Propagation of femtosecond light pulses in a dye solution: Nonadherence to the conventional group velocity

Abstract

Propagation of femtosecond light pulses in a resonant absorption medium is studied. The propagation time of the light pulses was measured in a dye solution by the optical-Kerr-gate method. Nonadherence to the conventional group velocity domega/dk , which is defined in weak absorption region, was found. The observed wavelength dependences of the group delay and the spectral change in the anomalous dispersion region are qualitatively in good agreement with the theoretical prediction obtained from the new definitions of group velocity, which can be clearly defined even in strong absorption region.

Testing the reliability of a velocity definition in a dispersive medium

“…We observe that the velocity definition of Peatross et al, relying on the Poynting vector average of the pulse, results in 3% discrepancy in the superluminal propagation region; see Since the arrival time introduced by Peatross et al also corresponds to the detector time [11], we reach the additional conclusion that the arrival time measurements [2][3][4][5][6] do not address a proper velocity for the flow. It is still an open problem to find a reliable velocity description consistent with the equivalence of the two approaches.…”

“…Unfortunately, when the original pulse is severely modified there is no direct way to test the validity of the propagation velocity. Several experiments [2][3][4][5][6] measure either the peak of the pulse or the mean absorption time. However, motion of the pulse peak or center may not correspond to a travel velocity, since the shape of the pulse is distorted by mutual act of gain or absorption.…”

“…Studies concerning the propagation of light in dispersive media dates back as far as Brillouin and Sommerfeld [1]. Nevertheless, new studies [2][3][4][5][6][7][8][9][10][11][12][13][14] on the concept of pulse velocity were stimulated by the famous experiment [15,16], where light seems to propagate over the speed of light in vacuum (superluminal). This effect takes place due to superluminal group velocities near the absorption resonance in dye solutions.…”

Applications and Limits of Time-to-Energy Mapping of Protein Crystal Diffraction Using Energy-Chirped Polychromatic XFEL Pulses

“…In the context of using shorter pump pulses and with the idea of pursuing femtosecond time resolution as well as Moffat's original proposal [34] of pumping within the probe pulse, chromatic dispersion of the laser light has to be considered: there is a physical limit dictated by the material dispersion of the optical pump pulse when propagating through the protein crystal and the delivery matrix or solution. Chromatic dispersion is larger in pulses with large bandwidths, and so will become significant if few-cycles ultrashort lasers are used for pumping; the fact that the protein sample is excited by the pump is also important, as group velocity dispersion (GVD) behavior differs at absorption [75]. Though crystal birefringence results in an orientation dependence of the GVD value [76], its contribution is not further considered here.…”

Observation of precursorlike behavior of femtosecond pulses in a dye with a strong absorption band

“…The breakdown of the conventional group velocity dispersion approximation in organic dyes with a sharp absorption resonance has been explored experimentally [9] but, to our knowledge, no direct observation of optical precursors has been obtained in such a dye solution. In this work we numerically and experimentally explore optical precursor phenomena in a linear, dispersive bulk organic dye solution with a sharp absorption resonance by means of simulation of an ultrashort pulse as it propagates through the material.…”