Efficient Second Harmonic Generation in a Hybrid Plasmonic

May 22, 2019 - Naomi J. Halas. Naomi J. Halas .... The Institute for Advanced Studies, Wuhan University, Wuhan 430072, China. ‡. School of Physics a...
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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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Efficient Second Harmonic Generation in a Hybrid Plasmonic Waveguide by Mode Interactions Junjun Shi,† Yang Li,‡ Meng Kang,‡ Xiaobo He,‡ Naomi J. Halas,§ Peter Nordlander,§ Shunping Zhang,*,‡ and Hongxing Xu*,†,‡ †

The Institute for Advanced Studies, Wuhan University, Wuhan 430072, China School of Physics and Technology, Center for Nanoscience and Nanotechnology, and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China § Department of Physics and Astronomy, Department of Electrical and Computer Engineering and Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States Downloaded by UNIV OF SOUTHERN INDIANA at 20:27:59:819 on May 24, 2019 from https://pubs.acs.org/doi/10.1021/acs.nanolett.9b01004.

‡

S Supporting Information *

ABSTRACT: Developing highly efficient nanoscale coherent light sources is essential for advances in technological applications such as integrated photonic circuits, bioimaging, and sensing. An on-chip wavelength convertor based on second harmonic generation (SHG) would be a crucial step toward this goal, but the light-conversion efficiency would be low for small device dimensions. Here we demonstrate strongly enhanced SHG with a high conversion efficiency of 4 × 10−5 W−1 from a hybrid plasmonic waveguide consisting of a CdSe nanowire coupled with a Au film. The strong spatial overlap of the waveguide mode with the nonlinear material and momentum conservation between the incident and reflected modes are the key factors resulting in such high efficiency. The SHG emission angles vary linearly with excitation wavelength, indicating a nonlinear steering of coherent light emission at the subwavelength scale. Our work is promising for the realization of efficient and tunable nonlinear coherent sources and opens new approaches for efficient integrated nonlinear nanophotonic devices. KEYWORDS: hybrid waveguide, semiconductor nanowire, second harmonic generation, momentum conservation, surface plasmon polariton

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surface plasmon polaritons (SPPs). The hybrid nature of SPPs enables the confinement of the electromagnetic field into arbitrarily small volumes, resulting in large local field enhancements. Strongly amplified SHG has been observed in plasmonic nanostructures.15−17 The overall conversion efficiency in bare plasmonic structures is still low (∼10−11 or smaller) owing to the lack of dipole-allowed bulk susceptibility in centrosymmetric crystal structure of most plasmonic metals.18 Recently, hybrid structures composed of nonlinear dielectric materials in close proximity to metallic structures have attracted growing interests.19−22 These structures benefit from both the strong near-field enhancements of the metal constituent and the high nonlinear susceptibilities of the dielectric constituent medium. In waveguide geometries, efficiencies can potentially be further optimized by satisfying momentum conservation23 and phase matching conditions, which allow for the coherent superposition of the generated second harmonic. Theoretical calculations predict conversion efficiencies as large as a few percent may be achievable in

mall footprint, bright, and tunable coherent sources based on nonlinear frequency conversion have long been pursued for integrated silicon photonics,1 nonlinear optical microscopy,2−4 and nonclassical light generation.5 Dielectric nanowires (NWs) are attractive components for achieving these goals, since they can guide light at the subwavelength scale6−8 and offer large built-in nonlinear susceptibilities, owing to their crystalline structures. Second harmonic generation (SHG) is a nonlinear process where two photons annihilate and generate a photon with twice the photon energy. SHG from dielectric NWs has been widely studied in materials such as ZnTe,9 GaAs,10 CdS,11 LiNbO3,12 excited by incident Gaussian beams or guided photons in the NWs. However, the conversion efficiency is generally low, typically on the order of 10−8 or lower, which hinders practical applications. The photonic modes become poorly confined within the NWs as their diameters approach the mode wavelength scale.13 As a consequence, the overlap between the electromagnetic field and the nonlinear materials (i.e., the NWs) drops significantly, resulting in low conversion efficiency. In contrast to dielectric NWs, plasmonic waveguides can guide light in the form of coupled electromagnetic field and electronic motions at metal−dielectric interfaces13,14 called © XXXX American Chemical Society

Received: March 11, 2019 Revised: May 22, 2019

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Figure 1. (a) Schematic illustration of the SHG from the guided modes in a hybrid plasmonic waveguide. The system consists of a CdSe NW on top of a Au film with a Al2O3 dielectric spacer layer. Inset: tilted-view SEM image of the hybrid structure showing the hexagonal cross section of the CdSe NW. Scale bar: 100 nm. (b) TEM micrograph of a CdSe NW grown along the c-axis. (c) The electron diffraction pattern of the CdSe NW, collected from the boxed selected region in (b). (d) Polarimetric SHG pattern obtained from the same NW by rotating the sample while the polarization of excited laser and collection polarizer fixed. α is the angle between the axis of the NW and the polarization of the incident laser. The red solid line is the fitting curve.

Figure 2. (a) Numerical simulations of normalized electric field distribution for the different modes in the hybrid system. Here, the incident wavelength is set to 800 nm. Structure parameters are w = 360 nm and t = 5 nm. (b) The effective refractive index of the five guided modes with the widths w. The triangles represent symmetric (with respect to y-axis) modes while the circles mean asymmetric modes. (c,d) Mode profiles of electric field components |Ez| (c) and |Ex| (d) along x = 0 direction.

hybrid plasmonic waveguides,24 but thus far, such large conversion efficiencies have not been realized. In this Letter, we demonstrate a remarkably high SHG efficiency in a hybrid plasmonic waveguide consisting of a CdSe NW coupled with a Au film (Figure 1a). This geometry takes full advantage of the high second-order nonlinear susceptibility of CdSe,25 strong mode confinement, and a long-range propagation length.26 Using end-scattering excitation with polarization control, either SPP-like or photonic-like modes can be selectively launched into the waveguide resulting in SHG conversion efficiencies of 2 × 10−6 W−1 and 4 × 10−5 W−1, respectively. These SHG conversion efficiencies are

several orders of magnitude higher than the best reported values in bare plasmonic systems.17,27 Measurements of the emission angles of the SH signal using Fourier imaging demonstrated that the two counter-propagating modes in the SHG process satisfy momentum conservation. The emission angles were found to shift linearly with excitation wavelength, demonstrating angular steering of the nonlinear output from the waveguide. Our findings demonstrate a significant step toward the implementation of dielectric NWs as highly compact nonlinear optics platforms. A schematic of the hybrid waveguide is shown in Figure 1a. It consists of a CdSe NW on a Au film, separated by a Al2O3 B

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Figure 3. (a) SEM image of a CdSe NW on an ultrasmooth Au film, with a length of 5 μm and a width of 150 nm. (b) A false color image of light propagation along the NW. The excitation wavelength is 800 nm with a power of 1 mW. The polarization of the laser is parallel to the NW axis, indicated by the arrow. (c) SHG image of the same NW. The marked regions A and B represent the excitation and output terminal, respectively. (d) Calculated y-component of the power flux at SH frequency for a NW laid on a Au film. (e) The intensity profiles along the NW, corresponding to the dashed lines in (c) and (d). (f) Spectra of the SH signal from region A and B in (c). Inset: Power dependence of the SHG from region B. A slope of 1.99 is close to the expected quadratic dependence. (g) Calculated and experimental values of SHG efficiency as a function of the spacer thickness. The red line represents the calculated SH power over the NW, reflecting the trend of SHG efficiency. The black spots represent the experimental data.

dielectric layer grown by atomic layer deposition. The Au film (thickness 200 nm) is prepared using template-stripping,28 yielding an ultrasmooth gold surface with a typical roughness of 0.32 nm.29 The CdSe NWs were synthesized by chemical vapor deposition (CVD)30 and were dropped onto the substrate after being dispersed in ethanol. The widths of the CdSe NWs typically range from 140 to 500 nm, and the lengths from 4 to 30 μm. The tilt-view scanning electron microscope (SEM) image of the hybrid waveguide shows that their cross sections are hexagonal (inset to Figure 1a). To correlate the crystal structure and the second-order susceptibility of the samples, the NWs were studied subsequently by SHG polarimetry and transmission electron microscopy (TEM) (Figures 1b−d). The selected area electron diffraction pattern in Figure 1c, collected from the boxed region in the TEM image, indicates that the c-axis is along the long axis of the NW.31,32 Figure 1d shows the polarimetric SHG pattern of the same CdSe NW obtained by rotating the sample, keeping the polarization of the excitation laser and the collection analyzer unchanged and parallel to each other. The sample was rotated around the y-axis (substrate normal) in steps of 10°, with an angle α with respect to the incident polarization. The results demonstrate that the strongest SHG is along the long axis of the NWs. This is because the largest second-order nonlinear coefficient (d33) of NW is along the c axis. To understand the characteristics of modes supported by the hybrid waveguide, we performed mode analysis using the finite element method (COMSOL Multiphysics V5.2a, for details see Supporting Information S1). Figure 2a shows the normalized electric field of five guided modes of the hybrid waveguide, for a NW of width w = 360 nm. The fundamental mode is the hybrid plasmonic (HP) mode resulting from the

hybridization of the SPP at the metal surface and the HE11 mode in an isolated CdSe NW. The HP, HE11b, and HE21 modes are symmetric with respect to the central plane containing the y-axis, while the HE11a and the TE01 modes are antisymmetric (see electric distribution in S1 in Supporting Information). Figure 2b shows the real part of the effective refractive index for these five modes as a function of NW width. For widths smaller than 190 nm, only the HP mode, with its energy confined in the dielectric gap (Al2O3 layer) and inside the NW, is supported. For w = 190−320 nm, there are four modes coexisting and for w > 320 nm, even more modes appear which can participate in the SHG process. To guide the experimental choice of incident polarization and further illustrate the overlap between the guided modes and nonlinear material, we plot the profiles of the electric field components Ez and Ex along the y-axis in Figure 2c,d. The profiles show that Ez is the dominant component for the HP and HE11b modes. The energy of these modes is strongly confined to the gap due to the discontinuity of the normal components of the electric field at the interfaces.33 The lateral electric field component Ex is dominant for the HE11a, TE01 and HE21 modes, which store more electromagnetic energy in the NWs (S2 in Supporting Information). These three modes can be excited with polarization perpendicular to the NW axis. The identification of the different modes is further confirmed by adding perfect magnetic (electric) conductor conditions (S3 in Supporting Information). Since the geometrical localization of the modes is different and their properties are controlled by the waveguide dimensions, this device represents a new type of photonic component. We can choose which modes should participate in the SHG process by simply C

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launched only at one terminal, there is no interference pattern at the SH frequency and only a rapidly decaying pattern remains (S4 in Supporting Information). Full wave electromagnetic calculation using Gaussian beam excitation at one end of the NW is shown in Figure S5. It reproduces the orthogonal SHG radiation at the excitation terminal in the experiments and the fast attenuated beating fringe near the excitation end. The absence of the interference pattern near the far end of the NW agrees with the calculations using one HP mode input, since the output terminal of the NW was extended into the perfect matched layer to remove the reflections. To estimate of the efficiency of the measured SHG, we calibrated our optical setup in two steps. First, the SHG intensity ISH was integrated over an area by the spectrometer after subtracting the background from the same area size. The incident power of the excitation laser is measured under the objective using a power meter. The peak power PFW was calculated taking account of the pulse duration. Second, another laser at SH frequency was coupled to the optical setup and traveled along exactly the same collection optics (objective, filters, mirrors, lens, and spectrometer, etc.). This calibrating laser was recorded by the spectrometer with intensity I2, and its peak power P2 was obtained by a power meter under the objective, taking into account the reflectivity of the mirror. For more details about the calibration, see S6 in Supporting Information. Finally, the SHG power from the I hybrid waveguide was determined by PSH = ISH × P2 , and the

changing either the polarization of the incident light and/or the width of the NW. Room-temperature optical measurements were carried out using a home-built nonlinear optical microscope.23 A wavelength-tunable picosecond laser beam (NKT Photonics, EXB4) was focused on one end of a CdSe NW using a 100× objective (NA = 0.9, Olympus). The second harmonic signal was collected by the same objective in a backscattering configuration and collected either by a spectrometer or by an imaging CCD. Two bandpass filters (380−420 nm) were placed in the collection path, rejecting the excitation laser at the fundamental frequency. Figure 3a shows the SEM image from a thin NW with a length of 5 μm and a width of 150 nm. According to the mode analysis in Figure 2, there is only one guided mode (i.e., the HP mode). To launch guided modes of different parities, we used end-scattering excitation34 with incident polarization either parallel to or perpendicular to the axis of the NW. Figure 3b shows the optical image of the NW when its bottom end is excited by a laser (power 1 mW, under the objective) at 800 nm, polarized parallel to the NW axis. The optical image at the SHG frequency (Figure 3c) shows that the SHG photons are emitted along the entire NW. At the excitation terminal, SHG radiates orthogonally to the NW due to the local excitation by the laser spot, since the excited polarization charges at the SH frequency behave like a dipole along the waveguide axis. This is a typical behavior for the SHG emission from a waveguide geometry when the excitation laser is incident onto the intermediate position of the NW and polarized parallel to the waveguide axis.9,35 Away from the excitation terminal, a periodic SHG pattern is observed along the NW, in addition to collimated beams of SHG propagating away from the NW. Figure 3f shows the spectra of the SHG signal collected from the input and output ends (regions A and B in Figure 3c), and shows sharp peaks at 400 nm (half the wavelength of the fundamental at 800 nm). The power dependence of the SHG signal follows a quadratic response to the excitation power (The R2 of the fitting is 0.995 in the inset in Figure 3f), confirming that the observed signal is indeed from a second-order nonlinear optical process. The periodic pattern in the SHG image is due to the interference between the counter-propagating modes in the waveguide,23 specifically, a forward propagating mode and its reflection at the output terminal. To confirm this mechanism, we calculated the electromagnetic power density of the SHG at a plane 50 nm above the top of the NW, shown in Figure 3d. In order to avoid calculating the whole NW that is too large, we used mode analysis and selectively launched the specific guided mode(s) into the simulated system.36 This method simplified the analysis of mode interactions. For example, both terminals were allowed to launch the HP mode simultaneously to model the counter-propagating modes (for details, see S4 in Supporting Information). The simulated optical imaging at the SH frequency shows a similar periodic pattern along the NW as observed in our experiments. The period of the pattern is ∼400 nm, in good agreement with the experimental images (Figure 3e). The assignment of the periodic pattern in the SHG image to the interference between the counterpropagating waveguide modes differs from the mechanism proposed in previous studies on GaAs10 and ZnSe37 NWs. This is because we used a tightly focused laser spot (∼1 μm) which only covers a small region around one end of the CdSe NW, different from the excitation configuration used in their studies. Our calculations also confirm that if the HP mode is

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nonlinear conversion efficiency was ηSH = PSH/P2FW. This calibration scheme can reduce the uncertainty in calibrating individual optical element and, to a large extent, eliminate the effect from their imperfect alignments. The conversion efficiency of the hybrid plasmon waveguide shown in Figure 3c is estimated to be 2 × 10−6 W−1, using a pump laser of 1 mW. Control experiments with a spacer layer of different thickness can support the hypothesis that the enhancement of SHG is due to the hybridization of plasmonic and photonic components. We chose a CdSe NW with width fixed around 150 nm and varied the thickness of the spacer layer (t = 5 nm, 10 nm, 35 nm). The results show that SHG efficiency falls off drastically when the thickness is increased (Figure 3g and S7 in Supporting Information). Electromagnetic modeling using counter-propagating modes input also predicts a similar trend. More precisely, it shows that the SHG intensity drop significantly when t is increased from 5 to 15 nm. For t > 15 nm, the SHG efficiency is basically unchanged since the main contribution to the SHG is from the excitation terminal, as can be seen from the SHG image (panel c in Figure S7). Compared with photonic waveguides with similar structural parameters, the enhanced SHG from hybrid plasmonic waveguide is partly due to the stronger field enhancement in the CdSe NW. When the spacer thickness t is decreased, the photonic mode on the dielectric NW is hybridized with the SPP on the metal film due to the near-field coupling. Such hybrid mode inherits a plasmonic characteristic because its energy can be stored partly in the collective motions of the conduction electrons in the metal. The consequence of this hybridization is the mode can have a better spatial confinement and so a higher field enhancement if the same amount of power is launched into this mode. For an isolated dielectric NW (t is large enough), the optical modes will extend outside D

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Figure 4. SHG intensity images and Fourier plane images for different hybrid waveguides, the width of the CdSe NWs is 150 nm (a), 270 nm (b) and 360 nm (c). The wavelength of the excitation laser is 800 nm with an averaged power of 1 mW under the objective. The white arrow shows the polarization of the fundamental light. The collection regions of the Fourier imaging are the boxed regions in the SH images. The emission angles are θ1 = 12°, θ2 = 18°, θ3 = 30°. Inset in (a): schematic of the Fourier imaging system using an air objective for collection.

image. The SH Fourier image for a hybrid waveguide with a 150 nm wide CdSe NW is shown in Figure 4a for incident polarization parallel to the NW axis. The angular distribution shows a major component of the SHG emission at kz = 0, which means that the SHG emission is in the plane perpendicular to the NW axis. As expected, the zero zcomponent of the wavevector implies a cancellation of the axial momentum between two counter-propagating modes (the HP modes). The line across the origin of the Fourier plane indicates a span in the emission angle, which describes transversely divergent SHG emission from an emitter with 2D confinement.23 For the thicker NWs in Figure 4b, two emission side lobes at kz = ± 0.22k0 appear in the Fourier image, but with a smaller amplitude than the central emission feature. This nonzero axial momentum emission indicates that more than one mode takes part in the interference process. If the width of the NW further increases, more side lobes appear (Figure 4c). Note that the above measurements were performed for incident polarization parallel to the NW axis for thin NW (Figure 4a) but perpendicular to the NW axis for thick NWs (Figure 4b,c). The mode analysis shows that symmetric modes (HP, HE11b) are excited with parallel polarization, while asymmetric modes (HE11a, TE01, and HE21) are excited by perpendicular polarization. The mode symmetries are also reflected in the emission pattern of the SHG. Symmetric modes (Figure 4a) have emission maxima at

the waveguide due to the diffraction limit, when the diameter of the NW is smaller than the cutoff size. Overall, the hybrid plasmonic waveguide (CdSe NW/Al2O3/Au) could utilize both advantages of large nonlinear susceptibility and the subwavelength confinement, which explains why the SHG conversion efficiency is higher for smaller t. Additional calculations also show that as t increases, the in-coupling efficiency from a focus laser to the HP mode is increased and the reflectivity at the output end of the NW is decreased (S7 in Supporting Information). These results rule out other possible explanationssuch as the SHG enhancement is from the improvement of the in-coupling coefficient when the thickness is decreasedand indicate that the counter-propagating HP mode from the end reflectance also plays an important role in enhancing the SHG. To further explore the emission properties of the SHG, we used Fourier plane imaging to map the wavevector distribution for the different modes. Figure 4 shows the SHG emission angles for different hybrid waveguides of widths 150 nm, 270 and 360 nm. The inset in Figure 4a shows a schematic of the Fourier imaging system,38 where the radial coordinate ρ in the Fourier image scales with the emission angle θ as ρ = n sin θ (n = 1 is the refractive index of air), and φ denotes azimuthal angle (0 < φ < 2π). In this configuration, the emission angle θ is limited by the numerical aperture of the objective (NA = 0.9) to ±64°, which determines the outer circle in the Fourier E

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Figure 5. (a) Schematic drawing of the momentum conservation in a SHG by counter-propagating waves with the same modes (i) and different modes (ii). (b) The dependence of the radiation angles of SH photons on the excitation wavelength. The circle markers represent the experiment results from Fourier imaging and triangle markers represent the FEM simulated results. The width of the CdSe NW is 360 nm. (c) Fourier transform analysis of the real-space profile in Figure 4c. The arrows mark the Fourier peaks of the beating modes.

kx = 0, while asymmetric modes (Figure 4b,c) have nodes at kx = 0 and side-lobe emission at titled angles (S8 in Supporting Information). From the SHG images it is clear that periodic and oblique beams appeared from both sides of the NWs. The SH Fourier image from these beams (S9 in Supporting Information) show that the origin of these beams is also due to the interference of counter-propagating waves of different modes. Because of multiple mode interference and the large spatial overlap of the source, we obtain a nonlinear conversion efficiency of ηSH = 4 −4 × 10−5 W−1 (averaged power ratio PSH ̅ /PFW ̅ = 6.15 × 10 %) from the 360 nm wide NW using a pump laser with an averaged power of 1 mW (peak power PFW = 0.17 W). This large conversion efficiency demonstrates that our hybrid device is a promising platform for SHG with an efficiency that is substantially larger than most previously reported nanostructures (Table S1 in Supporting Information). There are two types of mode interference in the present device. Figure 5a shows a schematic of SHG photons generated by the same counter-propagating modes (i) and by different modes (ii). When the two guided modes with wave-vectors k1(ω) and k2(ω) are counter-propagating, the direction of the SHG emission is modulated, since the SHG process must satisfy both momentum and energy conservation. The relationship between the SHG emission angle θ in Figure 5a (S11 in Supporting Information) and wave-vectors are k(2ω)sin θ = k1(ω) − k 2(ω)

that there is good agreement between experiments and simulations. Specifically, the two counter-propagating HE+11a and HE−11a modes generate the normal emission at kz = 0 (θ = 0°), forming a central bright line in the Fourier image of Figure 4c. In the same figure, the interference between HE+11a and HE−21 or between HE−11a andHE+21 results in emission at θ3= ± 30°, close to the θ′3 = ±28.3° obtained from mode analysis and eq 1. The emission at θ2 = ±18° is due to the interference of counter-propagating TE01 and HE21, and the emission at θ1= ± 12° is due to interfering HE11a and TE01. A striking feature in the real-space SHG images is the periodic pattern which also includes information about the interfering modes. We performed a Fourier transform analysis on the real-space line profiles in the SHG image (white line in Figure 4c). The corresponding Fourier transform profile is given in Figure 5c. The spectrum shows three main fringe periods Λ3 = 0.76 μm, Λ2 = 1.47 μm, and Λ1 = 2.13 μm. As shown in Figure S11, the SH radiated intensity will vary along the waveguide (z-direction) as cos[(k1(ω)−k2(ω))z]. The spatial period is thus Λ = 2π /Re(k1(ω) − k 2(ω))

where k1(ω) and k2(ω) are the wavevectors of the different modes. The existing three modes for 360 nm wide NW will introduce three spatial periods: Λ′3 = 2π /Re(k HE11a − k HE21), Λ′2 = 2π /Re(k TE01 − k HE21), Λ′1 = 2π /Re(k HE11a − k TE01), which all agree well with the measured spatial periods Λ3, Λ2, Λ1. The above results clearly demonstrate that the interference between different electromagnetic modes plays an important role in the SHG. The nonlinear conversion efficiency can be increased even further by tweaking the design of the device. The SHG is strongly influenced by the interference of counter-propagating modes. This interference is reduced by reflection loss at the end of the NW but could be alleviated by the introduction of a 1D mirror.39 The Ohmic loss of the Au film is very large at the SH wavelength, resulting in short propagation distances of the guided SHG light, making it difficult to meet the phase matching criteria across the entire length of the NW. The introduction of a single-crystal Al plate40,41 instead of the Au film would increase the propagation length for both fundamental and SHG modes. Finally, the efficiency depends strongly on mode coupling, which depends sensitively on the NW diameter (Figure. 2). A more precise tuning of the NW could be accomplished by FIB milling of CdSe nanobelts, for example.

(1)

or alternatively, in terms of the effective mode indices n1 − n2 = 2 sin θ

(3)

(2)

Here, k(2ω) is the SHG wavevector, and n1, n2 are the effective indices corresponding to wavevectors k1(ω) and k2(ω). When the counter-propagating modes are the same, normal emission with θ = 0° is for condition (i) (S10 in Supporting Information). The tilted SH emission is related to condition (ii), that is, to interference between different modes. We recorded SHG Fourier images for 360 nm wide NW, varying the excitation wavelength (760−820 nm). The emission angles extracted as a function of excitation wavelength are plotted in Figure 5b. The emission angles increase linearly with wavelength, revealing angular steering of the nonlinear output. For the short wavelength range (760−820 nm), the effective index can be approximated to change linearly with wavelength. From eq 1, the SHG emission angle increases linearly as the wavelength increases. Figure 5b shows F

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In summary, we have developed a high efficiency second harmonic source in a hybrid waveguide consisting of a CdSe NW coupled to an ultrasmooth Au film. The energy of the incident light is effectively concentrated within the nonlinear material. Multiple electromagnetic modes and their interference contribute to a high conversion efficiency which reaches values up to 4 × 10−5 W−1. Through Fourier imaging and Fourier transform analysis of the SHG emission, we find that the interference between counter-propagating modes allows for directional steering of the SH emission. The SHG emission angle depends linearly on the wavelength of the incident fundamental light. Our findings present a tunable, directional and highly efficient on-chip SHG source which could become an essential component of integrated nonlinear data processing devices.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +8627 68752219. *E-mail: [email protected]. Phone: +8627 68752253. ORCID

Naomi J. Halas: 0000-0002-8461-8494 Peter Nordlander: 0000-0002-1633-2937 Shunping Zhang: 0000-0002-8491-0903 Hongxing Xu: 0000-0002-1718-8834 Author Contributions

J.J.S. and Y.L. prepared the samples and performed the experiments. J.J.S. and M.K. performed the calculations. J.J.S. and S.P.Z. analyzed the data. X.B.H. helped with the data analysis. S.P.Z., J.J.S., P.N., and N.H. wrote the manuscript. S.P.Z. and H.X.X. supervised the project. All authors discussed and commented on the manuscript.

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MATERIALS AND METHODS Sample Preparation. Our hybrid system consisted of a CdSe NW coupled with a flat Au film, with an Al2O3 dielectric layer as a spacer. The flat Au film was fabricated by the template stripping method. A 200 nm Au film was deposited onto a polished silicon wafer by thermal evaporation (JSD400) with a rate of 0.5 Å/s. Pieces of transparent glass substrates were gently glued onto the fresh Au surface with ultraviolet curing glue (Norland Optical Adhesive 61). After that, the glass pieces with the ultrasmooth Au surface were stripped off the silicon wafer. CdSe NWs (140−500 nm wide, and 4−30 μm long) were grown by CVD. Optical Measurements. For the SHG measurements, a wavelength tunable supercontinuum laser (NKT Photonics, EXB - 4) with a repetition rate of 78 MHz was coupled to our homemade second harmonic (SH) microscope. The incident polarization was selected by an analyzer and then controlled by a half-wave plate. A dichroic mirror (FF670 - SDi01, Semrock.) was used to reflect the fundamental light into a 100× objective (MPLFLN - BD, Olympus) and transmit the light at the SH frequency. The generated SH signal could be sent either to the spectral measurement path or to the SH Fourier imaging path by using a flipping mirror. In the SH Fourier imaging path, the SH signal was cleaned by bandpass filters (FF01-400/40-25, Semrock). Then a set of lenses was used to achieve a back focal plane image onto the CCD camera (Retiga 3000, Q imaging Inc.). By using a 3:7 (transmission: reflection) beam splitter in the spectral measurement path, the SH signal was simultaneously delivered to another CCD camera (EXi Blue, Q imaging Inc.) and to a spectrometer (iHR320, Horiba Jobin Yvon) placed after the bandpass filters (FF01-380-420, Semrock).

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program (Grant No. 2015CB932400), the National Natural Science Foundation of China (Grants Nos. 91850207, 11674255, and 11674256), the National Key R&D Program of China (Grant No. 2017YFA0303504, 2017YFA0205800), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB30201000).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b01004. Detailed description of simulations; calculations of SHG using port inputs and focused Gaussian beam; estimation of the SHG conversion efficiency; and calculations of SHG efficiency when varying the thickness of Al2O3 layer and the analysis of SHG process with the counter-propagating modes (PDF) G

DOI: 10.1021/acs.nanolett.9b01004 Nano Lett. XXXX, XXX, XXX−XXX

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