Pulsed source, spectral

Terahertz imaging approaches have typically used either short-pulsed laser or continuous wave (CW) THz generation and detection. The short-pulsed method usually involves the generation and detection of sub-picosecond THz pulses using either photoconductive antenna structures or optical rectification in a non-linear crystal. Pulsed sources seem to be more favorable (in particular for close proximity applications) because they can be used for acquiring depth information. Spectral information is retrieved by a Fourier transform of the time-domain data to the frequency domain. 

Finally, it should be noted that dephasing dynamics may also be investigated with temporally incoherent pulses [30-32]. To the extent that the two- and three-pulse Tj measurements described in the preceding depend upon the electric field correlation and not upon the intensity autocorrelations, femtosecond temporal resolution may be obtained with any broadband source. Spectral cross-relaxation and a variety of effects, however, can reduce the effectiveness of this technique. 

The spectral resolution An of most time-resolved techniques is, in principle, confined by the Fourier limit Av = a/AT, where AT is the duration of the short light pulse and the factor a 1 depends on the profile / (i) of the pulse. The spectral bandwidth Av of such Fourier-limited pulses is still much narrower than that of light pulses from incoherent light sources, such as flashlamps or sparks. Some time-resolved coherent methods based on regular trains of short pulses even circumvent the Fourier limit Av of a single pulse and simultaneously reach extremely high spectral and time resolutions (Sect. 7.4). 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

In electron-spin-echo-detected EPR spectroscopy, spectral infomiation may, in principle, be obtained from a Fourier transfomiation of the second half of the echo shape, since it represents the FID of the refocused magnetizations, however, now recorded with much reduced deadtime problems. For the inhomogeneously broadened EPR lines considered here, however, the FID and therefore also the spin echo, show little structure. For this reason, the amplitude of tire echo is used as the main source of infomiation in ESE experiments. Recording the intensity of the two-pulse or tliree-pulse echo amplitude as a function of the external magnetic field defines electron-spm-echo- (ESE-)... 

Figure 5 shows the dependence of the total emission intensity on the excitation intensity and its spectral width obtained from DCM-encapsulated dendrimers. A nitrogen laser (wavelength of 337 run, pulse duration of 4 ns, and repetition rate of 10 Hz) was used as the excitation source. A cylindrical lens focused the excitation beam onto a stripe 200 pm wide on a quartz cuvette... 

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