High harmonic generation

About: High harmonic generation is a research topic. Over the lifetime, 11694 publications have been published within this topic receiving 222650 citations. The topic is also known as: HHG.

Terahertz pulse generation from noble gases

Abstract: Terahertz pulse generation in the laser-induced plasma from a series of noble gases (He, Ne, Ar, Kr, and Xe) was systematically investigated. Femtosecond laser pulses consisting of both a fundamental and its second-harmonic frequency were used for the terahertz generation. Experimental results reveal that terahertz generation efficiency of these noble gases increases with decreasing ionization potential.

Separating surface structure and surface charge with second-harmonic and sum-frequency scattering

Abstract: We analyze the effect of an electrostatic surface charge on the angular nonlinear light scattering pattern from spherical particles in solution. An electrostatic field near a charged interface leads to a bulk-allowed third-order process, the strength of which is proportional to the electrostatic surface potential. The commonly isotropic nature of the bulk leads to a fixed angular scattering pattern with fixed intensity ratios between polarizations. We show that second- and third-order scattering effects are separable due to their different angular radiation patterns. Furthermore, nonlinear light scattering from a third-order contribution is strongest in the ppp polar- ization combination. Colloidal suspensions cover a wide range of technological applications and are responsible for many biological phe- nomena. Considerable interest exists in the processes that form and stabilize such systems. In a typical colloidal sus- pension a number of interactions between the suspended par- ticles are present: 1 van der Waals attraction, 2 electro- osmotic repulsion due to surface charge, and 3 steric repulsion or chemical interaction due to surface chemistry. 1 Of these three effects, two originate from the surface of the particle. In order to study these surface-induced interactions, it is necessary to obtain information that is specific to the interface. Ideally, we seek a method that characterizes both surface chemistry and surface potential noninvasively. Soft matter science has greatly benefited from the emer- gence of nonlinear optical techniques, such as second- harmonic generation SHG and sum-frequency generation SFG, which allows for the noninvasive recording of interface-specific information while eliminating any influ- ences from the surrounding bulk. 2-19 In these techniques, two electromagnetic waves interact in a sample and can undergo a second-order interaction to generate light at the sum of their frequencies. This interaction is forbidden under spatial inversion symmetry. Near the interface, however, this inver- sion symmetry is broken so that second-harmonic SH or sum-frequency SF light can be generated exclusively at the interface.

High-Gain Harmonic-Generation Free-Electron Laser Seeded by Harmonics Generated in Gas

Abstract: The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (?rad=133nm) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators. © 2011 American Physical Society.

Laser-induced nonlinear excitation of collective electron motion in a cluster

Abstract: We consider the nonlinear collective electron dynamics inside a large cluster irradiated by a strong linearly polarized short (0.1 ?ps) laser pulse. The equation of the centre-of-mass motion of the electron cloud driven by the strong laser field is derived using the approximation of an incompressible medium. The analysis of this equation demonstrates the presence of odd harmonics of the fundamental frequency in the cluster dipole moment, and in both the internal and the scattered electric field. Both neutral and ionized clusters are considered. For clusters with radii R 100?? irradiated by a femtosecond titanium?sapphire laser with a peak intensity of I 1016?W?cm?2, the internal electric field strength near the tripled fundamental frequency is shown to be of the same order as the field of the fundamental. The reason is that both for metallic and for laser-ionized van der Waals clusters the Mie surface-plasmon energy ?M is around 5 eV, which is close to three times the energy of a titanium?sapphire laser-field quantum. On the other hand, the condition for first-order resonance with the Mie frequency is not met during the presence of the main laser pulse, but only temporarily, either at the first onset of inner ionization on the leading edge of the pulse (for van der Waals clusters) or during the subsequent Coulomb explosion. In both cases, the ion density is reduced. The presence of a strong third harmonic leads, in particular, to the enhanced production of multiply charged ions in clusters irradiated by a strong laser field, as compared with isolated atoms. This point is discussed in the light of recent experimental results on the production of multiply charged ions in laser?cluster experiments. Third-harmonic generation by a cluster in a strong laser field, as a function of both the cluster and the laser-field parameters, is also considered.

Two-Wavelength Displacement-Measuring Interferometer Using Second-Harmonic Light to Eliminate Air-Turbulence-Induced Errors

Abstract: In order to eliminate air-turbulence-induced errors in the measurement of linear displacement, an interferometer with two different laser beams using second harmonic generation (SHG) has been developed. These beams have wavelengths of 488 nm, emitted from an Ar+ laser, and 244 nm, which is the second harmonic generated by a nonlinear crystal β-BaB2O4 inserted in the Ar+ laser cavity. Demonstration experiments have proven the system's efficacy in eliminating air-turbulence-induced errors.