China
Africa
Explore chapters and articles related to this topic
Photonic Crystal Fibre
Published in Shyamal Bhadra, Ajoy Ghatak, Guided Wave Optics and Photonic Devices, 2017
Samudra Roy, Debashri Ghosh, Shyamal Bhadra
High harmonic generation (HHG) is one of the major nonlinear optical effects where new frequencies are generated in an ordered fashion when an optical system is pumped by a high-power laser pulse. The kagome-type HC-PCF is likely to have a major impact in generating higher harmonics and a soft x-ray by pumping noble gases using energetic femtosecond Ti-sapphire laser pulses [30]. A gas-filled HC-PCF offers a small mode area and a larger effective interaction length, which significantly reduce the threshold of different nonlinear processes such as HHG. In an experiment [31], it has already been shown that in a xenon-filled kagome fibre, higher harmonics are generated from the seventh to the thirteenth order, ranging from 120 to 60 nm in terms of wavelength. A 30 fs pulse at 800 nm with a repetition rate of 1 KHz and a pulse energy of 10.5 μJ is used in the experiment. The kagome-type HC-PCF reduces the threshold of generating this harmonic to as low as 440 nJ. The conversion efficiency (η) is measured to be η ∼ 2 × 10−9. However, the conversion efficiency may be improved by modulating the core diameter so that the phase-matching condition is achieved between the incident pump and the x-rays.
Light–Matter Interactions (Part 2)
Published in Marcos Dantus, Femtosecond Laser Shaping, 2017
Beyond the simple picture given for ionization above, there is an interesting phenomenon. As the electric field pulls on the electron, the electromagnetic field of the pulse reverses polarity and then accelerates the electron toward the atom from where it originated. This phenomenon is known as re-scattering. The light emerging from re-scattering is easily identified because it has a clear energy signature equivalent to the number of photons of the incident laser that were absorbed by the electron during its interaction with the field. Values exceeding 50 photons were observed quite early; presently, it has been determined that the maximum values observed are for helium and that longer-wavelength lasers achieve greater acceleration. Light with kilo-electron volt energies, corresponding to about 1000-photon acceleration, has been reported. This process, known as high harmonic generation (HHG), is now used routinely in laboratories to generate x-ray laser pulses (Figure 6.5). Interestingly, the bandwidth generated during HHG is sufficient to support attosecond pulse durations. The field of attosecond science emerged around the year 2000 and has already yielded measurements of the coherent motion of electrons in molecules.
High-order harmonic generation of pulses with multiple timescales: selection rules, carrier envelope phase and cutoff energy
Published in Molecular Physics, 2019
Ofer Neufeld, Avner Fleischer, Oren Cohen
High harmonic generation (HHG) is a nonlinear frequency up-conversion process that occurs when intense ultrashort laser pulses interact with matter [1–3]. In this process, the electric field of the driving pulse ionises bound electrons in the media, and subsequently accelerates them back towards the ion, where they recombine and emit harmonic photons [4]. An interesting property of HHG, is that it can be sensitive to the carrier-envelope phase (CEP) of the driving pulse [5,6]. This sensitivity is observed when the driving laser pulse is sufficiently short, i.e. a several-cycle pulse [5–8]. For example, a single-cycle pulse changes its peak intensity by over 50% if the CEP is changed from zero to π. Accordingly, both the plateau structure and the cutoff frequency of the emitted spectrum change significantly with the CEP. These effects vanish as the driver’s duration is increased beyond some threshold value, which is on the order of several cycles of the optical period [6–8]. It is worth mentioning that some CEP effects were also observed from slightly longer multi-cycle pulses (∼20 fs pulses at 800 nm central wavelength) in the form of spectral oscillations in-between adjacent harmonics [9,10]. This effect arises due to interference between short and long electron trajectories, and is very sensitive to the experimental conditions.
Spectrum modification of XUV radiation in the presence of a delayed weak control field
Published in Journal of Modern Optics, 2019
Khuong Ba Dinh, Khoa Anh Tran, Peter Hannaford, Lap Van Dao
High harmonic generation (HHG) is an extreme nonlinear optical process for the generation of coherent extreme ultraviolet (XUV) pulses on ultrashort time scales (1,2). The high order harmonics are emitted in a series of attosecond bursts with high spatial and temporal coherence. In terms of non-pertubative optics, a classical or quantum treatment through the time-dependent Schrödinger equation can be used to describe a simple picture of the high harmonic generation process (1–4). In the semi-classical three-step model, the intense laser field modifies the potential barrier so that the initially bound electron is ionized into the continuum. The free electron is then accelerated away from the ion core. When the electric field reverses its direction, the free electron is controlled and accelerated toward the parent ion. Under certain conditions the electron will recombine with the parent ion, emitting its kinetic energy plus the binding energy in the form of electromagnetic radiation (1–4). The process of recombination of the free electron and the parent ion in HHG can be used to study atomic and molecular structural dynamics and opens new directions for studies in atomic and molecular physics (5–8).
Enhanced attosecond pulse generation in the vacuum ultraviolet using a two-colour driving field for high harmonic generation
Published in Journal of Modern Optics, 2018
P. Matía-Hernando, T. Witting, D. J. Walke, J. P. Marangos, J. W. G. Tisch
High-harmonic generation (HHG) is a nonlinear interaction between matter and an intense laser field that results in the coherent emission of highly energetic photons. It was first observed in gases in the late 1980s [1,2], and since 2000 it has proven to be an invaluable tool of attosecond science due to the characteristic temporal confinement of harmonic emission to a small fraction of the driving laser cycle. The process of HHG can be used to generate pulses below 200 attoseconds of duration in the extreme ultraviolet and soft X-ray spectral regions [3], which can be used in pump-probe experiments. Furthermore, the high degree of coherence of the process can be applied to HHG spectroscopy, where we can perform structural measurements in small atoms and molecules as well as probe nuclear dynamics [4].