Linearly polarized pump pulses

The experimental set-up we used is shown in Fig. 2, A synchronously pumped mode-locked and cavity-dvunped dye laser, which can be timed to the D or Dj line of Cs, generates pulses of about 20 ps duration at a pulse rate of U MHz and peak powers of several hundred watt. They are split into linearly polarized pump pulses and stronger circularly polarized probe pulses, which pass an optical delay line. Both beams are focussed into a common interaction region where they act on the Cs vapor, which is contained in a cell at room temperature. The radiated wave propagating in pump pulse direction is detected by a photomultiplier, which measures the transmitted average intensity behind a crossed polarizer as a function of the delay time. 

Optical Kerr Effect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an intensity-dependent refractive index change. The sample is placed between crossed polarizers and a weak, typically tunable, continuous wave (cw) probe laser (usually at a different wavelength and polarized at 45° to the pump pulse) overlaps the pumped region. The increased transmission of the probe beam when the pump pulse arrives is proportional to (Xeff), a combination of elements of the tensor. Many... 

Optical Kerr Eftect. Another important method used to characterize polymers is the optical Kerr effect (OKE). The optical Kerr effect differs from the quadratic electrooptic effect in that the birefringence effects are induced solely by an optical field (37). In this measurement, an intense linearly polarized pump pulse induces birefringence in the nonlinear sample through an... 

FigureBl.5.16 Rotational relaxation of Coumarin 314 molecules at the air/water interface. The change in the SFI signal is recorded as a fimction of the time delay between the pump and probe pulses. Anisotropy in the orientational distribution is created by linearly polarized pump radiation in two orthogonal directions in the surface. (After [90].)...

Figure 11. Time-resolved PADs from ionization of DABCO for linearly polarized pump and probe pulses. Here, the optically bright S E state internally converts to the dark 5i state on picosecond time scales, (a) PADs at 200 fs time delay for pump and probe polarization vector both parallel to the spectrometer axis. The difference in electronic symmetry between S2 and Si leads to significant changes in the form of the PAD. (b) The PADs at 200 fs time delay for pump polarization parallel and probe polarization perpendicular to the spectrometer axis, showing the effects of lab frame molecular alignment, (c) and (d) The PADs evolve as a function of time due to molecular axis rotational wavepacket dynamics. Taken with permission from C. C. Hayden, unpublished.

In Sect. 6.3, we first provide the pulse-design scheme to induce and control 7T-electron rotation in a chiral aromatic molecule. Next, on the basis of dynamical simulations in a semiempirical model, we demonstrate that the initial direction of 7T-electron rotation depends on the spatial configuration of each enantiomer with respect to the polarization direction of a linearly polarized laser pulse and then 7T electrons continue to rotate clockwise and counterclockwise (or counterclockwise and clockwise) in turn. Moreover, a pump-dump control scheme to prevent the switching of the rotation direction and realize a consecutive unidirectional JT-electron rotation is presented. 

In this section, a pulse-design scheme to induce and control jt-electron rotation in a chiral aromatic molecule is provided within a frozen-nuclei approximation. We perform electron WP simulations and show that the initial direction of rr-electron rotation in a chiral aromatic molecule depends on the polarization direction of a linearly polarized laser pulse. A pump-dump method for performing unidirectional rotation of n electrons is also presented [15]. An ansa (planar-chiral) aromatic molecule with a six-membered ring, 2,5-dichloro[n](3,6)pyrazinophane (DCPH Fig. 6.1), was chosen. 

The optical anisotropy can be detected via changes of the optical polarization of linearly polarized probe pulses, which are delayed with respect to the pump pulses. Variation of the delay-time allows to sample the time evolution of the optical anisotropy and by this the atomic coherence. 

In the OKE method, an intense linearly polarized light pulse incident on a material induces birefringence in the material. The induced anisotropy causes a weaker linearly polarized probe pulse at the same frequency to be partially transmitted through crossed polarizers. The pump polarization is set at 45 relative to the two polarizers. A typical arrangement is shown in Fig. 5. 

In contrast to fluorescence, which probes molecules in the excited state only, TRPS dichroism may provide us with information about the dye molecular dynamics in the ground singlet state Sq. In the pump-probe scheme, a linearly polarized pumping optical pulse induces an iititial anisotropy in the ground-state population labeled with the suffix i =g. Then the decay of this anisotropy is followed by measuring at variable temporal delays the absorption of a probe beam of frequency z/pr. 

Linearly polarized, near-diffraction-hmited, mode-locked 1319 and 1064 nm pulse trains are generated in separate dual-head, diode-pumped resonators. Each 2-rod resonator incorporates fiber-coupled diode lasers to end-pump the rods, and features intracavity birefringence compensation. The pulses are stabilized to a 1 GHz bandwidth. Timing jitter is actively controlled to < 150 ps. Models indicate that for the mode-locked pulses, relative timing jitter of 200 ps between the lasers causes <5% reduction in SFG conversion efficiency. 

Fig. 2.6. Top panel spectral migration of SE measured in photoexcited films of mLPPP. The two spectra are SE just following excitation and after 10 ps. The inset shows the shift of the SE peak with time on a wavelength axis. Bottom panel photoinduced dichroism decay in mLPPP films following excitation with linearly polarized 390 nm pulses after 150 fs. The inset shows the pump-probe traces for parallel (higher) and perpendicular (lower) pump-probe polarization. Both data sets are assigned to the same phenomena, namely, energy migration within the conjugated segments or chains...

The noncollinear pump-probe experiment is depicted schematically in Fig. 13. The linearly polarized (P3) pump pulse is focused (LI) into the sample producing induced transmission changes. The polarization of the probe beam is adjusted to 45° relative to the pump with a half-wave plate (A./2) and a Gian polarizer (PI). By the help of an analyzer (P2) simultaneous detection of the parallel ( ) and perpendicular ( L) components of the energy transmission T(v, to) of the probe through the sample is installed. For blocked excitation (chopper, Ch) the sample transmission... 

In order to compare primary dynamics with secondary relaxation steps, we depict on the left-hand side of Fig. 15 the anisotropic spectra (a-c), which consist mainly of spectral components with the same linear polarization as directly induced by the pump pulse. On the right-hand side of the figure the corresponding isotropic spectra (d-f) are shown. In the latter spectral components can notably contribute that result from a relaxation process, where the initially orientation of the OH transition dipole is (partially) lost. 

The pump pulse in time-resolved pump-probe absorption spectroscopy is often linearly polarized, so photoexcitation generally creates an anisotropic distribution of excited molecules. In essence, the polarized light photoselects those molecules whose transition moments are nominally aligned with respect to the pump polarization vector (12,13). If the anisotropy generated by the pump pulse is probed on a time scale that is fast compared to the rotational motion of the probed transition, the measured anisotropy can be used to determine the angle between the pumped and probed transitions. Therefore, time-resolved polarized absorption spectroscopy can be used to acquire information related to molecular structure and structural dynamics. 

Figure 14 A model calculation of the 2D-IR spectra of a idealized system of two coupled vibrators. The frequencies of these transitions were chosen as 1615 cm-1 and 1650 cm-1, the anharmonicity as A = 16 cm the coupling as = 7 cm and the homogeneous dephasing rate as T2 = 2 ps. The direction of both transitions as well as the polarization of the pump and the probe pulse were set perpendicular. The spectral width of the pump pulses was assumed 5 cm-1. The figure shows (a) the linear absorption spectrum and (b) the nonlinear 2D spectrum. In the 2D spectra, light gray colors and solid contour lines symbolize regions with a positive response, while negative signals are depicted in dark gray colors and with dashed contour lines.

In its simplest implementation, OKE spectroscopy is a form of pump-probe polarization spectroscopy. A powerful pump pulse, linearly polarized at 45° to the vertical, is used to induce a transient birefringence in the liquid... 

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