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Coherent control of spontaneous emission by photonic crystals
Chemical Physics Letters 444 (2007) 287–291 www.elsevier.com/locate/cplett
Coherent control of spontaneous emission by photonic crystals Mingzhu Li, Andong Xia, Jingxia Wang, Yanlin Song *, Lei Jiang Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 9 February 2007; in final form 29 June 2007 Available online 12 July 2007
Abstract Rhodamine B (RB) doped photonic crystals (PCs) (RBPCs) are fabricated by introducing different concentrations of RB into PCs during the deposition procedure. Photoluminescence (PL) spectra of RBPCs show strong dependence on the dye concentrations and the corresponding stopbands of the PCs. Additionally, an obvious narrowing and enhancement of the emission are achieved with the increasing excitation intensity. These results indicate that a suitable combination of organic dye and the PC would offer a promising route towards low-threshold minilasers, light-emitting diodes and other optical applications. 2007 Elsevier B.V. All rights reserved.
1. Introduction Spontaneous emission is a fundamental process resulting from the interaction between radiation and matter, and its effective control is essential for diverse applications ranging from miniature lasers, light-emitting diodes to solar energy harvesting. The application performance of spontaneous emission depends on both the excited atomic system and the utilization environment [1,2]. Recently, increasing attention has been paid to PCs, because of their great potentials for manipulating the emission properties of internal light sources [3,4]. Particularly, more and more attention has been given to colloidal crystals, because of their facile and economical strategy in fabrication and design of three-dimensional PCs with feature sizes of the light’s wavelength [5–8]. Furthermore, the stopbands of colloidal crystals possess highly anisotropic and wavelength-dependent dispersion properties, which would produce some interesting phenomena even in the absence of a complete photonic band gap [9–11]. In this case, a few strategies have been reported to study the manipulation of the emission of fluorophore by PCs via the combination of dye and colloidal crystals [12,13,6].
*
Corresponding author. Fax: +86 10 62562935. E-mail address: [email protected] (Y. Song).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.07.020
Especially, Clays and co-workers [14,15] have successfully fabricated a nano-engineering sandwich-like photonic structure and photonic superlattice, which clearly demonstrates the effect of well-designed bandgap on the spontaneous emission of fluorophore. The methods offer an effective way to study the manipulation of spontaneous emission by various PC superlattices. Meanwhile, the concentration of dye is another crucial factor for the dye emission performance [1,2]. However, few literatures have explored the effect of dye concentration on the spontaneous emission of organic dye doped PCs. Here both effects of dye concentrations and stopbands of the PCs on the fluorescence emission were systematically investigated. RBPCs was fabricated by introducing different concentrations of RB into poly(styrene-methyl methacrylateacrylic acid) (P(St-MMA-AA)) PCs during the deposition stage. The manipulation of RB emission is systematically investigated by modifying the relative position between the PC stopband and RB emission peak. It is found that both the RB concentration dispersed in PC and the relative position between the stopband and RB emission peak play a key role in the emission of RBPCs. Furthermore, the effective enhancement and narrowing of the emission peak are observed with increasing excitation intensity. These phenomena demonstrate that PC can provide an efficient coherent feedback in the opal matrix of the periodic structure, which indicate that suitable combination of organic
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M. Li et al. / Chemical Physics Letters 444 (2007) 287–291
dye and PC can offer a promising route towards highly efficient optical applications such as low-threshold minilasers, light-emitting diodes and other optical devices. Additionally, systematic studies on the fundamental properties of these combinative materials also provide a reasonable strategy to investigate the spontaneous emission properties of atoms or molecules in three-dimensional (3D) PCs. 2. Experimental Monodispersed P(St-MMA-AA) latex spheres with diameters of 270 nm and 280 nm were prepared via batch emulsion polymerization following our previous literature [16]. The films of RB doped P(St-MMA-AA) PCs were fabricated by vertical deposition [17] from the mixture of P(StMMA-AA) latex spheres suspension with volume fraction of 0.15% and RB aqueous solution at the constant temperature of 60 C and humidity of 60%, respectively. The control sample was obtained by heating as-prepared film at the temperature higher than its Tg (112 C). UV–visible spectra were acquired by Hitachi UV-4100 spectrophotometer. The scanning electron microscope (SEM) images were obtained with a Hitachi S-4700 field emission scanning electron microscope at 3.0 kV. The size distribution of the P(St-MMA-AA) latex spheres was detected by ZetaPALS BI-90plus (Brookhaven Instrument). The laser pulses (7 ns, 1 Hz) with the wavelength of 532 nm, obtained from a frequency tripled Nd:YAG laser (New Wave, Tempest 300, USA), were used as the excitation source. PL spectra were obtained by a polychromator (Spectropro 550i, Acton) equipped with a liquidnitrogen cooled ( 113 C) CCD (charged-coupled device) camera (1340 · 400 pixels) detector (SPEC-10-400B/LN, Roper Scientific). And the collecting PL was focused onto the entrance slit of the polychromator. With grating (150 grooves/mm) and 10 lm slit, a spectral resolution of 1.2 nm can be obtained. The integration time was 20 s. 3. Results and discussion To evaluate the effect of different stopbands on RB emission, RBPCs are fabricated from different P(St-MMA-AA) latex spheres (270 nm – RBPCI and 280 nm – RBPCII in diameter) by vertical deposition method [18], respectively. The samples are assembled from the mixture of RB solution with different concentrations from 10 4 M to 5 · 10 6 M and monodispersed P(St-MMA-AA) latex spheres suspension (0.15%). Fig. 1 shows the transmission spectra of the P(St-MMAAA) PCs with different diameters and the emission spectrum of RB. As for the as-prepared colloidal crystals, the relative stop bandwidth of Dk/k0 are 6%, where Dk is the full width at half-minimum of the dip and the k0 is the center wavelength of the dip. This value is in accordance with the theoretical calculation result for a face-centered cubic crystal [19]. Additionally, the dips of transmission spectra are less than 5%. Aforementioned results further confirm
Fig. 1. Transmission spectra of the P(St-MMA-AA) PCs with sphere diameters of 270 (dotted line) and 280 nm (dashed line) together with the emission spectrum of RB (solid line). The corresponding sharp dip positions are at 617 and 652 nm, respectively measured with a light incident along the normal surface ((1 1 1) direction).
the good crystalline quality of as-prepared colloid crystals as shown in Fig. 2a, b. The deep, sharp dips in the transmission spectra predict sufficiently strong stopbands in period dielectric structures of the as-prepared colloidal crystals [20,21]. As shown in Fig. 1, the stopband of RBPCI (d = 270 nm) is situated at 617 nm (dotted line) and that of RBPCII (dashed line) takes red-shifting to 652 nm due to larger spheres (d = 280 nm), which are packed with larger lattice constants. The optical properties of as-prepared RBPCs lie on the well-order lattice structure resulting from the good monodispersity of latex spheres (Fig. 2c) and suitable RB concentration (Fig. 2d). As shown in Fig. 2c, the diameter of as-prepared P(St-MMA-AA) latex spheres is 279.3 nm with size distribution of 1.8%. Obviously, the diameters of latex spheres in water suspension are bigger than that (270.0 nm) measured in the films, which could be attributed to the separate swollen and shrunk states for latex spheres in the water and dry film. Meanwhile, RB concentration must be kept low enough to weaken the disturbance to the self-assembly of the latex spheres. As shown in Fig. 2d the well-order films could not be obtained when RB concentration is higher than 10 4 M. In our case, the RBPCs are prepared from the mixture solution of RB (5 · 10 5 M or 5 · 10 6 M, respectively) and P(St-MMA-AA) suspension (0.15%). As a result, these P(St-MMA-AA) latex spheres are crystallized into a cubic closed-packed lattice, with its (1 1 1) plane oriented parallel to the surface of the substrates (Fig. 2a, b). The RB molecules would be absorbed either on the surface of the P(St-MMA-AA) latex spheres or in interstitial void via the solution evaporation. Dye concentration is crucial factor for the dye emission performance [2]. In this work, we compare the effects of two different concentrations of RB in as-prepared RBPCs, 5 · 10 6 M (RBPCI) and 5 · 10 5 M (RBPCIII), respectively (Fig. 3). It is clear that higher RB concentration
M. Li et al. / Chemical Physics Letters 444 (2007) 287–291
289
Fig. 2. (a) Typical SEM top view of a sample self-assembled from the mixture of P(St-MMA-AA) latex spheres (270 nm in diameter) suspension with volume fraction of 0.15% and RB aqueous solution (5 · 10 6 M), (b) typical SEM side view of the same sample. Each facet of the PC has high degree of order over large areas, (c) hydrodynamic diameter and distribution of the latex spheres in water suspensions, measured by dynamic light scattering and (d) typical SEM top view of the sample fabricated from the mixture of P(St-MMA-AA) latex spheres suspension with volume fraction of 0.15% and RB aqueous solution (10 4 M).
Fig. 3. PL spectra of RBPCI (dotted line) and RBPCIII (dot-dashed line), respectively together with the transmission spectrum of RBPCI (solid line).
results in a blurring of the intensity dip and red-shift of the emission peak. As for RBPCIII, there is only a moderate attenuation in the emission spectrum at the wavelength of 630 nm, and the peak of the fluorescence (603 nm) takes red shift of ca. 11 nm compared to that of the control sample (592 nm) assigned to the aggregation of RB molecules. When the concentration of RB molecules dispersed in the latex solution is high, a great deal of RB molecules are
aggregated on the surface layer of the P(St-MMA-AA) PC during the solution evaporation. The emission light of these RB molecules contributes more substantially to the total fluorescence intensity, which is not affected by the 3D structure of the P(St-MMA-AA) PC. On the contrary, an obvious dip can be observed at the wavelength of 621 nm in the emission spectrum of RBPCI, which is in consistent with the position of the stopband of RBPCI (617 nm). The dip indicates the PL suppression inside the stopband [22]. RBPCI shows a drastically narrow spectrum, situated at the stopband edge around 588 nm. Its full width at half maximum (FWHM) is 29 nm, ca. 27 nm narrower than that of RBPCIII (FWHM = 56 nm). The effect can be ascribed to the spread configurations of the photonic density of state in PC, partial inhibition at the band gap [23,24]. Similarly, the stopbands of PCs also play a key role on emission properties of dye doped PCs in the aspects of the position and shape of emission peak [10]. Herein, the effects of different stopbands on the PL emission of RBPCs are investigated in Fig. 4. It has been theoretically demonstrated that the local density of optical states is reduced within the stopband and strongly enhanced near the band edge frequencies [23]. Such changes in the local density of optical states should influence the radiative rate of internal emitters [25], because the spontaneous emission rate of an atom or molecule depends on the electromagnetic mode
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M. Li et al. / Chemical Physics Letters 444 (2007) 287–291
The FWHM of 15 nm can be obtained when the excitation intensity approaches the maximum, which is ca. 33 nm narrower than that of the control sample (solid line in Fig. 4). The results indicate the coherent control of spontaneous emission near the edge of a PBG [26,27]. Consequently, the PC can provide an efficient coherent feedback of the radiated energy to the emitter and make an important step towards the realization of miniature laser [28,29]. 4. Conclusion
Fig. 4. PL spectra of RBPCI (dotted line), RBPCII (dashed line) and the control sample (solid line).
density at its spatial location. Fig. 4 shows emission properties of RBPCs with the different relative position between PCs stopband and RB emission peak. A sharp dip of PL intensity is clearly observed for RBPCI, where there is a good match between the RB emission peak (592 nm) and the corresponding stopband of PCs at 617 nm. Besides, the FWHM of the emission light from RBPC is effectively tuned and narrowed. The FWHM of RBPCI becomes 29 nm, 19 nm narrower than that of the control sample (48 nm). In contrast, the dip turns to flat and shifts toward longer wavelength region when the stopband of PC is 652 nm (RBPCII) and only 8 nm narrower than that of the control sample, which could be ascribed to the departure of the stopband position from RB emission peak. Fig. 5 demonstrates the dependence of PL spectra of RBPCI on excitation intensity. It could be clearly observed that the emission intensity increases with the enhancement of pump energy. Furthermore, a concomitant spectrum narrowing phenomenon can be observed in the procedure.
A series of RBPCs with different concentrations of RB are successfully fabricated. The effects of both RB concentration and the PC stopband on the PL emission are systematically investigated. Moreover, an obvious narrowing and enhancement of the emission are achieved along with the increasing excitation intensity in as-prepared RBPCs. The results indicate that the suitable combination of organic dye and the PCs can provide a promising strategy to fabricate and design the low-threshold minilasers, light-emitting diodes and other highly efficient optical applications. Acknowledgements The authors thank the support of the NSFC (Nos. 50625312, U0634004 and 20421101), the 973 Program (Nos. 2006CB806200 and 2006CB932100), CNKBRSF 2006CB921706 and the Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 5. The PL spectra of RBPCI at various pulsed excitation, using a pulsed laser excitation (Nd:YAG laser at 532 nm having 100 ps pulse duration). Inset shows the PL intensity dependence on the excitation intensity.
[19] [20] [21] [22]
E.M. Purcell, Phys. Rev. 69 (1946) 681. E.A. Hinds, Adv. At. Mol. Opt. Phys. 28 (1991) 237. E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. S. John, Phys. Rev. Lett. 58 (1987) 2486. D.M. Mittleman, J.F. Bertone, P. Jiang, K.S. Hwang, V.L. Colvin, J. Chem. Phys. 111 (1999) 345. L. Bechger, P. Lodahl, W.L. Vos, J. Phys. Chem. B 109 (2005) 9980. _ I. _ Tarhan, G.H. Watson, Phy. Rev. Lett. 76 (1996) 315. I. C.F. Blanford, R.C. Schroden, M. Al-Daous, A. Stein, Adv. Mater. 13 (2001) 26. M. Barth, A. Gruber, F. Cichos, Phys. Rev. B 72 (2005) 085129. A.F. Koenderink, W.L. Vos, Phys. Rev. Lett. 91 (2003) 213902. T. Yamasaki, T. Tsutsui, Appl. Phys. Lett. 72 (1998) 1957. M. Mu¨ller, R. Zentel, T. Maka, S.G. Romanov, C.M.S. Torres, Chem. Mater. 12 (2000) 2508. S.G. Romanov, T. Maka, C.M. Sotomayor Torres, M. Mu¨ller, R. Zentel, Appl. Phys. Lett. 75 (1999) 1057. K. Song, R.A.L. Valle´e, M. Van der Auweraer, F. De Schryver, A. Persoons, K. Clays, Chem. Phys. Lett. 421 (2006) 1. K. Baert, K. Song, R.A.L. Valle´e, M. Van der Auweraer, K. Clays, J. Appl. Phys. 100 (2006) 123112. J. Wang, Y. Wen, H. Ge, Z. Sun, Y. Song, L. Jiang, Macromol. Chem. Phys. 207 (2006) 596. S. Wong, V. Kitaev, G.A. Ozin, J. Am. Chem. Soc. 125 (2003) 15589. P. Jiang, J.F. Berton, K.S. Hwang, V.L. Colvin, Chem. Mater. 11 (1999) 2132. I.I. Tarhan, G.H. Watson, Phys. Rev. B 54 (1996) 7593. A. Richel, N.P. Johnson, D.W. McComb, Appl. Phys. Lett. 76 (2000) 1816. S.G. Romanov et al., Phys. Rev. E 63 (2001) 056603. E. Yablonovitch, J. Phys.: Condens. Matter 5 (1993) 2443.
M. Li et al. / Chemical Physics Letters 444 (2007) 287–291 [23] X. Wang, R. Wang, B. Gu, G. Yang, Phys. Rev. Lett. 88 (2002) 093902. [24] R. Wang, X. Wang, B. Gu, G. Yang, Phys. Rev. B 67 (2003) 155114. [25] N. Vats, S. John, K. Busch, Phys. Rev. A 65 (2002) 043808. [26] S. John, J. Wang, Phys. Rev. Lett. 64 (1990) 2418.
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[27] T. Quang, M. Woldeyohannes, S. John, G.S. Agarwal, Phys. Rev. Lett. 79 (1997) 5238. [28] J.R. Lawrence, Y. Ying, P. Jiang, S.H. Foulger, Adv. Mater. 18 (2006) 300. [29] H. Altug, J. Vucˇkovic´, Opt. Exp. 13 (2005) 8819.
Coherent control of spontaneous emission by photonic crystals Mingzhu Li, Andong Xia, Jingxia Wang, Yanlin Song *, Lei Jiang Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 9 February 2007; in final form 29 June 2007 Available online 12 July 2007
Abstract Rhodamine B (RB) doped photonic crystals (PCs) (RBPCs) are fabricated by introducing different concentrations of RB into PCs during the deposition procedure. Photoluminescence (PL) spectra of RBPCs show strong dependence on the dye concentrations and the corresponding stopbands of the PCs. Additionally, an obvious narrowing and enhancement of the emission are achieved with the increasing excitation intensity. These results indicate that a suitable combination of organic dye and the PC would offer a promising route towards low-threshold minilasers, light-emitting diodes and other optical applications. 2007 Elsevier B.V. All rights reserved.
1. Introduction Spontaneous emission is a fundamental process resulting from the interaction between radiation and matter, and its effective control is essential for diverse applications ranging from miniature lasers, light-emitting diodes to solar energy harvesting. The application performance of spontaneous emission depends on both the excited atomic system and the utilization environment [1,2]. Recently, increasing attention has been paid to PCs, because of their great potentials for manipulating the emission properties of internal light sources [3,4]. Particularly, more and more attention has been given to colloidal crystals, because of their facile and economical strategy in fabrication and design of three-dimensional PCs with feature sizes of the light’s wavelength [5–8]. Furthermore, the stopbands of colloidal crystals possess highly anisotropic and wavelength-dependent dispersion properties, which would produce some interesting phenomena even in the absence of a complete photonic band gap [9–11]. In this case, a few strategies have been reported to study the manipulation of the emission of fluorophore by PCs via the combination of dye and colloidal crystals [12,13,6].
*
Corresponding author. Fax: +86 10 62562935. E-mail address: [email protected] (Y. Song).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.07.020
Especially, Clays and co-workers [14,15] have successfully fabricated a nano-engineering sandwich-like photonic structure and photonic superlattice, which clearly demonstrates the effect of well-designed bandgap on the spontaneous emission of fluorophore. The methods offer an effective way to study the manipulation of spontaneous emission by various PC superlattices. Meanwhile, the concentration of dye is another crucial factor for the dye emission performance [1,2]. However, few literatures have explored the effect of dye concentration on the spontaneous emission of organic dye doped PCs. Here both effects of dye concentrations and stopbands of the PCs on the fluorescence emission were systematically investigated. RBPCs was fabricated by introducing different concentrations of RB into poly(styrene-methyl methacrylateacrylic acid) (P(St-MMA-AA)) PCs during the deposition stage. The manipulation of RB emission is systematically investigated by modifying the relative position between the PC stopband and RB emission peak. It is found that both the RB concentration dispersed in PC and the relative position between the stopband and RB emission peak play a key role in the emission of RBPCs. Furthermore, the effective enhancement and narrowing of the emission peak are observed with increasing excitation intensity. These phenomena demonstrate that PC can provide an efficient coherent feedback in the opal matrix of the periodic structure, which indicate that suitable combination of organic
288
M. Li et al. / Chemical Physics Letters 444 (2007) 287–291
dye and PC can offer a promising route towards highly efficient optical applications such as low-threshold minilasers, light-emitting diodes and other optical devices. Additionally, systematic studies on the fundamental properties of these combinative materials also provide a reasonable strategy to investigate the spontaneous emission properties of atoms or molecules in three-dimensional (3D) PCs. 2. Experimental Monodispersed P(St-MMA-AA) latex spheres with diameters of 270 nm and 280 nm were prepared via batch emulsion polymerization following our previous literature [16]. The films of RB doped P(St-MMA-AA) PCs were fabricated by vertical deposition [17] from the mixture of P(StMMA-AA) latex spheres suspension with volume fraction of 0.15% and RB aqueous solution at the constant temperature of 60 C and humidity of 60%, respectively. The control sample was obtained by heating as-prepared film at the temperature higher than its Tg (112 C). UV–visible spectra were acquired by Hitachi UV-4100 spectrophotometer. The scanning electron microscope (SEM) images were obtained with a Hitachi S-4700 field emission scanning electron microscope at 3.0 kV. The size distribution of the P(St-MMA-AA) latex spheres was detected by ZetaPALS BI-90plus (Brookhaven Instrument). The laser pulses (7 ns, 1 Hz) with the wavelength of 532 nm, obtained from a frequency tripled Nd:YAG laser (New Wave, Tempest 300, USA), were used as the excitation source. PL spectra were obtained by a polychromator (Spectropro 550i, Acton) equipped with a liquidnitrogen cooled ( 113 C) CCD (charged-coupled device) camera (1340 · 400 pixels) detector (SPEC-10-400B/LN, Roper Scientific). And the collecting PL was focused onto the entrance slit of the polychromator. With grating (150 grooves/mm) and 10 lm slit, a spectral resolution of 1.2 nm can be obtained. The integration time was 20 s. 3. Results and discussion To evaluate the effect of different stopbands on RB emission, RBPCs are fabricated from different P(St-MMA-AA) latex spheres (270 nm – RBPCI and 280 nm – RBPCII in diameter) by vertical deposition method [18], respectively. The samples are assembled from the mixture of RB solution with different concentrations from 10 4 M to 5 · 10 6 M and monodispersed P(St-MMA-AA) latex spheres suspension (0.15%). Fig. 1 shows the transmission spectra of the P(St-MMAAA) PCs with different diameters and the emission spectrum of RB. As for the as-prepared colloidal crystals, the relative stop bandwidth of Dk/k0 are 6%, where Dk is the full width at half-minimum of the dip and the k0 is the center wavelength of the dip. This value is in accordance with the theoretical calculation result for a face-centered cubic crystal [19]. Additionally, the dips of transmission spectra are less than 5%. Aforementioned results further confirm
Fig. 1. Transmission spectra of the P(St-MMA-AA) PCs with sphere diameters of 270 (dotted line) and 280 nm (dashed line) together with the emission spectrum of RB (solid line). The corresponding sharp dip positions are at 617 and 652 nm, respectively measured with a light incident along the normal surface ((1 1 1) direction).
the good crystalline quality of as-prepared colloid crystals as shown in Fig. 2a, b. The deep, sharp dips in the transmission spectra predict sufficiently strong stopbands in period dielectric structures of the as-prepared colloidal crystals [20,21]. As shown in Fig. 1, the stopband of RBPCI (d = 270 nm) is situated at 617 nm (dotted line) and that of RBPCII (dashed line) takes red-shifting to 652 nm due to larger spheres (d = 280 nm), which are packed with larger lattice constants. The optical properties of as-prepared RBPCs lie on the well-order lattice structure resulting from the good monodispersity of latex spheres (Fig. 2c) and suitable RB concentration (Fig. 2d). As shown in Fig. 2c, the diameter of as-prepared P(St-MMA-AA) latex spheres is 279.3 nm with size distribution of 1.8%. Obviously, the diameters of latex spheres in water suspension are bigger than that (270.0 nm) measured in the films, which could be attributed to the separate swollen and shrunk states for latex spheres in the water and dry film. Meanwhile, RB concentration must be kept low enough to weaken the disturbance to the self-assembly of the latex spheres. As shown in Fig. 2d the well-order films could not be obtained when RB concentration is higher than 10 4 M. In our case, the RBPCs are prepared from the mixture solution of RB (5 · 10 5 M or 5 · 10 6 M, respectively) and P(St-MMA-AA) suspension (0.15%). As a result, these P(St-MMA-AA) latex spheres are crystallized into a cubic closed-packed lattice, with its (1 1 1) plane oriented parallel to the surface of the substrates (Fig. 2a, b). The RB molecules would be absorbed either on the surface of the P(St-MMA-AA) latex spheres or in interstitial void via the solution evaporation. Dye concentration is crucial factor for the dye emission performance [2]. In this work, we compare the effects of two different concentrations of RB in as-prepared RBPCs, 5 · 10 6 M (RBPCI) and 5 · 10 5 M (RBPCIII), respectively (Fig. 3). It is clear that higher RB concentration
M. Li et al. / Chemical Physics Letters 444 (2007) 287–291
289
Fig. 2. (a) Typical SEM top view of a sample self-assembled from the mixture of P(St-MMA-AA) latex spheres (270 nm in diameter) suspension with volume fraction of 0.15% and RB aqueous solution (5 · 10 6 M), (b) typical SEM side view of the same sample. Each facet of the PC has high degree of order over large areas, (c) hydrodynamic diameter and distribution of the latex spheres in water suspensions, measured by dynamic light scattering and (d) typical SEM top view of the sample fabricated from the mixture of P(St-MMA-AA) latex spheres suspension with volume fraction of 0.15% and RB aqueous solution (10 4 M).
Fig. 3. PL spectra of RBPCI (dotted line) and RBPCIII (dot-dashed line), respectively together with the transmission spectrum of RBPCI (solid line).
results in a blurring of the intensity dip and red-shift of the emission peak. As for RBPCIII, there is only a moderate attenuation in the emission spectrum at the wavelength of 630 nm, and the peak of the fluorescence (603 nm) takes red shift of ca. 11 nm compared to that of the control sample (592 nm) assigned to the aggregation of RB molecules. When the concentration of RB molecules dispersed in the latex solution is high, a great deal of RB molecules are
aggregated on the surface layer of the P(St-MMA-AA) PC during the solution evaporation. The emission light of these RB molecules contributes more substantially to the total fluorescence intensity, which is not affected by the 3D structure of the P(St-MMA-AA) PC. On the contrary, an obvious dip can be observed at the wavelength of 621 nm in the emission spectrum of RBPCI, which is in consistent with the position of the stopband of RBPCI (617 nm). The dip indicates the PL suppression inside the stopband [22]. RBPCI shows a drastically narrow spectrum, situated at the stopband edge around 588 nm. Its full width at half maximum (FWHM) is 29 nm, ca. 27 nm narrower than that of RBPCIII (FWHM = 56 nm). The effect can be ascribed to the spread configurations of the photonic density of state in PC, partial inhibition at the band gap [23,24]. Similarly, the stopbands of PCs also play a key role on emission properties of dye doped PCs in the aspects of the position and shape of emission peak [10]. Herein, the effects of different stopbands on the PL emission of RBPCs are investigated in Fig. 4. It has been theoretically demonstrated that the local density of optical states is reduced within the stopband and strongly enhanced near the band edge frequencies [23]. Such changes in the local density of optical states should influence the radiative rate of internal emitters [25], because the spontaneous emission rate of an atom or molecule depends on the electromagnetic mode
290
M. Li et al. / Chemical Physics Letters 444 (2007) 287–291
The FWHM of 15 nm can be obtained when the excitation intensity approaches the maximum, which is ca. 33 nm narrower than that of the control sample (solid line in Fig. 4). The results indicate the coherent control of spontaneous emission near the edge of a PBG [26,27]. Consequently, the PC can provide an efficient coherent feedback of the radiated energy to the emitter and make an important step towards the realization of miniature laser [28,29]. 4. Conclusion
Fig. 4. PL spectra of RBPCI (dotted line), RBPCII (dashed line) and the control sample (solid line).
density at its spatial location. Fig. 4 shows emission properties of RBPCs with the different relative position between PCs stopband and RB emission peak. A sharp dip of PL intensity is clearly observed for RBPCI, where there is a good match between the RB emission peak (592 nm) and the corresponding stopband of PCs at 617 nm. Besides, the FWHM of the emission light from RBPC is effectively tuned and narrowed. The FWHM of RBPCI becomes 29 nm, 19 nm narrower than that of the control sample (48 nm). In contrast, the dip turns to flat and shifts toward longer wavelength region when the stopband of PC is 652 nm (RBPCII) and only 8 nm narrower than that of the control sample, which could be ascribed to the departure of the stopband position from RB emission peak. Fig. 5 demonstrates the dependence of PL spectra of RBPCI on excitation intensity. It could be clearly observed that the emission intensity increases with the enhancement of pump energy. Furthermore, a concomitant spectrum narrowing phenomenon can be observed in the procedure.
A series of RBPCs with different concentrations of RB are successfully fabricated. The effects of both RB concentration and the PC stopband on the PL emission are systematically investigated. Moreover, an obvious narrowing and enhancement of the emission are achieved along with the increasing excitation intensity in as-prepared RBPCs. The results indicate that the suitable combination of organic dye and the PCs can provide a promising strategy to fabricate and design the low-threshold minilasers, light-emitting diodes and other highly efficient optical applications. Acknowledgements The authors thank the support of the NSFC (Nos. 50625312, U0634004 and 20421101), the 973 Program (Nos. 2006CB806200 and 2006CB932100), CNKBRSF 2006CB921706 and the Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 5. The PL spectra of RBPCI at various pulsed excitation, using a pulsed laser excitation (Nd:YAG laser at 532 nm having 100 ps pulse duration). Inset shows the PL intensity dependence on the excitation intensity.
[19] [20] [21] [22]
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