Active metal strip hybrid plasmonic waveguide with low critical material gain

doi:10.1364/OE.20.011487

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Active metal strip hybrid plasmonic waveguide with low critical material gain

Linfei Gao, Liangxiao Tang, Feifei Hu, Ruimin Guo, Xingjun Wang, and Zhiping Zhou »View Author Affiliations

Optics Express, Vol. 20, Issue 10, pp. 11487-11495 (2012)
http://dx.doi.org/10.1364/OE.20.011487

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▸ Article Outline
– Abstract
1. Introduction
2. Waveguide structure
3. Simulation and analysis of the waveguide
4. Conclusion
– References and links

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Abstract

An active metal strip hybrid plasmonic waveguide (MSHPW) using gain materials as loss compensation is proposed with an extremely simple fabrication procedure. Gain materials are introduced either in the low-index layer or in the high-index layer of MSHPW. The effects of waveguide dimensions and material gain coefficients on loss compensation are analyzed at the communication wavelength. For one configuration presented here, a critical material gain as low as 3.8cm⁻¹ is sufficient for fully compensation of the loss when using a high-index gain material. The active MSHPW with low critical material gain opens up opportunities for practical plasmonic devices in active applications such as amplifiers, sources, and modulators.

1. Introduction

Plasmonics is drawing increased attention due to its outstanding capability of confining and controlling light beyond the diffraction limit [1

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

], thereby potentially enabling devices integration in nanoscale [2

E. Ozbay, “Plasmonics: Merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

–4

Q. Min, C. Chen, P. Berini, and R. Gordon, “Long range surface plasmons on asymmetric suspended thin film structures for biosensing applications,” Opt. Express 18(18), 19009–19019 (2010). [CrossRef] [PubMed]

]. As the building block of integrated circuits, various plasmonic waveguides have been proposed [5

D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]

]. However, a typical challenge for plasmonic waveguides is the trade-off between propagation loss and field confinement. In other words, they can perform either low propagation loss with diffused field (e.g., long-range surface plasmon polaritons (LRSPP) [6

R. Charbonneau, N. Lahoud, G. Mattiussi, and P. Berini, “Demonstration of integrated optics elements based on long-ranging surface plasmon polaritons,” Opt. Express 13(3), 977–984 (2005). [CrossRef] [PubMed]

]), or compact mode size at the expense of large losses (e.g., metal-insulator-metal waveguides (MIM) [7

L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express 13(17), 6645–6650 (2005). [CrossRef] [PubMed]

, 8

G. Veronis and S. H. Fan, “Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides,” Appl. Phys. Lett. 87(13), 131102 (2005). [CrossRef]

]). Recently, a new type of plasmonic waveguide called “hybrid plasmonic waveguide (HPW)” was proposed to attempt both low propagation loss and strong field confinement [9

R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics 2(8), 496–500 (2008). [CrossRef]

–14

I. Goykhman, B. Desiatov, and U. Levy, “Experimental demonstration of locally oxidized hybrid silicon- plasmonic waveguide,” Appl. Phys. Lett. 97(14), 141106 (2010). [CrossRef]

Many passive devices based on HPW have been demonstrated either theoretically or experimentally, such as splitters [15

J. Wang, X. Guan, Y. He, Y. Shi, Z. Wang, S. He, P. Holmström, L. Wosinski, L. Thylen, and D. Dai, “Sub-μm² power splitters by using silicon hybrid plasmonic waveguides,” Opt. Express 19(2), 838–847 (2011). [CrossRef] [PubMed]

] and ring resonators [16

D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, “Silicon hybrid plasmonic submicron-donut resonator with pure dielectric access waveguides,” Opt. Express 19(24), 23671–23682 (2011). [CrossRef] [PubMed]

]. By contrast, active plasmonic devices are of critical significance for future integrated plasmonic device technology, and it is beneficial to make active devices based on HPW whose intrinsic modal loss is low. However, there are only some limited researches focusing on active applications of HPW [17

R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature 461(7264), 629–632 (2009). [CrossRef] [PubMed]

–19

J. Zhang, L. Cai, W. Bai, Y. Xu, and G. Song, “Hybrid plasmonic waveguide with gain medium for lossless propagation with nanoscale confinement,” Opt. Lett. 36(12), 2312–2314 (2011). [CrossRef] [PubMed]

]. Furthermore, structures of previously proposed HPW with gain materials are complicated (e.g., semicylinders or ribs with low experimental tolerance are included), thus the feasibility in practice is seriously constrained. Meanwhile, some gain materials are very difficult to be etched, as a result, choices of gain materials for those structures with the etching process are heavily restricted. Consequently, a practical configuration suitable for active plasmonic devices is urgently needed.

In this paper we propose and analyze an active metal strip hybrid plasmonic waveguide (MSHPW) with gain materials in different regions. Compared to the previous work, MSHPW is particularly suitable to introduce gain materials for active applications due to its extremely simple fabrication procedure and low intrinsic modal loss. Effects of waveguide parameters and material gain coefficients on propagation properties are analyzed in detail. Our simulation indicates that the critical material gain to fully compensate the loss in MSHPW is so small that it can be easily obtained by current gain materials.

2. Waveguide structure

The waveguide geometry is shown in Fig. 1 . It is composed of a metal layer,a narrow low-index dielectric gap, a high-index dielectric layer, a buffer layer and substrate. In traditional HPW, the metal layer and the low index layer can be patterned to strips, and the high-index layer is usually patterned to a cylindrical [9

] or rectangular [10

D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express 17(19), 16646–16653 (2009). [CrossRef] [PubMed]

] core to provide lateral confinement. By contrast, in MSHPW, only the metal layer is patterned to strip, while the other layers are slabs (infinite in

x

direction). Therefore, the fabrication procedure for MSHPW is extremely simple.

Fig. 1 Schematic of the active metal strip hybrid plasmonic waveguide. Either the low-index dielectric gap or the high-index dielectric layer can be active region by introducing a gain material with proper refractive index.

On account of the significant status of Si photonics as one of the most promising candidates for optoelectronic integration [20

J. A. Dionne, L. A. Sweatlock, M. T. Sheldon, A. P. Alivisatos, and H. Atwater, “Silicon-based plasmonic for on-chip photonics,” IEEE J. Quantum Electron. 16(1), 295–306 (2010). [CrossRef]

], we choose materials in silicon series to introduce the fabrication procedure. Thus the structure could be fabricated on an SOI wafer and fabrication processes are compatible with CMOS technology. The corresponding substrate, buffer layer and high-index layer are Si, SiO₂ and Si, respectively. Processes we need to do are forming a low-index dielectric gap on the SOI wafer and patterning a metal strip on the low-index dielectric gap. First, the low-index material could be SiO₂ obtained by thermal oxidation of Si or deposition. The next step is to form a strip pattern by photolithography or e-beam lithography. Then deposit a metal layer on the top and obtain a metal strip in lift-off process. One sees that there is no etching process and only once lithography, which may avoid extra losses from fabrication (e.g., etched surface roughness and alignment error) and make the structure quite practical. The waveguide could be characterized by various methods. For example, dielectric waveguides with tapers could be fabricated at the two ends of the waveguide for coupling light in and out, and then the whole system is able to be tested on a common waveguide test platform [13

M. Wu, Z. Han, and V. Van, “Conductor-gap-silicon plasmonic waveguides and passive components at subwavelength scale,” Opt. Express 18(11), 11728–11736 (2010). [CrossRef] [PubMed]

]. Near filed imaging using near-field scanning optical microscope (NSOM) is also feasible [14

I. Goykhman, B. Desiatov, and U. Levy, “Experimental demonstration of locally oxidized hybrid silicon- plasmonic waveguide,” Appl. Phys. Lett. 97(14), 141106 (2010). [CrossRef]

]. In addition, scattered light from the waveguide can be detected by etching slits or grating at the metal layer, which has been proofed effectively [21

R. J. Walters, R. V. A. van Loon, I. Brunets, J. Schmitz, and A. Polman, “A silicon-based electrical source of surface plasmon polaritons,” Nat. Mater. 9(1), 21–25 (2010). [CrossRef] [PubMed]

Though a typical material system is taken for analysis, a wide range of materials, especially gain materials, can be utilized in this waveguide. For example, the low-index gain materials could be PMMA with PbS quantum dots [22

J. Grandidier, G. C. des Francs, S. Massenot, A. Bouhelier, L. Markey, J. C. Weeber, C. Finot, and A. Dereux, “Gain-assisted propagation in a plasmonic waveguide at telecom wavelength,” Nano Lett. 9(8), 2935–2939 (2009). [CrossRef] [PubMed]

], Er silicate [23

X. J. Wang, B. Wang, L. Wang, R. M. Guo, H. Isshiki, T. Kimura, and Z. Zhou, “Extraordinary infrared photoluminescence efficiency of Er_0.1Yb_1.9SiO₅ films on SiO₂ /Si substrates,” Appl. Phys. Lett. 98(7), 071903 (2011). [CrossRef]

] etc., while the high-index gain materials could be InGaAsP [24

M. Nezhad, K. Tetz, and Y. Fainman, “Gain assisted propagation of surface plasmon polaritons on planar metallic waveguides,” Opt. Express 12(17), 4072–4079 (2004). [CrossRef] [PubMed]

], Er-doped silicon [25

C. T. Huang, C. L. Hsin, K. W. Huang, C. Y. Lee, P. H. Yeh, U. S. Chen, and L. J. Chen, “Er-doped silicon nanowires with 1.54 μm light-emitting and enhanced electrical and field emission properties,” Appl. Phys. Lett. 91(9), 093133 (2007). [CrossRef]

], etc. MSHPW is practically suitable for active devices due to its unique properties. Firstly, there is no strict temperature limitation which is a typical obstacle for active plasmonic devices. Specifically, frequently-used metals for plasmonics such as Au and Ag cannot endure high temperature, while some gain materials such as nanocrystal silicon [26

S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O. I. Lebedev, G. Van Tendeloo, and V. V. Moshchalkov, “Classification and control of the origin of photoluminescence from Si nanocrystals,” Nat. Nanotechnol. 3(3), 174–178 (2008). [CrossRef] [PubMed]

] and Er silicate [23

] need high temperature (e.g., around 1000°C) annealing process to obtain a good active performance. Therefore, in some cases (e.g., MIM waveguides), researchers need to take very complicated fabrication process [21

R. J. Walters, R. V. A. van Loon, I. Brunets, J. Schmitz, and A. Polman, “A silicon-based electrical source of surface plasmon polaritons,” Nat. Mater. 9(1), 21–25 (2010). [CrossRef] [PubMed]

] to avoid the temperature’s incompatibility. By contrast, in MSHPW the metal layer is deposited at last and not affected by processes for other layers. Secondly, unlike some hybrid plasmonic waveguides which need to etch the low-index and high-index dielectric layers, MSHPW makes it possible to utilize those gain materials difficult to etch, such as the erbium silicate. In summary, MSHPW is compatible to a wide range of gain materials as well as chemical and physical processes, and this good compatibility makes it a promising candidate for practical active devices.

3. Simulation and analysis of the waveguide

As shown in Fig. 1, we suppose the propagation direction is along the

z

axis. There are two waveguide parameters that must be taken into account for optimization of the waveguide: the height of the low-index dielectric gap

h

and the width of the metal strip

w

. Apart from the metal layer, other layers are supposed to be infinite in

x

direction. For all cases below, heights of the metal strip and the high-index dielectric layer are fixed to be 100nm and 300nm, respectively. We set materials for the metal strip, the low-index gap, the high-index layer and the buffer layer as Ag, SiO₂, Si, and SiO₂, respectively. The telecommunication wavelength

λ

= 1550nm is chosen as the work wavelength and corresponding refractive indices for materials are

n_{A g} = 0.1453 + 11.3587 i

n_{S i O_{2}}

= 1.445 and

n_{S i}

= 3.455.

We simulated the structure by the finite element method (FEM). The key parameter that characterizes the mode is the effective modal index

n_{e f f}

defined as

n_{e f f} = k / k_{0}

, where

k

and

k_{0}

are the propagation constant of the hybrid mode and the free space wave number, respectively.

n_{e f f}

is a complex number written as

n_{e f f} = n_{e f f}' + i n_{e f f} "

. Here

n_{e f f} "

is positive for lossy materials and becomes negative when sufficient gain is induced in the structure. The modal propagation loss

l_{m}

(or gain

g_{m}

) is

l_{m} [d B / μ m] = - 2 \times k_{0} \times n_{e f f} " \times 4.34.

(1)

Here

k_{0} = 2 π / λ_{0}

, where

λ_{0}

is the vacuum wavelength in the unit of μm, the factor 4.34 converts the loss(or gain) value from 1/μm to dB/μm, and the negative sign makes the final value negative for loss and positive for gain. For the sake of simplicity, in the passive case where no gain appears, we take the absolute value of

l_{m}

for analysis, while in the active case, we take the signed value of

l_{m}

and

g_{m}

to show up the trend of modal propagation loss and gain under the influence of gain materials.

3.1 Field profile

Figure 2 illustrates the perpendicular electric field (E_y) distribution of the fundamental quasi-TM mode along

y

axis through the center of metal strip as the dashed line in the inset shows, while the inset shows the E_y field profile of the whole cross section. Geometrical dimensions are

w

= 300nm,

h

= 30nm as an example. One sees that there is a strong field enhancement in the gap layer, which is due to the combination of surface plasmon at the Ag-SiO₂ interface and the discontinuity in the E_y field at the Si-SiO₂ interface. This is a key advantage of this configuration because of its significance for various applications such as active devices [27

J. T. Robinson, K. Preston, O. Painter, and M. Lipson, “First-principle derivation of gain in high-index-contrast waveguides,” Opt. Express 16(21), 16659–16669 (2008). [CrossRef] [PubMed]

] and nonlinear effects [28

P. Muellner, M. Wellenzohn, and R. Hainberger, “Nonlinearity of optimized silicon photonic slot waveguides,” Opt. Express 17(11), 9282–9287 (2009). [CrossRef] [PubMed]

]. Moreover, it is notable that in MSHPW, although the high-index dielectric layer is not patterned to form a dielectric core waveguide as the common case in other hybrid plasmonic waveguides [10

D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express 17(19), 16646–16653 (2009). [CrossRef] [PubMed]

, 12

I. Avrutsky, R. Soref, and W. Buchwald, “Sub-wavelength plasmonic modes in a conductor-gap-dielectric system with a nanoscale gap,” Opt. Express 18(1), 348–363 (2010). [CrossRef] [PubMed]

], the residual power guided in the high-index silicon slab still has a finite modal width. The lateral confinement is obtained by the metal strip which generates an effective index difference between areas with metal and without metal along the

x

direction.

Fig. 2 Vertical line scan of the E_y field profile across the center of the metal strip as the dashed line in the inset shows. The inset shows the E_y field profile of the cross section with w = 300nm, h = 30 nm.

3.2 Dependence on the geometrical parameters

Geometrical parameters may have significant influence on waveguide performance, as discussed in [29

S. Massenot, J.-C. Weeber, A. Bouhelier, G. Colas des Francs, J. Grandidier, L. Markey, and A. Dereux, “Differential method for modeling dielectric-loaded surface plasmon polariton waveguides,” Opt. Express 16(22), 17599–17608 (2008). [CrossRef] [PubMed]

,30

J. Grandidier, G. C. des Francs, L. Markey, A. Bouhelier, S. Massenot, J.-C. Weeber, and A. Dereux, “Dielectric-loaded surface plasmon polariton waveguides on a finite-width metal strip,” Appl. Phys. Lett. 96(6), 063105 (2010). [CrossRef]

], which also refer to metal strips for surface plasmon. Here we carried out a detailed study about the effects of

h

and

w

on the effective index and propagation loss, and results are shown in Fig. 3 . One sees that

n_{e f f}'

rises as w increases or

h

decreases. The qualitative explanation is that dimensions affect the energy distribution in different areas of the structure. Specifically, when more power is confined in the area with high index, the corresponding

n_{e f f}'

will be larger, and when smaller area of electromagnetic field interacts with the metal,

l_{m}

will be smaller. Figure 3(b) suggests that

l_{m}

can be made very low by reducing

w

and enlarging

h

simultaneously.

Fig. 3 (a) The real part of effective index

n_{e f f}'

and (b) the propagation loss

l_{m}

(dB/μm) for varying waveguide widths

w

and gap heights

h

However, it is considerable whether the ability of confinement becomes weaker when decreasing waveguide width for lower losses. There are a few figures of merit to measure a mode’s confinement [31

P. Berini, “Figures of merit for surface plasmon waveguides,” Opt. Express 14(26), 13030–13042 (2006). [CrossRef] [PubMed]

–33

R. F. Oulton, G. Bartal, D. F. P. Pile, and X. Zhang, “Confinement and propagation characteristics of subwavelength plasmonic modes,” New J. Phys. 10(10), 105018 (2008). [CrossRef]

Here we calculated the effective mode area

A_{m}

as the merit for confinement ability, which is defined as the ratio of the total mode energy flux and the peak energy flux density [9

, 18

D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, “Gain enhancement in a hybrid plasmonic nano-waveguide with a low-index or high-index gain medium,” Opt. Express 19(14), 12925–12936 (2011). [CrossRef] [PubMed]

], such that

A_{m} = \frac{\int_{- \infty}^{\infty} P (x, y) d x d y}{\max [P (x, y)]},

(2)

P (x, y) = E (x, y) \times H (x, y),

(3)

where

P (x, y)

is the Poynting vector (mode energy flux density).

A_{m}

has been widely used and also proved to be effective to measure the confinement in HPW [9

, 17

, 18

, 33

R. F. Oulton, G. Bartal, D. F. P. Pile, and X. Zhang, “Confinement and propagation characteristics of subwavelength plasmonic modes,” New J. Phys. 10(10), 105018 (2008). [CrossRef]

]. In addition, it is inversely proportional to the spontaneous emission rate enhancement [33

R. F. Oulton, G. Bartal, D. F. P. Pile, and X. Zhang, “Confinement and propagation characteristics of subwavelength plasmonic modes,” New J. Phys. 10(10), 105018 (2008). [CrossRef]

]. This is also a reason why this measurement has been widely used, and it may be useful for our future work on active applications.

l_{m}

and

A_{m}

versus the waveguide width

w

at a fixed

h

of 70nm are shown in Fig. 4 . When

w

goes down from 500nm to 50nm,

l_{m}

experiences a gradual decrease from 0.00596dB/μm to 0.00179dB/μm while

A_{m}

increases from 0.17434μm² to 0.29207μm². Therefore, a trade-off between loss and confinement exists here. However, it is notable that when

w

reaches around 350nm and increases further,

A_{m}

levels off at 0.175μm², whereas

l_{m}

keeps rising continually. Therefore, the point

w

= 350nm is an optimum value for both low propagation loss and compact confinement. Accordingly, for different applications, we can effectively control the waveguide performance by tuning waveguide dimensions.

Fig. 4 The propagation loss

l_{m}

and the effective mode area

A_{m}

versus the waveguide width

w

at a fixed

h

of 70nm.

3.3 Overlap with the active region

Substantial mode confinement in the active region is necessary for effective loss compensation [34

J. Grandidier, S. Massenot, G. des Francs, A. Bouhelier, J.-C. Weeber, L. Markey, A. Dereux, J. Renger, M. González, and R. Quidant, “Dielectric- loaded surface plasmon polariton waveguides: Figures of merit and mode characterization by image and fourier plane leakage microscopy,” Phys. Rev. B 78(24), 245419 (2008). [CrossRef]

, 35

S. M. García-Blanco, M. Pollnau, and S. I. Bozhevolnyi, “Loss compensation in long-range dielectric-loaded surface plasmon-polariton waveguides,” Opt. Express 19(25), 25298–25311 (2011). [CrossRef] [PubMed]

]. In order to further describe the power distribution that paves the way to introduce gain materials in the next subsection, we calculated the power confinement factor

Г

for different regions.

Г

is defined as the ratio of the power confined in a certain region to the total power. Illustrated in Fig. 5 inset, the structure is divided into several regions: regions covered by metal strip are called “center” while other regions are called “side”. Figure 5 shows the power confinement factor of the high-index Si layer in center (

Г_S i

center) and side (

Г_S i

side) regions, and the low-index SiO₂ gap in center (

Г_S i O_{2}

center) and side (

Г_S i O_{2}

side) regions. There are two remarkable points. First, as

w

increases,

Г_S i

center and

Г_S i O_{2}

center go up while

Г_S i

side and

Г_S i O_{2}

side go down. These trends indicate that more power is confined in the center region for wider metal strips,resulting in a stronger confinement at larger width, which is in accordance with the trend of

A_{m}

in Fig. 4. Secondly,

Г_S i

is always larger than

Г_S i O_{2}

though there is a strong electric field enhancement in the SiO₂ gap as mentioned before. Overall, mode power is confined in the low-index region and the high-index region separately, which provides the possibility to utilize gain materials with low or high refractive index.

Fig. 5 The power confinement factor

Г

in different regions as the waveguide width

w

varies when

h

= 70nm. The inset shows schematic of different regions.

3.4 Loss compensation in MSHPW

A gain material is introduced either in the low-index gap or in the high-index layer to compensate the propagation loss or even obtain net gain. For simplicity, it is assumed that the gain obtained in the active region is uniform. The gain coefficient of the bulk material

g_{b}

is written in terms of the imaginary part of the refractive index

n_{b} "

g_{b} = 2 k_{0} n_{b} "

[27

J. T. Robinson, K. Preston, O. Painter, and M. Lipson, “First-principle derivation of gain in high-index-contrast waveguides,” Opt. Express 16(21), 16659–16669 (2008). [CrossRef] [PubMed]

]. By doing so, we are able to treat the gain as a perturbation to the modal effective refractive index

n_{e f f}

and thus the modal propagation loss (or gain).

First, we introduce gain in the low-index dielectric gap. Figure 6(a) shows the propagation loss (or gain) versus

g_{b}

at varying widths. One sees that for a certain width, the modal loss decreases (or gain increases) linearly as

g_{b}

increases. When

g_{b}

is large enough, the modal propagation loss can be fully compensated, i.e.

n_{e f f} "

= 0. This special point of bulk material gain is defined as critical material gain

g_{c}

. When

g_{b}

increases further to be larger than

g_{c}

, net modal gain is obtained, which corresponds to the region above the dashed line in Fig. 6(a). For different widths, the modal gain (or loss) and

g_{b}

are all in linear relationship. However, slopes of these lines are different—larger slope at larger width. This can be partly expained by the power distribution in Fig. 5. As

w

goes up,

Г_S i O_{2}

center rises and begins to lead the trend of the total power in the gap. Therefore, when the metal strip is wider, a larger overlap between the electromagnetic field and the low-index gain material apears and results in a more effective loss compensation. In consequence, a competetion exsits for the net propagation loss (or gain): on one hand, the intrinsic modal propagation loss is larger for wider waveguides; on the other hand, the gain material works more effectively when the width increases.

Fig. 6 The modal gain or loss for varying widths when gain materials are introduced in (a) low-index gap and (b) high-index layer, respectively. (c) Critical material gain

g_{c}

required for lossless propagation verses waveguide width

w

. The gap height

h

is fixed to 70nm.

In order to provide a merit for the final competetion result, we calculated the critical material gain

g_{c}

for varying widths. As shown by the blue curve in Fig. 6(c), for the case of introducing a low-index gain material,

g_{c}

undergoes an increase as

w

rises, from 33.73cm⁻¹ (

w

= 50nm) to 59.09 cm⁻¹ (

w

= 500nm).

In the next place, we fill the high-index layer with a gain material. Figure 6(b) presents the propagation loss (or gain) for varying material gain coefficients when

w

is 100, 300, and 500nm, respectively. Trends in Fig. 6(b) are similar to those in Fig. 6(a). A more interesting fact is that loss compensation from the high-index gain material is much more effective than that from the low-index gain material. For example, when

g_{b}

is 40.5 cm⁻¹ and w is 100nm, the propogation loss is 3.18e⁻⁴dB/μm for the case of introducing gain in the low index gap, while a pure gain of 1.67e⁻²dB/μm is obtained if gain is introduced in the high-index layer. One reason account for this phenomenon is that much more power is confined in the high-index layer than the low-index gap, which means there will be a larger overlap and stronger interaction between the electricmagentic field and the gain material if gain is introduced in the high-index layer. The critical material gain of the high-index gain material is also shown in Fig. 6(c). Its trend is similar to that of the low-index gain material but with a much lower level of values. For instance, when

w

= 50nm,

g_{c}

of the high-index gain material is only 3.8cm⁻¹, and around a ninth of the value (33.7 cm⁻¹) for the low-index gain material. When

w

rises to 500nm

g_{c}

of the high-index gain material is still as low as 14.1 cm⁻¹. It is notable that the critical gain values discussed above could be easily obtained by many current gain materiels such as quantum dots [22

], III-V semiconductors [24

M. Nezhad, K. Tetz, and Y. Fainman, “Gain assisted propagation of surface plasmon polaritons on planar metallic waveguides,” Opt. Express 12(17), 4072–4079 (2004). [CrossRef] [PubMed]

], and dyes [36

I. De Leon and P. Berini, “Amplification of long-range surface plasmons by a dipolar gain medium,” Nat. Photonics 4(6), 382–387 (2010). [CrossRef]

4. Conclusion

In conclusion, we have proposed and investigated an active metal strip hybrid plasmonic waveguide. The extremely simple fabrication procedure not only avoids extra losses from fabrication but also widens the scale of gain material selection. The propagation loss and mode confinement of the waveguide are effectively controlled by tuning waveguide dimensions. This is beneficial to meet various demands of applications. In the passive case, an optimum value of the metal strip width is demonstrated to be 350nm for both low-loss propagation and compact confinement. In the active case, gain materials could be introduced either in the low-index gap or the high-index layer for loss compensation. Specifically, the high-index gain material works more effectively than the low-index gain material. For one configuration simulated, the critical material gain for the high-index gain material is as low as 3.8cm⁻¹. Consequently, apart from providing an active waveguide with loss compensation, the results from this investigation also facilitate the designing of other plasmonic devices for active applications such as amplifiers, sources and modulators.

Acknowledgments

This work was partially supported by the Peking University 985 startup fund and the National Natural Science Foundation of China under Grant No. 61036011.

References and links

1.	W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]
2.	E. Ozbay, “Plasmonics: Merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]
3.	M. Mansuripur, A. R. Zakharian, A. Lesuffleur, S.-H. Oh, R. J. Jones, N. C. Lindquist, H. Im, A. Kobyakov, and J. V. Moloney, “Plasmonic nano-structures for optical data storage,” Opt. Express 17(16), 14001–14014 (2009). [CrossRef] [PubMed]
4.	Q. Min, C. Chen, P. Berini, and R. Gordon, “Long range surface plasmons on asymmetric suspended thin film structures for biosensing applications,” Opt. Express 18(18), 19009–19019 (2010). [CrossRef] [PubMed]
5.	D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]
6.	R. Charbonneau, N. Lahoud, G. Mattiussi, and P. Berini, “Demonstration of integrated optics elements based on long-ranging surface plasmon polaritons,” Opt. Express 13(3), 977–984 (2005). [CrossRef] [PubMed]
7.	L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express 13(17), 6645–6650 (2005). [CrossRef] [PubMed]
8.	G. Veronis and S. H. Fan, “Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides,” Appl. Phys. Lett. 87(13), 131102 (2005). [CrossRef]
9.	R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics 2(8), 496–500 (2008). [CrossRef]
10.	D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express 17(19), 16646–16653 (2009). [CrossRef] [PubMed]
11.	P. D. Flammer, J. M. Banks, T. E. Furtak, C. G. Durfee, R. E. Hollingsworth, and R. T. Collins, “Hybrid plasmon/dielectric waveguide for integrated silicon-on-insulator optical elements,” Opt. Express 18(20), 21013–21023 (2010). [CrossRef] [PubMed]
12.	I. Avrutsky, R. Soref, and W. Buchwald, “Sub-wavelength plasmonic modes in a conductor-gap-dielectric system with a nanoscale gap,” Opt. Express 18(1), 348–363 (2010). [CrossRef] [PubMed]
13.	M. Wu, Z. Han, and V. Van, “Conductor-gap-silicon plasmonic waveguides and passive components at subwavelength scale,” Opt. Express 18(11), 11728–11736 (2010). [CrossRef] [PubMed]
14.	I. Goykhman, B. Desiatov, and U. Levy, “Experimental demonstration of locally oxidized hybrid silicon- plasmonic waveguide,” Appl. Phys. Lett. 97(14), 141106 (2010). [CrossRef]
15.	J. Wang, X. Guan, Y. He, Y. Shi, Z. Wang, S. He, P. Holmström, L. Wosinski, L. Thylen, and D. Dai, “Sub-μm² power splitters by using silicon hybrid plasmonic waveguides,” Opt. Express 19(2), 838–847 (2011). [CrossRef] [PubMed]
16.	D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, “Silicon hybrid plasmonic submicron-donut resonator with pure dielectric access waveguides,” Opt. Express 19(24), 23671–23682 (2011). [CrossRef] [PubMed]
17.	R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature 461(7264), 629–632 (2009). [CrossRef] [PubMed]
18.	D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, “Gain enhancement in a hybrid plasmonic nano-waveguide with a low-index or high-index gain medium,” Opt. Express 19(14), 12925–12936 (2011). [CrossRef] [PubMed]
19.	J. Zhang, L. Cai, W. Bai, Y. Xu, and G. Song, “Hybrid plasmonic waveguide with gain medium for lossless propagation with nanoscale confinement,” Opt. Lett. 36(12), 2312–2314 (2011). [CrossRef] [PubMed]
20.	J. A. Dionne, L. A. Sweatlock, M. T. Sheldon, A. P. Alivisatos, and H. Atwater, “Silicon-based plasmonic for on-chip photonics,” IEEE J. Quantum Electron. 16(1), 295–306 (2010). [CrossRef]
21.	R. J. Walters, R. V. A. van Loon, I. Brunets, J. Schmitz, and A. Polman, “A silicon-based electrical source of surface plasmon polaritons,” Nat. Mater. 9(1), 21–25 (2010). [CrossRef] [PubMed]
22.	J. Grandidier, G. C. des Francs, S. Massenot, A. Bouhelier, L. Markey, J. C. Weeber, C. Finot, and A. Dereux, “Gain-assisted propagation in a plasmonic waveguide at telecom wavelength,” Nano Lett. 9(8), 2935–2939 (2009). [CrossRef] [PubMed]
23.	X. J. Wang, B. Wang, L. Wang, R. M. Guo, H. Isshiki, T. Kimura, and Z. Zhou, “Extraordinary infrared photoluminescence efficiency of Er_0.1Yb_1.9SiO₅ films on SiO₂ /Si substrates,” Appl. Phys. Lett. 98(7), 071903 (2011). [CrossRef]
24.	M. Nezhad, K. Tetz, and Y. Fainman, “Gain assisted propagation of surface plasmon polaritons on planar metallic waveguides,” Opt. Express 12(17), 4072–4079 (2004). [CrossRef] [PubMed]
25.	C. T. Huang, C. L. Hsin, K. W. Huang, C. Y. Lee, P. H. Yeh, U. S. Chen, and L. J. Chen, “Er-doped silicon nanowires with 1.54 μm light-emitting and enhanced electrical and field emission properties,” Appl. Phys. Lett. 91(9), 093133 (2007). [CrossRef]
26.	S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O. I. Lebedev, G. Van Tendeloo, and V. V. Moshchalkov, “Classification and control of the origin of photoluminescence from Si nanocrystals,” Nat. Nanotechnol. 3(3), 174–178 (2008). [CrossRef] [PubMed]
27.	J. T. Robinson, K. Preston, O. Painter, and M. Lipson, “First-principle derivation of gain in high-index-contrast waveguides,” Opt. Express 16(21), 16659–16669 (2008). [CrossRef] [PubMed]
28.	P. Muellner, M. Wellenzohn, and R. Hainberger, “Nonlinearity of optimized silicon photonic slot waveguides,” Opt. Express 17(11), 9282–9287 (2009). [CrossRef] [PubMed]
29.	S. Massenot, J.-C. Weeber, A. Bouhelier, G. Colas des Francs, J. Grandidier, L. Markey, and A. Dereux, “Differential method for modeling dielectric-loaded surface plasmon polariton waveguides,” Opt. Express 16(22), 17599–17608 (2008). [CrossRef] [PubMed]
30.	J. Grandidier, G. C. des Francs, L. Markey, A. Bouhelier, S. Massenot, J.-C. Weeber, and A. Dereux, “Dielectric-loaded surface plasmon polariton waveguides on a finite-width metal strip,” Appl. Phys. Lett. 96(6), 063105 (2010). [CrossRef]
31.	P. Berini, “Figures of merit for surface plasmon waveguides,” Opt. Express 14(26), 13030–13042 (2006). [CrossRef] [PubMed]
32.	R. Buckley and P. Berini, “Figures of merit for 2D surface plasmon waveguides and application to metal stripes,” Opt. Express 15(19), 12174–12182 (2007). [CrossRef] [PubMed]
33.	R. F. Oulton, G. Bartal, D. F. P. Pile, and X. Zhang, “Confinement and propagation characteristics of subwavelength plasmonic modes,” New J. Phys. 10(10), 105018 (2008). [CrossRef]
34.	J. Grandidier, S. Massenot, G. des Francs, A. Bouhelier, J.-C. Weeber, L. Markey, A. Dereux, J. Renger, M. González, and R. Quidant, “Dielectric- loaded surface plasmon polariton waveguides: Figures of merit and mode characterization by image and fourier plane leakage microscopy,” Phys. Rev. B 78(24), 245419 (2008). [CrossRef]
35.	S. M. García-Blanco, M. Pollnau, and S. I. Bozhevolnyi, “Loss compensation in long-range dielectric-loaded surface plasmon-polariton waveguides,” Opt. Express 19(25), 25298–25311 (2011). [CrossRef] [PubMed]
36.	I. De Leon and P. Berini, “Amplification of long-range surface plasmons by a dipolar gain medium,” Nat. Photonics 4(6), 382–387 (2010). [CrossRef]

OCIS Codes
(130.2790) Integrated optics : Guided waves
(230.7370) Optical devices : Waveguides
(240.6680) Optics at surfaces : Surface plasmons
(230.4480) Optical devices : Optical amplifiers

ToC Category:
Optics at Surfaces

History
Original Manuscript: February 29, 2012
Revised Manuscript: April 19, 2012
Manuscript Accepted: April 27, 2012
Published: May 4, 2012

Citation
Linfei Gao, Liangxiao Tang, Feifei Hu, Ruimin Guo, Xingjun Wang, and Zhiping Zhou, "Active metal strip hybrid plasmonic waveguide with low critical material gain," Opt. Express 20, 11487-11495 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-10-11487

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References

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature424(6950), 824–830 (2003). [CrossRef] [PubMed]
E. Ozbay, “Plasmonics: Merging photonics and electronics at nanoscale dimensions,” Science311(5758), 189–193 (2006). [CrossRef] [PubMed]
M. Mansuripur, A. R. Zakharian, A. Lesuffleur, S.-H. Oh, R. J. Jones, N. C. Lindquist, H. Im, A. Kobyakov, and J. V. Moloney, “Plasmonic nano-structures for optical data storage,” Opt. Express17(16), 14001–14014 (2009). [CrossRef] [PubMed]
Q. Min, C. Chen, P. Berini, and R. Gordon, “Long range surface plasmons on asymmetric suspended thin film structures for biosensing applications,” Opt. Express18(18), 19009–19019 (2010). [CrossRef] [PubMed]
D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics4(2), 83–91 (2010). [CrossRef]
R. Charbonneau, N. Lahoud, G. Mattiussi, and P. Berini, “Demonstration of integrated optics elements based on long-ranging surface plasmon polaritons,” Opt. Express13(3), 977–984 (2005). [CrossRef] [PubMed]
L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express13(17), 6645–6650 (2005). [CrossRef] [PubMed]
G. Veronis and S. H. Fan, “Bends and splitters in metal-dielectric-metal subwavelength plasmonic waveguides,” Appl. Phys. Lett.87(13), 131102 (2005). [CrossRef]
R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics2(8), 496–500 (2008). [CrossRef]
D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express17(19), 16646–16653 (2009). [CrossRef] [PubMed]
P. D. Flammer, J. M. Banks, T. E. Furtak, C. G. Durfee, R. E. Hollingsworth, and R. T. Collins, “Hybrid plasmon/dielectric waveguide for integrated silicon-on-insulator optical elements,” Opt. Express18(20), 21013–21023 (2010). [CrossRef] [PubMed]
I. Avrutsky, R. Soref, and W. Buchwald, “Sub-wavelength plasmonic modes in a conductor-gap-dielectric system with a nanoscale gap,” Opt. Express18(1), 348–363 (2010). [CrossRef] [PubMed]
M. Wu, Z. Han, and V. Van, “Conductor-gap-silicon plasmonic waveguides and passive components at subwavelength scale,” Opt. Express18(11), 11728–11736 (2010). [CrossRef] [PubMed]
I. Goykhman, B. Desiatov, and U. Levy, “Experimental demonstration of locally oxidized hybrid silicon- plasmonic waveguide,” Appl. Phys. Lett.97(14), 141106 (2010). [CrossRef]
J. Wang, X. Guan, Y. He, Y. Shi, Z. Wang, S. He, P. Holmström, L. Wosinski, L. Thylen, and D. Dai, “Sub-μm2 power splitters by using silicon hybrid plasmonic waveguides,” Opt. Express19(2), 838–847 (2011). [CrossRef] [PubMed]
D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, “Silicon hybrid plasmonic submicron-donut resonator with pure dielectric access waveguides,” Opt. Express19(24), 23671–23682 (2011). [CrossRef] [PubMed]
R. F. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L. Dai, G. Bartal, and X. Zhang, “Plasmon lasers at deep subwavelength scale,” Nature461(7264), 629–632 (2009). [CrossRef] [PubMed]
D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, “Gain enhancement in a hybrid plasmonic nano-waveguide with a low-index or high-index gain medium,” Opt. Express19(14), 12925–12936 (2011). [CrossRef] [PubMed]
J. Zhang, L. Cai, W. Bai, Y. Xu, and G. Song, “Hybrid plasmonic waveguide with gain medium for lossless propagation with nanoscale confinement,” Opt. Lett.36(12), 2312–2314 (2011). [CrossRef] [PubMed]
J. A. Dionne, L. A. Sweatlock, M. T. Sheldon, A. P. Alivisatos, and H. Atwater, “Silicon-based plasmonic for on-chip photonics,” IEEE J. Quantum Electron.16(1), 295–306 (2010). [CrossRef]
R. J. Walters, R. V. A. van Loon, I. Brunets, J. Schmitz, and A. Polman, “A silicon-based electrical source of surface plasmon polaritons,” Nat. Mater.9(1), 21–25 (2010). [CrossRef] [PubMed]
J. Grandidier, G. C. des Francs, S. Massenot, A. Bouhelier, L. Markey, J. C. Weeber, C. Finot, and A. Dereux, “Gain-assisted propagation in a plasmonic waveguide at telecom wavelength,” Nano Lett.9(8), 2935–2939 (2009). [CrossRef] [PubMed]
X. J. Wang, B. Wang, L. Wang, R. M. Guo, H. Isshiki, T. Kimura, and Z. Zhou, “Extraordinary infrared photoluminescence efficiency of Er0.1Yb1.9SiO5 films on SiO2 /Si substrates,” Appl. Phys. Lett.98(7), 071903 (2011). [CrossRef]
M. Nezhad, K. Tetz, and Y. Fainman, “Gain assisted propagation of surface plasmon polaritons on planar metallic waveguides,” Opt. Express12(17), 4072–4079 (2004). [CrossRef] [PubMed]
C. T. Huang, C. L. Hsin, K. W. Huang, C. Y. Lee, P. H. Yeh, U. S. Chen, and L. J. Chen, “Er-doped silicon nanowires with 1.54 μm light-emitting and enhanced electrical and field emission properties,” Appl. Phys. Lett.91(9), 093133 (2007). [CrossRef]
S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O. I. Lebedev, G. Van Tendeloo, and V. V. Moshchalkov, “Classification and control of the origin of photoluminescence from Si nanocrystals,” Nat. Nanotechnol.3(3), 174–178 (2008). [CrossRef] [PubMed]
J. T. Robinson, K. Preston, O. Painter, and M. Lipson, “First-principle derivation of gain in high-index-contrast waveguides,” Opt. Express16(21), 16659–16669 (2008). [CrossRef] [PubMed]
P. Muellner, M. Wellenzohn, and R. Hainberger, “Nonlinearity of optimized silicon photonic slot waveguides,” Opt. Express17(11), 9282–9287 (2009). [CrossRef] [PubMed]
S. Massenot, J.-C. Weeber, A. Bouhelier, G. Colas des Francs, J. Grandidier, L. Markey, and A. Dereux, “Differential method for modeling dielectric-loaded surface plasmon polariton waveguides,” Opt. Express16(22), 17599–17608 (2008). [CrossRef] [PubMed]
J. Grandidier, G. C. des Francs, L. Markey, A. Bouhelier, S. Massenot, J.-C. Weeber, and A. Dereux, “Dielectric-loaded surface plasmon polariton waveguides on a finite-width metal strip,” Appl. Phys. Lett.96(6), 063105 (2010). [CrossRef]
P. Berini, “Figures of merit for surface plasmon waveguides,” Opt. Express14(26), 13030–13042 (2006). [CrossRef] [PubMed]
R. Buckley and P. Berini, “Figures of merit for 2D surface plasmon waveguides and application to metal stripes,” Opt. Express15(19), 12174–12182 (2007). [CrossRef] [PubMed]
R. F. Oulton, G. Bartal, D. F. P. Pile, and X. Zhang, “Confinement and propagation characteristics of subwavelength plasmonic modes,” New J. Phys.10(10), 105018 (2008). [CrossRef]
J. Grandidier, S. Massenot, G. des Francs, A. Bouhelier, J.-C. Weeber, L. Markey, A. Dereux, J. Renger, M. González, and R. Quidant, “Dielectric- loaded surface plasmon polariton waveguides: Figures of merit and mode characterization by image and fourier plane leakage microscopy,” Phys. Rev. B78(24), 245419 (2008). [CrossRef]
S. M. García-Blanco, M. Pollnau, and S. I. Bozhevolnyi, “Loss compensation in long-range dielectric-loaded surface plasmon-polariton waveguides,” Opt. Express19(25), 25298–25311 (2011). [CrossRef] [PubMed]
I. De Leon and P. Berini, “Amplification of long-range surface plasmons by a dipolar gain medium,” Nat. Photonics4(6), 382–387 (2010). [CrossRef]

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