emission during high-order harmonic generation from aligned molecules.

We theoretically investigate the selective enhancement of high-harmonic generation (HHG) in a small spectral range when an orthogonal-polarized two-color laser field interacts with aligned O(2) molecules. It is shown clearly that the enhanced narrow-bandwidth emission near the cutoff of the HHG spectrum can be effectively controlled by the molecular alignment angle, laser intensity and the relative phase of two-color laser fields. Furthermore, the strong dependence of narrow-bandwidth HHG on molecular alignment angle indicates that it encodes information about O(2) molecular orbitals, so it may be an alternative method for reconstruction of O(2) molecular orbitals.

Enhanced harmonic emission from a polar molecule medium driven by few-cycle laser pulses.

We investigate theoretically the enhancement of the low-order harmonic emission from a polar molecular medium. The results show that, by using a control laser field, the intensity of the spectral signals near fourth-order harmonics will increase over 25 times as a result of the four-wave mixing process. Moreover, the enhancement effects depend strongly on the carrier-envelope phase of the initial laser fields, which cannot be found in a symmetric system.

Generation of isolated attosecond pulses of sub-atomic-time durations with multi-cycle chirped polarization gating pulses.

We theoretically investigate the X-ray supercontinuum generated by interaction of multi-cycle, chirped polarization gating pulses with the helium gas. It is shown that with this scheme, an isolated sub-50-attosecond pulse can be obtained straightforwardly without any phase compensation. Interestingly, if one selects an extremely broad spectral range near the high-order harmonic cutoff, an isolated and intense sub-24-attosecond pulse can be generated after phase compensation, which could be used to detect and control the electronic dynamics inside the atoms.

Harmonic-seeded remote laser emissions in N₂-Ar, N₂-Xe and N₂-Ne mixtures: a comparative study.

We report on the investigation on harmonic-seeded remote laser emissions at 391 nm wavelength from strong-field ionized nitrogen molecules in three different gas mixtures, i.e., N₂-Ar, N₂-Xe and N₂-Ne.

Dynamics of femtosecond laser filamentation in argon with non-uniform density distribution.

We numerically investigated the femtosecond laser filamentation in a cell filled with argon of non-uniform density distribution. By comparison with the case of uniform density distribution, we demonstrated the crucial differences in the dynamics between the two cases. We found that the pulse-splitting appeared earlier due to the sensitivity of rear-part refocusing to the plasma density and a double Λ shape appeared in the spatio-temporal intensity profile in the non-uniform density case.

Femtosecond filamentation in argon and higher-order nonlinearities.

We numerically investigate the femtosecond filamentation dynamics in argon by considering the higher-order nonlinearities measured by a recent experiment. Our result indicates that, instead of plasma, these higher-order nonlinearities can provide main defocusing effect as shown by a recent theoretical work. However, when the higher-order nonlinearity is so strong to provide a main defocusing effect, some phenomena inconsistent with previous experiments such as pulse splitting and pulse self-compression can appear.

Dependence of dynamic Lorentz frequency shift on carrier-envelope phase and including local field effects.

We investigate the local field effects in a ZnO dense medium. Our results show due to the local-field effects, the Lorentz shifts can be found in the reflected spectra driven by the few-cycle laser pulse. Moreover, the dynamic Lorentz shifts depend sensitively on the carrier-envelope phase (CEP) of the few-cycle laser pulse, which provides a useful means to obtain the CEP information by the frequency shifts.

Carrier-envelope phase control of carrier-wave Rabi flopping in asymmetric semiparabolic quantum well.

We investigate the carrier-wave Rabi flopping effects in an asymmetric semiparabolic semiconductor quantum well (QW) with few-cycle pulse. It is found that higher spectral components of few-cycle ultrashort pulses in the semiparabolic QW depend crucially on the carrier-envelope phase (CEP) of the few-cycle ultrashort pulses: continuum and distinct peaks can be achieved by controlling the CEP. Our results demonstrate that by adjusting the CEP of few-cycle ultrashort pulses, the intersubband dynamics in the asymmetric semiparabolic QW can be controlled in an ultrashort timescale with moderate laser intensity.