Control chirped pulse

The second example is the quadratically chirped pump-dump scheme. Since the pioneering work by Tannor and Rice [119], the pump-dump method has been widely used to control various processes. However, since it is not possible to transfer a wave packet from one potential energy surface to another nearly completely by using the ordinary transform limited or linear chirped pulses, the... 

Figure 39. Pump-dump control of NaK molecule by using two quadratically chirped pulses. The initial state taken as the ground vibrational eigenstate of the ground state X is excited by a quadratically chirped pulse to the excited state A. This excited wavepacket is dumped at the outer turning point at t 230 fs by the second quadratically chirped pulse. The laser parameters used are = 2.75(1.972) X 10-2 eVfs- 1.441(1.031) eV, and / = 0.15(0.10)TWcm-2 for the first (second) pulse. The two pulses are centered at t = 14.5 fs and t2 = 235.8 fs, respectively. Both of them have a temporal width i = 20 fs. (See color insert.) Taken from Ref. [37].

Figure 40. Pump-dump control of NaK by using two quadraticaUy chirped pulses. The initial state and the first step of pump are the same as in Fig. 39. The excited wave packet is now dumped at R 6.5cio on the way to the outer turning point. The parameters of the second pulse are a ) = 1.929 X 10 eVfs , = 1.224eV, and I = 0.lOTWcm . The second pulse is centered at...

A negative chirped pulse is shown in Figure 6.4c. Experiments and theoretical studies on coherent control of ultrafast electron dynamics by intense chirped laser pulses will be discussed in Sections 6.3.2.3 and 633.2. 

In the following, we will discuss two basic - and in a sense complementary [44] - physical mechanisms to exert efficient control on the strong-field-induced coherent electron dynamics. In the first scenario, SPODS is implemented by a sequence of ultrashort laser pulses (discrete temporal phase jumps), whereas the second scenario utilizes a single chirped pulse (continuous phase variations) to exert control on the dressed state populations. Both mechanisms have distinct properties with respect to multistate excitations such as those discussed in Section 6.3.3. 

Since the development of titanium-sapphire (Ti Sa) femtosecond laser source, the domain of research fields or development covered by the use of ultrafast lasers is in continuous expansion. In femtochemistry, it was realized that, when in a photochemical reaction different pathways lead to a given final state, the presence of a well-controlled frequency pulse chirp might greatly enhance the probability with which this final state is reached [1,2]. Chirp control is needed and mastering the phase of the laser pulse is the key point. 

Fig. 1. Demonstration of the phase control of femtosecond chirped pulses. Solid lines spectral phase of the diffracted pulse for two delays X and -X between the writing pulses. Thin dotted lines spectral phase difference between the 2 writing pulses for the two cases x and -X. The thin dashed line represents the spectral amplitude of the diffracted pulse. Insert spectral interference fringes between the unchirped and chirped writing pulses at time delay equal 0 at 800nm. The chirp is formed by propagation through a SF58 glass plate of thickness 1.7 cm.

Summary. An effective scheme for the laser control of wavepacket dynamics applicable to systems with many degrees of freedom is discussed. It is demonstrated that specially designed quadratically chirped pulses can be used to achieve fast and near-complete excitation of the wavepacket without significantly distorting its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura (ZN) theory of nonadiabatic transitions. The scheme is applicable to various processes, such as simple electronic excitations, pump-dumps, and selective bond-breaking, and, taking diatomic and triatomic molecules as examples, it is actually shown to work well. 

Fig. 5.7. Pump-dump control of NaK molecule using two quadratically chirped pulses. The initial state is taken as the ground vibrational eigenfunction of the ground state X1S+ and this is excited by a quadratically chirped pulse to the excited state A1E+. The excited wavepacket is dumped at the outer turning point t cs 230 fs by the second quadratically chirped pulse. The laser parameters used are...

As demonstrated above, bond-selective dissociation can be achieved with high efficiency by using an initial displaced-position and/or a directed-momentum wavepacket. The latter wavepacket can be prepared via the sequence of quadratically chirped pulses or by using semiclassical optimal control theory [34,35],... 

In order to control elementary process (i), an effective scheme based on the concept of quadratic chirping has been proposed [12-17]. It has been demonstrated that this idea can be applied to process (i) and that fast and near-complete selective excitation of a wavepacket can be achieved without significant distortion of its shape through the utilization of specially designed quadratically chirped pulses [18,19]. This method is discussed in the first part... 

Finally, by combining the laser control of electronic transitions of wavepackets using quadratically chirped pulses [18,19] with semiclassical op-... 

Tanabe T, Ohno K, Okamoto T, Yamanaka M, Kannari F (2004) Feedback control for accurate shaping of ultrashort optical pulses prior to chirped pulse amplification. Jpn. J. Appl. Phys. 43 1366-1375... 

Optimal chirped-pulse schemes for achieving population inversion ( molecular 7r pulses ) and to explain the chirp-dependence of multiphoton absorption yields have been described by Cao (Cao and Wilson, 1997 Cao, et al., 1998 Cao, et al., 2000). The learning algorithm approach has been reviewed by Levis, et al., (2001) and Rabitz, et al., (2000). The use of masks, arrays, and computer controlled liquid crystal devices for phase and amplitude control has been described by Kawashima, et al., (1995) Weiner, (1995) Krause, et al., (1997) and Tull, et al., (1997). Schemes for storing information in the rotation-vibration levels of diatomic molecules have been implemented by Ballard, et al., (2002) and Stauffer, et al., (2002). 

The main advantage of the phase-modulated pulse is its capability of having very broad spectrum in a controlled manner. A preliminary experiment of four-wave mixing in dye solutions has been tried in our group using a broad-band chirped pulse produced in an optical fiber, and confirmed the achievement of subpicosecond resolution time much shorter than the pulse width, though T2 is too short to be observed. 

Vala, J., Duheu, O., Masnou-Seeuws, F., Pillet, R, and Kosloff, R., Coherent control of cold-molecule formation through photoassociation using a chirped-pulsed-laser field,... 

---