Incoherent pump

To clarify whether the 532 nm pulse pumping is essential to produce the x anisotropy, the samples were also pumped by a continuous wave (cw) 442 nm He-Cd laser. The use of an incoherent pump also excludes polar alignment of molecules and, therefore, 355 nm sum-frequency generation that can occur with coherent w and 2cu beams. 

Since the fluorescent transitions must obey certain selection rules, it is often possible to populate a selected level m) by fluorescence from the laser-pumped upper level (Fig. 5.1b). Even with a weak pumping intensity large population densities in the level m) may be achieved. In the pre-laser era, the term optical pumping was used for this special case because this scheme was the only way to achieve an appreciable population change with incoherent pumping sources. 

Plotted in Fig. 12 is the zero-frequency output spectrum F(0) as a function of A. The incoherent pump rate A, the atomic decay rate yi and the cavity loss rate k (the cavity loss rates for the two modes are assumed to be equal to k) are scaled in units of 72. The cooperativity parameter is defined as Co=2g AY(Ky2). The parameters are chosen as Cq=200, yi =0.02 (solid), 0.03 (dotted), 0.05(dashed). When large cooperativity parameter is large (Co l), the system operates... 

P3/2), and 3)= D3/2,5/2). However, the imidirectional incoherent pumping is a stringent requirement. This perhaps may be achieved by using ultrafast pulses or coherently controlled adiabatic passage [116,117]. 

We assume that ysnji, Y o=A, and Yo/=Y2+A, /=1,2, where Yi,2 are spontaneous decay rates while A is incoherent pump rate. The cavity loss term is the same as in the previous scheme. 

Major breakthroughs in early ultrafast VER measurements were made in 1972 by Laubereau et al [22], who used picosecond lasers in an SRS pump-incoherent anti-Stokes Raman probe configuration, to study VER of C-H... 

If the dephasing time of the coherent phonons depend critically on the carrier density, photo-injection of carriers with the second pump pulse can annihilate them partially or completely, depending on its fluence but not on its relative timing. Such incoherent control was demonstrated for the LO phonons of GaAs [37],... 

This experimental work on the dissociation of excited Nal clearly demonstrated behavior one could describe with the vocabulary and concepts of classical motions.The incoherent ensemble of molecules just before photoexcitation with a femtosecond laser pump pulse was transformed through the excitation into a coherent superposition of states, a wave packet that evolved as though it represented a single vibrationally activated molecule. 

The CIS trapping experiment monitors the population in level 2 by an incoherent ionization step. In the weak-fleld limit of the pump laser this can be compared to an experiment detecting the fluorescence from level 2 to a spectator state (here level 1) at the field-free resonance frequency between levels 1 and 2. In such an experiment no fluorescence is expected at this frequency due to the strong shift of level 2 for resonant dump laser frequency. 

The pump induced transient polarisation of the medium modifies the polarisation state of a time delayed probe pulse. Phenomenologically, this process can be regarded as a transient pump induced linear or circular birefringence, also called the Specular Optical Kerr Effect (SOKE) and the Specular Inverse Faraday Effect (SIFE) [18], These are cubic non-linear effects and are predicted to exist from symmetry arguments. Both effects consist of coherent and incoherent parts. For the coherent part, the pump drives the coherent electron-hole pair that affects the probe polarisation. The effect depends upon the probe phase relative to that of the electron-hole pair, and hence, that of the pump. For the incoherent part of the SIFE and the SOKE, the relative pump-probe phase is not important, since the probe pulse polarisation is modified by the pump induced sample polarisation that survives after the decoherence of the electron-hole pair. 

Equations (3.19) and (5.27) represent two extremes, where the result is eitfc fully coherent or fully incoherent. It is enlightening to consider [188] the effeqf partially coherent laser sources through the example of the pump-dump scenario]) Section 3.5 in the case where the laser is not fully coherent. 

A major breakthrough in the measurement of VER occurred in 1972. Laubereau et al. (32) used picosecond laser pulses to pump molecular vibrations via stimulated Raman scattering (SRS) and time-delayed incoherent anti-Stokes probing to study VER of C-H groups in ethanol and methanol ( " -3000 cm-1). Alfano and Shapiro (33) used the same technique to monitor both the decay of the initially excited (parent) C-H stretch excitation and the appearance and subsequent decay of a daughter vibration,... 

A tunable mid-IR pulse at frequency >ir pumps vibrational excitations in a polyatomic liquid (all work discussed here is at ambient temperature 295 K). A time-delayed visible probe pulse at frequency >l generates incoherent anti-Stokes Raman scattering. For an instantaneous pump pulse arriving at time t = 0, the change in the anti-Stokes intensity of transition i, with frequency o) the anti-Stokes transient, is (44)... 

Because the effects of VER are contained in the incoherent anti-Stokes signal (2), any coherent emission resulting from coupling between the pump and probe pulses may be regarded as an artifact. Coherent coupling artifacts are well known in pump-probe measurements of population dynamics (78). In the IR-Raman experiment, the dominant artifact in the anti-Stokes... 

Examples of anti-Stokes data contaminated by an SFG artifact are shown in Fig. 11a and c, where the higher energy C-H stretching transition of neat methanol is pumped at u>ir = 3020 cm-1. The artifact will be centered at col + anti-Stokes emission from methanol vibrational transitions at 3020 cm-1 (actually the higher energy tail of the C-H stretch transition at 2940 cm-1). The spectral and temporal properties of the artifact can be independently characterized by purposely generating SFG in a thin ( 50 pm) slab of KTP placed at the location of the sample. However, the amplitude of the SFG artifact in the spectrum is unknown. 

A 10- to 20-Hz Nd YAG laser is very convenient as an excitation source for this experiment, since the doubled output at 532 nm is near the broad 550-nm ruby absorption and the laser pulse is short (5 to 10 ns) compared to the excited Cr radiative lifetime. The student should note the cavity construction of such a laser and, following the directions of the laboratory instructor, adjust the doubling crystal for optimum green output. A 532-nm output of 0.1 to 1 mJ is adequate for the experimeut, so a small and relatively inexpensive flashlamp- or diode-pumped pulsed laser is sufficient. Also suitable as an excitation source would be a dye laser operated near 550 nm and pumped by a nitrogeu or excimer laser. (An incoherent pulsed source such as a strobe light can also be used if the pulse is about 10 /ts or less and if appropriate band-pass filters are used.) For all laser experiments, safety goggles must be worn to minimize hazard due to the high intensity of these sources. The instructor will provide instructions about any special features of the lasers and their safe operation. 

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