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Plasmonically enhanced Kerr frequency combs Rigoberto Castro-Beltran, Vinh M. Diep, Soheil Soltani, Eda Gungor, and Andrea M. Armani ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00808 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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Plasmonically enhanced Kerr frequency combs Rigoberto Castro-Beltrán‡1,2, Vinh M. Diep‡1, Soheil Soltani3, Eda Gungor1 & Andrea M. Armani1,3,*

1

Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, USA.

2

Departamento de Ingeniería Física, División de Ciencias e Ingenierías, Universidad de Guanajuato Campus León 37150, México.

3

Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, USA.

ABSTRACT Optical frequency combs are high repetition rate, broad spectral bandwidth coherent light sources.

These devices have numerous applications in many fields, ranging from

fundamental science to defense. Recently, low threshold and small footprint frequency combs have been demonstrated using ultra-high quality factor (Q) whispering gallery mode resonant cavities. The majority of research in cavity-based combs has focused on optimizing the Q. An alternative strategy is to engineer the cavity material to enhance the underlying nonlinear process for comb generation. In this work, we demonstrate that gold nanorods coated with a nonlinear material reduce the comb generation threshold when

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decorated on the surface of the resonant cavities. The enhancement mechanism is explored with finite element method modeling and can be explained in terms of photonic-plasmonic mode hybridization. A comb span of ~300 nm in the near-IR range is observed with incident intensity 60% decrease in the circulating intensity (1.85 GW cm-2). Considering the symmetry of the comb around the pump, it is probable that the comb extends beyond 1750 nm. However, the working range of the optical spectrum analyzer used in this study was limited to 1700 nm. A comb spanning 350 nm would be comparable to combs generated by other high χ(3) materials10-12. Additionally, the individual comb lines are more uniformly generated throughout the range of analysis, indicating a higher efficiency process (Figure 5b – 5d). While an improvement in comb span was observed for the devices with CTABfunctionalized gold nanorods, the enhancement was not as significant (~250 nm span at 3.6 GW cm-2, Figure S6). The FEM modeling presented in Figure 2 demonstrates an increased optical intensity at the surface of the metal nanoparticles where the PEG is located. Previous work has shown that organic molecules, including PEG, have large third order non-linear


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coefficients compared to silica (n2,PEG ~10-14 cm2/W and n2,silica ~10-16 cm2/W)36-38. Therefore, the PEG can increase the effective third order nonlinearity of the cavity. The small interaction length of the nanoparticles does not disturb the phase-matching conditions that are critical for optical parametric oscillation to be realized. This allows the photons from the parametric process to be emitted and supported in the hybrid cavity at very low circulating optical power19.

Figure 6. (a) Threshold for parametric processes in resonators coated at three different concentrations of PEG-functionalized gold nanorods compared to an uncoated device. (b) Threshold for parametric processes in resonators coated with CTAB-functionalized gold nanorods and PEG-functionalized gold nanorods at the same colloidal concentration. (c) Comparison between the threshold power for gold nanorod-coated devices with different concentrations and surface functionalizations.

To illustrate the role of the PEG in the comb generation process and differentiate it from a purely plasmonic enhancement process, the OPO threshold curves for the PEGfunctionalized gold nanorod-coated devices are presented alongside the curves for a device coated with CTAB-functionalized gold nanorods and for an uncoated silica device (Figure 6a,b). All values are also included in Table 1. The signal vs. pump curves in Figure 6a show a clear dependence on the PEGfunctionalized gold nanorod concentration. Specifically, for the coated devices the threshold decreases from 1.5 mW to 148 µW (0.439 GW cm-2 to 0.041 GW cm-2) as the PEG-functionalized gold nanorod concentration increases from 0.070 mM to 0.125 mM. These values represent an enhancement factor of ~15 with respect to both the uncoated


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silica devices studied here and to previously published devices with similar characteristics30. Furthermore, Figure 6b shows that it is the combination of both the nanorods and PEG that results in the greatest enhancement. For the same colloidal concentration of gold nanorods (0.125 mM), the threshold for PEG-functionalized nanorod devices is 148 µW compared to 1.32 mW for the CTAB-functionalized nanorod devices. The threshold for parametric oscillation for each device studied is shown in Figure 6c. While the threshold decreases when high concentrations of CTAB-functionalized nanorods are used, the most dramatic effect is observed when the PEG-functionalized gold nanorod coating is present. Additionally, there is a clear dependence on the PEGfunctionalized gold nanorod concentration, providing further evidence of the role of the PEG as a contributor to the decrease in threshold of the parametric processes. The quality factor, threshold, and comb span of all devices are summarized in Table 1. The decrease in the required threshold and the enhancement in the comb span are evident with the increasing concentration of gold nanorods and the functionalization with PEG. This performance improvement comes despite the decrease in quality factor due to the increased surface scattering by the presence of the particles on the surface. Furthermore, the signalidler power dependence of all devices tested shows a linear behavior with correlation factors ~0.95±0.03 (Figure S5). Table 1. Comparison of Microresonators with Different Surface Coatings Device

Quality Factor (Q)

OPO Threshold (mW)

Comb Span @ Circulating Intensity (Input Power*)

Uncoated Device PEG-NR (0.070 mM) PEG-NR (0.080

1.2 × 108

2.2

7.7 × 107

1.5

5.6 × 107

0.677

180 nm @ 5.2 GW cm-2 (~18 mW) 210 nm @ 4.60 GW cm-2 (~13.5 mW) 260 nm @ 2.90 GW cm-2


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mM) (~11 mW) 7 PEG-NR (0.125 6.5 × 10 0.148 280 nm @ 1.85 GW cm-2 mM) (~6.1 mW) 7 CTAB-NR 6.3 × 10 1.32 245 nm @ 3.82 GW cm-2 (0.125 mM) (~12.8 mW) *Input power is measured before the tapered fiber waveguide. Conclusions In conclusion, we have demonstrated a 15× improvement in whispering gallery mode frequency comb performance by leveraging plasmonic enhancement of nonlinear organic polymers. Modeling shows that the phenomenon responsible for this improvement in efficiency is a surface plasmon-polariton between the surfaces of the resonator and nanorods, which interacts with the highly nonlinear PEG layer. By utilizing this structure, it is possible to reduce the threshold and increase frequency comb span without pursuing higher quality factor or increasing input powers. This materials-first strategy will enable a wide range of application areas including sensing39, quantum computing40, and communications41.

ASSOCIATED CONTENT Supporting Information Experimental details including gold nanorod synthesis and device characterization, simulation details, additional experimental results. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]

Author Contributions R. C-B. designed experiments, performed device fabrication and characterization measurements, and analyzed data. V. M. D. performed device characterization measurements and analyzed data. S. S. performed finite element method modeling. E. G. synthesized the nanoparticles and performed surface chemistry. A. M. A. designed experiments and oversaw project. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Funding R.C-B. was supported by CONACYT-USC Post-doctoral fellowship. V.M.D. was supported by the NSF GRFP and USC Viterbi Fellowship. The work was supported by the Northrop Grumman Institute of Optical Nanomaterial and Nanophotonics and the Office of Naval Research [N00014-11-1-0910, N00014-17-1-2270].

ACKNOWLEDGEMENTS The authors would like to thank Dr. Xiaoqin Shen for valuable discussions in the data analysis.

Methods Gold Nanorod Synthesis. Synthesis of gold nanorods was carried out using the seedmediated growth method according to previously published protocols with some


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modifications42. The seed solution was prepared by mixing 0.25 mL of an aqueous 0.01 M solution of HAuCl4.3H2O and 7.5 mL of a 0.10 M CTAB solution. Then, 0.6 mL of a 0.01 M ice-cold NaBH4 solution was added into the mixture, and kept at 30 °C for 2 h. 100 mL of 0.1 M CTAB, 5 mL of 0.01 M HAuCl4·3H2O, 1 mL of 10 mM AgNO3, 2 mL of 0.5 M H2SO4 and 800 µL of 0.1 M ascorbic acid were added in sequence to prepare the growth solution. Next, 0.24 mL of the seed solution was added to the reaction mixture to initiate the growth, and kept at 30 °C for 12 h. Subsequently, the gold nanorods were purified by centrifugation. Finally, the gold nanorods were redispersed in DI water. PEGylation of Au NRs took place according to a previously published paper21. The PEGylated gold nanorods were first redispersed in methanol. The final concentration was adjusted to 2.5 mM, and three dilutions of gold nanorods (0.125 mM, 0.080 mM, 0.070 mM) were prepared. An additional solution of non-PEGylated gold nanorods was also prepared (0.125 mM). Additional details on the synthesis/purification and characterization are in the Supporting Information.

Resonant Cavity Fabrication and Functionalization Silica resonators are fabricated by melting the tip of a cleaved optical fiber using a CO2 laser beam. The gold nanorods are coated on the surface of the resonators using a dip coating method. Dip coating has been utilized in many applications including creating uniform optical coatings on ultra-high-Q devices due to the simplicity of the process. Additional details of this procedure are available in the Supporting Information.

Optical Characterization.


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A CW tunable 1550 nm laser is coupled into the cavity using a tapered optical fiber waveguide. The position between the waveguide and cavity is controlled using a 3-axis motorized nanopositioning stage and monitored using top and side view cameras. The output from the resonator is sent to a 90/10 splitter, with 90% going to an optical spectrum analyzer (OSA) and the rest going to a photodetector (PD). The PD is connected to a highspeed digitizer/oscilloscope, allowing the transmission spectra to be recorded. The optical signal is recorded on an oscilloscope, and the spectrum is fit to a Lorentzian to determine the linewidth (ߜߣ). The loaded quality factor (Q) is calculated from ܳ = ߣ/ߜߣ, and the intrinsic Q is calculated using a resonator coupling model43. The comb spectrum is measured on the OSA. Additional details are in the Supporting Information.

FEM Modeling. We use COMSOL Multiphysics finite element method software to model the plasmon-polariton field and the hybridized mode between surfaces. The model is similar to one described in a previously published paper21. All dimensions and constants used in the model were based on the experiment. The mesh size was λ/10. Additional details are in the Supporting Information.

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For Table of Contents Use Only

Plasmonically enhanced Kerr frequency combs Rigoberto Castro-Beltrán‡1,2, Vinh M. Diep‡1, Soheil Soltani3, Eda Gungor1 & Andrea M. Armani1,3,*


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(a) A whispering gallery mode resonator coated with gold nanorods. (b) The gold nanorods on the device surface are functionalized with a layer of PEG polymer with highly nonlinear optical properties. 30x11mm (600 x 600 DPI)


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(a) Cross-sectional field profile of a gold nanoparticle near the surface of the whispering gallery mode resonator (rresonator=65 µm). The nanoparticle is located 15 nm from the surface and has an aspect ratio of 3.8 (width ~17 nm). (b) 1-D field profile in the radial direction of a resonator without (top) and with (bottom) a gold nanoparticle 15 nm from the surface. (c) The maximum field enhancement of a gold nanoparticle vs. distance from the surface of the resonator. The inset shows the field profile for a 30 nm separation between the particle and silica surface, where the hybrid mode has nearly disappeared. 198x547mm (600 x 600 DPI)


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Schematic representation of the experimental setup used to characterize both the transmission and the nonlinear optical phenomena from the microresonator. The inset shows a transmission spectrum of one of the devices. Based on this spectrum, the linewidths (δλ) of the clockwise and counterclockwise modes can be determined (5.35×10-5 nm and 4.81×10-5 nm), which correspond to Q values of 2.9×107 and 3.2×107. 57x42mm (600 x 600 DPI)


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Parametric gain is observed for a ~130 µm diameter silica microsphere coated with functionalized gold nanorods (0.080 mM colloidal concentration). (a) At a circulating intensity of 0.09 GW cm-2, the first and second order parametric gains are located symmetrically about the pump line. This intensity is higher than the OPO threshold. The inset shows a typical transmission spectrum when nonlinear behavior is observed. (b) Signal-idler relation with a correlation coefficient of 0.95. 113x168mm (600 x 600 DPI)


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Optical frequency comb formation in (a) an uncoated microsphere and microspheres coated with PEGfunctionalized gold nanorods at colloidal concentrations of (b) 0.070 mM, (c) 0.080 mM and (d) 0.125 mM. 71x28mm (600 x 600 DPI)


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(a) Threshold for parametric processes in resonators coated at three different concentrations of PEGfunctionalized gold nanorods compared to an uncoated device. (b) Threshold for parametric processes in resonators coated with CTAB-functionalized gold nanorods and PEG-functionalized gold nanorods at the same colloidal concentration. (c) Comparison between the threshold power for gold nanorod-coated devices with different concentrations and surface functionalizations. 164x335mm (600 x 600 DPI)


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