Letter pubs.acs.org/NanoLett
Multiresonant Broadband Optical Antennas As Efficient Tunable Nanosources of Second Harmonic Light Heykel Aouani,*,†,∥ Miguel Navarro-Cia,†,‡,∥ Mohsen Rahmani,¶,§,∥ Themistoklis P. H. Sidiropoulos,† Minghui Hong,§ Rupert F. Oulton,† and Stefan A. Maier† †
The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom ¶ Data Storage Institute, (A*STAR) Agency for Science, Technology and Research, 5 Engineering Drive 1, Singapore 117608 § Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576 ‡
S Supporting Information *
ABSTRACT: We report the experimental realization of efficient tunable nanosources of second harmonic light with individual multiresonant logperiodic optical antennas. By designing the nanoantenna with a bandwidth of several octaves, simultaneous enhancement of fundamental and harmonic fields is observed over a broad range of frequencies, leading to a high second harmonic conversion efficiency, together with an effective second order susceptibility within the range of values provided by widespread inorganic crystals. Moreover, the geometrical configuration of the nanoantenna makes the generated second harmonic signal independent from the polarization of the fundamental excitation. These results open new possibilities for the development of efficient integrated nonlinear nanodevices with high frequency tunability. KEYWORDS: Plasmonic, nanoantenna, broadband optical antenna, field enhancement, nonlinear optics, second harmonic generation
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light-emitting devices,12 and photovoltaics.13 Nonlinear interactions with optical antennas have been extensively studied for individual nanoparticles.14−16 core−shell nanocavities,17 metallic nanowires,18,19 plasmonic dimers,20,21 and planar nanostructures patterned using lithography techniques.22−24 While even order nonlinear processes are forbidden in bulk centrosymmetric materials,25 localized second harmonic generation from optical antennas is allowed at interfaces due to the symmetry breaking.26 To achieve a significant second harmonic response in the far field, the overall shape of the optical antenna must be noncentrosymmetric. A high second harmonic conversion efficiency was recently reported for metal-capped hemispherical nanoparticles with a second-order nonlinear susceptibility χ(2) within the range of values provided by widespread inorganic crystals.27 However, because of their dipolar properties, such optical antennas exhibit a narrow-band response, which severely limits their use over a wide operating bandwidth. Moreover, the intrinsic broadband nature of multiphoton processes could also benefit from a simultaneous enhancement of fundamental and higher-order scattered fields generated in the near field of multifrequency plasmonic systems in order to reach highfrequency conversion rates. Despite significant progress, developing robust optical nanodevices with a significant
onlinear optical processes are generated via coherent interactions of electromagnetic waves from induced dipoles and play a central role in many areas of science and technology.1−5 Since the first experimental demonstration of multiwave mixing introduced by Franken et al. in 1961,6 optical frequency conversion has taken a central role in laser-based science, as it provides the basis to extend the available wavelength range to the short-wavelength region of the visible spectrum. Efficient frequency conversion is commonly accomplished at the macroscopic scale in inorganic birefringent crystals, where the contributions from individual atoms or molecules in the crystal add up constructively, a process referred to as phase matching. 7 However, the typical dimensions of nonlinear crystals, many wavelengths in size, as well as the polarization dependence and the phase matching condition hinder the development of chip-scale tunable nonlinear optical materials for future integrated nanocomponents. Metallic nanodevices based on surface plasmon polaritons represent an interesting approach to bridge the gap between conventional optics and highly integrated nanophotonic components.8 In this context, special attention has been recently devoted to optical antennas, counterparts of radio and microwave antennas in the optical regime.9,10 By reversibly converting propagating electromagnetic waves into localized hot spots, optical antennas open new routes to manipulate and control incident and scattered light in nanoscale volumes with major applications in molecular sensing and spectroscopy,11 © 2012 American Chemical Society
Received: July 18, 2012 Revised: August 16, 2012 Published: August 23, 2012 4997
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Figure 1. (a) Schematic geometry of a three-arm trapezoidal silver optical antenna. (b) Extinction cross section (blue solid line) and intensity enhancement of the electric field (red solid line) with respect to the incident field (|E(0,0)|2/|E0|2) at the center of the antenna when illuminated by a x-polarized plane-wave. (c) Normalized local field components Ex(x,y) (top row) and Ey(x,y) (bottom row) at the middle cross section plane of the nanoantenna for all resonance peaks shown in panel (b); from left to right: 933, 1266, 1733, and 2480 nm. (d) Charge density distribution at the plane of the middle cross section for the same four resonances and in the same order as panel (c). (e) SEM image of a fabricated optical antenna with geometrical parameters identical to (a). (f) Measured extinction spectrum of an optical antenna array at normal incidence.
generation enhancement from a Kerr medium localized at the gap of the two elements of the antenna. Despite these theoretical results, fabrication and characterization of the broadband properties of this antenna are still lacking, as well as an experimental demonstration of efficient nonlinear lightmatter interactions in a broad range of frequencies. In this Letter, we report the experimental realization and observation of tunable second harmonic light with polarizationindependent three-arm trapezoidal silver nanoantennas. Ex-
bandwidth of operation remains an open challenge in nonlinear plasmonics. Double-resonant optical antennas based on two arms of different lengths have been recently proposed to enhance three-28 and four-wave mixing processes,29 thus introducing the concept of multiresonant optical antennas. Furthermore, a broadband response from a single optical antenna was theoretically demonstrated via a multifrequency log−periodic trapezoidal design,30 predicting efficient third harmonic 4998
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Figure 2. (a) Experimental setup for the nonlinear investigations. (b) Configuration of the excitation and second harmonic field emitted from the nanoantenna.
2400 nm and remains larger than 100 well down to 1230 nm (Figure 1b). At shorter wavelengths, a weaker almost 80-fold enhancement occurs near 933 nm, corresponding to the resonant response in the extinction cross section at around such wavelength. This intensity enhancement, although useful for many applications, is meaningless for second harmonic generation unless the local vectorial field of the fundamental and the harmonic signal overlap. To investigate this issue, the electric-field components Ex(x,y) and Ey(x,y) at the central part of the nanoantenna and at its middle cross section for the four peaks visible in panel 1b are plotted in Figure 1c. These color filled contour plots demonstrate that the local vectorial fields overlap significantly for 1266, 1733, and 2480 nm, but these have less correlation with 933 nm. Hence, we will see that this leads to a much smaller second harmonic conversion efficiency in the experimental spectral window from 940 to 1480 nm. Please also note that the local fields indicate that the maximum intensity enhancement does not happen at the center of the nanoantenna but at the vertices of the arms.30 Finally, the charge density distribution at the plane of the middle cross section of the nanoantenna is plotted in Figure 1d for all resonance peaks shown in Figure 1b. We notice in Figure 1d how different pairs of opposite neighboring teeth are active at each resonance in a dipolar fashion. As the wavelength shifts from longer to shorter values, the pair of teeth excited move from the outermost to the innermost ones. These pairs of localinduced dipoles oriented almost parallel to the incoming polarization intercept the incoming wave and the captured energy is guided to the vertices, creating an extremely confined multipolar configuration at the gap responsible for the intensity enhancement shown in Figure 1b. Trapezoidal silver nanoantennas were fabricated on a barium fluoride (BaF2) substrate by electron beam lithography (Elonix 100KV EBL system). A thin Ti film (3 nm thick) was deposited on the substrate by e-beam evaporation to ensure good adhesion between the evaporated 40 nm Ag layer and the BaF2 substrate. To define the nanoantenna patterns, a 50 nm hydrogen silsesquioxane (HSQ) was used as a negative electroresist. After baking the sample at 200 °C for 2 min, a combined process of e-beam exposure, chemical development, and ion milling was performed to obtain three-arm trapezoidal silver nanoantennas with geometrical parameters identical to those described in Figure 1a. The surface morphology of the optical antennas was characterized by high-resolution scanning electron microscopy (SEM) and atomic force microscopy
perimental results are supported by numerical simulations using the finite difference time domain method (FDTD), and the multiresonant performance of the optical antenna is qualitatively interpreted via a multidipolar scenario. We demonstrate that a broadband efficient conversion efficiency and second-order nonlinear susceptibility χ(2) in the range of a pm/ V can be achieved when the fundamental and harmonic fields are simultaneously resonant with the multifrequency nanoantenna, providing new opportunities for the development of integrated nonlinear optical devices with high frequency tunability. To begin with, let us introduce the design and the FDTD linear calculations (Lumerical 7.5) that guided us for the nonlinear experiment. The three-arm trapezoidal nanoantenna is displayed in Figure 1a together with the geometrical parameters: Rm+1/Rm = (0.49)1/2 with m = 1, 2...5, R1 = 1000 nm; inner and outer angle θi = 30° and θo = 60°, respectively; metal thickness t = 45 nm; separation between arm vertices g = 10 × √3 nm. These dimensions match those inferred from the high-resolution scanning electron microscopy (SEM) images of the fabricated samples, but neglect fabrication asymmetries and imperfections. In addition, the nanoantenna lies on a 3 nm thick titanium (Ti) adhesion layer and a semi-infinite barium fluoride (BaF2) substrate. The optical dielectric function of silver and Ti were modeled using a 3 and 6 Drude-Lorentzian term fit to Johnson and Christy, respectively.31,32 The BaF2 was assumed to be dispersionless with index of refraction n = 1.465.33 A highly nonuniform discretization mesh was used to treat accurately the different length scales involved in the solution of Maxwell’s equations. The default cubic grid was set to 6 nm × 4 nm × 5 nm, and fine details of the vertices (Ti adhesion layer) were mapped with a cubic grid of 1 nm × 1 nm × 5 nm (0.5 nm). The convergence of the calculations against the residual energy in the calculation volume, the mesh, and the performance of the perfect matched layers at the boundaries was checked following the same procedure as in ref 30. In the first set of simulations, we illuminated the nanoantenna from the semi-infinite air space with a x-polarized plane-wave propagating along z. In doing so, we find that the extinction cross section of the three-arm nanoantenna displays four resonance peaks within the bandwidth of interest, which overlap with each other preventing the extinction cross section from decreasing to zero (Figure 1b). Likewise, we find that the local electric field intensity enhancement at the center of the nanoantenna |E(0,0)|2/|E0|2 reaches a peak of 535 at around 4999
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(AFM). A representative SEM image of an individual nanoantenna is presented in Figure 1e. The extinction spectrum of the fabricated structures (defined as 1 - transmission) was measured by Fourier transform infrared spectroscopy (FTIR, Bruker Hyperion 2000) through a square array of nanoantennas (pitch of 10 μm) at normal incidence under linear polarization. The 10 μm pitch ensured that coupling between neighboring nanoantennas was insignificant, as confirmed by numerical simulations. The measured extinction spectrum of the broadband optical antenna is presented in Figure 1f. A slight red shift of the fundamental (2480 nm) and second (1733 nm) resonances can be observed compared to theoretical predictions, which can be attributed to minor defects related to the fabrication process. The resonances at shorter wavelengths (933 and 1266 nm) are not highlighted with our experimental configuration because of the relative low signal-to-noise ratio. However, the overall shape of the measured extinction cross section agrees well with the simulated spectrum of Figure 1b. The second harmonic response from individual optical antennas was generated by focusing a tunable pulsed Ti:Sapphire laser delivering 200 fs pulses at 80 MHz repetition rate (Chameleon Ultra II + Compact OPO, Coherent, Santa Clara, CA) with a 0.5NA reflective microscope objective (Figure 2a). This configuration provides a spot diameter of about 2.3 μm at λ = 940 nm, largely covering the whole surface of the nanoantenna (necessary to achieve the multiresonant effect). The optical antenna acts as a nonlinear element that converts the input signal at a frequency ω into an output signal at the second harmonic 2ω (Figure 2b). The backward-emitted signal at 2ω was collected via the same objective and filtered from the scattered laser light by a dichroic mirror. Second harmonic emission was then measured by a spectrometer (PI Acton Spec-10:100BR/LN, Princeton Instruments) with adequate filtering. At least six different broadband silver nanoantennas have been tested to ensure statistical validity, leading to variation in second harmonic signal of approximatively 10%, due to variations in antennas dimensions and reproducibility of the experimental measurements. Our first set of experimental results focuses on second harmonic conversion efficiency ηSHG, defined as the ratio of the second harmonic radiated power P(2ω) relative to the fundamental incident power P(ω), ηSHG = P(2ω)/P(ω). Experimental determination of ηSHG was performed for an incident fundamental wavelength ranging from 940 to 2330 nm, corresponding to a harmonic signal generated in the 470− 1165 nm spectral window. To ensure a direct comparison between the different wavelengths, the average power density was kept constant (on the order of 3.3 × 104 W/cm2, corresponding to peak intensity of 1.9 GW/cm2) during all multiwavelength measurements, and the average power was normalized by the area of the antenna, considering that the dimensions of the incident focused beam increases with wavelength. Let us also emphasize that the experimental data have been computed by taking into account the transmission/ reflection coefficients of the different optical elements and the quantum efficiency of the CCD detector. The evolution of second harmonic conversion efficiency versus excitation wavelength is presented in Figure 3a. As depicted, two regions can be distinguished. In the first region (940−1480 nm), low conversion efficiencies ηSHG ranging from 1 × 10−12 to 4 × 10−12 are determined, which is comparable to conversion efficiency values measured for gold nanowires.34
Figure 3. (a) Evolution of the second harmonic conversion efficiency ηSHG versus incident wavelength obtained with the nanoantenna (the error bars displayed indicate the standard deviations of our measurements). Region 1 corresponds to only excitation of the fundamental incident field, while in region 2, fundamental and second harmonic fields are simultaneously enhanced. (b) Evolution of the second order susceptibility χ(2) from 1880 to 2330 nm (the error bars displayed indicate the standard deviations of our measurements) and summary table of the determined experimental values.
These experimental results can be explained by the moderate enhancement of the fundamental field in the vicinity of the nanoantenna but are mostly a consequence of the low correlation between the local vectorial fields of the fundamental and the second harmonic (Figure 1c) and the fact that the frequency of the second harmonic electric field does not coincide with the resonances of the nanoantenna (Figure 1b), leading to no significant enhancement of the emitted harmonic signal. However, in the second region explored (1880−2330 nm), since the fundamental and second harmonic fields are resonant with the broadband optical antenna, the two enhanced fields overlapping in the vicinity of the antenna induce a strong second order susceptibility χ(2) , resulting in increased conversion efficiencies ηSHG between 0.91 × 10−9 and 1.25 × 10−9 (enhancement up to 3 orders of magnitude compared to the first region), within the range of highest values recently reported for other structures.27 Confirming our assumption based on which we designed such configuration, the multiresonances of the nanoantenna allow a broad tunability of the generated second harmonic light, over a spectral range of 450 nm, and possibly more, as a high-intensity enhancement is also expected beyond 2330 nm. From these results, we deduce that 5000
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Figure 4. (a) Normalized second harmonic signal generated from the three-arm trapezoidal nanoantenna for incident polarization angles ranging from 0 to 360° at incident wavelength λ = 1000 nm. (b) Numerical simulation of the electric field intensity distribution (|E(x,y)|2/|E0|2) at the cross sectional plane of the nanoantenna for different polarization angles (0, 15, 30, and 45°) at incident wavelength λ = 1183 nm. The sample is fixed and the incident plane-wave, initially x-polarized, is rotated.
rotated from 0 to 360° (Figure 4a) in steps of 10° at the excitation wavelength of 1000 nm. Interestingly, the second harmonic plasmonic response is not affected by the polarization of the incident field, as the global second harmonic signal remains almost constant for different angle measurements. We point out that the previous statement does not mean that the second harmonic signal is unpolarized, as the detected second harmonic emission from the nanoantenna was integrated for all possible polarization angles. To support these experimental data, numerical simulations of the electric field intensity enhancement distribution (|E(x,y)2|/|E0|2) have been performed. As a consequence of the geometrical configuration of the antenna, we limited our study to polarization angles ranging from 0 to 45° in Figure 4b. From these data, it is apparent that the intensity distribution of the fundamental resonance in the extinction cross section is almost invariant at the center of the nanoantenna for the different angles studied. A decrease of the electric field intensity in the vicinity of the excited bottom arm can be observed when the incident polarization is rotated from
in terms of efficiency the nonlinear response of the optical antenna is essentially determined by the linear spectral response, as previously reported.21,35 We next determine the evolution of the effective second order susceptibility χ(2) versus excitation wavelength (Figure 3b). Effective susceptibilities χ(2) greater that 1.0 pm/V can be achieved in the 1880−2330 nm region with a maximum value χ(2) = 1.9 pm/V at 2330 nm, which is the same order of magnitude as second order susceptibility provided by widespread nonlinear crystals.7 The major limitations of our antenna are currently related to the quick oxidation of silver, and to the challenging dimensions required to tune the fundamental resonance in the visible part of the electromagnetic spectrum. Lastly, we investigate the nonlinear polarization dependence of this class of optical antennas by using a structure with slightly different dimensions compared to the nanoantenna described in Figure 1a (see Supporting Information for more details on the dimensions of the nanoantenna). The second harmonic generation signal was measured for incident linear polarization 5001
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0 to 45°, which is compensated by an increase of the intensity distribution near the left arm. This observation seems to be corroborated by the polarization-independent characteristic of the harmonic signal. Therefore, the three-arm trapezoidal configuration seems to be a useful design to overcome limitations related to polarization dependence of nonlinear signal generated by optical antennas. In conclusion, we report the experimental realization of a nonlinear broadband optical antenna with a significant bandwidth of several octaves. By simultaneously enhancing both fundamental and second harmonic fields, this multiresonant nanoantenna provides high second harmonic conversion efficiencies and effective second order susceptibilities χ(2) within the range of values reported with inorganic crystals, over a bandwidth of 450 nm, and possibly more. Stronger nonlinear effects are expected for higher order processes (four-wave mixing, higher harmonic generation, and so forth), as the frequency of the different fields involved can be resonant due to the broadband response of the nanoantenna. Multiresonant optical antennas thus provide a promising means to develop future integrated nonlinear polarization-independent media with high-frequency tunability.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Additional information and figures are available. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Tyler Roschuk for his help during the preparation of the samples. This work was supported by the U.S. Army International Technology Centre Atlantic (USAITC-A), the Office of Naval Research (ONR and ONR Global), and the Engineering and Physical Sciences Research Council (EPSRC). M.N.-C. acknowledges support from the Leverhulme Trust.
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REFERENCES
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