**3. Characterisation of samples and experimental setup**

Nanodisperse globules of silica dioxide were synthesized using a modified Stöber method (Stöber et al., 1968) through hydrolysis of tetraethoxysilane Si(OC2H5)4 at high values of water concentration. Bulk synthetic opals were obtained by natural sedimentation of α-SiO2 globules with the following annealing of samples at 800 0C during several hours. Annealing was performed in order to remove organic residua, extra- and intra-globular ethoxygroups, and chemically bound water (Samarov et al., 2006). Dimensions of obtained samples were about 1.0x1.0x0.2 cm3.

Characterization of initial opals was performed by analyzing the surface structure with the use of X-Ray Microanalyzer JEO JXA 8200 and by measuring transmission and reflection spectra within a visible spectral range (Fig. 4, 5). Opal samples in these studies were composed of hexagonal close-packed layers of monodisperse α-SiO2 globules which are arranged in the face centred cubic lattice. The value of globules diameter *D* in various samples was from 250 nm to 270 nm, and the distance *d* between the (111) planes was from 204 nm to 220 nm.

Fig. 4. Image of the opal surface in [111] direction. Diameter of globules *D* = 270 nm

Parameters of photonic stop-band in [111] direction (spectral position of stop-band centre *λ<sup>c</sup>* and its spectral width *Δλg*) were determined from transmission and reflection spectra as the parameters of non-transmission or reflection band (spectral position of maximum and spectral band width) formed in accordance with Bragg diffraction mechanism. The spectral position of reflection maximum (or transmission minimum) assigned as stop-band centre *λ<sup>c</sup>* is dependent of an incident angle *θ*, effective refractive index *neff* and distance *d* between the (111) planes as follows (Podolskyy et al., 2006):

$$\mathcal{A}\_{\varepsilon}(\theta) = 2d \sqrt{\mathbf{r}\_{\text{eff}}^2 - \sin^2 \theta} \ . \tag{10}$$

The effective refractive index *neff* is determined by the refractive index *ns* of α-SiO2 globules (*ns* = 1.47), the refractive index *np* of substance in opal pores (for initial opal it is air, *np* = 1) and the volume fraction *f* occupied by α-SiO2 globules (in our case, *f* 0.74) as follows:

92 Quantum Optics and Laser Experiments

Nanodisperse globules of silica dioxide were synthesized using a modified Stöber method (Stöber et al., 1968) through hydrolysis of tetraethoxysilane Si(OC2H5)4 at high values of water concentration. Bulk synthetic opals were obtained by natural sedimentation of α-SiO2 globules with the following annealing of samples at 800 0C during several hours. Annealing was performed in order to remove organic residua, extra- and intra-globular ethoxygroups, and chemically bound water (Samarov et al., 2006). Dimensions of obtained samples were

Characterization of initial opals was performed by analyzing the surface structure with the use of X-Ray Microanalyzer JEO JXA 8200 and by measuring transmission and reflection spectra within a visible spectral range (Fig. 4, 5). Opal samples in these studies were composed of hexagonal close-packed layers of monodisperse α-SiO2 globules which are arranged in the face centred cubic lattice. The value of globules diameter *D* in various samples was from 250 nm to 270 nm, and the distance *d* between the (111) planes was from

Fig. 4. Image of the opal surface in [111] direction. Diameter of globules *D* = 270 nm

(111) planes as follows (Podolskyy et al., 2006):

Parameters of photonic stop-band in [111] direction (spectral position of stop-band centre *λ<sup>c</sup>* and its spectral width *Δλg*) were determined from transmission and reflection spectra as the parameters of non-transmission or reflection band (spectral position of maximum and spectral band width) formed in accordance with Bragg diffraction mechanism. The spectral position of reflection maximum (or transmission minimum) assigned as stop-band centre *λ<sup>c</sup>* is dependent of an incident angle *θ*, effective refractive index *neff* and distance *d* between the

*<sup>c</sup>* ( ) 2 sin *eff*

The effective refractive index *neff* is determined by the refractive index *ns* of α-SiO2 globules (*ns* = 1.47), the refractive index *np* of substance in opal pores (for initial opal it is air, *np* = 1) and the volume fraction *f* occupied by α-SiO2 globules (in our case, *f* 0.74) as follows:

  2 2

 

*d n* . (10)

**3. Characterisation of samples and experimental setup** 

about 1.0x1.0x0.2 cm3.

204 nm to 220 nm.

22 2 (1 ) *neff <sup>s</sup> <sup>p</sup> f n f n* . (11)

Fig. 5. Typical transmission and reflection spectra of initial opals (*D* = 255 nm). Transmission spectrum was measured at normal incidence to (111) plane (*θ* = 00), reflection spectrum was measured at *θ* = 70

Photonic crystals based on synthetic opals were obtained by further infiltration of initial opals with organic luminophores (rhodamine 6G, 2,5-bis(2-benzoxazolyl)hydroquinone, pironin G, astrofloksin) or nonlinear optical substances (Ba(NO3)2, LiIO3, KH2PO4, Li2B4O7). In most cases the infiltration was performed by a multiple soaking of samples in corresponding supersaturated solutions at room temperature. For example, synthetic opals were filled with rhodamine 6G by soaking samples in a dilute ethanol solution with laser dye concentrations of 10-4 M or 510-3 M. After soaking the obtained samples were in the air until ethanol was evaporated. In case of infiltration with Ba(NO3)2, LiIO3, KH2PO4 an additional annealing of samples was performed at temperatures lower than melting ones (595 oC for Ba(N03)2 and 120 oC for LiIO3) to remove water. In case of Li2B4O7 the initial opal was in Li2B4O7 melt at 860 0C.

Reflection and transmission spectra of opals after infiltration were measured to prove the existence of corresponding substance in pores. Two types of changes in the spectra were registered. First, in opals with organic luminophores, an additional non-transmission band caused by absorption of embedded molecules was observed (Fig. 6). Second, the band caused by Bragg diffraction was shifted if a quantity of embedded substance was enough to change essentially the value of *neff*, according to expression (11) (Podolskyy et al., 2006).

In some experiments, in order to diminish (or exclude) the photonic stop-band effects and to study phenomena in a regular matrix of nano-emitters the opal samples were additionally soaked in water-glycerine solutions or pure glycerine. Opal infiltration with any waterglycerine solution yields in decreasing dielectric contrast in the synthetic opal photonic crystals as the refractive index of a water-glycerin solution *np* (variable from 1.39227 till 1.47399 in our experiments) is close to that of SiO2 globules *nS*. It causes the shift of the stopband center *λc* to the longer wavelengths (10) and the narrowing of stop-band region Δ*λ<sup>g</sup>* with increasing glycerin concentration.

Quantum Optics Phenomena in Synthetic Opal Photonic Crystals 95

absent. Nevertheless, the use of opals as containers of emitting organic molecules allows changing the irradiative transitions probabilities remarkably (Bechger et al., 2005). It may be actual for substances with intra-molecular proton transfer, such as 2,5-bis(2 benzoxazolyl)hydroquinone, to control the probabilities of transitions with and without

Luminescence spectra of laser dyes (rhodamine 6G and pironin G) are shown in Fig. 7 (Moiseyenko et al., 2008, 2010). As seen from Fig. 7, for both molecules embedded into opal matrix a partial inhibition of luminescence intensity takes place within a region corresponding to the stop-band. For the rhodamine 6G spectrum, enhancement of the shortwavelength tail of the emission band is observed, while the long-wavelength tail of the spectrum is not altered essentially (Fig. 7, a). At the same time, the opal-pironin G spectrum is concentrated inside a long-wavelength region though it is rather not amplified (Fig. 7, b).

Fig. 7. Luminescence spectra of rhodamine 6G (a) and pironin G (b) put into optical cell with an ethanol (1) and infiltrated into opals (2) at a 517 nm diode excitation. Rectangles point to

Following Bechger et al., 2005, observed transformations may be explained in such a way. When the light with wavelength *λem* shorter than a stop-band centre *λc* is emitted in the [111] direction it encounters Bragg diffraction at higher angles. Because of diffuse propagation more light is detected in the [111] direction at these higher angles. For the light of any longer wavelengths (*λem* > *λc*) all the directions in opal volume are equivalent and that light escapes the sample without being enhanced. As mentioned above, spectral distribution of spontaneous emission is defined by density of photon states *g*(*ω*). In both cases the spectral intensity maximum is near by the stop-band edge where the density of states *g*(*ω*) has maximum (Fig. 7). An absence of total inhibition inside stop-band region can be connected with a structure disorder that results in appearance of local states in the stop-band (Kaliteevskii et al., 2005). From these points of view luminescence spectra of rhodamine 6G and pironin G in opals with additional water-glycerine solutions (Fig. 8) may be interpreted

proton transfer.

stop-band positions.

**4.1.1 Laser dyes molecules** 

For studying luminescence and light scattering phenomena in synthetic opals photonic crystals the incoherent and coherent light sources were used. The incoherent light sources were two light emitting diodes Edixeon EDST-3LAx (*λ* = 400 nm and 517 nm, *Δλ1/2* 30 nm, and the average power *P* = 30 mW). The coherent light sources were the pulsed nitrogen laser (*λ* = 337 nm, *Δλ1/2* = 0.1 nm) with the pulse repetition frequency of 100 Hz and *P* = 3 mW, the semiconductor laser (*λ* = 407 nm, *Δλ1/2* = 1 nm, *P* = 60 mW), and the diode-pumped solid state laser (*λ* = 532 nm, *Δλ1/2* = 1 nm, *P* = 120 mW). As a rule, the forward and back scattering geometry along the [111] direction were used. Length of samples along the excitation direction was from 2 mm to 3 mm. Some experiments on light scattering were performed in the right angle geometry. The secondary emission from the sample surface was collected along the [111] direction by using lens with an aperture of about 0.17π sr. The angular dependences of emission spectra were obtained within an angles region from 10 to 50 with the use of circle diaphragms. Spectral analysis was performed by using modernized spectrometer DFS-12. Signal registration was carried out in a regime of photon counting with accumulation.

Fig. 6. Transmission spectra of initial opal (1) and the same opal infiltrated with pironin G (2). Spectra were measured at normal incidence to (111) plane (*θ* = 00)

#### **4. Results and discussion**

In emission spectra of synthetic opal photonic crystals under optical excitation three typical regions are clearly observed (Gaponenko et al., 1999; Gorelik, 2007; Gruzintsev et al., 2008). One of them takes place in the opal-luminophores spectra, and is beyond doubt caused by the irradiative transitions between luminophore molecule levels. The other regions are inherent in emission spectra of opals filled with nonlinear optical substances. The first one is in a spectral range typical for Raman scattering region. The position of the second one is more distanced from an exciting line and is rather correlated with a stop-band position.

#### **4.1 Luminescence of organic molecules in synthetic opals**

As mentioned above synthetic opals are characterised by a presence of band gap in one space direction. This is why a complete inhibition of spontaneous emission should be absent. Nevertheless, the use of opals as containers of emitting organic molecules allows changing the irradiative transitions probabilities remarkably (Bechger et al., 2005). It may be actual for substances with intra-molecular proton transfer, such as 2,5-bis(2 benzoxazolyl)hydroquinone, to control the probabilities of transitions with and without proton transfer.

#### **4.1.1 Laser dyes molecules**

94 Quantum Optics and Laser Experiments

For studying luminescence and light scattering phenomena in synthetic opals photonic crystals the incoherent and coherent light sources were used. The incoherent light sources were two light emitting diodes Edixeon EDST-3LAx (*λ* = 400 nm and 517 nm, *Δλ1/2* 30 nm, and the average power *P* = 30 mW). The coherent light sources were the pulsed nitrogen laser (*λ* = 337 nm, *Δλ1/2* = 0.1 nm) with the pulse repetition frequency of 100 Hz and *P* = 3 mW, the semiconductor laser (*λ* = 407 nm, *Δλ1/2* = 1 nm, *P* = 60 mW), and the diode-pumped solid state laser (*λ* = 532 nm, *Δλ1/2* = 1 nm, *P* = 120 mW). As a rule, the forward and back scattering geometry along the [111] direction were used. Length of samples along the excitation direction was from 2 mm to 3 mm. Some experiments on light scattering were performed in the right angle geometry. The secondary emission from the sample surface was collected along the [111] direction by using lens with an aperture of about 0.17π sr. The angular dependences of emission spectra were obtained within an angles region from 10 to 50 with the use of circle diaphragms. Spectral analysis was performed by using modernized spectrometer DFS-12. Signal registration was carried out in a regime of photon counting

Fig. 6. Transmission spectra of initial opal (1) and the same opal infiltrated with pironin G

In emission spectra of synthetic opal photonic crystals under optical excitation three typical regions are clearly observed (Gaponenko et al., 1999; Gorelik, 2007; Gruzintsev et al., 2008). One of them takes place in the opal-luminophores spectra, and is beyond doubt caused by the irradiative transitions between luminophore molecule levels. The other regions are inherent in emission spectra of opals filled with nonlinear optical substances. The first one is in a spectral range typical for Raman scattering region. The position of the second one is more distanced from an exciting line and is rather correlated with a stop-band position.

As mentioned above synthetic opals are characterised by a presence of band gap in one space direction. This is why a complete inhibition of spontaneous emission should be

(2). Spectra were measured at normal incidence to (111) plane (*θ* = 00)

**4.1 Luminescence of organic molecules in synthetic opals** 

with accumulation.

**4. Results and discussion** 

Luminescence spectra of laser dyes (rhodamine 6G and pironin G) are shown in Fig. 7 (Moiseyenko et al., 2008, 2010). As seen from Fig. 7, for both molecules embedded into opal matrix a partial inhibition of luminescence intensity takes place within a region corresponding to the stop-band. For the rhodamine 6G spectrum, enhancement of the shortwavelength tail of the emission band is observed, while the long-wavelength tail of the spectrum is not altered essentially (Fig. 7, a). At the same time, the opal-pironin G spectrum is concentrated inside a long-wavelength region though it is rather not amplified (Fig. 7, b).

Fig. 7. Luminescence spectra of rhodamine 6G (a) and pironin G (b) put into optical cell with an ethanol (1) and infiltrated into opals (2) at a 517 nm diode excitation. Rectangles point to stop-band positions.

Following Bechger et al., 2005, observed transformations may be explained in such a way. When the light with wavelength *λem* shorter than a stop-band centre *λc* is emitted in the [111] direction it encounters Bragg diffraction at higher angles. Because of diffuse propagation more light is detected in the [111] direction at these higher angles. For the light of any longer wavelengths (*λem* > *λc*) all the directions in opal volume are equivalent and that light escapes the sample without being enhanced. As mentioned above, spectral distribution of spontaneous emission is defined by density of photon states *g*(*ω*). In both cases the spectral intensity maximum is near by the stop-band edge where the density of states *g*(*ω*) has maximum (Fig. 7). An absence of total inhibition inside stop-band region can be connected with a structure disorder that results in appearance of local states in the stop-band (Kaliteevskii et al., 2005). From these points of view luminescence spectra of rhodamine 6G and pironin G in opals with additional water-glycerine solutions (Fig. 8) may be interpreted

Quantum Optics Phenomena in Synthetic Opal Photonic Crystals 97

the short-wavelength tail of the luminescence band is being enhanced. At glycerine concentrations close to 85 volume per cents (marked by the arrow in Fig. 9), the dielectric contrast vanishes (*∆λ<sup>g</sup>* = 0) and luminescent band position becomes just the same as in the optical cell (a so-called solvent effect). After passing through this concentration, the stopband width *∆λg* starts growing due to increasing refractive index *np*. In this case we have an inversion of photonic bands, something like that occurring in narrow-gap semiconductors.

2,5-bis(2-benzoxazolyl)hydroquinone belongs to a class of substances that manifests intramolecular excited-state proton transfer. This substance is tautomerized in the conditions of ultraviolet excitation and shows a pronounced luminescence in green-red region with a large Stokes shift. When 2,5-bis(2-benzoxazolyl)hydroquinone in a hexane solution is excited within a main absorption band (280 nm – 420 nm), the irradiative transitions in both structural forms appear the spectrum (curve 1 in Fig. 10). The band in the 430 nm – 470 nm region is correspondent to the transitions without proton transfer, the band in the 580 nm – 620 nm region is due to the transitions with proton transfer. In condensed states these bands may be shifted towards the greater wavelength region. Thus a wide intensive band observed in the polycrystalline state spectrum within a 600 nm – 750 nm region is a result of the shift of a "proton-transfer" band. A shoulder of this band (in the 490 nm – 560 nm region) is most likely due to the impurity luminescence. It is proved by diminishing this band intensity in amorphous state (impurity-free state, according to our obtaining

Fig. 10. Luminescence spectra of 2,5-bis(2-benzoxazolyl)hydroquinone in a hexane solution at a 400 nm diode excitation (1), in polycrystalline state at a 350 nm wide-band mercury lamp excitation (2), in polycrystalline (3) and amorphous (4) states, and into synthetic opal

The luminescent band in such inverse opal is shifting towards a "blue" side.

**4.1.2 Intra-molecular proton transfer substances** 

procedure) and by results presented by Chayka et al., 2005.

volume (5) at a 337 nm nitrogen laser excitation.

as follows. Infiltration with any water-glycerine solution results in lowering dielectric contrast. It causes a shift of stop-band centre *λc* to longer wavelengths. In accordance with our calculations by using expressions (10, 11), the stop-band shift is equal to 6 nm by varying glycerine volume concentration from 66 % till 100 %. It corresponds exactly to the luminescence maximum shift observed in the opal-rhodamine 6G spectrum (Fig. 8, a). In case of pironin G, we have somewhat different behaviour (Fig. 8, b, and Fig. 9).

Fig. 8. Luminescence spectra of rhodamine 6G (a) and pironin G (b) placed into opals filled with a water-glycerin solution and in the optical cell with pure glycerine. In case (a) glycerine volume concentrations are 66 % (1), 75 % (2), 100 % (3), and curve 4 is the spectrum in pure glycerine. In case (b) glycerine volume concentrations are a 40% (1), 60% (2), 80% (3), 100% (4), and curve 5 is the spectrum in pure glycerine.

Fig. 9. Concentration dependences of stop-band center *λc* (1), pironin G luminescent maxima in opals (2) and water-glycerine solution in optical cell (3). The bars are stop-band widths.

Without infiltrating opals with a solution the condition *λem* > *λc* takes place and we have a weak emission in the long-wavelength region discussed above (Fig. 7, b). By increasing glycerine concentration the relation between *λem* and *λc* becomes the opposite (*λem* < *λc*) and 96 Quantum Optics and Laser Experiments

as follows. Infiltration with any water-glycerine solution results in lowering dielectric contrast. It causes a shift of stop-band centre *λc* to longer wavelengths. In accordance with our calculations by using expressions (10, 11), the stop-band shift is equal to 6 nm by varying glycerine volume concentration from 66 % till 100 %. It corresponds exactly to the luminescence maximum shift observed in the opal-rhodamine 6G spectrum (Fig. 8, a). In

Fig. 8. Luminescence spectra of rhodamine 6G (a) and pironin G (b) placed into opals filled with a water-glycerin solution and in the optical cell with pure glycerine. In case (a) glycerine volume concentrations are 66 % (1), 75 % (2), 100 % (3), and curve 4 is the spectrum in pure glycerine. In case (b) glycerine volume concentrations are a 40% (1), 60%

Fig. 9. Concentration dependences of stop-band center *λc* (1), pironin G luminescent maxima in opals (2) and water-glycerine solution in optical cell (3). The bars are stop-band widths.

Without infiltrating opals with a solution the condition *λem* > *λc* takes place and we have a weak emission in the long-wavelength region discussed above (Fig. 7, b). By increasing glycerine concentration the relation between *λem* and *λc* becomes the opposite (*λem* < *λc*) and

(2), 80% (3), 100% (4), and curve 5 is the spectrum in pure glycerine.

case of pironin G, we have somewhat different behaviour (Fig. 8, b, and Fig. 9).

the short-wavelength tail of the luminescence band is being enhanced. At glycerine concentrations close to 85 volume per cents (marked by the arrow in Fig. 9), the dielectric contrast vanishes (*∆λ<sup>g</sup>* = 0) and luminescent band position becomes just the same as in the optical cell (a so-called solvent effect). After passing through this concentration, the stopband width *∆λg* starts growing due to increasing refractive index *np*. In this case we have an inversion of photonic bands, something like that occurring in narrow-gap semiconductors. The luminescent band in such inverse opal is shifting towards a "blue" side.

#### **4.1.2 Intra-molecular proton transfer substances**

2,5-bis(2-benzoxazolyl)hydroquinone belongs to a class of substances that manifests intramolecular excited-state proton transfer. This substance is tautomerized in the conditions of ultraviolet excitation and shows a pronounced luminescence in green-red region with a large Stokes shift. When 2,5-bis(2-benzoxazolyl)hydroquinone in a hexane solution is excited within a main absorption band (280 nm – 420 nm), the irradiative transitions in both structural forms appear the spectrum (curve 1 in Fig. 10). The band in the 430 nm – 470 nm region is correspondent to the transitions without proton transfer, the band in the 580 nm – 620 nm region is due to the transitions with proton transfer. In condensed states these bands may be shifted towards the greater wavelength region. Thus a wide intensive band observed in the polycrystalline state spectrum within a 600 nm – 750 nm region is a result of the shift of a "proton-transfer" band. A shoulder of this band (in the 490 nm – 560 nm region) is most likely due to the impurity luminescence. It is proved by diminishing this band intensity in amorphous state (impurity-free state, according to our obtaining procedure) and by results presented by Chayka et al., 2005.

Fig. 10. Luminescence spectra of 2,5-bis(2-benzoxazolyl)hydroquinone in a hexane solution at a 400 nm diode excitation (1), in polycrystalline state at a 350 nm wide-band mercury lamp excitation (2), in polycrystalline (3) and amorphous (4) states, and into synthetic opal volume (5) at a 337 nm nitrogen laser excitation.

Quantum Optics Phenomena in Synthetic Opal Photonic Crystals 99

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

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

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),

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.

volume, and also, as a result of surface enhanced conditions inside opal pores.

component (Cardona, 1975) (Fig. 12, b).

Ba(NO3)2 (3), and LiIO3 (4)

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