Pulses linearly polarized probe

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. 

The linearly polarized probe pulse can be described as a superposition of a - and a -light. Therefore the pulses, acting with the atoms, measure the actual population differences between left (mj=- /2 ) and right mj= /z) sublevels in ground and excited state. The optical polarization of the probe pulse is changed due to dichroism and can sensitively be detected behind a crossed polarizer. The calculated signal for observation on the Di-line gives a transmitted intensity for the probe pulses behind the crossed polarizer ... 

Both measurements show a fast rising beat structure at negative delaytimes. The origin of this signal results from optical coherence generated by the linearly polarized probe pulse and corresponds to a first-order Free-Induction-Decay. An interpretation of this signal is found in ref. [2]. 

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. 

FIGURE 5 Optical Kerr effect setup. The induced anisotropy causes a weaker linearly polarized probe pulse to be partially transmitted through crossed polarizers 1 and 2. 

An ultrafast time-resolved near- and mid-IR absorption spectrometer was designed to achieve high sensitivity, ultrafast time resolution, and broad tunability in the near- and mid-IR regions (see Fig. 2). The details of this spectrometer are described elsewhere (9). Briefly, MbCO was photolyzed with a linearly polarized laser pulse, whose polarization direction was controlled electronically by a liquid crystal polarization rotator. The photolyzed sample was probed with an optically delayed, linearly polarized IR pulse whose transmitted intensity was spectrally resolved with a monochromator and detected with either a Si photodiode (near-IR RilO cm-1 bandpass) or a liquid nitrogen-cooled InSb photodetector (mid-IR 3 cm-1 bandpass). To measure the sample transmission, this signal was divided by a corresponding signal from a reference IR pulse... 

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... 

Pulsed fluorescence spectroscopy can be applied to investigations of dynamics and structure of biopolymers [14.28]. If the fluorescence emission, excited by a linearly polarized laser pulse, is probed with respect to its polarization, the anisotropy is related to the orientation... 

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].)...

Fig. 21. Calculated linear and 2D spectra for Ala -Ala-Ala using fi = 12.3 cm-1 and = 52°, which corresponds to an a-helical structure. Linear spectrum (a) and 2D scans for parallel (b) and perpendicular (c) polarizations of the pump and probe pulses. From Woutersen and Hamm (2001)./. Chem. Phys. 114, 2727-2737, 2001, Reprinted with permission from American Institute of Physics.

Linear Dichroism. In this technique, the DNA molecules are aligned either by an applied electric field pulse or in a flow gradient. The orientation of the aromatic residues of the metabolite model compounds bound to the DNA (either covalently or non-covalently) relative to the orientation of the DNA bases is probed utilizing linearly polarized light. The linear dichroism A A can be either negative or positive, and is defined as... 

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.

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... 

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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