Femtosecond laser pulses filamenting in various gases are shown to generate long- lived quasi-stationary cylindrical depressions or 'holes' in the gas density. For our experimental conditions, these holes range up to several hundred microns in diameter with gas density depressions up to ~20%. The holes decay by thermal diffusion on millisecond timescales. We show that high repetition rate filamentation and supercontinuum generation can be strongly affected by these holes, which should also affect all other experiments employing intense high repetition rate laser pulses interacting with gases.
Separate theoretical and experimental investigations of the effect of lattice anharmonicity on the A 1g phonon dynamics in photoexcited bismuth are presented. First-principles density functional calculations show that the anharmonic contribution to the phonon period is negligible for an excitation of 1.25% or less of the valence electrons, corresponding to electronic frequency softening from 2.9 to 2.3 THz. Experiments using optical double-pump-probe excitation of coherent phonon motion clearly separate the role of lattice anharmonicity from electron-hole plasma and other dynamics, confirming that the effect of anharmonicity on the phonon period is much smaller than the observed softening.Anharmonicity in solids manifests itself in a variety of ways including thermal expansion and phonon decay. Coupling between vibrational modes in a solid involves a continuum of states, in contrast with molecules where anharmonicity leads simply to a set of discrete levels with unequal spacings. Excitation of coherent atomic displacements have opened the prospect of observing anharmonic atomic vibrations in real-time using femtosecond pulsed lasers. 1 In particular, high amplitude coherent optical phonon generation in bismuth has been extensively studied. [2][3][4][5][6][7][8][9] In a number of these experiments, anharmonic effects have been claimed in the form of an amplitude dependent phonon frequency, 7 quantum effects such as collapse and revival 8 and even Bose-Einstein condensation. 9 In understanding the complex dynamics of this system, the relative importance of lattice anharmonicity ͑in the form of an explicit dependence of vibration frequency on vibration amplitude͒ and electronic softening ͑i.e., a weakening of the interatomic restoring forces due to excitation of electrons from the valence bands into the conduction bands͒ has been particularly controversial. 10,11 In this Rapid Communication, we resolve this controversy, clearly separating the effects of lattice anharmonicity on the phonon period from other effects, including those of plasma dynamics and electronic softening. We demonstrate, both in first-principles density functional theory calculations and, separately, in optical double-pump-probe experiments, that lattice anharmonicity has a negligible effect on the period of oscillation for photoexcited electron-hole plasma densities of less than 1.25% of the valence electrons. We infer that electronic softening is the primary cause of the timedependent period of reflectivity oscillations observed in this and previous experiments.Following high-density photo-excitation of carriers, the equilibrium positions of the atoms and the electronic restoring forces are substantially altered and the atoms then oscillate in a coherent, large amplitude motion about their new equilibria. The spatio-temporal dynamics of the phonons and the photoexcited carriers are intricately coupled. On the timescale of the pump-probe experiments, several processes are important: coherent motion of the atoms and nonlinear effects in the optic...
The absolute time-dependent nonlinear response of O 2 , N 2 , N 2 O, and Ar to intense nonionizing, ultrashort optical pump pulses is measured with single-shot spectral interferometry. The instantaneous and delayed rotational responses are distinguished as a function of pump-pulse duration and probe central wavelength. Our measurements are central to the modeling and understanding of nonlinear propagation of intense ultrashort laser pulses in gases.
We demonstrate that femtosecond filaments can set up an extended and robust thermal waveguide structure in air with a lifetime of several milliseconds, making possible the very-long-range guiding and distant projection of high-energy laser pulses and high-average power beams. As a proof of principle, we demonstrate guiding of 110-mJ, 7-ns, 532-nm pulses with 90% throughput over ∼15 Rayleigh lengths in a 70-cm-long air waveguide generated by the long time-scale thermal relaxation of an array of femtosecond filaments. The guided pulse was limited only by our available laser energy. In general, these waveguides should be robust against the effects of thermal blooming of extremely high-average-power laser beams.
We present the first experimental evidence, supported by theory and simulation, of spatiotemporal optical vortices (STOVs). A STOV is an optical vortex with phase and energy circulation in a spatiotemporal plane. Depending on the sign of the material dispersion, the local electromagnetic energy flow is saddle or spiral about the STOV. STOVs are a fundamental element of the nonlinear collapse and subsequent propagation of short optical pulses in material media, and conserve topological charge, constraining their birth, evolution, and annihilation. We measure a self-generated STOV consisting of a ring-shaped null in the electromagnetic field about which the phase is spiral, forming a dynamic torus that is concentric with and tracks the propagating pulse. Our results, here obtained for optical pulse collapse and filamentation in air, are generalizable to a broad class of nonlinearly propagating waves.
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