Near-field nanoimprinting using colloidal monolayers 1,3,6 F. Javier Garc´ıa de Abajo,4,5,∗ and ¨ Christin David,1,2,6 Paul Kuhler, Jan Siegel1,7

´ de Optica, Consejo Superior de Investigaciones Cient´ıfcas, Madrid, Spain de Qu´ımica-F´ısica “‘Rocasolano”’, Consejo Superior de Investigaciones Cient´ıfcas, Madrid, Spain 3 Photonics and Optoelectronics Group, Ludwig-Maximilians-University Munich, Germany 4 ICFO - Institut de Ciencies Fotoniques, Mediterranean Technology Park, Castelldefels, Spain 5 ICREA - Instituci´ o Catalana de Recerca i Estudis Avanc¸ats, Barcelona, Spain 6 These authors contributed equally to the work 7 [email protected] 1 Instituto

2 Instituto

[email protected],

Abstract: We experimentally and theoretically explore near-field nanopatterning obtained by irradiation of hexagonal monolayers of micron-sized polystyrene spheres on photosensitive Ge2 Sb5 Te5 (GST) films. The imprinted patterns are strongly sensitive to the illumination conditions, as well as the size of the spheres and the orientation of the monolayer, which we change to demonstrate control over the resulting structures. We show that the presence of multiple scattering effects cannot be neglected to describe the resulting pattern. The experimental patterns imprinted are shown to be robust to small displacements and structural defects of the monolayer. Our method enables the design and experimental verification of patterns with multiple focii per particle and complex shapes, which can be directly implemented for large scale fabrication on different substrates. © 2014 Optical Society of America OCIS codes: (350.4600) Optical engineering; (160.2900) Optical storage materials; (050.5298) Photonic crystals; (000.2190) Experimental physics; (000.6800) Theoretical physics.

References and links 1. E. Mcleod and C. B. Arnold, “Subwavelength direct-write nanopatterning using optically trapped microspheres,” Nat. Nano 3, 413 (2008). 2. X. A. Zhang, J. Elek, and C.-H. Chan, “Three-Dimensional Nanolithography Using Light Scattering from Colloidal Particles,” ACS Nano 7 (7), 6212–6218 (2013). 3. A. Sundaramurthy, P. J. Schuck, N. R. Conley, D. P. Fromm, G. S. Kino, and W. E. Moerner, “Toward NanometerScale Optical Photolithography: Utilizing the Near-Field of Bowtie Optical Nanoantennas,” Nano Lett. 6, 355 (2006). 4. I. Mart´ın-Fabiani, J. Siegel, S. Riedel, J. Boneberg, T. A. Ezquerra, A. Nogales, “Nanostructuring Thin Polymer Films with Optical Near Fields,” ACS Appli. Mater. Interfaces 5 (21), 11402–11408 (2013). 5. O. Watanabe, T. Ikawa, M. Hasegawa, M. Tsuchimori, and Y. Kawata, “Nanofabrication induced by near-field exposure from a nanosecond laser pulse,” Appl. Phys. Lett. 79, 1366–1368 (2001). 6. Z. B. Wang, M. H. Hong, B S. Luk’yanchuk, Y. Lin, Q. F. Wang, and T. C. Chong, “Angle effect in laser nanopatterning with particle-mask,” J Appl. Phys. 96, 6845 (2004). 7. D. Brodoceanu, L. Landstr¨om, and D. B¨auerle, “Laser-induced nanopatterning of silicon with colloidal monolayers,” Appl. Phys. A 86, 313 (2007). 8. T. Sakai, N. Nedyalkov, and M. Obara, “Positive and negative nanohole-fabrication on glass surface by femtosecond laser with template of polystyrene particle array”, J. Phys. (Paris) D: Appl. Phys. 40, 2102 (2007).

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Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8226

9. A. Pereira, D. Grojo, M. Chaker, P. Delaporte, D. Guay, and M. Sentis, “Laser-fabricated porous alumina membranes for the preparation of metal nanodot arrays,” Small 4, 572–576 (2008). 10. R. Morarescu, L. Englert, B. Kolaric, P. Damman, R. A. L. Vallee, T. Baumert, F. Hubenthal, and F. Trager , “Tuning nanopatterns on fused silica substrates: a theoretical and experimental approach,” J. Mater. Chem 21, 4076 (2011). 11. L. Li, W. Guo, Z. B. Wang, Z. Liu, D. J. Whitehead, and B. Luk’yankchuk, “Large-area laser nano-texturing with user-defined patterns,” J. Micromech. Microeng. 19, 054002 (2009). 12. Z. B. Wang, W. Guo, B. Luk’yankchuk, D. J. Whitehead, L. Li, and Z. Liu, “Optical Near-field Interaction between Neighbouring Micro/Nano-Particles,” J. Laser Micro/Nanoeng. 3 (1), 14–18 (2008). 13. T. Miyanishi, Y. Tsunoi, M. Terakawa, and M. Obara, “High-intensity near-field generation for silicon nanoparticle arrays with oblique irradiation for large-area high-throughput nanopatterning,” Appl. Phys. B 107, 323–332 (2012). 14. P. K¨uhler, F. J. Garc´ıa de Abajo, J. Solis, M. Mosbacher. P. Leiderer, C. Afonso, and J. Siegel, “Imprinting the Optical Near Field of Microstructures with Nanometer Resolution,” Small 5, 1825 (2009). 15. N. Yamada, E. Ohno, K. Nishiuchi, N. Akahira, and M. Takao, “Rapid-phase transitions of GeTe-Sb2 Te3 pseudobinary amorphous thin films for an optical disk memory,” J. Appl. Phys. 69, 2849 (1991). 16. J. Siegel, A. Schropp, J. Solis, C. N. Afonso, and M. Wuttig, “Rewritable phase-change optical recording in Ge2 Sb2 Te5 films induced by picosecond laser pulses,” Appl. Phys. Lett. 84, 2250 (2004). 17. J. Siegel, W. Gawelda, D. Puerto, C. Dorronsoro, J. Solis, C. N. Afonso, J. C. G. de Sande, R. Bez, A. Pirovano, and C. Wiemer, “Amorphization dynamics of Ge2 Sb2 Te5 films upon nano- and femtosecond laser pulse irradiation,” J. Appl. Phys. 103, 023516 (2008). 18. P. K¨uhler, F. J. Garc´ıa de Abajo, P. Leiprecht, A. Kolloch, J. Solis, P. Leiderer, and J. Siegel, “Quantitative imaging of the optical near field,” Opt. Express 20, 22063–22078 (2012). 19. B. Lee, J. Abelson, S. Bishop, D. Kang, B. Cheong, and K. Kim, “Investigation of the optical and electronic properties of Ge2 Sb5 Te5 ,” J. Appl. Phys. 97, 18 (2005). 20. N. Stefanou, V. Yannopapas, and A. Medinos, “Heterostructures of photonic crystals: frequency bands and transmission coefficients,” Nat. Nano 3, 413 (2008). 21. F. J. Garc´ıa de Abajo, “Multiple scattering of radiation in clusters of dielectrics,” Nat. Nano 3, 413 (2008). 22. N. Stefanou, V. Yannopapas, and A. Medinos, “MULTEM 2: A new version of the program for transmission and band-structure calculations of photonic crystals,” Nat. Nano 3, 413 (2008). 23. R. Sainidou, N. Stefanou, I. Psarobas, and A. Medinos, “A layer-multiple-scattering method for phononic crystals and heterostructures of such,” Nat. Nano 3, 413 (2008).

1.

Introduction

Optical near-fields in the vicinity of metal or dielectric microstructures upon external illumination display complex patterns that can be used for nanoscale imprinting on substrate surfaces.[1, 2, 3, 4] This is a promising concept for nanoprocessing large regular patterns, for example by arranging colloidal particles to form closed-packed layers that act as a mask. This approach has been used with different types of surfaces and for various particle materials and shapes.[5, 6, 7, 8, 9, 10] Engineered structures exploiting the focal points of the colloids become realizable in practice by angular beam scanning[11]. It is generally known that the resulting patterns not only correspond to a linear superposition of the optical near-fields of the single particles, but also that particle interaction and mutliple scattering effects become important for touching particles in a colloidal monolayer.[12] Recent FDTD simulations for periodic boundary conditions in a Si particle monolayer have been used to identify optimum particle array and irradiation parameters for maximum field enhancement at the substrate surface near the contact point with the particle. [13] Moreover, collective effects in the resulting 2D photonic crystal produce a wealth of features, beyond a single region of field enhancement as typically reported and exploited, which enlarge the suite of achievable imprinting structures. In this combined experiment-theory study, we address the optical near-fields imprinted by colloidal particle arrays to gain insight into the underlying mechanisms of pattern formation. 2.

Experiments

We use chalcogenide phase-change substrates to record and subsequently image the near-field light intensity with subwavelength resolution.[14] Upon irradiation with short laser pulses, a #203796 - $15.00 USD (C) 2014 OSA

Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8227

(c) ­



(c)

ka

(a)

(d) (d) ­ (e) ­

(b) Reflectance |r|



1

(e)

0

0 0



kjja



min

I/I0 max

Fig. 1. (a) Hexagonal lattice of polystyrene spheres with an underlying calculated near-field pattern, which is imprinted into a GST substrate. The calculation details are as in (d) (see below). The incident light is p-polarized and has wave vector along a nearest-neighbors bond direction (φ = 0◦ ). (b) Dispersion diagram showing the reflection coefficient |r| of a closed-packed monolayer of polystyrene spheres on an fcc GST planar substrate. (c)-(e) Near-field associated with specific points in the dispersion diagram for a = 817 nm (see corresponding symbols in (b)). Wavelengths and angles of incidence used: (c) λ = 409 nm, θ = 40.8◦ ; (d) λ = 799 nm, θ = 52.2◦ ; (e) λ = 925 nm, θ = 30.0◦ .

change from the crystalline to the amorphous phase is induced in regions where the local intensity is large enough, effectively mapping complex intensity patterns for varying illumination conditions with great detail.[15, 16, 17] The phase transition induced is also accompanied by a change in material density, topography, and electric conductivity, which makes scanning electron microscopy (SEM) a suitable high-resolution read-out technique.[18] We have applied this method to study the near-field distributions of 2D hexagonally arranged arrays of polystyrene (PS) spheres (nPS = 1.58, κ = 0.003 at λ = 800nm) as a function of several experimental parameters, as illustrated in Fig. 1(a). The substrates consist of 40-nm-thick, face-centered-cubic (fcc) polycrystalline Ge2 Sb5 Te5 (GST) films[15, 16, 17] sputter-deposited on Si [001] wafers covered by a 10-nm-thick amorphous SiO2 buffer layer (Numonyx, Italy). The complex index of refraction (n + iκ) of these materials at the experimental laser wavelength (800 nm) is 5.72 + 4.09i for fcc-GST[19], 4.74 + 1.45i for amorphous GST[19], 1.453 + 0i for amorphous SiO2 , and 3.69 + 0.006i for Si. Closed-packed monolayers of spherical PS particles (diameters 817 nm and 1704 nm, Microparticles GmbH, polydispersity 2.6%) were self-assembled on a wanPS d ter surface and then deposited onto the substrate. Given their large Mie parameter 2π λ 2  1, each particle produces collimating lensing, involving the participation of many multipoles up to a high order. Laser irradiation of the particle covered films was performed in air using a regeneratively amplified Ti:sapphire laser system operating at 800 nm central wavelength with a pulse duration of 350 ps. The laser beam was focused onto the sample at an angle of incidence θ = 52.2◦ to a measured elliptical spot size of 270 × 150 µm2 (1/e2 diameter). A single pulse was selected from a 100 Hz pulse train by means of an electromechanical shutter to irradiate the targeted area. The sample was mounted on a motorized 3D translation stage and observed with a home-built microscope for in-situ control. Subsequent to laser irradiaton, the particles were removed with a scotch tape in order to access the patterns because the laser irradiation at the fluences required for imprinting the near-field patterns did not lead to removal of the particles. Additionally,

#203796 - $15.00 USD (C) 2014 OSA

Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8228

(a)

Imax=0.84

Imax=0.61

(b)

q = 20o

(d)

Imax=0.28

q = 30o

Imax=0.16

(e)

q = 50o

0

(c)

q = 40o

(f)

q = 60o

I/I0

Imax=0.30

Imax=0.15

q = 70o

Imax

Fig. 2. Imprinted near-field intensity calculated for increasing angles of incidence with fixed azimuthal sample lattice rotation φ = 21◦ , particle size a = 1730 nm, and p-polarized light of wavelength λ = 900 nm. Gray circles signal particle positions. The maximum of intensity beneath the GST-air interface is given in each figure, while the color scale is normalized as indicated in the lower color bar. The imprinted intensity drops for increasing inclination.

images were taken using a Gemini Ultra Plus field emission scanning electron microscope (SEM) operated at 5 kV and yielding 5 nm spatial resolution. 3.

Results and interpretation

We performed multiple scattering calculations using a layer-KKR approach[20, 21, 22, 23] for periodic particle arrays, which are converged with respect to both the number of multipoles used for each sphere and the number or reciprocal lattice vectors in the layer-substrate coupling. Fig. 1(b) shows the calculated band diagram for a hexagonal PS sphere monolayer on crystalline GST. The upper triangular region above the light line (k = ωc nPS ≡ kk ) allows us to identify configurations of particle diameter a (equal to the lattice spacing in the closed-packed structures), incident wavelength λ = 2π k , and incidence angle (related to the parallel wave vector through kk = k sin θ ) associated with resonant optical modes, where high local field enhancement is expected. Incidentally, the kinematical small-particle bands given by |kk − Gnm | = k, where Gnm runs over reciprocal lattice vectors (superimposed curves in Fig. 1(b)), differ from the numerical bands due to inter-particle interaction (see Supporting Information (SI) for more details). The remarkable diversity in the near-field distribution patterns at the GST-air interface for

#203796 - $15.00 USD (C) 2014 OSA

Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8229

(a) (a)

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FIG. FIG. 3. 3. Influence Influence of of lattice lattice rotation rotation φ φ on on FIG. 2. incident Influence ofpolarization incident light polarization on thefor imFig. 3. Influence lightof on thepolarization imprinted near-field λ = 799 nm,for field a = 817 nm and p-polarized ligh field for a = 817 nm and p-polarized ligh FIG. of 2. Influence incident light on the im◦ ◦ printed near-field for λ = 799 nm, a = 1704 nm, and 799 a = 1704 nm, and θnear-field = 52.2 . Gray in the calculated graphsnm, indicate 799 nm nm and and θθ = = 52.2 52.2◦.. Gray Gray circles circles sign sign printed for λcircles = 799 nm, a = 1704 andthe sphere◦ θ = 52.2 circlesSEM in the calculated graphs indicate the (c,d) the calculated images. Measured ◦ . Gray substrate contact points. Measured images (a,b) are compared with theory for the calculated images. Measured SEM SEM im im θ = 52.2 . Gray circles in the calculated graphs indicate the ◦ sphere-substrate contact points. Measured SEM images (a,b) with the following parameters: (a),(c) s-polarization, and lattice SEM rotation φ = 22.3 withtoaa sideways sideways detector detector that that is is insensit insensit sphere-substrate contact points. Measured images (a,b)(see inset ◦ are compared with theory (c,d) for the following parameters: point compared with theory (c)); (b),(d)are p-polarization, φ = 20.6 . (c,d) point defects) defects) are are compared with theory for the following parameters: ◦ compared with theory (a),(c) s-polarization, and lattice rotation φ = 22.3◦◦ (see inset (a,c) and φ = 23 ◦ in (b,d). (a,c) and φ = 37 in (b,d). (a),(c) s-polarization, and lattice rotation φ = 22.3 (see inset to (c)); (b),(d) p-polarization, φ = 20.6◦ . to (c)); (b),(d) p-polarization, φ = 20.6◦ .

selected points of the band diagram (see Fig. 1(c)-(e)) clearly illustrates the large sensitivity of the imprinted structures to geometrical and illumination parameters. This allows apresumably rich variety as a result of slightly varyi ◦ Fig. 4(a) and (c). This demonstrates The left (right) side corresponds to φ = 19 (φ = of patterns to be3.created that display the periodicity of the colloidal monolayer mask. Notice preparation. Furthermore, ing sample The dependence on the lattice orientation relative to ◦ produced with regular colloidal monola 37 the ).projected Symmetry is reduced respect to theφ)original that the change in particle size-to-wavelength ratio of the patterns 1calculated varies ima with the array superposition the light incidencewith direction (angle isshown dis- in Fig. riodicity of the particle and cannm monolayer for these directions of incidence. Significant the complexity of the near-fields. TheThe near-field intensity is given as the ratio intensity siderably different shapes and cussed in Fig. 3. left (right) side corresponds to of the simple interference of incomingamplitud and sca differences between both of them also observable in field ◦ I directly beneath the41 GST normalized by is theare intensity of the incident .measured This observes those in Fig. or calcul φ = (φsurface = 23◦ ). Symmetry reduced with respect eachI0particle the4(a) scattered fiel the detailed shape of position of the near-field maxima model. This demonstrates that the p indicates the absorbed intensityaxis thatand produces phase-change nanoimprinting. to thefield horizontal significant differences between Another interesting feature can be relative to the spheres. cannot be In Fig. 2 we compare for fixed azimuthal sample orientation and increasing an-explained by simple interfe both of simulations them are observable in the detailed shape of the 4(a). The absence of a single contact p [[[[[[[[ We want to emphasize that the intensity pattern and scattered position of the near-field maxima relative to the gles of p-polarized light incidence. The imprinted intensity drops withspheres. increasing angle of inci- light because each partic tion marked by a dashed circle) indicat produced by a colloidal monolayer cannot be described icant contributions originating in dence, but it acquires more complex structure, due pattern to the involvement a field richer Wea want to emphasize thatpresumably the intensity proatofthis position might have been missin by as a simple superposition of patterns by sinneighbors. duced by a colloidal monolayer cannotofproduced be by structure of modes, observed in the dispersion diagram Fig.described 1(b) with increasing k . k illumination. Such lattice defe during spheres, without considering particle interactions. To A remarkable featureobserved can be observe aglesimple superposition of patterns produced by polarization, single Additionally, pattern complexity is influenced by the incident-light see Fig.are 3. frequently cancies in col demonstrate this, considering we have performed an experiment usspheres, without particle interactions. absence of a single contact pointDespit (at th From here on, light is considered to be coming from the left in all figures.ToThe upper panels as can be seen in Fig. 4(b). ing conditionsthis, for we imprinting a relatively simple pattern by a dashed circle), indicating that th demonstrate have performed an experiment show measured SEM images of structures created with a single laser pulseusat an angle of inciimprinted pattern shown in Fig. 4(a) (Fig.conditions 4(a)), which we comparea to the result of ourpattern model ing for imprinting relatively simple position might have been missing in the dark spots at position II in Fig. 3(a) can also be found in regions that were dence of 52.2◦ . The responding main maximum next to th (Fig. 4(a)), 4(c)) which and to the result with of a simple superposition Such lattice defects inthat for (Fig. compare calculations both for (seeillumination. not irradiated, and thus,calculation they are notwe associated with light irradiation effects SI, Fig. 9 there was2).no defect. We speculate of the result for a single particle , shown in multiple scattering method noted above (Fig. 4(c)), frequently observed in colloidal monol They can only bethe detected with the inlens detector, indicating they are small modifications of tive interaction between the particles Fig.well 4(d). The alatter wassuperposition done for a hexagonal lattice Fig. 4(b). thismonolayer defect, therobu im asfrom with simple of spheres the scattered the GST surface as that11 result adsorbants deposited at the PS contact region. They are Despite optical response of the of × 11 particles. Neglecting 9 thus collective effects field of each individual particle (single scattering, Fig. Fig. 4(a) features the corresponding ma therefore suitablesuch to determine the position during illumination of the spheres even theirdefects, featuring photonic of after lattice asThe mutliple scattering from neighboring particles, to the defect, just as if there was no de 4(d)). latter was carried out for a hexagonal lattice Yet, we cannot exclude the p removal. In contrast, the dark regionsmodel with adoes brighter circle inside the (position I) are a erties. direct result the11superposition not reproduce relative of × 11 particles, neglecting collective effects such as that the strong interaction defect was not acollective vacancy but a particlb of irradiation. Bypositions comparison with optical micrographs, the observed modifications of the GST of the intensity maxima, which is predicted to cles might render the optical response multiple scattering from neighboring particles. This suabove the substrate, thus not formin film can be attributed to amorphization of the otherwise crystalline film (outer ring) and ablation lie much nearer to does the contact points thanposition found experperposition model not reproduce the of the robust against theinpresence lattice but contributing a slightlyofdifferen imentally and correctly predicted by our model. Note collective photonic crystal-like properti dominant intensity maxima, which are predicted to lie cal response. More studies are needed that the contact arepoints more than pronounced here than much closer to thepoints contact found experimenexclude the possibilityofthat the defect degree robustness monolayers to #203796 - $15.00 USD Received 30 Dec 2013; revised 12 caused Mar 2014;by accepted 14differences Mar 2014; published 1 Apr of 2014 in Fig. 2 which is presumably slight but a particle slightly elevated above t tally. At the same time, the multiple scattering model (C) 2014 OSA 7sample April 2014 | Vol. 22, No. 7Furthermore, | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8230 during preparation. the calculated In conclusion, we have experimentall predicts a distance between maxima and contact points not forming a contact point but contrib intensity showwith a considerable in terms cal near-fields of the colloidal monolayers different way to optical response.b that is inmaxima agreement experiment.difference Notice that the of shape and amplitude with respect to those found in on thin GST films. The measured inte contact points are more pronounced here than in Fig. 2, explanations imply that the imprinted

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Fig. 4. Influence of lattice rotation φ on the imprinted near-field for a = 817 nm and pFIG.incident 3. Influence of 799 lattice rotation φ on the imprinted near◦ . Gray light with λ = nm and θ = 52.2 circles signal contact points of incident light polarization on thepolarized imfield for a = 817 nm and p-polarized light incident with λ = detector the calculated images. Measured◦ SEM images (a,b) (acquired with a sideways for λ = 799 nm, a = 1704 nm,inand 799 nm and θ = 52.2 . Gray circles signal contact points in that is insensitive to the contact-point surface modifications) are compared with theory (c,d) rcles in the calculated graphs indicate the the calculated images. Measured SEM images (a,b) (acquired ◦ φ = 41 in (a,c) and φ = 23◦ in (b,d). ntact points. Measured SEM images for (a,b) with a sideways detector that is insensitive to the contacttheory (c,d) for the following parameters: point defects) are compared with theory (c,d) for φ = 19◦ in ◦ n, and lattice rotation φ = 22.3 (see inset (a,c) and φ = 37◦ in (b,d). olarization, φ = 20.6◦ . (enclosed by the bright rim). [18] A relevant parameter is the lattice orientation angle φ relative

to the light incidence direction (see inset of Fig. 3(c)). Regions of similar orientation φ were selected in Fig. 3, so the main differences between panels (a) and (b) is the light polarization ◦ Fig. 4(a) and (c). Thisonly demonstrates that theshaped patterns t) side corresponds to φ =(s19 (φ = and p, respectively). While displaying a simple elliptically maximum at I for s monolayers reflectthethe pe- maximum at IV, is reduced with respect to the original the produced polarization, imprint is with more regular complexcolloidal for p polarization. Beside main of theseveral particle andfeatures cannotare berevealed explained byIII), including an ese directions of incidence. which Significant is still wellriodicity pronounced, lessarray intense (see simple interference of incoming and scattered light, since en both of them are also observable in auxiliary maximum at the contact point V, which can not be observed for s polarization. The reeach particle observes the scattered fields of its neighbors. e of position of the near-field maxima markable modifications observed in the spatial patterns when the light polarization is changed interestingoffeature observed in given Fig. light frequency heres. can be traced backAnother to the involvement several can arraybe modes for any 4(a). The absence of a single contact point (at the posito emphasize that the intensity and pattern direction of incidence. Their excitation strongly depends on the orientation and amplitude tion marked by a dashed circle) indicates that particle lloidal monolayer cannot beofdescribed the in-plane electric field vector of the incoming light, see Fig.the 1 (b). Essentially, different at this position might have been missing in the monolayer rposition of patterns produced by sin- couple with different strengths to these modes, thus producing different spatial polarizations during illumination. Such lattice defects in form of vaut considering particle interactions. patterns. To The same arguments apply to the variations on the produced patterns with the angle cancies are frequently observed in colloidal monolayers, we have performed an experiment usof incidence (Fig. 2) and the lattice rotation that we discuss in the following. as can be seen in Fig. 4(b). Despite this defect, the imprinting a relatively simpleThe pattern lower panels in Fig. 3pattern depict our corresponding calculations imprinted shown in Fig. 4(a) featureswith the grey cor- circles superimwe compare to the result of our model posed to indicateresponding the positionsmain of contact points. The agreement between theory maximum next to the defect, just as if and experiment o the result of a simple superposition is good. In particular, the occurence multiple localthat maxima as well collecas their detailed shape there was no defect.ofWe speculate the strong result for a single particle9 , shown in and position depending on the laser polarization is well reproduced in the calculations. As intive interaction between the particles might render the atter was done for a hexagonal lattice dicated in Fig. 1(c), the number of near-field maxima is not limited by the number of particles optical response of the monolayer robust to the presence les. Neglecting thus collective effects and can exceed it. of lattice defects, featuring photonic crystal-like propscattering from neighboring particles, dependence on theYet, lattice relativethe to the projectedthat light the incidence direction erties. we orientation cannot exclude possibility model does not reproduce theThe relative ◦ (φ = 23◦ ). Sym(angle φ ) is discussed in Fig. 4. The left (right) side corresponds to φ = 41 defect was not a vacancy but a particle slightly elevated ntensity maxima, which is predicted to metryexperis reducedabove with respect to the horizontal axis forming and significant differences the substrate, thus not a contact point between both of o the contact points than found but contributing in a slightly different way to the optirrectly predicted by our model. Note cal response. More studies are needed to investigate the points are more pronounced here than #203796 $15.00 USD Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014 degree of robustness of monolayers to defects. ]]]]]]]] presumably caused by slight differences 2014 OSA 7 April 2014 we | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 OPTICS EXPRESS 8231 eparation. Furthermore, the(C) calculated In conclusion, have experimentally imaged the |optishow a considerable difference in terms cal near-fields of colloidal monolayers by imprinting them plitude with respect to those found in on thin GST films. The measured intensity distributions

particular, the number of near-field ma by the number of particles and can be proper choice of illumination condition of the lattice with respect to the light i is found to have a significant impact on tribution, thus providing an additional to tailor patterned imprinted structure

(b)

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ACKNOWLEDGMENT

2µm

10µm

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(d)

I/I0

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1.33

II 0.12

0.63

The authors thank E. Varesi, A. Pir for supplying the GST films and Ma help with the SEM measurements. We derer and J. Solis for stimulating disc was performed within a Joint Project b Konstanz University funded by the Sp and the DAAD. We would like to ack support from Spanish National Resear solider NanoLight.es, MAT2010-1488 22422). C.D. acknowledges a FPU fe Spanish Ministerio de Educaci´on. 1 E.

Mcleod and C. B. Arnold, Nat Nano 3, A. Zhang, J. Elek, and C.-H. Chang, A Fig. 5. (a) Observation of an imprinted pattern imaged with a SEM inlens mode detector. , (2013). FIG. 4. (a) Observation of an imprinted pattern imaged The samplewith and aillumination parameters are λ =The 799 sample nm, a =and 1730 nm, θ = 52.23◦A. , p-Sundaramurthy, P. J. Schuck, N. R. C SEM inlens mode detector. illumina◦ G. S. Kino, and W. E. Moerner, Nano L ◦ No perturpolarization, and φ = 26.4 . The single lattice defect is marked by an arrow. tion parameters are λ = 799 nm, a = 1730 nm, θ = 52.2 , 355 (2006). bation in the imprinted near-field is ◦observed. (b) Optical microscopy of a p-polarization, and φpattern = 26.4 . The single lattice defect is image4 Z. B. Wang, M. H. Hong, B. S. Luk’yanchuk different, larger region of the same monolayer before irradiation, showing the presence marked by an arrow. No perturbation in the imprinted nearand T. C. Chong, J. Appl. Phys. 96, 6845 5 D. of single defects (vacancies). (c) Multiplethe corresponding fullBrodoceanu, L. Landstr¨om, and D. B¨au field pattern is observed. (b)scattering Optical calculation microscopyofimage of a lattice (no defect). (d)larger Simpleregion superposition model.monolayer before irradiaA, 86, 313 (2007). different, of the same 6 T. Sakai, N. Nedyalkov, and M. Obara, J tion, showing the presence of single defects (vacancies). (c) Applied Physics 40, 2102 (2007). Multiple- scattering calculation of the corresponding full lat7 A. Pereira, D. Grojo, M. Chaker, P. Delap them are observable in the detailed the position of the near-field maxima relative to tice (no defect). (d)shape Simpleofsuperposition model. M. Sentis, Small 4, 572 (2008). the spheres. 8 R. Morarescu, L. Englert, B. Kolaric, P. We want to emphasize that the intensity pattern produced by a colloidal monolayerVallee, cannotT. be Baumert, F. Hubenthal, and Chem. 21, 4076 (2011). described by a simple superposition of patterns produced by single spheres, without considering 9 P. K¨ uhler, F. J. Garc´ıa de Abajo, J. Solis, M too sensitive to small and structural de-using conditions particle interactions. To demonstrate this,displacements we have performed an experiment derer, C. Afonso, and J. Siegel, Small 5, 1 fects of the colloids. Further analysis of the degree of 10 N. Yamada, for imprinting a relatively simple pattern (Fig. 5(a)), which we compare with calculations both E. Ohno, K. Nishiuchi, N. Aka monolayer is still needed. for the multiple tolerance scattering against method noted abovedefects (Fig. 5(c)), as well as with a simpleJ.superpoAppl. Phys. 69, 2849 (1991). 11 J. Siegel, A. Schropp, J. Solis, C. N. Afon sition of the scattered field of each particle[14] imaged (single scattering, The In conclusion, weindividual have experimentally the opti- Fig. 5(d)). Appl. Phys. Lett. 84, 2250 (2004). latter was carriedcal out for a hexagonal lattice of 11 × 11 particles, neglecting collective effects 12 near-fields of colloidal monolayers by imprinting them J. Siegel, W. Gawelda, D. Puerto, C. Dorro such as multipleon scattering from neighboring particles. This superposition model does not reAfonso, J. C. G. de Sande, R. Bez, A. Pirov thin GST films. The measured intensity distributions J. Appl. Phys. 103, 023516 (2008). produce the position of excellent the dominant intensitywith maxima, which are predicted to lie much closer are in agreement electromagnetic simula13 P. K¨ uhler, F. J. Garc´ıa de Abajo, P. Leiprec to the contact points thanThis foundsimple experimentally. At theconcept same time, model tions. yet effective is the wellmultiple suited scattering lis, P. Leiderer, and J. Siegel, Opt. Expres predicts a distance maxima and points that is in agreement experiment. for between nanostructuring andcontact mapping arbitrary intensity dis-with 14 B. Lee, J. Abelson, S. Bishop, D. Kang, B. C Notice that the contact points are more here than The in Fig. 3, presumably J. as Appl. a result tributions with high pronounced spatial resolution. imprinted Phys. 97, 18 (2005). 15 N. Stefanou, V. Yannopapas, and A. of slightly varying conditions duringthe sample preparation. Furthermore, the intensity maxima near-fields inherit full translational invariance from Physics Communications 113, 49 (1998). calculated with the model display considerably different shapes and16amplitudes thesuperposition colloidal monolayer. At the same time, by studying F. J. G. de Abajo, Phys. Rev. B 60, 6086 the influence setup parameters, with respect to those measuredofinvarious Fig. 5(a) or calculated with we the conclude full model. This 17 N. demonStefanou, V. Yannopapas, and A. that the detailed near-field distribution is not only de- of incoming strates that the patterns produced cannot be explained by simple interference Physicsand Communications 132, 189 (2000). 18 R. Sainidou, N. Stefanou, I. Psarobas, an termined the particle butcontributions also strongly scattered light because eachbyparticle reacts toarrangement significant field originating in scatputer Physics depends on polarization and angle of incidence. tering from its neighbors. Thelight experimental conditions in Figs. 3 and 5 areInvery similar, with Communications 166, 197 (2 2 X.

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Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8232

only a small variation in both lattice rotation and particle size. However, the imprinted patterns look different, thus emphasizing the sensitivity small parameter changes. A remarkable feature can be observed in Fig. 5(a): the absence of a single contact point (at the position marked by a dashed circle), indicating that the particle at this position might have been missing in the monolayer during illumination. Such lattice defects in form of vacancies are frequently observed in colloidal monolayers, as shown in Fig. 5(b). Despite this defect, the imprinted pattern in Fig. 5(a) features the corresponding main maximum next to the defect, just as if there was no defect. We speculate that the strong collective interaction between the particles might render the optical response of the monolayer robust against the presence of lattice defects, featuring collective photonic crystal-like properties. Yet, we cannot exclude the possibility that the defect was not a vacancy but a particle slightly elevated above the substrate, thus not forming a contact point but contributing in a slightly different way to the optical response. Nonetheless, both explanations imply that the imprinted patterns are not too sensitive to small displacements and structural defects of the colloids. Further analysis of the degree of tolerance against monolayer defects is still needed. 4.

Conclusion

In conclusion, we have experimentally imaged the complex optical near-fields of colloidal monolayers by imprinting them on thin GST films. The measured intensity distributions are in excellent agreement with electromagnetic simulations. This simple yet effective concept is well suited for nanostructuring and mapping arbitrary intensity distributions with high spatial resolution, which can be implemented directly for large scale fabrication on other substrates. The imprinted near-fields inherit the full translational invariance from the colloidal monolayer. At the same time, by studying the influence of various setup parameters, we conclude that the detailed near-field distribution is not only determined by the particle arrangement but also strongly depends on light polarization and angle of incidence. In particular, the number of nearfield maxima is not limited by the number of particles and can be increased by the proper choice of illumination conditions. The orientation of the lattice with respect to the light incidence direction is found to have a significant impact on the near-field distribution, thus providing an additional degree of freedom to tailor patterned imprinted structures. Acknowledgments The authors thank E. Varesi, A. Pirovano, and R. Bez for supplying the GST films and Matthias Hagner for help with the SEM measurements. We also thank P. Leiderer and J. Solis for stimulating discussion. This work was performed within a Joint Project between CSIC and Konstanz University funded by the Spanish Government and the DAAD. We would like to acknowledge national support from Spanish National Research Projects (Consolider NanoLight.es, MAT2010-14885, and TEC2011-22422). C.D. acknowledges a FPU fellowship from the Spanish Ministerio de Educaci´on.

#203796 - $15.00 USD (C) 2014 OSA

Received 30 Dec 2013; revised 12 Mar 2014; accepted 14 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008226 | OPTICS EXPRESS 8233

Near-field nanoimprinting using colloidal monolayers.

We experimentally and theoretically explore near-field nanopatterning obtained by irradiation of hexagonal monolayers of micron-sized polystyrene sphe...
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