We directly measure the nonlinear optical response in argon and nitrogen in a thin gas target to laser intensities near the ionization threshold. No instantaneous negative nonlinear refractive index is observed, nor is saturation, in contrast with a previous measurement [Loriot et al., Opt. Express 17, 13429 (2009)] and calculations [Brée et al., Phys. Rev. Lett. 106, 183902 (2011)]. In addition, we are able to cleanly separate the instantaneous and rotational components of the nonlinear response in nitrogen. In both Ar and N2, the peak instantaneous index response scales linearly with the laser intensity until the point of ionization, whereupon the response turns abruptly negative and ∼constant, consistent with plasma generation.The optical Kerr effect, the intensity-dependent refractive index experienced by an optical pulse in a transparent medium, plays an important role in phenomena from nonlinear propagation in optical fibers [1] to modelocking in pulsed lasers [2] to filamentary propagation in condensed media and the atmosphere [3]. A recent transient birefringence measurement in the components of air reported by Loriot et al. [4] purported to show that the optical Kerr effect saturates and then becomes negative for intensities greater than 26 TW/cm 2 . A strong higher-order Kerr effect, with a crossover from positive to negative nonlinear index at intensities well below the ionization threshold, would have a huge impact on the nonlinear optics of transparent media, and has inspired theoretical works predicting plasma-free light filamentation [5] and exotic new effects in light propagation [6]. It would overturn the picture most have of the mechanism behind long-range filamentary propagation of intense ultrashort pulses -as arising from an interplay between self-focusing due to the positive optical nonlinearity from bound electrons and defocusing due to the plasma generated by ionization. The existence of a higher-order Kerr effect would also have implications for the general nonlinear susceptibility in transparent media [7,8], including harmonic generation [9][10][11].Subsequent experimental studies of light filaments [12][13][14][15] have not supported the higher-order Kerr model, with one exception [16]. One measurement [13] found that the electron density was two orders of magnitude higher than predicted by a calculation including higher-order nonlinearities, but agreed with a simulation based on plasma defocusing alone [5]. A physical mechanism for the saturation and negative response was proposed based on the nonlinear response near the threshold of ionization [17,18]. What is missing from this debate is a direct measurement of the nonlinearity that corroborates or refutes the intensity dependence observed by Loriot et al. Here, we describe such a measurement in Ar and N 2 using spectral interferometry. We find no saturation and no negative instantaneous nonlinear phase, in contrast to the original experiment [4].The technique we use, single-shot supercontinuum spectral interferometry [19], provides...
Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic processes leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics developed at Maryland for ultrafast measurements of laser-gas interactions. V C 2014 AIP Publishing LLC.
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