Coherent pumping

Another clear example of a system generating second harmonics is the one employing an external coherent pump held = constant without dumping (Y = 0) and frequency mismatch ( ,- = 0). The system belongs to the class of Hamiltonian systems. The function (Hamiltonian)... 

Figure 5.3 demonstrates the effect of vibrational relaxation on photo-induced ET (PIET) in an ultrashort pulse experiment where more than one vibronic level are coherently pumped and only the single-level rate... 

A main feature of ultrafast processes under consideration takes place in the time scale shorter than picoseconds. Thus, it is necessary to employ the laser with pulse-duration 10 fsec to study these ultrafast processes. From the uncertainty principle AE At h/2 it can be seen that using this pulse-duration, numerous vibronic states can be coherently pumped (or excited) and thus the probing signal in a pump-probe experiment will contain the information of the dynamics of both population and coherence (or phase). In other words, in order to obtain the information of ultrafast dynamics it is... 

The first term in (11) is the fV-photon Kerr Hamiltonian [35], giving rise to optical bistability, and % is related to the (2/V — l)-order susceptibility of the medium. The second term in (11) represents coherent pumping modulated by classical function/(f). Similarly, as in the previous section, we assume that the excitation has a constant envelope i.e. /(f) = 1. Applying the procedure analogous to that described in the previous section we get the following equations... 

We assume strong coherent pumping of laser modes and replace the operators aLj with corresponding complex amplitudes [Pg.558]

Figure 15.5 (a) - (c) Time-re solved change of O-D stretching absorbance as a function of the pump-probe delay for 3 different probe frequencies (solid lines). Around delay zero, coherent pump-probe coupling leads to a strong signal. The absorbance changes for positive delay times consist of rate-like components due to population relaxation of the... 

There are several unresolved issues in the problem of coherent vibrational pumping by shock fronts. These include (1) to what degree can a shock front be viewed as a coherent phonon source (2) can a shock front coherently drive vibrational excitations, and (3) could shock front coherent pumping cause selective bond breaking, especially bonds other than those broken by ordinary thermochemical reactions One way to look at the first issue is to look at the shock front as a superposition of phonons. Since phonons form a complete set... 

Things are not quite as simple as they seem. In order for the constructive interference, which is at the core of wavepacket interferometry, to occur, not only must (t + At) = (t), but also the phases of apump and aprobe> which depend on the optical phase of the femtosecond laser rather than the molecular phase, must match. A rigorous treatment of the phase coherent pump/probe scheme using optically phase-locked pulse pairs is presented by Scherer, et al., [1990, 1991, 1992] and refined by Albrecht, et al., (1999), who discuss the distinction between and consequences of pulse envelope delays vs. carrier wave phase shifts (see Fig. 9.6). A simplified treatment, valid only for weak optical pulses is presented here. 

J. Bergou, M. Orszag, and M. O. Scully. Correlated-emission laser Phase noise quenching via coherent pumping and the effect of atomic motion. Physical Revew A 1988 Jul 15 38 (2) 768-772. 

T. C. Ralph, C. M. Savage. Squeezed light from a coherently pumped four-level laser Physical Revew A 1991 Dec 1 44(11) 7809-7814. 

Summary Optical parametric oscillators are coherent devices similar to lasers. There are, however, important differences. While lasers can be pumped by incoherent sources, OPOs require coherent pump sources. Often diode laser-pumped solid state lasers are used. While in lasers coherent amplification can last until the inversion in the active medium has fallen below threshold, in OPO s the time dependence of the coherent output is directly coupled to that of the pump laser. Since the pump photon is split into signal and idler photon with u> = u>i, the energy of the output equals that of the input i.e. there is no energy, i.e. heat deposited in the active crystal. The spectral tuning range is by far wider than for tunable lasers. Most OPOs operate in the near infrared but can be tuned from the visible region to the far infrared. 

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