Redshifted surface plasma resonance-inducedenhancement of third-order opticalnonlinearities in silver nanoclusters

embedded in Bi2O3-B2O3-TiO2pseudo-ternary glasses

Feifei Chen,1,* Shixun Dai,1 Tiefeng Xu,1 Xiang Shen,1 Baoan Song,1 Changgui Lin,1

Xunsi Wang,1 Chao Liu,2 Kai Xu,2 and Jong Heo2

1College of Information Science and Engineering, Ningbo University, Fenghua Load No. 818, Ningbo 315211, China2Center for Information Materials and Department of Materials Science and Engineering,Pohang University of Science and Technology, Pohang, Gyeongbuk, 790-784, South Korea

*Corresponding author: chencyin@sina.com

Received December 6, 2010; revised March 27, 2011; accepted March 28, 2011;posted March 29, 2011 (Doc. ID 139121); published April 27, 2011

A series of silver (Ag0) nanoclusters embedded in bismuthate glasses (Bi2O3-B2O3-TiO2) were synthesized by thethermochemical reduction method. When B2O3 was substituted with TiO2, the refractive index of the encapsulat-ing medium increased, leading to redshifting of the surface plasmon resonance (SPR) absorption band of Ag0,which was initially located at 612nm. Z-scan measurements with femtosecond laser pulses at 800nm showed thatthird-order optical nonlinearities (TONs) were significantly enhanced in the SPR redshifted samples, and calcula-tion of the figure of merit manifested excellent performances of present Ag0-bismuthate nanocomposites forTON-based all-optical switching. © 2011 Optical Society of America

OCIS codes: 160.4236, 160.4330, 190.3970.

1. INTRODUCTIONIn recent years, noble metallic particles (e.g., Au, Ag, and Cu)[1] embedded in dielectric composites have attracted consid-erable attention because of their unique optical and dielectricproperties. When the sizes of these embedded metallic parti-cles are in nanoscale, optical excitations at special wave-lengths activate electronic oscillations between interbandsor intrabands of the metal atoms, which are well known assurface plasmon resonance (SPR)[2,3]. Notably, enhancementof various optical properties (e.g., nonlinear effects [4,5],luminescence [6], and so on) can be induced in the SPR cover-ing region, thus leading to better performance of their relatedapplications [7]. Usually, fabrication procedures of such nano-composites include Solgel, magnetron sputtering deposition,femtosecond laser irradiation, and a heat treatment method,which are complex, costly, or take a lot of energy/time. Somand Karmakar [6,8–10] discovered the self-reductions of gold(Au0) and silver (Ag0) nanoclusters in heavy metal oxide(antimony-based) glasses during one-step melting–quenchingprocess, leading to novel nanocomposites that possess someoutstanding advantages, such as simple fabrication, low cost,high mechanical strength, and chemical stability.

Previously, we successfully prepared Ag0 nanoclusters(ANCs) embedded in bismuthate glasses [11] accordingto the standard reduction potential values (Bi3þ=Bi0, E0 ¼0:3175V; Agþ=Ag0, E0 ¼ 0:7996V), and the Ag0 clusters inbismuthate glasses exhibited an SPR absorption band withcentral wavelength λmax above 600nm, which is evidentlyredshifted as compared to those in conventional zinc oxide

and silica substrates (λmax at ∼400nm) [4,12]. In theory, thelocation of the SPR band of metal particles is related to(1) shape, (2) size, and (3) surrounding environments [13], andthe redshifted SPR in our case could be attributed to the highlinear refractive index n0 of the Bi2O3-based precursor [14].The relationship between λmax and n0 can be expressed as [8]

λmax ¼ ½ð2πcÞ2mNe2ðεc þ 2n20Þ=ε0�1=2; ð1Þ

where c is light speed in vacuum, m is effective mass of con-duction electrons,N is the free electron concentration, e is theelectronic charge, ε0 is the free space permeability, and εc isthe optical dielectric function of the metal. Accordingly, mod-ification to the SPR of such metal–glass composites could besimplified by changing compositions of their host mediums.

On the other hand, present nonlinear devices based onoptical communications require operating wavelengths atnear-IR regions (e.g., 1310 and 1550nm). Heavy metal oxide(Bi2O3-, PbO-, or TeO2-based) and chalcogenide (S- orSe-based) glasses are near-IR transparent and considered tobe promising materials for these devices due to their highTON and ability to be formed or processed easily into almostany shape. However, as compared to nonlinear crystal andorganic materials, most nonlinear homogeneous glassespresent unfortunately lowTON intensities at the near-IR regionsince their electronic transition wavelengths (fundamentalabsorption edges) [15,16] are far away from this region;selenium-based glasses may have fundamental absorptionedges near the required wavelengths, but there are environ-mental concerns, and the low mechanical strength, complex

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fabrication processes, and combination with arsenic nega-tively affect their mass practical applications. Alternatively,introduction of nanoscaledmetallic particles to a glassmediumprovides an effective way to obtain high TON at certainwavelengths, taking advantage of the strong SPR-induced elec-tromagnetic fields between host medium and metallic parti-cles, which were able to promote various nonlinear opticalbehaviors. Therefore, we could expect such SPR-assist third-order nonlinearities at the near-IR region by shifting the SPRcoverage to longer wavelengths, which can be achieved by tai-loring the n0 value of the host medium. Furthermore, it is alsobe worthwhile to investigate the detailed TON dependence onthe location of the SPR that remain unexploited so far.

In this work, we synthesized ANCs embedded in bismuthate(Bi2O3-B2O3-TiO2) composites by using a one-step melting–quenching method, and refractive index n0 of the host wasmodified by a variation of titanium dioxide (n0 ¼ 2:5) contentin a range of 0–15mol:%. TON behaviors of the samples wereinvestigated by the Z-scan method using a femtosecond laserat 800 nm, and the relationship between TON and SPR isdiscussed.

2. EXPERIMENTALHost glasses in molar compositions of 60Bi2O3-ð40-xÞB2O3-xTiO2 (x ¼ 0; 5; 10; 13; 15 abbreviated as BBT-Ag1, 2,3, 4, 5 below) were selected and 0:3wt:% AgCl was dopedto precipitate ANCs. Reagent grade chemicals of Bi2O3,H3BO3, TiO2, and AgCl were used as starting materials.Batches of 15 g were mixed in corundum crucibles and meltedat 1000–1200 °C for 30 min in an electric furnace. The meltwas then poured onto a stainless steel plate and annealedwith a cooldown rate of 10 °C=h to reduce stress. The sampleplates with thicknesses of 1:5mm were cut and opticallypolished for subsequent measurements. A scanning electronmicroscope (SEM) study was carried out by coating the sam-ples with platinum in an argon atmosphere using the HitachiS-4800 SEM. The morphology images of ANCs were takenusing a TEM (FEI, Tecnai F20, 200kV).

Linear refractive indices n0 at 632:8 nm were measuredusing the SAIRON SPA4000 prism coupler. Absorption spectraof the samples from 400 to 2000 nm were measured by using aPerkin-Elemer-Lamda 950UV/VIS/NIR spectrophotometer.TON properties were investigated by the Z-scan technique(the detailed experimental setup used had already beendescribed elsewhere [17]). The laser source was a mode-locked Ti:sapphire laser (Coherent Mira 900-D) at 800nm witha pulse width of 200 fs operated at a 76MHz repetition rate.Nonlinear refraction (NR) γ of the samples was obtained fromthe closed aperture (CA) Z-scan curves, which correspond tothe transmittance of a tightly focused Gaussian beam througha finite aperture S (linear transmission, S ¼ 0:05) in the farfield as a function of the sample position Z with respect tothe focal plane, while removing the aperture, open aperture(OA) Z-scan curves were yielded to estimate the nonlinearabsorption (NA) coefficient β. The calculations of theseTON parameters were done according to the well-establishednonlinear curve fitting procedures [18,19]. All above measure-ments were conducted at room temperature.

3. RESULTS AND DISCUSSION

A. SEM and TEM StudiesSEM images taken from BBT-Ag3 and its host glass (in molarcomposition of 60Bi2O3-30B2O3-10TiO2) are shown in Fig. 1.In the SEM image of the latter [Fig. 1(a)], no microscopicstructures could be seen, indicating the homogeneity of thehost glass. From Fig. 1(b), a large number of light-coloredspots (diameters of 20–50nm) can be observed on the surfaceof the composite sample. As demonstrated in a previous study[11], phase separation originated from the introduction ofAgCl is responsible for the formation of these “dots,” whichare aggregated multicomponent clusters with high silver con-centrations.

For further investigation, transmission electron microscope(TEM) images taken from BBT-Ag3 are shown in Fig. 2. Asindicated in the lowest resolution image [Fig. 2(a)], the com-posite consists of a large number of uniformly distributed grayand dark cluster structures, which confirmed the observationin its SEM image, but the real cluster sizes (diameters of100–300 nm) are much larger. On magnification of the imageshown in Fig. 2(b), one can clearly observe that these clustersexhibit an encapsulated structure with black “cores” inside,

Fig. 1. SEM images taken from (a) host glass in molar composition of60Bi2O3-30B2O3-10TiO2 and (b) BBT-Ag3.

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and these cores are big silver metallic clusters coming fromOstwald’s ripening during the annealing process in whichadjacent silver clusters merged with each other and aggre-gated to large sizes, and they might be responsible for thelight-colored spots in the SEM images for their similar sizesand dispersions. When the clusters were further magnified,the microstructure of the ANCs was observed in Fig. 2(c): theyare mainly spherical shaped with diameters from 5 to 23nm.Figure 2(d) shows the corresponding cluster diameter histo-gram. By fitting the histogram with a curve of Gaussian shape,the average diameter of ANC is estimated to be 12:2nm, whichis small enough to induce surface plasmon absorption inthe visible region as demonstrated in Subsection 3.B. Further,the selected area electron diffraction (SAED) image in theinset of Fig. 2(a) has a single diffraction ring responding toAg (200) with a lattice distance of 0:204nm.

B. Absorption Spectra and Linear Refractive IndicesThe absorption spectra of the samples are shown in Fig. 3, andit is clear to observe the presence of SPR bands with centralwavelengths λmax above 600nm, signifying the formation ofANCs. These Lorentzian-shaped bands exhibit broad FWHMover 260 nm and their tails extend up to 1300nm. Size confine-ment effects [3] (shorter electron mean free path) induced bythe large number of small ANCs (≤ 10nm) is responsible forthese very broad SPR bands, and the large Ag metallic clusters

(several tens of nanometers) with irregular sizes and shapescoming from Ostwald’s ripening process caused the broaden-ing for their multipolar behaviors. Further, as seen in thenormalized absorption spectra of Fig. 3(b), the SPR bands ex-perienced a redshifting with an increase of TiO2 content up to10mol:%, but then blueshifted for the further increases. Thistrend is consistent with that of the refractive index n0 at632:8 nm (as seen in Table 1), which exhibits a maxima of2.1666 in BBT-Ag3. Since silver concentration remains con-stant, the unexpected decreasing tendency of n0 after the10mol:% TiO2 addition can be attributed to the transformationof layers of alternating BO3 triangles andBO4 tetrahedra [20] toisolated BO3 triangles when an excess amount of Ti4þ ionswere introduced to the glassmatrix, which opened up the glassnetwork and consequently weakened the linear refractive be-havior. However, themonotonous relationship betweenn0 andλmax manifesting in Fig. 4 agrees well with Eq. (1).

On the other hand, the steep fundamental absorption edgescorresponding to electronic transitions between bismuth andoxygen ions were found at ∼400 nm, thus those compositesare dichroic: they transmitted light green light and reflectedpink, violet, or blue light depending on the location of theSPR band. Further, the optical bandgap Eopg [21] of the sam-ples were defined by the linear absorption coefficient of100 cm−1 at the UV edges. As listed in Table 1, it is notablethat the Eopg values also exhibit an extremum (minimum of

Fig. 2. (Color online) (a)–(c) TEM images taken from BBT-Ag3 in three resolutions. The inset in (a) is a SAED image; the red rings in (b) areindicative of the encapsulated structure. (d) Silver cluster diameter histogram.

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3:03 eV) in 10mol:% TiO2-contained sample (BBT-Ag3), and itis consistent with Dimitrov’ prediction [22] for solids, whichindicated that n0 would empirically exhibit a negative depen-dence on energy bandgap Eg, namely Eopg in present case.

C. Third-Order Optical Nonlinearities (TONs)The formation of ANCs has been confirmed by SEM, TEM, andabsorption spectra, thus it would be an interesting subject to

investigate their TON properties in such a high dielectric med-ium. As the CA curves show in Fig. 5, the peaks following val-leys are indicative of positive NR γ, and their normalizeddistances were estimated to be 1:7Z=Z0 (where Z0 ¼3:01mm is the diffraction length of the laser beam), demon-strating that the nonlinear response was third-order type[18], i.e., negligible components from thermal lensing or freecarriers effect. The linear dependency of NR intensity ΔT

(distance of normalized transmittance between valley andpeak) on optical powers (120.5, 175.2, and 224:1mW wereused in the measurements), as illustrated in Fig. 6, also con-firmed the TONs. Meanwhile, it is very interesting to see thatΔT was strengthened with the progressive increase of TiO2

content up to 10mol:% and attenuated for further addition,which is exactly the same tendency to that of λmax. The max-imum γ value in BBT-Ag3 is 20:82 × 10−18 m2=W, which is al-most 10 times higher than that of BBT-Ag1. However, thisvalue is relatively small compared to those of silver nanopar-ticles embedded in composites reported [4,11] due to theirhigher silver concentration and stronger SPR intensity. By in-troduction, more AgCl to host glasses could obtain much astronger SPR as well as a larger γ value [11], but, in this case,the cumulative thermal effect would be an evident componentand influence the optical–electronic nonlinear process se-verely. Moreover, high AgCl content (above 1wt:%) couldsuppress the phase separation, which consequently reducesthe size of ANCs and results in the blueshifting of the SPRbands. Therefore, a low AgCl concentration (0:3wt:%) waschosen in the present work.

Figure 7 illustrates the evolution of NA obtained from threerepresentative samples (BBT-Ag2, 3, 5). With a progressive in-crease of TiO2 content, the NA profiles converted from satu-rated absorption (SA; peak in the center) to reverse SA (RSA;valley in the center) type, and, notably, no NA signal was ob-served in BBT-Ag3. In general, SA in metal particles embeddedin materials originated from the so-called bleaching of ground-state surface plasmas [23], which attenuated the SPR absorp-tion at high optical irradiation intensity, while its oppositebehavior RSA was due to the inherent two-photon absorption(TPA) [24] when normalized incident photon energy hv=Eopg

(hv ¼ 1:55 eV is incident photon energy at 800nm) was be-tween 0.5 and 1. In the present case, it is clear that the NAwas interplayed between SA and TPA: the Ti4þ ions withlow lying and empty 3d electronic states [25] could generatestrong TPA through electronic transitions between their

Fig. 3. (Color online) (a) Absorption spectra of ANCs embeddedin bismuthate composites with the inset showing the enlargedspectra at the SPR region and (b) normalized absorption spectra atthe SPR region (a, BBT-Ag1; b, BBT-Ag2; c, BBT-Ag3; d, BBT-Ag4;e, BBT-Ag5).

Table 1. Linear Refractive Index n0, Optical Bandgap Eopg, SPR Peak Wavelength λmax, NR γ, NA Coefficient β,Third-Order Nonlinear Susceptibilities χ �3�, Optical Density for 2π Nonlinear Phase Shifting I2π,

and Figure of Merit F of the Composites Studied and References for Comparison

Sample No.n0

�0:001Eopg

(eV)λmax

(nm)γð10−18 m2=WÞ

�30%βð10−12 m=WÞ

�30%jχð3Þjð10−12 esuÞ

�40%I2πðGW=cm2Þ

�30%F

�30%

BBT-Ag1 2.0588 3.15 612 2.09 −5:86 2.28 26.03 −4:49BBT-Ag2 2.0988 3.11 630 7.11 −3:63 7.93 8.85 −0:82BBT-Ag3 2.1666 3.03 668 20.82 0 24.74 2.56 0BBT-Ag4 2.1399 3.05 657 7.45 0.45 8.64 8.30 0.09BBT-Ag5 2.1298 3.07 630 2.45 3.05 2.82 20.45 1.99ZnO-0.5

Ag [4]— 400 44 1555 — — 37.6

Au3Ag6A[12]

— 480 −16000 130000 920 — 8.3

`Z-scan wavelengths used in [4,12] were 532 and 527nm, respectively.

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valence and conduction bands, and this two-photon processprogressively compensated the SPR beaching; when the num-ber of Ti4þ ions reached the threshold (i.e., 10mol:%), the SAwas completely neutralized. For further addition of TiO2, TPAbegan to take over.

To calculate third-order nonlinear susceptibilities χð3Þ, bothγ and β were applied to following formulas:

Reðχð3ÞÞ ¼ n20ε0cπ γ; ð2Þ

Imðχð3ÞÞ ¼ n20ε0c2λ4π2 β; ð3Þ

jχð3Þj ¼ ½Reðχð3ÞÞ2 þ Imðχð3ÞÞ2�1=2; ð4Þ

where Reðχð3ÞÞ and Imðχð3ÞÞ are the real and image parts of χð3Þwhose modulus can be calculated by Eq. (4) and λ is the ex-citation wavelength of 800 nm. As concluded in Table 1, thelargest jχð3Þj value in the order of 10−11 esu was obtained inBBT-Ag3 with the furthest redshifting SPR, and it might notbe the maximum because there is still 132nm from λmax tothe measuring wavelength. Besides, it should be noted thatthe increase of Ti4þ ions in the glass matrix could alsoenhance χð3Þ, but an early study [26] revealed that theχð3Þ of bismuthate glass in the molar composition of

70Bi2O3-15B2O3-15TiO2 was 2:73 × 10−12 esu at 800nm. There-fore, the high TON obtained in present silver–bismuthate com-posites mainly originated from the resonance effect inducedby the introduction of ANCs. Since the minimum gap energy ofsilver was 4:0 eV [27] larger than the incident photon energyhv (1:55 eV), intraband (discrete s–p conduction bands of theANC) transitions caused by electronic oscillations could pro-mote the electromagnetic field in the vicinity of the ANCs.This so-called local field effect contributed significantly toχð3Þ, and the redshifting of SPR extended this behavior to long-er wavelengths.

To evaluate the performance of a nonlinear material for all-optical switching (AOS), two criteria verified by figure of mer-its [28,29] were proposed when consider linear (W) and NA(T) as negative effects:

W ¼ γI2π=αλ > 1; ð5Þ

T ¼ 2βλ=γ < 1; ð6Þ

where I2π is the optical density for achieving nonlinear phaseshifting of 2π and α is the linear absorption coefficient. Forpresent silver–bismuthate composites as summarized inTable 1, (i) the maximum I2π value of 26:1GW=cm2 was ob-tained from BBT-Ag1 and the minimum value was 2:6GW=cm2

for BBT-Ag3, while no optical damage was observed inthe Z-scan measurements (I0 ¼ 4:2� 1:3GW=cm2 at focus),indicating that BBT-Ag3 had the best capacity to overcomethe SPR-induced linear absorption at 800 nm; (ii) the SA that

Fig. 4. Relationship between the linear refractive index (n0) and theSPR peak wavelength (λmax).

Fig. 5. (Color online) CA curves of the silver–bismuthate nanocom-posites measured at 800nm.

Fig. 6. (Color online) Dependence of NR intensity ΔT on opticalpower for three representative samples (BBT-Ag1, 3, 4).

Fig. 7. (Color online) OA curves of the silver–bismuthate nanocom-posites (a, BBT-Ag2; b, BBT-Ag3; c, BBT-Ag5) measured at 800nm.

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enhanced light transmittance at high irradiant intensity couldimprove T values of BBT-Ag1 and BBT-Ag2 due to their nega-tive sign of β; (iii) BBT-Ag3 possessed the best performancefor TON-based AOS due to its largest γ and minimum β value;(iv) the relative weak TPA in BBT-Ag4 manifested its suitabil-ity for AOS when considering TPA as a limitation; (v) BBT-Ag5unfortunately dissatisfied the second criterion due to its highTPA at 800 nm, which came from the high concentrated emptyd orbits in Ti4þ ions; (vi) as compared to other similarnanocomposites reported, the smaller T values and simplerfabrication process of present Ag0–bismuthate compositessuggested that they have better optical switching capacitiesand are promising for further applications.

4. CONCLUSIONSIn summary, ANCs embedded in bismuthate glasses were pre-pared by a thermochemical reduction method. The redshiftingof the SPR of Ag clusters as far as 56nm was achieved by theaddition of 10mol:% TiO2 to the bismuth–borate host medium;however, excess TiO2 content exhibited a negative trend. Z-scan measurements at 800nm showed that TONs contributedfrom intraband transitions in Ag0 possessed a monotonous de-pendence on the location of the SPR band: third-ordernonlinear susceptibilities χð3Þ of the SPR furthest redshiftedsample was more than 10 times higher than that of thenon-TiO2-containing sample. The simple fabrication processand excellent TON performances suggest present silver–bismuthate nanocomposites are promising candidates forAOS and related applications.

ACKNOWLEDGMENTSThis work was supported by the National Natural ScienceFoundation of China (NSFC; grants 60978058, 60908032,and 60972064), the National Natural Science Foundation ofZhejiang Province (grant Y1090996), the Outstanding Disser-tation Engagement Foundation of Graduate School of NingboUniversity (grant PY2009001), the K. C. Wong Magna Fund inNingbo University, and the Program for Innovative ResearchTeam in Ningbo City.

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