Second-Harmonic Generation Imaging of Semiconductor Nanowires

Letter pubs.acs.org/NanoLett

Second-Harmonic Generation Imaging of Semiconductor Nanowires with Focused Vector Beams Godofredo Bautista,*,† Jouni Mak̈ italo,† Ya Chen,‡ Veer Dhaka,‡ Marco Grasso,† Lasse Karvonen,‡ Hua Jiang,§ Mikko J. Huttunen,∥ Teppo Huhtio,‡ Harri Lipsanen,‡ and Martti Kauranen† †

Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland Department of Micro and Nanosciences, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland § Department of Applied Physics and Nanomicroscopy Center, Aalto University, P.O. Box 15100, FI-00076 Aalto, Finland ∥ COMP Centre of Excellence, Department of Applied Physics, Aalto University, FI-00076 Aalto, Finland ‡

ABSTRACT: We use second-harmonic generation (SHG) with focused vector beams to investigate individual vertically aligned GaAs nanowires. Our results provide direct evidence that SHG from oriented nanowires is mainly driven by the longitudinal field along the nanowire growth axis. Consequently, focused radial polarization provides a superior tool to characterize such nanowires compared to linear polarization, also allowing this possibility in the native growth environment. We model our experiments by describing the SHG process for zinc-blende structure and dipolar bulk nonlinearity.

KEYWORDS: Second-harmonic generation, semiconductor, radial polarization, nonlinear imaging, modeling

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the substrate. For real applications of nanowires, however, it has been shown that pristine vertically aligned nanowires are beneficial due to the ease of fabrication, maximization of surface-to-volume ratio, and high integrability to components and devices.24−26 To this date, however, there is a lack of optical tools that can address vertically aligned nanowires. In order to couple light into vertically aligned nanowires, the excitation field needs to have a field component along the wire axis. This implies that the excitation geometry be modified compared to the standard normal incidence with respect to the sample substrate. This can be accomplished by exposing the nanowires to a uniform field at an oblique incidence angle. However, such geometry is not applicable to nonlinear optical techniques due to difficulties in signal detection. An emerging approach for exciting symmetric nanostructures at normal incidence is to use cylindrical vector beams such as radial (RP) or azimuthal (AP) polarizations.27 As shown earlier, tightly focused RP results in a strong field component along the direction of propagation of the beam, usually referred to as the longitudinal component.28−32 Owing to the strong longitudinal component, linear and nonlinear microscopies with vector beams have proven to be particularly useful in orientation and structure-sensitive imaging.33−44

he salient features of semiconductor nanowires, such as high crystallinity,1,2 polarization anisotropy,3 and high nonlinear optical responses,4 make them potential building blocks for future nonlinear photonic5 and plasmonic devices.6 Indeed, a plurality of essential results has been achieved regarding the fundamental properties and potential applications of semiconductor nanowires.7−13 To harness the full potential of nanowires, novel characterization tools are needed that are capable of scrutinizing their optical properties on the level of individual nanowires. Optical second-harmonic generation (SHG) is the simplest and most common nonlinear optical phenomenon that can be used to study semiconductor nanowires. SHG refers to the conversion of an incident optical field at frequency ω to second-harmonic field at frequency 2ω and arises from the second-order nonlinear optical susceptibility of the material.14 The use of SHG is advantageous because of its inherent sensitivity to structural symmetry, which is not easy to access using other optical techniques.15 Indeed, it has been shown that SHG-based techniques are useful in investigating semiconductor nanowires with different composition and crystallinity.16−22 Most of the previous works, however, have investigated nanowires that are mechanically or chemically removed from their native substrate and then horizontally placed on a host substrate. In addition, the samples have been excited with optical fields that could have far- or near-field character,23 with the latter prone to imaging artifacts, and subsequent detection of scattered signals along the normal of © XXXX American Chemical Society

Received: October 16, 2014 Revised: January 18, 2015

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DOI: 10.1021/nl503984b Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters In this Letter, we demonstrate that vertically aligned GaAs nanowires up to 10 μm long and grown on top of a GaAs substrate can be characterized by SHG imaging with focused vector beams. Using different incident beam polarizations, we find that SHG from an individual vertically aligned nanowire is highest when tightly focused RP is used. The highest SHG signal is achieved due to the preferential coupling of the longitudinal field component along the nanowire growth axis. Finally, we verify our experiments using SHG modeling of GaAs nanowires described with dipolar bulk nonlinearity. Although SHG was observed recently from vertically aligned GaAs45 and GaP nanowires,46 these works did not fully account for the vectorial nature of the excitation. In contrast, we will show both experimentally and numerically that the vectorial properties of the beam are crucial in understanding the favorable coupling of incident light to the individual nanowires. Our technique has been earlier used to characterize subwavelength metal nanoparticles, whose second-order response has surface origin.39 The present results thus extend the technique to much larger objects and bulk nonlinearity, which brings additional challenges to the experiments and their modeling. Au-catalyzed vertical GaAs nanowires were grown on GaAs (111)B substrates in a 1 in. horizontal flow atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) system using trimethylgallium (TMG) and tertiarybutylarsine (TBA) as precursors for gallium (Ga) and arsenic (As), respectively.47,48 Hydrogen was used as carrier gas, and the total reactor gas flow rate was ∼5 slm. For cleaning, the samples were soaked in an ultrasonic bath with acetone and isopropanol for 2 min each, followed by 5 min deionized water flow immersion. Subsequently, the substrates were treated with polyL-lysine (PLL) solution for about 1 min, followed by 2 min deposition of 40 nm diameter colloidal gold (Au) nanoparticles solution. Prior to growth, the GaAs substrate was annealed in situ at 650 °C for 10 min under hydrogen ambient to desorb surface contaminants. MOVPE growth was started by switching on TMG and TBA sources simultaneously for 5 min at the temperature of 450 °C. The nominal V/III ratio during the growth was ∼25 and the TMG and TBA flows were 7 and 75 sccm, respectively. High-resolution transmission electron microscopy (TEM), electron diffraction (ED), and scanning electron microscopy (SEM) were used to determine the dimensions and crystallinity of the fabricated nanowires. The grown nanowires were typically 40 nm in diameter and 10 μm in length (Figure 1a,d). TEM-ED revealed that the nanowires have a pure zinc-blende structure free from structural defects (Figure 1b,c). It is well-known that SHG from noncentrosymmetric cubic media, like GaAs, is dominated by the electric-dipole-allowed bulk response. This response, however, can be suppressed using appropriate choice of experimental geometries.49 GaAs can exhibit zinc-blende50 and wurtzite51 symmetries, where the former is energetically more favorable in the bulk form. In the nanoscale, however, GaAs may display both crystal symmetries, as shown theoretically and experimentally.52−56 As the nonlinear susceptibility tensor dictates the efficiency of the SHG process, it is highly imperative to determine the crystal symmetry of the nanowire. Indeed, it was recently shown that crystal orientation significantly influences SHG in oriented GaAs photonic crystal cavities.57 According to TEM-ED measurements (Figure 1c), our GaAs nanowires exhibit zinc-blende structure, hence, the second-

Figure 1. TEM images (a,b) of a GaAs nanowire grown on a GaAs substrate. The arrow in (a) indicates the growth direction of the GaAs nanowire. (c) Typical indexed ED pattern taken at different sections along the nanowire axis that confirms a pure zinc-blende structure. (d) Side-view SEM image of the GaAs nanowires used in the SHG study.

order susceptibility has only three nonvanishing tensor (2) (2) components: χ(2) xyz, χyzx, and χzxy, where x, y, and z refer to the principal-axis system of the crystal ([100], [010], [001]). For a nanowire that preferentially grows along the ⟨111⟩ direction, it is necessary to perform appropriate coordinate transformations between the crystal and laboratory frames in analyzing the results. To experimentally investigate the nanowires, we used a custom-built point-scanning SHG microscope operating in reflection.39,40 We used a mode-locked femtosecond Nd:glass laser (wavelength 1060 nm, pulse length 200 fs, repetition rate 82 MHz) for excitation. After routine beam isolation and conditioning, the beam was directed to the back-aperture of an infinity-corrected (50×, 0.8 NA) and strain-free microscope objective. The objective was used to focus the beam onto the sample, mounted on a three-axis piezo-scanning stage. The reflected fundamental and SHG signals were collected by the same objective and discriminated by a dichroic mirror. To extract the backscattered SHG signal, appropriate spectral band filters, a tube lens, and a cooled photomultiplier tube were used. The 800 nm diameter of the fundamental beam focus permitted us to collect signals from individual nanowires that are free from interwire coupling effects. To control the linear polarization (LP) input, we used a motorized half-wave plate. To achieve RP or AP with high polarization purity before the objective, a polarization mode converter (ARCoptix, S.A.) with built-in phase compensation in tandem with a spatial Fourierfilter were used. The quality of RP and AP was further confirmed by verifying that their vortex structure is maintained after propagation through the focus and recollimation. Throughout the imaging experiments, average power levels of less than 2 mW were used. Here, the background SHG signals from the bare ⟨111⟩ GaAs substrate are well below 10% of the SHG signals detected from the vertically oriented GaAs nanowire when the beam is focused on the substrate. In the B

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We then imaged the GaAs nanowires using tightly focused LP, RP, and AP. For focused LP (Figure 4a), we obtained SHG

actual experiments, we focused the beam near the nanowire tips, that is, about 10 μm from the substrate, resulting in background levels several orders weaker than the actual signals. All SHG measurements were carried out at room temperature. Prior to imaging, we measured and verified the SHG signal from an individual vertical GaAs nanowire. As depicted in Figure 2, the signal exhibits quadratic power dependence (see

Figure 4. Observed far-field SHG images of vertically aligned GaAs nanowires (diameter = 40 nm, length = 10 μm) using focused LP along x (a), RP (b), and AP (c) under the same experiment settings. The images are normalized to (b).

intensities that resemble two-lobed patterns with a dark spot at the center, which coincides with the location of the vertical nanowires. Furthermore, the two-lobed patterns are symmetric with respect to the incident LP axis, due to the distribution of the longitudinal component of a focused LP beam.58,59 For focused RP (Figure 4b), we found that the SHG intensity patterns resemble a pointlike distribution at the location of the nanowires. These intensity patterns are due to the strong longitudinal field at the center of the focused RP.29 Generally, both LP and RP produce appreciable SHG signals from the nanowires. More interestingly, the resulting SHG hotspots from the vertical nanowires under RP are stronger than under LP. Altogether, these results suggest that the longitudinal field and its strength significantly influence the SHG signal. Although there are some differences in the SHG images for LP and RP, we attribute it to variations in height (Figure 1d). We also excited the nanowires using focused AP and found no significant SHG patterns from the vertical nanowires due to the absence of any longitudinal fields at the focus (Figure 4c). The signals here are mainly attributed to the coupling of the azimuthal fields with bent nanowires (Figure 1d). To understand the origin of SHG from the nanowires, we calculated the SHG response of an idealized GaAs nanowire with zinc-blende symmetry under tightly focused LP, RP and AP. Our model is based on the frequency-domain boundary element method (BEM), previously extended for surface SHG in the undepleted-pump approximation in ref 60. For this work, we extended the Poggio−Miller−Chang−Harrington−Wu− Tsai (PMCHWT) formulation of BEM with Rao−Wilton− Glisson (RWG) basis functions for modeling dipolar bulk SHG described by a volume polarization source P = ϵ0χ(2): ee, where e is the fundamental electric field. The BEM calculations are performed in the laboratory frame (x′, y′, z′), where the wire is oriented along the z′-direction. To calculate the nonlinear polarization source P in the laboratory frame, a transformation between the laboratory and crystal frames is used. It is then necessary to integrate G⃡ P, where G⃡ is the Green dyadic, over the volume of the particle. We used the appropriate focused fields of ref 61 as excitation. The scattered SHG signal in reflection was integrated over the aperture of the lens as in ref 39. We stored the LU-factorizations of the system matrices for both fundamental and SH fields, so that the solutions for multiple beam positions were obtained efficiently. We also utilized the three plane symmetries for the wires and by applying the group representation theory we were able to significantly decrease memory requirements and CPU time. For the simulated sample, we used a nanowire of GaAs62,63 with a

Figure 2. Power dependence of the SHG signal from an individual GaAs nanowire using focused RP and LP. The red and blue data points show the quadratic dependence of the unpolarized SHG signal from the GaAs nanowire illuminated by RP and LP, respectively. The solid curves show the theoretical quadratic fits for both cases.

theoretical fits) on the input laser power regardless of the incident polarization used. This result suggests that both RP and LP are useful in driving SHG from such nanostructures. We also found that the SHG signal is always higher for the incident RP than LP, suggesting that the longitudinal field produced by focused RP couples better than LP to the nonlinear response of the nanowire. As other two-photon processes can also result in quadratic power dependence, we verified the nature of the measured signals by replacing the photomultiplier tube with a spectrometer. Shown in Figure 3 are the detected signals from an individual nanowire using focused RP. The prominent peak at 530 nm corresponds to the SHG wavelength for the fundamental wavelength of 1060 nm. Clearly, there are no other features, validating the good spectral discrimination of the setup.

Figure 3. Measured back-reflected signal from an individual GaAs nanowire using focused RP and LP (inset). The remnant of the fundamental (after addition of several IR filter blocks) is also shown. C

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Nano Letters diameter of 40 nm and height of 10 μm and used air as the surrounding medium. Shown in Figure 5 are the calculated far-field SHG images from a vertical nanowire under tightly focused LP, RP and AP

hypothesis, we repeated our simulations for a very short nanowire (diameter = 40 nm, length = 100 nm), where propagation effects are much less important. This nanowire geometry does result in a spotlike SHG intensity distribution for LP instead of a two-lobed pattern (data not shown) suggesting that the nonlinear response involves a complicated interplay between the material properties and the nanowire geometry, which is particularly challenging for modeling. Our experimental results (Figures 2−4) are in qualitative agreement with the SHG calculations (Figure 5). Furthermore, the use of RP is advantageous because the longitudinal fields at the focus are stronger than for LP and AP excitations at equivalent experimental settings. Although the experimental and modeling results agree qualitatively, we found a difference in the ratio of the SHG intensities taken with RP and LP in the experiment (3- to 5-fold) and simulations (30-fold). Note, however, that the modeling is based on the calculation of field amplitudes rather than detected intensities. This immediately reduces the discrepancy to a factor of ∼3. We believe that this discrepancy is mainly because our model neglects full electromagnetic treatment of the fundamental and SHG fields reflected by the substrate. The refractive index of GaAs for the fundamental wavelength of 1060 nm is about 3.5. In consequence, the driving fundamental field intensity can be modulated by a factor of ∼10 along the nanowire through interference between the forward and backward propagating field components. This results in a similar modulation of the SHG field amplitude produced by different parts of the nanowire. In addition, the GaAs wire samples have a gold nanoparticle at the top that may yield surface SHG but as even the relative values of the nonlinear susceptibilities of gold and GaAs are unknown, these nanoparticles were neglected in the model. Considering these unknown factors, we believe that the predicted and experimental SHG images are in good agreement. Of course, even more quantitative agreement would be desirable but will require significant advances in the modeling of additional sample details, for example, the growth substrate, metal catalyst, nanowire defects, and possible coupling between the nanowires. Altogether, our results suggest that the vertical nanowires are preferentially excited by longitudinal fields resulting from focused beam geometries. This is in accordance to earlier results where nanowires lying on a substrate were illuminated by LP along the nanowire axis.3,18,65 Additionally, our work supports previous reports that advocated the importance of longitudinal (or out-of-plane) field components in the nonlinear responses of semiconductor assemblies.66−70 It is also worth noting that our technique is general and widely applicable to studying other semiconductor nanowires with unique properties. At this point, we have only investigated vertical GaAs nanowires with zinc-blende structure. The lattice structure, nanowire orientation, structural features, and presence of external fields can significantly influence the SHG response. We therefore anticipate that it will also be interesting to investigate and demonstrate novel optical or electrical phenomena in vertical GaAs or other semiconductor nanowires that exhibit pure wurtzite symmetry or polytypism, that is, combinations of zinc-blende and wurtzite structures.71 Furthermore, as the role of gold,72 or in general catalysts, in the performance of semiconductor nanowires is still under debate, it will also be important to scrutinize self-assisted semiconductor nanowire growth using our technique.

Figure 5. Calculated far-field SHG images of a vertically aligned GaAs nanowire (diameter = 40 nm, length = 10 μm) in air using focused LP along x (a), RP (b) and AP (c). The calculated images are normalized to the incident beam amplitude and their relative intensity values (with respect to panel b) are shown. The parameters used in the calculations simulate the experiment settings.

under the same experiment parameters. Note that the SHG emission from the nanowire occurs predominantly in the forward direction. Hence, the signal detected backward is dominated by the forward emission reflected by the substrate. Under LP (Figure 5a), a two-lobed SHG pattern that is symmetric with respect to the transverse axes is observed, closely resembling the intensity distribution of the longitudinal electric field components. In contrast, the SHG pattern for RP is spotlike and localized at the center, again closely resembling the intensity distribution of the longitudinal field component.28 Although significant SHG intensities arose from focused LP beams, we found that focusing RP provides a 30-fold increase in the predicted SHG intensity (Figure 5b) as compared to LP. This observation is reasonable for moderately tight focusing conditions (NA = 0.8), where the strength of the longitudinal field components becomes significant. As further evidence, we also calculated the far-field SHG image for AP (Figure 5c) that produces a strictly transverse field distribution at the focus. The signal weakness is a direct consequence of the absence of longitudinal fields and the fact that the transverse field components couple only very weakly. Altogether, our simulations suggest that SHG from the vertical GaAs nanowires can only be excited by longitudinal fields as predicted accurately by using the dipolar bulk nonlinearity. The results for LP are actually surprising considering the structure of the GaAs nanowires. They have three-fold symmetry when viewed along the growth axis and should therefore be SHG active for LP at normal incidence. Although the predicted SHG image of the nanowires under focused LP (Figure 5a) seems to exhibit a node at the location of the nanowire, the SHG signal between the two prominent intensity lobes is nonzero suggesting that transverse field components at the center of focused LP can also excite the nanowire. This is also evident in the experimental SHG image of the nanowires under focused LP (Figure 4a). However, such a transverse source emits SHG predominantly along the wire and its growth is constrained by phase-matching and losses in propagation along the wire. On the other hand, the RP as well as LP that is not centered on the wire give rise to axial sources, which can only emit to off-axis directions, thereby coupling SHG light out of the wire. This allows Cerenkov-type phase matching of the SHG signal into off-axis directions,64 which are still well collected by the microscope objective. In order to validate this D

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(6) Kauranen, M.; Zayats, A. V. Nat. Photonics 2012, 6, 737. (7) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (8) Borgstrom, M. T.; Zwiller, V.; Muller, E.; Imamoglu, A. Nano Lett. 2005, 5, 1439. (9) Saxena, D.; Mokkapati, S.; Parkinson, P.; Jiang, N.; Gao, Q.; Tan, H. H.; Jagadish, C. Nat. Photonics 2013, 7, 963. (10) Priolo, F.; Gregorkiewicz, T.; Galli, M.; Krauss, T. F. Nat. Nanotechnol. 2014, 9, 19. (11) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885. (12) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. Nano Lett. 2007, 7, 1003. (13) Yu, H.; Fang, W.; Wu, X.; Lin, X.; Tong, L.; Liu, W.; Wang, A.; Shen, Y. R. Nano Lett. 2014, 14, 3487. (14) Franken, P. A.; Hill, A. E.; Peters, C. W.; Weinreich, G. Phys. Rev. Lett. 1961, 7, 118. (15) Boyd, R. W. Nonlinear Optics, 3rd ed.; Academic Press: Burlington, MA, 2008. (16) Chen, R.; Crankshaw, S.; Tran, T.; Chuang, L.; Moewe, M.; Chang-Hasnain, C. Appl. Phys. Lett. 2010, 96, 051110. (17) Zhang, C. F.; Dong, Z. W.; You, G. J.; Zhu, R. Y.; Qian, S. X.; Deng, H.; Cheng, H.; Wang, J. C. Appl. Phys. Lett. 2006, 89, 042117. (18) Barzda, V.; Cisek, R.; Spencer, T. L.; Philipose, U.; Ruda, H. E.; Shik, A. Appl. Phys. Lett. 2008, 92, 113111. (19) Prasanth, R.; van Vugt, L. K.; Vanmaekelbergh, D. A. M.; Gerritsen, H. C. Appl. Phys. Lett. 2006, 88, 181501. (20) Long, J. P.; Simpkins, B. S.; Rowenhorst, D. J.; Pehrsson, P. E. Nano Lett. 2007, 7, 831. (21) Grange, R.; Brönstrup, G.; Kiometzis, M.; Sergeyev, A.; Richter, J.; Leiterer, C.; Fritzsche, W.; Gutsche, C.; Lysov, A.; Prost, W.; Tegude, F. J.; Pertsch, T.; Tünnermann, A.; Christiansen, S. Nano Lett. 2012, 12, 5412. (22) Liu, W.; Wang, K.; Liu, Z.; Shen, G.; Lu, P. Nano Lett. 2013, 13, 4224. (23) Johnson, J. C.; Yan, H. Q.; Schaller, R. D.; Petersen, P. B.; Yang, P. D.; Saykally, R. J. Nano Lett. 2002, 2, 279. (24) Anttu, N.; Xu, H. Q. Opt. Express 2013, 21, A558. (25) Li, Q.; Creighton, J. R.; Wang, G. T. J. Cryst. Growth 2008, 310, 3706. (26) Krogstrup, P.; Jørgensen, H.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Fontcuberta i Morral, A. Nat. Photonics 2013, 7, 306. (27) Kogelnik, H.; Li, T. Appl. Opt. 1966, 5, 1550. (28) Quabis, S.; Dorn, R.; Eberler, M.; Glöckl, O.; Leuchs, G. Opt. Commun. 2000, 179, 1. (29) Dorn, R.; Quabis, S.; Leuchs, G. Phys. Rev. Lett. 2003, 91, 233901. (30) Youngworth, K. S.; Brown, T. G. Opt. Express 2000, 7, 77. (31) Zhan, Q. Adv. Opt. Photon. 2009, 1, 1. (32) Wolf, E. Progress in Optics; Elsevier: New York, 2011. (33) Novotny, L.; Beversluis, M. R.; Youngworth, K. S.; Brown, T. G. Phys. Rev. Lett. 2001, 86, 5251. (34) Failla, A. V.; Qian, H.; Qian, H.; Hartschuh, A.; Meixner, A. J. Nano Lett. 2006, 6, 1374. (35) Ishitobi, H.; Nakamura, I.; Hayazawa, N.; Sekkat, Z.; Kawata, S. J. Phys. Chem. B 2010, 114, 2565. (36) Fleischer, M.; Stanciu, C.; Stade, F.; Stadler, J.; Braun, K.; Heeren, A.; Häffner, M.; Kern, D. P.; Meixner, A. J. Appl. Phys. Lett. 2008, 93, 111114. (37) Yoshiki, K.; Ryosuke, K.; Hashimoto, M.; Hashimoto, N.; Araki, T. Opt. Lett. 2007, 32, 1680. (38) Yew, E. Y. S.; Sheppard, C. J. R. Opt. Commun. 2007, 275, 453. (39) Bautista, G.; Huttunen, M. J.; Mäkitalo, J.; Kontio, J. M.; Simonen, J.; Kauranen, M. Nano Lett. 2012, 12, 3207. (40) Huttunen, M. J.; Lindfors, K.; Andriano, D.; Mäkitalo, J.; Bautista, G.; Lippitz, M.; Kauranen, M. Opt. Lett. 2014, 39, 3686.

Our results have important implications for two complementary applications. First, as a characterization tool, the technique can be extended to 3D imaging of nanowires. By combining this with more advanced control of the vector fields, both the nanowire orientation as well as the crystal axis directions should become accessible. The other important application is in nanoscale harmonic sources that need to be excited using optimum incident fields as already demonstrated here. Finally, the possibility of exciting vertical nanowires in their native environment by RP provides a relatively general new platform that can be adapted to other characterization schemes, each important for their particular applications. We anticipate that the new excitation geometry will be applicable to established characterization schemes based on photoluminescence,3 sum-frequency generation,73 third-harmonic generation,74 and photocurrent26 detection in pristine nanowires. In addition, our work can impact the emerging applications of nanostructures for all-optical beam manipulation,75,76 engineering,77 and orientation probing78 in the nanoscale. In conclusion, we have demonstrated that pristine vertically aligned GaAs nanowires are preferentially excited by the longitudinal field arising from the focusing of vector beams. Strong second-harmonic signals were achieved with the strongest longitudinal fields as provided by focused radial polarization. We supported our experiments by modeling the SHG process for zinc-blende structure and dipolar bulk nonlinearity. This new excitation geometry has direct implications for similar characterization protocols for vertical nanowires but is also relevant to photonics, plasmonics, and photovoltaics and in tailoring nonlinear optical phenomena in semiconductor nanowires in general.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: godofredo.bautista@tut.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants 134973 (to M.K.) and 267847 (to G.B.) from the Academy of Finland, TUT’s Strategy Funding 2014 (84010), MOPPI project of Aalto Energy Efficiency Research Programme and TEKES FiDiPro NP-Nano project. J.M. acknowledges the graduate school of TUT for funding. M.J.H. acknowledges support from the Academy of Finland through its Centers of Excellence Programme (2012−2017) under Project No. 251748 and the European Research Council (ERC-2013-AdG-340748-CODE). Nanowire fabrication was performed at the Micronova Nanofabrication Centre of Aalto University. This work was performed in the context of the European COST Action MP1302 Nanospectroscopy.

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REFERENCES

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Nano Letters

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DOI: 10.1021/nl503984b Nano Lett. XXXX, XXX, XXX−XXX