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Gap plasmon enhanced metasurface third harmonic generation in transmission geometry Mohammadreza Sanadgol Nezami, Daehan Yoo, Ghazal Hajisalem, Sang-Hyun Oh, and Reuven Gordon ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00038 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 26, 2016

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Gap plasmon enhanced metasurface third harmonic generation in transmission geometry Mohammadreza S. Nezami†, Daehan Yoo‡, Ghazal Hajisalem†, Sang-Hyun Oh‡, Reuven Gordon*,† †

University of Victoria, Electrical and Computer Engineering P.O. Box 3055, Victoria, BC, CAN V8W 3P6

‡

University of Minnesota, Electrical and Computer Engineering, 200 Union St. SE, 5-119 Keller Hall, Minneapolis, MN, USA 55455, 612-625-0125

KEYWORDS: metasurfaces, gap plasmon, third harmonic generation, nonlinear scattering theory. ABSTRACT: The influence of gap plasmons in the third harmonic generation of annular and H-shaped aperture arrays has been investigated. Surprisingly, the maximum third harmonic in the annular ring structures is not from the smallest gap, but rather appears for a gap of 14 nm. We interpret this result in terms of resonant plasmonic antenna design to match scattering and absorption cross sections. Unlike the annular aperture, continuous gold apertures, like the H-structure, achieve 0.45% con2 version efficiency for 2 mW/µm CW pump power of a 100 fs pulse; which is 9 times larger than rectangular apertures we have reported in the past. Nonlinear scattering theory has been used to estimate the transmitted third harmonic signal and is in generally good agreement with the experimental results.

Metasurfaces offer great potential for optical functionality in a flat layer1-8. Nonlinear optical metasurfaces are promising structures for applications such as optical switching9, 10, wavelength conversion11, near field imaging12,13, subwavelength lithography14, and spectroscopy15. Due to their ability to localize the electric field while removing heat through a continuous metal layer of high thermal conductivity, apertures in metal films have been considered as favorable structures for the third harmonic generation 16-18. There is a continuing effort to optimize the conversion efficiency from apertures in metal films, ideally approaching ~1% to 10% conversion efficiency to enable the aforementioned applications 17-32 . The aperture’s localized surface plasmon resonance can be manipulated by the aperture shape18,24 and surrounding medium25,26, which consequently leads to a higher conversion efficiency. Propagating surface plasmon polaritons can be tuned to the Bragg resonance to improve the nonlinear optical response 27-34. Moreover, by tuning the localized surface plasmons resonance

to the fundamental wavelength and propagating surface plasmons of a rectangular aperture array to the third harmonic wavelength, 0.05% conversion efficiency was achieved with 2 mW/µm2 pump intensity 35. It is possible that even larger intensities can be obtained with more intense pulse sources; however, material damage (for 100 nm gold on a fused silica substrate at around 10 mW/µm2 36,37 and close to 1 mW/µm2 for nanostructures with tiny gaps 38) and saturation of the nonlinear response39 are limiting factors. The fundamental beam power can be enhanced locally at the metal surface gap plasmons. So far, it has been both experimentally and theoretically proven that the field enhancement in the gap region is inversely proportional to the gap size40. This field enhancement facilitates nonlinear optical response of the metasurfaces41,42,43. Quantum tunneling, however, limits this enhancement at the subnanometer regime44. There are four main reasons that we focus on the third harmonic. SHG is dipole forbidden in gold due to its centrosymmetry45. THG has a cubic dependence on the

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fundamental wavelength beam’s power45. So, higher conversion efficiencies are achievable at increased power levels44. Interband transition of the gold are close to the THG energy at the fundamental of 1500~1600 nm, so it is possible to use high-power fiber-based lasers for compact wavelength conversion46. Kerr-like switching is allowed in the χ ( 3) susceptibility of gold that might be of interest of future switching applications47. Moreover, high sensitivity of THG to the near-field intensity, makes it very efficient in near-field spectroscopy applications48. Recently a functional metasurface using the nonlinear response to steer and focus beams was demonstrated49. These concepts can be extended to achieve subwavelength lithography using the concept of radiationless electromagnetic interference50. Furthermore, achieving a high conversion efficiency is desired for energy-efficient applications; and this will certainly be assisted in metals by having an optimal slit structure. Of course there is a trade-off when making the gap too small, because losses also increase with smaller gaps which limits conversion efficiency51. This is particularly problematic for the transmission geometry where the light must travel through the aperture. It has also been shown that if the extinction resonance wavelength of a plasmonic structure matches the source centre wavelength, the maximum THG is achieved43,52. The novelty of this study is to show a previously unstudied result: that gap apertures have an optimal gap size related to their coupling properties. One may naively expect that the gap should be narrowed to the smallest dimension before the onset of tunneling, which we have shown gives the highest third harmonic generation44. On the contrary, when a plasmonic resonance is present (which can be thought of as a resonant circuit53,54,55) there is an optimal gap size at which the third harmonic generation is maximized. This gap size is typically much larger than the onset of tunneling, which also benefits ease of fabrication in many cases. In this work, we investigate various gap sizes while tuning the localized plasmon resonance for annular ring structures to find the optimal operating gap for 150 nm thick films. The annular ring structures have a disconnected central island, and this impacts the ability to remove heat from the centre and ultimately lowers the damage threshold.

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Consequently, we also investigate H-shape apertures with small gaps in the middle for their ability to efficiently localize the field (albeit with larger gaps than the annular structures due to fabrication challenges). Here we show that by tuning the gap plasmon resonance to the fundamental beam wavelength at 1570 nm, the transmitted third harmonic signal can be enhanced. The periodicity of the fabricated arrays in direction of the fundamental beam polarization is close to the third harmonic wavelength. This also leads to the third harmonic field enhancement and also directivity improvement. We investigated these effects in annular ring arrays, H-shaped aperture arrays, double nanohole (DNH) arrays and compared our results with the rectangular aperture array investigated previously. All structures show higher conversion efficiencies than rectangular aperture arrays seen the past. The greatest conversion efficiency results from the H-shaped structure and is around 0.45%. Our experimental results are in generally good agreement with nonlinear scattering theory, and paths towards even higher conversion efficiency are discussed. While in the present case, we consider arrays of identical elements, it is easy to envision how these findings may inform functional metasurface design where each aperture element can be tuned independently to achieve a desired result. Optimal gap size for the 1D case

While narrowing the gap in a metal nanostructure certainly leads to higher field concentration and a greater local density of optical states56, it does not necessarily lead to the optimal third harmonic conversion. It has been shown previously for optical antennas that the optimal coupling to the antenna arises when the radiative and absorption rates are matched54. For the antenna case, this was achieved by tuning the ground plane separation and feedgap size. Here we consider the same

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Figure 2. (a) Schematic of third harmonic generation from an aperture-based metasurface (b) simulated configuration based on for incident fundamental and time reversed harmonic beam

Figure 1. (a) Absorption (blue curve) and scattering (red curve) cross section spectra for different gap sizes, w, of the slit configuration depicted in the inset. The polarization of the incident light is parallel to the width, w, of the slit. The thickness of the gold film is 100 nm. (b) Resonant THG prediction based on nonlinear scattering theory (red) and corresponding absorption and scattering (black) for different gap sizes, w, of the slit. Note that the optimal THG occurs when the absorption and scattering cross sections are equal.

result, particularly as it will relate to optimal third harmonic generation in later sections, but for a simple slit configuration (inset in Figure 1.a). We calculate both the absorption and scattering cross sections for the slit, as shown in Figure 1.a. It is noted that these do not show monotonic behavior. Also, it is noted that the wavelength shifts with changing gap size, so in each case, we are taking the peak value. Figure 1.b shows the peak results as compared with the nonlinear scattering theory prediction for the THG in each case. It is clear that the highest THG comes at the point where the scattering matches the absorption coupling, for a 4 nm gap. (The gap size changes in the 3D configurations studied below). This is the case that gives the maximal local field enhancement54. In that past work, however, only a linear measurement was made, so the local field enhancement could not be easily distinguished from distributed absorption over the antenna structure. Nonlinear scattering theory

Figure 2.a shows the schematic of the third harmonic generation from the metasurfaces in the transmission geometry. Figure 2.b shows the configuration that has been used for FDTD simulations. Since the dielectric region plays an important role in THG, particularly at the smallest gap, it should be considered in calculating the total THG from the annular ring structure. The total out-coupled THG from the annular ring structure can be estimated by THG ∝ 





  .    

 



   .  

(1) Where  is the induced third harmonic electric field vector,  is the fundamental electric field vector in the structure. According to the Lorentz reciprocity theory, the scattered plane wave field leaving a structure can be modeled by the equivalent but time reversed field incident on the structure 57. Nonlinear scattering theory 58,59 has been developed based on the Lorentz reciprocity theorem and it is quite efficient in predicting the nonlinear optical response of the metamaterials 35,60. Physically, Eq. 1 shows that the fundamental electric field induces a nonlinear current source   ∝  that generates a third harmonic electric fields through the power source term   .  . Therefore, by using Lorentz reciprocity, the outcoupled third harmonic wave can be calculated by the integration of overlapping fields over the

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material volume.  is the third order suscep-

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since there is no alumina in the gaps.



tibility of gold and   is the third order susceptibility of alumina that are 7.6×10-19 (m/v)2 45 and 2.3×10-23 (m/v)2 61. For other structures such as the H-shaped aperture, DNH and rectangular arrays, we used equation below: 

THG ∝    .  

(2)

Figure 3. Calculated normalized THG versus inner ring diameter in annular ring structures using equation (1) for different gap sizes: (a) 6 nm (b) 8 nm (c) 10 nm (d) 12 nm (e) 14 nm (f) 16 nm.

Figure 3 shows the calculated THG from the annular ring structures versus the inner ring diameter and for different gap sizes. It can be seen from the results that the peak conversion efficiency

occurs at smaller ring diameters for the smaller gap sizes. Maximum conversion efficiency is from the 14 nm gap at 150 nm inner ring diameter.

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Figure 4. (a) Schematic of the experimental setup used to measure THG of the fabricated structures where LPF is low pass filter; BS is beam splitter, HWP is half wave plate, FM is flip mirror, C is collimator and OF is optical fiber. (b) THG incident power dependence is shown on a logarithmic scale with a slope of 3.07. (c) Measured THG signal from the spectrometer. (d) Linear transmission spectra of the annular ring structures for different gap sizes and fixed ring diameter of 90 nm. (e) SEM image of the annular ring structure of 6 nm gap size before THG measurement.

Nonlinear Measurements and Discussion

Figure 4.a shows the schematic of the experimental setup for transmitted third harmonic measurement. A He-Ne laser beam at 633 nm used on top of the fundamental beam at 1550 nm. Since the transmitted beam of the guide laser is detectable on the camera, the fabricated structures can be found on the CMOS camera (Thorlabs, DCC1545M). The guide laser was off during the nonlinear measurement. Figure 4.b

shows the power dependence of the third harmonic signal to the input power on a logarithmic scale with the slope of 3.07. Figure 4.c shows the measured spectrum of the third harmonic signal on the spectrometer, shown a peak at a third of the fundamental wavelength, as expected from THG. Figure 4.d shows measured linear transmission spectra of the fabricated annular ring structures for different gap sizes and fixed ring diameter of 90 nm.

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Figure 5. Measured THG signals versus inner ring diameter in the annular ring structures for different gap sizes: (a) 6 nm (b) 8 nm (c) 10 nm (d) 12 nm (e) 14 nm (f) 16 nm

Figure 5 shows the measured THG in transmission geometry versus inner ring diameter in annular ring structures with different gap sizes. The trend shows a good agreement with the simulation results except for two outliers: 1- For the smallest gap size and at the smallest diameter, the THG is quite high whereas it was weak in the simulation results. This may be the result of shape imperfections in the fabricated structure, which may be improved with further iterations. 2- The larger gaps have peaks at smaller diameters than predicted by theory. This might be related to the thermal damage to the the samples at these high powers which we have observed in disconnected structures in the past (Figure S3)..

Figure 6. (a) Measured THG spectra for different aperture geometries and periodicities; array 1: array of

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rectangular aperture, w= 20 nm , l= 343 nm, Px=518 nm; array 2: DNH, g=60 nm, Px=518 nm; array 3:rectangle, w=40 nm, l=343 nm, Px=518 nm; array 4: DNH, g=60 nm, Px=410 nm , array 5:H-shaped, g=40 nm, Px=410 nm; array 6: H-shaped, g=40 nm, , Px=530 nm where w is the width of rectangular aperture, l is the length of rectangular aperture, g is the gap size of the DNH and H-shaped aperture and Px is the periodicity of the array in the direction of the fundamental wave polarization (b) SEM image of the optimum H-shaped array structure for THG (c) SEM image of the optimum DNH array structure for THG.

Figure 6.a shows the measured THG from aperture arrays with different geometries and fabrication parameters. We fabricated H-shaped, DNH and rectangular arrays on the same sample. Figures 6.b and c show the SEM images of the optimum aperture array structures for THG. The figures shown are for the best results found among a wider range of parameters scanned. Previously, we found that the rectangular aperture is an efficient structure for third harmonic generation, since it can scatter light beyond its single channel limit under TM polarized light exposure. We got into 0.05% conversion efficiency. These new results show that the H-shaped aperture can even show higher conversion efficiencies by a factor of 9 due to the smaller gap and local confinement in the center. The DNH also has a narrow gap, but is not as good candidate for THG enhancement. From these results we found that the gap plasmon resonance in the H-shaped and DNH apertures can improve third harmonic conversion efficiency further. Figure S4 of supporting information shows the electric field distribution of these structures at the fundamental wavelength of 1570 nm. Table 1 shows a quantitative comparison of the experimental and simulation results for the fabricated structures. Table 1. Comparison of the optimum THG for fabricated structures.

Structure tructure Rectangular array

DNH array H-shaped array

Normalized exNormalized experiment periment

Normalized simuNormalized simulation equations 1 ,2

1 ,2



0.10

0.13

0.20

0.24

1

1

Annularring array

0.43

0.30

Conclusions

Here we have investigated nanogap structures for enhanced nonlinear conversion from aperture metasurfaces operating in the transmission geometry. We have found that while narrow gaps enhance nonlinear conversion, there is a limit when operating in the transmission geometry due to losses. So far, we have achieved 0.45% conversion efficiency using H-shaped apertures. Nonlinear scattering theory generally shows the same trends as in the experiments. Disconnected ring structures, like the annulus presented here, show damage at these powers (Figure S3), which we suspect is from the inability to effectively remove heat from the centre island. It is possible that this may be improved with future works. These results are promising for nonlinear metasurfaces with >1-10% efficiency for applications including optical switching, wavelength conversion, near field imaging, subwavelength photolithography, and spectroscopy. To achieve these higher efficiencies, it is expected that other parameters such as film thickness, aperture shape, materials (considering optical, nonlinear and thermal characteristics), and structure (e.g., adding encapsulation to help stabilize the structure and manage thermal damage), should be explored. Methods FDTD simulation

We used Lumerical FDTD 8.12.501 to perform simulations to feed into the nonlinear scattering theory. Anti-symmetric boundary conditions in x direction and symmetric boundary condition in y direction were used. We used two 3D monitors at 1570 nm and 523 nm around the aperture region to collect the induced fundamental and third harmonic beam in the gold and dielectric region. A 2D index monitor in the xy plane collects the refractive index of the material in xy plane and helps to discriminate dielectric and metal region. Using the index monitor, we defined two overlay masks to extract fundamental and third harmonic electric fields for dielectric and gold regions. For the annular ring structure, We found the THG

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from equation (1) by superposition of the resulted THG from gold volume around the aperture and dielectric volume inside the aperture. For other fabricated structures (H-shaped, rectangular, DNH), just the gold volume around the apertures was taken into account. A mesh override region with the same dimensions as the 3D monitors was used. We used 1 nm mesh size in all directions The conformal mesh 1 refinement technology that is reliable in metal dielectric structures has been also utilized. We also performed 2D simulations for the single slit configuration within PML boundaries. We used a wide-band totalfield scattering-field (TFSF) source. Two boxes of frequency-domain field monitors inside and outside of the source box used to calculate absorption and scattering cross sections. A mesh size of 0.1 nm was used in x and y directions. Fabrication Methods

Annular ring arrays fabricated using ALD similar to the past works 62-64. The thickness of the fabricated annular ring arrays is 150 nm. We used focused ion beam milling technique to fabricate arrays of H-shaped, DNH and rectangular array. An acceleration voltage of 40 kV, a Beam current of 0.01064 nA and a dwell time of 10 µs have been used to fabricate the structures on a 100 nm thick gold film on a glass substrate

ASSOCIATED CONTENT Supporting Information

Additional SEM characterization after nonlinear measurement of samples not showing obvious damage. SEM of sample with damage. Simulations showing near-field distribution.

AUTHOR INFORMATION Corresponding Author

*Email: [email protected] Present Addresses

Department of Electrical and Computer Engineering, University of Victoria, Victoria, BC, Canada, V8P5C2 Author Contributions

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M. S. Nezami prepared samples, measured samples, and performed calculations. D. Yoo prepared and characterized annulus samples, under the supervision of S.-H. Oh. Nezami and Yoo contributed equally to this work. G. Hajisalem measured samples. R. Gordon supervised the project. All authors contributed to writing the manuscript. Notes

The authors declare no competing financial interest, ACKNOWLEDGMENT

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For Table of Contents Use Only: Gap plasmon enhanced metasurface third harmonic generation in transmission geometry Mohammadreza S. Nezami†, Daehan Yoo‡, Ghazal Hajisalem†, Sang-Hyun Oh‡, Reuven Gordon*,† The paper investigates gap plasmon enhanced third harmonic generation in transmission geometry (graphic on the right). THG form several structure such as array of H-shaped, double nanohole, rectangular and annular ring apertures has been investigated. The maximum THG in this work is resulted from the array of H-shaped apertures (SEM image on the left).

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