Efficient Nonlinear Metasurface Based on Nonplanar Plasmonic

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Efficient nonlinear metasurface based on non-planar plasmonic nanocavities Feng Wang, Alex B. F. Martinson, and Hayk Harutyunyan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00094 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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Efficient nonlinear metasurface based on non-planar plasmonic nanocavities Feng Wang,† Alex B. F. Martinson,‡ and Hayk Harutyunyan∗,† †Department of Physics, Emory University, Atlanta, GA 30322, USA ‡Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA E-mail: [email protected]

Abstract. Since their discovery in the 1960s, nonlinear optical effects have revolutionized optical technologies and laser industry. Development of efficient nanoscale nonlinear sources will pave the way for new applications in photonic circuitry, quantum optics and biosensing. However, nonlinear signal generation at dimensions smaller than the wavelength of light brings new challenges. The fundamental difficulty of designing an efficient nonlinear source is that some of the contributing factors involved in nonlinear wave-mixing at the nanoscale are often hard to satisfy simultaneously. Here, we overcome these limitations by developing a new type of non-planar plasmonic metasurfaces, which can greatly enhance the second harmonic generation (SHG) at visible frequencies and achieve conversion efficiency of ∼6×10−5 at a peak pump intensity of ∼0.5 GW/cm2 . This is 4-5 orders of magnitude larger than the efficiencies observed for nonlinear thin films and doubly-resonant plasmonic antennas. The proposed metasurface consists of an array of metal-dielectric-metal (MDM) nanocavities formed by conformally cross-linked nanowires separated by an ultrathin nonlinear material layer. The non-planar MDM geometry minimizes the destructive interference of nonlinear emission into the far-field, provides strongly-enhanced independently tunable resonances both for fundamental and harmonic frequencies, a good mutual overlap of the modes and a 1 ACS Paragon Plus Environment

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strong interaction with the nonlinear spacer. Our findings enable the development of efficient nanoscale single photon sources, integrated frequency converters and other nonlinear devices. Keywords: Plasmonics, nonlinear optics, metamaterials

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Nonlinear optical effects are inherently weak resulting in bulky nonlinear devices that typically require high operation power 1 . Concentration of optical energy into nanometric volumes can strongly enhance the light-matter interaction. To this end, metal plasmonic 2–8 and dielectric metamaterials 9–11 have been employed to provide locally enhanced electromagnetic fields. To be used for efficient nonlinear signal generation these local modes have to satisfy several conditions. First, nanosystems need to exhibit strong field enhancement at all the frequencies participating in the nonlinear process 12–15 . Next, the near fields of these modes at different frequencies need to have a significant spatial overlap with each other and with the nonlinear material 16–19 . Finally, the nonlinear polarization currents generated in different parts of the nanosystem need to add up constructively and efficiently out-couple to the far field 17,20–22 . Finding a viable approach that would efficiently combine all these contributions has not yet emerged since some of the contributions can be incompatible with each other 20 . Among various plasmonic nanostructures used for field enhancement, vertically aligned MDM nanocavities 7,18,23,24 and patch nanoantennas 25–28 , possess some unique advantages. The dielectric layer thickness of these nanocavities can be precisely controlled at a single atom level by using thin film deposition techniques. Thus, ultrathin dielectric gap between metal walls can be achieved, enabling extreme confinement of light into ultra-small mode volumes for strongly enhanced local field intensity 23 . These nanocavities can support a plethora of higher order modes 27 , and when used for nonlinear signal generation, some of these modes can be designed to match the nonlinear signal frequencies, making the cavities doubly-resonant. However, the modes of simple planar MDM cavities do not satisfy the efficient nonlinear emission criteria listed above. First, SHG from common optical gap antennas or dimer antennas is severely silenced due to the destructive interference of SH waves at the opposite side of the dielectric gap 20,29 . Thus, quadrupolar optical modes are necessary for producing non-zero SHG 20 . Second, the dispersion relation of gap plasmon is sublinear at shorter wavelengths (Supporting Information, Fig. S1) especially for very


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thin dielectric gaps 27 . This makes it impossible to match the SHG wavelength with the quadrupolar modes when using dipolar modes for field enhancement at the fundamental wavelength. For symmetric nanoantennas, although the SHG wavelengths can be designed to approach other higher order modes, these modes cannot facilitate strong SHG for either weaker resonances and mode overlapping, or being forbidden within the dielectric dipole approximation in centrosymmetric materials 30,31 .

Figure 1: a) Schematic of one unit of the crossbar plasmonic nanocavity network (red arrows indicate the locations of the local probes used in numerical simulations); b) representative SEM of the fabricated metasurface.

In this paper, we report efficient SHG using a metasurface formed by an array of plasmonic MDM nanocavities. The cavities are formed at the intersections of conformally linked nanowires separated by a thin nonlinear dielectric layer (Fig. 1). Compared to planar MDM cavities, the resulting non-planar design of the nanocavity improves the spatial overlap of the modes at the SHG and fundamental frequencies by a factor of ∼3, as will be discussed below. A strong quadrupole resonance inside the cavity is induced at the SHG wavelengths by interfering period-dependent propagating surface plasmons with the cavity modes. Thus, our platform supports spatially overlapped strong resonances at both frequencies involved in the nonlinear wave-mixing process. In detail, the metasurface is composed of perpendicularly oriented Au and Ag nanowires fabricated on top of each other. An insulating layer (12 nm thick Al2 O3 or ZnO) separates the wires at their intersection, forming an MDM plasmonic nanocavity (Fig. 1). The samples


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were fabricated using standard two-step electron-beam lithography process to respectively define the bottom-layer and top-layer nanowire gratings (See Supporting Information for complete details on sample fabrication). Au was deposited for the bottom nanowires (40 nm thick) and Ag for the top nanowires (60 nm thick). Silver has lower losses and is in general better material for plasmonic applications. However it tends to tarnish in the ambient and thus gold was chosen as a material of bottom nanowires to avoid deterioration of their top surface during the fabrication process. A gap dielectric or a nonlinear material was deposited using atomic layer deposition (ALD). The interplay between different electromagnetic modes is controlled by the widths, the periods of the nanowires, and by the spacer thickness, enabling independent tuning of parameters such as field-enhancement, mode symmetry and overlap. ALD creates a conformal dielectric film that covers the entire surface of the bottom layer nanowires including their sides. As we will demonstrate below, the resulting non-planar design of the nanocavity effectively contributes to enhancing the SHG polarization currents that can couple to the far field. In the first part of this work we use a dielectric spacer (alumina) with negligible second order nonlinearity and study the impact of the near field distribution of the modes at fundamental and nonlinear frequencies on the SHG. The effect of the nonlinearity of the spacer (ZnO) is explored in the second part of the manuscript.

Figure 2: Linear characterization of metasurfaces under normal incidence. a) experimental transmission spectra, b) calculated transmission spectra and c) calculated cavity field enhancement for 410 nm bottom layer period, 320 nm top layer period and 65 nm top nanowire width with varying widths of bottom nanowires (74 nm, 98 nm and 135 nm). Dashed lines in b) correspond to the peaks of the local field enhancements shown in c).


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In order to measure the linear optical response of the metasurface design, we performed white-light transmission spectroscopy at normal incidence (See Supporting Information for complete details on experimental setup). Here the polarization of the incident light is set perpendicular to the bottom layer nanowires. Fig. 2a shows transmission measurements on three different samples with varying widths of bottom layer nanowires for tuning of the fundamental resonance. Here the bottom layer grating period (x-direction), the top layer grating period (y-direction) and the widths of top layer nanowires were fixed at 410 nm, 320 nm and 65 nm respectively. Spectra for varying grating periods can be found in Supporting Information Fig. S2. As illustrated in Fig. 2a and Fig. 2b, all the measured transmission spectra feature a wide dip in the infrared range and several narrower dips in the visible range. With increasing the widths of the bottom nanowires from 74 nm to 135 nm, the wide dips experience a large red-shift from 1050 nm to 1350 nm. Our numerical simulations calculated for plane wave incidence confirm these general trends and are in good agreement with the experimental spectra, Fig. 2b. To gain more insight into the nature of the modes we also simulate the wavelength-dependent local field enhancement by placing field probes (Fig. 1a) in the nanocavities. Our simulations indicate that the broad resonances are due to the dipolar modes of the MDM cavities (Fig. 2c and Fig. 3b). These modes provide field enhancement at the fundamental frequency in our SHG experiments. Note that the wavelengths of the broad transmission dips do not exactly match with the local resonant peak wavelengths because of the Fano line-shape of the transmission spectra (Fig. 2b and 2c). Next, we identify resonances in the visible part of the spectrum that provide field enhancement at SHG frequency. The two modes featured in the transmission spectra in Fig. 2 cannot be used for SHG enhancement since they do not possess quadrupolar mode distribution. Both of these modes are excited under near-normal plane wave incidence conditions and feature dipolar mode distributions. As it is discussed in the Supporting Information Fig. S2, these modes correspond to the dipolar resonances of the bare bottom nanowires and the


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hexapole resonances of the cavities (i.e. the (3,0) cavity mode with 3 wave nodes along the −L to L line and 0 wave mode along y-axis). For centrosymmetric materials, these modes are usually thought to strongly suppress the SHG signal. As it has been extensively discussed in the literature, efficient SHG in nanostructures can only be induced by quadrupole resonances because their overlap integral with the dipolar fundamental modes induces nonlinear polarization currents that add up constructively in the far field 20,32 .

Figure 3: a) Local electric field enhancement of the quadrupole cavity modes at different bottom grating periods (380 nm, 410 nm and 460 nm), with top layer grating period of 320 nm, top/bottom nanowires widths of 60 nm/130 nm; b)-d) Lateral view of cavity field distribution at λ=1330 nm; λ=805 nm and λ=665 nm, respectively; e) Radiation directivity at 665 nm wavelength. b), c), d) and e) are for 410 nm bottom grating period.

The general agreement between the measured spectra in Fig. 2a and the calculated spectra in Fig. 2b confirm that our sample was properly fabricated, however both the experimental and theoretical transmission spectra cannot provide us with the excitation conditions for quadrupolar (dark) modes since the latter cannot be excited under normally incident plane waves. Quadrupole cavity modes can either be excited by tilted incidence with p-polarization, 8 ACS Paragon Plus Environment

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or be locally excited 33 . For this, we simulate local field enhancement in our nanocavities using localized dipoles as excitation sources. As shown in the Fig. 3a, for each specific period, we can excite the quadrupole cavity modes (i.e. the (2,0) cavity mode) at two different wavelengths (peaks A and peaks B). Both the A and B resonances possess similar mode field distribution as shown in Fig. 3c and d. However, the resonant wavelengths of mode A can hardly be affected by varying the period of the bottom layer grating, while the resonance peaks of mode B are primarily period-dependent. Thus, mode A around the 805 nm wavelength is a pure cavity mode and its resonant wavelength only depends on the width and height of the nanowires and the dielectric gap thickness. Mode B, however, is a coupling mode between the propagating surface plasmon modes on the Ag nanowires and the gap plasmons inside the cavities. This can be more clearly seen observing the field distribution in Fig. 3d. We emphasize that mode B is very different from mode 3 visible in transmission spectra in Fig. 2 and Fig. S2. Both modes are hybrid modes between propagating plasmons and MDM cavity modes. However, the cavity mode involved in mode B is a quadrupolar mode and the one involved in mode 3 is a dipolar mode. To confirm the quadrupolar nature of mode B we calculate the radiation directivity of the mode and observe the characteristic 4-lobe emission pattern, Fig. 3e. It should be noted that the scattered quadrupole field is primarily p-polarized, i.e. is polarized perpendicular to the bottom nanowires. The SHG signal shows a strong polarization dependence in accordance to the polarization-dependent excitation conditions of the linear modes discussed above (Supporting Information Fig. S4). Having identified the linear resonances of the metasurface at the fundamental and nonlinear frequencies, we next performed wavelength-dependent SHG measurements by sweeping the pump wavelengths between 1200 nm and 1560 nm as shown in Fig. 4a. Based on the simulations (Fig. 2c), the strongest fundamental resonance for 130 nm wide nanocavities appears at 1400 nm which should lead to a strongest SHG around 700 nm. Within the scanned wavelength range, however, the strongest SHG appears nearby 660 nm wavelength which is very close to the wavelength of mode B (Fig. 3a). Compared with the SHG from


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Figure 4: a) SHG for different wavelengths for 65/130 nm top/bottom nanowire widths, 320/410 nm top/bottom nanowires period; b) SHG at 660nm for varying widths and periods of bottom nanowires. Top nanowire width/period is fixed at 65/320 nm.

45 nm thick gold film the SH signal have been enhanced by 3×105 times (Supporting Information, Fig.S3). As mentioned above, the wavelength of mode B is sensitive to the period of bottom gratings while the fundamental wavelength is sensitive to the bottom nanowires’ width and height. In order to further confirm that the strong enhancement of SHG could be attributed to the fundamental dipolar modes and the period-dependent quadrupole modes, we measured the SHG intensity at 660 nm for different grating periods and nanowires widths. As expected, upon increasing the period of the bottom layer nanowires, the SHG starts to drop (Fig. 4b, blue line) since the larger period would red-shift the quadrupole resonances away from 660 nm wavelengths. Similarly, SHG signal drop is also observed when reducing the bottom wires widths from 130 nm, because of the shift of the fundamental resonance wavelengths (Fig. 4b, red line). The SHG signal shows a strong polarization dependence in accordance to the polarization-dependent excitation conditions of the linear modes discussed above (Supporting Information, Fig. S4). It is worth noting that we collect the SHG signal in reflection whereas the linear response was measured in transmission mode. In spite of the different signal collection configuration, the far field transmission spectra provide us with correct information regarding the sample’s resonant wavelengths for obtaining enhanced


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SHG.

(2)

Figure 5: The profile of normalized χ⊥⊥⊥ E⊥ (r, ω)E⊥ (r, ω), normalized E⊥ (r, 2ω) and E⊥ (r, ω)E⊥ (r, ω) · E⊥ (r, 2ω). a) planar patch nanoantennas with 60/188 nm patches width/length, 420/340 nm x/y period); b) non-planar crossed nanowire cavities with 65/130 nm top/bottom nanowire widths, 320/410 nm top/bottom nanowires periods.

It should also be pointed out that the enhanced SHG also benefits from the improved local field overlap between mode B and the fundamental dipolar modes due to the non-planar design of our plasmonic cavity. The total scattered far field of SHG from the plasmonic nanocavities can be expressed as 17,18 :

Enl (2ω) ∝

Z Z

(2)

χ⊥⊥⊥ E⊥ (r, ω)E⊥ (r, ω) · E⊥ (r, 2ω)dS

(1)

where E⊥ (r, ω) and E⊥ (r, 2ω) represent the enhanced local electric field just inside the (2)

metals at the fundamental and SHG frequencies respectively; χ⊥⊥⊥ is the surface nonlinear susceptibility. Here, ⊥ denotes normal to the surface. Since the resonant electric fields are primarily polarized perpendicular to the metal surface, we only consider the normal components in Eq. 1 20,34–36 . Good mode overlap not only depends on the spatial superimposition (2)

between χ⊥⊥⊥ E⊥ (r, ω)E⊥ (r, ω) and E⊥ (r, 2ω), but also depends on their relative sign. In order to clearly show how the non-planar nanocavities improve the field overlap between the


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fundamental dipolar mode and the period-dependent quadrupole mode, we made a comparison between the planar MDM cavities and our non-planar structures (Fig. 5). Based on the simulations, the planar Ag-Al2 O3 -Au patch nanoantennas with 12 nm Al2 O3 thickness, have the dipolar fundamental modes at 1330 nm and the period-dependent quadrupole mode at 665 nm. We plot the profiles of the normalized E⊥ (r, ω) · E⊥ (r, ω) and the normalized E⊥ (r, 2ω) for both the planar and the non-planar cavities along the dashed line from point −L to L (Fig. 5). The products of E⊥ (r, ω) · E⊥ (r, ω) · E⊥ (r, 2ω) are also plotted in Fig. 5a and b. Since the field enhancement factors are significantly higher inside the MDM cavity, the SHG originating at grating surface outside the cavities can be estimated to be negligible. For our non-planar structure, the product E⊥ (r, ω) · E⊥ · E⊥ (r, 2ω) has negligible negative values so that the integral of Eq. 1 can be maximized. However, for the planar structure, the positive values of E⊥ (r, ω) · E⊥ · E⊥ (r, 2ω) can be partially cancelled out by the substantial negative values. Thus, the calculated integral of our structure is ∼3.1 times larger than for a planar structure, due to the improved mode overlap in the non-planar nanocavity. It is worth noting that the field enhancement of the fundamental mode of our non-planar crossbar nanocavities is slightly weaker than the field enhancement of planar patch nanoantennas, owing to the large radiation loss of its hollow design. Transmission loss of patch nanoantennas can be effectively reduced because of its metal ground. However, the improved mode overlap of the non-planar design makes up for this loss. In addition, the radiation loss of our crossbar nanocavities can also be effectively reduced by depositing or spin-coating anti-reflection dielectric films to cover the nanowires. Thus far, we used surface nonlinearities of metals to generate SHG emission. Another way to induce nonlinear signal generation in metal nanostructures is to place nonlinear materials in the hot spots of plasmonic systems. In recent studies this approach was successfully demonstrated in various nano-architectures 37–40 . In these experiments an additional lithography step was typically used to place the nonlinear material in a region of enhanced fields. Our geometry provides an advantage that the nonlinear material can be simply deposited as


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a spacer layer without the need of second nanofabrication step and the associated alignment issues. Therefore, in order to further enhance SHG from our plasmonic MDM nanocavities, instead of Al2 O3 we deposited a layer of nonlinear material as dielectric gap spacer. ZnO was chosen as the nonlinear material since its Wurtzite structure possesses a C6v symmetry with non-vanishing χ(2) zzz element, which is ideal for z-polarized fields in the proposed vertical cavity 1,41 . For fabricating the ZnO nanocavities, we adopted the same design parameters as in Fig. 2a and used ALD to deposit a 12 nm thick layer of ZnO. The measured wide field transmission spectrum for ZnO cavities with ∼100 nm wide bottom nanowires grating is shown in Fig. 6a. As a comparison, we also show in Fig. 6a the linear transmission spectra of crossed Al2 O3 nanocavities with the same periods and similar widths and heights of the nanowires. It can be clearly seen that the ZnO nanocavities possess very similar resonance features as the Al2 O3 nanocavities. We also observed that the maximum SHG for both the ZnO and the Al2 O3 samples were obtained around 650 nm wavelength. However, SHG at 650 nm for ZnO nanocavities is ∼6.3 times larger than for the Al2 O3 nanocavities. As shown in the Supporting Information Fig. S5, the difference of the local field enhancement between the two samples is very small, therefore we attribute the observed SHG enhancement to the larger second-order susceptibility of ZnO film. It should be noted that the previously discussed dipolar modes in the visible range (Fig. 2) could also contribute to the enhanced SHG in the ZnO sample, since the ZnO possesses bulk non-linear susceptibility. In addition, we also measured SHG signal from a 45 nm Au film and from a 12 nm ZnO film on top of 120 nm Au substrate. Compared to these samples, the ZnO nanocavities exhibit an enhancement factor of ∼2×106 and ∼4.6×105 respectively. In our experiments, the peak power density of the femtosecond pump laser is ∼0.5 GW/cm2 . At this power, the non-planar plasmonic cavities demonstrate a SHG conversion efficiency of ∼1×10−5 (inactive Al2 O3 spacing layer) and ∼6×10−5 (active ZnO spacing layer) at the designed resonant wavelengths. This conversion efficiency is slightly larger than the efficiency observed for 3rd order nonlinear process that does not have symmetry constraints 10 and 4 orders of magnitude stronger than the SHG


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conversion efficiency in plasmonic doubly-resonant nanoantennas 13 .

Figure 6: a) Transmission spectra of crossed nanowire cavities with Al2 O3 and ZnO gap layer (65/100 nm top/bottom nanowire widths, 320/410 nm top/bottom nanowire period); b) SHG at 650 nm for the 45 nm Au film, 12 nm ZnO on 120 nm Au film, Al2 O3 and ZnO nanocavities.

In conclusion, we have demonstrated a conceptually new platform that overcomes the typical limitations of nonlinear nanosources by simultaneously enhancing all the contributing factors of optical nonlinear signal generation. We have shown that non-planar plasmonic nanocavities can be utilized to greatly enhance the nonlinear optical signals by the independent tuning and enhancement of the optical modes at the nanoscale and minimizing the destructive interference of nonlinear polarization emission. By placing a nonlinear material inside the plasmonic cavity we were able to boost the SHG even further achieving many orders of magnitude larger nonlinear signals than in nonlinear thin films and doubly-resonant plasmonic nanoantennas. The general approach outlined here can be applied to the enhancement of other nonlinear processes including third harmonic generation and multiphoton-excited photoluminescence. Our results will enable highly efficient nanoscale nonlinear sources and integrated nanophotonic devices.


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Acknowledgements. This work was supported by startup funds from Emory University. Atomic layer deposition work performed by A.B.F.M. was supported by the Argonne Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DE-SC0001059. The authors thank Dr. Qi-Huo Wei for fruitful discussions and invaluable input. Supporting Information Available: Dispersion curve of MDM plasmons, linear transmission spectra and mode field distributions for varying grating periods, SHG efficiency comparison for thin gold films and nonlinear metamaterial, polarization dependence of SHG signal, field enhancement comparison for ZnO and alumina spacers. This material is available free of charge via the Internet at http://pubs.acs.org


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Efficient Nonlinear Metasurface Based on Nonplanar Plasmonic

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