Low-loss metal-insulator-semiconductor waveguide with an air core for on-chip integration

Optics Communications 285 (2012) 3604–3607

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Low-loss metal-insulator-semiconductor waveguide with an air core for on-chip integration Jing Xiao, Jiansheng Liu n, Zheng Zheng, Yusheng Bian, Guanjun Wang, Shuna Li School of Electronic and Information Engineering, Beihang University, 37 Xueyuan Rd, Beijing 100191, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 November 2011 Received in revised form 24 April 2012 Accepted 24 April 2012 Available online 17 May 2012

In this paper, a novel metal-insulator-semiconductor waveguide with an air core in the insulator layer is proposed in order to improve the on-chip integration. Compared with the structure with homogeneous insulator, the introduced air core provides an additional degree of freedom for tuning the characteristics of the hybrid plasmonic mode. Simulation results demonstrate that the proposed structure exhibits better tradeoff between the mode confinement and the propagation length. The relatively low crosstalk between adjacent proposed waveguides could be achieved and the coupling efficiency between conventional silicon waveguide and the proposed structure maintain. The proposed waveguide could be used to build ultra-compact photonic components and enable on-chip integration of photonic circuits. & 2012 Elsevier B.V. All rights reserved.

Keywords: Plasmonics Nanostructure Hybrid plasmonic waveguide with air core Low propagation loss Crosstalk Coupling efficiency

1. Introduction The development of future photonic integrated circuits has put increasing demands on the downscaling of physical size of photonic components [1]. During the last decade, surface plasmon polaritons (SPPs) have attracted considerable attention due to its capacity for breaking the diffraction limit and manipulation of light at the subwavelength scale [2]. Much effort has been put on the design and experimental demonstration of various SPP waveguides. However, for many conventional SPP waveguides, the compromise between the mode confinement and propagation loss could not be well balanced. For example, the traditional longrange SPP waveguide could achieve ultra-low loss propagation but come at a price of rather weak mode confinement [3,4]. On the other hand, the metal-insulator–metal structure could support tightly confined plasmonic modes but its high loss has limited its propagation distance, and thus formed a limitation to its practical applications [5–7]. To overcome the above challenge, the recent proposed hybrid plasmonic waveguide consisting of a high-index semiconductor nanowire separated from a metal substrate by a low-index insulator gap, has been proposed [8] and demonstrated [9,10], which features simultaneously subwavelength mode confinement and long-range propagation. Such a novel hybrid concept has also been applied to the design of many

n

Corresponding author. E-mail address: [email protected] (J. Liu).

0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.04.035

other metal-insulator-semiconductor waveguiding structures, including slab [11–13], coaxial [14], dielectric-loaded [15], wedgetype [16] and many other waveguide components [17–23]. Here in this paper, a novel metal-insulator-semiconductor waveguide is been proposed and studied. In contrast to the hybrid structure in [8,10], the insulator layer between the metal surface and the semiconductor structure consists of a dielectric with an air core rather than a homogeneous low-index material. Due to the combination of hybrid and dielectric slot effects, this closeform structure could provide strong field enhancement. Moreover, the propagation loss could be significantly reduced due to the existence of the air core in the insulator layer. The proposed waveguiding structure could be fabricated by using deposition and the air core could be formed by using the chemical etching method, similar as that of the dual-slot waveguides [24].

2. Structure and physical model In the following text, the on-chip integration properties of the proposed hybrid plasmonic waveguide with the air core (HPWC) are investigated systematically. In order to demonstrate the advantage of the proposed waveguide, the traditional hybrid plasmonic waveguide (HPW) without the air core is also investigated comparatively. The geometries of the conventional HPW and proposed HPWC are shown in Fig. 1(a) and (b). For either of the waveguide, the metal substrate is made of silver and the silicon nanowire has a width of a and a height of b. Also, the

J. Xiao et al. / Optics Communications 285 (2012) 3604–3607

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do not interfere with the solutions. According to [25], to ensure strong coupling between the dielectric mode and the plasmonic mode, the size of the silicon strip is chosen as a¼b¼ 200 nm. Simulation results reveal that the proposed HPWC could support a quasi-TM hybrid plasmon polarizon (HPP) mode. The electric field distributions of the fundamental HPP mode of the proposed structure are shown in Fig. 1(c). It is clear that ultra-strong field enhancement could be observed in the air core region. Then the properties of the proposed HPWC are investigated and shown in Fig. 2, which include the propagation length (LP) and the confinement factor (Cf) in the intermediate layer. The propagation length is given by LP ¼ l/[4pIm(neff)] [12]. Cf is defined as the ratio of the power inside the intermediate layer to the total power of the waveguide. In the simulations, the wavelengths of the incident

thickness of the insulator layer is h which is made of silicon nitride with refractive index n1, the width of the air core in it is w.

3. Simulation result and discussion 3.1. The modal property The mode properties are investigated by means of the finiteelement method (FEM) using COMSOLTM. The eigenmode solver is used with the scattering boundary condition which is one of the selectable boundaries in this COMSOLTM. In the model, the simulation area is set as large as 20  20 mm2. Convergence tests were made to ensure that the numerical boundaries and meshing

a

×1010 3 Si

Si

b

n1

Air

2

n1 y z

Ag

x

n1

Air w

n1

1

h

Fig. 1. Schematic diagram of (a) the conventional HPW and (b) the proposed HPWC. (c) 9E9 distributions of the fundamental modes of the proposed HPWC (a¼ b¼ 200 nm, w¼ 120 nm, h ¼ 50 nm).

Fig. 2. (a) The dependence of propagation length (LP) and confinement factor (Cf) for two waveguides on the operating wavelength of 1100 nm–1550 nm; (b) the dependence of Lp and Cf on the width w of the air core.

D

D Max: 1.3e10 1 0 -1 ×1010 0 -1 -2 Min: -2.2e10

Fig. 3. Ey field distributions of (a) the anti-symmetric, two HPWs; (b) the symmetric,two HPWs; (c) the anti-symmetric, two HPWCs and (d) symmetric, two HPWCs with the distance of D ¼ 400 nm.

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light are chosen within the range of 1000 nm–1700 nm. The dielectric permittivity for pure silver was chosen from Ref. [26]. Si are characterized by the optical constants from Ref. [27] while the refractive index of silicon nitride is 1.8. Dashed line shown in Fig. 2(a) exhibit relationship between incident wavelength and confinement factor. Meanwhile, the dependence of propagation length on wavelength is shown with the solid curve. Results indicate that the trend of Cf and Lp is similar for both waveguides. In the HPWC, the propagation length increases 50% while the confinement factor only decreases 9%. In details, HPWC waveguide raised the propagation length from 53.5 mm to 81.0 mm, but the confinement power only dropped from 38% to 35% at wavelength of 1550 nm. The outstanding tradeoff performance of the HPWC waveguide can be explained as follows. First, the relatively long propagation length is due to the presence of the air core which helps to reduce the imaginary part of the effective refractive index. Second, high confinement ability is maintained because of the existence of the closely spaced metal substrate and the silicon strip, and an amount of power could be confined in the nanoscale region.The wavelength is set at l ¼1550 nm, the permittivities of silicon nitride, Si and Ag

Fig. 4. The dependences of the normalized Maximum transfer power and the coupling length on the CTC distance D between two coupled hybrid plasmonic waveguides. rectangle: HPW; circle: HPWC.

-0.2

X (um) 0

are 3.24, 12.25 and  129þ 3.3ni, respectively. The dependence of Lp and Cf on the width w of the air core can be shown in Fig. 2(b).

3.2. The property of crosstalk To determine the ultimate integration density of the planar photonic circuits [12,28–30], the crosstalk between two parallel straight proposed HPWC is investigated. The crosstalk can be determined by the coupling length (Lc) and the maximum transfer power (Pmax). The Lc is the length where the energy completely transfers from one waveguide to the other. The coupling length Lc is calculated by using Lc ¼ p/9ks  ka9, where ks and ka are the wavenumbers of the symmetric and anti-symmetric modes of two coupled waveguides, respectively [30]. The maximum power transfer Pmax in the coupling length Lc is a function of x: [29] Pmax ¼ expð2nxnarctanðx1 ÞÞ=ð1 þ x2 Þ, where x ¼2 Lc/(pLp) and Lp is the mean attenuation length of the symmetric and anti-symmetric modes of two coupled waveguides. The system consisting of two identical waveguides with a center-to-center separation distance D is schematically drawn in Fig. 3. The Ey field distributions of symmetric and anti-symmetric mode is shown in Fig. 3(a), (b) supported by two traditional HPWs and (c), (d) by two HPWCs. According to the equations above, the dependence of Lc and Pmax on the center-to-center spacing D of the two kinds of waveguides are illustrated in Fig. 4. The solid curves are Pmax and the hollow curves are Lc. The Lc increases exponentially as the separation distance D increase and the trend is more obvious in the proposed waveguide. Furthermore, the Pmax decrease with the separation distance D and the proposed HPWC has a more quick decreasing trend. It is clear that the two HPWCs has a much lower crosstalk than the HPWs because the larger LC and smaller Pmax can obtain in the same separation distance D. The lower crosstalk is due to the subwavelength mode confinement in the air core and the much weaker mode overlap between the two HPWCs than that between the HPWs. The above calculated results concerning crosstalk reveal that, the proposed hybrid waveguides could enable higher overall packing density when being used in integrated photonic circuits.

0.2

X (um)

(i) 0.36

-0.1

0

0.1 1

Y (um)

0.25 Y (um)

(ii)

0 -0.15

0.1 0 -0.1

0

0

Y (um)

0.25

1

0 -0.15 0 -3.0

-1.5

0 Z (um)

1.5

3.0

Fig. 5. Ey field distributions for the direct coupling between a silicon waveguide and the HPWC waveguide. (i) the Ey distribution of TM mode in the Si waveguide at Z¼  1.5 mm. (ii) the Ey distribution of cross section in the HPWC waveguide at Z¼ 1.5 mm.

J. Xiao et al. / Optics Communications 285 (2012) 3604–3607

3.3. The coupling efficiency In general, the coupling efficiency between the silicon waveguide and the proposed HPWC waveguide is essential in the integrated circuit [31,32]. To verify this, the coupling characteristic is investigated by a 3D finite-difference time-domain (FDTD) simulation. Fig. 5 depicts the direct coupling process from pure Si stick to the HPWC at wavelength of 1550 nm. The pure Si has a cross section of 400  400 nm and the bottom draw level with the metal interface of the HPWC waveguide. The Ey distribution in the cross-section of Si is shown in panel (i) of Fig. 5, which is stimulated by the TM mode source. The electric field be coupled into the intermediate layer of the HPWC waveguide where the Ey distribution is shown as panel (ii) of Fig. 5. The coupling efficiency can be calculated by the transmission at Z ¼  1.5 mm and 1.5 mm. The values of the coupling efficient is 58% in HPWC and 60% in HPW, respectively. About 60% is the result of the waveguide mode mismatch and interface reflection between pure Si and two kinds of waveguide. The HPWC coupling efficiency is lower than that of HPW. The reason is that the confinement factor in HPWC is decreased slightly by the air-core structure. But the ability of collecting light in the intermediate layer is maintained.

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4. Conclusion

[22]

In conclusion, the characteristics of novel plasmonic waveguides incorporating a air core in the insulator layer have been investigated in this paper. The proposed hybrid waveguide could achieve two-dimensional tight mode confinement with low propagation loss. Investigations on the coupling between adjacent proposed waveguides demonstrate that ultra-low crosstalk could be obtained. Coupling effciencies between conventional silcon waveguide and proposed structure could be as high as 58%. It could play an important role in high-density integration photonic circuits.

[23] [24] [25]

Acknowledgements This work was supported by National Instrumentation Program (2011YQ0301240502), the Aviation Science Foundation of China (2011ZD51049) and 973 Program (2009CB930701).

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