Diploma thesis

Figure 3.6: Different Λ Figure 3.7: Higher order phasematching
Eq. 3.30 also reveals that the square wave grating orders provide several phasematching
curves, m = ±1 as the two main periods and several higher order grating
periods m = ±3, ±5, ±7 . . . some of which are visualized in figure 3.7. However the
higher order modes are suppressed by 1
m
in amplitude. Additionally, these higher
order phasematching modes soon drift into regions, where the produced photons
can’t be detected any more.
Waveguides
Waveguiding structures as depicted in Figure 3.8 exhibit two main advantages over
bulk crystal. Firstly, the propagation of the signal, idler and pump beams is now
Figure 3.8: Embedded 2D waveguiding structure
strictly collinear. This restricts the momentum mismatch to one dimension, as already
assumed in Equation 3.23. Because of the collinear propagation of all electric
fields, this two-photon source is now much more convenient to handle experimentally.
In contrast to bulk crystal, where angular spread, which is a result of angular
phasematching, has to be considered. Secondly, the pump beam undergoes a high
modal confinement in waveguides. These constrictions lead to a high photon flux
of signal and idler in a narrow solid angle. The conversion efficiency from pump
photons to downconverted photons is increased by several orders of magnitude. The
waveguides restrict the fields to certain well defined spatial modes (see Figures 3.9
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