Femtosecond laser printing of single Ge and SiGe nanoparticles with

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Femtosecond laser printing of single Ge and SiGe nanoparticles with electric and magnetic optical resonances Denis Zhigunov, Andrey B. Evlyukhin, Alexander Sergeevich Shalin, Urs Zywietz, and Boris N. Chichkov ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01275 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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Femtosecond Laser Printing of Single Ge and SiGe Nanoparticles with Electric and Magnetic Optical Resonances Denis M. Zhigunov1,*, Andrey B. Evlyukhin2, Alexander S. Shalin3, Urs Zywietz2 and Boris N. Chichkov2,4 1

Faculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory 1, 119991 Moscow, Russia 2

Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany 3

4

ITMO University, 49 Kronversky Ave., 197101, St. Petersburg, Russia

Institut für Quantenoptik, Leibniz Universität Hannover, D-30167 Hannover, Germany *

E-mail: [email protected]

ABSTRACT: Recently introduced femtosecond laser printing technique was further developed for the fabrication of crystalline single Ge and SiGe nanoparticles. Amorphous Ge and SiGe thin films deposited by e-beam evaporation on a transparent substrate were used as donors. The developed approach is based on laser-induced forward transfer process, which provides an opportunity of NP controlled positioning on different types of receiver substrates. The size of the generated nanoparticles can be varied from about 100 to 300 nm depending on the laser pulse energy and wavelength. The crystallinity and composition of nanoparticles are both confirmed by the Raman spectroscopy measurements. The experimental visible scattering spectra of single nanoparticles are found to be well coincident with theoretical simulations performed on the basis of Mie theory. It is demonstrated that Ge and SiGe nanoparticles are characterized by electric and magnetic dipole resonances in the visible and near-infrared spectral ranges, which is promising for photonic applications. KEYWORDS: all-dielectric metamaterials, nanoparticles, Mie resonances, nanophotonics

laser-induced

forward

transfer,

spherical

At present more and more attention is paid to all-dielectric low-loss metamaterials. They are supposed to replace plasmonic metamaterials based on metallic nanostructures which suffer from non-radiative dissipative losses at optical frequencies leading to large absorption and heating effects hence limiting possible applications of plasmonic metamaterials in nanophotonics.1,2 Alternatively, dielectric nanostructures based on high refractive index materials may be used to manipulate light at the nanoscale.3-5 The physics behind is related to the excitation of electric and magnetic Mie-type resonances in dielectric nanoparticles providing possibilities to control the system optical response.6,7 Being elementary building blocks for all-dielectric metamaterials, individual nanoparticles (NPs) act as nanoresonators whose response is generally determined by their sizes and shapes. For this reason, fabrication of NPs with precise control of these parameters is of current importance. Among the different available fabrication methods, femtosecond (fs) laser printing of nanoparticles is one of the most advantageous, since it allows the production of single size-controlled spherical NPs with a smooth surface, including their precise positioning and ACS Paragon Plus Environment

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patterning on a sample surface. This method has been initially applied for the fabrication of metallic (gold) micro- and nanostructures.8,9 The basic principle of the method is known as laserinduced backward or forward transfer (LIBT or LIFT). The laser-induced transfer implies the melting of a donor substrate material (bulk or thin film) by a pulsed fs laser to generate and transfer nanoparticles onto a receiver substrate placed either between the laser source and the donor substrate (LIBT) or underneath the donor substrate opposite to the laser source (LIFT). Following the trend towards all-dielectric metamaterials the fabrication of Si NPs by LIBT has been demonstrated recently.10,11 Initially bulk silicon was proposed as a donor substrate. However, the experiments demonstrated that using a single fs laser pulse with a Gaussian intensity distribution it is impossible to generate and transfer single Si NPs from the crystalline silicon wafer target in a controllable way: always several Si NPs were generated in this case. Two solutions for the laser-induced controllable fabrication of single Si NPs were suggested. First, an employment of ring-shaped laser intensity distribution resulted in transfer of single Si NP by single fs laser pulse, which was called femtosecond laser printing to be distinguished from more general LIBT and LIFT processes.10 The second and a more progressive known to date technique for the generation of single spherical Si NPs with precisely controlled sizes and positions on a receiver substrate is the fs laser printing using silicon-on-insulator (SOI) as donor substrate.11 The latter allowed one to fabricate Si NPs with size of ~ 200 nm, which is important for nanophotonic applications since such small NPs possess pronounced electric and magnetic dipole Mie resonances in the visible spectrum.3,4,12 Summarizing the data on fs laser printing one can conclude that the size of the transferred NPs can be controlled mainly by the laser beam spot size, donor film thickness and fs laser pulse energy. It is worth noting also another method of NP precise positioning known as ‘laser printing’, which allows one to transfer single NPs from the colloidal suspension onto a substrate employing optical forces acting on colloidal particles in a tightly focused laser beam.13 In opposite to femtosecond laser printing described above, this method requires the colloidal suspension with NPs to be prepared anyhow in advance and implies the utilization of continuous wave laser to operate with NPs in a similar way like in optical tweezer systems. Going beyond silicon, the development of novel all-dielectric nanostructures which could be prospective for nanophotonics, as well as, fully compatible with the technological processing is of current importance. In particular, certain interest is focused nowadays on Ge and SiGe nanoparticles, which are characterized by higher refractive index as compared to Si-based counterparts. Hence an advantage of such nanoparticles is higher achievable degree of electromagnetic field enhancement, which is particularly attractive for non-linear optics.14 In addition, SiGe solid alloys provide a possibility of tuning their physical properties by the variation of their composition.15 As shown recently, arrays of SiGe-based hemispherical nanoislands show pronounced resonant behavior at visible wavelengths in spite of their slightly higher optical losses.16 In turn, pure Ge nanostructures possess even higher refractive index, though also significantly larger absorption in the visible. However, in the near-IR spectral region, the absorption of Ge becomes much less prominent, which provides an opportunity of employment of Ge-based nanostructures as Mie resonators and elements for non-linear optics.17,18 In the present study, we further developed fs laser printing approach and realized the laser-induced transfer of crystalline single Ge and SiGe NPs from the corresponding initially amorphous thin films. In contrast to previous experiments aimed at laser printing of Si and Au NPs by means of laser-induced backward transfer,8,11 we used LIFT geometry which allows one ACS Paragon Plus Environment

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to transfer NPs onto different types of receiver substrates. Scattering spectra of generated single Ge and SiGe NPs were measured and compared with the corresponding theoretical calculations in order to explain the observed resonant behavior. Experimental details Thin Ge and Si1-xGex (x ≈ 0.5) films were deposited on 150 µm-thick standard borosilicate wafers (Borofloat 33, Plan Optik AG) by the e-beam evaporation at room temperature using ultra high vacuum growth system.19 Two e-beam evaporators were used for deposition of Si and Ge, and the growth rate for both Ge and Si1-xGex films was fixed at 1 Å/s. The thickness of deposited films was equal to about 12, 30, 60 or 100 nm. A commercial femtosecond laser system (Spectra Physics) with λ = 800 nm central wavelength (λ2ω = 400 nm in frequency doubling regime), τ = 50 fs pulse duration, Emax = 3 mJ maximum pulse energy and 1 kHz repetition rate was used for laser printing of NPs. Laser irradiation was focused on a donor substrate with Ge or Si1-xGex thin film by means of long-distance 50× microscope objective (Nikon) with a numerical aperture (NA) of 0.45. NPs were transferred on a receiver glass substrate located directly underneath the thin film. The distance between the donor and receiver substrates was equal to about 10 µm. Raman spectra of fabricated nanoparticles were measured using a micro Raman setup (Horiba Jobin Yvon LabRAM HR800) in backscattering geometry with an excitation source at 632.8 nm line of a He-Ne laser. On the basis of test experiments, laser intensity was appropriately reduced down to 0.1 kW/cm2 to avoid in-situ laser-induced crystallization of initial amorphous films and NPs under study. Scattering spectra were collected in forward-scattering geometry using a microscopic setup (Zeiss Microscope Axio Scope) attached to the optical fiber spectrometer (HR 2000, Ocean Optics) equipped with CCD camera. An illumination of generated NPs was performed by means of dark field condenser (Zeiss) with NA of 0.9. The scattered light was collected by a 50× microscope objective (Zeiss) with NA of 0.45. Theoretical calculations of scattering cross sections for ideal spherical Ge and Si1-xGex NPs were performed using Mie theory, while dispersion relations of real and imaginary parts of complex dielectric permittivity were taken from the respective database.20 All experiments were carried out at room temperature. Results and discussion NP generation and patterning A schematic representation of femtosecond laser printing of Ge and Si1-xGex NPs is given in Fig. 1. In order to find out optimum conditions for fs laser printing, different combinations of experimental parameters, such as donor thin film thickness, laser wavelength, and aperture of objective or focusing lens, were tried. The controllable transfer of single NPs was only observed from the 60 nm thin films, thus hereafter we mainly present the experimental results obtained using such donor Ge and Si1-xGex films, if not stated otherwise.

Fig. 1. Scheme of femtosecond laser printing of Ge and Si1-xGex NPs by means of LIFT.

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Fig. 2. SEM images of 60 nm thin Ge film surface irradiated by single fs laser pulses (λ = 800 nm) with gradually increasing energies: (a) 1 nJ, (b) 1.1 nJ, (c) 1.3 nJ, (d) 1.5 nJ. Scale bar is common for all images. In Fig. 2, a set of SEM images of the donor Ge film surface after irradiation with single 800 nm fs laser pulses, having different energies, is shown. At low pulse energy a typical bulb can be seen (Fig. 2a), which is a result of surface tension after the solid-liquid phase transition induced by the fs laser pulse heating. The diameter of the modified film area is about 1 µm. Further increase of pulse energy leads to the formation and solidification of a droplet, which can either be ejected or remain attached to the substrate (Figs. 2b and 2c). At high enough laser pulse energies multiple small droplets and a consolidated Ge ring at the edge of the irradiated area are observed (Fig. 2d). Germanium (as well as silicon) belongs to a group of materials with abnormally higher density in a liquid state as compared to the solid phase. This results in a material depression within the molten area and radial motion of liquid germanium towards the edge of the focal spot due to the temperature-dependent gradient of the surface tension induced by the Gaussian intensity distribution of the fs laser pulse.10 The same behavior is generally observed for similarly treated Si1-xGex films, whose basic properties are found to be varied between those of bulk Si and bulk Ge. Fig. 3 shows dark-field microscopic image as well as SEM images of Ge NPs generated by 800 nm fs laser pulses. The size of NPs is varied in the range of about 100 to 300 nm, while their shape is supposed to be close to spherical with some minor imperfections, as can be seen in insets of Fig. 3. However, the full control over the NP size and position on a receiver substrate was not achieved, in particular, often multiple or even no NP generation occurred by single fs laser pulse irradiation (see Fig. 3). The laser pulse energy was varied from very low values below Ge film melting threshold up to levels high enough for the material ablation. At some intermediate pulse energies the generation of Ge NPs was always observed, nevertheless, single NP transfer occurred irregularly and not in a precise controllable manner. Similar situation occurred earlier in the case of laser-induced transfer of Si NPs from bulk Si target.10 Finally, after a number of experiments with films of different thicknesses, laser focusing conditions and pulse energies, the laser wavelength was switched to 400 nm (second harmonic), which resulted in reduction of irradiated area (see below) and allowed one to generate and transfer single NPs in a reproducible and controlled way, i.e. to implement fs laser printing regime. The fs laser printing of single Ge NPs from 60 nm thick amorphous Ge film was observed eventually using 400 nm laser pulses with energies in some narrow range from about 6.6 till 7.3 nJ. The size of generated single NPs increased from ~ 225 to 260 nm following to the laser pulse energy rise. At slightly higher or lower pulse energies mostly multiple NP transfer occurred, moreover, NP position on a receiver substrate was not controlled anymore in a ACS Paragon Plus Environment

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Fig. 3. Dark-field microscopic image (main) and SEM images (insets) of Ge NPs transferred from 60 nm donor Ge film by means of 800 nm single femtosecond laser pulses with energies in the range of 1-1.5 nJ. Scale bar is common for all inset images.

Fig. 4. Dark-field microscopic image of Ge NP array printed by 400 nm, 7 nJ single fs laser pulses using 60 nm Ge donor film. The inset shows characteristic SEM images of Ge nanoparticle (upper image) and irradiated donor film surface (lower image).

reproducible manner, similar to the situation shown in Fig. 3. The fs laser printing method shows hence a threshold dependence on the laser pulse energy, while the latter is one of the key characteristic defining NP size at fixed donor film thickness. Although we didn’t not accomplish fs laser printing of NPs in case of other films used, we believe that in principle it should be possible to be realized by matching the proper parameters combination (which is limited within the certain experimental conditions) for a given donor film thickness. A dark-field microscopic image of an array of Ge NPs produced by laser printing with 400 nm, 7 nJ fs laser pulses is shown in Fig. 4. The mean size of Ge NPs is about 240 nm, while the irradiated area is approx. 450 nm in diameter (see insets of Fig. 4). Such a roughly two times reduction of modified film area as compared to the described above observation for 800 nm laser pulses is in consistence with the twofold wavelength decrease upon change to the laser second harmonic. Noticeably, in general, the formation and ejection of Ge NPs was not accompanied by the film perforation, while the irradiated spot took a form of a bump (see lower inset of Fig. 4), similar to previously observed one in case of fs laser printing of Si NPs from SOI target.11 Comparing to the utilization of 800 nm pulses, with the laser second harmonic we didn’t observe the formation of the ring from the solidified molten germanium around the exposed area (see Fig. 2), even in case of film perforation. As shown in previous works on silicon, the breakup of such liquid ring due to its instability resulted generally in generation of multiple small Si NPs instead of a bigger single one.10 Hence, according to our current findings, to achieve the regime of single NP ejection, a proper combination of such parameters like film thickness, focal spot size, laser pulse energy and wavelength should be chosen to prevent an excessive material melting followed by its radial motion and ring formation at the edge of the irradiated spot. It is important that often the task of choice of optimal parameters cannot be solved by adjusting solely the laser pulse energy. In turn, the variation of Ge film thickness alone also ACS Paragon Plus Environment

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didn’t change the situation radically. The only another workable combination of parameters, which resulted in realization of fs laser printing approach, included 100 nm thin Ge film irradiated by single 800 nm fs laser pulses focused with the 20 mm focal length lens, while the generated single Ge particles possessed the size in a microscale range. Hence it was necessary to change several parameters at once in order to obtain single particle transfer from 100 nm Ge film. The molten film area was about 4.5 µm in diameter and the mean microparticle size was about 1.5 µm. The study of such relatively large particles, however, fell Fig. 5. Dark-field microscopic image of Si1-xGex beyond the scope of the present paper. One NP array produced using 400 nm single fs laser of the important results of these experiments pulses with the energy in the range from about 5 is that once the laser wavelength was to 10 nJ using 60 nm Si1-xGex donor film. The changed to 400 nm, the modified area insets show SEM images of selected Si1-xGex decreased down to ~ 3.5 µm in diameter and nanoparticles (scale bar is common for all inset the ability of single NP controlled transfer images). was lost whatever laser pulse energies were used. This is another confirmation that fs laser printing approach needs the simultaneous adjustment of such seemingly primary parameters as donor film thickness and focal spot size in order to provide some optimal proportion between the depth and the lateral size of the molten area. The latter may be controlled either by the aperture of the focusing objective/lens or by the laser wavelength selection. The laser-induced transfer of Si1-xGex NPs demonstrated less degree of controllability as compared to the case of Ge NPs. Fig. 5 shows a dark-field microscopic image of Si1-xGex NPs array printed by 400 nm single fs laser pulses with the variable energy in the range of approx. 5 to 10 nJ. The donor film was treated with fs laser in a form of 5x5 equidistant array with the constant energy step between adjacent array units. The laser pulse energy increases from the top to the bottom of each column in array, as well as from left to right upon switch to the next column. Each array unit should be presented in principle by 3x3 NP sub-array (10x10 µm in size, with 10 µm distance to adjacent units) having in total nine Si1-xGex NPs produced at fixed pulse energy in order to trace the NP reproducibility. While the partial ordering of NP arrangement can be seen in the right part of the microscopic image shown in Fig. 5, the precise control of NP size and position still was not achieved. It is also evident that the size of Si1-xGex NPs gradually increases with the fs laser pulse energy (i.e. from left to right part of the image). The size of the NPs generated at high pulse energies in the range of about 8-10 nJ, when the better control over the NP position on a receiver substrate was observed, is ~ 200-250 nm (see Fig. 5 and insets therein). In turn, the use of 800 nm fs laser pulses or donor films with different from 60 nm thicknesses led to the formation of mostly multiple Si1-xGex NPs with random positions on the receiver substrate.

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Fig. 6. Typical Raman spectra of (a) donor thin Ge film (dots) and single Ge NP (line), and (b) donor thin Si1-xGex film (dots) and single Si1-xGex NP (line). NP characterization by Raman and optical spectroscopy In order to study the structure and composition of the generated NPs, Raman spectroscopy was performed. Fig. 6 shows typical inelastic light scattering spectra for single Si and Si1-xGex NPs in comparison with those for initial thin films. As expected, from the growth conditions (low deposition temperature) donor films possess amorphous structure, since only broad amorphous type peaks are observed in the respective spectra. In contrast, Raman spectra for both Ge and Si1-xGex NPs are characterized by sharp peaks whose spectral positions coincide well with the positions for Ge-Ge (~ 300 cm-1), Si-Ge (~ 405 cm-1) and Si-Si (~ 498 cm-1) vibrational modes in crystalline Ge21 and SiGe alloys.22 It means that both formation and crystallization of Ge and Si1-xGex NPs occur during laser printing process, since no posttreatment (e.g. additional laser annealing) of the printed NPs was performed. While the Raman spectrum of single Ge NP is characterized by a single peak and can be straightforwardly interpreted, the corresponding spectrum of single Si1-xGex NP possesses more features and needs to be discussed in detail. First of all, such spectrum containing three distinct peaks due to Ge-Ge, Si-Ge and Si-Si modes is typical for Si1-xGex alloys with some intermediate composition (i.e. far from almost pure Si or Ge), which agrees in general with our expectation based on the initial amorphous film composition (x ≈ 0.5). A more thorough quantitative analysis, e.g. determination of composition and/or strain in Si1-xGex NP, can be done by the calculation of relative integrated peak intensities or extracting peak positions (i.e. phonon frequencies) for different vibrational modes followed by application of the respective models.23 We applied first the refined model which takes into account the phonon frequencies dependence on both composition and strain parameters described elsewhere.22 Using the system of two equations (see Eqs. 6 and 7 in Ref. 22) and the experimentally obtained frequencies of Ge-Ge and Si-Si modes the Si1-xGex NP composition was calculated then to be equal to x ~ 0.64 by eliminating the second unknown parameter (strain). The obtained in this way higher Ge vs. Si content in Si1-xGex NPs is corroborated also by a typically 3-4 times higher integrated intensity of Ge-Ge Raman peak as compared to its Si-Si counterpart for a number of different measured Si1-xGex NPs. Indeed, taking into account the proportionality of the given Raman peak intensity to the relative number of corresponding bonds (which is x2 and (1 - x)2 in case of Ge-Ge and Si-Si, respectively) as well as to respective Bose factors and phonon frequencies23 one can calculate the composition parameter x from the ratio between Ge-Ge and Si-Si mode intensities. We estimated hence x values in the range from ~ 0.56 to 0.6 for different observed integrated intensity ratios, which ACS Paragon Plus Environment

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Fig. 7. Experimental (dots) and theoretical (lines) scattering spectra for Ge NPs with the diameter of (a) 120 nm and (b) 230 nm. The contributions of ED and MD components in the scattering spectra are shown by the corresponding marked lines. matches well with the above discussion. This estimation is made under the assumption of randomly arranged Si and Ge atoms inside a NP, moreover, the difference in scattering cross sections for Ge-Ge and Si-Si modes due to photon energy dependence is neglected since the used excitation (1.96 eV) is far from both resonances of the Raman scattering in pure Si (~ 3.4 eV) and pure Ge (~ 2.2 eV), as well as in Si1-xGex (~ 2.7 eV) with x ~ 0.6.23-25 The obtained difference between expected x ≈ 0.5 and calculated x ~ 0.56-0.64 values might be explained either by the theoretical models uncertainty or by some laser printing approach regularity (e.g. formation of Ge clusters within Si1-xGex NP), which should be clarified during the oncoming laser printing experiments with Si1-xGex films of different compositions. Thus, an opportunity of individual crystalline NP generation by means of laser printing from amorphous donor films, demonstrated in our work, increases the potential applications of the femtosecond laser transfer method. Optical properties of the fabricated NPs were studied using light scattering spectroscopy accompanied by theoretical simulations of scattering cross sections on the basis of Mie theory. Fig. 7a shows experimental and theoretical scattering spectra including electric and magnetic dipole contributions for Ge NP (diameter ~ 120 nm, SEM image is demonstrated in Fig. 3a). As can be seen, the local maximum at around 650 nm is related to the magnetic dipole (MD) resonance, while the electric dipole (ED) contribution provides a monotonic background. For comparison, Fig. 7b shows experimental and theoretical scattering spectra for Ge NP (diameter ~ 230 nm, SEM image is shown in the inset of Fig. 4). In contrast, for this NP the detected local resonance at around 630 nm is mainly determined by the ED contribution and much less pronounced. Presumably due to lower absorption of Si1-xGex NP compared to pure Ge, a more pronounced resonance peaked at 640 nm was observed for Si1-xGex NP with the diameter of about 200 nm (see Fig. 8). The best agreement between the theory and experiment was obtained for the compositional parameter x = 0.65. This value agrees well with the Si1-xGex NP composition calculated above using Raman spectroscopy showing the deviation from the expected value of x ≈ 0.5 towards higher Ge content. The observed in Figs. 7 and 8 differences between the calculated scattering cross section and the experimental data are most probably due to deviations of the NP shape from the ideal sphere.

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Fig. 8. Experimental (dots) and theoretical (lines) scattering spectra for Si1-xGex NP. Parameters of the model: NP diameter d = 200 nm, composition x = 0.65. The contributions of ED and MD components in the scattering spectrum are shown by the corresponding marked lines.

Fig. 9. Calculated scattering cross sections σ for Ge NP (line) and for Si NP (dots). The diameter of both NPs is equal to 400 nm. The contributions of different multipoles in the scattering cross section for Ge NP are shown by the corresponding marked lines.

Finally, we calculated the scattering cross section for Ge nanosphere with 400 nm diameter and compared results with the equally large Si nanosphere in near-IR spectral region, where the absorption of Ge becomes comparable with that of Si (see Fig. 9). From this comparison it is clear that Ge nanospheres have at least as strong Mie ED and MD resonances in near-IR as the Si nanospheres, whose positions are shifted towards higher wavelengths due to higher refractive index of Ge. It is also worth noting the absence of pronounced contributions from MQ (magnetic quadrupole) and EQ (electric quadrupole) components in the scattering cross section of Ge nanosphere. However, if only the real part in Ge refractive index is taken into account and, moreover, assumed to be a constant, unreasonably strong MQ term in the extinction cross section of Ge sphere (with the diameter of 480 nm) appears as shown by Gómez-Medina et al.17 Note that in our calculations we took into account the dispersion of real and imaginary parts of dielectric permittivity. Conclusions Laser-induced forward transfer of crystalline Ge and Si1-xGex NPs from an amorphous donor film has been demonstrated for the first time. The size of the generated NPs has been controlled by changing the laser wavelength and/or pulse energy. Single nanoparticle femtosecond laser printing regime has been realized using second harmonic of a femtosecond laser at 400 nm. The best reproducibility has been achieved with Ge NPs as shown by respective ordered array fabrication. The generated single Ge and Si1-xGex NPs are characterized by dipole Mie resonances, whose spectral positions match well with the results of theoretical calculations based on Mie theory. The scattering spectra of single Ge NPs with diameters of 120 and 230 nm contain local maxima in the visible spectral range corresponding to resonant magnetic or electric dipole contributions, respectively. The larger (400 nm) Ge NPs possess strong ED and MD resonances in the near-IR spectral region according to theoretical calculations. The demonstrated femtosecond laser printing of size-controlled Ge and composite Si1-xGex NPs with resonant optical responses provides new possibilities for application of this method in nanophotonics and optoelectronics. ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS The authors are grateful to A.V. Novikov (Institute for Physics of Microstructures RAS, Russia) for thin films deposition and to V.A. Volodin (Rzhanov Institute of Semiconductor Physics SB RAS, Russia) for helpful discussion of result on Raman scattering. D.Z. gratefully acknowledge the financial support from the German Academic Exchange Service (DAAD). The partial financial support from Russian Science Foundation (RSF) (Grant No. 16-12-10287) and German Research Foundation (DFG) (Grant Nos. EV 220/2-1, CH179/34-1, RE3012/4-1) is also acknowledged. REFERENCES (1) Zhao, Q.; Zhou, J.; Zhang, F.; Lippens, D. Mie Resonance-based Dielectric Metamaterials. Mater. Today 2009, 12, 60-69. (2) Staude, I.; Schilling, J. Metamaterial-inspired Silicon Nanophotonics. Nat. Photonics 2017, 11, 274-284. (3) Evlyukhin, A. B.; Reinhardt, C.; Seidel, A.; Luk’yanchuk, B. S.; Chichkov, B. N. Optical Response Features of Si-nanoparticle Arrays. Phys. Rev. B 2010, 82, 045404. (4) Evlyukhin, A. B.; Novikov, S. M.; Zywietz, U.; Eriksen, R. L.; Reinhardt, C.; Bozhevolnyi, S. I.; Chichkov, B. N. Demonstration of Magnetic Dipole Resonances of Dielectric Nanospheres in the Visible Region. Nano Lett. 2012, 12, 3749-3755. (5) Baryshnikova, K. V.; Petrov, M. I.; Babicheva, V. E.; Belov, P. A. Plasmonic and silicon spherical nanoparticle antireflective coatings. Sci. Rep. 2016, 6, 22136. (6) Spinelli, P., Verschuuren, M. A.; Polman, A. Broadband omnidirectional antireflection coating based on subwavelength surface Mie resonators. Nat. Comm. 2012, 3, 692. (7) Babicheva, V. E.; Evlyukhin, A. B. Resonant Lattice Kerker Effect in Metasurfaces With Electric and Magnetic Optical Responses. Laser Photonics Rev. 2017, 1700132. (8) Kuznetsov, A. I.; Koch, J.; Chichkov, B. N. Laser-induced Backward Transfer of Gold Nanodroplets. Opt. Express 2009, 17, 18820-18825. (9) Willis, D. A.; Grosu, V. Microdroplet Deposition by Laser-induced Forward Transfer. Appl. Phys. Lett. 2005, 86, 244103. (10) Zywietz, U.; Reinhardt, C.; Evlyukhin, A. B.; Birr, T.; Chichkov, B. N. Generation and Patterning of Si Nanoparticles by Femtosecond Laser Pulses. Appl. Phys. A 2014, 114, 45–50. (11) Zywietz, U.; Evlyukhin, A. B.; Reinhardt, C.; Chichkov, B. N. Laser Printing of Silicon Nanoparticles with Resonant Optical Electric and Magnetic Responses. Nat. Comm. 2014, 5, 3402. (12) García-Etxarri, A.; Gómez-Medina, R.; Froufe-Pérez, L. S.; López, C.; Chantada, L.; Scheffold, F.; Aizpurua, J.; Nieto-Vesperinas, M.; Sáenz, J. J. Strong magnetic response of submicron silicon particles in the infrared. Opt. Express 2011, 19, 4815−4826. (13) Urban, A. S.; Lutich, A. A.; Stefani, F. D.; Feldmann, J. Laser Printing Single Gold Nanoparticles. Nano Lett. 2010, 10, 4794-4798. (14) Kapitanova, P.; Ternovski, V.; Miroshnichenko, A.; Pavlov, N.; Belov, P.; Kivshar Y.; Tribelsky M. Giant Field Enhancement in High-index Dielectric Subwavelength Particles. Sci. Rep. 2017, 7, 731.

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(15) Littlejohns, C. G.; Nedeljkovic, M.; Mallinson, C. F.; Watts, J. F.; Mashanovich, G. Z.; Reed, G. T.; Gardes, F. Y. Next Generation Device Grade Silicon-Germanium on Insulator. Sci. Rep. 2015, 5, 8288. (16) Wood, T.; Naffouti, M.; Berthelot, J.; David, T.; Claude, J.-B.; Métayer, L.; Delobbe, A.; Favre, L.; Ronda, A.; Berbezier, I.; Bonod, N.; Abbarchi, M. All-Dielectric Colour Filters Using SiGe-based Mie Resonator Arrays. ACS Photonics 2017, 4, 873-883. (17) Gómez-Medina, R.; García-Cámara, B.; Suárez-Lacalle, I.; González, F.; Moreno, F.; Nieto-Vesperinas, M.; Sáenz, J. J. Electric and Magnetic Dipolar Response of Germanium Nanospheres: Interference Effects, Scattering Anisotropy, and Optical Forces. J. Nanophotonics 2011, 5, 053512. (18) Grinblat, G.; Li, Y.; Nielsen, M. P.; Oulton, R. F.; Maier, S. A. Enhanced Third Harmonic Generation in Single Germanium Nanodisks Excited at the Anapole Mode. Nano Letters 2016, 16, 4635-4640. (19) Drozdov, M. N.; Drozdov, Yu. N.; Zakharov, N. D.; Lobanov, D. N.; Novikov, A. V.; Yunin, P. A.; Yurasov, D. V. A New Approach to the Diagnostics of Nanoislands in GexSi1-x/Si Heterostructures by Secondary Ion Mass Spectrometry. Tech. Phys. Lett. 2014, 40, 601–605. (20) Jellison Jr., G.E. Refractive index database. https://refractiveindex.info/. (21) Parker, J. H.; Feldman, D. W.; Ashkin, M. Raman Scattering by Silicon and Germanium. Phys. Rev. 1967, 155, 712-714. (22) Volodin, V. A.; Efremov, M. D.; Deryabin, A. S.; Sokolov, L. V. Determination of the Composition and Stresses in GexSi(1−x) Heterostructures from Raman Spectroscopy Data: Refinement of Model Parameters. Semiconductors 2006, 40, 1314-1320. (23) Tsang, J. C.; Mooney, P. M.; Dacol, F.; Chu, J. O. Measurements of alloy composition and strain in thin GexSi1-x layers. J. Appl. Phys. 1994, 75, 8098. (24) Renucci, J. B.; Tyte, R. N.; Cardona, M. Resonant Raman scattering in silicon. Phys. Rev. B 1975, 11, 3885. (25) Cerdeira F.; Dreybrodt W.; Cardona M. Resonant Raman scattering in germanium. Solid State Comm. 1971, 10, 591.

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Femtosecond Laser Printing of Single Ge and SiGe Nanoparticles with Electric and Magnetic Optical Resonances D. M. Zhigunov, A. B. Evlyukhin, A. S. Shalin, U. Zywietz and B. N. Chichkov

(Left) Dark-field microscopic image of Ge nanoparticle array printed using single femtosecond laser pulses from 60 nm donor Ge film. The inset shows characteristic SEM image of Ge nanoparticle. The image demonstrates a principled opportunity of controlled laser-induced transfer of single semiconductor nanoparticles from a donor thin film. (Right) Typical Raman spectra of donor thin SiGe film (dots) and single SiGe nanoparticle (line). The spectra confirm, first, the crystallinity of the generated nanoparticles and amorphous state of corresponding donor film. Moreover, the Raman spectrum of SiGe nanoparticle contains peaks corresponding to Ge-Ge, Si-Ge and Si-Si modes, which proves the ability of femtosecond laser printing of composite nanoparticles.

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TABLE OF CONTENTS GRAPHIC 88x34mm (300 x 300 DPI)

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Scheme of femtosecond laser printing of Ge and Si1-xGex NPs by means of LIFT 84x48mm (300 x 300 DPI)

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SEM images of 60 nm thin Ge film surface irradiated by single fs laser pulses (λ = 800 nm) with gradually increasing energies: (a) 1 nJ, (b) 1.1 nJ, (c) 1.3 nJ, (d) 1.5 nJ. Scale bar is common for all images 160x34mm (300 x 300 DPI)

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Dark-field microscopic image (main) and SEM images (insets) of Ge NPs transferred from 60 nm donor Ge film by means of 800 nm single femtosecond laser pulses with energies in the range of 1-1.5 nJ. Scale bar is common for all inset images 73x81mm (300 x 300 DPI)

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Dark-field microscopic image of Ge NP array printed by 400 nm, 7 nJ single fs laser pulses using 60 nm Ge donor film. The inset shows characteristic SEM images of Ge nanoparticle (upper image) and irradiated donor film surface (lower image) 84x84mm (300 x 300 DPI)

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Dark-field microscopic image of Si1-xGex NP array produced using 400 nm single fs laser pulses with the energy in the range from about 5 to 10 nJ using 60 nm Si1-xGex donor film. The insets show SEM images of selected Si1-xGex nanoparticles (scale bar is common for all inset images) 83x83mm (300 x 300 DPI)

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Typical Raman spectra of (a) donor thin Ge film (dots) and single Ge NP (line), and (b) donor thin Si1-xGex film (dots) and single Si1-xGex NP (line). 152x59mm (300 x 300 DPI)

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Experimental (dots) and theoretical (lines) scattering spectra for Ge NPs with the diameter of (a) 120 nm and (b) 230 nm. The contributions of ED and MD components in the scattering spectra are shown by the corresponding marked lines 156x58mm (300 x 300 DPI)

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Experimental (dots) and theoretical (lines) scattering spectra for Si1-xGex NP. Parameters of the model: NP diameter d = 200 nm, composition x = 0.65. The contributions of ED and MD components in the scattering spectrum are shown by the corresponding marked lines 79x55mm (300 x 300 DPI)

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Calculated scattering cross sections σ for Ge NP (line) and for Si NP (dots). The diameter of both NPs is equal to 400 nm. The contributions of different multipoles in the scattering cross section for Ge NP are shown by the corresponding marked lines 80x55mm (300 x 300 DPI)

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