Photonic stop band effect in ZnO inverse photonic crystal

Photonic stop band effect in ZnO inverse photonic crystal

Optical Materials 33 (2011) 466–474 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Ph...

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Optical Materials 33 (2011) 466–474

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Photonic stop band effect in ZnO inverse photonic crystal Sunita Kedia a, R. Vijaya a,⇑,1, A.K. Ray b, Sucharita Sinha b a b

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India Laser and Plasma Technology Division, Bhabha Atomic Research Center, Mumbai 400 085, India

a r t i c l e

i n f o

Article history: Received 12 April 2010 Received in revised form 11 September 2010 Accepted 12 October 2010 Available online 9 November 2010 Keywords: Photonic crystals Self-assembly Emission Photoluminescence

a b s t r a c t Three-dimensional photonic crystals are fabricated using inward-growing self-assembly technique from polymethyl methacrylate (PMMA) colloids. Their air voids are infiltrated with zinc oxide (ZnO) by sol–gel method and the PMMA template is removed by the two independent processes of heat treatment and wet chemical method resulting in ZnO inverse photonic crystal. The inversion is confirmed from the structural characterization. In X-ray diffraction (XRD) experiment, the ZnO inverse photonic crystals obtained by both the techniques do not show any signature of single-crystalline ZnO. The inverse photonic crystals obtained by chemical method are further heated at different temperatures and XRD confirms crystalline nature of ZnO for temperature treatment at 400 °C. Laser-induced emission studies on ZnO inverse photonic crystals are carried out at two different excitation wavelengths. Excitation with 355 nm enables the observation of the stop band effect for emission at 45° from the inverse crystal obtained by the inexpensive chemical method. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Three dimensional (3D) opal photonic crystals control the propagation of electromagnetic waves in a selective manner. The position of the stop band in these structures can be modified by changing the lattice constant or the effective refractive index. It has been suggested that the photonic crystal with a complete photonic band gap (CPBG) is a hopeful design for the fabrication of low-threshold lasers [1]. A CPBG can be achieved in a 3D face centered cubic (fcc) lattice with a high refractive index contrast (about 2.8) of the materials constituting the crystal [2]. While 3D photonic crystals can be fabricated by the two approaches of the top-down method and the bottom-up technique, the latter is relatively less expensive and does not require any sophisticated instrument for the crystal fabrication and it includes the self-assembly method. Attaining a large value for the refractive index contrast is difficult in the case of materials such as silica (SiO2) and organic polymers like polymethyl methacrylate (PMMA) and polystyrene (PS) that are commonly used to get direct photonic crystals. In earlier works, a high index contrast in the self-assembled photonic crystals has been achieved by infiltrating the opals with a higher index material followed by removal of the original template. The resultant structure is known as inverse opal. In the present work, 3D photonic crystals are fabricated by selfassembly method using the colloidal solution of PMMA. Since the ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (R. Vijaya). Present address: Department of Physics, Indian Institute of Technology Kanpur, Kanpur 208016, India. 1

0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.10.020

index contrast of the constituent materials in the crystal is 0.49, which is the difference in the refractive index of PMMA and air present in the voids of the crystal, the crystal shows a pseudo band gap. Zinc oxide (ZnO) is chosen for infiltration of the air voids of the photonic crystal because it has a high refractive index 2.0 in the visible range. Many methods such as atomic layer deposition [3], chemical vapor deposition [4], electro-deposition [5–7] and sol– gel chemistry [8] are known for successful infiltration of ZnO into the photonic crystals. In this work, the PMMA photonic crystals are infiltrated with ZnO using inexpensive sol–gel chemistry [8]. Subsequently, ZnO inverse photonic crystals are successfully fabricated from the infiltrated opals by removing the original PMMA template using heat treatment [6] and chemical method [5]. The inverse crystals are studied for their emission characteristics under excitation with 325 nm and 355 nm laser light. To our knowledge, this is the first report on laser-induced emission studies in ZnO inverse photonic crystal obtained from PMMA photonic crystal infiltrated with ZnO using the simple sol–gel method. The presence of the photonic stop band effect on its emission indicates the possibility of achieving a miniature ZnO laser.

2. Fabrication of direct and inverse photonic crystals 2.1. PMMA photonic crystals The direct photonic crystals are fabricated by inward-growing self-assembly technique [9] using commercially available aqueous suspensions of PMMA with a mean colloidal sphere diameter of 287 nm (from M/s Micro-particles, Germany). Simple glass slides

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of (2.5  2.5 cm2) dimension are used as substrates and crystals are grown within 3–4 h in the ambient conditions. Details of the fabrication process can be found elsewhere [10]. This method results in fcc arrangement of the spherical globules. To increase the index contrast of the direct PMMA photonic crystal, the voids of the crystal are infiltrated with ZnO by sol–gel method [8].

2.2. Infiltration of PMMA photonic crystals To make the sol of ZnO, 1g of zinc acetate is mixed with 19 ml of 2-propanol. The solution is stirred for 10 min on a magnetic stirrer and 0.6 ml of diethanolamine is added drop-wise to the mixture after 15–20 min of stirring to dissolve zinc acetate further. The final mixture is stirred for 2 h. The sol is kept undisturbed for 1 h. The PMMA photonic crystals are heated at 50 °C for 10 min and dipped into the sol and withdrawn after 30 s in a controlled way. Since the crystals are at a slightly higher temperature than the sol, the crystal soaks the sol into its voids. The infiltrated samples are allowed to dry in the air for 12 h. The color of the infiltrated sample changes when it dries completely. The infiltration cycle was repeated three times with fresh sol. The samples required longer times of 3-days and 1 week, to completely dry out, after second and third infiltration cycles, respectively. The samples look opaque after the third infiltration cycle, perhaps due to a layer of ZnO left on the surface of the photonic crystal after all the voids are fully infiltrated. To increase the index contrast further, ZnO inverse crystals are fabricated by removing the original PMMA template from the infiltrated crystals.

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2.3. Inversion of infiltrated PMMA photonic crystals Two methods are tried for inverting the photonic crystal, namely the heat treatment [6] and the chemical method [5]. In the heat treatment, the infiltrated photonic crystals are heated in a furnace at a rate of 7 °C min 1 and kept at 400 °C for 4 h to remove the PMMA colloids. The inverted crystals are then cooled back to the room temperature at the same rate. ZnO inverse crystals are also fabricated by an alternative chemical method, in which the infiltrated crystals are dipped in toluene for 30 min and then dried in air. The final structure looks blue in color to the naked eye. The ZnO inverse photonic crystals contain periodically arranged air spheres surrounded by the solid ZnO filling the interconnecting voids of the original template.

3. Results and discussion 3.1. Structural characterization The morphology of the fabricated photonic crystals is studied by their structural characterization. For this purpose, scanning electron microscope (SEM), atomic force microscope (AFM) and optical microscope (OM) images are recorded to understand the lattice arrangement and surface quality of the crystals and transmission electron microscope (TEM) image is used to observe the arrangement of the underlying layers. The results in Fig. 1 represent well-arranged colloidal spheres in cubic lattice form. In Fig. 1(a), spheres are arranged in hexagonal

Fig. 1. (a) SEM image of 3D PMMA photonic crystal of sphere diameter 287 nm; hexagonal arrangement of spheres represents the (1 1 1) plane of the cubic lattice [scale bar 100 nm]. (b) AFM image of the 3D PMMA photonic crystal shows the same hexagonal arrangement of spheres in the (1 1 1) plane [scan area (1  1 lm]. (c) OM image of the (1 1 1) plane of the 3D PMMA photonic crystal. Dark lines are grain boundaries which separate different domains [scale bar 500 lm].

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geometry on the top surface of the crystal representing the (1 1 1) plane of the fcc lattice. The hexagonal arrangement of the spheres can also be noticed in Fig. 1(b) which is the AFM image of the same plane of the crystal. The average sphere diameter of PMMA colloids measured in AFM by scanning (1  1 lm2) area is 283 nm. The measured diameter is nearly the same as the sphere diameter of the colloids provided by the manufacturer. Fig. 1(c) is the OM image of the (1 1 1) plane of the PMMA photonic crystal showing domains of size of about (2.0  0.5 mm2). The black lines in the OM image are the grain boundaries which separate different domains in the crystal. The SEM images of the ZnO inverse photonic crystal obtained by heat treatment method are shown in Fig. 2 at different magnifications. The ZnO inverse photonic crystals obtained by this process contain periodically arranged air spheres surrounded by ZnO net. In Fig. 2(a), a number of domains of the ZnO inverse photonic crystal are visible which are separated by wide cracks. Fig. 2(b) is the SEM image of a single domain which shows broken ZnO net (indicated by white arrows). In Fig. 2(c), hexagonal arrangement of air spheres and the underlying layer of the crystal can be noticed in a larger magnification. The average diameter of the air spheres is measured as 212 nm from the SEM image. The diameter of the air spheres has reduced considerably from its original value due to shrinkage during the heating process. Fig. 3 has the SEM images of ZnO inverse photonic crystal obtained by chemical method. In Fig. 3(a), a number of domains of the crystal are separated by comparatively narrower cracks. Fig. 3(b) is the SEM image of a single domain; well-connected

ZnO net is observed throughout the domain. Periodicity in a large area of about (9  6 lm2) size is achieved in this case. Abramova et al. in 2009 infiltrated the PS photonic crystal with ZnO followed by inversion [11]. In that work, sol of ZnO infiltrated the cracks of the crystal along with the voids. The ZnO inverse photonic crystal with filled cracks was considered as large-scale periodic crystal. In the present work, the infiltration is done by the same sol–gel method but infiltration of cracks is not observed (shown with white arrow in Fig. 3(b)). In Fig. 3(c), hexagonal arrangement of the air spheres and underlying layers are visible. The average air sphere size found in this case is 277 nm which is nearly equal to the diameter of the original PMMA spheres measured from AFM image (283 nm). The problem of shrinkage and broken net in the domain due to heat is eliminated in this process. Hence the inverse photonic crystals obtained by the wet chemical method are found to be superior to the inverse crystals obtained by heat treatment method. Fig. 4(a) is the SEM image of the ZnO inverse photonic crystal obtained by chemical method and subsequently heated at 400 °C for 4 h and annealed to the room temperature. The structure retains its hexagonal arrangement of air spheres with interconnected ZnO net. The air sphere diameter found in this case is 212 nm. Even though the diameter of the air spheres has reduced due to the high-temperature treatment, well-connected ZnO net formed during the chemical treatment is still seen throughout the domain. Fig. 4(b) is the TEM image of ZnO inverse photonic crystal obtained by chemical method in which the ZnO net and underlying layer are visible. The image clearly indicates the stacking of the layers in (ABCABC) sequence as required in fcc arrangement.

Fig. 2. SEM images of the ZnO inverse photonic crystal obtained by heat treatment: (a) a number of domains are separated by wide cracks [scale bar 1 lm]. (b) Within a single domain, broken ZnO net is visible which is indicated by white arrows [scale bar 300 nm]. (c) Hexagonal arrangement of air spheres and the presence of the underlying layer are noticeable [scale bar 100 nm].

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Fig. 3. SEM images of the ZnO inverse photonic crystal obtained by chemical method: (a) a number of domains are separated by narrow cracks [scale bar 1 lm]. (b) Wellconnected ZnO net is observed throughout the domain, empty crack is marked by white arrow [scale bar 300 nm]. (c) Hexagonal arrangement of air spheres and the underlying layer of the inverse photonic crystal are visible [scale bar 100 nm].

Fig. 4. (a) SEM images of the ZnO inverse photonic crystal obtained by chemical method and heated at 400 °C for 4 h [scale bar 100 nm]. (b) TEM image of the ZnO inverse photonic crystal obtained by chemical method [scale bar 200 nm].

3.2. Optical characterization Optical characterization of the photonic crystal gives the position of the stop band and its angle-dependent behavior. For optical characterization, reflectance and transmittance are recorded from the (1 1 1) plane of the crystals. In Fig. 5(a), the thin solid line is the reflection spectrum of PMMA photonic crystal for 8° incident angle which shows a max-

imum reflectance of 25% at a wavelength of 597 nm. The wavelength of the reflectance peak is found to be 631 nm when calculated from the modified Bragg’s law [12] by considering the colloidal sphere diameter to be 283 nm, filling fraction of PMMA as 74% and the effective refractive index of the photonic crystal as 1.37. The difference in these values of wavelength is perhaps due to the slight dispersity in the colloidal sphere diameter and the disordered packing in some areas. When the angle of incident

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light increases, the reflection spectrum shifts towards the lower wavelength range. The reflection spectrum of PMMA photonic crystal for 15°, 30° and 45° are also plotted in the same figure. The full width at half maximum of the reflection spectra for different angles is nearly constant at 67 nm. The shift of the reflection spectrum with increasing angles of incident light is pointed out by an arrow in the figure. The shift in the reflection spectrum indicates the pseudo band gap nature of the crystal. The difference in the wavelength values of peak reflectance at 8° and 45° is 76 nm. Fabry–Perot (F–P) oscillations are observed on both sides of the reflection maximum. The estimated thickness of the crystal from F–P oscillation is 2 lm with nine numbers of arranged layers [13]. The transmission spectrum of PMMA photonic crystal for normal incidence of light is also shown with open circles in the figure. The peak in reflection at near-normal incidence matches with the dip in transmission. The index contrast of the PMMA photonic crystal is increased by infiltrating the crystal with ZnO. The infiltration cycle is repeated and the reflection spectrum is recorded at 8° after each infiltration cycle. The reflection spectrum of ZnO infiltrated sample shifted towards higher wavelength range as compared to the bare PMMA photonic crystal (thick line in Fig. 5(b)) because the filling fraction of ZnO increases after each cycle. The dashed curve, the dotted curve and the thin line curve in Fig. 5(b) are the reflection spectra of the ZnO-infiltrated PMMA photonic crystal after 1st, 2nd and 3rd infiltration cycles respectively. After 3rd cycle, the surface of the sample looks opaque, perhaps due to a thick layer of ZnO left on the top of the sample after all the voids are filled. The maximum value of reflectance decreases substantially after the infiltration. The effective refractive index of the infiltrated sample is calculated

as 1.63 for 26% of filling fraction of ZnO in fcc lattice. The reflection spectra of ZnO-infiltrated PMMA photonic crystal for different angles of incidence are shown in Fig. 5(c). The reflection spectrum shifts towards lower wavelength region when angle of incidence increases as governed by the modified Bragg’s law [12]. This again represents a pseudo band gap material because the refractive index contrast is not sufficient for CPBG condition [2]. The peaks in reflection at 8° and 45° are separated by 73 nm which is comparable to the case of direct PMMA photonic crystal. The full width at half maximum of each reflection spectrum is nearly the same at 78.7 nm. The dotted line in Fig. 6(a) is the reflection spectrum of ZnO inverse photonic crystal obtained by chemical method. The reflection is measured at 8° and the spectrum shows a peak value of 10% at 462 nm. The reflection spectra of direct and infiltrated PMMA photonic crystals are also shown in the same figure with solid and dashed lines respectively, for comparison. The refractive index contrast and the effective refractive index of the ZnO inverse photonic crystal are calculated as 1.0 and 1.34, respectively. The reflection peak of inverse crystal shifted about 172 nm towards lower wavelength compared to ZnO-infiltrated PMMA photonic crystal due to lower effective index. Fig. 6(b) is the reflection spectrum of ZnO inverse photonic crystal recorded at different angles. The reflection spectrum shifts towards lower wavelengths when the angle of incidence is increased and the difference in wavelength between the reflectance peaks at 8° and 45° is 107 nm. The RI contrast has increased in this case but the effective RI of the inverse crystal is less than the direct and infiltrated photonic crystals due to lesser filling fraction of the solid ZnO. The full width at half maximum of the reflection spectra of the inverse photonic crystal is 77.8 nm.

Fig. 5. (a) Reflection spectrum of PMMA photonic crystal at different angles and transmission for normal incidence. (b) Reflection spectrum of ZnO-infiltrated PMMA photonic crystal after different infiltration cycles recorded at 8°. (c) Reflection spectrum of ZnO-infiltrated PMMA photonic crystal (after 3rd infiltration cycle) at different angles.

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Fig. 6. (a) Reflection spectrum of PMMA photonic crystal (thick line), ZnO-infiltrated PMMA photonic crystal (dashed line) and ZnO inverse photonic crystal (dotted line) recorded at 8°. (b) Reflection spectrum of ZnO inverse photonic crystal obtained by chemical method at different angles.

3.3. X-ray diffraction (XRD) The ZnO infiltrated into the voids and then inverted has been obtained from a sol–gel process. Hence its crystalline nature is to be ascertained. Towards this purpose, X-ray diffraction (XRD) spectrum is recorded on the inverse crystals. The solid line and dotted curve in Fig. 7(a) are the XRD spectra of ZnO inverse photonic crystal obtained by heat treatment and chemical method, respectively. No XRD peaks are observed in the range of 20–70°. Both types of inverse photonic crystals do not show any signature of crystalline ZnO. The sample obtained by chemical method is heated at different temperatures of 100 °C, 200 °C, 300 °C and 400 °C for 4 h and XRD is recorded for each of them. The spectrum in Fig. 7(b) is the XRD pattern of ZnO inverse photonic crystal heated at 400 °C at a rate 7 °C/min after chemical treatment. The spectrum shows a peak at 34.6° which corresponds to reflection from the (0 0 2) plane of crystalline ZnO as confirmed from the JCPDS XRD software. This result confirms that the ZnO sol infiltrated into the photonic crystals is crystallized only after the high-temperature treatment. 3.4. Laser-induced emission studies The photoluminescence (PL) of ZnO is studied under laser excitation on ZnO inverse photonic crystals obtained by both the techniques. Experiments are carried out at two excitation wavelengths, 325 nm and 355 nm, provided by He–Cd laser and third-harmonic of Nd:YAG laser, respectively. The emission results for excitation at 325 nm are shown in Fig. 8.

The thick solid line in Fig. 8(a) is the emission spectrum of ZnOinfiltrated PMMA photonic crystal when excited with He–Cd laser at 325 nm. It shows a UV band with peak at 368 nm. The dotted line and the thin line in the figure are the emission spectra of ZnO inverse photonic crystal obtained by chemical method and heating process, respectively. The intensity of the emission spectrum is decreased more drastically in the latter case. The peak position is observed to shift to 372 nm for chemically treated crystal and to 384 nm for the crystal obtained by heating process. Yang et al. in 2006 infiltrated PS photonic crystal with ZnO and inverse photonic crystals were fabricated by dissolving the PS template in toluene and by combustion method [7]. The emission spectra of both the samples were recorded by exciting them with He–Cd laser at 325 nm and a shift in the peak position of the UV band was observed in the inverse photonic crystal obtained by heating method. This is similar to the result obtained in our work. In earlier publications, the emission spectrum of ZnO inverse photonic crystal showed a broad visible band along with the UV band [3,5]. Fig. 8(b) is the emission spectrum of the ZnO inverse photonic crystal obtained by chemical method and subsequently heated at 400 °C. For this crystal, the emission spectrum has a sharp UV peak centered at 371 nm with a broad visible spectrum. The UV peak arises due to the band-to-band transition of ZnO and the visible emission is because of deep level defects due to oxygen vacancies and interstitial ZnO ions [7]. The effect of photonic stop band on the emission of ZnO is not observed when excited with He–Cd laser. The ZnO inverse photonic crystal is also excited with the third harmonic of a pulsed Nd:YAG laser at 355 nm. Fig. 9(a) is the sche-

Fig. 7. (a) XRD spectrum of inverse photonic crystal obtained by heat treatment (solid line) and by chemical method (dotted curve). (b) XRD of ZnO inverse photonic crystal heated at 400 °C after chemical treatment.

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Fig. 8. (a) Laser-induced emission spectrum of ZnO-infiltrated PMMA photonic crystal (thick line), ZnO inverse photonic crystals obtained by chemical method (dotted line) and by heat treatment (thin line). (b) Emission spectrum of ZnO inverse photonic crystal obtained by chemical method and subsequently heated at 400 °C. Excitation wavelength is 325 nm and emission is recorded at 45° to the excitation direction. The angle of incidence of the excitation laser beam on the sample is 0°.

Fig. 9. (a) Schematic diagram of experimental set-up used for laser-induced emission studies. (1) Nd:YAG laser with harmonic generators, (2) Pellin–Broca prism (dotted line is 532 nm and solid line is 355 nm), (3) mirror, (4) beam dump, (5) focusing lens, (6) rotating stage, (7) sample, (8) detector and (9) data acquisition system. (b) Thin and thick lines are the emission spectra of ZnO inverse photonic crystal obtained by chemical method when excited at 355 nm and 325 nm respectively. (c) Emission spectrum of ZnO inverse photonic crystal obtained by chemical method (thick line) and by heating method (thin line) when excited with third harmonic of Nd:YAG laser at 355 nm.

matic of the experimental set-up, in which the sample is kept fixed at the center of a rotating stage and the laser beam is incident normally on the sample after passing through a number of optical components. The detector is attached to the rotating stage and collects the emission from the sample at different angles. To compare the results of the excitations at 325 nm and 355 nm, the emission spectra of ZnO inverse photonic crystal recorded at 45° at both these excitation wavelengths are plotted in Fig. 9(b). The thin line is the emission spectrum of ZnO inverse photonic crystal obtained by chemical method for 355 nm excitation. The spectrum contains both UV and visible bands. The result is similar to the result presented by Scharrer et al. in 2005 where the emission of ZnO inverse

photonic crystal showed a sharp UV band along with a broad band in the visible range [3]. The thick line in the figure is the emission spectrum of the same crystal when excited with He–Cd laser at 325 nm. The emission has a broad UV peak but visible emission is feeble. The UV spectrum is broader in the case of the 325 nm excitation, perhaps due to the larger line width of the He–Cd laser as compared to the Nd:YAG laser. Additionally, the higher peak power of excitation at 355 nm has enabled the band gap effect to be observed in emission for this excitation wavelength. Fig. 9(c) is the emission at 8° of the ZnO inverse photonic crystals obtained by chemical method and heat treatment, when excited at 355 nm. The ZnO inverse photonic crystal obtained by

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Fig. 10. Emission measured at an excitation wavelength of 355 nm from ZnO inverse crystal obtained by chemical method. (a) The reflection spectrum (dotted line) for 8° incident light and the emission of the ZnO inverse photonic crystal recorded at 8° (thick line). (b) The reflection spectrum (dotted line) for 15° incident light and the emission of the ZnO inverse photonic crystal recorded at 15° (thick line). The effect of the reflection band is not observed on the emission spectrum at these two angles. (c) The reflection spectrum (dotted line) for 30° incident light and the emission of the ZnO inverse photonic crystal recorded at 30° (thick line). (d) The dotted line is the reflection spectrum at 45° and the thick line is the emission spectrum of the ZnO inverse photonic crystal recorded at 45°. The intensity of the UV band of emission decreases because the stop band of the photonic crystal overlaps with the UV band at this angle. The emission spectrum of ZnO thin film at 15° considered as the intrinsic emission is shown with the thin line in all the figures.

chemical method has a sharp UV emission along with a broad visible band (thick line). The green emission is not significant in the case of the ZnO inverse photonic crystal obtained by heat treatment process and only the UV emission is observed (thin line). This result is similar to the result obtained by Yang et al. in 2006 [5]. The ZnO inverse photonic crystal obtained by chemical method has the stop band in the range of 350 nm and 460 nm for incidence angles of 8–45°. The ZnO sol is spread on glass substrate as a thin film and dried in air. The resultant sample is considered as a reference sample. The emission from the reference sample is referred to as intrinsic emission of ZnO. The emission from ZnO inverse photonic crystal is recorded at different angles to check the effect of photonic stop band on it under an excitation wavelength of 355 nm. The range of emission of ZnO inverse photonic crystal which matches with the photonic stop band is not allowed to transmit out of the crystal completely and the emission is thus inhibited in that range. The emission can be enhanced at the photonic band edge because of more allowed states. The thick line is the emission of the ZnO inverse photonic crystal and the dotted line is the reflection spectrum of the sample, both at the same angle. The range of photonic stop band overlaps with the visible band of the ZnO inverse photonic crystal for 8° (Fig. 10(a)) and 15° (Fig. 10(b)). The effect of stop band is not observed on the visible band because the structure has only 10% of reflectance in this wavelength range. At 30°, the position of the photonic stop band has shifted towards the UV band of the emission spectrum. Decrease in the intensity of the UV band is observed at this angle which can be seen in Fig. 10(c). The effect is more prominent when

the emission is recorded at 45°, shown in Fig. 10(d) with a reduction in the emission intensity in the region of 360–380 nm since the reflection spectrum overlaps the UV band of the emission spectrum completely at this angle. In conclusion, 3D PMMA photonic crystals fabricated by selfassembly method are successfully infiltrated with ZnO by simple sol–gel technique. The ZnO inverse photonic crystals are obtained from the infiltrated samples by both heat treatment and chemical technique. The position of the photonic stop band at normal incidence is different for the PMMA photonic crystal, ZnO infiltrated crystal and the ZnO inverse photonic crystal due to changes in the effective refractive index of the crystal. The sample heated at 400 °C after chemical treatment shows the signature of crystalline ZnO in XRD experiment. Laser-induced emission studies are done on both types of ZnO inverse photonic crystals under two different excitation wavelengths. The results show the effect of photonic stop band on the emission spectrum at 45° for ZnO inverse photonic crystal obtained by chemical method when excited at 355 nm. Acknowledgements The authors gratefully acknowledge several useful discussions with Dr. K. Dasgupta of Laser and Plasma Technology Division of BARC, Prof. S.S. Major of the Physics Department of IIT Bombay for PL set-up facility and the Electrical Engineering department of IIT Bombay for technical support. This work was financially supported by the Board of Research in Nuclear Sciences, DAE.

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References [1] Y. Yamamoto, R.E. Slusher, Phys. Today 46 (1993) 66–73. [2] G.I.N. Waterhouse, M.R. Waterland, Polyhedron 26 (2007) 356–368. [3] M. Scharrer, X. Wu, A. Yamilov, H. Cao, R.P.H. Chang, Appl. Phys. Lett. 86 (2005) 151113-3. [4] G.A. Emelchenko, A.N. Gruzintsev, V.V. Masalov, E.N. Samarov, A.V. Bazhenov, E.E. Yakimov, J. Opt. A: Pure Appl. Opt. 7 (2005) 213–218. [5] Y. Yang, H. Yan, Z. Fu, B. Yang, J. Zuo, S. Fu, Solid State Commun. 139 (2006) 218–221. [6] K.H. Yeo, L.K. Teh, C.C. Wong, J. crystal. Growth 287 (2006) 180–184.

[7] Y. Yang, H. Yan, Z. Fu, B. Yang, J. Zuo, Appl. Phys. Lett. 88 (2006) 191909-1– 191909-3. [8] R.V. Nair, R. Vijaya, J. Phys. D: Appl. Phys. 40 (2007) 990–997. [9] Q. Yan, Z. Zhou, X.S. Zhao, Langmuir 21 (2005) 3158–3164. [10] R.V. Nair, R. Vijaya, Appl. Phys. A 90 (2008) 559–563. [11] V. Abramova, A. Sinitskii, Superlattices Microstruct. 45 (2009) 624–629. [12] A. Reynolds, F. Lopez-Tejeira, D. Cassagne, F.J. Garcia-Vidal, C. Jouanin, J. Sanchez-Dehesa, Phys. Rev. B 60 (2000) 11422–11426. [13] S.G. Romanov, C.M.S. Torres, M. Egen, R. Zentel, Photonics Nanostruct. Fundam. Appl. 4 (2006) 59–68.