Input beam

Figure Bl.5.15 SFG spectrum for the water/air interface at 40 °C using the ssp polarization combination (s-, s- and p-polarized sum-frequency signal, visible input and infrared input beams, respectively). The peaks correspond to OH stretching modes. (After [ ].)...

When the spectral characteristics of the source itself are of primary interest, dispersive or ftir spectrometers are readily adapted to emission spectroscopy. Commercial instmments usually have a port that can accept an input beam without disturbing the usual source optics. Infrared emission spectroscopy at ambient or only moderately elevated temperatures has the advantage that no sample preparation is necessary. It is particularly appHcable to opaque and highly scattering samples, anodized and painted surfaces, polymer films, and atmospheric species (135). The interferometric... 

This first step makes necessary a correction of the atmosphere aberrations by means of an adaptive optics or at the minimum a tip tilt device. If the turbulence induces high aberrations the coupling efficiency is decreased by a factor VN where N is the number of spatial modes of the input beam. Note that tilt correction is also mandatory in a space mission as long as instabilities of the mission platform may induce pointing errors. Figure 10 (left) illustrates the spatial filtering operation. This function allows a very good calibration of... 

Figure 10. Left Monomode optical fibre acts as a spatial filter. The coupling efficiency is 1/N where N is the input beam number of modes. Right Using a wavefront corrector the coupling efficiency is significant and quite stable (K band CFHT/ GHANA) (Perrin et al., 2000).

To determine /, g, and h the second-harmonic intensity is measured as a function of the angle of the quarter waveplate for different experimental geometries. The experimental setup is shown in Figure 9.12. A thin film is irradiated with an infrared Nd YAG laser (1064 nm, 50 Hz, 8 ns). The polarization of the input beam (p polarized) is continuously varied with a quarter waveplate... 

From the form of the polarization it is clear that in order to observe any nonlinear optical effect, the input beams must not be copropagating. Furthermore, nonlinear optical effects through the tensor y eee requires two different input frequencies (otherwise, the tensor components would vanish because of permutation symmetry in the last two indices, i.e., ytfl eee = Xijy ) For example, sum-frequency generation in isotropic solutions of chiral molecules through the tensor y1 1 1 has been experimentally observed, and the technique has been proposed as a new tool to study chiral molecules in solution.59,61 From an NLO applications point of view, however, this effect is probably not very useful because recent results suggest that the response is actually very low.62... 

Although in some cases monochromators are classified as dispersive or nondis-persive monochromators, in this book we are only dealing with dispersive monochromators, since they are most commonly used in optical spectroscopy. In dispersive monochromators, a spatial separation is obtained for the different spectral components of the input beam. As shown in Figure 3.1, the simplest monochromators consist of the following elements ... 

Sum-frequency generation (SEG) is also widely recognized as a powerful tool for intrinsically surface-specific spectroscopy (Shen 2000). As shown in Figure 10.16a, SFG spectroscopy, which requires two color input beams, is technically rather complicated, but one can expect additional features. For example, SFG is now expected to be a new probe for molecular chirality (Belkin et al. 2000 Champagne et al. 2000 Ji et al. 2006). From the viewpoint of biomolecular imaging, this feature is more attractive. Therefore, we hereinafter focus on sum-frequency (SF) chiral... 

SF (chiral) spectroscopy can be easily modihed for vibrational spectroscopy as shown in Figure 10.16b. This SF vibrational spectroscopy can beneht from plas-mon resonances of a metallic tip by tuning the input beam frequency and/or the output frequency tUspo- Therefore, spatial resolution of SF vibrational spectroscopy could reach to the order of several nanometers. Development of tip-enhanced SF chiral/vibrational spectroscopy is awaited for biomolecular imaging. 

For an OB system driven by a combination of the stochastic reference beam and the periodically modulated input beam (Im(t) = /m I A cosilt), the equation for the phase takes on the form... 

Fig. 6.9. A Spontaneous Raman spectrum of d62-DPPC lipids and its decomposition into Lorentzian line profiles. B Normalized multiplex CARS spectra (dots) of a planar-supported bilayer and monolayer formed by d62-DPPC on a glass-water interface for parallel-polarized input beams, together with the fit using the center frequency and line width parameters extracted from the decomposition analysis in (A) (solid line). The spectrum exposure time was 0.64 s. Error bars indicate the shot-noise standard deviation (Copyright American Chemical Society [70])...

So far, a modulator device has been described, not a filter. This can be used on a monochromatic (for example laser) input beam to vary the Doppler-shifted frequency or direction or intensity of an output beam as a function of input RF frequency and power. In order to understand how an AOTF device goes further than this, it is necessary to look at the effect of Bragg regime diffraction and the impact of the anisotropy of the AOTF crystal medium. 

What happens to the diffracted beam in an anisotropic medium (e.g. as in an AOTF device) As given above, the birefringence induced by the acoustic wave causes the diffracted beam polarisation to change. So if, for example, the input beam is P-polarised, then... 

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