META 2014 CONFERENCE, 20 – 23 MAY 2014, SINGAPORE

Highly Efficient 3D Nanofocusing Plasmonic Waveguide

Hyuck Choo*

Department of Electrical Engineering, Division of Engineering and Applied Science, California Institute of Technology Pasadena, California USA

* E-mail: hchoo@caltech.edu

Abstract We have demonstrated experimentally a highly efficient

on-chip three-dimensional (3D) linearly tapered metal-insulator-metal (MIM) nanoplasmonic photon compressor (3D NPC) with a final aperture size of 14 x 80 nm2. An optimized and linearly tapered MIM gap plasmon waveguide could theoretically reduce the excessive losses that would occur during nanofocusing processes. In simulation study, this approach could enable nanofocusing into a 2 x 5 nm2 area with the coupling loss and maximum E2 enhancement of 2.5 dB and 3.0 × 104, respectively. We fabricated the 3D NPC on a chip employing electron beam-induced deposition and demonstrated its highly localized light confinement using a two-photon photoluminescence (TPPL) technique. From the TPPL measurements, we experimentally estimated an intensity enhancement of 400 within a 14 x 80 nm2 cross-sectional area and a coupling efficiency of -1.3 dB (or 74% transmittance).

1. Introduction Rapid advances in integrated circuits (ICs) have

revolutionized computing, communications, and data management in the last thirty years. In that time, digital electronic devices such as computers and mobile phones have become an indispensable part of our society. As the applications of electronic devices continue to expand, the device bandwidths need to be increased. For example, within ten years, inter-chip optical communication is expected to operate at modulation speeds of over 70 GHz while consuming less than 10 fJ of power per bit [1]. To meet this demand beyond the limits imposed by the large interconnect delays and power consumption inherent in IC technologies [2, 3], photonic integrated circuits (PICs) have been explored as an alternative solution to ICs [4, 5]. Some PIC approaches have shown excellent improvement over conventional complementary metal oxide semiconductor (CMOS) ICs, with approximately 20 GHz of bandwidth and about 1 pJ of power consumption per bit. However, PIC performance still falls short of the bandwidth and energy efficiency demanded by future computing and communication technologies. [6]. Unfortunately, due to optical diffraction, it is impractical to pursue the performance improvement of dielectric-based PIC components by producing smaller optical modal volumes.

Recently, nanoscale plasmonic devices have been explored as a feasible mechanism for overcoming the diffraction limits that prevent the further improvement of PICs [7-9]. Perpendicularly confined propagating electromagnetic surface waves at the metal-dielectric interface, called surface plasmon polaritons (SPPs), enable the guiding, controlling, and confining of electromagnetic waves on the nanometer scale, which is much smaller than the corresponding wavelength [7]. If light can be confined and manipulated efficiently within sub-100 nm spaces inside the nanoplasmonic light sources and detectors, the reduced physical dimensions and resulting smaller optical modal volumes will enable 1) on-chip integration densities easily surpassing those of current state-of-the-art PICs; 2) superior modulation speeds and bandwidths; and 3) lower power consumption. Efficiently focusing and confining light in deep sub-wavelength spaces is still a major challenge, even for nanoplasmonic devices, due to large resistive and scattering losses associated with the plasmonic light focusing and confining process. Furthermore, physically adjusting and optimizing the structural geometries of on-chip plasmonic devices for different applications are not trivial tasks, given the fabrication challenges associated with their tiny nanoscale dimensions.

2. 3D Tapered Gap Plasmon Waveguide As a potential solution to these challenges, we have

demonstrated the design, fabrication, and characterization of a highly efficient on-chip three-dimensional (3D) metal-insulator-metal (MIM) nanoplasmonic photon compressor (3D NPC), as shown in Fig. 1. Our device can be readily integrated with other on-chip nanophotonic components [10]. Among various nanofocusing approaches, we have further developed the linearly tapered MIM concept proposed by Conway and Yablonovitch [11] and Pile and Gramotnev [12]. An optimized and linearly tapered MIM gap plasmon waveguide could theoretically reduce the excessive losses that would occur during nanofocusing processes. Based on simulation, the coupling loss and maximum E2 enhancement of the 3-D NPC are predicted to be 2.5 dB and 3.0 × 104, respectively, for a case in which light was compressed from a 200 x 500 nm2 area into a 2 x 5 nm2 area. We produced the 3-D NPC on a chip by employing electron beam-induced deposition (EBID). In addition, we demonstrated highly localized light

2

confinement using two-photon photoluminescence (TPPL) techniques (Fig. 2). From the TPPL measurements, we experimentally estimated an intensity enhancement of 400 within a 14 x 80 nm2 cross-sectional area and a coupling efficiency of -1.3 dB (or 74% transmittance).

Figure 1: (a) Schematic illustration of a 3-D NPC structure. The 3-D NPC is essentially configured as a 3-D linearly tapered MIM-based SPP guide composed of upper and lower gold plates with an intermediate dielectric SiO2 layer. (b) SEM image of a fabricated 3-D NPC structure.

Figure 2: (a)-(d) Electron-Multiplying-CCD (EMCCD) images of TPPL emissions for four different excitation scenarios. The red dots indicate the positions of laser excitation. Here, the laser beams were x-polarized, and the excitation power was fixed at 210 µW. (e) Unsaturated intensity map of Fig. 2(d) superimposed onto the contours of the SEM image of the device. (f) Top view SEM image of the characterized sample, and the simulated |E|2 intensity profile for a sample with the dimensions of the fabricated sample. (g) Individual bright and dark bands shown in simulation results are not resolved in the experimental measurement due to the diffraction limit.

The tip behaves as a nanoscale optical resonance cavity due to its sub-100 nm finite size (Fig. 2(g)), and could serve as the core of a nanophotonics device such as a nanoscale LED and detector. By carefully optimizing the cavity properties (such as the radiation or absorption rates), impedances (or loss-rates) can be realized which match and dramatically increase the electromagnetic energy coupled from the microscale body of the waveguide into the nanoscale tip for maximum field enhancement. To accomplish this delicate tuning task, we investigated the integration of the 3-D NPC structure with MIM plasmonic crystals [13, 14]. These crystals possess properties similar to those of photonic crystals. Our preliminary simulation results show that this arrangement represents a simple yet highly effective engineering approach to further improve the degree of energy concentration within an extremely small volume with excellent energy coupling efficiency.

3. Conclusions In summary, we demonstrated a highly efficient

approach to accomplish and tune on-chip 3D nanofocusing. Optimal tapering angles and lengths were found in simulation to minimize the intrinsic scattering and resistive absorption losses observed during nanofocusing. The optimized design was realized on a chip by employing an EBID technique, and the implemented 3D NPC showed highly localized light confinement. We suggest that this highly efficient 3D nanoplasmonic photon compressor will be useful in a variety of on-chip nanoscale photonic and plasmonic applications.

Acknowledgements This work was funded by the DARPA, the Powell Foundation, and the EAS Division of Caltech. The author also thanks all the collaborators involved in this project.

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