Plasmonic Resonance-Induced Effects on Stopband and EmissionCharacteristics of Dye-Doped Opals

Dipak Rout & R. Vijaya

Received: 28 September 2014 /Accepted: 4 December 2014# Springer Science+Business Media New York 2014

Abstract Three-dimensional colloidal photonic crystals con-taining a fluorophore, and fabricated by inward growing self-assembly method, are infiltrated with gold nanoparticles of5-nm diameter. The effects of localized surface plasmon res-onance of gold nanoparticles on the stopband of the photoniccrystal are investigated by tuning the reflection band to over-lap with the plasmonic band. Interestingly, reflection spectraof gold-infiltrated PhC show a clear suppression in the reflec-tion band around the stopband center at higher angles ofincidence due to the efficient coupling between the plasmonand photon modes inside the crystal. The resonant absorptionappears only when there is a significant overlap between thestopband and the plasmonic resonance. The role of plasmonicresonance towards the enhancement of emission at the bandedges is also discussed from the experimental results on thedielectric and metallo-dielectric photonic crystals.

Keywords Photonic crystals . Localized surface plasmonresonance .Metallo-dielectric photonic crystals . Resonantabsorption . Band-edge emission

Introduction

Photonic crystals (PhCs) are conventionally made from aperiodically varying dielectric medium. The period of dielec-tric repetition must be comparable to the wavelength of thelight for intended application. This feature of PhC leads to a

range of frequency being blocked in transmission, whileallowing others. The forbidden range of frequency forms thestopband. If the stopband is present in all directions, it is calleda complete photonic bandgap (CPBG) material, or else, it is apseudo bandgap material. The bandgap properties are decidedby parameters such as the refractive index contrast betweenthe individual dielectric constituents, lattice parameters, andthe crystal structure.

The optical properties of PhCs can be tailored by introduc-ing different kinds of materials into the structure. The sponta-neous emission from an emitter can be controlled by usingPhCs as a host material, since the density of states is maximumat the band edge and minimum inside the stopband [1–3]. Amaterial with a complex dielectric constant such as silver andgold can be a suitable choice for certain applications as itsupports both localized and propagating surface plasmonmodes. A thin silver film coated over the spherical colloidsused in self-assembled monolayer supports localized surfaceplasmon resonance (LSPR) whose peak position can be tunedby varying the sphere diameter, the silver film thickness, orboth [4]. Hybrid metal-dielectric photonic structures are usedto achieve the diversion of light efficiently from their incidentdirections [5]. Metallo-dielectric photonic crystal has beenshown to possess CPBG in the visible wavelength range [6].A monolayer of dielectric spheres containing a gain mediumgrown over a gold film can show directional emission [7]. Asreported by one of us, a thin film of gold coating on an activePhC can reduce the lasing threshold [8]. LSPR in the metallo-dielectric structure has a significant role in increasing thelight-matter interaction and also gives freedom over the tun-ability of resonant frequency. Apart from metallic thin films,metal nanoparticles are also used to increase the light-matterinteraction in the PhCs. Gold or silver nanoparticles are usedfor enhanced light harvesting due to their inherent plasmonicresonances [9]. A redshift in the stopband center is observedafter the infiltration of metal nanoparticles into the voids of

D. Rout : R. VijayaDepartment of Physics, Indian Institute of Technology Kanpur,Kanpur 208016, India

R. Vijaya (*)Center for Laser and Photonics, Indian Institute of TechnologyKanpur, Kanpur 208016, Indiae-mail: rvijaya@iitk.ac.in

PlasmonicsDOI 10.1007/s11468-014-9857-2

opals [10, 11]. Besides the tunability in the stopband, one canstudy the effects of plasmon resonance on the stopband.

This work studies the optical properties of PhC grown fromrhodamine B (RhB) dye-doped polystyrene spheres, followedby infiltration of gold nanoparticles of 5-nm diameter into itsvoids. The diameter of gold nanoparticles in this metallo-dielectric PhC was specifically chosen since it supports plas-mon resonance near 520 nm [12, 13]. This ensures an overlapbetween the absorption spectra of gold nanoparticles and theRhB dye doped in the colloids. Hence, one can expect anenhancement in the emission spectrum of the dye as a result ofincreased absorption.

The motivation is to study the modification in stopbandcharacteristics of PhC as well as the emission from the dyeinfluenced by the infiltrated gold nanoparticles in the photonicenvironment. The diffraction resonance of PhC gets modifiedaccording to the extent of its overlap with the plasmon reso-nance band. By varying the angle of incidence, the PhCstopband was made to overlap with the LSPR band of goldnanoparticles in this work. Maximum suppression in the pho-tonic stopband was observed for a large overlap between theplasmon band and the PhC stopband. The emission measure-ments confirm the participation of LSPR towards increasingthe light-matter interaction, leading to an enhancement in theemission spectrum.

Experimental Details

The PhCs are grown using inward growing self-assemblymethod [14] frommono-dispersed polystyrene colloids dopedwith rhodamine B (PS-RhB) dye with a mean sphere diameterof 277 nm from M/s Microparticles GmbH. The crystals aregrown on a cleaned glass substrate of area 2×2 cm2 in a closedroom with no air flow. The grown samples were heated at50 °C to remove the residual chemicals and water vapor. Theheat-treated samples were dipped in 2.5 ml of gold chloridesolution and kept at 45 °C for nearly a week [11]. Thediameter of the gold nanoparticles in the gold chloride solu-tion is 5 nm with a polydispersity of 15 % supplied by SigmaAldrich. Angle-dependent reflection measurement of PhCs,both before and after infiltration, was done using thePerkinElmer Lambda 950 spectrophotometer with a universalreflectance accessory. Unpolarized light of beam area 5×5 mm2 from a halogen lamp was used for the optical mea-surements. The structural characterization includes a fieldemission scanning electron microscopy (FESEM) measure-ment on the PhC. The FESEM measurement confirms theperiodic hexagonal arrangement of the colloidal spheres andthe extent of ordering. A 532-nm source beam from Nd-YAGlaser is used to excite the RhB dye embedded in PhC and gold-infiltrated PhC (GIPhC) to validate the LSPR effects of gold

nanoparticles towards enhancement of absorption and henceemission.

Results and Discussion

In Fig. 1a, the FESEM image shows periodic arrangement ofthe colloidal spheres over a length scale of several microns. Inlower magnification, as in Fig. 1b, the crystal shows manynoticeable cracks, signifying the distinct grain boundaries.These are formed due to the post-growth heat treatment ofthe PhC. The plane shown here is the top surface of the PhC.

The gold nanoparticles were characterized in a wavelengthrange of 300 to 800 nm. The absorption measurement forthese particles gives a spectrum which lies over a broad rangeof wavelength from 500 to 550 nm with the peak at 512 nmshown in Fig. 2a. The wavelength of absorption peak obtainedfrom the experiment is comparable to the aforementionedliterature value (520 nm) for gold nanoparticles with 5-nmdiameter. The arrow in Fig. 2a indicates the peak. Figure 2bshows the specular reflection measured at 8° for PhC andGIPhC, with the GIPhC recording a much lower reflection.In addition, there is a redshift (emphasized with vertical lines)observed for the reflection spectrum of GIPhC, as reported inearlier works [10, 11]. Strong Fabry-Perot (F-P) oscillationsand a high reflectance value confirms about the good qualityof the PhC. The full width at half maximum was found to be40 nm leading to a gap to mid gap ratio of 7 %. The number ofordered layers in the PhC is calculated using the expressiongiven in [15] to be 27. Since the number of layers is fairlylarge, the good quality of the crystal is ensured.

Figure 3a shows angle-dependent reflection measured forPhC at lower angles of incidence. The stopband moves to-wards lower wavelengths with the increase in angle of inci-dence. This is in accordance with the Bragg’s law. A similarmeasurement on GIPhC shows the same trend but gives areduced peak reflectance value when the angle of incidence isincreased as compared to PhC as shown in Fig. 3b. This couldbe due to the diffused scattering and inhomogeneous distribu-tion of gold nanoparticles after infiltration.

The crystal surface shows small gold patches after theinfiltration. The uneven scattering from these patches will alsoreduce the reflectance significantly, leading to an overalldecrement in the reflectance. At higher angles of incidence,the stopbands of PhC, and GIPhC undergo Bragg wave split-ting. The higher index planes start contributing to the reflec-tion spectrum beyond a certain angle of incidence as a result ofwhich the spectral shape is changed. These contributions aremore significant for incident angles starting from 55° for PhCas shown in Fig. 3c. Since the angle between (111) and (200)plane of fcc crystal structure is 55° [16], the prominent con-tribution towards the change in the spectral shape would comefrom (200) plane from 55° onwards. On the other hand,

Plasmonics

GIPhC contains much more interesting features in thestopband region along with the signature of higher indexplanes, as shown in Fig. 3d which will be elaborated in Fig. 4.

A suppression around the stopband center is observed inthe reflection spectrum for GIPhC, whereas the reflectionspectrum of PhC remains unaffected in that wavelength rangeas shown magnified in Fig. 4a, b. To the best of our knowl-edge, the suppression in the stopband near the band center dueto LSPR effects of embedded gold nanoparticles inside thePhC is not discussed in any earlier work. Bare PS PhC showsa monotonous shift in wavelength of the peak without anyadditional features when the angle of incidence is increased asshown in Fig. 4a. Infiltration of gold nanoparticles into thisPhC allows the modification of the stopband properties selec-tively, if a proper choice is made for their diameter. Interest-ingly, the suppressed stopband features are present only for aset of angles of incidence as shown in Fig. 4b. The suppres-sion is due to the interaction between the LSPR of the infil-trated gold nanoparticles inside the PhC and the incidentelectromagnetic radiation whose wavelength lies within the

stopband range. The plasmon-photon coupling increaseswhen photonic stopband overlaps with the plasmonic bandof gold nanoparticles. The LSPR can be excited by leakymodes as well as propagating modes lying outside thestopband. But the excitation due to leaky modes would sustainlonger than the other, since it lies within the stopband,resulting in an increased light-matter interaction. Hence, thedip due to resonant absorption would be significant whenthere is a maximum overlap between the stopband and theLSPR band.

It is possible to corroborate the suppression in stopband tothe plasmonic resonant absorption of gold nanoparticles. Wedefine a functionF(R,A,λ,θ) tomodel the reflection influencedby resonant absorption as

F R;A;λ; θð Þ ¼ R λ; θð Þ 1−βA λð Þð Þ ð1Þ

where R(λ,θ) is the reflectance from the PhC measured atdifferent wavelengths λ and different angles of incidence θ,

Fig. 1 FESEM images of PhC with a colloidal diameter of 277-nm diameter showing the a ordered hexagonal close-packed structure taken in a 300-nmscale bar and b the distinct grain boundaries taken in a 1-μm scale bar

a bFig. 2 a Absorption spectrum ofgold nanoparticles (5-nmdiameter) in water. b Reflectionspectrum of PhC (solid line) andGIPhC (dash-dotted line) at 8°angle of incidence

Plasmonics

while A(λ) is the absorption spectrum of gold nanoparticles.The constant β is required to scale the independently mea-sured absorption A. The absorption shown in Fig. 2a is fit to aGaussian function in the wavelength range where the LSPRabsorption is prominent and used for A(λ)while R(λ,θ) is useddirectly from the data in Fig. 3c. The resulting A(λ) obtainedfrom the fitting of Gaussian function to the absorption isshown by dash-dotted line in Fig. 5a. F(R,A,λ,θ) representsthe modeled reflectance for GIPhC. The modeled reflectanceresembles the trend of experimentally measured spectrum forGIPhC in the relevant wavelength range. Several trial valuesof β are used in the calculation of F(R,A,λ,θ). It is found thatβ=12 gives a good match of the trends of the experimentaland the modeled reflection spectra of GIPhC.

The calculation was done for all four angles of incidencehighlighted in Fig. 4. The calculated F(R,A,λ,θ) is in agree-ment with the experimental results for all these cases, and thecomparison is shown for an incident angle of 51° in Fig. 5b.

The intensity of modeled reflectance is larger than that ofexperimentally measured case, which is expected becausethe calculation of F(R,A,λ,θ) does not account for the scatter-ing losses. Since the stopband always gets blueshifted withincreasing angles of incidence, there exists a set of angles forwhich it will overlap with the plasmonic resonance band ofgold nanoparticles (centered at 512 nm). For the angles ofincidence of 47° to 53°, the tuned stopband overlaps with theplasmonic resonance band leading to a strong resonant ab-sorption in the reflection spectrum. Hence, a suppression inthe photonic stopband is observed.

Only in the presence of the stopband, the fields get moreconcentrated and localized in the vicinity of nanoparticlesleading to a strong resonant absorption [17]. The photonicstopband helps in increasing the field localisation in a catalyticway when the plasmon resonance lies within the bandgap.Hence, our results show a clear dependence of stopbandcharacteristic on the plasmon resonance of gold nanoparticles,

a b

c d

Fig. 3 Reflection spectra of abare PS PhC and b GIPhC takenat lower incident angles. c and dare the reflection spectra of PhCand GIPhC taken at larger anglesof incidence

baFig. 4 The reflection spectra of aPhC and b GIPhC plotted for aselected range of wavelength

Plasmonics

when there is overlap between the plasmon band and thestopband. GIPhC thus provides a suitable platform for theintegration of plasmonic and photonic characteristics.

At lower angles of incidence, there is no clear signature ofresonance dip near 512 nm in the reflection spectrum(Fig. 3b). This could be due to the presence of strong andmultiple F-P oscillations in this wavelength range at theseangles. The effect is only observed when the resonant dip iswell within the stopband. However, the transmissionmeasure-ment at normal incidence showed a small reduction at 518 nmdue to this and another small dip at 550 nm due to theabsorption of RhB dye, apart from the expected prominentreduction due to the stopband at 604 nm.

Apart from the above, the plasmonic resonance is alsoexpected to influence the emission characteristics of an emitterin its vicinity. To investigate this, the emission spectrum ofRhB dye in PhC and GIPhC is studied. The emission mea-surements were done in reflection geometry [18]. Afrequency-doubled Nd-YAG laser operating at 532 nm withnanosecond-pulsed output is used as the excitation source.The detector is fixed to a rotational stage which can rotate

over 0–360°. The emission was measured in the first quadrantfor the detection angles of 20°, 25°, 30°, and 40° by rotatingthe detector. A minimum detection angle of 15–20° wasaccessible in reflection geometry using our setup. For angleslarger than 40°, the stopband is away from the emission bandand hence not relevant to the present discussion. To accesslower angles, one can prefer the transmission geometry, but inthat case, the stopband effect will not be as dominant as in thecase of reflection geometry [18].

The measured emission spectra for PhC and GIPhC at theabove-mentioned angles differ significantly in their peak in-tensity. GIPhC (line of medium thickness) shows an enhancedemission compared to PhC (thin line) as shown in Fig. 6.Incorporating gold nanoparticles into the photonic structureoffers the additional plasmonic resonant absorption towardsincreasing the light-matter interaction. The prominent LSPRabsorption adds more excitation energy to RhB dye embeddedin GIPhC, leading to a larger enhancement in emission. Thewavelength corresponding to the maximum emission en-hancement is marked by a dotted vertical line in Fig. 6. Theemission is seen to be enhanced selectively when there is a

baFig. 5 a Gaussian fit (dash-dotted line) to the absorption ofgold nanoparticles in water (solidline). b Comparison ofexperimental (solid line) andmodeled (dashed line) reflectionspectrum of GIPhC at 51°

a b

c d

Fig. 6 The emission spectra ofGIPhC (line of medium thickness)and PhC (thin line) measured at20°, 25°, 30°, 40° (a–d,respectively) are shown alongwith the corresponding stopbandof PhC (very thick line). Thewavelengths of emissionenhancement are shown by dottedvertical lines

Plasmonics

maximum overlap of emission band and the photonicstopband. For angles such as 20° and 25°, this effect issignificant. When the photonic stopband (very thick line)overlaps maximally with the emission band of the RhB dye,an asymmetric as well as selective suppression around theband center is observed as seen in Fig. 6a, b. An enhancementat 565 nm is seen in Fig. 6a whereas Fig. 6b shows theemission being enhanced at two different wavelengths suchas 558 and 590 nm for GIPhC. This is mainly because of thenonuniform distribution of photon density of states inside thePhC. In these cases, the photonic stopband influences theemission enhancements to be wavelength selective.

The effect of stopband gradually diminishes as it movesaway from the emission band for higher angles. Detectionangles from 40° onwards will mostly not carry the bandgapeffect due to a large separation between the stopband andemission spectrum. One may note that there is no abruptchange in the shape of the spectrum in Fig. 6c, d. Thewavelength-selective enhancement occurs only due to thecontributions from the PhC towards the nonuniform photondensity of states.

However, the enhancement in emission is noted at alldetection angles for GIPhC compared to PhC (Fig. 6c, d).Interestingly, the emission enhancement for GIPhC is seen tobe increased for larger detection angles as viewed from theemission intensity. This is due to a two-step effect. At largerangles, the stopband approaches the plasmonic resonanceband of gold nanoparticle (shown in Fig. 2a). The LSPRmodeand photonic mode couple strongly to each other in an overlapregion of stopband and the plasmon band. As a result, theabsorption becomes prominent leading indirectly to an en-hancement in emission. As described earlier in Fig. 3a, thestopband moves towards the lower wavelength side for higherangles, thus moving away from emission band while ap-proaching towards the LSPR band. Thus, the measurementtaken at 40° for GIPhC shows a larger enhancement in emis-sion than that of PhC as compared to other angles. A carefulobservation of Fig. 6 shows that the presence of stopbandreduces the emission intensity inside the bandgap region whileenhancing it at other wavelengths marked by dotted verticallines (Fig. 6a, b). This suggests that the selective enhance-ment in emission can only be achieved by a suitableand calculated amount of tuning of the stopband to thewavelength region of interest. The presence of stopbandstrengthens the plasmonic contribution selectively to-wards the emission enhancement.

The presence of plasmonic resonance within the absorptionrange of RhB dye influences its emission features, in awavelength-selective approach inside the stopband and mo-notonously outside the stopband. An increased light-matterinteraction is enabled inside the structure by the LSPR of gold.Hence, our structure can be a good choice for low threshold,room temperature, miniaturized PhC lasers.

Conclusion

Our results demonstrate the modifications in the stopbandfeatures of PhC due to the LSPR of gold nanoparticles incor-porated in it. In the wavelength region of overlap between theplasmon resonance and the stopband, the resonant absorptionis prominent. Hence, the stopband characteristics can be se-lectively manipulated by an appropriate choice of the diame-ters of colloidal sphere and gold nanoparticles. Our structurespecifically shows an enhanced emission from the dye-embedded GIPhC than PhC, due to inclusion of gold nano-particles. Our work shows that by a careful choice of appro-priate diameters for the gold nanoparticles and the colloidsalong with a suitable fluorophore, the absorption and LSPRwavelength can be tuned exactly to one band edge, while theemission is tuned to the other band edge of the stopband, inorder to fully utilize the increased photon density of states forapplications such as miniaturized lasers.

Acknowledgments The authors would like to acknowledge the De-partment of Physics of IIT Kanpur for the support to carry out the work.Dipak Rout thanksM. Srinivas Reddy for the valuable discussions duringthe work.

References

1. Schriemer HP, vanDriel HM,KoenderinkAF, VosWL (2000)Modifiedspontaneous emission spectra of laser dye in inverse opal photoniccrystals. Phys Rev A 63:011801–1. doi:10.1103/PhysRevA.63.011801

2. Lodahl P, van Driel AF, Nikolaev IS, Irman A, Overgaag K,Vanmaekelbergh D, Vos WL (2004) Controlling the dynamics ofspontaneous emission from quantum dots by photonic crystals.Nature 430:654–657. doi:10.1038/nature02772

3. Petrov EP, Bogomolov VN, Kalosha II, Gaponenko SV (1998)Spontaneous emission of organic molecules embedded in a photoniccrystal. Phys Rev Lett 81:77–80. doi:10.1103/PhysRevLett.81.77

4. Cushing SK, Hornak LA, Lankford J, Liu Y, Wu N (2011) Origin oflocalized surface plasmon resonances in thin silver film over nanospherepatterns. Appl Phys A 103:955–958. doi:10.1007/s00339-011-6393-1

5. Ding B, Bardosova M, Pemble ME, Korovin AV, Peschel U,Romanov SG (2011) Broadband omnidirectional diversion of lightin hybrid plasmonic-photonic heterocrystals. Adv Funct Mater 21:4182–4192. doi:10.1002/adfm.201100695

6. Moroz A (1999) Three-dimensional complete photonic-band-gapstructures in the visible. Phys Rev Lett 83:5274–5277. doi:10.1103/PhysRevLett.83.5274

7. López-García M, Galisteo-López JF, Blanco A, Sanchez-Márcos J,Lopez C, García-Martín A (2010) Enhancement and directionality ofspontaneous emission in hybrid self-assembled photonic–plasmoniccrystals. Small 6:1757–1761. doi:10.1002/smll.201000216

8. Kedia S, Vijay R, Ray AK, Sinha S (2011) Spectral narrowing andlasing threshold in self-assembled active photonic crystal. OpticsCommun 284:2056–2060. doi:10.1016/j.optcom.2010.12.056

9. Xu M-F, Zhu X-Z, Shi X-B, Liang J, Jin Y, Wang Z-K, Liao L-S(2014) Plasmon resonance enhanced optical absorption in invertedpolymer/fullerene solar cells with metal nanoparticle-doped solution-processable TiO2 layer. ACS Appl Mater Interfaces 5:2935–2942.doi:10.1021/am4001979

Plasmonics

10. Pérez N, Hüls A, Puente D, González-Viñas W, Castaño E, OlaizolaSM (2007) Fabrication and characterization of silver inverse opals.Sensors Actuators B 126:86–90. doi:10.1016/j.snb.2006.10.057

11. Khokhar AZ, Rahman F, Johnson NP (2011) Preparation and prop-erties of gold-infiltrated polystyrene photonic crystals. J Phys ChemSolids 72:185–189. doi:10.1016/j.jpcs.2010.12.010

12. Haiss W, Nguyen TKT, Aveyard J, Fernig DG (2007) Determinationof size and concentration of gold nanoparticles from UV–vis spectra.Anal Chem 79:4215–4221. doi:10.1021/ac0702084

13. Jain PK, Huang X, El-Sayed IH, El-Sayed MA (2007) Review ofsome interesting surface plasmon resonance-enhanced properties ofnoble metal nanoparticles and their applications to biosystems.Plasmonics 2:107–118. doi:10.1007/s11468-007-9031-1

14. Yan Q, Zhou Z, Zhao XS (2005) Inward-growing self-assembly ofcrystal films on horizontal surface. Langmuir 21:3158–3164. doi:10.1021/la047337p

15. Bohn JJ, Tikhonov A, Asher SA (2010) Colloidal crystal growthmonitored by Bragg diffraction interference fringes. J ColloidInterface Sci 350:381–386. doi:10.1016/j.jcis.2010.04.072

16. Galisteo-Lopez JF, Palacios-Lidon E, Castillo-Martínez E, Lopez C(2003) Optical study of the pseudogap in thickness and orientationcontrolled artificial opals. Phys Rev B 68:115109–1. doi:10.1103/PhysRevB.68.115109

17. Sánchez-Sobrado O, Lozano G, Calvo ME, Sánchez-Iglesias A, Liz-Marzán LM, Míguez H (2011) Interplay of resonant cavity modeswith localized surface plasmons: optical absorption properties ofbragg stacks integrating gold nanoparticles. Adv Mater 23:2108–2112. doi:10.1002/adma.201004401

18. Reddy MS, Vijaya R, Rukhlenko ID, Premaratne M (2012) Spatialand spectral distributions of emission from dye-doped photonic crys-tals in reflection and transmission geometries. J Nanophotonics 6:063526–1. doi:10.1117/1.JNP.6.063526

Plasmonics