Nonlinear Optical Properties of some Au Nanostructures

Transcription

1 Nonlinear Optical Propertie of ome Au Nanotructure S. Couri Intitute of Chemical Engineering and High Temperature Chemical Procee (ICEHT) Foundation for Reearch and Technology-Hella (FORTH), and Department of Phyic, Univerity of Patra Patra 6500, Greece Tel: (0) , Fax: (0) , ABSTRACT The invetigation of the nonlinear optical repone of variou Au nanotructure ha been recently accomplihed in our laboratory and ome elected reult concerning the nonlinear optical propertie of thee nanotructure will be preented and dicued. In particular, the third-order nonlinear optical repone of nanometer thick Au film and Au nano-iland film, both depoited on tranparent dielectric ubtrate, are going to be preented and dicued. In addition, the nonlinear optical repone of Au nano-cluter encapulated in block copolymer micelle will be alo preented and compared with the other nanotructure. In all cae, the influence of the morphological characteritic of thee tructure on the nonlinear optical repone will be dicued. Keyword: nonlinear optic, third-order uceptibility, nonlinear aborption, nonlinear refraction, thin film, Au nanoparticle. 1. INTRODUCTION Material with large third-order optical nonlinearitie and fat nonlinear optical repone are uually conidered promiing candidate for ue in optoelectronic and photonic application uch a optical telecommunication, optical computing, optical data torage, information proceing, etc., where light-controlled phae and refractive index modulation are required. The major requirement of uch material, in addition to large nonlinear optical (NLO) repone, are low loe at the wavelength() of interet, good optical quality and mechanical tability, together with eay preparation procedure and proceability and low cot. In that context, metallic nanoparticle are known to be among the mot uitable and promiing material. In recent year thin film of noble metal nanoparticle (Au, Ag, etc) depoited on or embedded in different dielectric hot [1]-[] have attracted a lot of interet ince, in addition to their inherent large third-order NLO repone, the localized urface plamon reonance (SPR) that they exhibit may be exploited to further enhance their NLO repone. The ize and the hape of thee nanoparticle and their dielectric environment are among the mot important parameter determining the urface plamon frequency and their NLO propertie a well. In thi talk, a ummary of recently obtained reult will be preented concerning the third-order NLO repone of ome Au nanotructure ranging from continuou nanometer thick film, to ultra-thin nano-iland film and nanocluter embedded into block copolymer. The influence of the morphological parameter and preparation condition (i.e. ize and hape of nanoparticle, volume fraction, pre- or pot treatment of the film, etc.) of the tudied nanotructure on their NLO propertie will be dicued and compared with other reearch report.. EXPERIMENTAL.1 Material and Characteriation The continuou Au film were depoited on quartz ubtrate by magnetron DC puttering uing a gold target (99.999%). The ubtrate were degreaed in hot organic olvent and in itu Ar + ion cleaned prior to depoition. The prepared film had thickne ranging from 5 to 5 nm. AFM image have hown lack of any iland formation confirming the continuou character of the film. The average urface roughne wa found to be 0.5, 4.5 and.5 nm for the 5, and 5 nm thick film repectively. The crytallinity of the film wa checked by mean of X-ray-diffraction (XRD) meaurement, while their tranmittance and reflectance have been meaured by a UV Vi NIR pectrophotometer. More detail on the preparation and characteriation of the puttered film are given elewhere [4]. The Au iland film were prepared by thermal evaporation at low depoition rate of nm -1 onto gla ubtrate, either untreated or pre-coated with a thin layer of -aminopropyl trimethoxyilane for improved adheion of the gold to the gla. The gla lide were cleaned and ubequently ilanized. The nominal thickne of the evaporated Au film ranged from 0.5 to 15 nm. Pot-depoition annealing of ome of the Aucovered lide wa carried out for 0 h at 00 0 C. The morphology of both unannealed and annealed film wa tudied by combining high-reolution canning electron microcopy (HRSEM) and atomic force microcopy (AFM), a hown in Fig. 1 confirming that both annealed and unannealed film conited of iland whoe Thi work wa partially upported by the European Union IST Program NANOPHOS (No. IST ) and the Greek Minitry of Development Program PENED (Grant No. 0E 888) /08/$ IEEE 1

2 Figure 1. HRSEM (A, B) and AFM (C, D) image of 5 nm Au iland film on pre-treated gla lide, unannealed (A, C) and annealed (B, D). A repreentative cro-ection i hown under each AFM image. average area increae a the film thickne increae. Annealed film have larger average iland area, average iland height and ditance between iland compared to the correponding unannealed film. Some characteritic UV-Vi-NIR aborption pectra of the ilanized gla/annealed iland film are hown in Fig. a. A can be een, a pronounced feature in the aborption pectra of all the film in the nm thickne range i a well-defined localized SPR peak, a property characteritic of nanoparticulate metal tructure urrounded by a dielectric hot [5] below the percolation threhold. A the film nominal thickne increae, an increae of the optical denity and a red hift of the SPR peak are oberved, while the SPR diappear completely for thick enough film, e.g. 15 nm, correponding to a tranition to continuou, metallic film [6]. Thi reult i in accordance with the HRSEM image of 15 nm film where a near-percolation ituation occur. In addition, it wa found that annealing of the film promote a blue hift of the SPR peak and a decreae of the optical denity, aociated with the morphological change occurring in the iland film. Silanization of the gla ubtrate reulted generally in modification of the aborption pectra attributed to the improved adheion of Au nanoparticle to the pre-treated (ilanized) ubtrate. The preparation of the Au nanocluter embedded into block copolymer uch a poly(ioprene-b-acrylic acid) (PI-PAA) occurred in olution, uing a elective olvent for one of the polymer block cauing the formation of micelle, which were then loaded by the addition of variou amount of alt precuror containing the metal cation. The alt were preferentially diolved into the micelle core due to the polar environment. Addition of a reducing agent, e.g. hydrazine, caued the reduction of the cation to metal nanoparticle. Poly(ioprene-b-acrylic acid) block copolymer (PI-PAA) were yntheized by anionic polymerization and pot polymerization hydrolyi. The particular block copolymer form micelle in THF with poly(acrylic acid) core and poly(ioprene) corona, which can be ued a nanoreactor for the templated ynthei of Au nanocluter from alt precuror, via chemical reduction. Figure b preent the aborption pectra of two polymer/au nanoparticle ytem which although have the ame concentration in Au, they exhibit different polymer to gold ratio a well a different acrylic acid to ioprene ratio regarding the polymer.. Experimental Technique Figure. Aborption pectra of: a) Au iland film, annealed, depoited on ilanized gla ubtrate, and b) IAA-5:Au 1:1 and IAA-:Au :1 in THF.

3 For the invetigation of the NLO propertie of the Au nanotructure two experimental technique have been ued, namely the Optical Kerr Effect (OKE) and the Z-can technique employing a mode-locked 5 p and a Q-witched 5 n Nd:YAG laer repectively, both operating at 5 nm at 10 Hz. In the Optical Kerr Effect technique [7], an intene linearly polarized beam (c.a. the pump) i focued in a nonlinear medium and induce a nonlinear polarization in the direction of it polarization which reult in an induced optical birefringence, which i then probed by a weak laer beam (i.e. the probe). Delaying the probe in repect to the pump, e.g. by mean of a time delay line, the temporal repone of the nonlinearity of the medium can be oberved. In thi cae, the ignal time evolution i decribed by the following relation: πl It ( τ) = Eprobe ( t τ) in δn dt λ (1) where E probe i the probe electric field, τ i the time delay between the pump and the probe, l i the length of the ample, λ i the wavelength of the laer and δn = n I pump, where I pump i the intenity of the pump. The maximum ignal occur at zero time delay (e.g. τ = 0) between the pump and the probe beam. For low value of δn, the ignal I t (0) i proportional to (δn). In that cae, the nonlinear refractive index n of the medium can be determined with repect to the known n of a reference material. Then, the effective third-order uceptibility χ () of the ample i determined by the equation: 1 ( ) αl I n lr α l / r e Ir nr l α l ( 1 e ) χ ωωω ;,, ω = χ ωωω ;,, ω where the ubcript and r, refer to the ample and the reference material repectively, I i the meaured OKE ignal intenity (i.e. I t (0)), n i the linear refractive index, α i the linear aborption coefficient of the ample and l i the interaction length of the beam with the material. Z-can technique i baed on the meaurement of the tranmiion of a focued Gauian laer beam through a ample a it move along the laer propagation direction around the focal plane. Two different tranmiion meaurement are performed; the cloed-aperture z-can, where the tranmitted laer beam i meaured after it pae through an aperture placed in the far field and the open-aperture one, where the tranmitted beam i meaured after it i totally collected. Dividing the cloed-aperture z-can curve with the correponding openaperture z-can, the o called divided z-can i obtained. Open-aperture and divided z-can allow for the imultaneou determination of both the magnitude and the ign of the nonlinear aborption (β) and refraction (γ ) parameter of the ample. β and γ are directly related to the imaginary and real part of the third-order uceptibility χ () repectively through the following relation: ( ) 7 eu c n0 6 Im χ = 10 β 96π ω and Re χ eu = 10 c n γ 480π () 0 where γ i given in cm /W, β in cm/w, c i the peed of light in cm/ec and ω i the fundamental frequency in cycle/ec. The nonlinear refractive indexe n (eu) and γ (m /W) are related through the equation: ( m/ 40π) γ ( m /W) n eu = c n (4) 0 The detail of the procedure for the determination of β and γ from the experimental z-can meaurement have been given elewhere [8], [9].. RESULTS AND DISCUSSION The continuou puttered Au film were meaured by both OKE and Z-can technique, uing 5 nm, 5 p laer pule. Figure preent the time dependence of the OKE ignal of the film and the reference material CS at an intenity of 140 MW/cm. All repone were found to be fat, being limited by the laer pule width. The intenity dependence of the OKE ignal of the Au film and CS were all found to exhibit a quadratic power dependence on the pump intenity (the probe intenity kept contant) in agreement with theoretical aumption [7], characteritic of a third-order proce. Then the third order uceptibility χ () of the film wa determined. Figure 4 preent the variation of the figure of merit χ () /α, where α i the linear aborption coefficient, a a function of the film thickne. From the Z-can meaurement the nonlinear aborption parameter of the 5 nm thick film wa determined OKE ignal (a.u) 1 CS Au film 5 nm Au film nm Au film 55 nm x time delay τ (p) () Figure. Time dependence of the OKE ignal of the 5, and 5 nm puttered Au film together with the reference material CS.

4 to be cm/w correponding to an imaginary part of the third order nonlinear uceptibility Imχ () = (4±) 10-9 eu. Then the NLO repone of the Au iland film wa invetigated and compared with that of the continuou film. The effective third-order uceptibility χ () eff of each film wa determined uing CS a reference material. Meaurement of the untreated and uncoated gla ubtrate performed under exactly the ame experimental condition have not revealed any NLO repone. Since the film Au content, and conequently the aborption, varie with the thickne of the film, the figure of merit χ () eff/α wa introduced, defined a previouly. Figure 5 preent the dependence of the determined χ () eff/α value of everal Au iland film veru film thickne. In the ame figure the value obtained for the continuou, puttered Au film are alo hown for comparion. It i obviou from figure 5, that tranition from dicontinuou (i.e. dielectric) Au film to Figure 5. χ () eff/α of the nano-iland Au film veru thickne. χ /α (eu cm) 7,0x10-15 Au film 6,0x ,0x ,0x10-15,0x10-15,0x thickne (nm) Figure 4. χ () /α of the puttered Au film veru thickne. continuou (i.e. metallic) one lead to a decreae of the effective third-order uceptibility by almot two orderof-magnitude. Both the magnitude and the ign of the imaginary and real part of the third-order uceptibility of the Au iland film were determined from Z-can meaurement. The obtained divided z-can curve of all tudied film, except thoe of 15 nm thickne, were found to exhibit a pre-focal tranmiion valley followed by a tranmiion peak (i.e. a valley-peak configuration), revealing the poitive ign of Reχ () correponding to elffocuing. The 15 nm film, being nearpercolation threhold, exhibited flat cloedaperture curve due to trong aborption. The open-aperture z-can trace obtained for the film below the percolation threhold were found to exhibit a tranmiion peak, due to a decreae of the aborption at the highet incident intenity, correponding to a aturable aborption behaviour and therefore a negative ign of Imχ (). The 15 nm film were found to exhibit tranmiion valley uggeting a revere aturable aborption ituation and a poitive Imχ (). Figure 6 preent the open-aperture trace (a, c) of a 5 and a 15 nm thick Au iland film repectively, a well a the divided (b) z-can trace of a 5 nm thick Au iland film. The correponding laer intenitie of Fig. 6a,b and c being 0.5 and 0. GW/cm repectively. The change of the ign of the Imχ (), correponding to the tranition of the film from dicontinuou (dielectric) to continuou (metallic) one i apparent in Fig. 6a,c. A can be een, the repone of the film witche from aturable aborption (SA) to revere aturable aborption (RSA). The magnitude of the Reχ () value a obtained from the Z-can technique for the ilanized, annealed film ranged from (4.±1.1) 10-7 eu to (5.±1.1) 10-7 eu, wherea that of Imχ () ranged from (0.44±0.1) 10-7 eu to (.6±0.76) 10-7 eu repectively. Finally the NLO repone of everal different hybrid block copolymer micelle which are encapulating metal nanoparticle wa invetigated, employing the Z-can technique under 5 p and 5 n laer pule. Several ytem with varying polymer to Au ratio a well a with different concentration of gold nanoparticle in the olution have been tudied. It wa found that increaing the gold content of the ample generally reulted in an increae of the NLO repone, for both pule duration regime. Then, by keeping the metal concentration, a well a the polymer to gold ratio unchanged, two different ytem have been tudied exhibiting varying acrylic acid to ioprene ratio. In particular ratio of about 1/5.5 (IAA-5) and /1 (IAA-) were invetigated. It wa found that the former ytem wa exhibiting ignificantly larger NLO repone, which i attributed to the formation of maller ize and much more table micelle. 4

5 Comparion of the reult of the experiment carried out under 5 p and 5 n laer pule revealed that although the Reχ () maintained it poitive value, the Imχ () changed ign from poitive to negative, indicating a tranition from revere aturable aborption to aturable aborption type behavior, a it i obviou in Fig. 6d,e and f. The Reχ () and Imχ () value were found to be quite large for all the polymer/au ytem tudied reaching value of (.±0.) 10-1 eu, (.1±0.) 10-1 eu under p excitation and (10.6±0.7) 10-1 eu, (109.±1.6) 10-1 eu under n excitation. Figure nm Au nano-iland film: (a) open-aperture (SA behaviour) and (b) divided z-can at 0.5 GW/cm. 15 nm Au nano-iland film: (c) open-aperture z-can (RSA behaviour) at 0. GW/cm. Both film were annealed and ilanized. Au nano-cluter in block copolymer: IAA-:Au :1 olution (6 mm) in THF (d) open-aperture at 5 p, 9.1 GW/cm, (e) 8 n, 4. MW/cm, (f) divided z-can at 5 p, 9.1 GW/cm REFERENCES [1] N. Pincon, et al.: Third-order nonlinear optical repone of Au:SiO thin film: Influence of gold nanoparticle concentration and morphologic parameter, Eur. Phy. J. D., vol. 19 pp , 00. [] I. Tanahahi, et al.: Optical nonlinearitie of Au/SiO compoite thin film prepared by a puttering method, J. Appl. Phy., vol. 79 pp , [] H.B. Liao, et al.: Large third-order optical nonlinearity in Au:SiO compoite film near the percolation threhold, Appl. Phy. Lett., vol. 70 pp. 1-, [4] E. Xenogiannopoulou, et al.: Third-order nonlinear optical propertie of thin puttered gold film, Opt. Commun., vol. 75 pp. 17-, 007. [5] D. Stauffer, A. Aharony, in Introduction to Percolation Theory, Taylor & Franci, 199. [6] I. Doron-Mor, et al.: Ultra thin gold iland film on ilanized gla. Morphology and optical propertie, Chem. Mater., vol. 16 pp , 004. [7] P. P. Ho, R. R. Alfano: Optical Kerr effect in liquid, Phy. Rev. A, vol. 0 pp , [8] S. Couri and E. Koudouma, in Nonlinear Optical Repone of Molecule, Solid and Liquid: Method and Application, Ed: M. G. Papadopoulo, Reearch Signpot, Trivandrum, India, 00. [9] S. Couri, et al: Concentration and wavelength dependence of the effective third-order uceptibility and optical limiting of C 60 in toluene olution, J. Phy. B: At. Mol. Opt. Phy., vol. 8 pp ,