On-Chip Multiple Electromagnetically Induced Transparencies in

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On-chip multiple electromagnetically-induced transparencies in photon-plasmon composite nanocavities Zhen Chai, Xiaoyong Hu, Chong Li, Hong Yang, and Qihuang Gong ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00399 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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On-chip multiple electromagnetically induced transparencies in photon-plasmon composite nanocavities

Zhen Chai1, Xiaoyong Hu1,2∗∗, Chong Li1, Hong Yang1,2 and Qihuang Gong1,2

1

State Key Laboratory for Mesoscopic Physics & Department of Physics, Collaborative Innovation Center of Quantum Matter, Peking University, Beijing 100871, P. R. China

2

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, P. R. China

∗

Corresponding author. Email: [email protected] ACS Paragon Plus Environment

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ABSTRACT

Chip-integrated multiple electromagnetically-induced transparencies are realized

using a photon-plasmon composite nanocavity (composed of a single photonic crystal nanocavity coupled with a single plasmonic nanocavity), i.e. comprehensively utilizing the superiorities of photonics as well as plasmonics. The destructively interferential coupling of one broad-band plasmonic nanocavity mode with two narrow-band silicon photonic crystal nanocavity modes generates two electromagnetically-induced transparency windows. The two transparency windows simultaneously have a narrow linewidth of 15 nm, a high transmission contrast of 70%, and a strong slow light effect. A large group refractive index of more than 400 is obtained at the transparency window center, indicating a tenfold increase in comparison with earlier results. The photon-plasmon composite nanocavity has a feature size of only 2 µm. The transparency window shifts 30 nm for the photon-plasmon composite nanocavity covered with a monolayer of graphene excited by a -30 V external voltage. The strategy offers one approach to constructing multifunctional integrated photonic circuits.

Keywords: On-chip multiple electromagnetically-induced transparencies, Photon-plasmon composite nanocavity, Single layer graphene, Integrated photonic circuits

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As an essential strategy to achieving slow light effect, electromagnetically-induced transparency (EIT) has important applications in nanoscale optoelectric devices.[1] Various schemes have been proposed to demonstrate EIT effects in both photonics and plasmonics, such as using atomic systems,[2-4] ultrahigh quality-factor dielectric microsphere (or microtoroid) resonators,[5-7] metamaterials,[8-10] metallic nanoparticle arrays,[11,12] and asymmetrically split rings.[13,14] Atomic systems as well as microsphere (or microtoroid) resonators (having a large feature size of more than 100 microns) are very difficult to be integrated into a photonic chip.[2-7] Plasmonic microstructures, including metamaterials, metallic nanoparticle arrays, and asymmetrically split ring matrixes, have a large lateral dimension around 100 µm order, and only permit the vertical incidence of the signal light, i.e. providing off-chip EIT effect.[8-14] On-chip EIT (also called chip-integrated EIT) could directly provide EIT effect in integrated photonic circuits.[15] Several essential characteristics, i.e., small feature size, strong slow light effect, and multiple transparency windows, are stringent requirements for on-chip EIT microstructures.[15] Traditionally, in the field of photonics, various photonic microstructures, including dielectric microring resonators,[16-18] photonic crystal (PC) microcavities,[19.20] and self-coupled optical waveguide resonators[21,22] are adopted to demonstrate on-chip EIT. Even though a strong slow light effect with a group refractive index (defined as the quotient of group velocity dividing light velocity in vacuum) of larger than 1000, and a narrow linewidth for the transparency window of less than 10 nm, can be obtained, the large feature size, on the order of dozens of microns, restricts the practical on-chip integration applications of these photonic microstructures.[16-22] In the field of plasmonics, plasmonic nanostructures consisting of two coupled plasmonic asymmetric nanocavities are conventionally used to demonstrate on-chip EIT based on the phase-coupled mechanism[23,24] or the coupled-microcavity mechanism.[25-27] Although plasmonic nanostructures possess the unique properties of small feature size, on the order


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of hundreds of nanometers, strong light confinement into the subwavelength scale, and enormous near-field enhancement, the slow light effect is weak because of intrinsic Ohmic losses of noble metals.[23-27] More often than not, the group refractive index is less than 100.[27,28] Usually, only one transparency window can be obtained in the abovementioned photonic and plasmonic microstructures.[16-28] The electrical tunability of on-chip multiple EIT transparency windows using a monolayer of graphene plays an important role in ultrahigh speed on-chip optical connection systems and cascaded logic processing chips. However, an on-chip EIT microstructures simultaneously having a small feature size, strong slow light effect, and multiple transparency windows are difficult to obtain when using photonics (or plasmonics) alone. In this letter, on-chip multiple EIT windows are realized using a photon-plasmon composite nanocavity via integrated usage of the superiorities of photonics as well as plasmonics. The photon-plasmon composite nanocavity (Fig. 1(a)) consists of a plasmonic nanocavity coupled with a two-dimensional silicon photonic crystal nanocavity via a hybrid waveguide (formed by an air groove between a 220-nm-thick gold film and a 220-nm-thick two-dimensional silicon photonic crystal). Only the unaltered photonic crystal can be used to construct the hybrid waveguide. The plasmonic nanocavity provides a broad-band nanocavity mode, while the two-dimensional silicon photonic crystal nanocavity provides three narrow-band nanocavity modes. Two EIT windows are obtained simultaneously based on destructively coherent coupling of one broadband plasmonic nanocavity mode with two narrow-band silicon photonic crystal nanocavity modes. The narrow linewidth of 15 nm for the transparency window, large transmission ratio contrast of 70%, and strong slow light effect are achieved simultaneously for the two transparency windows. The group refractive index at the transparency window center reaches more than 400, i.e. a tenfold increase in comparison with earlier results using coupled plasmonic asymmetric nanocavities.[23-28] Moreover,


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an ultracompact feature size of 2 µm is maintained for the photon-plasmon composite nanocavity. When the photon-plasmon composite nanocavity is covered with a single layer of graphene, the center of the transparency window moves 30 nm under excitation of an applied voltage of -30 V. The fabrication process related to the photon-plasmon composite nanocavity is given in detail in the Supplementary Information. The plasmonic nanocavity (Figs. 1(b) and (c)) comprised a single 265-nm-long, 120-nm-wide, and 220-nm-deep air nanogroove fabricated within a gold film having a thickness of 220 nm. The two-dimensional silicon PC comprised periodic arrays of triangular lattices of air holes embedded within a silicon slab having a thickness of 220 nm, as shown in Figs. 1(a)–(c), which is the type of photonic crystal studied for proving the phenomenon of on-chip multiple EITs in a single planar photon-plasmon composite nanocavity. The silicon PC had a lattice constant of 420 nm, air hole radius of 120 nm, and air hole depth of 220 nm. There was an interval of 230 nm from the uppermost air-hole centers to the hybrid waveguide. Only the altered photonic crystal could be used to construct the photonic crystal nanocavity, which was formed by removing three air holes in line and adjusting six neighboring air holes, i.e., the radius of the six neighboring air holes was reduced to 108 nm; the centers of air holes 1 and 6 shifted 50 nm in the horizontal direction, while that of air holes 2 and 5 (Figs. 1(b) and (c)) shifted 50 nm in the vertical direction. There was an interval of 200 nm from air hole 1 of the silicon photonic crystal nanocavity to plasmonic nanocavity along horizontal axis. Hybrid waveguide, supporting photon-plasmon hybrid modes, had a width of 200 nm and depth of 220 nm. The waveguide properties of the hybrid waveguide are discussed in detail in the Supplementary Information. Yu et al pointed out that the propagation loss of hybrid waveguides were less than that of traditional plasmonic slot waveguides.[29] The calculated coupling efficiency (Fig. 1(e)) with different wavelengths of signal light was obtained for the input-coupling port, simulated based on the finite-element method. The


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input-coupling efficiency of more than 12% (from 1500 to 1650 nm) was reached, with a maximum input-coupling efficiency of 16% at 1550 nm, indicating a superb input-coupling function. The dispersion relations (Fig. 1(f)) were simulated based on the finite element method for the hybrid waveguide, using a refractive index of 3.45 for silicon and 1 for air. The complex dielectric function of gold was extracted from measured results of Johnson et al.[30] It is clear that the photon-plasmon hybrid waveguide could provide wideband guide modes. For the composite nanocavity sample (shown in Fig. 1(c)), the linear transmission spectrum (Fig. 2(a)) was measured using a micro-spectroscopy measurement system. We adopted a continuous wave (CW) fiber laser system (Model TSL-510, SANTEC Company, Japan) with a tunable wavelength range from 1500 to 1635 nm. The diameter of the beam output from the fiber laser system was shrunk down to 3 µm. Then the laser beam propagated through the substrate (shown in Fig. 1(d)) and beat down on the input-coupling port. The scattered light from the output-coupling port was gathered by an objective lens (Mitutoyo 20, NA=0.58) before detected by a charge coupled device (CCD). We normalized the linear transmittance using a control sample with the identical parameters, while not etching the photon-plasmon composite nanocavity.[31] Two large transmittance peaks were obtained in the stop band, indicating the generation of EIT.[31] The physical mechanism lies in the destructively interferential coupling of two photonic crystal nanocavity modes with the plasmonic nanocavity mode.[31] The linewidth, transmittance, and central wavelength of the transmission peak were 15 nm, 76%, and 1620 nm, respectively, for transparency window 1. The transmission, central wavelength, and linewidth of the transmission peak were 80%, 1508 nm, and 15.5 nm, respectively, for transparency window 2. The measured results correspond to those simulated ones (Fig. 2(b)) utilizing the finite element method (using a software COMSOL Multiphysics). Our calculated linewidths (4 nm for window 1, and 7 nm for


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window 2) was less than the measured ones. This result originates from the imperfectly fabricated sample, including the polycrystalline configuration of the gold film and imperfectly etched photonic crystal structure, which induce additional losses. Therefore, two transparency windows were realized in a photon-plasmon composite nanocavity possessing a small lateral dimension around 2 µm (shown in Fig. 1(c)). The magnetic-field distributions (Figs. 2(c)-(g)) in photon-plasmon composite nanocavity for a CW signal light at various wavelengths were simulated utilizing the finite element method. For signal light with wavelength of 1620 nm (situated in the peak of window 1) or 1508 nm (situated in the peak of window 2), magnetic-field distributions (Figs. 2(c) and (e)) were localized within both plasmonic nanocavity and photonic crystal nanocavity, and the signal light can propagate through the hybrid waveguide. For signal light with wavelength of 1547 nm or 1450 nm (both situated in the forbidden band), magnetic-field distributions (Figs. 2(d) and (f)) were mostly localized within the plasmonic nanocavity, while signal light could not propagate through the hybrid waveguide. Therefore, the formation of transparency windows was related to the plasmonic nanocavity modes and the silicon photonic crystal nanocavity modes. We also simulated the group refractive index (Fig. 2(g)) with different incident light wavelengths for the hybrid waveguide coupled with the photon-plasmon composite nanocavity utilizing the finite element method. The simulated group refractive index reached 410 at a wavelength of 1620 nm (situated in the peak of window 1) and 470 at 1508 nm (situated in the peak of window 2). This indicates that an on-chip EIT microstructure with small feature size, strong slow light effect, and two transparency windows was realized. The linear transmittance spectrum (Fig. 3(a)) of a hybrid waveguide coupled with a single plasmonic nanocavity (without PC nanocavity) in the lateral side, in order to study the resonant responses of a single plasmonic nanocavity, was simulated utilizing the finite element method. A


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broad transmission forbidden band (centered at 1500 nm) having a wavelength range of 200 nm was obtained, which corresponds to the wideband resonance mode of the plasmonic nanocavity. The magnetic-field distributions (Fig. 3(b)) at 1500 nm for the hybrid waveguide coupled with the plasmonic nanocavity in the lateral side were also calculated, which were mostly localized within plasmonic nanocavity, while signal light could not propagated through hybrid waveguide. Therefore, the wideband transmission forbidden band originates from the plasmonic nanocavity mode. To study the resonant responses of the single photonic crystal nanocavity, the linear transmittance properties (Fig. 3(c)) of hybrid waveguide only coupled with the silicon photonic crystal nanocavity (without plasmonic nanocavity) was calculated utilizing the finite element method. Three transmission minimums appeared in the linear transmission spectrum, centered at 1620, 1508, and 1400 nm. The calculated magnetic-field distributions (Figs. 3(d)-(f)) of hybrid waveguide coupled with the silicon photonic crystal nanocavity in the lateral side at wavelengths of 1620, 1508, and 1400 nm were mostly confined in the silicon photonic crystal nanocavity, and no signal light propagated through the hybrid waveguide. Only altered photonic crystals were used in Fig. 3(d)–(f). The silicon photonic crystal nanocavity can provide three nanocavity modes, with resonant wavelengths of 1620, 1508, and 1400 nm. The linewidths were 1, 9, and 3 nm for the silicon photonic crystal nanocavity modes with resonant wavelengths of 1620, 1508, and 1400 nm, respectively. These results confirm that the measured transparency windows centered at 1620 nm and 1508 nm originate from destructively interferential coupling of wideband plasmonic mode with narrow-band silicon photonic crystal nanocavity modes. The silicon photonic crystal nanocavity mode with a resonant wavelength of 1400 nm was very close to the transmission pass band edge, as shown in Fig. 3(c). Therefore, this transparent window was generated through destructively interferential coupling of 1400-nm resonance silicon photonic crystal nanocavity mode with the


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plasmonic nanocavity mode, and dropped in the transmission pass band edge. This is why only two transparency windows were obtained. To study the tunability of the on-chip EIT, we deposited a 10-nm-thick HfO2 layer on the upper surface of the photon-plasmon composite nanocavity sample; then, a monolayer graphene was deposited on the HfO2 layer, as shown in Fig. 4(a). The fabrication process is given in detail in the Supplementary Information. The 10-nm-thick HfO2 layer was used as a dielectric insulation layer that separates the graphene from the gold film and silicon photonic crystal. An external electric voltage was applied between the uppermost gold electrode and the bottom silicon substrate. The linear transmittance spectra (Fig. 4(b)) were calculated for a hybrid waveguide coupled in the lateral side with photon-plasmon composite nanocavity coated with HfO2 layer and a single layer of graphene under various applied voltages. There was a transparency window centered at 1590 nm in the linear transmission spectrum without applied voltage. The HfO2 and graphene layers resulted in an increase in the ambient dielectric material around the plasmonic nanocavity and an increment in the effective refractive-index of silicon PC nanocavity because of a large linear refractive index of HfO2, 1.95, and that of graphene, 2.4, in the optical communication range.[32] This makes the plasmonic nanocavity mode and the silicon photonic crystal nanocavity modes move in the direction of low frequency.[24] Accordingly, transparency windows caused red-shift. Therefore, transparency window 2 moved to 1590 nm when the photon-plasmon composite nanocavity was coated with an HfO2 layer and a single layer of graphene. The transparency window center shifted from 1590 nm to 1560 nm if applied voltage decreased from zero to −30 V, as shown in Fig. 4(b). Hanson and Xu et al. noted that the linear dielectric characteristics of graphene decreases under the excitation of a negative applied voltage in the optical communication range.[33,34] The measured data corresponds to those simulated ones (Fig. 4(c)). Therefore, the transparency window center moves


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30 nm under an applied voltage of -30 V. According to our calculations, when a small change of less than 10 nm was introduced in the plasmonic nanocavity, and a small change of less than 15 nm was introduced in the air hole radius (or lattice constant) of the photonic crystal nanocavity, perfect multiple narrow EIT windows could be obtained. Because of the protective effect of the single layer of graphene, possible contaminations with a size of less than 30 nm did not influence the perfect multiple EIT windows. This indicates that the multiple narrow EIT windows in the photon-plasmon composite nanocavity sample were very robust, reliable, and immune to small changes in dimensions and possible contamination. In summary, an ultracompact on-chip EIT was realized using a photon-plasmon composite nanocavity directly integrated into photonic circuits. Two transparency windows were obtained, with strong slow light effect, a narrow linewidth of 15 nm, and high transmission contrast of 70%. Group refractive index at transparency window center reached more than 400, and an ultracompact feature size of 2 µm was maintained. When a monolayer graphene coating photon-plasmon composite nanocavity, the transparency window center moved 30 nm under an applied voltage of −30 V. The strategy offers one approach to constructing multifunctional integrated photonic circuits.

Acknowledgements

This work was supported by the 973 Program of China under grant nos. 2013CB328704 and 2014CB921003 and the National Natural Science Foundation of China under grant nos. 11225417, 61475003, 11134001, 11121091, and 90921008.


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Figure Captions Figure 1. Three-dimensional schematic structure (a) and top-view schematic structure (b) of the photon-plasmon composite nanocavity coupled with a photon-plasmon-hybrid waveguide. (c) Top-view SEM image of the photon-plasmon composite nanocavity coupled with the hybrid waveguide. Inserts show the enlarged areas labeled using a square. (d) Three-dimensional schematic structure of the photon-plasmon composite nanocavity sample. The arrow indicates the incident direction of the signal light. (e) Calculated coupling efficiency of the input-coupling port as a function of signal light wavelength. (f) Calculated dispersion relations of the hybrid waveguide. Figure 2. Measured (a) and calculated (b) linear transmission spectra of the hybrid waveguide coupled with the photon-plasmon composite nanocavity. Calculated magnetic field distributions of the photon-plasmon composite nanocavity under excitation by a CW incident light at wavelengths of 1620 nm (c), 1547 nm (d), 1508 nm (e), and 1450 nm (f). (g) Calculated group refractive index as a function of incident light wavelength for the hybrid waveguide coupled with the photon-plasmon composite nanocavity. Figure 3. Calculated linear transmission spectra (a) and magnetic field distribution at 1500 nm (b) of the hybrid waveguide side-coupled to the plasmonic nanocavity. Calculated linear transmission spectra (c) and magnetic field distribution of the hybrid waveguide side-coupled to the silicon photonic crystal nanocavity at wavelengths of 1620 nm (d), 1508 nm (e), and 1400 nm (f). Figure 4. (a) Top-view SEM image of the photon-plasmon composite nanocavity sample covered with a single layer of graphene. Measured (b) and calculated (c) linear transmission spectra of the hybrid waveguide side-coupled to the photon-plasmon composite nanocavity covered with 10-nm-thick HfO2 and a single layer of graphene under external electric voltages of 0, −10 V, −20 V, and −30 V. ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)


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ACS Photonics

(b)

(c)

(d) ACS Paragon Plus Environment

ACS Photonics

(e) 0.20

Coupling Efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.15 0.10 0.05 0.00

1400

1600

1800

Wavelength(nm)


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Frequency(THz)

(f) 280 260 240 220 200 180 160 0.0

0.1

0.2

0.3

0.4

0.5

K (2π/ π/a) π/ x Figure 1 Zhen Chai

1.0

1.0

(a)

2

1

Transmission

0.8

Transmission

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ACS Photonics

0.6 Measured 0.4 0.2 0.0

1400

1500

1600

1700

0.8

1620 nm

Au

1

2

0.6 0.4 0.2 0.0

Wavelength (nm)

(c)

(b)

Calculated

1400

H

Si photonic crystal


1500

1600

Wavelength (nm)

1700

ACS Photonics

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(d) 1547 nm

Au

H

Si photonic crystal

(e) 1508 nm

Au

H

Si photonic crystal

(f) 1450 nm

Au

H

Si photonic crystal


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ng

400

(g)

2

1

200

0

-200

1400

1500

1600

1700

Wavelength (nm)

Figure 2 Zhen Chai

0.6

Transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

(a)

Only SPP cavity

0.4

0.2

0.0

1400

1500

1600

1700

Wavelength (nm)

(b) 1500 nm

H


ACS Photonics

Au Si photonic crystal

1.0

Transmisssion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8 0.6 0.4

3

0.2

Only PC Cavity

0.0

1400

1500

1

2

(c)

1600

1700

Wavelength (nm)

(d) 1620 nm

Au

H

Altered Si photonic crystal

(e)


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ACS Photonics

Au

1508 nm

H


(f) Au

1400 nm

H


Figure 3

Zhen Chai

(a)


ACS Photonics

1.0

Transmission

0.8 0.6 0.4

(b)

Measured

0V -10 V -20 V -30 V

0.2 0.0

1500

1550

1600

1650

Wavelength (nm)

1.0

Transmission

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0.8

(c)

0V -30 V

Calculated

0.6 0.4 0.2 0.0

1400

1500

1600

1700

Wavelength (nm) Fiure 4 Zhen Chai

For Table of Contents Use Only


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

On-chip multiple electromagnetically-induced transparencies in photon-plasmon composite nanocavities ACS Paragon Plus Environment

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Zhen Chai, Xiaoyong Hu, Chong Li, Hong Yang, and Qihuang Gong

On-chip multiple electromagnetically-induced transparencies was realized in a photon-plasmon composite nanocavity in integrated photonic circuits directly, realized based on combining the advantages of both photonics and plasmonics. The electrical tunability of the transparency is also reached.


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On-Chip Multiple Electromagnetically Induced Transparencies in

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