Letter pubs.acs.org/NanoLett
Squeezing Photons into a Point-Like Space Myung-Ki Kim,*,†,§ Hongchul Sim,†,§ Seung Ju Yoon,† Su-Hyun Gong,† Chi Won Ahn,‡ Yong-Hoon Cho,† and Yong-Hee Lee*,† †
Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea National Nanofab Center, 291 Deahak-ro, Yuseong-gu, Daejeon 305-806, South Korea
‡
S Supporting Information *
ABSTRACT: Confining photons in the smallest possible volume has long been an objective of the nanophotonics community. In this Letter, we propose and demonstrate a three-dimensional (3D) gap-plasmon antenna that enables extreme photon squeezing in a 3D fashion with a modal volume of 1.3 × 10−7 λ3 (∼4 × 10 × 10 nm3) and an intensity enhancement of 400 000. A three-dimensionally tapered 4 nm air-gap is formed at the center of a complementary nanodiabolo structure by ion-milling 100 nm-thick gold film along all three dimensions using proximal milling techniques. From a 4 nm-gap antenna, a nonlinear secondharmonic signal more than 27 000-times stronger than that from a 100 nm-gap antenna is observed. In addition, scanning cathodoluminescence images confirm unambiguous photon confinement in a resolution-limited area 20 × 20 nm2 on top of the nano gap. KEYWORDS: Plasmonics, nano-optics, 3D gap-plasmon antennas, 3D nanofabrications
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lateral gap sizes (or horizontal photon squeezing in two dimensions). However, along the vertical direction, photon energy spreads rather uniformly over the thickness of metal film. In order to squeeze photons more tightly and maximize the photon density, three-dimensional (3D) photon confinement is the ultimate goal. However, because of the challenges involved in near-field engineering of tiny antenna structures and 3D nanofabrications, genuine 3D gap-plasmon antennas have yet to be demonstrated. In this Letter, we propose and demonstrate a 3D gapplasmon antenna that enables extreme photon squeezing on a chip within a volume of 1.3 × 10−7 λ3 (∼4 × 10 × 10 nm3) with an intensity enhancement of 400 000. To achieve this feat, we introduced carefully engineered 3D 4 nm air-gaps at the center of complementary nanodiabolo structures. Then, we fabricated 3D gap-plasmon antennas by ion-milling a 100 nm-thick gold film along all three dimensions using proximal techniques. Further, we experimentally confirmed strongly enhanced field intensity and tight field localization by comparing nonlinear second-harmonic signals and taking cathodoluminescence images. Gap-plasmon antennas based on fundamental metal− insulator−metal (MIM; Au−air−Au) antisymmetric SPP modes were chosen in our study (Figure 1 and Supporting Section 1-1). The MIM-type structure has no cutoff frequency
ecent advances in nanofocusing techniques have allowed us to focus light into deep-subwavelength space by utilizing surface plasmon polaritons (SPPs).1−9 The resultant strong field enhancement has opened up various potential applications, such as super-resolution optical imaging,10−12 sensitive photodetection,13 nanoscale high-speed optical communication,14−23 single molecule spectroscopy,10,24−26 and efficient single photon sources. 27,28 Among many plasmonic devices, gap-plasmon antennas with nanometer-size air-gaps,29−35 such as dipole/bowtie antennas29−33 and nanohole/pore antennas,34,35 have been investigated competitively by many research groups because of the huge enhancement of electric fields in the vicinity of the metallic nanogaps. Thanks to recent fabrication techniques such as focused-ion-beam (FIB) milling36−39 and electron beam lithography (EBL),40−43 impressive progress has been made in gap-plasmon antennas, including achievement of lateral gap sizes below 10 nm. In 2014, Kollmann et al. developed gold bowtie antennas with 6 nm gaps and single-nanometer accuracy by employing Ga+-FIB and milling-based He+-ion lithography (HIL) techniques.29 They achieved photon squeezing in an area approximately 6 × 10 nm2 and observed 3-fold enhancement of nonlinear intensity from a 6 nm-gap antenna compared to that from a 20 nm-gap antenna. In 2012, photoresist-based EBL was also used to fabricate single and array antennas with gap sizes below 10 nm under special fabrication conditions of high-energy electron beams (>100 keV) and thin SiN membrane substrates.40 To increase the amount of field enhancement, most recent studies of gap-plasmon antennas have focused on reducing the © XXXX American Chemical Society
Received: March 28, 2015 Revised: May 19, 2015
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Figure 1. (a) Schematic of the 3D gap-plasmon antenna. The 3D nanodiabolo structure squeezes photons horizontally at the central air-gap in the xy-plane and vertically along the z-direction. (b) |E|2 profiles of the 3D gap-plasmon antenna in the xy-, xz-, and yz-planes. The size of the central airgap (g) is 5 nm, and the resonant wavelength is 1560 nm. A nanocandle-like extreme photon density profile is observed.
Figure 2. (a) Cross-sectional |E|2 profiles in the xy-plane with different central air-gaps (g = 150, 100, 25, 5 nm). Here, W, L, and vertical taper angle (θ) are 150 nm, 350 nm, and 80°, respectively. (b) Cross-sectional |E|2 profiles in the xz-plane at the center of the antenna with different vertical taper angles (θ = 90° and 80°). (c) Mode volume and |E|2 enhancement as a function of g at a fixed θ of 80°. When g = 4 nm, the mode volume and |E|2 enhancement are calculated to be 1.3 × 10−7 λ3 and 400 000, respectively. Here, all the simulation data were taken at resonant wavelengths.
of the antenna were 5 and 150 nm, respectively. We chose the length of antenna L to be 320 nm in order to match the resonance with the pump laser wavelength. The central air-gap g is the most critical parameter in reducing the mode volume (Supporting Section 2). Considering the fabrication limit, we chose g to be 5 nm in the simulations. In our computations, the SiO2 layer in the air-gap was set to be slightly sunk to reflect the overmilling effect of the SiO2 layer underneath the air-gap, as shown in Figure 1b. Figure 2a shows the cross-sectional |E|2 profiles along the xyplane with different central air-gaps. This figure exhibits photon squeezing with decreasing central air-gap g, at a fixed θ of 80°. The SPP resonant mode originates from two counterpropagating antisymmetric MIM SPP guided modes polarized along the x-direction. When the shape of a gap-plasmon antenna is made rectangular (g = w = 150 nm), the amplitude of the electric field along the y-direction has a cosinusoidal distribution (Figure 2a). However, as one modifies it into a diabolo shape (g < w), the electric field tends to be concentrated in the vicinity of the central gap. When the central air-gap g is reduced from 150 to 5 nm, the modal area
and is generally robust against thermal issues. To construct 3D diabolo nanostructures, we tapered 100 nm-thick gold film along both the horizontal and vertical directions, as schematically shown in Figure 1a. The 3D nanodiabolo structure squeezes photons horizontally at the central air-gap in the xyplane and vertically along the z-direction. The width (W) and length (L) of the diabolo shape in Figure 1a were set to 320 and 150 nm, respectively. The horizontal taper angle (φ in Figure 1b) was less than 30° and reduced losses in the taper section (Supporting Section 1-3). The vertical tapering (θ in Figure 1b) was introduced to push photons down to the bottom of the V-groove. When θ = 80°, the mode volume is reduced by a factor of 12 with respect to that of the 90° structure (Supporting Section 1-2). The proposed 3D gap-plasmon antennas are characterized by three-dimensional (3-D) finite-difference time-domain (FDTD) methods. As shown in Figure 1b, a nanocandle-like extreme photon density profile was obtained. The mode volume was calculated to be 3.2 × 10−7 λ3, where λ is the resonant wavelength of 1560 nm and the quality factor is seven (Supporting Section 2). The central air-gap g and the width W B
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Figure 3. (a) Proximal focused-ion-beam milling technique. By controlling the distance s between two triangular milling patterns, the FIB resolution limit is improved and 3D nanodiabolo shapes are fabricated with minimum feature sizes of 4 nm. (b) Top and 45°-tilted SEM images of the fabricated 3D gap-plasmon antenna and (c) 85°-tilted SEM.
Figure 4. (a) Second-harmonic generation (SHG) measurement setup. (b) SEM images of 3D gap-plasmon antennas with g of 4, 19, and 63 nm. (c) EMCCD images of the generated SHG signals for antennas with different g and polarization directions. (d) SHG intensities as a function of polarization directions. (e) Measured and calculated SHG intensities as a function of g. The inset shows a typical spectrum of a gap-plasmon antenna. Each air-gap value g contains 27 measured data points taken from nine groups of different gap-plasmon antennas.
decreases from 3.0 × 10−3 λ2 to 5.3 × 10−5 λ2 (a 56-fold reduction). Similarly, vertical photon squeezing also occurs in the vertical V-groove. Figure 2b shows side views of the |E|2 distribution in the xz-plane at the center of the antenna. The vertical taper pushes photons downward to the narrowest airgap, where the surface charge density is highest. Here, the width of the top air-gap is fixed to 40 nm and the vertical angle θ is controlled. As θ is varied from 90° to 80°, the modal area along the xz-plane is reduced from 5.2 × 10−4 λ2 to 1.9 × 10−5 λ2 (a 25-fold reduction). In addition, compared to the structure with g = 5 nm and θ = 90°, the modal area of the 3D antenna with g = 5 nm and θ = 80° is five times smaller (Supporting Section 1-2).
The mode volume and |E|2 enhancement of the 3D gapplasmon antenna are plotted as a function of g in Figure 2c. In the plot, θ is fixed to 80°. The pump beam (beam diameter = 1.0 μm) is illuminated from the SiO2 substrate side for the better coupling with the antenna and the higher field enhancement (Supporting Section 2-3). The |E|2 enhancement is defined as the ratio between the maximum |E|2 values of the antenna mode and the incident wave. Photon confinement is clearly noticeable when g is less than 20 nm. When g is reduced from 100 to 2 nm (a 50-fold decrease), the mode volume decreases from 7.5 × 10−5 λ3 to 3.4 × 10−8 λ3 (a 2205-fold decrease) and the |E|2 enhancement increases from 1.3 × 103 to 2.0 × 106 (a 1538-fold increase). For an antenna with g = 4 nm, the mode volume and |E|2 enhancement are calculated to be 1.3 C
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Figure 5. (a,b) SEM and SEM-CL images of the 3D gap-plasmon antennas with g of approximately 10 and 60 nm, respectively. (c) CL intensity profiles at the center of the antenna along the x- and y-directions. The inset is the SEM-CL profile overlapped with the SEM picture, taken from a sample with g ≈ 10 nm.
× 10−7 λ3 and 400 000, respectively. The |E|2 enhancement is dependent on the mode volume, coupling efficiency, and joule losses. Here, the coupling efficiency is estimated to be 20%, and the ratio of the metal absorption among the total loss from the antenna is 25%. We fabricated 3D gap-plasmon antennas on 100 nm-thick gold films deposited on fused quartz substrate using focused ion-beam (FIB) milling techniques. The Ga+-based FIB equipment typically has patterning resolution greater than 10 nm, which is limited by the size of the ion-beam and the Gaussian proximity effect.44 Although the He+-based FIB machine could provide spatial resolution less than 5 nm,29 its high price and rarity prevented us from easily accessing it. In our experiment, we took advantage of proximal milling techniques in Ga+-based FIB milling. As shown in Figure 3a, we intentionally off-designed the milling pattern to take proximity effects into account. By controlling the distance (s) between two triangular milling patterns (Figure 3a), we improved the FIB resolution limit (Supporting Section 3) and were able to fabricate 3D gap-plasmon antennas with minimum feature sizes of sub-5 nm, as shown in Figure 3b. The smallest gap size was measured to be 4 nm from the scanning electron microscope (SEM) image. The length (L) and width (W) of the fabricated antenna were 320 and 160 nm, respectively. The 100 nm-thick gold film sputtered on fused quartz substrate had a surface roughness of 0.8 nm. This gold film was opaque enough to block direct transmission through the film. The vertical taper angle of approximately 80° along the z-direction was formed through the proximity effect, as shown in Figure 3c. To take this image of a highly tilted sample (85°), we removed the base of the bottom triangular part sufficiently through additional ion milling (Supporting Section 3). The second-harmonic generation (SHG) measurement was performed in a confocal setup (Figure 4a) to study the electricfield enhancement. A femtosecond pulsed laser (λ = 1.56 μm) was used as a pump source with approximately 3 kW peak power. The pump beam was focused from the SiO2 side, and the spot diameter was approximately 2 μm. Nonlinear SHG signals were collected from the air side. The far fields were monitored using an electron-multiplication charge-coupled
device (EMCCD) with a narrow bandpass filter (λpass = 780 ± 10 nm) (Supporting Section 4). Nonlinear SHG intensities were measured as a function of the central air-gap g to compare the strength of the electric fields in the gap. All the SHG data were taken from a single 3D gap-plasmon antenna for fair comparison. Three samples with g of 4, 19, and 63 nm, respectively, were prepared (Figure 4b) and tested (Figure 4c). The incident pump beam was polarized along the x-direction, which is the polarization of the fundamental antenna mode in Figure 2. The SHG intensity was very weak for large-gap samples (g = 63 nm). However, for g = 19 nm, the SHG signals were detectable by an EMCCD. For the antenna with g = 4 nm, the SHG signal was more than 100 times (>1000-times) brighter than that with g = 19 nm (g = 63 nm). The distinct polarization dependency of the SHG intensity (Figure 4d) agrees with that of the fundamental antenna mode. The polarization extinction ratio of the SHG signal was more than 1/10 000. The relative intensities of both measured and calculated SHG are plotted as a function of g in Figure 4e. The calculation of SHG intensities was conducted by integrating |En|4 over the metallic medium,45 where En is the electric field component normal to the metal surface. SHG signals from the antennas with g > 100 nm were too weak to be detected with our EMCCD. However, for the antenna with g < 100 nm, the SHG intensity grew stronger than the detection limit of the EMCCD and was very bright for the sample with g < 20 nm. Overall, the behaviors of measured SHG data were congruent with those of the simulations, proving that the enhanced SHG intensities at a small g originate from the strong field enhancement of the 3D gap-plasmon antenna. The SHG intensity from a 4 nm-gap antenna was measured to be 2.7 × 104-times stronger than that from a 100 nm-gap antenna. In other words, the ratio of the electric-field intensities between the 4 nm-gap and 100 nm-gap antennas is 1.6 × 102, which is also in agreement with that of the simulation in Figure 2c. The asymmetry of the vertical-taper is one of the main reasons responsible for the strong SHG effect.46 In order to directly confirm photon localization in the 3D gap-plasmon antenna, we employed the scanning cathodoluminescence (CL) technique (Supporting Section 5) with a D
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spatial resolution of 20 nm.47 The measured SEM images and SEM-CL profiles of the 3D gap-plasmon antennas with g of approximately 10 and 60 nm are shown in Figures 5a,b, respectively. The CL signals originated from the quartz substrate supporting the antenna and were enhanced by the gap-plasmon antenna mode. In order to observe the CL emission from the quartz at wavelength λ < 1.0 μm, 3D gapplasmon antennas with a shorter length of 170 nm were newly prepared to shift the resonant wavelength to approximately 900 nm. Note that in Figure 5a,b only the samples with g = 10 nm produce resolution-limited CL images. CL intensity profiles along the x- and y-directions are plotted in Figure 5c. Note that photons in the antenna are confined along both the x- and ydirections. Full-width at half-maximum of the CL intensities is less than 20 nm, limited by the SEM. The inset in Figure 5c is the SEM-CL profile of the 3D gap-plasmon antenna with g of approximately 10 nm, overlapped with the corresponding SEM image. It is an unambiguous and direct demonstration of sub-20 nm photon localization in a 3D gap-plasmon antenna. In summary, we proposed and demonstrated a 4 nm-gap 3D gap-plasmon antenna that confines photons three-dimensionally in a volume of 1.3 × 10−7 λ3 and enhances the field intensity by a factor of 400 000. By tapering the metallic nanostructure along both the horizontal and vertical directions, a 3D nanocandle with a mode volume of ∼4 × 10 × 10 nm3 was demonstrated. From the 4 nm-gap plasmon antenna, the SHG signal was enhanced by a factor greater than 27 000 compared to that of a 100 nm-gap antenna. In addition, we demonstrated direct and unambiguous sub-20 nm imaging from a 3D gap-plasmon antenna via CL imaging techniques. We strongly believe that our 3D gap-plasmon antennas could open a new research theme based on 3D nanometallic structures and small space nonlinear optics.
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ASSOCIATED CONTENT
S Supporting Information *
Photon squeezing in a metal−insulator−metal (MIM) mode; mode characteristics of 3D gap-plasmon antenna; proximal focused ion-beam milling; second-harmonic generation from 3D gap-plasmon antennas; cathodoluminescence imaging. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b01204.
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Letter
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (2014M3C1A3052567, 2007-0093863 and 2013K1A1A2035662). M.K. acknowledges support received from the Basic Science Research Program through NRF (2014R1A1A1008604). C.A. acknowledges support received from IBS-R004-D1 and the Global Frontier Project (CAMM2014M3A6B3063706) through NRF. Y.C. and S.G. acknowledge support from NRF (2013R1A2A1A01016914). E
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