Third harmonic generation intensities

In the following sections, we will briefly discuss second-order and third-nonlinea-rities and some of the optical phenomena originating from them, namely, second harmonic generation, third harmonic generation, intensity-dependent refraction, and two-photon absorption. A general description of these optical processes will be given and some of the experimental results on ZnO will be discussed. Before we delve further into the discussion, the physical units of the parameters used in this section are given in both SI and cgs systems in Table 3.11. 

If any of the frequencies (w, 2m or 3w in third harmonic generation or similar frequency combinations in three wave mixing experiments) pproach the region of intense absorption situated around 16000 cm appreciable changes in the values of are expected... 

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 two important consequences of the third-order optical nonlinearities represented by x are third-harmonic generation and intensity dependence of the refractive index. Third-harmonic generation (THG) describes the process in which an incident photon field of frequency (oj) generates, through nonlinear polarization in the medium, a coherent optical field at 3a>. Through x interaction, the refractive index of the nonlinear medium is given as n = nQ+n I where n describes intensity dependence of the refractive index ana I is the instantaneous intensity of the laser pulse. There is no symmetry restriction on the third-order processes which can occur in all media including air. 

The third-harmonic generation method has the advantage that it probes purely electronic nonlinearity. Therefore, orientational and thermal effects as well as other dynamic nonlinearities derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the time-response of optical nonlinearity. Another disadvantage of the method is that one has to consider resonances at oj, 2w and 3o> as opposed to degenerate four wave mixing discussed below which utilizes the intensity dependence of refractive index and where only resonances at a) and 2a) manifest. 

To date, results have only been acquired for the ethoxy-substituted poly(ester) with a pentamethylene spacer. A 2.5 pm film produced a third-harmonic signal at 632.8 nm 29 times the intensity of a quartz signal, corresponding to a of approximately 8 x 10 13 esu. Degenerate four-wave mixing experiments are underway on this and other materials, and preliminary results are comparable to the values from third harmonic generation. 

One of the more commonly measured components of x is third harmonic generation. For this interaction, a single laser beam of the necessary intensity is focused into the nonlinear material, and can be viewed as three simultaneously... 

Figure 2. Hystereses curves for third harmonic generation in PDN6S vs temperature at two different fundamental wavelengths. At 1.9 /mi there is a change of intensity of about a factor of two whereas at 1.06 /mi there is a change of intensity of about six due to resonance effects.

Since many atmospheric pollutants such as ozone have absorption bands in the ultraviolet, measurements have also been performed in this spectral region. Recently, it was shown that phase locking within the filaments results in enhanced third harmonic generation [46]. Then, the build-up of the ultraviolet supercontinuum was characterized over both the laboratory and the atmospheric scales. The UV-visible part of the continuum measured in the laboratory with a single filament is presented in Fig. 15.7. At the beginning of filamentation, a third harmonic band with 20 nm bandwidth is generated around 270 nm. Two meters further, the intensity of the third harmonic is reduced and a plateau appears in the UV-Visible region be-... 

F. Theberge, Q. Luo, W. Liu, S.A. Hosseini, M. Sharifi, S.L. Chin, Long-range third-harmonic generation in air using ultrashort intense laser pulses, Applied Physics Letters 87, 081108 (2005)... 

Optical processing systems are proposed based on relative intensity changes of second harmonic generation in sorbate complexes of p-nitroaniline and 2-methyl-p-nitroaniline in molecular sieve hosts [98]. Optical data storage would be one of a variety of potential electro-optical applications. In addition, molecular optical effects by third harmonic generation are suggested for optical data storage based on the possible formation of bistabilities for the local fields [65,99]. 

I i Intensity of incident light, 1 sample path length, 1 c coherent length, n, refractive index If phase matching condition A k =0 does not hold, I decreases, i.e., 1 m depends on the incident angle 0. m = 2 second harmonic generation (SHG) m=3 third harmonic generation (THG)... 

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