Tuning of Hybrid Oligomers via Nanoscale fs-Laser Reshaping
Sergey Lepeshov, Alexander Krasnok, Ivan Mukhin, Dmitry Zuev, Alexander Gudovskikh, Valentin Milichko, Pavel Belov, Andrey Miroshnichenko
TTuning of Hybrid Oligomers via Nanoscale fs-Laser Reshaping
Sergey Lepeshov, Alexander Krasnok,
1, 2
Ivan Mukhin,
1, 3
Dmitry Zuev, Alexander Gudovskikh, Valentin Milichko, Pavel Belov, and Andrey Miroshnichenko Laboratory of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, Russia Department of Electrical and Computer Engineering,The University of Texas at Austin, Austin, Texas 78712, USA ∗ Laboratory of Renewable Energy Sources, St. Petersburg Academic University, St. Petersburg, Russia Nonlinear Physics Centre, Australian National University, Canberra ACT 2601, Australia
Various clusters of metallic or dielectric nanoparticles can exhibit sharp Fano resonances orig-inating from at least two modes interference of different spectral width. However, for practicalapplications such as biosensing or nonlinear nanophotonics, the fine-tuning of the Fano resonancesis generally required. Here, we propose and demonstrate a novel type of hybrid oligomers consistingof asymmetric metal-dielectric (Au/Si) nanoparticles with a sharp Fano resonance in visible range,which has a predominantly magnetic origin. We demonstrate both, numerically and experimen-tally, that such hybrid nanoparticle oligomers allow fine-tuning of the Fano resonance via fs-laserinduced melting of Au nanoparticles at the nanometer scale. We show that the Fano resonancewavelength can be changed by fs-laser reshaping very precisely (within 15 nm) being accompaniedby a reconfiguration of its profile.
Introduction
Plasmonic oligomers consisting of nanoparticles of no-bel metals (e.g. silver and gold) are the cornerstoneof modern nanophotonics due to a sharp effect of reso-nant scattering originating from destructive interferencebetween super-radiant and sub-radiant modes , whichcan be described in terms of the
Fano resonances . Inaddition to a strong local field enhancement, the asym-metric profile of the Fano resonance in such structuresallows to control the radiative damping of the localizedsurface plasmon resonance. This superior feature is veryuseful for applications of nanophotonics, although suchplasmonic nanostructures suffer from high dissipativelosses in visible . Recently, all-dielectric oligomers basedon high-index dielectric and semiconductor nanoparti-cles (e.g. silicon) have been proposed theoretically ,and realized experimentally as a more efficient coun-terpart to the plasmonic ones. It has been shown thatthe all-dielectric oligomers can exhibit not only an elec-tric type of Fano resonance, but also a magnetic one ,which is associated with the optically induced magneticdipole mode of individual high-index nanoparticle .An existence of the resonant magnetic response in suchstructures makes it possible to control the electric andmagnetic response individually. It is worth noting thatthe plasmonic oligomers also can provide resonant mag-netic response including more complicated metal-insulator-metal structures , where the insulator hasa low refractive index (SiO ). However, such resonantplasmonic structures suffer from dissipative losses inher-ent to metals in the visible range.Currently, both all-dielectric and more sophisticatedplasmonic oligomers are used to achieve the near-field enhancement and associated nonlinear optical ef-fects , biosensing , surface-enhanced Raman scat-tering , graphene electronics , strong optical activity ,which potentially can be applied for quantum optics as well. In terms of these practical applications, it isnecessary to have a possibility for a fine-tuning of thespectral features of the Fano resonances in the fabricated nanoparticle oligomer structures. The recently proposedapproaches to tune the Fano resonances in the clustersof metallic nanoparticles are based on a changing of theirgeometry during a fabrication process or electro-magnetic properties of their environment . Moreover,the near-field distribution and absorption properties ofoligomers with rotational symmetry can be tuned via apolarisation of incident light, leaving the scattering prop-erties unchanged . Although these methods show signif-icant performance, they can not be applied to fabricatedoligomers for fine-tuning of their modes and scatteringproperties.The purpose of this paper is twofold. First, we pro-pose to combine two paradigms of plasmonic and all-dielectric oligomers and form a hybrid metal-dielectricclusters to have benefits and advantages of both ofthem. Recently, unique properties of asymmetric hy-brid nanoparticles made them as a very promising plat-form for nanophotonics . However, oligomers basedon resonant plasmonic and dielectric nanoparticles havenot been studied yet. Here, we suggest and implement anovel type of oligomers consisting of resonant asymmetricmetal-dielectric (Au/Si) nanoparticles realizing the con-cept of hybrid oligomers. We show that the proposedoligomers exhibit a sharp Fano resonance in the visiblerange. Based on the multipole expansion analysis (forthe method of multipole expansion in vector sphericalharmonics see Supplementary Information ), we demon-strate that the Fano resonance has a predominantly mag-netic origin owing to magnetic Mie-type modes of theSi nanoparticles. Second, being inspired by our recentexperimental work on a new technique for fabricationof asymmetric hybrid (Au/Si) nanoparticles, we proposeand realize an original approach for tuning of the mag-netic Fano resonance in the oligomers. The approach is a r X i v : . [ phy s i c s . op ti c s ] D ec - Au - Si - SiO ( )a (b) (c) Au nanoparticle reshaping
Figure 1: Sketch of the hybrid oligomers composed of asymmetric hybrid (Au/Si) dimer nanoparticles with different shapes ofAu components, which correspond to different stages of laser reshaping: (a) nanodiscs, (b) nanocups, and (c) nanospheres. based on a fs-laser induced melting of Au part of hybriddimer nanoparticles at the nanometer scale (as schemat-ically shown in Figure 1). We show that the Fano res-onance wavelength can be changed by fs-laser reshapingvery precisely being accompanied by a reconfiguration ofits profile.
Results
We start our analysis by comparing the scattering crosssections of a single silicon nanocone and hybrid nanopar-ticles (see Figure 2). We assume the incident planewave ispropagating along the axis of symmetry of the nanopar-ticles. Figure 2(a) shows the light scattering spectrumof the single Si truncated nanocone. The geometric pa-rameters of the cone are taken from the Ref. , namely,the diameter of the upper base is a = 60 nm, and thecone height is h = 200 nm. The lower base ( b ) of the Sinanocone is b = 190 nm; the results for Si nanocones ofother sizes are presented in Supplementary Information .The scattering cross section of the single Si nanoconehas two distinct resonances, around the wavelengths550 nm and 650 nm (Figure 2(a), points A and B). Byusing the multipole expansion , we reveal that theseresonances are of the electric dipole type at 550 nm (ED,red dashed curve) and magnetic dipole type at 650 nm(MD, blue dashed curve). Higher-order multipoles pro-duce a negligible contribution for given parameters. Theelectric near-field distribution profiles at these resonancesare presented in Figure 2(a)A,B. It is known that themagnetic dipole Mie resonance condition for the dielec-tric (such as silicon) nanoparticle depends on its size. Forthe conical particle under investigation we obtain the fol-lowing equation for the wavelength of magnetic resonance λ res ≈ . bn d , where λ res is the resonant wavelength, n d is the refractive index of the silicon nanoparticle . Inprevious experimental articles it has been shown that the refractive index of balk crystalline silicon works well withnanoparticles of such sizes . The last equation isa good approximation in the domain close to selected geo-metric parameters. Thus, the reduction of b from 190 nmto 150 nm leads to the blue-shift of the magnetic reso-nance from λ res = 640 nm to λ res = 570 nm (see Sup-plementary Information ). This feature was recently usedfor controlling over the wavelength of Fano resonance inall-dielectric oligomers at the manufacturing stage .Now we consider the scattering properties of a singlehybrid nanoparticle consisting of Si nanocone and Aunanodisc (see inset in Figure 2(b)). We assume thatthe diameter of Au nanodisc is equal to the diameterof the lower base of the Si nanocone, which is dictatedby the lithography process . We also take the thicknessof the Au nanodisc is equal to d = 20 nm. By addingthe gold nanodisc on the upper base of Si nanocone, anadditional resonance appears in the scattering spectra ofthe resulting hybrid nanoparticle, which is shown in Fig-ure 2(b), and where the resonance depicted by point C .This resonance has a plasmonic nature and manifests it-self in strong local electric field enhancement around thenanodisc. Moreover, the modes of Si nanocone and Aunanodisc begin to hybridize. The hybridization of Mieand plasmonic modes causes their mutual perturbation(see multipole expansion for this particle in Figure 2(b)).The magnetic Mie resonance still has a resonant be-haviour (Figure 2(b), point D ). The electric near-fielddistribution at the wavelength of the plasmonic resonance( λ = 800 nm) is presented in Figure 2(b)C. The existenceof the Au nanodisc perturbs the electric near-field of thenanocone at its magnetic resonance (Figure 2(b)D) dueto their effective coupling. The scattering properties ofthe hybrid nanoparticles in the optical frequency rangeand electric near-field distributions hereinafter were nu-merically calculated by using CST Microwave Studio. Anonuniform mesh was used to improve the accuracy inthe vicinity of the Au nanoparticle where the field con-
50 120508040 3.5040 50
A CGEB DF H S ca tt e r i ng C r o ss S ec ti on , a . u . S ca tt e r i ng C r o ss S ec ti on , a . u . S ca tt e r i ng C r o ss S ec ti on , a . u . S ca tt e r i ng C r o ss S ec ti on , a . u . ( )a(c) (d) total EDMDEQ
AB CEF GH
Wavelength, nmWavelength, nm Wavelength, nmWavelength, nm D E k E k E kE k (b) |E|/|E0| |E|/|E0||E|/|E0| |E|/|E0| Figure 2: The scattering cross sections (black curves) and results of the multipole expansion for (a) a single Si nanocone, andhybrid Au/Si nanoparticles: Si nanocone with (b) Au nanodisc, (c) Au nanocup, and (d) Au nanosphere. The diameters ofthe lower base of the Si nanocone and the Au nanodisc are equal 190 nm. The spectra are normalised identically. (A–H) Theelectric field profiles (in terms of field amplitude) at the corresponding resonance; the corresponding points are marked on thespectra. The incident wave propagates along the axis of symmetry of the nanoparticles. centration was significantly large. The dispersion modelfor the Au and Si materials was taken from the litera-ture .The plasmon resonance of Au nanoparticles arises froman excitation of localized surface plasmon modes, whichare strongly depended on the geometrical shape of thenanoparticle . It has been shown that under irra-diation of a Au nanoparticle by femtosecond laser pulsewith energy density of 40–50 mJ/cm (depending on theAu particle size), the Au nanoparticle changes its shapefrom a disc to a cup . At lower intensities, thereis no detectable shape deformation. We emphasise thatit is necessary to use a truncated nanocone to properlychange the Au nanoparticles shape. At the same time,the Si nanocone is not affected by the fs-laser radiationdue to the higher melting temperature and enthalpy offusion (about 1687 K and 50.21 kJ/mol for crystallinesilicon in contrast to 1337 K and 12.55 kJ/mol for gold).The plasmon resonance of the deformed nanoparticle [seescattering spectra in Figure 2(c)] shifts to shorter wave-lengths (from 800 nm to 690 nm, in our case). Nowit is difficult to separate the response of whole hybridnanoparticle to responses of dielectric and metallic parts.It results in dramatically changing in the near field distri- bution of the hybrid nanoparticle [see Figure 2(c)E] andappearance of hot-spots of the locally enhanced electricfield at the edges of the nanocup where E/E reaches 8, E is the exciting field strength. Upon the Au nanoparti-cle reshaping, the wavelength of the magnetic resonanceof Si nanocone shifts to 630 nm (see Figure 2(c)F). More-over, in this case we observe the notable contribution ofthe electric quadrupole mode (EQ) in the total scattering(see Figure 2(c), green doted curve).By increasing the energy density of the laser radiationup to 70–80 mJ/cm , the nanocup transforms its shape toa nanosphere (in our case the radius of resulting sphereis 51 nm). The scattering cross section of such hybridnanoparticle as well as the results of multipole expansionare presented in Figure 2(d). The scattering cross sec-tion is similar to the single Si nanocone, due to the Aunanosphere scatters much less of light energy than theSi nanocone. Thus, the position of the Au nanoparticleplasmon resonance as well as response of the whole hy-brid nanoparticle can be controlled via fs-laser inducedreshaping.Let us consider an all-dielectric oligomer consisting ofSi nanocones and having a 6-fold rotational axis ( C ). Todemonstrate that the oligomer has a Fano resonance, we S ca tt e r i ng C r o ss S ec ti on , a . u .
450 500 550 600 650 700 750 800
Wavelength, nm (a) (b) t opv i e w s i d e v i e w iiiiii @ 588 nm @ 560 nm E/E0 E/E0Arg(E) Arg(E)
Figure 3: (a) Scattering cross sections of (i) all-dielectric hexamer based on Si nanocones, (ii) single Si nanocone with smallerlower base, and (iii) all-dielectric heptamer. The gaps between the central cone and the boundary ones are 30 nm. The dashedline shows the position of the Fano resonance. (b) The electric field profiles (in terms of field amplitude) in the vertical andtop cross-sections calculated at the scattering intensity dip of Fano resonance ( λ = 588 nm) and outside of the resonance( λ = 560 nm). calculate the scattering spectra of hexamer and single Sinanocone separately as well as scattering spectra of wholeoligomer (see Figure 3). The hexamer structure is basedon the nanocones with a diameter of the lower base of b = 190 nm. The gap between nanocones (the distancesbetween the neighboring lower bases) is 10 nm, whichleads to their effective interaction resulting in the appear-ance of low-Q collective modes. The scattering spectraof these modes overlap forming a non-resonant scatter-ing channel [see Figure 3(a)i]. To obtain a heptamer, weplace a Si nanocone with the diameter of lower base of150 nm and with a relatively narrow magnetic resonance(see Figure 3(a)ii) in the center of the hexamer. Thegap between the central Si nanocone and the hexamer’sones in the resulting structure is 30 nm. Figure 3(a)iiishows the scattering spectrum of the resulting heptamer.This spectrum has a resonant dip at the wavelength ofmagnetic Mie resonance of the central nanocone (around λ = 590 nm) with a pronounced asymmetric profile. InRefs. it has been shown that this dip is associatedwith the magnetic Fano resonance , which is caused bythe scattered wave interference of two modes – the spec-trally narrow magnetic dipole Mie mode of the centralnanocone and the broadband collective magnetic modesof the hexamer. The Fano resonance dip at 588 nm iscaused by antiphase oscillating of the magnetic dipolesof heptamer and magnetic dipole of the central nanocone(dark mode). Outside of this resonance ( λ = 560 nm)these modes oscillate in phase (bright mode) (see Fig-ure 3(b)). We also note that due to the rotational sym-metry of the all-dielectric oligomer the scattering crosssection does not depend on the incident wave polariza-tion . The shape of the Fano resonance depends onthe distances between the nanocones. The results of de- tailed study of this effect are presented in SupplementaryInformation .Our goal now is to use the melting of the Au nanopar-ticles placed on the Si nanocones to tune the magneticFano resonance in the hybrid oligomer. For this pro-pose we show that the hybrid oligomer has a pronouncedFano resonance, even in the presence of Au nanocups,i.e. when the Au nanoparticle is resonant in vicinity ofthe Fano resonance wavelength. In other words, we showthat the Au nanoparticles perturb the Fano resonance,but does not destroy it. We demonstrate it in the samemanner as for the all-dielectric oligomers. Namely, thescattering spectra of the the hybrid Au/Si hexamer withAu nanocups has a broad and nonresonant wing of col-lective modes (see Figure 4(a)i). The interaction of thesemodes with the narrow resonance of the single Au/Sinanoparticle (see Figure 4(a)ii) results in appearance ofthe asymmetrical dip in the scattering spectrum (see Fig-ure 4(a)iii). The electric field distribution profiles in theside and top views calculated at the scattering intensitydip of Fano resonance ( λ = 584 nm) and outside of theresonance ( λ = 566 nm) are presented in Figure 4(b).At the Fano resonance wavelength ( λ = 584 nm) themodes of central particle and hexamer oscillate in op-posite phase, forming a dark mode of the whole hybridoligomer.Now we consider the optical properties of hybridoligomers composed of Au/Si nanoparticles for differentstages of reshaping (see Figure 5). We study the hy-brid oligomers with the diameters of the lower base ofthe hexamer’s Si nanocones and the central nanocone of190 nm and 150 nm, respectively. The gap between thecentral Si nanocone and the hexamer’s ones is 30 nm. Weconsider the Fano resonance of the oligomer composed of S ca tt e r i ng C r o ss S ec ti on , a . u .
450 500 550 600 650 700 750 800
Wavelength, nm t opv i e w s i d e v i e w iii (a) (b) iii @ 584 nm @ 566 nm E/E0 E/E0Arg(E) Arg(E)
Figure 4: (a) Scattering cross sections of (i) hybrid Au/Si hexamer, (ii) single hybrid nanoparticle with smaller lower base, and(iii) heptamer, with Au nanoparticles in form of nanocups. (b) The electric field profiles (in terms of field amplitude) in thevertical and top cross-sections calculated at the scattering intensity dip of Fano resonance ( λ = 584 nm) and outside of theresonance ( λ = 566 nm). hybrid nanoparticles with Au nanodiscs that appears at580 nm (see Figure 5(a), blue curve). The resonanceis caused mainly by the responses of Si nanocones, andits wavelength corresponds to the wavelength of the Fanoresonance of all-dielectric oligomer (see Figure 3(a)iii) be-cause of the weak coupling between Au nanodiscs and Sinanocones. For this case of Au/Si nanoparticle oligomersthe Fano resonance is less pronounced compared to theall-dielectric counterpart (see Figure 3(a)).Next, we numerically show that the profile and itsspectral position of this resonance can be changed by fs-laser induced reshaping of the Au nanodiscs. When theAu nanoparticles take the form of nanocups, the mini-mum of the Fano resonance shifts to λ = 585 nm be-ing accompanied by a constriction of its profile (see Fig-ure 5(a), green curve). It has been shown above (see Fig-ure 4(a)) that this very pronounced dip in the scatteringspectrum of hybrid oligomers with Au nanocups corre-sponds to the Fano resonance. Upon further reshapingof the Au nanoparticles to nanospheres, the Fano reso-nance becomes broader again and its minimum shifts to λ = 595 nm (see Figure 5(a), red curve). Thus, the laserreshaping of the Au nanoparticles can be applied for fine-tuning of the Fano resonance of the hybrid oligomers.In order to prove the concept of fine-tuning of the mag-netic Fano resonance we provide a series of dark-fieldscattering spectra measurements from hybrid oligomerswith different degree of the Au nanoparticle reshaping.First, we have fabricated the hybrid oligomers based onthe gold nanodiscs placed on the upper base of the trun-cated silicon nanocones by means of a combination of e-beam lithography (25 kV), metal evaporation, lift-off pro-cedure, and gas-phase chemical etching. Recently, it hasbeen shown that under the electron-beam processing stepwith 25 kV acceleration voltage the amorphous silicon gets the nanocrystalline structure . This method of hy-brid nanostructures fabrication is developed in Ref. . Atthe first step, an a-Si:H layer with a thickness of ≈
200 nmwas deposited on a properly cleaned substrate of fused sil-ica by the plasma-enhanced chemical vapor deposition ofSiH gas. Then, the arrays of metal nanodiscs consistingof Cr/Au layers with thicknesses of ≈ and O gases. The etchingwas carried out with temperature of 265 K to fabricateSi nanostructures in the shape of nanocones. The typi-cal SEM images of fabricated hybrid oligomers with Aunanodiscs, Au nanocups, and Au nanospheres are repre-sented in Figure 5(c)–(e); the scale-bar is 200 nm.For fs-laser melting a commercial femtosecond lasersystem (Ytterbium-doped Femtosecond Solid-State LaserTeMa 150, Avesta Poject) was used, providing laserpulses at a central wavelength of 1050 nm, with a maxi-mum pulse energy of 85 nJ, and a pulse duration of 150 fsat a repetition rate of 80 MHz. The laser energy was var-ied and controlled by an optical attenuator (Avesta Po-ject) and a power meter (FielfMax II, Coherent), respec-tively. Laser pulses were focused on the fabricated sampleby an objective (Mitutoyo M Plan Apo NIR 10X) witha numerical aperture (NA) of 0.26. In order to adhesionof the Au nanodisk and Si nanocone a thin Cr layer isused. According to the results of molecular dynamic sim-ulations , the Cr layer (with a thickness of 1-2 nm) pro-vides a desired shape of the Au nanoparticle during laserreshaping without affecting the electromagnetic proper-ties of the hybrid nanoparticle. Moreover, this Cr layerprevents formation of the Au-Si alloy.
500 550 600 650
Wavelength, nm S ca tt e r i ng C r o ss S ec ti on , a . u . r e s h a p i ng
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Wavelength, nm r e s h a p i ng simulation measured (c) (e)(d) (b)(a) S ca tt e r i ng s i gn a l , a . u . Figure 5: (a) Calculated scattering cross section spectra and (b) experimentally measured scattering dark-field signals of thehybrid Au/Si heptamer with Au nanodiscs (blue curve), Au nanocups (green curve), and Au nanospheres (red curve).Thespectral region with strong Fano resonane response is highlighted by yellow stripe. (c)–(e) The SEM images (viewing angle is45 ◦ ) of typical of fabricated hybrid oligomers with Au nanodiscs (c), Au nanocups (d), and Au nanospheres (e); the scale-baris 200 nm. Measurements of the scattering spectra were carriedout in a dark-field scheme, where the arrays irradiationwas performed by p-polarized light from a halogen lamp(HL-2000-FHSA) at an angle of incidence of 70 ◦ withthe surface normal. Scattered signal collection was per-formed by means of a Mitutoyo M Plan APO NIR ob-jective (NA = 0.7), which directed light to a commercialspectrometer (Horiba LabRam HR) with a CCD cam-era (Andor DU 420A-OE 325). The confocal opticalscheme was optimized for signal collection from individ-ual nanoparticles. A sketch of the experimental setup forthe polarization-resolved dark-field spectroscopy is rep-resented in the Supplementary Information .The results of fine-tuning of the Fano resonance inthe fabricated hybrid oligomers are summarized in Fig-ure 5(b). Our experimental results clearly show thespectral shift of the Fano resonance minimum from λ = 650 nm to λ = 660 nm with increasing of thepower of external laser field from 0 mW (blue curve)to 40 mW (green curve). Following our previous resultsin this regime of reshaping the Au nanoparticles takesthe form of nanocones . Upon further increase of thelaser power up to 90 mW the Au nanocones reshape tonanospheres with spectral shifting of the Fano resonancedip to λ = 665 nm (see Figure 5(b), red curve). Themeasured damage threshold of the Au nanoparticles isabout 130 mW. At this power the Fano resonance of thehybrid oligomer disappears. The slight mismatching ofthe numerical and experimental results is explained bythe presence of SiO substrate and accuracy of nanos-tructures fabrication. Conclusion
In summary, we have proposed and implemented anovel type of hybrid oligomers consisting of resonantasymmetric metal-dielectric (Au/Si) nanoparticles andexhibiting a sharp Fano resonance in visible range,which has a predominantly magnetic nature. We havedemonstrated, numerically and experimentally, that sucholigomers allow irreversible fine-tuning of the Fano res-onance via fs-laser melting of Au nanoparticles at thenanometer scale. We have shown that the Fano reso-nance wavelength can be changed by fs-laser reshaping very precisely (within 15 nm) being accompanied by areconfiguration of its profile. We believe that our resultspave the way to realization of nanophotonic elements thatcan be adjusted after their manufacturing.
Acknowledgements
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