Nanoscale Generation of White Light for Ultrabroadband

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Nanoscale generation of white light for ultrabroadband nanospectroscopy Sergey V. Makarov, Ivan S. Sinev, Valentin A. Milichko, Filipp E Komissarenko, Dmitry A. Zuev, Elena V. Ushakova, Ivan Mukhin, Yefeng Yu, Arseniy I. Kuznetsov, Pavel A. Belov, Ivan V. Iorsh, Alexander N. Poddubny, Anton Samusev, and Yuri S. Kivshar Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04542 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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Nanoscale generation of white light for ultrabroadband nanospectroscopy S.V. Makarov,∗,† I.S. Sinev,† V.A. Milichko,† F.E. Komissarenko,† D.A. Zuev,† E.V. Ushakova,† I.S. Mukhin,† Y.F. Yu,‡ A.I. Kuznetsov,‡ P.A. Belov,† I.V. Iorsh,† A.N. Poddubny,†,¶ A.K. Samusev,† and Yu.S. Kivshar∗,†,§ †Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg 197101, Russia ‡Data Storage Institute, A*STAR (Agency for Science, Technology and Research), 138634, Singapore ¶Ioffe Institute, St. Petersburg 194021, Russia §Nonlinear Physics Centre, Australian National University, Canberra ACT 2601, Australia E-mail: [email protected]; [email protected]

Abstract Achieving efficient localization of white light at the nanoscale is a major challenge due to the diffraction limit, and nanoscale emitters generating light with a broadband spectrum require complicated engineering. Here we suggest a simple, yet highly efficient, nanoscale white-light source based on a hybrid Si/Au nanoparticle with ultrabroadband (1.3–3.4 eV) spectral characteristics. We incorporate this novel source into a scanning-probe microscope and observe broadband spectrum of photoluminescence that allows fast mapping of local optical response of advanced nanophotonic structures with submicron resolution, thus realizing ultrabroadband near-field nanospectroscopy.

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Keywords Nonlinear nanophotonics, white light emission, nanoparticles, near-field microscopy Creation of efficient broadband nanoscale emitters operating at optical frequencies is a major fundamental challenge that is crucially important for many applications, including ultracompact optical chips, bioimaging, and active nanophotonics. An efficient nanoscale white-light source would be a powerful tool for nanospectroscopy 1 , broadband probing of the near fields of nanophotonic devices or mapping their optical properties correlated with the local density of states (LDOS) 2,3 , as shown schematically in Fig. 1A. Spectral maps of local optical properties outside nanostructures are usually measured by a near-field scanning optical microscope (NSOM) equipped with an optically active near-field probe 4 . In this case, the near-field excitation of a nanostructure is provided by the emission of the subwavelength light source embedded in the probe, e.g. by quantum dots 5 , active molecules 6 , ions 7 , or NV-centers 8 . In general, all such probes suffer from a narrow spectral band (3.4 eV) are injected into conductance band of Si, as well as hot holes are injected into its valence band 21 , as schematically shown in Fig.S8 17 . Third, the hot electron-hole pairs relax nonradiatively and recombine by emitting photons. Observed broadband stationary emission indicates efficient PL of hot charge carriers. This is quite challenging for bulk silicon due to its indirect band gap nature, that makes phonon-assisted optical transitions (Fig. 2K) inefficient as compared to the energy relaxation. We propose several mechanisms that can explain hot PL in considered nanoparticles. (i) Nanosecond PL in the blue spectral range is known for Si nanocrystals and may originate from the Si–O, Si–H, or Si–C surface bonds 13,15,23 (Fig. 2L). (ii) Another mechanism of hot PL proposed in Ref. 13 is based on the Auger recycling processes 24,25 that re-populate high-

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energy states of electrons and pump quasi-direct Γ − Γ transitions (see Fig. 2I). Such hot PL emission exhibits redshift for smaller nanocrystals, and it broadens in an inhomogeneous ensemble. (iii) Hot carriers in silicon can also be generated due to ponderomotive acceleration by several consecutive pump pulses 16 (see Fig. 2J). (iv) Optically-induced heating of nanoparticle elevates the lattice temperature, and hence it leads to the Bose stimulation of optical phonon-assisted indirect emission, see Fig. 2K. Namely, the exciton emission rate in a nanocrystal is increased about 3 times at 1000 K 26 . Moreover, (v) phonons emitted during the energy relaxation process in nanoparticles can be re-absorbed. This slows down the hot carrier energy relaxation by several times 27 and increases probability of light emission. (vi) Finally, we put forward the concept of plasmon-assisted hot PL emisison, see Fig. 2M. the proposed mechanism differs from the plasmonic Purcell-effect enhancement 14 , and it is specific for indirect bandgap materials with free carriers, when the momentum mismatch between electrons and holes is compensated by plasmons rather than phonons. For spatially separated plasmons and excitons, similar ideas were suggested in Refs. 28,29. Here, the effect is stronger because all carriers are confined in the same nanoparticle. As shown in Ref. 17, for realistic concentrations of optically-induced electron-hole plasma (such as ∼ 1020 cm−3 30 ) the plasmon-assisted transitions are more efficient than phonon-assisted transitions by several orders of magnitude. This boosts hot PL efficiency explaining the observed broadband emission. All mentioned processes of hot PL emission can be attributed to the experimentally measured sub-ns times of the PL decay (τhot ≈0.6 ns), whereas the observed nanosecond-scale decay (τhot ≈2.9 ns) can be assigned to defect-assisted processes. For optimizing performance of the white-light emission from hybrid Si/Au nanoparticles, we find the range of incident intensities where the observed PL signal is stable, and its spectral shape does not depend on the intensity. According to Fig. 2E, the three-photon PL emission is observed for 2–10 GW/cm2 , whereas the dependence becomes quadratic for 10— 30 GW/cm2 . Also, the SHG signal drops significantly for 8–20 GW/cm2 , as shown in Fig. 2E. A recovery of the SH signal is observed only for the intensities larger than 20 GW/cm2 . Since

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SHG in Si nanospheres requires nanocrystallinity for inducing local symmetry breaking 31 , we relate the threshold-like decrease of the SH signal to annealing of the silicon part. Indeed, as follows from Fig. 2F, the out-coupling integral PL efficiency is saturated around the annealing threshold. After uncovering the exceptional PL characteristics of the hybrid Si/Au nanoparticles, we employ a single nanoparticle as an ultrabroadband light source for near-field nanospectroscopy. To do so, we functionalize a common atomic force microscopy cantilever by attaching a hybrid nanoparticle to its very tip, as shown in Fig. 3A (for details, see Ref. 17) similarly to the approach proposed in Ref. 12. The modified tip of the cantilever can then be irradiated by a laser light yielding localized white-light generation. The emitted light intensity is high enough, so that it is clearly visible with a microscope videocamera (see the inset in Fig. 1A and Supplementary Movie S1) and allows to measure the broadband spectra with subsecond acquisition time (0.1 second per point for the presented data), which is critical for the implementation of ultrabroadband scanning optical nanospectroscopy. As a test for near-field measurements, we consider a dimer consisting of two a-Si:H nanodisks with diameter 330 nm, see Fig. 3B. Such a structure is capable of providing the enhancement of both electric and magnetic fields at optical frequencies 32 , and it is therefore well-suited for the demonstration of the concept of ultrabroadband nanospectroscopy enabled by nonlinear PL with hybrid nanoparticles. While measuring PL intensity in a broad spectrum range, we map the information about local PL enhancement in the vicinity of the silicon dimer by scanning the functionalized AFM cantilever over the structure in the tapping mode 17 . To interpret the observed PL modifications, we employ a numerical model with a number of assumptions. First, we replace the emitting nanoparticle by point dipoles with three orthogonal orientations, which is a fair approximation taking into account its subwavelength size. We further assume that PL of the hybrid nanoparticle is incoherent, and we neglect the influence of the cantilever tip. Under these conditions, we model the measured PL maps as a

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sum of non-coherent radiation intensity of three mutually orthogonal dipole emitters scanned within a plane above the dimer. Owing to the experimental signal collection geometry, we only account for the emission of dipoles into the substrate. Spectral evolution of the PL maps near a silicon dimer is illustrated in Fig. 3(d). Both calculations (top row) and experimental data (bottom row) reveal conspicuous enhancement of the PL signal above the dimer near 760 nm wavelength. This is further supported by experimental and numerical spectra of normalized mean PL signal above the dimer shown in Fig. 3C, which demonstrate qualitative agreement with aperture-type NSOM 4 , as shown in Fig.S7 17 . Essentially, we demonstrate that hybrid nanoparticle attached to an AFM cantilever can be used as a nanoscale light source for probing the local optical properties of photonic nanostructures. However, as compared to other solutions for probes with embedded light sources, 5–8 the hybrid nanoparticle displays superior radiance combined with extremely broadband emission spectra, which offers unique versatility in near-field measurements. Among the drawbacks of the proposed nanospectroscopy method is that since the whole volume of the particle contributes to the light emission, the spatial resolution of the nearfield signal is limited by the particle size, which in our case is approximately 180 nm (we estimate the spatial resolution of the collected PL signal to be 200-250 nm in the measured spectral region). This cannot match the resolution provided by scattering-type probes 11 , however, we believe that our NSOM probes can be further enhanced by using smaller particles in combination with optimized signal collection scheme. In summary, we have demonstrated a bright white-light nanoscale source with the external quantum efficiency exceeding 0.1% based on hybrid Si/Au nanoparticles. The superb PL characteristics of the hybrid nanoparticles readily suggest multiple applications of such a source for probing local optical properties of advanced nanoscale systems. The major advantage of the method utilizing nonlinear PL of nanoparticles is the possibility to measure the near-field response in a broad spectral range, essentially covering the whole visible spectrum, thus drastically decreasing the required scanning time. This type of ”nanoscale light bulb”

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can be employed as a versatile on-chip light source for broadband active media.

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Acknowledgments The authors thank A. Gudovskikh, A. Mozharov, E. Ubyvovk, and G. Zograf for their assistance with samples and experiments, and also M. Belkin, T. Gregorkiewicz, S. Kruk, K. Kuipers, and M. Stockman for their interest to this work and useful comments. The work was supported by the Russian Foundation for Basic Research (grants 17-03-00621, 1702-00538, 16-29-05317, 16-02-00684), Russian Ministry of Education and Science (Projects 13


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16.8939.2017/8.9, 3.8891.2017/8.9, and 2.2267.2017/4.6), the Australian Research Council, and A*STAR SERC Pharos program, Grant No. 152 73 00025 (Singapore).

Supporting Information Available The following files are available free of charge. • Filename: Suppl This material is available free of charge via the Internet at http://pubs.acs.org/.

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Nanoscale Generation of White Light for Ultrabroadband

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