**4.2 Light scattering by synthetic opals filled with nonlinear optical dielectrics**

Emission spectrum of synthetic opals with dielectrics observed within a wide spectral region under ultraviolet excitation may be divided into two parts. The first part is the spectrum located in the vicinity of the excitation line. The second one is in a region of 440 nm – 650 nm including a stop-band. By now the secondary emission of initial and dielectrics infiltrated opals has been given no universal explanation which should be satisfied by all experimental facts (Gorelik, 2007; Gruzintsev et al., 2008; Moiseyenko et al., 2009a, 2009b, 2012).

#### **4.2.1 Enhanced spontaneous Raman scattering**

Consider a total emission spectrum of initial synthetic opals; some of them were in air for a long time. The others were excited just after high temperature annealing. In all spectra a quite intensive band in the vicinity of the excitation line was observed. Its spectral position was independent of the stop-band position and previous technology conditions (Fig. 11). In the spectrum of a long time air-conserving sample a wide band with maximum at 570 nm was also observed. The fact that the band has vanished after annealing, i.e. after removing water in opal pores, reveals the impurity OH-groups luminescence origin of this band.

Fig. 11. Emission spectra of two different initial opals after thermal annealing at 800 0C (a). Emission spectra of opal conserved for a long time in the air at 70 % moisture (b) before (1) and just after (2) annealing at 700 0C. The rectangles point to the stop-band positions.

98 Quantum Optics and Laser Experiments

Spectral intensity distribution in the spectrum of 2,5-bis(2-benzoxazolyl)hydroquinone in synthetic opal is like to that in amorphous state (curves 4, 5 in Fig. 10). It allows assuming amorphous state of the substance in opal pores. The "blue" shift observed in this case may be explained in the following way. As a "proton-transfer" band is near by the stop-band region (600 nm – 640 nm in opal under study), the probability of these transitions decreases. It may result in increasing probabilities of impurity irradiative transitions and transitions without proton transfer. The latter transitions have not been observed in a "free" condensed state (Chayka et al., 2005). Another reason to make these processes observable is an accumulation of the shorter wavelength radiation because of Bragg reflection from the {111}

**4.2 Light scattering by synthetic opals filled with nonlinear optical dielectrics** 

facts (Gorelik, 2007; Gruzintsev et al., 2008; Moiseyenko et al., 2009a, 2009b, 2012).

Emission spectrum of synthetic opals with dielectrics observed within a wide spectral region under ultraviolet excitation may be divided into two parts. The first part is the spectrum located in the vicinity of the excitation line. The second one is in a region of 440 nm – 650 nm including a stop-band. By now the secondary emission of initial and dielectrics infiltrated opals has been given no universal explanation which should be satisfied by all experimental

Consider a total emission spectrum of initial synthetic opals; some of them were in air for a long time. The others were excited just after high temperature annealing. In all spectra a quite intensive band in the vicinity of the excitation line was observed. Its spectral position was independent of the stop-band position and previous technology conditions (Fig. 11). In the spectrum of a long time air-conserving sample a wide band with maximum at 570 nm was also observed. The fact that the band has vanished after annealing, i.e. after removing water in opal pores, reveals the impurity OH-groups luminescence origin of this band.

Fig. 11. Emission spectra of two different initial opals after thermal annealing at 800 0C (a). Emission spectra of opal conserved for a long time in the air at 70 % moisture (b) before (1) and just after (2) annealing at 700 0C. The rectangles point to the stop-band positions.

planes at higher incident angles (Bechger et al., 2005).

**4.2.1 Enhanced spontaneous Raman scattering** 

In order to understand the nature of the band near by the excitation line an influence of infiltrated substance on the emission spectrum has been studied (Moiseyenko et al., 2009a, 2009b). These spectra measured under spectral correct conditions with a 2 cm-1 resolution are presented in Raman shift scale after subtracting excitation line profile (Fig. 12, a). As seen from Fig. 12, spectral intensity distribution is dependent of kind of substance into opal pores. This fact together with mentioned above regularities allows us to suppose that the band observed within a typical vibrational spectrum range is caused by Raman scattering in substances forming photonic crystal. Such process becomes possible to be detected owing to an essential increase of field due to a slow diffuse motion of exciting photons into opal volume, and also, as a result of surface enhanced conditions inside opal pores.

However, obtained spectra are too wide compared with the usual Raman spectra. It may be explained, if remember, that band spectral profile is determined by spectral profile of excitation line and Raman spectrum of substance. In our initial experiments we have used a source with a significant width of the exciting line (*Δλ1/2* ≈ 30 nm). Another reason for spectrum broadening is a possible amorphous state of substances which form the sample structure. In case of amorphous state, a density of vibrational states *g*(*Ω*) can be quantitatively described by calculating reduced Raman spectrum *JR*(*Ω*) for the Stokes component (Cardona, 1975) (Fig. 12, b).

Fig. 12. Emission spectra (a) in the vicinity of the 400 nm exciting line and the corresponding reduced Raman spectra (b) for initial synthetic opal (1) and opals infiltrated with CuCl2 (2), Ba(NO3)2 (3), and LiIO3 (4)

To diminish a role of exciting radiation parameters in forming measured spectrum, we have used a 532 nm laser radiation with *Δλ1/2* ≈ 1 nm to excite emission in opal filled with KH2PO4 (Fig. 13). The significant band width in this case may testify amorphous state of substance in opal pores. The presence of the anti-Stokes component should be pointed out.

Quantum Optics Phenomena in Synthetic Opal Photonic Crystals 101

In order to experimentally prove the enhancement effects in synthetic opal photonic crystals Raman spectra in opal-Li2B4O7 and single Li2B4O7 crystal were measured in the lowfrequency region under the same conditions (Fig. 15). As seen from Fig. 15, integral scattering intensity in the opal-Li2B4O7 spectrum is about of a three times higher than the one in the single Li2B4O7 crystal spectrum. Taking into account the lesser quantity of lithium tetraborate in opal matrix in the same scattering volume (no more than 26 % from total volume, as lithium tetraborate is situated only in opal pores) we can estimate the Raman enhancement coefficient as high as 10. Two enhancement mechanisms can be proposed. The first one is a photon slowing in accordance with a dispersion law in photonic crystals and the second is a multiple reflection from disordered planes resulting to diffuse photon

Fig. 15. Low-frequency region in non-polarised Raman spectra of opal infiltrated with Li2B4O7 (1) and single Li2B4O7 crystal (2) under the same conditions at the 532 nm diode pumped solid state laser excitation. The right angle geometry was used. The longer

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

wavelength tale of excitation line was subtracted.

**4.2.2 Spontaneous parametric down-conversion** 

motion (Gorelik, 2007).

Fig. 13. Emission spectrum in the vicinity of the 532 nm laser exciting line for synthetic opal infiltrated with KH2PO4

The emission spectral distribution typical for Raman spectrum was observed in the opal-Li2B4O7 spectrum in the right angle geometry (Fig. 14). The A1(TO) Raman Li2B4O7 spectrum measured earlier (Moiseyenko et al., 2006) at the excitation of a 532 nm Q-switched Nd:YAG laser with mean power of 250 mW is also presented in Fig. 14.

Fig. 14. The emission spectrum of opal infiltrated with Li2B4O7 (1) and A1(TO) Raman Li2B4O7 spectrum (2) at the 532 nm laser excitation

Both spectra have a similar structure in the 100 cm-1 – 550 cm-1 spectral range, but the bands in the opal-Li2B4O7 spectrum are shifted towards the excitation line and have a greater halfwidth. However, the values of bands halfwidths (no more than 30 cm-1) give no reason to conclude amorphous state of the substance in opal pores. The broadening of bands is rather caused by structural disordering and the existence of polydomain structure. The bands shifts are most probably due to the small sizes of the unit Li2B4O7 scattering volume defined by the pores sizes (no more than 100 nm in our samples). The coincidence of highfrequency Raman range and the stop-band spectral region results in a crucial decrease of emission intensity at Raman shifts higher than 600 cm-1.

100 Quantum Optics and Laser Experiments

Fig. 13. Emission spectrum in the vicinity of the 532 nm laser exciting line for synthetic opal

The emission spectral distribution typical for Raman spectrum was observed in the opal-Li2B4O7 spectrum in the right angle geometry (Fig. 14). The A1(TO) Raman Li2B4O7 spectrum measured earlier (Moiseyenko et al., 2006) at the excitation of a 532 nm Q-switched Nd:YAG

Fig. 14. The emission spectrum of opal infiltrated with Li2B4O7 (1) and A1(TO) Raman

Both spectra have a similar structure in the 100 cm-1 – 550 cm-1 spectral range, but the bands in the opal-Li2B4O7 spectrum are shifted towards the excitation line and have a greater halfwidth. However, the values of bands halfwidths (no more than 30 cm-1) give no reason to conclude amorphous state of the substance in opal pores. The broadening of bands is rather caused by structural disordering and the existence of polydomain structure. The bands shifts are most probably due to the small sizes of the unit Li2B4O7 scattering volume defined by the pores sizes (no more than 100 nm in our samples). The coincidence of highfrequency Raman range and the stop-band spectral region results in a crucial decrease of

laser with mean power of 250 mW is also presented in Fig. 14.

Li2B4O7 spectrum (2) at the 532 nm laser excitation

emission intensity at Raman shifts higher than 600 cm-1.

infiltrated with KH2PO4

In order to experimentally prove the enhancement effects in synthetic opal photonic crystals Raman spectra in opal-Li2B4O7 and single Li2B4O7 crystal were measured in the lowfrequency region under the same conditions (Fig. 15). As seen from Fig. 15, integral scattering intensity in the opal-Li2B4O7 spectrum is about of a three times higher than the one in the single Li2B4O7 crystal spectrum. Taking into account the lesser quantity of lithium tetraborate in opal matrix in the same scattering volume (no more than 26 % from total volume, as lithium tetraborate is situated only in opal pores) we can estimate the Raman enhancement coefficient as high as 10. Two enhancement mechanisms can be proposed. The first one is a photon slowing in accordance with a dispersion law in photonic crystals and the second is a multiple reflection from disordered planes resulting to diffuse photon motion (Gorelik, 2007).

Fig. 15. Low-frequency region in non-polarised Raman spectra of opal infiltrated with Li2B4O7 (1) and single Li2B4O7 crystal (2) under the same conditions at the 532 nm diode pumped solid state laser excitation. The right angle geometry was used. The longer wavelength tale of excitation line was subtracted.
