**4.2.2 Spontaneous parametric down-conversion**

Emission spectra of photonic crystals infiltrated with any nonlinear optical substances mentioned above are similar after subtracting the longer wavelength tale of excitation line and the bands corresponding to Raman scattering processes (Fig. 16). The spectra contain a wide asymmetric band within a 410 – 600 nm range. This band spectral position is different for opals with different infiltrators but it is correlated with the stop-band position. The emission intensity decreases within a stop-band region but it does not vanish completely because of the existence of point defects and structural disordering in photonic crystals.

Photos of secondary emission made far from sample surface reveal the angular distribution of spectral intensity (Moiseyenko et al., 2009b). Emission spectra measured at different scattering angles and treated by subtracting the longer wavelength tale of excitation line and

Quantum Optics Phenomena in Synthetic Opal Photonic Crystals 103

Fig. 17. Emission spectra of opal infiltrated with Ba(NO3)2 at a 407 nm laser excitation in the forward scattering geometry. The spectrum (1) was measured within a full angle range. The spectra (2) and (3) were registered at the angles of 50 and 30 to the pump propagation direction, respectively. The spectrum (4) was measured within a full angle range after

factor and by the existence of polydomain structure which forms additional superlattice. In our samples typical domain size was about of 70 mkm. Then the phase quasi-synchronism condition becomes true for the greater number of directions and wavelengths. It results in broadening the parametric down-conversion spectrum like that occurred in chirped structures with quadratic nonlinearity (Nasr et al., 2008). Besides spectrum broadening effects, nonlinear diffraction by 3D grating of quadratic optical susceptibility and bi-photon field interference may result in more complicated changes in spatial and frequency distribution of far field compared with that observed in polydomain crystals (Kitaeva &

In accordance with Fermi's golden rule, modification of luminescence spectra of organic molecules in synthetic opals is determined by the density of optical states in the vicinity of photonic stop-band. Inside stop-band region the partial inhibition of spontaneous emission is observed for all substances. If the wavelength, corresponding to the intensity maximum in spectrum of "free-state" substance, is shorter than wavelength, corresponding to the stopband center, the amplification of dye luminescence at the high-energy edge of stop-band occurs. In the contrary case a weak luminescence is observed at the low-energy edge of stop-

additional sample infiltration with glycerine.

Penin, 2004).

**5. Conclusion** 

band without any amplifying.

Fig. 16. Emission spectra of opals infiltrated with Ba(NO3)2 (1), LiIO3 (2), and KH2PO4 (3) at a 400 nm diode excitation in the forward scattering geometry. Spectral stop-band positions are 525 nm – 590 nm (1), 560 nm – 615 nm (2, 3).

bands in Raman scattering region are presented in Fig. 17 (Moiseyenko et al., 2012). The spectral intensity distribution and the emission maximum position are quite different at various scattering angles. When dielectric contrast becomes negligible (by infiltrating opal with pure glycerine) a shifted symmetric emission band is observed (curve 4 in Fig. 17). In this case the sample may be considered as practically transparent matrix with periodic distributed nonlinear substance which is responsible for generating secondary emission. The symmetric form of spectral distribution is typical for spontaneous parametric downconversion in uniform media (Kitaeva & Penin, 2005).

Thus, all elicited regularities together with chosen conditions of the samples heat treatment (Samarov et al., 2006) are the reasons to exclude the fluorescence of nano-composite components and the OH-groups fluorescence observed in synthetic opals within a 520 nm – 650 nm range (Gruzintsev et al., 2008). Taking into account angular dependences of spectral intensity, the emission observed may be interpreted as spontaneous parametric downconversion in spatially nonuniform nonlinear optical media. Additional contribution to the emission within a 407 – 437 nm region may be given by enhanced Raman scattering discussed above.

As shown earlier, spontaneous parametric down-conversion intensity per a unit angle and spectral interval is determined by the value of quasi-synchronism *Δm* for the *m*-th order nonlinear diffraction. In case of 3D photonic crystals based on synthetic opals this magnitude is defined by the structure disordering degree, the nonlinear substance filling 102 Quantum Optics and Laser Experiments

Fig. 16. Emission spectra of opals infiltrated with Ba(NO3)2 (1), LiIO3 (2), and KH2PO4 (3) at a 400 nm diode excitation in the forward scattering geometry. Spectral stop-band positions are

bands in Raman scattering region are presented in Fig. 17 (Moiseyenko et al., 2012). The spectral intensity distribution and the emission maximum position are quite different at various scattering angles. When dielectric contrast becomes negligible (by infiltrating opal with pure glycerine) a shifted symmetric emission band is observed (curve 4 in Fig. 17). In this case the sample may be considered as practically transparent matrix with periodic distributed nonlinear substance which is responsible for generating secondary emission. The symmetric form of spectral distribution is typical for spontaneous parametric down-

Thus, all elicited regularities together with chosen conditions of the samples heat treatment (Samarov et al., 2006) are the reasons to exclude the fluorescence of nano-composite components and the OH-groups fluorescence observed in synthetic opals within a 520 nm – 650 nm range (Gruzintsev et al., 2008). Taking into account angular dependences of spectral intensity, the emission observed may be interpreted as spontaneous parametric downconversion in spatially nonuniform nonlinear optical media. Additional contribution to the emission within a 407 – 437 nm region may be given by enhanced Raman scattering

As shown earlier, spontaneous parametric down-conversion intensity per a unit angle and spectral interval is determined by the value of quasi-synchronism *Δm* for the *m*-th order nonlinear diffraction. In case of 3D photonic crystals based on synthetic opals this magnitude is defined by the structure disordering degree, the nonlinear substance filling

525 nm – 590 nm (1), 560 nm – 615 nm (2, 3).

conversion in uniform media (Kitaeva & Penin, 2005).

discussed above.

Fig. 17. Emission spectra of opal infiltrated with Ba(NO3)2 at a 407 nm laser excitation in the forward scattering geometry. The spectrum (1) was measured within a full angle range. The spectra (2) and (3) were registered at the angles of 50 and 30 to the pump propagation direction, respectively. The spectrum (4) was measured within a full angle range after additional sample infiltration with glycerine.

factor and by the existence of polydomain structure which forms additional superlattice. In our samples typical domain size was about of 70 mkm. Then the phase quasi-synchronism condition becomes true for the greater number of directions and wavelengths. It results in broadening the parametric down-conversion spectrum like that occurred in chirped structures with quadratic nonlinearity (Nasr et al., 2008). Besides spectrum broadening effects, nonlinear diffraction by 3D grating of quadratic optical susceptibility and bi-photon field interference may result in more complicated changes in spatial and frequency distribution of far field compared with that observed in polydomain crystals (Kitaeva & Penin, 2004).
