Development of Ultrashort Pulse Lasers and Their Applications to Ultrafast Spectroscopy in the Visible and Nir Ranges

Abstract

Both intensity-dependent nonlinear processes and field-only dependent processes are involved in the process of our discussion of ultrashort pulses Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by UNIVERSITY OF OTTAWA on 09/11/16. For personal use only. Development of Ultrashort Pulse Lasers and their Applications 159in the visible range in this subsection. The former group includes SHG and OPA, and the latter includes SPM. For ultrashort pulse generation in the visible spectral range, including a part of the near infrared (NIR) range, the second-order nonlinear polarization effect has been used in a second-order nonlinear crystal without an inversion symmetry. Even though OPF can be used in principle to generate a pulse with a spectrum sufficiently broad to be an ultrashort pulse, it is too weak to have utility for some experimental purposes because it originates from vacuum noise; therefore, OPA is used instead of OPF to obtain a pulse of sufficient intensity. As mentioned previously, an intense pump light (ω p ) amplifies a weak signal (ω s ) without real population inversion in OPA process. In this case, the energy conversion is satisfied among the intense pump light, the weak signal of frequency ω s , and an idler of frequency ω I = ω p − ω s , which is generated simultaneously by means of second-order nonlinear polarization. An idler pulse is generated from a vacuum, which is coupled with the pump and associatively amplifies the signal at a frequency of ω s = ω p − ω i . The spatial and temporal overlaps of the three are needed to be maintained for efficient conversion. In the beginning, the intensity of the pump is much higher than that of signal, signal amplification continues along with pump propagation through the nonlinear crystal, and the energy of the pump photon is consequently divided into the signal photon and the idler photon. This process can be considered to be a parametric down-conversion (PDC) process from the high-frequency pump photon to the low-frequency signal and idler photons by means of an increase in the photon numbers of the signal and idler at the expense of the pump.As mentioned above, in the case of OPA, pump pulse, signal pulse, and idler pulse must meet the following angular frequency dependence:Here, ω j are the angular frequencies with the suffixes p, s, and i corresponding to the pump, signal, and idler, respectively. Since this is the relation among pulses the angular frequencies has some width corresponding to the inverse of the temporal shapes of the pulses. Here the angular frequencies are set to be the center frequencies. Because of the finite widths of the pulses, the relation of (5) has some ambiguity. The relation can be rephrased in the photon view. The corresponding to the three components in the OPA process Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by UNIVERSITY OF OTTAWA on 09/11/16. For personal use only. Advances in Multi-Photon Processes and Spectroscopy Downloaded fr...

Development of Ultrashort Pulse Lasers for Ultrafast Spectroscopy

“…Thus, ultrafast electronic dynamics and vibrational dynamics can be studied simultaneously in the common experimental condition [32][33][34][35][36][37].…”