ARTICLE pubs.acs.org/JPCC
Large Raman Gain in a Stable Nanocomposite Based on Niobiosilicate Glass Pasquale Pernice,*,† Luigi Sirleto,‡ Alessandro Vergara,§ Antonio Aronne,† Massimo Gagliardi,‡ Esther Fanelli,† and Giancarlo C. Righini|| †
Dipartimento di Ingegneria dei Materiali e della Produzione, Universita di Napoli Federico II, P.le V. Tecchio, I-80125 Napoli, Italy Consiglio Nazionale delle Ricerche (CNR), Istituto per la Microelettronica e Microsistemi, Via Pietro Castellino, 111 I-80131 Napoli, Italy § Dipartimento di Chimica “Paolo Corradini”, Universita di Napoli Federico II, Complesso Universitario Monte Sant’Angelo, Via Cintia, I-80126 Napoli, Italy Consiglio Nazionale delle Ricerche (CNR), Istituto di Fisica Applicata Nello Carrara, Via Madonna del Piano10, I-50019 Sesto Fiorentino, Italy
)
‡
ABSTRACT: The optical properties of composite materials can be adjusted by controlling the constituents and morphology of the composite structure. The optical nanocomposite approach offers opportunities to produce high-performance and relatively low-cost optoelectronic media suitable for many applications. In this paper, a nanocomposite material obtained by a high niobium content glass belonging to the K2ONb2O5SiO2 (KNS) glass-forming system is prepared and characterized. We focus on the investigation of spontaneous and stimulated Raman scattering. Strong changes of Raman spectra in nanocomposite material with respect to bulk sample are proved and discussed, while a significant enhancement of Raman gain (up to 25 times higher) and of its bandwidth with respect to SiO2 glass is reported. This family of glasses is a promising candidate for next generation Raman gain applications for use in the format of integrated optics as well.
’ INTRODUCTION Nanocomposities are random media containing domains or inclusions that are on the nanometric size scale. The optical properties of composite materials can be adjusted by controlling the constituents and morphology of the composite structure. The optical nanocomposite approach offers opportunities to produce high-performance and relatively low-cost optoelectronic media suitable for many applications.1 “Stimulated Raman scattering (SRS)” is one of the first discovered nonlinear optical effects: a pump laser beam enters a nonlinear medium and spontaneous generation and amplification lead to a beam at a frequency different from the pump. SRS belongs to a class of nonlinear optical processes that can be called quasi-resonant. Although none of the fields is in resonance with the vibrations in the lattice of the medium (optical phonons), the difference between the pump and generated beam equals the transition frequency.2 SRS is used in tunable laser development, high energy pulse compression, etc. One of the most important perspectives is the realization of micro/nanosources, with improved performances. In bulk semiconductors, lasing by SRS was first discovered in GaP.3 Most recently, Raman lasers have been demonstrated in silicon microwaveguides.4 SRS from spherical droplets and microspheres, with diameters 520 μm, has been also observed using both pulsed and continuous wave probe beams.5 Except for r 2011 American Chemical Society
a report of SRS from individual single walled carbon nanotubes,6 the observation of SRS from semiconductor nanowires7 and from silicon nanocrystals,8,9 we find no other evidence for this important nonlinear optic effect in nanostructured materials. In this paper, the microstructural and the optical characterizations of glasses with composition 30K2O 3 30Nb2O5 3 40 SiO2 (KNS 303040) with different thermal treatments were carried out by differential thermal analysis (DTA), highresolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), ellipsometry, and Raman spectroscopy. Our findings prove that as a consequence of the thermal treatment, a nanostructuring process is obtained, which can be considered a powerful tool to make functional nanocomposite glassy materials especially for optical applications.10,11 In addition, strong changes in the Raman spectra of nanocomposite material with respect to KNS glass are proved and discussed. Finally, a significant enhancement of Raman gain (up to 25 times higher) and of its bandwidth of both KNS glasses and nanocomposite material with respect to SiO2 glass is reported. Received: May 10, 2011 Revised: July 28, 2011 Published: August 01, 2011 17314
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Table 1. Refraction Indices, Peak Positions, Full Width at Half-Maximum, and Amplitudes Normalized to the Silica Standard of the Most Representative Raman Bands of the 30K2O 3 30Nb2O5 3 40SiO2 Glassesa sigma/ I/ g/ sigma ISiO2 gSiO2
sample
refractive index
peak position (cm1)
fwhm (cm1)
SiO2 flass
1.46
450
190
1
1
1
initial glass
1.77
665
175
70
38
25
825
176
104
57
905
50
18
34
655
166
61
35
805 885
125 58
27 15
21 26
655
166
66
38
805
125
28
22
885
58
16
26
2h
10 h
1.88
1.87
16
18
a
The last column shows the peak Raman gain measured on the KNS glasses and normalized to the values obtained in silica glass.
’ EXPERIMENTAL SECTION Preparation of Samples. KNS glass of molar composition 30K2O 3 30Nb2O5 3 40SiO2 was synthesized from reagent grade KNO3, Nb2O5, and SiO2 by melting to 1550 °C and subsequent quenching. A set of transparent samples was prepared covering the full range of the key stages of bulk nanostructuring, e.g., asquenched glass (no annealing), early stages of nanostructuring (2 h of annealing at Tg), and fully nanostructured glass (10 h of annealing at Tg). DTA curves were recorded on bulk specimens of about 50 mg in air at a heating rate of 10 K min1 using a Netzsch thermoanalyzer high temperature DSC 404 with Al2O3 as reference material. The samples intended for HRTEM imaging were ground into fine powders, and the as-ground powders were imaged at the grain boundaries in transmission mode on a Philips CM20 FEG field emission TEM. Raman Characterization. A confocal Raman microscope (Jasco, NRS-3100) was used to obtain Raman spectra. The 514 nm line of an air-cooled Ar+ laser (Melles Griot, 35 LAP 431220), 125 mW, was injected into an integrated Olympus microscope and focused to a spot size of approximately 3 μm by a 20 objective with a final 2.5 mW power at the sample. A holographic notch filter was used to reject the excitation laser line. The Raman backscattering was collected at 180°, using a 0.1 mm slit and a diffraction lattice of 1200 grooves/mm, corresponding to an average spectral resolution of 8 cm1. It took 60 s to collect a complete data set by a Peltier-cooled 1024 128 pixel CCD photon detector (Andor DU401BVI). Wavelength calibration was performed by using polystyrene as a standard. Raman microscopy measurements on KNS samples were performed on the smoothest surface, and spectra were recorded in three distinct locations of the sample to test for homogeneity and at three different depths for test of reproducibility of the Raman intensity. Ellipsometric Characterization. Spectroscopic ellipsometry is an essential technique for nondestructive testing and characterization of thin films or structures. Our measurements were carried out using a Jobin Yvon UVISEL-NIR phase modulated spectroscopic ellipsometer apparatus, with an angle of 70° in the spectral range 4501600 nm. As a reference, the refractive index
Figure 1. DTA curves recorded in air at 10 °C/min.
of pure silica was measured, and excellent agreement was found with the well-known data within an absolute error of 1 103. In the transparency region above 450 nm, ellipsometric data were analyzed using a Cauchy model, and surface roughness was also taken into account. The refractive indices (n) of samples at 514 nm are reported in Table 1.
’ RESULTS AND DISCUSSION Among the advanced materials for Raman amplification, one of the most interesting classes is oxide glasses, particularly silicon dioxide based glasses due to their compatibility with the existing optical fibers technology. Until now, silica-base glasses containing suitable dopants (heavy metal oxide as Ta2O5, Bi2O3, Nb2O5) have been prepared showing a Raman gain only twice that of pure silica.12 The glasses belonging to the tellurite family combine large scattering intensity and bandwidth giving very high Raman gains, up to 40 times larger than the SiO2;1316 however their fabrication demands particular wise precaution including controlled melting atmosphere. For glasses belonging to the K2ONb2O5SiO2 (KNS) glass-forming system, an interesting glass nanostructuring process has been investigated. The process, involving two partially overlapped processes, namely, phase separation and crystallization, produces a transparent material.1719 In this paper the investigated glasses, whose composition corresponds to the crystalline phase K3Nb3O6Si2O7, have the highest niobium content among the KNS glasses prepared in our laboratory.1719 It is anyway an easily formed stable glass of optical quality. In order to obtain transparent nanostructured glasses, it is necessary that amorphous or crystalline phase grow from the glass exclusively with a bulk nucleation mechanism. By means of a proper nucleation heat treatment, it is possible to produce a high number of bulk nuclei and, consequently, a high number of nanocrystals small enough to avoid any light scattering. Usually the highest nucleation rate is in the glass transition range; therefore thermal annealing in this range allows control of the size of the amorphous and/or crystalline nanoinhomogeneity obtaining transparent nanostructured glasses.1720 The DTA curve of the KNS 303040 initial glass exhibits a slope change of the baseline in the endothermic versus followed by a sharp exothermic peak at 727 °C (Figure 1). The slope change may be related to the glass transition, and the inflection 17315
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Figure 2. HRTEM micrographs and XRD patterns of the initial glass and of the glass samples heated 2 h and 10 h at Tg.
point at the slope change was taken as the glass transition temperature, Tg = 663 °C. These thermal effects disappear in the DTA curves of the samples annealed at Tg for 2 and 10 h. These curves show that the behavior of the annealed samples strongly differs from that of the initial glass (Figure 1), indicating that for the KNS 303040 glass, the nanostructuring produced by annealing, induces a very deep structural transformation of the amorphous matrix. The HRTEM micrograph of the initial glass attests to its amorphous nature, since no planar fringes appear all over the investigated area (Figure 2). Selected area electron diffraction (SAED) (not shown) and high-angle XRD patterns confirm the morphological observation, exhibiting only the halos characteristic of the amorphous state. The HRTEM micrograph of the sample annealed for 2 h shows a few zones containing nanosized circular domains with not well definite boundaries, the size of which can be estimated lower than 5 nm. They can be related to spherical nanocrystals dispersed into the amorphous matrix according to high-angle XRD and SAED patterns (Figure 2).
In the subsequent stage of annealing, the nanocrystals show more intense planar fringes and better-defined boundaries, and they look larger (from 10 to 15 nm), more numerous, and uniformly distributed in the matrix than the nanocrystals formed after 2 h of annealing. SAED patterns, showing well-defined crystalline spots, together with the narrowing of the high angle XRD peaks, confirm that at this stage of nanostructuring the crystallization prevails. As the intensity of the Raman active modes depends on the temperature and on the frequency of the vibrational modes,2124 the measured Stokes Raman intensity I(ω) has been reduced according to the relation RðωÞ ¼
ω 1 IðωÞ Nðω, TÞ þ 1 ðω0 ωÞ4
where ω is the Stokes Raman shift (in cm1 units), ω0 is the laser excitation frequency, N(ω,T) is the BoseEinstein mean occupation number, and T is the temperature. Afterward, to correctly 17316
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Figure 3. Reduced Raman spectra and Raman gain spectra normalized to the silica standard. From the top: initial glass, and glass samples heated 2 h and 10 h at Tg.
compare the Raman spectra of KNS glasses with the silica glass standard, the measured Raman spectra have been corrected also for the differences in reflection and angle of collection following
the procedure reported in refs 2124. Reduced and corrected Raman spectra have been fitted to several Gaussian components, and the results are listed in Table 1. The reduced Raman spectra 17317
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The Journal of Physical Chemistry C of the initial glass as well as of the samples annealed at Tg for 2 and 10 h are displayed in Figure 3. Three main bands are present in the spectrum of the initial glass, lying in the ranges 250350, 750850, and 850 950 cm1 with features at about 690 and 1060 cm1 (Figure 3). A lower frequency region (200250 cm1) is not considered in our discussion. The bands into the range 250950 cm1 are related to vibration of NbO bonds in NbO 6 octahedra with a different distortion degree, while the feature at 1060 cm1 is related to vibration of SiO bonds in SiO4 tetrahedra.1719 For the initial glass seven Gaussian components are found, which can be related to SiO4 tetrahedra (∼1090 cm1) as well as to NbO6 octahedra whose distortion degree increases with increasing wavenumbers.1719 So, Gaussian features at ∼495 cm1, VLD, and at ∼650 cm1, LD, can be related to the presence of clustering of NbO6 and of NbO6 having all bridging oxygens, respectively. The highest Gaussian component occurs in the range 800825 cm1 and indicates the presence of more distorted octahedra, MD, i.e., NbO6 octahedra sharing a corner with at least one more NbO6 octahedra. Gaussian components in the ranges 885905 cm1, HD, and 965995 cm1, VHD, are related to highly distorted NbO6 and/or NbO6 having at least a short terminal NbO bond pointing toward a modifier ion.1719 Finally, the Gaussian feature at ∼280 cm1 is related to the bending modes of the NbONb bonds. Upon annealing, nanostructuring process result in an important redistribution of the niobium octahedra, so that a significant modification of the Raman spectra is obtained. For the annealed samples a slight red shift of the bands VHD, HD, and MD indicates a relative lowering of the octahedra distortion degree due to the formation of bridging bonds NbONb, involving the partial substitution of NbO(Si) bonds with NbO(Nb) ones. This result, with the appearance of an additional band in the region where the stretching modes of SiO bonds belonging to SiO4 tetrahedra (∼1040 cm1) are active, indicates that during the nanostructuring, observed by HRTEM/XRD, the most remarkable structural transformations concern the amorphous matrix giving rise to a phase separation. For the annealed samples the highest intensity occurs at about 650 cm1, whereas a strong reduction of the intensity of the band lying in the 850950 cm1 takes place. Particularly, the fraction of MD octahedra decreases with respect to the LD and HD ones. From the analysis of Table 1, the relative intensity of the MD band versus LD and HD can be obtained. For the as-quenched the ratio is 1.5 and 1.6, respectively, whereas for the 2 h annealed sample it is 0.6 and 0.8. The Raman gain spectrum is related to the spectral and differential Raman cross section by means of the equation25,26 ! λS 3 ∂2 σ gðωÞ ¼ 2 2 c hn ∂Ω∂ω 0
where
! ∂2 σ 1 ∂2 σ Þ ¼ ∂Ω∂ω 0 q þ Nðω, TÞ ∂Ω∂ω
is the Raman cross section at T = 0 K (i.e., corrected for the thermal population factor), λS is the Stokes wavelength (in meters), c is the velocity of light in vacuum, and n is the refraction index at the excitation wavelength.
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As evident in Figure 3, the resulting peak Raman gain coefficient turned out to be for all the samples at least 16 times and up to 25 times higher than in silica glass. So, our findings prove that Raman gain of the initial and the nanostructured samples is of the same order of magnitude. We note that in KNS glasses the Raman gain is higher than in silica glass on a wavenumbers range spanning from 400 to 1050 cm1, so these glasses show a bandwidth as wide as about 650 cm1, which is significantly greater than that of silica glasses (about 400 cm1) and is comparable to that observed in tellurite and phosphotellurite glasses.1417 The usable gain bandwidth of the glasses, i.e., the full width at half-maximum of the most intense Raman spectral portion, is approximately 400 cm1, again higher than in silica glasses (approximately 190 cm1). Our results prove that due to the presence of a high niobium content in silicon dioxide based glasses, both the gain and the bandwidth are increased. It is noted that Raman cross section enhancement in silicon dioxide based glasses with respect to silica glass is due to the higher polarizability of NbO bonds with respect to SiO bonds. This high polarizability can also contribute to an increase of the refractive index (Table 1). It is known that high intensity Raman bands are produced by high atomic number elements owing to more extended electronic clouds; this makes also the polarizability more sensitive to bonds stretching. In our glass, niobium is coordinated 6-fold in NbO6 octahedra with different degree of distortion. Consequently, distorted NbO6 octahedra, that is, with different NbO bond lengths, give rise to several Raman bands covering a wide range of wavenumbers. Moreover, the disorder typical of glasses manifests itself in an enlargement of the Raman bandwidth. Consequently, the Raman spectra of our glass are the result of the strong overlap of several broad bands. Concerning the physical origin of Raman gain in nanocomposite materials, although the structure of the interfaces, the stoichiometric material disorder, and the dimensionality of the clusters are expected to influence Raman amplification significantly, a theoretical understanding of their respective roles remains to be established. Finally we point out that a proper choice of the annealing parameters, hence the degree of crystallization, can allow us to find the best compromise between the highest Raman gain (achieved in the amorphous glass) and the highest nonlinear coefficients (achieved in the nanostructured glass).27 Therefore, this family of glasses would represent a promising candidate for next generation of devices for nonlinear optics in the format of integrated optics as well.
’ CONCLUSIONS Our results prove that in a glass with composition 30K2O 3 30 Nb2O5 3 40SiO2 as a consequence of the thermal treatment, a nanostructuring process is obtained, which can be considered a powerful tool for making functional nanocomposite glassy materials especially for optical applications. In addition, strong changes are produced in the Raman spectra of KNS nanocomposite materials with respect to SiO2 glass. Finally, a significant enhancement of Raman gain (up to 25 times higher) and of its bandwidth of both KNS glasses and nanocomposite material with respect to SiO2 glass is reported. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 17318
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’ ACKNOWLEDGMENT We gratefully acknowledge Nobuyoshi Miyajima of the Bayerisches Geoinstitut, Universitaet Bayreuth, Germany, for the help in TEM observations. ’ REFERENCES (1) Prasad, P. N. Nanophotonics; John Wiley & Sons: New York, 2004. (2) Shen, Y. R.; Bloembergen, N. Phys. Rev. 1965, 137, A1787. (3) Nishizawa, J.; Suto, K. J. Appl. Phys. 1980, 51, 2429. (4) Rong, H. S.; Liu, A. S.; Jones, R.; Cohen, O.; Hak, D.; Nicolaescu, R.; Fang, A.; Paniccia, M. Nature 2005, 433, 292. (5) Spillane, S. M.; Kippenberg, T. J.; Vahala, K. J. Nature 2002, 415, 621. (6) Zhang, B. P.; Shimazaki, K.; Shiokawa, T.; Suzuki, M.; Ishibashi, K.; Saito, R. Appl. Phys. Lett. 2006, 88, 241101. (7) Jian Wu, Awnish K.; Gupta, Humberto R.; Gutierres, P. C. Eklund Nano Lett. 2009, 9, 3252. (8) Sirleto, L.; Ferrara, M. A.; Rendina, I.; Basu, S. N.; Warga, J.; Li, R.; Dal Negro, L. Appl. Phys. Lett. 2008, 93, 251104. (9) Sirleto, L.; Ferrara, M. A.; Rendina, I.; Nicotra, G.; Spinella, C. Appl. Phys. Lett. 2009, 94, 221106. (10) Ivanda, M.; Furic, K.; Music, S.; Ristic, M.; Gotic, M.; Ristic, D.; Tonejc, A. M.; Djerdj, I.; Mattarelli, M.; Montagna, M.; Rossi, F.; Ferrari, M.; Chiasera, A.; Jestin, Y.; Righini, G. C.; Kiefer, W.; Gonc-alves, R. R. J. Raman Spectrosc. 2007, 38, 647. (11) S. Berneschi, S.; Soria, G. C.; Righini, G.; Alombert-Goget, A.; Chiappini, A.; Chiasera., Y.; Jestin, M.; Ferrari, E.; Moser, S. N. B.; Bhaktha, B.; Boulard, C.; Duverger Arfuso, S. Turrell. Opt. Mater. 2010, 32, 1644. (12) Manolescu, G.; Poumellec, B. Glass Technol.: Eur. J. Glass Sci. Technol., Part A 2009, 50, 143. (13) Yang, Q.; Qian, Q.; Chen, D. D.; Zhang, Q. Y.; Jiang, Z. H. Optoelectron. Adv. Mater., Rapid Commun. 2009, 3, 565. (14) Jackson, J.; Smith, C.; Massera, J.; Rivero-Baleine, C.; Bungay, C.; Petit, L.; Richardson, K. Opt. Express 2009, 17, 9071. (15) O’Donnell, M. D.; Richardson, K.; Stolen, R.; Rivero, C.; Cardinal, T.; Couzi, M.; Furniss, D.; Seddon, A. B. Opt. Mater. 2008, 30, 946. (16) Jose, R.; Ohishi, Y. Appl. Phys. Lett. 2007, 90, 211104. (17) Aronne, A.; Sigaev, V. N.; Champagnon, B.; Fanelli, E.; Califano, V.; Usmanova, L. Z.; Pernice, P. J. Non-Cryst. Solids 2005, 351, 3610. (18) Aronne, A.; Sigaev, V. N.; Pernice, P.; Fanelli, E.; Usmanova, L. Z. J. Non-Cryst. Solids 2004, 337, 121. (19) Bergese, P.; Alessandri, I.; Bontempi, E.; Depero, L. E.; Aronne, A.; Fanelli, E.; Pernice, P.; Boffa Ballaran, T.; Miyajima, N.; Sigaev, V. N. J. Phys. Chem. B 2006, 110, 25740. (20) Shivakiran Bhaktha, B. N.; Kinowski, C.; Bouazaoui, M.; Capoen, B.; Robbe-Cristini, O.; Beclin, F.; Roussel, P.; Ferrari, M.; Turrell, S. J. Phys. Chem. C 2009, 113, 21555. (21) Shuker, R.; Gammon, R. W. Phys. Rev. Lett. 1970, 25, 222. (22) Galeener, F. L.; Sen, P. N. Phys. Rev. B 1978, 17, 1928. (23) Galeener, F. L.; Mikkelsen, J. C., Jr; Geils, R. H.; Mosby, W. J. Appl. Phys. Lett. 1978, 32, 34. (24) Sirleto, L.; Donato, M. G.; Messina, G.; Santangelo, S.; Lipovskii, A. A.; Tagantsev, D. K.; Pelli, S.; Righini, G. C. Appl. Phys. Lett. 2009, 94, 031105. (25) Stolen, R. H. In Raman Amplifiers for Telecommunications 1; Islam, M .N., Ed.; Springer-Verlag: New York, 2004; p 35. (26) Donato, M. G.; Gagliardi, M.; Sirleto, L.; Messina, G.; Lipovskii, A. A.; Tagantsev, D. K.; Righini, G. C. Appl. Phys. Lett. 2010, 97, 231111. (27) Aronne, A.; Fanelli, E.; Pernice, P.; Malvestuto, M.; Bergese, P.; Bontempi, E.; Colombi, P.; Depero, L. E.; Bignardi, L.; Giannetti, C.; Ferrini, G.; Parmigiani, F. J. Non-Cryst. Solids 2011, 357, 1218. 17319
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