Nanoplasmonic antennas are well established for focusing light from the far field into subwavelength-sized dimensions, while simultaneously increasing strongly the local electromagnetic field—an important ingredient for boosting nonlinear optical effects. Here, we study both the optical and structural properties of gold bowtie nanoantennas under illumination with femtosecond laser pulses and observe a pronounced metamorphism of the antennas, while the upconverted incoherent nonlinear emission increases simultaneously. Scanning electron microscopy images recorded before and after illumination show a clear modification of the antenna feedgap, accompanied by an increase of the nonlinear signal. This is caused by laser-induced electromigration of gold nanoparticles, a process that is feedgap-size-dependent, self-limiting, and irreversible. Moreover, it is identified as the root cause for the strong enhancement of the nonlinear conversion efficiency by a factor of as compared with an unpatterned gold film. By experimentally quantifying the electric field enhancement inside the nanoantenna feedgap to be , we demonstrate consistency with the observed enhancement of the nonlinear signal. Complete switching off of the nonlinear response of such metamorphic nanoantennas with a degree of polarization indicates their potential for novel, nonlinear all-optical devices. Furthermore, we envision the controlled, laser-induced modification of plasmonic nanoantennas may provide a promising route to realize antennas with even higher field enhancements and, moreover, might enable deterministic quantum plasmonic experiments that require sub-nanometer-sized feedgaps.
© 2016 Optical Society of America
Plasmonic nanostructures  have gained much interest due to their nano-focusing capability , accompanied by local electric field enhancements , which significantly enhance nonlinear optical effects . Although nonlinear light conversion can already be achieved using bulk materials, plasmonic nanoparticles facilitate new regimes of light–matter interaction [5–10] due to downscaling to sub-10-nm dimensions, while the strong field localization relaxes phase-matching requirements . Furthermore, the ultrafast response times of plasmonic antennas  make them highly attractive for novel nano-optical devices, such as all-optical switches [4,13,14] or frequency converters . Numerous nonlinear effects, such as second- [8,9,16,17], third- [18,19], and higher-  harmonic generation, multi-photon luminescence [6,8,21–24], and frequency mixing  have been demonstrated using various plasmonic nanostructures. Although nanoparticle dimers provide stronger maximum electromagnetic energy confinement compared to single particles, the nonlinear light conversion efficiencies remain relatively weak due to the relatively low electric field penetration into the metals .
In this paper, we investigate the enhanced nonlinear light upconversion of lithographically defined, metamorphic gold (Au) bowtie nanoantennas in both experiment and simulation. We observe a self-optimization behavior when exciting the antennas with 100 fs near-infrared optical pulses, resulting in a progressive enhancement of the nonlinear two-photon photoluminescence (TPPL) generation [6,8,21–23,27] by factors of up to . We attribute this enhancement to a combination of laser-induced electromigration  of Au nanoparticles into the feedgap and nanoparticle re-shaping [29–31], and support this conclusion using both experiment and numerical simulations. We demonstrate that this self-optimization process is self-limiting, permanent, and strongly feedgap-size dependent. Moreover, we measure an enhancement of the nonlinear conversion efficiency by as compared to a planar Au film, corresponding to an electromagnetic intensity enhancement inside the antenna feedgap of . This effect can be completely switched off via control of the excitation polarization. Besides the strong enhancement of the nonlinear signal, the controlled optical metamorphosis of the nanoantennas is envisioned as a novel approach toward deterministic quantum plasmonic  studies in lithographically defined nanoplasmonic systems.
2. ENHANCED NONLINEAR RESPONSE AND METAMORPHOSIS
Single bowtie nanoantennas were realized on a glass substrate using electron beam lithography and thermal evaporation of a 35 nm Au film on top of a 5 nm titanium (Ti) adhesion layer. For further details on the nano-fabrication of bowtie nanoantennas, we refer to Section 1 of Supplement 1 and Ref. . After fabrication, we determined the antenna size and feedgap size using high-resolution scanning electron microscopy (SEM). A representative SEM image of an individual antenna is shown in the inset of Fig. 1(a). Maximum nonlinear light generation efficiency requires spectral matching between the localized surface plasmon polariton (LSPP) resonance energy and the fundamental pump laser energy . Therefore, we experimentally determined of individual bowties with and by measuring their differential reflectivity spectra using a broadband super-continuum white-light laser, where and denote the reflectivity signal recorded spatially on and off the bowtie, respectively . Typical spectra for three different bowtie nanoantennas with (black symbols), (red symbols), and (blue symbols) are presented in Fig. 1(a). We observe a clear resonance behavior, attributed to the dipolar mode of the bowtie [34–36], and a redshift from to with decreasing from 30 to 6 nm [34,36]. The solid lines show corresponding numerical finite-difference time-domain (FDTD) simulations  of the scattering cross sections, yielding good agreement with the experimental data.
To probe the nonlinear optical properties of the bowtie nanoantennas, we mounted the sample into a confocal luminescence microscope in vacuum (), as depicted in Fig. 1(b), and resonantly excited () individual antennas with linearly polarized, ultrashort () laser pulses generated from a Ti:sapphire laser (repetition rate 82 MHz) at room temperature. For direct comparability and to achieve maximum field enhancement, we studied only resonant bowtie nanoantennas with [blue shaded region in Fig. 1(a)]. This corresponds to antennas with and . For further information on the experimental setup, we refer to Section 2 of Supplement 1.
In a first experiment, we spectrally integrated the nonlinear optical signal, generated by the bowtie nanoantenna, for energies , where denotes the cut-off energy of the employed short-pass filter, as indicated by the green hashed region in Fig. 1(a). During the first ramp-up of the laser peak power density (continuous wave equivalent) to , we observe a quadratically increasing nonlinear optical intensity, indicated by the lower dashed line in Fig. 1(b). This quadratic dependence clearly indicates that we probe second-order nonlinear optical effects, such as incoherent TPPL and coherent second-harmonic generation (SHG). Surprisingly, the detected signal increases in a super-quadratic manner when exceeding this laser power threshold (i.e., ) up to the maximum applied . Multiple additional repetitions of this power-dependent experiment [labeled 2 and 3 in Fig. 1(b)] clearly show that the initial quadratic behavior is established over the entire power range, but at a signal level that has increased by a factor of . Moreover, we observe that those additional power sweeps preserve the generated nonlinear signal to much lower peak power densities. In total, we investigated 35 bowtie nanoantennas, each of which exhibited the unexpected super-quadratic increase during the first power ramp-up, while showing a stable quadratic trend accompanied with an increased signal level for all additional power sweeps.
To shed light on the origin of the enhanced nonlinear response, we compare the morphology of the same bowtie nanoantennas before (denoted as a “pristine” antenna) and after (denoted as a “fs-illuminated” antenna) illumination with the femtosecond (fs)-pulsed laser excitation, by recording high-resolution SEM images. We observe a pronounced structural modification of the feedgap region of the fs-illuminated antennas, as shown in Fig. 1(c). This we attribute to accumulation of Au (nanoparticles) in the antenna hotspot due to electromigration , nanoparticle re-shaping [29–31], and optical tweezing effects  triggered by the ultra-short () and high-intensity () laser illumination. Since we conducted all experiments in vacuum and due to the much higher melting point of Ti () as compared to Au (), we assume trapping of other materials, such as dust particles from the air or Ti from the adhesion layer, as unlikely. However, we note that small amounts of residuals of electron beam resist and the organic solvents used might be found on the sample after the fabrication process, although the samples have been carefully cleaned prior to measurements. We observe that the spectrally dispersed nonlinear response of the fs-illuminated antennas (cf. Fig. 3) is in good agreement with the form of the TPPL generated from Au [8,21–23]. Since we observe the material accumulating along the electric field distribution in the fs-illuminated antennas for larger feedgaps (cf. Fig. S2), we conclude that the observed structural modification of the antenna hotspots stems from the presence of the LSPP.
3. MATERIAL ACCUMULATION IN THE FEEDGAP: SIMULATION VERSUS EXPERIMENT
To test our hypothesis of Au accumulation in the antenna feedgap, we performed detailed numerical simulations of the plasmonic response for both pristine and fs-illuminated nanoantennas and compared our theoretical findings to differential reflectivity measurements . For reference, we plot the simulated scattering cross section of a pristine bowtie nanoantenna (, ) in blue in Fig. 2(a). We observe a peak-like response, which is attributed to the dipolar resonance of the antenna with a resonance energy , and find excellent agreement with the corresponding differential reflectivity measurements shown as the blue curve in Fig. 2(b). Turning our attention to fs-illuminated antennas, we notice that the detailed distribution of the material within the feedgap of the antenna cannot be resolved due to the limited spatial resolution of the SEM along with charging effects on the insulating substrate. Therefore, we discuss in the following two possible scenarios. In case (I), we assume a homogeneous Au film of thickness 8 nm, continuously connecting both triangles of the bowtie nanoantenna, as indicated in inset (i) of Fig. 2(a), while in case (II) we model the gap of fs-illuminated antennas with several Au nanoparticles [inset (ii) in Fig. 2(a)]. For case (I), the scattering cross section exhibits two energetically separated modes at and , as shown in green in Fig. 2(a). Deeper insights into the nature of both modes is gained from the corresponding charge carrier distribution, plotted in insets (iii) and (iv) of Fig. 2(a) for and , respectively. For the lower energy mode , we find that the negative and positive charges accumulate in opposing nanotriangles. Therefore, we interpret this mode as the dipolar resonance of the formed super-particle, since electrons can now freely move between the nanotriangles due to the connecting Au film. In strong contrast, the charge carrier distribution for the high-energy mode at indicates the formation of a so-called charge transfer plasmon , as recently observed in electron energy loss spectroscopy studies [40,41]. Additional simulations of varied Au-film thickness showed only a minor influence on both the resonance energy and intensity of the observed modes.
For case (II), we filled the feedgap with several () cylindrically shaped Au particles with diameter and height , taking special care that the particles do not short circuit the two nanotriangles forming the bowtie, as shown schematically in inset (ii) of Fig. 2(a). The corresponding scattering cross section [red curve in Fig. 2(a)] exhibits an energetic redshift of as compared to the pristine antenna, which we attribute to an effective decrease of the feedgap size and, thus, an increased near-field coupling between the two triangles . We compare this result to the experimentally determined plasmonic response of the fs-illuminated antenna shown in red in Fig. 2(b). We also observe a redshift of the resonance of as compared to the pristine antenna (blue curve), in excellent agreement with the corresponding simulations. These findings strongly suggest that the modified region of the fs-illuminated antennas does not consist of a homogeneous Au film, but rather consists of several spatially separated Au particles. Further evidence is given by the optical study of intentionally connected bowties, as depicted in inset (iii) of Fig. 2(b), where the experimentally determined plasmonic response yields a resonance at . This is in good agreement with the simulation of a bowtie nanoantenna, where the gap is connected via a homogeneous Au film [green curve in Fig. 2(a)]. We support this finding by studying multiple intentionally connected and fs-illuminated nanoantennas and present a histogram of the corresponding energy shift in Fig. 2(c). Here, and denote the resonance energy of the either intentionally connected or fs-illuminated antennas, and the corresponding pristine antennas, respectively. We observe a clearly distinct behavior for both types of antenna; while the fs-illuminated antennas (red) exhibit an average redshift of , the intentionally connected antennas (green) show a distribution that is blueshifted by . This statistical analysis of intentionally connected and fs-illuminated antennas, in combination with our numerical simulations, strongly suggest that the accumulated material in the feedgap does not short circuit the triangles of the fs-illuminated nanoantennas. Our findings rather indicate the formation of ultra-small feedgap antennas due to laser excitation, which renders fs-illuminated bowtie nanoantennas promising candidates for future applications in quantum plasmonics [43,44]. In addition to the resonance position, we also analyzed the full width at half-maximum of after fs-illumination; however, we do not find a clear change, which would indicate, for example, annealing of the antennas . Moreover, we note that the absolute intensity of increases slightly after fs-illumination for the same antennas. This might be due to small changes in the optical setup, e.g., improved collection efficiency or better focusing of the white-light laser and, thus, we refrain from correlating this intensity increase with the enhanced nonlinear signal in this study. We further note that the observation of the corresponding dipolar mode of the intentionally connected nanoantennas  was hindered by the limited quantum efficiency of the Si-CCD camera used.
4. PROPERTIES OF METAMORPHIC NANOANTENNAS: REPRODUCIBILITY, STABILITY, AND FIELD ENHANCEMENT
In the following, we consider the formation process and reproducibility of the fs-illuminated nanoantennas by studying their nonlinear optical response in a time-resolved experiment. In Fig. 3(a), we present the temporal evolution of the spectrally integrated nonlinear signal for five nominally identical bowties with . To adiabatically change the applied excitation conditions, we exponentially increase the peak power density from zero to a maximum value of , while simultaneously recording the generated from the antennas. During the power ramp-up phase (), we observe a rather complex increase of with time, changing from a quadratic (, indicated by the gray dashed line) to an unexpected super-quadratic () behavior. After reaching at , the nonlinear signal saturates temporally delayed by at a saturation intensity and stays constant afterward (). The sudden change in at from a quadratic to a clear super-quadratic dependence is attributed to the onset of the accumulation of Au particles inside the feedgap of the antennas. Assuming a strict quadratic signal increase up to at , as expected for a second-order nonlinear process such as TPPL, we would expect to reach a maximum signal of cps [horizontal gray dashed line in Fig. 3(a)]. In our experiment, the laser-induced modification of the antenna feedgap results in cps and, thus, an enhancement of the by . Moreover, upon switching off and on the excitation laser at and , a full recovery of is observed, suggesting stable operation for after the initial fs-illumination. Due to the redshift of after fs-illumination, as shown in Fig. 2(b), the fundamental laser at is slightly off-resonance with respect to the mode and, thus, the observed enhancement of is only a lower bound, and fully resonant excitation could potentially lead to even higher enhancement factors.
After the formation of the fs-illuminated antennas, we studied the spectral dependence of their nonlinear response as a function of increasing excitation power densities , as shown in Fig. 3(b). The spectra exhibit the typical shape expected for TPPL, and scales quadratically with over the whole emission energy range studied (cf. Fig. S4 in Supplement 1), in contrast to other reports found in the literature . We note that these results clearly show that it is physically sound to study the antennas by spectrally integrating their nonlinear response, without any loss of information.
Next, we experimentally quantify the electromagnetic intensity enhancement () of a fs-illuminated antenna by studying as a function of and compare it to a planar Au film, as shown in blue and red in Fig. 3(c). For both the fs-illuminated antenna and planar Au film we observe a quadratic increase of the spectrally integrated with , according to , with for the bowtie nanoantenna and for a planar Au film. This observation supports our identification of the signal as arising from TPPL. We note that the filling factor of the bowtie nanoantenna has been taken into account and refer the reader to Supplement 1 for further details. We observe that the fs-illuminated nanoantenna yields comparable nonlinear emission as the planar Au film, but at a reduced . In other words, the fs-illuminated antenna exhibits a increase of the nonlinear light intensity with respect to the planar Au film at a comparable excitation level. This strong enhancement is attributed to the concentration of the electromagnetic field inside the modified antenna feedgap . Our experimental findings enabled us to estimate a lower limit for the nonlinear optical light conversion efficiency of at (see Supplement 1), clearly demonstrating the superior frequency conversion efficiency of the studied self-optimized fs-illuminated nanoantennas and complementing the observation of an nonlinear intensity enhancement.
We gain additional information about the precise spatial position at which the nonlinear optical signal is generated by investigating the spectrally integrated as a function of the linear excitation polarization angle . In Fig. 3(d), we present a polar plot of as a function of . We observe an enhanced signal for excitation along the principle axes of the antenna (i.e., ). In strong contrast, completely vanishes for the excitation polarization aligned perpendicular to the antenna (i.e., ), corresponding to a degree of linear polarization , limited only by the detector noise. This high confirms that the nonlinear signal stems from the antenna feedgap for the near-field coupled mode  and, thus, is strongly linked to the structurally modified region of the fs-illuminated antenna.
5. ORIGIN OF THE ENHANCED NONLINEAR RESPONSE
After demonstrating enhanced nonlinear light generation from the structurally modified feedgap of fs-illuminated antennas, we continue to interpret its origin by studying the nonlinear signal as a function of . We present in Fig. 4(a) the saturation intensities for several fs-illuminated antennas as a function of on a double-logarithmic scale. We observe a monotonic increase of with decreasing , as indicated by the blue dashed line, which is approximated by , where denotes a constant. We attribute this dependence to a combination of electric dipole coupling between the two adjacent nano-triangles  and the quadratic behavior of the studied second-order nonlinear process. Figure 4(b) shows a simulation of the intensity enhancement for a pristine bowtie nanoantenna (, ), where and denote the electric field of the scattered and incoming light, respectively, reaching in the feedgap. In contrast, the intensity inside the Au nanotriangles, i.e., the area where TPPL is generated, is only of the order of .
Placing a cylindrically shaped Au particle into the center of the feedgap yields enhanced electric fields inside this particle  with , as shown in Fig. 4(c). Moreover, the presence of the Au nanoparticle in the feedgap pushes the electric field deeper inside the adjacent tips of the nanotriangles composing the bowtie antenna, which is clearly shown in Fig. 4(d) as the ratio for a fs-illuminated bowtie nanoantenna to for a pristine antenna. This is a direct measure for the additional intensity enhancement of the antennas after fs-illumination. The additional field inside the Au nanoparticle and the increased field inside the nanotriangle tips are interpreted to give rise to the observed enhancement of the nonlinear optical signal. To distinguish between the two effects and compare them with our experiments, we calculate the integral over the squared additional intensity enhancement , for the volume of the Au particle and the nanotriangle tips . The results are plotted as red symbols in Fig. 4(a), with circles and squares, respectively, for the Au nanoparticle and the nanotriangle tips. For both mechanisms, we obtain a monotonic increase with decreasing , which follows a dependence, in good agreement with the experimental data. The finding that is lower as compared to strongly suggests that the dominating contribution leading to the enhanced nonlinear signal of fs-illuminated antennas is due to the additional electric field inside the tips of the nanotriangles forming the bowtie nanoantenna (additional numerical studies of the particle shape and number dependence are shown in Fig. S8 of Supplement 1). We conclude that our simple theoretical considerations provide a good qualitative understanding of the increased intensity enhancement and, thus, the enhanced nonlinear optical intensity of the fs-illuminated antennas, although the coarse mesh size (1 nm) and the non-consideration of quantum mechanical effects does not allow quantitative predictions.
6. DISCUSSION AND CONCLUSIONS
Further insights on the type, shape, and size of the accumulated material might be gained by probing their vibrational dynamics by applying ultrafast transient pump–probe spectroscopy  or by realizing similar metamorphic antennas on few 10 nm thin SiN membranes for electron energy-loss spectroscopy (EELS) studies. Furthermore, a controlled transition from ultra-small feedgaps to kissing particles for quantum plasmonic applications  might become accessible by monitoring the dc conductivity via local electrical gates [49,50] while engineering the antenna morphology via fs-illumination. Last but not least, integration of metamorphic nanoantennas on substrates with high nonlinearities, such as GaAs () , () , or even monolayer transition metal–dichalgonenides () , might significantly boost second-order nonlinear processes like SHG, in analogy to the strongly enhanced third-harmonic upconversion in indium–tin-oxide (ITO)-loaded dimer antennas [46,18].
In summary, we observed a 100-fold increase of TPPL generated from Au bowtie nanoantennas upon first fs-illumination. High-resolution SEM images and surface plasmon resonance spectroscopy before and after excitation suggest the accumulation of small () Au nanoparticles in the feedgap due to a combination of laser-induced electromigration , nanoparticle re-shaping [29–31], and optical tweezer effects , rather than a continuous Au film. The process is found to be self-limiting and stable and can, therefore, be used to reach an effective reduction of into the sub-10-nm regime. We determined the electric field intensity enhancement of such fs-illuminated antennas to exceed by comparing its TPPL generation efficiency to a planar Au film. Furthermore, the nonlinear light generation can be controlled by turning the excitation polarization, giving rise to , clearly demonstrating that the enhanced nonlinear signal is generated from the antenna feedgap. Numerical simulations suggest that the strong enhancement of the nonlinear signal originates mainly from an increase of the electric field inside the tips of the nanotriangles composing the bowtie, induced by the presence of the Au nanoparticles in the feedgap. The presented metamorphic and self-optimized nanoantennas show high potential to optically tailor the nonlinear optical response at the nanoscale. Furthermore, the demonstrated concept of laser-induced electromigration promises a new route to study quantum plasmonic phenomena [43,44] in lithographically tailored plasmonic nanostructures by assembling particles below the spatial resolution amenable to electron beam lithography and without the need for electrical accessibility [54,55].
Deutsche Forschungsgemeinschaft (DFG) (SFB 631-B3); Nanosystems Initiative Munich (NIM); Institute for Advanced Study, Technical University of Munich (TUM-IAS); International Graduate School of Science and Engineering, Technical University of Munich (IGSSE).
See Supplement 1 for supporting content.
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