Third harmonic intensity

Third-order susceptibilities of the PAV cast films were evaluated with the third-harmonic generation (THG) measurement [31,32]. The THG measurement was carried out at fundamental wavelength of 1064 nm and between 1500 nm and 2100 nm using difference-frequency generation combined with a Q-switched Nd YAG laser and a tunable dye laser. From the ratio of third-harmonic intensities I3m from the PAV films and a fused quartz plate ( 1 thick) as a standard, the value of x(3) was estimated according to the following equation derived by Kajzar et al. [33] ... 

The third harmonic intensity from a transparent slab is... 

To treat an absorbing medium with a complex it is necessary to resort to eq. 17 including appropriate transmission factors accounting for absorption of the fundamental and/or third harmonic fields (31). In this case the third harmonic intensity is (again suppressing transmission coefficients, t)... 

The solute concentration dependence of the THG intensity is sensitive to whether y for the solute is real or complex. This can be shown qualitatively as follows. The third harmonic intensity is 1(3 )... 

The sample was mounted on a goniometer and rotated about an axis perpendicular to the laser beam. The laser beam was linearly polarized in a direction parallel to the rotational axis. The generated third harmonic was passed through a fundamental wave cutting filter and was detected by a photomultiplier tube. Third harmonic intensities were measured as a function of the incident angle for PAV films and a silica glass standard whose was reported to be 2.8 x 10 14 esu(17). The details of the experiments were reported else where (IS). 

Figure 3 is a log-log plot f141 of the third harmonic intensity (in W/cm2) as a function of fundamental intensity (in... 

Fig. 3. Log-log plot of the third harmonic intensity (W/cm2) as a function of the fundamentai intensity (MW/cm2). The solid iine indicates a cubic dependence of the third harmonic intensity on the fundamental intensity. The inset shows that the spectral content of the third harmonic is resolution limited at 355 nm Ref. 14.

Figure 3. P olarization d ependences of the third-harmonic intensity for the PS layers of different porosities grown on p -Si substrate as well as the c-Si (a) s-polarized third harmonic and (6) p-polarized third harmonic.

Note that the waves can propagate along the helical axis in either direction, and, correspondingly, the wave vectors k can be positive or negative. The generated third-harmonic intensity is given by Eq. (7), which is of course independent of the coordinate system we choose. If the beam has a finite cross section, then the total third-harmonic power is ... 

FIG. 8. Phase-matching peaks for mode combination 12 observed with a sample 130 Mm thick. The peak at the lower temperature is generated by right circularly polarized fundamental waves and the one at the higher temperature by left circularly polarized fundamental waves. The solid line is the theoretical phase-matching curve and the dots are experimental data points. The uncertainty in the experimental third-harmonic intensity is about 20%. 

The dielectric constants are comparable to those of the cholesterol-derived materials and hence the pitches for phase matching are approximately the same. A pitch of 17 pm is realizable by adjusting the concentration of PBLG, and hence phase-matched third-harmonic generation 12 should be possible. We did not use temperature to tune the pitch because of the slow response of PBLG samples. Instead, we used many samples with different concentrations to yield different pitches. We did observe a peak in the third-harmonic intensity around /> = 17 pm with the laser beam polarized to feed efficiently into the plus mode, and no peak for the opposite laser polarization. There was a moderate amount of scattering in the data, presumably due to the use of many different samples. 

Fig. 13 Third harmonic intensity in function of incidence angle for...

Figure 25 Third harmonic intensity in function of rotation angle 0 from a bioriented 1300 A thick film (pDCH) grovm by epitaxy on KBr (open squares). Solid line is a least square fit of Eq. (45) to experimental data. Closed circles show TH intensity from an unoriented film at the same experimental conditions (Le Moigne et al.(1988)).

Figure 26 Third harmonic intensity as a function of rotation angle 0 from a mono-oriented pDCH thin film on KAP single crysteil substrate Solid lines shows calculated values whereas the points the measured ones (Le Moigne et al., (1990)).

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