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Spectral properties of self-assembled polystyrene nanospheres photonic crystals doped with luminescent dyes
Optical Materials 35 (2013) 1538–1543
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Spectral properties of self-assembled polystyrene nanospheres photonic crystals doped with luminescent dyes A. Yadav a, R. De Angelis a, M. Casalboni a, F. De Matteis a,⇑, P. Prosposito a, F. Nanni b, I. Cacciotti b a b
Physics Department and INSTM, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy Industrial Engineering Department and INSTM, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy
a r t i c l e
i n f o
Article history: Received 17 January 2013 Accepted 21 March 2013 Available online 13 April 2013 Keywords: Photonic crystals Polymeric colloids Self-assembly Light emission Lasing
a b s t r a c t Three-dimensional ordered photonic crystals have been fabricated on solid substrates with a self-assembling method starting from a suspension of dye-doped polymeric nanospheres in water. These photonic crystals showed angle-dependent stop band for light transmission and, correspondingly, Bragg reflection peak due to the photonic crystal lattice. Polystyrene nanoparticles of 306 nm and 288 nm diameter, respectively in the case of the Rhodamine B and Fluored dye, were used to obtain self-assembled photonic crystals. They show 40% reflectance at 610 nm for Rhodamine B and 35% reflectance at 574 nm for Fluored doped materials. The size of the spheres calculated by Bragg’s law are in good agreement with those evaluated by scanning electron microscopy (SEM). The appropriate choice of sphere diameters results in an overlap between the photonic stop-band and the dye emission spectrum. The photonic crystals showed angle-dependent suppression of spontaneous emission of the dye in the wavelength range of the photonic stop band and enhancement near the band edge. In reflection geometries, spectral narrowing and directional emission, all indicative of stimulated emission, were observed from the active photonic crystal matrix. The results of laser induced emission studies on the dye doped photonic crystals are presented. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Photonic crystals (PCs) [1–3] are optical materials for which the dielectric constant undergoes a spatial periodic variation with a period comparable to the wavelength of light. They are currently attracting much attention for their possible applications in optoelectronics, optical sensors, etc. [4–6]. The most striking property of such crystals is the appearance of wavelength ranges in which the propagation of light is forbidden, the so called photonic stopbands. By means of proper band-gap engineering these materials can strongly confine and control the propagation of light and thus they are appealing for a wide range of applications in optoelectronics [7,8] among which the possibility to produce low-threshold lasers [9,10]. The rate of spontaneous emission depends on the density of allowed photonic states which can be strongly affected by the periodic modulation in the PC. When an embedded emitter (can be a dye molecule, a quantum dot or a rare earth ion) which absorbs light out of the photonic stop-band, presents an emission which overlaps with the stop-band, the spontaneous emission from the emitter is inhibited in the stop band region due to the lowered density of allowed photon states, while it is enhanced at the band edge which presents instead a higher density of states ⇑ Corresponding author. Tel.: +39 0672594521. E-mail address: [email protected] (F. De Matteis). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.03.020
[11]. In some systems, a tunable spectral narrowing with a sharp power threshold and a highly directional emission can be observed when the excitation energy density is increased [12]. Among the different techniques used to synthesize three dimensional PCs, self-assembly from monodispersed colloidal spheres is receiving great attention due to the ease of fabrication [13]. This technique is based on the well-known property of monodispersed sub-micrometric spheres to self-organize into a facecentered-cubic (fcc) lattice with the (1 1 1) plane parallel to the substrate [14,15]. In this work, the optical properties of Rhodamine B (RB) and Fluored (F) dyes are studied when they are uniformly doped in polystyrene (PS) monodispersed nanospheres. Photonic crystal (PC) matrices are self-assembled by horizontal growth starting from colloidal solutions of the dye-doped nanospheres deposited on glass substrates. We will refer to photonic crystals obtained from RB- and F-doped PS nanoparticles, respectively, as PS–RB– PC and PS–F–PC samples. 2. Experimental 2.1. Photonic crystal synthesis Three-dimensionally ordered photonic crystals are grown using commercially available [Microparticles Gmbh] colloidal solutions
A. Yadav et al. / Optical Materials 35 (2013) 1538–1543
(2.5% w/v aqueous suspension) of dye-doped PS colloidal spheres by horizontal self-assembly on BK7 substrates (Corning glass). The nominal diameter is 306 nm for spheres doped by Rhodamine B (PS–RB) and 288 nm for those doped by Fluored (PS–F). This method provides a face centered cubic (fcc) arrangement to the PC with the (1 1 1) plane oriented parallel to the substrate [16]. This is an inexpensive method and crystals can be grown in ambient conditions. It yields well-ordered crystals within 3 h. The substrates were carefully cleaned in order to improve the wettability of the surface by the colloidal solution and obtain homogenous layers during the crystal growth. The glass substrates (2.5 2.5 cm2) were washed in a mixture containing concentrated sulfuric acid (95–98%) and hydrogen peroxide (40%) (H2SO4:H2O2 = 3:1, volume ratio) for 10 min. Then they were treated in an ultrasonic bath containing ammonium hydroxide solution (40%), hydrogen peroxide and deionized water with a volume ratio of NH4OH:H2O2:H2O = 1:0.6:0.8 for 5 min. Finally, the substrates were immersed in a mixture containing hydrochloric acid (37%), hydrogen peroxide and deionized water (HCl:H2O2:H2O = 1:2:7, volume ratio) for another 5 min. Thereafter, the substrates were washed with copious deionized water and dried in nitrogen gas flow before use [16]. Few drops of colloidal suspension (about 50 ll/cm2) were put on the horizontal substrate and were carefully spread to fully cover the surface. It was observed that the edge of the substrate began to exhibit iridescent color after few minutes and the iridescent color gradually moved from the periphery towards the center of the substrate. The solution is allowed to slowly dry at room temperature in ambient conditions. SEM images taken on gold sputtered surfaces (FEG-SEM, Leo Supra 35) of the two different samples are showed in Fig. 1. The typical close-packing arrangement of the (1 1 1) plane of an fcc lattice is clearly visible. The self-assembly of the spheres gives rise to a well ordered organization. Sphere diameters, as estimated from SEM analysis, are 280 nm and 246 nm respectively for the PS–RB and the PS–F colloids. The dimension of the spheres results 10% reduced from the nominal value. It has to be noted, however, that the two values have been obtained in rather different environmental condition: while the nominal value has been presumably obtained by measurements in aqueous suspension, the SEM measurements (and all the measurements which will be presented herein) are performed on dry samples.
Fig. 2. Sketch of the Bragg-reflection measurement.
repetition rate of 10 Hz and a pulse duration of 6 ns. A half-wave plate (HW) and a Glan-Taylor polarizer prism (P) are used in the path to control the laser power. A frequency doubling crystal thermally stabilized is used to generate the visible light beam for luminescence excitation at 532 nm. The scheme of the experimental set-up is shown in Fig. 3. The sample is fixed at the center of a circular stage which allows the luminescence detection around the crystal virtually over 360°. The laser spot on the sample was approximately 10 mm2 broad. The collected emission was filter by means of a 532 nm-notch filter and coupled to a fiber-bundle with a lens at a distance of 10 cm from the sample in order to achieve an adequate angular resolution. 3. Results 3.1. Reflection measurements We have characterized the deposited PCs by means of Braggtype reflection spectroscopy. Fig. 4 shows the reflection spectra for s-polarized light incident on the (1 1 1) growing surface at different angles for both materials. For increasing values of the incident angle h (with respect to the normal at the sample surface), the reflection peaks are shifted toward the shorter wavelengths. With reference to Fig. 2, the angular dependence can be described by a Bragg’s law which takes the form, for the first order [17].
kmax 2.2. Optical characterization The reflection spectra were performed with a variable angle spectroscopic VASEÒ32 ellipsometer by J.A. Woollam company. A sketch of the Bragg-type reflection measurement is shown in Fig. 2. Absorption spectra at normal incidence were collected by means of a commercial spectrophotometer (Perkin-Elmer k15). The dye fluorescence was optically excited by the second harmonic of the 1064 nm-line of a Q-switched Nd:YAG laser with a
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rffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 D n2eff sin h ¼ 2dð1 1 1Þ neff sin h ¼ 2 3
where kmax indicates the wavelength value of the reflection peak, d(111) the distance between adjacent (1 1 1) lattice planes, D the sphere diameter, h the angle of incidence and neff is the effective refractive index of the structure which can be expressed by
n2eff ¼ ð1 f Þ n2air þ f n2s Here nair and ns represent the refractive index of air and sphere material respectively, while f is the sphere filling fraction, equal to
Fig. 1. SEM image of typical PS–RB PC (left side) and PS–F–PC (right side) samples.
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Fig. 3. Sketch of the experimental set-up used in laser emission studies. HW is a half wave plate, P a Glan-Taylor polarizer, M a mirror, SH a frequency doubling crystal, DM a dichroic mirror, N a 532 nm-notch filter connected, by means of a lens-coupled optical fiber-bundle, to the spectrometer for the luminescence detection.
Absorption spectra at normal incidence were collected by means of a commercial spectrophotometer (Perkin-Elmer k15). The spectra of both samples, after background subtraction, are shown in Fig. 5. The reflection at normal incidence appears as a strong absorption band at wavelength larger than 600 nm. The
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Wavelength (nm) 60
20deg 30deg 40deg 50deg 60deg
PS-F-PC
eff
3.2. Absorption measurements
20
400
k1 k2 1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k1 k2 2 n2 sin2 #
50
Reflectance (%)
where k1 and k2 are the wavelengths corresponding to two subsequent maxima. We estimate a thickness of 4.9 lm, corresponding to 22 layers, for PS–RB–PC. The reflection spectra of the opaline PS–F–PC measured in specular configuration at different angles of incidence are presented in Fig. 4, too. The PS–F–PC sample presents a maximum reflectance of 55% at 574 nm for an incident angle of 20°. The reflection spectrum of the crystal shows the expected shift towards lower wavelength range for larger angles of incidence. The peak reflectance and the FWHM (37 nm) of the reflection spectra remain almost constant when measured with an angle of incidence from 20° to 60°. This indicates a well ordered photonic crystal with few defects. The thickness of the PC layer is estimated from the analysis of the Fabry–Perot fringes to be 3.5 lm, corresponding to 18 layers.
30
10
refractive index of the medium and h is the external angle of incidence. Therefore the PC thickness d can be estimated as:
d¼
20deg 30deg 40deg 50deg 60deg
40 PS-RB-PC
Reflectance (%)
0.74 for an ideal fcc lattice. The above equation allows an estimation of sphere diameters of 267 nm and 234 nm respectively for PS–RB– PC and PS–F–PC in good agreement with SEM measurements. The PS–RB–PC shows a maximum reflectance of 41% at 628 nm when measured at an incident angle of 20°. The peak reflectance and the FWHM (39 nm) of the reflection spectrum remain constant when measured along incident directions from 20° to 60°. This indicates a well ordered photonic crystal with few defects. The Fabry–Perot oscillations on the long wavelength side of the reflection peak occur as a consequence of the interference between the light reflected by the upper and lower interfaces of PC with air and substrate, respectively. The maximum intensity is detected for wavelengths which satisfy the relation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 mk ¼ 2d n2eff sin # ðm ¼ 1; 2; . . . ; NÞ, where neff is the effective
40 30 20 10
400
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600
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Wavelength (nm) Fig. 4. Reflection spectra of PS–RB–PC and PS–F–PC samples.
532 nm excitation line (frequency doubled Nd-YAG laser line) falls in the absorption region for both dyes. 3.3. Fluorescence measurements We studied the fluorescence of PS–RB–PC and PS–F–PC optically excited by the second harmonic of the 1064 nm-line of a Nd:YAG
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2
1
0 500
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Wavelength (nm) 2
1 x10
0 500
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Wavelength (nm) Fig. 5. Absorption spectra of PS–RB–PC and PS–F–PC samples. The peak above 600 nm in both cases is the effect of the reflection at normal incidence. The dashed vertical line indicates the laser excitation wavelength used for the luminescence measurements.
laser. Luminescence spectra at low excitation power have been measured in the range of collection angles from 20° to 50° with respect to the direction of the excitation beam (normal to the
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PS-RB-PC 30deg
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PS-RB-PC 50deg 40
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Fig. 6. Spontaneous emission and reflectance of PS–RB–PC at different angles.
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Reflectance (%)
PS-RB-PC 40deg
Reflectance(%)
Emission Intensity (arb.units)
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sample). The spectra relative to PS–RB–PC and PS–F–PC are reported, respectively, in Figs. 6 and 7 together with the reflection spectra at the same angles for reference. By varying the angle we observed the appearance of a dip in the emission spectra in the region where the dye emission and the photonic stop-band overlap. The position of the dip in the emission spectrum exactly matches that of the peak in the reflection spectrum. Within the stop-band, there are no photonic states available for the propagation of the light. The emission cannot emerge from the crystal at such angle and thus the spontaneous emission is inhibited in correspondence of the reflection range. For PS–RB–PC, we chose to fix the position of the detector at an angle of 32° in a direction where there is a partial overlap between the RB-dye spontaneous emission and the stop-band. We studied the luminescence band at increasing pump power as reported in Fig. 8a. The red line in the figure represents the peak position of the reflection spectrum at an angle of 32°. A narrow emission band emerges in the long-wavelength side of the band on top of the normal (low power) emission lineshape as the excitation power is increased. We extrapolated the narrow band lineshape subtracting the low power lineshape from the total luminescence band at all the excitation powers. The peak intensity and the corresponding full width at half maximum (FWHM) of the extrapolated narrow band as a function of the pump power are plotted in Fig. 8b. From both plots, it clearly shows up the onset of a nonlinear response for pump powers higher than about 150 mW (35 MW/cm2). For the case of PS–F–PC, we measured luminescence spectra at increasing pump power with a fixed detector angle position of 22°. The appearance of a narrow peak in the long wavelength side of the luminescence band is observed when the pump power is increased as shown in Fig. 9a. The sharp peak is centered at 574 nm, on the short wavelength side of the stop-band region for that value of the collection angle. The FWHM of the spectrum reduces from 15 nm to 11.5 nm for increasing pump power. The peak value and the FWHM of the emission band are shown in Fig. 9b for PS– F–PC. A power threshold at about 120 mW (28 MW/cm2) is present in both plots.
Emission Intensity (arb.units)
Absorbance (OD)
PS-RB
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Wavelength (nm)
Fig. 7. Spontaneous emission and reflectance of PS–F–PC at different angles.
Emission Intensity (arb.units)
5 12 mW 51 mW 128 mW 214 mW 230 mW 250 mW
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The appearance of a narrow peak in the stimulated emission spectrum in this type of samples has been attributed, by some authors [11], to the photonic crystal lasing originating from multiple reflections in the PC structure along a particular lattice direction. In the vast majority of cases it is expected that the lasing wavelength corresponds to the stop-band edges due to the enhanced density of allowed states. However also lasing wavelength occurring within the photonic stop band, which does not necessarily correspond to stop-band edges nor to the PL-peak, is reported by Kedia et al. [18], which shows a specific dependence of photonic crystal lasing on the dye emission cross-section. Laser feedback from a scattering material containing fluorescent emitters can be produced also by a different mechanism, the so called random laser effect. In this case it may yield light amplification by random multiple light scattering in high gain disordered structures with low refractive index. The light that is generated by fluorescence and propagates in a scattering material, makes a long random walk before it can leave the medium and can be significantly amplified in between the scattering events. Unlike the ordinary lasers, the resulting light emission is multidirectional and spectrally broad, due to the superposition of many independent lasing lines within the gain frequency windows [19]. Moreover such mechanism appears at higher laser threshold respect to photonic crystal lasing [20]. In our case we observe a spectral narrowing in the emission of PC for both dyes with a threshold lower than that observed with similar dye loadings of 0.09 wt.% [18]. The narrowing process was observed only for specific angles (22° and 32° for PS–F–PC and PS–RB–PC, respectively) where the reflection bands of the PC overlap the high wavelength side of the emission bands. The effect was very angle-dependent since changing the angles by few degrees around the indicated values and maintaining the excitation power light at the proper level the effect disappears rapidly. In addition in our case the narrowing closely corresponds with the short-wavelength side of the photonic stop-band, where an enhanced DOS is expected [1]. On the basis of the low lasing
threshold, the narrow spectral width and the clear connection with the reflection band for both PS–F–PC and PS–RB–PC we can conclude that the observed behavior is a photonic crystal lasing.
Peak Intensity (arb.units)
4. Discussion
0 0
25
50
Power Density (MW/cm2) Fig. 8. (a) Emission spectrum of PS–RB–PC at 32° for different power. The straight line (red line) indicates the peak position of the reflection spectrum of the crystal at 32°. (b) Normalized peak intensity and full width at half maximum (FWHM) of the narrow band versus the pump power. The line connecting the experimental points is a guide for the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Emission Intensity (arb.units)
A. Yadav et al. / Optical Materials 35 (2013) 1538–1543
which is almost constant within the investigated angular range. We studied the spontaneous emission of the embedded dye along different directions of the crystal. Within the stop-band spectral region, the spontaneous emission of the dyes is inhibited resulting in the appearance of a dip in the emission spectrum at low pump power whose position depends on the angle of luminescence collection. By increasing the pump power and measuring the emission at a fixed angle respect to sample normal (of 32° for the PS–RB and 22° for the PS–F PCs) where the dye emission overlaps the stopband we observe a spectral narrowing. The sharp peak occurs at a specific wavelength of 601 and 574 nm for the PS–RB–PC and PS–F–PC, respectively, where the photonic crystal density of states and the dye emission efficiency combine to yield lasing effect.
16 mW 45 mW 116 mW 172 mW 257 mW 328 mW
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Acknowledgment
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(b)
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1 10 0 0
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Power Density (MW/cm2) Fig. 9. (a) Emission spectrum of PS–F–PC at 22° for different power. The straight line (red line) indicates the peak position of the reflection spectrum of the crystal at 22°. (b) Normalized peak intensity at 574 nm versus the pump power. The line connecting the experimental points is a guide for the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions We have studied the spectral properties of two types of active PC matrix fabricated by horizontal self-assembly of dye doped polystyrene colloids in ambient conditions. We used nanospheres doped with Rhodamine-B and Fluored dyes with a diameter of 280 nm and 246 nm respectively, as determined by SEM observation and Bragg reflection analysis. PC quality has been characterized by means of variable angle reflection spectroscopy and resulted in good quality PC with reflectance of 40% (PS–RB–PC) and 55% (PS–F–PC) at the maximum of the photonic stop-band
This work was supported by the Italian CARIPLO foundation through the Project Number 2010-0525. References [1] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [2] S. John, Phys. Rev. Lett. 58 (1987) 2486. [3] D.J. Norris, E.G. Arlinghaus, L.L. Meng, R. Heiny, L.E. Scriven, Adv. Mater. 16 (2004) 1393–1399. [4] O. Painter, R.K. Lee, A. Scherer, A. Yariv, J.D.O. Brien, P.D. Dapkus, I. Kim, Science 284 (1999) (1819). [5] J.D. Joannopoulos, J.N. Winn, R.D. Meade, Photonic crystals: molding the flow of light, second ed., Princeton University Press, Princeton, 2008. [6] I. Venditti, I. Fratoddi, C. Palazzesi, P. Prosposito, M. Casalboni, C. Cametti, C. Battocchio, G. Polzonetti, M.V. Russo, J. Colloid. Int. Sci. 348 (2010) 424–430. [7] G. Pan, R. Kesavamoorthy, S.A. Asher, Phys. Rev. Lett. 78 (1997) 3860–3863. [8] K. Vynck, D. Cassagne, E. Centeno, Opt. Express 14 (2006) 6668–6674. [9] F. Marlow, M. Muldarisnur, P. Sharifi, R. Brinkmann, C. Mendive, Angew. Chem. Int. Ed. 48 (2009) 6212. [10] S.H. Kim, S. Kim, W.C. Jeong, S. Yang, Chem. Mater. 21 (2009) 4993–4999. [11] R.V. Nair, A.K. Tiwari, S. Mujumdar, B.N. Jagatap, Phys. Rev. A 85 (2012) 023844. [12] S. John, T. Quang, Phys. Rev. A 50 (1994) 1764. [13] J.F. Galisteo, F. Garcia-Santamaria, D. Golmayo, B.H. Jurez, C. Lopez, E. Palacious, J. Opt. A: Pure Appl. Opt. 7 (2005) S244. [14] I.I. Tarhan, G.H. Watson, Phys. Rev. Lett. 76–2 (1996) 315–318. [15] S. Schutzmann, I. Venditti, P. Prosposito, M. Casalboni, M.V. Russo, Opt. Express 16 (2) (2008) 897. [16] Q. Yan, Z. Zhou, X.S. Zhao, Langmuir 21 (2005) 3158–3164. [17] S. Schutzmann, P. Prosposito, M. Casalboni, I. Venditti, M.V. Russo, Phys. Stat. Sol. C 5 (5) (2008) 1403. [18] S. Kedia, R. Vijaya, A.K. Ray, S. Sinha, J. Nanophotonics 4 (2010) 049506. [19] S. Gottardo, R. Sapienza, P.D. Garcia, A. Blanco, D.S. Wiersma, C. Lopez, Nature Photonics 2 (7) (2008) 429–432. [20] M.N. Shkunov, Z.V. Vardeny, M.C. DeLong, R.C. Polson, A.A. Zakhidov, R.H. Baughman, Adv. Funct. Mater. 12 (2002) 21–26.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Spectral properties of self-assembled polystyrene nanospheres photonic crystals doped with luminescent dyes A. Yadav a, R. De Angelis a, M. Casalboni a, F. De Matteis a,⇑, P. Prosposito a, F. Nanni b, I. Cacciotti b a b
Physics Department and INSTM, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy Industrial Engineering Department and INSTM, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy
a r t i c l e
i n f o
Article history: Received 17 January 2013 Accepted 21 March 2013 Available online 13 April 2013 Keywords: Photonic crystals Polymeric colloids Self-assembly Light emission Lasing
a b s t r a c t Three-dimensional ordered photonic crystals have been fabricated on solid substrates with a self-assembling method starting from a suspension of dye-doped polymeric nanospheres in water. These photonic crystals showed angle-dependent stop band for light transmission and, correspondingly, Bragg reflection peak due to the photonic crystal lattice. Polystyrene nanoparticles of 306 nm and 288 nm diameter, respectively in the case of the Rhodamine B and Fluored dye, were used to obtain self-assembled photonic crystals. They show 40% reflectance at 610 nm for Rhodamine B and 35% reflectance at 574 nm for Fluored doped materials. The size of the spheres calculated by Bragg’s law are in good agreement with those evaluated by scanning electron microscopy (SEM). The appropriate choice of sphere diameters results in an overlap between the photonic stop-band and the dye emission spectrum. The photonic crystals showed angle-dependent suppression of spontaneous emission of the dye in the wavelength range of the photonic stop band and enhancement near the band edge. In reflection geometries, spectral narrowing and directional emission, all indicative of stimulated emission, were observed from the active photonic crystal matrix. The results of laser induced emission studies on the dye doped photonic crystals are presented. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Photonic crystals (PCs) [1–3] are optical materials for which the dielectric constant undergoes a spatial periodic variation with a period comparable to the wavelength of light. They are currently attracting much attention for their possible applications in optoelectronics, optical sensors, etc. [4–6]. The most striking property of such crystals is the appearance of wavelength ranges in which the propagation of light is forbidden, the so called photonic stopbands. By means of proper band-gap engineering these materials can strongly confine and control the propagation of light and thus they are appealing for a wide range of applications in optoelectronics [7,8] among which the possibility to produce low-threshold lasers [9,10]. The rate of spontaneous emission depends on the density of allowed photonic states which can be strongly affected by the periodic modulation in the PC. When an embedded emitter (can be a dye molecule, a quantum dot or a rare earth ion) which absorbs light out of the photonic stop-band, presents an emission which overlaps with the stop-band, the spontaneous emission from the emitter is inhibited in the stop band region due to the lowered density of allowed photon states, while it is enhanced at the band edge which presents instead a higher density of states ⇑ Corresponding author. Tel.: +39 0672594521. E-mail address: [email protected] (F. De Matteis). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.03.020
[11]. In some systems, a tunable spectral narrowing with a sharp power threshold and a highly directional emission can be observed when the excitation energy density is increased [12]. Among the different techniques used to synthesize three dimensional PCs, self-assembly from monodispersed colloidal spheres is receiving great attention due to the ease of fabrication [13]. This technique is based on the well-known property of monodispersed sub-micrometric spheres to self-organize into a facecentered-cubic (fcc) lattice with the (1 1 1) plane parallel to the substrate [14,15]. In this work, the optical properties of Rhodamine B (RB) and Fluored (F) dyes are studied when they are uniformly doped in polystyrene (PS) monodispersed nanospheres. Photonic crystal (PC) matrices are self-assembled by horizontal growth starting from colloidal solutions of the dye-doped nanospheres deposited on glass substrates. We will refer to photonic crystals obtained from RB- and F-doped PS nanoparticles, respectively, as PS–RB– PC and PS–F–PC samples. 2. Experimental 2.1. Photonic crystal synthesis Three-dimensionally ordered photonic crystals are grown using commercially available [Microparticles Gmbh] colloidal solutions
A. Yadav et al. / Optical Materials 35 (2013) 1538–1543
(2.5% w/v aqueous suspension) of dye-doped PS colloidal spheres by horizontal self-assembly on BK7 substrates (Corning glass). The nominal diameter is 306 nm for spheres doped by Rhodamine B (PS–RB) and 288 nm for those doped by Fluored (PS–F). This method provides a face centered cubic (fcc) arrangement to the PC with the (1 1 1) plane oriented parallel to the substrate [16]. This is an inexpensive method and crystals can be grown in ambient conditions. It yields well-ordered crystals within 3 h. The substrates were carefully cleaned in order to improve the wettability of the surface by the colloidal solution and obtain homogenous layers during the crystal growth. The glass substrates (2.5 2.5 cm2) were washed in a mixture containing concentrated sulfuric acid (95–98%) and hydrogen peroxide (40%) (H2SO4:H2O2 = 3:1, volume ratio) for 10 min. Then they were treated in an ultrasonic bath containing ammonium hydroxide solution (40%), hydrogen peroxide and deionized water with a volume ratio of NH4OH:H2O2:H2O = 1:0.6:0.8 for 5 min. Finally, the substrates were immersed in a mixture containing hydrochloric acid (37%), hydrogen peroxide and deionized water (HCl:H2O2:H2O = 1:2:7, volume ratio) for another 5 min. Thereafter, the substrates were washed with copious deionized water and dried in nitrogen gas flow before use [16]. Few drops of colloidal suspension (about 50 ll/cm2) were put on the horizontal substrate and were carefully spread to fully cover the surface. It was observed that the edge of the substrate began to exhibit iridescent color after few minutes and the iridescent color gradually moved from the periphery towards the center of the substrate. The solution is allowed to slowly dry at room temperature in ambient conditions. SEM images taken on gold sputtered surfaces (FEG-SEM, Leo Supra 35) of the two different samples are showed in Fig. 1. The typical close-packing arrangement of the (1 1 1) plane of an fcc lattice is clearly visible. The self-assembly of the spheres gives rise to a well ordered organization. Sphere diameters, as estimated from SEM analysis, are 280 nm and 246 nm respectively for the PS–RB and the PS–F colloids. The dimension of the spheres results 10% reduced from the nominal value. It has to be noted, however, that the two values have been obtained in rather different environmental condition: while the nominal value has been presumably obtained by measurements in aqueous suspension, the SEM measurements (and all the measurements which will be presented herein) are performed on dry samples.
Fig. 2. Sketch of the Bragg-reflection measurement.
repetition rate of 10 Hz and a pulse duration of 6 ns. A half-wave plate (HW) and a Glan-Taylor polarizer prism (P) are used in the path to control the laser power. A frequency doubling crystal thermally stabilized is used to generate the visible light beam for luminescence excitation at 532 nm. The scheme of the experimental set-up is shown in Fig. 3. The sample is fixed at the center of a circular stage which allows the luminescence detection around the crystal virtually over 360°. The laser spot on the sample was approximately 10 mm2 broad. The collected emission was filter by means of a 532 nm-notch filter and coupled to a fiber-bundle with a lens at a distance of 10 cm from the sample in order to achieve an adequate angular resolution. 3. Results 3.1. Reflection measurements We have characterized the deposited PCs by means of Braggtype reflection spectroscopy. Fig. 4 shows the reflection spectra for s-polarized light incident on the (1 1 1) growing surface at different angles for both materials. For increasing values of the incident angle h (with respect to the normal at the sample surface), the reflection peaks are shifted toward the shorter wavelengths. With reference to Fig. 2, the angular dependence can be described by a Bragg’s law which takes the form, for the first order [17].
kmax 2.2. Optical characterization The reflection spectra were performed with a variable angle spectroscopic VASEÒ32 ellipsometer by J.A. Woollam company. A sketch of the Bragg-type reflection measurement is shown in Fig. 2. Absorption spectra at normal incidence were collected by means of a commercial spectrophotometer (Perkin-Elmer k15). The dye fluorescence was optically excited by the second harmonic of the 1064 nm-line of a Q-switched Nd:YAG laser with a
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rffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 D n2eff sin h ¼ 2dð1 1 1Þ neff sin h ¼ 2 3
where kmax indicates the wavelength value of the reflection peak, d(111) the distance between adjacent (1 1 1) lattice planes, D the sphere diameter, h the angle of incidence and neff is the effective refractive index of the structure which can be expressed by
n2eff ¼ ð1 f Þ n2air þ f n2s Here nair and ns represent the refractive index of air and sphere material respectively, while f is the sphere filling fraction, equal to
Fig. 1. SEM image of typical PS–RB PC (left side) and PS–F–PC (right side) samples.
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Fig. 3. Sketch of the experimental set-up used in laser emission studies. HW is a half wave plate, P a Glan-Taylor polarizer, M a mirror, SH a frequency doubling crystal, DM a dichroic mirror, N a 532 nm-notch filter connected, by means of a lens-coupled optical fiber-bundle, to the spectrometer for the luminescence detection.
Absorption spectra at normal incidence were collected by means of a commercial spectrophotometer (Perkin-Elmer k15). The spectra of both samples, after background subtraction, are shown in Fig. 5. The reflection at normal incidence appears as a strong absorption band at wavelength larger than 600 nm. The
500
600
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Wavelength (nm) 60
20deg 30deg 40deg 50deg 60deg
PS-F-PC
eff
3.2. Absorption measurements
20
400
k1 k2 1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k1 k2 2 n2 sin2 #
50
Reflectance (%)
where k1 and k2 are the wavelengths corresponding to two subsequent maxima. We estimate a thickness of 4.9 lm, corresponding to 22 layers, for PS–RB–PC. The reflection spectra of the opaline PS–F–PC measured in specular configuration at different angles of incidence are presented in Fig. 4, too. The PS–F–PC sample presents a maximum reflectance of 55% at 574 nm for an incident angle of 20°. The reflection spectrum of the crystal shows the expected shift towards lower wavelength range for larger angles of incidence. The peak reflectance and the FWHM (37 nm) of the reflection spectra remain almost constant when measured with an angle of incidence from 20° to 60°. This indicates a well ordered photonic crystal with few defects. The thickness of the PC layer is estimated from the analysis of the Fabry–Perot fringes to be 3.5 lm, corresponding to 18 layers.
30
10
refractive index of the medium and h is the external angle of incidence. Therefore the PC thickness d can be estimated as:
d¼
20deg 30deg 40deg 50deg 60deg
40 PS-RB-PC
Reflectance (%)
0.74 for an ideal fcc lattice. The above equation allows an estimation of sphere diameters of 267 nm and 234 nm respectively for PS–RB– PC and PS–F–PC in good agreement with SEM measurements. The PS–RB–PC shows a maximum reflectance of 41% at 628 nm when measured at an incident angle of 20°. The peak reflectance and the FWHM (39 nm) of the reflection spectrum remain constant when measured along incident directions from 20° to 60°. This indicates a well ordered photonic crystal with few defects. The Fabry–Perot oscillations on the long wavelength side of the reflection peak occur as a consequence of the interference between the light reflected by the upper and lower interfaces of PC with air and substrate, respectively. The maximum intensity is detected for wavelengths which satisfy the relation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 mk ¼ 2d n2eff sin # ðm ¼ 1; 2; . . . ; NÞ, where neff is the effective
40 30 20 10
400
500
600
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Wavelength (nm) Fig. 4. Reflection spectra of PS–RB–PC and PS–F–PC samples.
532 nm excitation line (frequency doubled Nd-YAG laser line) falls in the absorption region for both dyes. 3.3. Fluorescence measurements We studied the fluorescence of PS–RB–PC and PS–F–PC optically excited by the second harmonic of the 1064 nm-line of a Nd:YAG
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2
1
0 500
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Wavelength (nm) 2
1 x10
0 500
600
700
800
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Wavelength (nm) Fig. 5. Absorption spectra of PS–RB–PC and PS–F–PC samples. The peak above 600 nm in both cases is the effect of the reflection at normal incidence. The dashed vertical line indicates the laser excitation wavelength used for the luminescence measurements.
laser. Luminescence spectra at low excitation power have been measured in the range of collection angles from 20° to 50° with respect to the direction of the excitation beam (normal to the
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PS-RB-PC 30deg
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PS-RB-PC 50deg 40
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Fig. 6. Spontaneous emission and reflectance of PS–RB–PC at different angles.
800
Reflectance (%)
PS-RB-PC 40deg
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Absorbance (OD)
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sample). The spectra relative to PS–RB–PC and PS–F–PC are reported, respectively, in Figs. 6 and 7 together with the reflection spectra at the same angles for reference. By varying the angle we observed the appearance of a dip in the emission spectra in the region where the dye emission and the photonic stop-band overlap. The position of the dip in the emission spectrum exactly matches that of the peak in the reflection spectrum. Within the stop-band, there are no photonic states available for the propagation of the light. The emission cannot emerge from the crystal at such angle and thus the spontaneous emission is inhibited in correspondence of the reflection range. For PS–RB–PC, we chose to fix the position of the detector at an angle of 32° in a direction where there is a partial overlap between the RB-dye spontaneous emission and the stop-band. We studied the luminescence band at increasing pump power as reported in Fig. 8a. The red line in the figure represents the peak position of the reflection spectrum at an angle of 32°. A narrow emission band emerges in the long-wavelength side of the band on top of the normal (low power) emission lineshape as the excitation power is increased. We extrapolated the narrow band lineshape subtracting the low power lineshape from the total luminescence band at all the excitation powers. The peak intensity and the corresponding full width at half maximum (FWHM) of the extrapolated narrow band as a function of the pump power are plotted in Fig. 8b. From both plots, it clearly shows up the onset of a nonlinear response for pump powers higher than about 150 mW (35 MW/cm2). For the case of PS–F–PC, we measured luminescence spectra at increasing pump power with a fixed detector angle position of 22°. The appearance of a narrow peak in the long wavelength side of the luminescence band is observed when the pump power is increased as shown in Fig. 9a. The sharp peak is centered at 574 nm, on the short wavelength side of the stop-band region for that value of the collection angle. The FWHM of the spectrum reduces from 15 nm to 11.5 nm for increasing pump power. The peak value and the FWHM of the emission band are shown in Fig. 9b for PS– F–PC. A power threshold at about 120 mW (28 MW/cm2) is present in both plots.
Emission Intensity (arb.units)
Absorbance (OD)
PS-RB
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Emission Intensity (arb. units)
Emission Intensity (arb. units)
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Wavelength (nm)
Fig. 7. Spontaneous emission and reflectance of PS–F–PC at different angles.
Emission Intensity (arb.units)
5 12 mW 51 mW 128 mW 214 mW 230 mW 250 mW
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Wavelength (nm) 3
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(b)
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FWHM (nm)
The appearance of a narrow peak in the stimulated emission spectrum in this type of samples has been attributed, by some authors [11], to the photonic crystal lasing originating from multiple reflections in the PC structure along a particular lattice direction. In the vast majority of cases it is expected that the lasing wavelength corresponds to the stop-band edges due to the enhanced density of allowed states. However also lasing wavelength occurring within the photonic stop band, which does not necessarily correspond to stop-band edges nor to the PL-peak, is reported by Kedia et al. [18], which shows a specific dependence of photonic crystal lasing on the dye emission cross-section. Laser feedback from a scattering material containing fluorescent emitters can be produced also by a different mechanism, the so called random laser effect. In this case it may yield light amplification by random multiple light scattering in high gain disordered structures with low refractive index. The light that is generated by fluorescence and propagates in a scattering material, makes a long random walk before it can leave the medium and can be significantly amplified in between the scattering events. Unlike the ordinary lasers, the resulting light emission is multidirectional and spectrally broad, due to the superposition of many independent lasing lines within the gain frequency windows [19]. Moreover such mechanism appears at higher laser threshold respect to photonic crystal lasing [20]. In our case we observe a spectral narrowing in the emission of PC for both dyes with a threshold lower than that observed with similar dye loadings of 0.09 wt.% [18]. The narrowing process was observed only for specific angles (22° and 32° for PS–F–PC and PS–RB–PC, respectively) where the reflection bands of the PC overlap the high wavelength side of the emission bands. The effect was very angle-dependent since changing the angles by few degrees around the indicated values and maintaining the excitation power light at the proper level the effect disappears rapidly. In addition in our case the narrowing closely corresponds with the short-wavelength side of the photonic stop-band, where an enhanced DOS is expected [1]. On the basis of the low lasing
threshold, the narrow spectral width and the clear connection with the reflection band for both PS–F–PC and PS–RB–PC we can conclude that the observed behavior is a photonic crystal lasing.
Peak Intensity (arb.units)
4. Discussion
0 0
25
50
Power Density (MW/cm2) Fig. 8. (a) Emission spectrum of PS–RB–PC at 32° for different power. The straight line (red line) indicates the peak position of the reflection spectrum of the crystal at 32°. (b) Normalized peak intensity and full width at half maximum (FWHM) of the narrow band versus the pump power. The line connecting the experimental points is a guide for the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Emission Intensity (arb.units)
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which is almost constant within the investigated angular range. We studied the spontaneous emission of the embedded dye along different directions of the crystal. Within the stop-band spectral region, the spontaneous emission of the dyes is inhibited resulting in the appearance of a dip in the emission spectrum at low pump power whose position depends on the angle of luminescence collection. By increasing the pump power and measuring the emission at a fixed angle respect to sample normal (of 32° for the PS–RB and 22° for the PS–F PCs) where the dye emission overlaps the stopband we observe a spectral narrowing. The sharp peak occurs at a specific wavelength of 601 and 574 nm for the PS–RB–PC and PS–F–PC, respectively, where the photonic crystal density of states and the dye emission efficiency combine to yield lasing effect.
16 mW 45 mW 116 mW 172 mW 257 mW 328 mW
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Acknowledgment
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(b)
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1 10 0 0
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75
Power Density (MW/cm2) Fig. 9. (a) Emission spectrum of PS–F–PC at 22° for different power. The straight line (red line) indicates the peak position of the reflection spectrum of the crystal at 22°. (b) Normalized peak intensity at 574 nm versus the pump power. The line connecting the experimental points is a guide for the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions We have studied the spectral properties of two types of active PC matrix fabricated by horizontal self-assembly of dye doped polystyrene colloids in ambient conditions. We used nanospheres doped with Rhodamine-B and Fluored dyes with a diameter of 280 nm and 246 nm respectively, as determined by SEM observation and Bragg reflection analysis. PC quality has been characterized by means of variable angle reflection spectroscopy and resulted in good quality PC with reflectance of 40% (PS–RB–PC) and 55% (PS–F–PC) at the maximum of the photonic stop-band
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