Nonlinear propagation of ultrashort pulses in cyanine dye solution investigated by SHG FROG Atsushi Yabushita * , Takao Fuji 1 , Takayoshi Kobayashi Department of Physics, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan Received 2 May 2004; in final form 7 September 2004 Available online 18 October 2004 Abstract Changing the chirp property of ultrashort pulses, the spectra of intensity and phase of the pulses after transmitting through eth- anol solution of cyanine dye were measured with second-harmonic-generation frequency-resolved optical gating (SHG-FROG) method. Anomalous dispersion due to the absorption of the dye solution was not observed when the pulses were negatively chirped and their intensity were large. This phenomenon is well explained by suppression of the absorption by pump–dump process based on wave packet motion along the shifted harmonic potential model. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Manipulating chemical reaction with laser beam is one of the most actively studied topics in molecular physics. In particular, ultrashort laser pulses are quite useful for the purpose because the time scale of molecu- lar motion is femtosecond. For several years, it has been demonstrated that interaction between electric field and condensed matter strongly depends on the phase of ultrashort pulses, for example, coherent control of ex- cited state and ground state population by use of phase locked pulse trains [1–4], or by changing chirp of the pulses [5,6]. In most of these experiments, only power spectra of the pulses transmitted through a sample were measured, and the pulse shape was usually assumed to be un- changed even after interacting with the sample at the data analysis. On the other hand, in order to investigate nonlinear phenomenon in highly absorptive media (OD > 0.3), such as Rabi oscillation [7], absorption sat- uration [8], and self-induced-transparency [9,10], it is basically necessary to know how the pulse shape changes after interacting with the sample. Event in the investigations of the other general nonlinear phenom- ena, the temporal profile or phase information of the pulses would be important for deeper understanding of interaction between the highly absorptive media and the ultrashort pulses. Frequency resolving optical gating (FROG) is one of the most popular pulse characterization method, which can measure the intensity and phase of the pulse in the same time [11–16]. A typical and simplest FROG is SHG FROG. Recently, the method was also applied for spectroscopic purpose to observe self-induced- transparency in quantum dot [17], femtosecond pulse propagation anisotropy in a quantum nanostructure [18], ultrafast phase distortion of the pulses in quantum wells [19], coherent population transfer in a three-level atom [20], intensity-dependent atomic dipole phase of a high harmonic [21], and dipolar molecular responses of isotopically diluted water [22]. In this Letter, the intensities and phases of the ultra- short pulses transmitted through ethanol solution of 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.128 * Corresponding author. Fax: +81 3 5841 4240. E-mail address: yab@femto.phys.s.u-tokyo.ac.jp (A. Yabushita). 1 Present address: Max-Planck-Institute of Quantum Optics, Hans- Kopfermann-Strasse 1, D-85748 Garching, Germany. www.elsevier.com/locate/cplett Chemical Physics Letters 398 (2004) 495–499