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Register a new userSpectral control of high order harmonics through non-linear propagation effects
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High harmonic generation (HHG) in crystals has revealed a wealth of perspectives such as all-optical mapping of the electronic band structure, ultrafast quantum information and the creation of novel all-solid-state attosecond sources. Significant efforts have been made to understand the microscopic aspects of HHG in crystals, whereas the macroscopic effects, such as non-linear propagation effects of the driving pulse inside the dense solid media and its impact on the HHG process is often overlooked. In this work, we study macroscopic effects by comparing two materials with distinct optical properties, silicon (Si) and zinc oxide (ZnO). By scanning the focal position of 85 fs, 2.123 $mu$m wavelength pulses inside the crystals (Z-scan) we reveal spectral shifts in the generated harmonics. We interpret the overall blueshift of the emitted harmonic spectrum as an imprint of the driving field spectral modulation occurring during the propagation inside the crystal. This is supported with numerical simulations. This study demonstrates that through manipulation of the fundamental driving field through non-linear propagation effects, precise control of the emitted HHG spectrum in solids can be realised. This method could offer a robust way to tailor HHG spectra for a range of spectroscopic applications.
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We theoretically investigate the spectral caustics of high-order harmonics in solids. We analyze the 1-dimension model of solids HHG and find that, apart from the caustics originated from the van Hove singularities in the energy-band structure, another kind of catastrophe singularities also emerge when the different branches of electron-hole trajectories generating high-order harmonics coalesce into a single branch. We solve time-dependent Schrodinger equation in periodic potential and demonstrate the control of this kind of singularities in HHG with the aids of two-color laser fields. The diffraction patterns of the harmonic spectrum near the caustics agree well with the inter-band electron-hole recombination trajectories predicted by the semiconductor semi-classical equation. This work is expected to help to understand the HHG dynamics in solids and manipulate the harmonic spectrum by adjusting driving field parameters.
Light beams carrying orbital angular momentum (OAM) have led to stunning applications in various fields from quantum information to microscopy. In this letter, we examine OAM from the recently discovered high-harmonic generation (HHG) in semiconductor crystals. HHG from solids could be a valuable approach for integrated high-flux short-wavelength coherent light sources. The solid state nature of the generation medium allows the possibility to tailor directly the radiation at the source of the emission and offers a substantial degree of freedom for spatial beam shaping. First, we verify the fundamental principle of the transfer and conservation of the OAM from the generation laser to the harmonics. Second, we create OAM beams by etching a spiral zone structure directly at the surface of a zinc oxide crystal. Such diffractive optics act on the generated harmonics and produces focused optical vortices with nanometer scale sizes that may have potential applications in nanoscale optical trapping and quantum manipulation.
We have derived the corresponding equations and found their solutions both for nonparaxial and paraxial beams. The paraxial solutions we have presented in the form of the generalized Hermite-Gaussian beams propagating perpendicular to the optical axis of a uniaxial crystal. We have also constructed the generalized Laguerre-Gaussian beams at the z=0 plane and analyzed their evolution in a homogeneous isotropic medium. Comparing it with the evolution of the standard Laguerre-Gaussian beams with and in the crystal we have revealed that the additional elliptic deformation of the extraordinary beam results in topological reactions that essentially distorts field structure for the account of different rotation rates of the vortex row originated from the centered degenerate optical vortex and the conoscopic pattern. We have predicted conversion of the vortex topological charge at the beam axis similar to that in astigmatic lenses and analyzed the radical differences with this process. We have revealed the synchronic oscillations of the spin angular momentum and the sign of the vortex topological charge at the beam axis.
High-order harmonics generated by bicircular laser field have helicities which alternate between $+1$ and $-1$. In order to generate circularly polarized high-harmonic pulses, which are important for applications, it is necessary to achieve asymmetry in emission of harmonics having opposite helicities. We theoretically investigated a wide range of bicircular field component intensities and found areas where both the harmonic intensity is high and the helicity asymmetry is large. We investigated the cases of $omega$--$2omega$ and $omega$--$3omega$ bicircular fields and atoms having the $s$ and $p$ ground states, exemplified by He and Ne atoms, respectively. We have shown that for He atoms strong high harmonics having positive helicity can be generated using $omega$--$3omega$ bicircular field with a much stronger second field component. For Ne atoms the helicity asymmetry can be large in a wider range of the driving field component intensities and for higher harmonic orders. For the stronger second field component the harmonic intensity is higher and the helicity asymmetry parameter is larger for higher harmonic orders. The results for Ne atoms are illustrated with the parametric plots of elliptically polarized attosecond high-harmonic field.
We demonstrate a new design principle for unidirectionally invisible non-Hermitian structures that are not only invisible for one specific wavelength but rather for a broad frequency range. Our idea is based on the concept of constant-intensity waves, which can propagate even through highly disordered media without back-scattering or intensity variations. Contrary to already existing invisibility studies, our new design principle requires neither a specific symmetry (like $mathcal{PT}$-symmetry) nor periodicity, and can thus be applied in a much wider context. This generality combined with broadband frequency stability allows a pulse to propagate through a disordered medium as if the medium was entirely uniform.
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