Pump radiation

Since there is a definite phase relation between the fiindamental pump radiation and the nonlinear source tenn, coherent SH radiation is emitted in well-defined directions. From the quadratic variation of P(2cii) with (m), we expect that the SH intensity 12 will also vary quadratically with the pump intensity 1 ... 

The linear and nonlinear optical responses for this problem are defined by e, 2, e and respectively, as indicated in figure Bl.5.5. In order to detemiine the nonlinear radiation, we need to introduce appropriate pump radiation fields E(m ) and (co2)- If these pump beams are well-collimated, they will give rise to well-collimated radiation emitted tlirough the surface nonlmear response. Because the nonlinear response is present only in a thin layer, phase matching [37] considerations are unimportant and nonlinear emission will be present in both transmitted and reflected directions. 

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].)...

The are two other important advantages of the X-ray excitation technique (a) Levels lying within the optical absorption band of the host may be excited and (b) filters are not required to eliminate leakage of the pumping radiation from the detector. 

Figure 10. Optical configuration for differentially arranged, thermal lens detected CD. P, beam steering prism M, beam steering mirror BS, polarizing beam splitter HR, half-wave rhomb QR, quarter-wave rhomb L, focusing lens DM, dichroic mirror C, converging sample cell (before probe focus) D, diverging sample cell (after probe focus) PD, aperture/photodiode combination LF, line filter (to isolate the probe laser from extraneous pump radiation). Solid line, probe laser optical path broken line, pump beam path.

Fig. 2.4. (a) The excitation process. The pump radiation (solid line) populates a high-lying vibronic level. Relaxation (dashed line) populates lower-lying states. The population is redistributed black circles correspond to occupied states and white circles to empty states, (b) Probing. Photobleaching (dotted lines), photoinduced absorption (PA) and stimulated emission (SE) (solid lines). Only one vibrational replica is shown (tilted arrow)... 

Momentum also plays a role in ordinary spontaneous Raman spectroscopy. When the pump radiation at 532 nm is passed through a sample, the aE term of Eq. (2) produces scattering and, for the first Stokes case shown in Fig. la, the frequency is i si = where is the Raman-active vibration excited in the sample. It should be noted that there is an exchange between the radiation field and molecule not only of energy but also of momentum, represented by the vector ky. The direction and magnitude of k, are determined by the photon-scattering direction, which is random for this spontaneous event. The result is scattering in all directions so that there is no coherent addition of photon amplitudes, as expressed in the summation /(i si) = C8q The net intensity from this inco-... 

Coherent Anti-Stokes Raman Scattering (CARS). In addition to the nonpara-metric SRS process to generate Stokes-shifted beams that propagate collinearly with the vq pump radiation, a second parametric process can occur that generates noncollinear beams at frequencies Vsi = vq + I yand vsi = vq - Vj. The energy level diagram for... 

Whilst the above is perfectly adequate for the description of processes observed with continuous-wave (cw) input, proper representation of the optical response to pulsed laser radiation requires one further modification to the theory. It is commonly thought difficult to represent pulses of light using quantum field theory indeed, it is impossible if a number state basis is employed. However by expressing the radiation as a product of coherent states with a definite phase relationship, it is relatively simple to construct a wavepacket to model pulsed laser radiation [39]. The physical basis for this approach is that pulses necessarily have a finite linewidth and therefore in fact entail a large number of radiation modes, so that for the pump radiation, it is appropriate to construct a coherent superposition... 

The net result of incorporating all these modifications in the theory of harmonic emission, or any other process entailing the annihilation of n photons from the pump radiation, is that we now have the following prescription ... 

Consider a system in which, prior to the input of the pump radiation responsible for the detected harmonic output, irradiation with a beam of the appropriate resonant frequency produces a significant population of the electronic level u among the optical centers in each particle or domain. With first-order decay kinetics, the probability that a certain center c is excited at time t is given by... 

The coefficients W determine the probabilities of third-order nonlinear optical processes in an unbounded crystal. An analogous expression can be derived for the coefficients determining the probabilities of fourth-order nonlinear optical processes. As already mentioned the derivation for multilevel molecules is rather complicated and has not yet been obtained. However, the simplicity of the final result, that is the simplicity of the nonlinear Hamiltonian, determines the simplicity of the calculations of nonlinear processes. Note also that a similar polariton approach can be applied for consideration of nonlinear processes in low-dimensional nanostructures (chains, quantum wells). For such structures just resonances of the pumping radiation with polaritons of low-dimensional structure and not with excitons will determine the resonances in the absorption of light as well as resonances in nonlinear processes. 

Figure 6.19 Stark quantum beats in BaO A1 +(u = 2, J = 1). The J = 1 level is excited via the R(0) line by radiation from an N2-laser-pumped dye laser. The pump radiation is linearly polarized at 45° to the 5-field direction in order to produce a coherent superposition of At = 0 with M = 1 components. The top trace shows the signal resulting when the polarization of the detected fluorescence is selected to be at 45° to and at 90° to the excitation polarization. The middle trace is for parallel excitation and detection polarizations. The bottom trace is the difference between the two detection geometries. [From Schweda, et ai.(1985).J...

Measurements of optieal amplification were carried out in a pump and probe experimental setup. The pump radiation was provided by an optical parametric oscillator (OPO) sintonized at 975nm with high energy pulses between 50 and 115 mJ/cm. The probe beam was obtained by a lOOOW lamp, giving a signal power density of 195 pW/cm at 650 nm. 

The differential transmission spectra DT(X)=[T(A,)-To(X)]/To(A,), where T(Z) and T0(X) are the transmission spectra of excited and unexcited films of QDs, were measured. Wide-band picosecond pulse of white light (picosecond continuum) obtained by four-wave mixing of the part of pump radiation focused in the cell with heavy water was used as a probe pulse. The central part of the QD sample s excited area was probed. The application of the optical delay line has allowed to measure the differential transmission spectra of QDs at different time delay with respect to the excitation by ultrashort pulses. 

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