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Electrically driven unidirectional optical nanoantennas Surya Prakash Gurunarayanan, Niels Verellen, Vyacheslav S Zharinov, Finub James Shirley, Victor V Moshchalkov, Marc
Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to
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Heyns, Joris Van de Vondel, Iuliana P Radu, and Pol Van Dorpe Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03312 • Publication Date (Web): 25 Oct 2017 Downloaded from http:// pubs.acs.org on October 26, 2017
Just Accepted
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“Just Accepted” manuscripts have been peer-r online prior to technical editing, formatting for p Society provides “Just Accepted” as a free dissemination of scientific material as soon as appear in full in PDF format accompanied by an fully peer reviewed, but should not be considere readers and citable by the Digital Object Identifi to authors. Therefore, the “Just Accepted” We in the journal. After a manuscript is technically Accepted” Web site and published as an ASAP changes to the manuscript text and/or graphic
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and ethical guidelines that apply to the journ or consequences arising from the use of inform
Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to
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Electrically driven unidirectional optical nanoantennas Surya Prakash Gurunarayanan,1, 2 Niels Verellen,2, 3 Vyacheslav S. Zharinov,3 Finub James Shirley,3, 2 Victor V. Moshchalkov,3 Marc Heyns,2, 1 Joris Van de Vondel,3 Iuliana P. Radu,2 and Pol Van Dorpe2, 3 1
KU Leuven, Department of Materials Engineering, B-3001 Leuven, Belgium 2 IMEC, Kapeldreef 75, B-3001 Leuven, Belgium 3 KU Leuven, INPAC-Institute for Nanoscale Physics and Chemistry, Department of Physics and Astronomy, Celestijnenlaan 200D, B-3001 Leuven, Belgium. ABSTRACT: Directional antennas revolutionized modern day telecommunication by enabling precise beaming of radio and microwave signals with minimal loss of energy. Similarly, directional optical nanoantennas are expected to pave the way towards on-chip wireless communication and information processing. Currently, on-chip integration of such antennas is hampered by their multi-element design or the requirement of complicated excitation schemes. Here, we experimentally demonstrate electrical driving of in-plane tunneling nanoantennas to achieve broadband unidirectional emission of light. Far-field interference, as a result of the spectral overlap between the dipolar emission of the tunnel junction and the fundamental quadrupole-like resonance of the nanoantenna, gives rise to a directional radiation pattern. By tuning this overlap using the applied voltage, we record directivities as high as 5 dB. In addition to electrical tunability, we also demonstrate passive tunability of the directivity using the antenna geometry. These fully configurable electrically driven nanoantennas provide a simple way to direct optical energy on-chip using an extremely small device footprint. KEYWORDS: Optical nanoantenna, directional emission, electrically driven, antenna-coupled tunnel junction, electromigration, surface plasmon resonance
anoantennas provide an efficient way to generate and manipulate light at the nanoscale. Metallic nanoantennas support collective oscillations of their free electrons known as localized surface plasmon resonances (LSPRs). LSPRs shape and enhance the local density of optical states, known as the Purcell effect, thereby controlling local emitters such as quantum dots, fluorophores or nitrogen vacancies in diamond1–5 . Nanoantennas also shape the omnidirectional radiation pattern of such emitters to result in functional patterns. Directional nanoantennas were first demonstrated using scaled versions of conventional antenna designs that multiple parasitic elements which are phase-tuned to constructively interfere in a certain spatial direction. Seminal examples include Yagi-Uda6–9 and log-periodic optical antennas10 . The phase-tuning technique is wavelength-specific and hampers applications requiring broadband operation. Symmetry breaking started gaining popularity as a technique to obtain directionality with a few or a single element antenna design. For instance, asymmetry in the material composition11 or asymmetry in the shape of the nanoantenna12–18 . The asymmetric antenna shape enables excitation of different multipolar plasmonic modes that interfere in the far-field to yield directional emission and scattering of light. Electrically driven nanoantennas bridge the gap between optical and electrical components at the nanoscale. Tunneling two-wire nanoantennas support pronounced plasmonic resonances and simultaneously allow transport of electrical energy directly to the feed point of the antenna. Upon electrical biasing, these antenna-coupled tunnel junctions act as sub-diffraction sources of light supporting broadband light emission ranging from IR to the visible regime19–21 . The light emission is shaped by these nanoantennas to result in a bidirectional dipole-like
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radiation pattern. Recently, unidirectional light emission was demonstrated by electrically driving asymmetric nanowires20 . The authors electromigrate a long nanowire to create a nanogap that is typically positioned off-center. The inherent asymmetry in the position of the nano-gap with respect to the electrodes results in a redirection of light emission. Though the nanowire system emits unidi-
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Voltage (V) Figure 2: Electrical activation and characterization. (a) Scanning electron micrograph of an in-plane tunneling V-antenna with dimensions, Larm = 90 nm, w = 50 nm, h = 50 nm and α = 90◦ . A nanogap is formed between the two arms of the V-antenna using controlled electromigration. Upon electrical biasing, electrons tunneling across the nanogap spontaneously emit light, hν. Inset, electroluminescence spot; V=2 V, acquisition=10 s. Scale bar: 60 µm. (b) a typical I-V plot taken after electromigration (blue circles). The red line shows a fit with Simmons single tunnel junction model25 using parameters: barrier height, 2.0 eV and gap size, 1.1 nm. Inset, Time evolution of the normalized conductance (G/G0 ) during the final stages of the electromigration procedure. As G becomes smaller than the conductance quantum G0 , the junction transport evolves from a ballistic to the tunneling regime and the EM process is stopped.
rectionally, the degree of directivity tunability and junction reproducibility is rather low. Unidirectional emission was also achieved by electrically driving nanoantennas using external schemes such as STM tips12,13,22 and tightly focused electron beams23 . In these works, breaking the system symmetry by positioning the localized excitation off-center results in a directional light emission and directional launching of plasmon polaritons. In this article, we demonstrate unidirectional emission of light by electrically driving in-plane tunneling two-wire nanoantennas in the shape of the letter V supported by a transparent substrate. A schematic of the antenna is shown in Figure 1. A polycrystalline gold V-antenna is connected with narrow gold leads (electrodes) at chargeminima points, which support electrical currents leaving the antenna resonance unperturbed24 . A nanoscopic tunneling gap is formed between the two arms of the Vantenna through a controlled electromigration (EM) procedure performed at room temperature under ambient conditions. Upon electrical biasing, tunneling electrons in the nanogap couple to the fundamental quadrupolelike antenna resonance resulting in a spontaneous, unidirectional emisison of light. We show that the directivity can be tuned using the geomtery of the antenna (passive) and the applied voltage (active). The electrically driven two-wire V-antennas feature ultra-small designs (∼λ3 /1000), yet achieve unidirectional light emission by exploiting electron-photon interactions at the nanoscale. A schematic of the electrical setup used for the EM
procedure is shown in Figure 2a. Details on electrical connection and activation are provided in Supporting Information Section 1. The inset of Figure 2b shows the time evolution of the normalized conductance during the final moments of EM. A nanoscopic gap is successfully formed when the normalized junction conductance falls below the conductance quantum, G0 = 2e2 /h. Here, e is the charge of a single electron and h is the Planck’s constant. The formation of the nanogap between the arms of the V-antenna is ensured by careful design and control of the fabrication procedure. A brief overview of the EM process is outlined in the methods section. Details on the nanogap’s position and its effect on the optical response are provided in Supporting Information Section 3 . The final abrupt opening of the nanogap from a barely connected junction (here, starting from 4 G0 ) is difficult to control in ambient conditions. As a result, the size of the nanogap varies from device to device. After the formation of a nanogap, the devices are electrically driven by applying a DC voltage across the two arms of the V-antenna. A typical I-V curve obtained after EM is shown in Figure 2b. The current first increases linearly with voltage and then super linearly for V > VT =2.0 V. As the applied voltage crosses the threshold voltage VT , the junction transport transitions from a tunneling to the field emission regime. A Simmons fit25 to the experimental I-V curve yields a barrier height of 2.0 eV and a gap size on the order of 1 nm. Ideal Auair-Au tunnel junctions are expected to show a barrier
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Figure 3: Optical response of electrically driven V-antennas. (a) Experimental full-spectrum back-focal plane image of a Larm = 90 nm V-antenna revealing a directional radiation pattern; V=2.0 V, I=∼10 nA, acquisition=60 s; The image is corrected for sample tilt and no polarization filter is used. (b) Simulated full-spectrum radiation pattern of the V-antenna. Black inner circle corresponds to the glass-air critical angle (NA=1, θ = 41.1◦ ) and outer dotted white line to the maximum acquisition angle of the objective (NA=1.49, θ = 78.9◦ ). Directivity (bottom right corners), is calculated by integrating pixel intensities in the two regions marked by the blue arcs; Polar angle span, ∆θ = 15◦ and Azimuthal angle span, ∆φ = 20◦ . (c) Experimental electroluminescence (EL) spectra of Larm = 90 nm and 145 nm V-antennas (open symbols); α = 90◦ . The spectra are corrected for the calibrated detection efficiency. Simulated Purcell enhancement spectra (dash lines) show the corresponding fundamental resonances of the antennas. (d) and (e) Simulated near-field intensity and charge density maps extracted at the resonance of the Larm = 90 nm antenna, 1.8 eV.
height of 5.1 eV26 . However, reduced barrier heights are commonly observed for tunnel junctions operating under ambient conditions27 . For sufficiently large voltages, V > 2 V , we observe an optical response from the device in the form of a diffraction-limited electroluminescence spot as shown in the inset of Figure 2a. The emitted photons are collected using a high numerical aperture objective (NA=1.49 and 60x magnification) and directed towards a sensitive detection setup (Supporting Information Figure S1b). To map the spatial distribution of the emitted photons, i.e., the radiation pattern, we image the conjugate Fourier plane of the objective (also called as back-focal plane, BFP). This is schematically illustrated in Figure 1 and detailed in Supporting Information Figure S1a. The experimentally obtained full-spectrum BFP image for a V-antenna is shown in Figure 3a. It features two lobes (left and right) diametrically opposite to each other and located just beyond the critical angle. Such a pattern is similar to that of a dipole emitter placed in close proximity to a glass substrate. Interestingly, most of the light is directed to the right (+x) and very little to the left (-x). This is indicative of a strong modulation from the nanoantenna which results in an asymetrical directional pattern. A directivity of +1.3 dB is calculated by taking a logarithmic ratio of the pixel intensities in two small regions around the point with maximum emitted power and the point diametrically opposite in the radiation pat-
tern. These two regions are indicated by the blue arcs in Figure 3a. Here, similar to previous works15,16,28 , directivity is defined as, R R (θ+∆θ,π+∆φ) ∗
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where, I(φ, θ) corresponds to the pixel intensities in the radiation pattern. We refer to this directivity D as F/B*. Note that, this definition deviates from the original definition of forward-to-backward figure of merit (F/B ) where only the point with maximum emission and the point diametrically opposite in the radiation pattern is considered6 . The F/B value can be readily obtained from the F/B* definition by setting ∆θ = ∆φ = 0. Both formulations mentioned above deviate from the definition of directivity used in standard antenna theory (Dstandard ) where, directivity is calculated relative to a hypothetical isotropic emitter29 . For more information on the different directivity formulations, see Supporting Information section 6. To understand the spectral distribution of the emitted photons, we direct the photons towards a spectrometer setup (Supporting Information Figure S1c). Figure 3c shows the experimental electroluminescence (EL) spectra for two V-antennas with different arm lengths. The spectrum of the shorter antenna (Larm = 90 nm) features a broad resonance with a single prominent peak
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near 1.9 eV. As expected, the resonance peak of the longer antenna is significantly red-shifted compared to the 90 nm antenna. The applied voltage determines the energy of the tunneling electrons and acts as a cut-off that limits the maximum energy of the emitted photons such that, hνmax ≤ eVapplied . By correcting the EL spectrum with the full transfer function of the detection setup (including the objective, optical elements and the spectrometer), the external quantum efficiency is estimated to be between 10−5 and 10−6 . Such low values are expected for tunnel junctions featuring nanoscopic gaps19 . To explain the experimental results and to elucidate the directional behavior, we performed numerical FDTD simulations (Lumerical Solutions, Inc., Vancouver, Canada) of gold V-antennas with connected leads. The V-antenna is driven by a y-polarized point dipole source placed in the nanogap between the two arms of the antenna; For details on simulation methodology, see Methods section. Figure 3b shows the simulated far-field radiation pattern of the antenna. An excellent match is obtained with the experimental pattern shown in panel a. The directional behavior is well predicted by the simulation and a F/B* value of +1.2 dB is calculated. The simulated Purcell enhancement spectra (Figure 3c, dashed lines) show that both antennas feature prominent resonance peaks that align well with the corresponding experimental EL peaks. Simulated near-field distribution extracted at resonance of the 90 nm V-antenna (Figure 3d) shows that, the fields are highly localized mainly at the gap and at the edges of the arms. A maximum near-field intensity enhancement of 600 is achieved at the center of the nanogap. The charge density map extracted at resonance is shown in panel e. By breaking the rotational symmetry of the two-wire geometry to form the V-antenna, an asymmetrical quadrupole-like mode is obtained. Such an asymmetrical quadrupole mode results in a directional radiation pattern15 . To prevent undesired energy leakage and damping of the antenna resonance, the leads are connected at right angles to the antenna arms at the charge minima points24 as shown in panel e. To exclude the leads from contributing to the directional behavior, we performed additional simulations of an unconnected V-antenna (without leads) excited by the point dipole source. From Supporting Information Figure S7 we see that the unconnected antenna also produces a directional radiation pattern similar to its connected counterpart. These results confirm that the leads play a negligible role and the dominant contribution to the directional behavior comes from the quadrupole-like antenna mode. We compare our electrically driven V-antennas with fluorophore-labelled V-antennas reported by Vercruysse et al.16 . They report similar directional behavior for the antennas excited by local emitters. As is the case for our electrically driven V-antennas, the observed directivity is to the right (+x). The weak directivity of the quadrupole-like mode is enhanced through far-field interference with the uncoupled dipolar emission of the local
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Figure 4: Electrical and geometrical tuning of the directivity. (a) Experimental full-spectrum back-focal plane images depicting the evolution of the directivity with voltage for two V-antenna geometries: Larm = 90 nm and 145 nm. w = 50 nm and α = 90◦ . Applied voltage and directivity values are mentioned for each image. No polarization filter is used. (b) Evolution of the directivity with the opening angle of the V-antenna. Experimental back-focal plane images and the corresponding simulated patterns are shown for three different angles: 90◦ , 120◦ and 180◦ . Here, all antennas have Larm = 145 nm, w = 50 nm and operated at V=∼5 V. Acquisition time=60 s. No polarization filter is used.
emitter. A well tuned antenna geometry that ensures a good spectral overlap between the emitter fluorescence and the antenna mode is necessary for a strong enhancement in directivity. Here, by forming the nanogap precisely at the feed point of the antenna (between the arms of the V), we create an antenna-coupled tunnel junction.
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The coupled system allows us to efficiently modulate the light emission from the tunnel junction through the plasmon mode of the V-antenna. Furthermore, the electroluminescence spectrum can be conveniently tuned using the applied voltage in order to obtain a strong spectral overlap with the antenna resonance. For more information on the effect of voltage on the electroluminescence spectrum, see Supporting Information section 5. The electrical tunability of the directivity is illustrated in Figure 4a. For the shorter Larm = 90 nm V-antennas, as the applied voltage is increased from 2.0 V to 3.0 V, the experimental directivity (F/B* ) almost tripled from 1.3 to 3.4 dB. As the voltage increases, the tunneling electrons carry more energy to populate the plasmon modes better and consequently, an increase in the directivity. However, we expect the directivity values to saturate for voltages much larger than 2.4 V. This is because, the interband transitions in gold start playing a crucial role and subsequently quench excess energy that could be injected into plasmon modes20 . The reduced directivity for these larger voltages (3.5 V) can thus be attributed to the isotropic background increase due to the auto-luminescence of gold6 . In contrast, for the longer Larm = 145 nm V-antennas, a high directivity (F/B* ) of 5 dB is obtained for a much smaller voltage, 1.4 V. This is because, longer antennas feature resonances that are spectrally red-shifted (Figure 3c, red circles) and relatively smaller voltages are sufficient to resonantly excite the antennas. Antennas operated close to resonance result in a high directivity: Larm = 90 nm at 3.0 V and Larm = 145 nm at 1.4 V. When operated off-resonance, the antennas result in low directivity due to weak far-field interference (at lower voltages) and isotropic background (at higher voltages). To enable meaningful comparison with other works, the directivity of the Larm = 145 nm V-antenna at 1.4 V is also calculated using the standard definition (Dstandard =∼18 dB) and using the forwardto-backward figure of merit (F/B=∼9 dB). For details on directivity calculation, see Supporting Information Section 6. Therefore, for a fixed geometry of the V-antenna, the applied voltage serves as a convenient tool to actively tune the directivity of the device. To further illustrate the tunability of the directivity, we fabricated Larm = 145 nm V-antennas with varying opening angles. Figure 4b shows the experimental radiation patterns along with the corresponding simulated patterns. As the angle increases from 90◦ to 120◦ , the obtained directivity (F/B* ) drastically reduced from 3.7 dB to 1.2 dB. A similar trend is observed for the simulated patterns. As expected, when the angle is further increased to 180◦ (axially symmetric two-wire antenna), the unidirectional behavior is completely lost and a bidirectional radiation pattern featuring two identical lobes is obtained. This resembles the radiation pattern of a dipole emitter in close proximity to a glass substrate. In conclusion, we experimentally demonstrated electrical driving of in-plane tunneling V-antennas to achieve broadband unidirectional emission of light. By precisely
forming a nanogap between the two arms of the Vantenna, an antenna-coupled tunnel junction is created. The V-antenna features a fundamental quadrupole-like resonance as a result of the broken rotational symmetry. Far-field interference between the dipolar emission of the tunnel junction and the antenna resonance results in a directional radiation pattern. The directivity can be electrically tuned using the applied voltage and passively tuned using the geometry of the V-antenna. Firstly, the arm length of the V-antenna predominantly determines the spectrum of the emitted photons, and the applied voltage serves as a convenient tool to tune the spectrum. Antennas operated close to resonance result in a high directivity. Directivity was found to be lower for off-resonance operation due to weak far-field interference (at low voltages) and isotropic background (at high voltages). Shorter antennas are resonantly excited at larger voltages and produce a high directivity. On the other hand, due to the spectrally red-shifted resonances, high directivity values are achieved for longer antennas for relatively smaller applied voltages. For a V-antenna with an arm length of 145 nm operated at 1.4 V (Figure 4a), a high directivity of 5 dB (Dstandard =∼18 dB, F/B=∼9 dB) was obtained. Secondly, the opening angle of the V-antenna plays a significant role in determining the directivity. V-antennas with an angle of 90◦ feature an unidirectional radiation pattern, while the antennas with 180◦ feature a dipole-like bi-directional radiation pattern. Resonant antennas also affect the polarization state of the emitted light. Polarization studies of the reported antennas (Supporting Information Sections 8 and 9) reveal that the emitted light is predominantly polarized along y-direction (the tunneling axis of the antenna). Numerical FDTD simulation results show excellent agreement with the experimental results and elucidate the directional behavior of the V-antennas. For practical applications, it is important to improve the stability and quantum efficiency of the devices. This can be achieved for instance, by using monocrystalline materials30 in combination with integrated scatterers/molecules in the tunnel junction31 . Stable nanogaps can be achieved through alternative fabrication procedures such as Focused Ion Beam (FIB)17 or AFM manipulation19 . Optimized junctions are expected to show quantum efficiency as high as 10% along with superior stability32,33 . Unidirectional emission in the IR regime should be possible by driving longer V-antennas at relatively smaller voltages (
