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.

Use of aperiodic quasi-phase-matched gratings in ultrashort pulse sources

Abstract: A chirped pulse amplification system employs chirped quasi-phase-matched (QPM) gratings as dispersive delay lines for stretching and/or compressing ultrashort pulses. QPM gratings with periods varying along the beam propagation direction produce simultaneous second-harmonic generation and, in general, both amplitude and phase modulation of this second harmonic output. The aperiodic QPM gratings are designed to provide stretching or compression of the output second harmonic pulse with respect to the fundamental-wavelength input pulse. The chirped QPM gratings are also used for simultaneous harmonic generation and compressing of the chirped output from a femtosecond laser oscillator. In general, the aperiodic QPM gratings can be used to efficiently produce arbitrarily shaped second-harmonic pulses.

Experimental investigation of finite amplitude distortion-based, second harmonic pulse echo ultrasonic imaging

Abstract: The computational predictions for the imaging potential of the second harmonic produced by finite amplitude distortion were investigated with a simple experiment. A focused transducer containing concentric 2.5 MHz and 5.0 MHz elements was used to obtain a sequence of radio-frequency (r-f) backscattered signals using a tissue equivalent phantom. The 2.5 MHz element was used as the transmitter and the 5.0 MHz element was used as the receiver. At 0.68 cm in front of the geometric focal point of the transducer, the phantom contained a 0.6 cm diameter cylindrical volume which contained no scatterers. Each of these r-f signals was then processed to produce the corresponding fundamental (2.5 MHz-centered) and second harmonic (5.0 MHz-centered) envelopes. The contrast resolution obtained for the scatterer-free or cyst region of the envelopes was compared against the computed prediction and good agreement was obtained. The results of this experiment also suggest that the simple one-pulse scheme may be adequate for second harmonic imaging.

Model calculations of high-harmonic generation in molecular ions

Abstract: One electron bound by a three-dimensional two-center zero-range potential is embedded in an electric field with sinusoidal time dependence and arbitrary polarization and orientation with respect to the axis of the two-center potential. In the absence of the field, the model supports up to two bound states, which have a large transition dipole moment. Hence, the physical systems best described by the model are molecular ions such as ${\mathrm{H}}_{2}^{+}.$ Rates for high-harmonic emission are calculated analytically up to one final quadrature. In terms of the rescattering picture, harmonic emission can be attributed to two different mechanisms: electrons recombine either at the center they started from or at the other one. The latter case allows for three topologically different classes of orbits, which lead to different spectral ranges of harmonics. Two of them are similar to atomic (one-center) harmonic generation, but have different cutoff laws that are no longer proportional to the ponderomotive potential. In the third the electron moves directly from one center to the other. This leads to strong harmonic emission at comparatively low frequencies similar to emission from a two-level atom with the cutoff proportional to the field amplitude rather than the intensity. The molecular dipole phase in this case is almost independent of the field intensity and, at constant intensity, the phases of neighboring harmonics are locked. Different orientations of the two-center system with respect to the field with various polarization configurations are investigated. Most of the observed features lend themselves to interpretation in terms of the simple man's model.

Interference between second-harmonic generation from a substrate and from an adsorbate layer

Abstract: Several experiments on surface second-harmonic generation are presented to show how the contribution from a transparent substrate can interfere with that from an adsorbate monolayer. The interference depends on the relative phase of the two contributions, which varies with the molecular orientation, the laser frequency, the polarization, and the optical geometry.