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Physica B ] (]]]]) ]]]–]]]

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

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Pleas

journal homepage: www.elsevier.com/locate/physb

Integration of plasmonics into nanoelectronic circuits

Ping Bai �, Hong Son Chu, Mingxia Gu, Oka Kurniawan, Erping Li

Computational Electronics and Photonics, Institute of High Performance Computing, 1 Fusionopolis Way #16-16 Connexis, Singapore 138632, Singapore

a r t i c l e i n f o

Keywords:

Plasmonics

Subwavelength waveguide

Plasmonic converter

Nanoantenna

26/$ - see front matter & 2010 Elsevier B.V. A

016/j.physb.2010.01.017

esponding author. Tel.: +65 64191242; fax:

ail address: baiping@ihpc.a-star.edu.sg (P. Bai

e cite this article as: P. Bai, et al., Ph

a b s t r a c t

Plasmonic waveguides are proposed to transmit information optically from one electronic component

to another in integrated nanoelectronic circuits. A metal–insulator–metal (MIM) plasmonic waveguide,

rather than a traditional dielectric waveguide or an electric wire, is used for data transmission. The MIM

plasmonic waveguide confines plasmonic (electromagnetic) waves to a dielectric slot with a cross-

section of 50 nm�50 nm, and propagates them close to the speed of light. Two nanorods that form a

nanoantenna are employed to receive the optical power from the waveguide, and localize as well as

concentrate the received power in the proximity around the two nanorods. The localized optical power

is converted to electric signals by a nanoscale plasmonics-to-electronics converter, which has a

nanosized active volume and a bandwidth of up to 1 THz. Both the plasmonic waveguide and the

converter have nanoscale dimensions comparable to those of modern nanoelectronic counterparts. The

proposed method could be used to improve the performance of existing nanoelectronic systems by

exploiting the strength of optics.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Integration of optical and electronic circuits leads to remark-able benefit in data transmission and processing by combining theadvantages of the large bandwidth of optics and the compactnessof electronics [1]. Continuous reduction in size of electroniccomponents has resulted in better, faster, and smaller electroniceveryday products for society. However, interconnects on a chiphave become a bottle-neck preventing further improvement ofthe performance of electronic products due to the increasingsignal delay and power consumption associated with the inter-connects [2]. Optical interconnects are superior to the electronicinterconnects by virtue of their large operational bandwidth, andhuge data transmission capability. However, the dimensions of atraditional optical device are fundamentally limited by the law ofdiffraction. The large mismatch in size prevents integrating opticswith electronics for a better performance.

Plasmonics allows manipulating the flow of light in ananometer scale well below the diffraction limit, by exploitingthe unique optical properties of metallic structures [3]. Muchattention has been attracted to the study of plasmonics, from thefundamentals to applications in recent years [4–8], includingnanoscale optical waveguides [4], perfect lenses, subwavelengthlithography, highly sensitive biosensors and ultra-fast modulators[5]. Plasmonics could provide a new way for optical datatransmission near light speed requiring a size well below the

ll rights reserved.

+65 64635176.

).

ysica B (2010), doi:10.1016

free-space wavelength, with efficient power delivery. Plasmonicswill be the potential technology to achieve information transmis-sion between nanoscale electronic devices at optical frequencies,and bridge the gap between photonics and electronics in thenanoscale regime [9].

We explore the convergence of optics and electronics by usingplasmonic waveguides for on-chip nanoscale optical intercon-nects. To optically transmit data between electronic devices, weneed to develop a number of plasmonic devices, includingplasmonic sources, modulators, waveguides, switches, filters anddetectors. As a first step, we focus on the waveguide and detector.We use plasmonic subwavelength waveguides as optical inter-connects. The plasmonic waves carry information, and propagatealong the waveguide close to the speed of light. At the end of thewaveguide, the plasmonic waves are converted into electronicsignals through a plasmonic converter. Fig. 1 shows a schematicdiagram of a metal–insulator–metal subwavelength waveguidecoupled to a plasmonic converter.

2. Subwavelength waveguides

The major challenge for the on-chip optical data transmissionis to confine the electromagnetic waves in a waveguide ofnanometer size compatible with that of the electric components.Previous studies show that plasmonics could be confined in, andpropagated along, a waveguide with dimensions below one-tenthof the free-space wavelength [4]. Different plasmonic waveguideshave been studied, including metal–insulator–metal (MIM),

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insulator-metal-insulator, and dielectric-loaded structures. TheMIM waveguide is most suitable for the on-chip data transmissionas it provides a better trade-off between the lateral confinementand the propagation loss [10].

For the purpose of this paper we have considered a subwave-length MIM waveguide. Fig. 2a shows the schematics of the MIMwaveguide, which consists of a dielectric slot between two thinmetallic films. The width and the thickness of each metal film are1mm and 50 nm, respectively. The gap between the two metal filmsis 50 nm. Fig. 2b shows the electric field propagating along the Ag–air–Ag waveguide at a free-space wavelength of 1.55mm, simulatedwith CST Microwave Studio [11]. Plasmonic waves are well confinedwithin the nanoscale slot. The distribution of the electric field in thexz-plane across the center of the waveguide is shown in Fig. 2c. Wenotice that the magnitude of the electric field decreases along thewaveguide. Fig. 3 shows the relationship between the propagationdistance and the wavelength. It shows that the propagation distanceincreases when the free-space wavelength of the input field isincreased. This can be attributed to the fact that the metal becomesless lossy at lower frequency. That is why the metal can be treated asa perfect conductor in microwave regime. Therefore, shorter free-space wavelength corresponds to a larger propagation loss.

PlasmonicConverter

Substrate

SiO2

X

Y

Z

PlasmonicWaveguide

Fig. 1. Schematic illustration shows plasmonic waves propagating along a metal–

insulator–metal subwavelength waveguide and detected by a plasmonics-to-

electronics converter.

Fig. 2. Plasmonic waves propagating along an MIM waveguide: (a) 2-D illustration

waveguide with a cross-section of 50 nm�50 nm and at a free-space wavelength of 1.55

the waveguide.

Please cite this article as: P. Bai, et al., Physica B (2010), doi:10.101

The length of an on-chip interconnect can range from sub-micrometers (local interconnects) to centimeters (global intercon-nects) in an advanced electronic chip. However, the MIM plasmonicwaveguide is very lossy due to high absorption of light in the opticalfrequency regime. The propagation distance in the MIM waveguidecan only reach a distance of deca-micrometers with the field beingreasonably confined. The performance of existing plasmonicwaveguides is not good enough for on-chip interconnect applica-tions yet. There is urgent need to increase the propagation distanceof plasmonic waveguides while maintain a high confinement inorder to integrate them into semiconductor-based electroniccircuits. Fortunately, studies on novel materials, such as gainmaterials, plasmonic metamaterials and epsilon-near-zero materi-als, and on novel structures have shown a very promising future forthe implementation of subwavelength waveguides with ultra-lowpropagation loss [8].

of the MIM waveguide, (b) 3-D electric fields propagating along the Ag–air–Ag

mm and (c) the distribution of the electric fields in the xz-plane across the center of

Fig. 3. Propagation distance of the Ag–air–Ag waveguide (slot cross-section of

50 nm by 50 nm) as a function of free-space wavelength.

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3. Plasmonics–electronics conversion

In order to integrate a plasmonic subwavelength waveguideinto an electronic circuit, a novel plasmonic converter with a highbandwidth is required to convert plasmonic signals to electronicones. Various photodetectors have been developed to detectelectromagnetic waves propagating along a conventional di-electric waveguide at the micrometer scale [12]. The plasmonicconverter, however, cannot be obtained by simply scaling downthe traditional micrometer-scaled waveguide photodetector.Firstly, plasmonics, unlike traditional optics which is based ondiffraction phenomena in dielectrics, confines electromagneticfields along the interface between metal and dielectric. Theplasmonic waves are generally very weak at the end of thewaveguide as the plasmonic waveguide is very lossy comparedwith the dielectric one. Secondly, the plasmonic waves decay intosurrounding dielectrics very rapidly at the open end of theplasmonic waveguide. Only a small fraction of the plasmonicenergy can reach the active region of a plasmonic converter if theconverter is simply attached to the end of the waveguide. Fig. 4ashows the electric field in the xy-plane, at 50 nm away from theend of an MIM waveguide. The field is too weak to be detected.Thirdly, there will also be a large reflection at the interfacebetween the waveguide and the converter due to a large contrastof permittivities of the waveguide and the converter. Lastly, thenanoscale photodetector does not have enough space to form aresonant cavity, which needs a size of (n+1)*l/2 where n is aninteger. Directly scaling down of the traditional waveguidephotodetector cannot provide enough photoresponsivity todetect the weak plasmonic waves. Therefore, the couplingbetween the plasmonic waveguide and the converter becomes amajor challenge for the design of a plasmonics-to-electronicsconverter.

In our study, metallic nanorods are used to couple plasmonicwaves from the subwavelength waveguide to the plasmonicconverter, and enhance the electromagnetic near-fields aroundthe nanorods at the same time. The optical properties of thenanoscale metallic particles enable localized surface plasmonmodes around the particles. The metallic nanorods can also beconsidered to form a dipole nanoantenna at optical frequenciesanalogue to that at radio or microwave frequencies [13]. We placetwo metallic nanorods near the end of the plasmonic waveguideas shown in Fig. 1. The two nanorods, which act as a dipolenanoantenna, receive the plasmonic waves from the waveguide.As a result, the electromagnetic fields are coupled from the

Fig. 4. Electric fields in the xy-plane 50 nm from the end of the waveguide: (a)

without nanoantenna and (b) with the nanoantenna.

Please cite this article as: P. Bai, et al., Physica B (2010), doi:10.1016

plasmonic waveguide to the nanorods, and the near-fields areformed and concentrated in the area around the nanorods [14].

The dimensions of the metallic nanorods have a significanteffect on the enhancement of the near-field around the nanorods.In our study, the plasmonic waves in the MIM waveguide areexcited with light at a free-space wavelength of 1.55mm. Two Agnanorods are used to collect the plasmonic waves. Each nanorodhas a dimension of 150�50�50 nm3; and the gap between twonanorods is optimized to be 50 nm to give the highest electricfields in the gap region. The total length between two extremeends of the nanorods is the most sensitive parameter affecting theenhancement of the near-field around the nanorods. This isanalogue to the effective length of a half-wavelength dipoleantenna in microwave applications. However, the optimal lengthof this nanorod antenna is found to be 350 nm, which is muchshorter than one-half of 1.55mm. The half-wave dipole resonancehere might correspond to a shorter effective wavelength, which ismuch shorter than that in free space owing to the plasmoniceffects of the metallic nanorods [15].

Fig. 4 shows the calculated electric field in the xy-plane acrossthe center of the nanorods without and with the nanoantenna.Results show that, the near-field around the nanorods is greatlycondensed by applying the nanoantenna. The electric fields in thegap region can be enhanced by more than 100 times with theoptimized geometry of the nanoantenna.

Once the optical power is concentrated in a nano-scaledvolume, the plasmonic converter can be designed by using thestructures of traditional photodetectors with its active regionenclosing the volume. A PIN photodetector structure is used in ourstudy as shown in Fig. 1. An absorption material InGaAs is filled inthe volume around the two nanorods where the near-fields arelocally enhanced. The P-I-N layered structure is formed in the y-direction with two electrodes located at the bottom and the top ofthe PIN layers, respectively. The photonic energy from theplasmonic waves, which can be treated as photons incident inthe semiconductor material InGaAs, is absorbed by the electronsin the valence band. These high-energy electrons are excited tobecome free electrons in the conduction band, and leave holes inthe valence band. As a result, electron–hole pairs are generated inthe absorption material, and the electrons and holes drift to thetwo electrodes separately to form photocurrents under anexternal bias voltage.

The electronic characteristics of the plasmonic converter aresimulated based on the Silvaco TCAD software [16]. A two-dimensional model is built in the xy-plane across the center of thenanorods. The photogeneration rate in the absorption material ismodeled with

G¼ ZjEj2 la2p‘ c

ð1Þ

where

a¼ 4pk=l

Z is the internal quantum efficiency; ‘ is Planck’s constant; l isthe free space wavelength; c is the speed of light; a is theabsorption coefficient; k is the imaginary part of the refractionindex of the absorption material; and E is the integrated value ofthe electric field in the z-direction. Fig. 5 shows the calculatedphotogeneration in the active area of the converter. It can be seenthat the density distribution of the excited carriers is proportionalto the distribution of the electromagnetic field as shown in Fig. 4b.The calculated photocurrent of the converter as a function of thebias voltage is shown in Fig. 6.

This proposed plasmonic converter would have very highresponsivity and large bandwidth. The weak plasmonic waves atthe end of the plasmonic waveguide have been coupled and

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1e-09

8e-10

6e-10

4e-10

2e-10

0

0 0.1 0.2 0.3 0.4 0.5

Cathode Voltage (V)

Cathode Current (A)

Fig. 6. Photocurrent of the plasmonic converter as a function of the bias.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

-0.2 -0.1 0 0.1 0.2

X (Microns)

Y (

Mic

rons

)

p+

n-

Photogeneration 1.87e+21

1.5e+21

1.12e+21

7.48e+20

3.74e+20

0

Fig. 5. Photogeneration in the cross section via the nanorods in the plasmonic

converter. The value has been integrated in the z-direction.

P. Bai et al. / Physica B ] (]]]]) ]]]–]]]4

highly enhanced within the small volume around the nanorods.This leads to a high responsivity of the plasmonic waves. Theactive area of the plasmonic converter is at a nanometer scale asthe field is concentrated in a volume approximately 500 nm�200nm�100 nm. At such a small size, the capacitance of theconverter is about 15af as predicted by a simple parallel-platemodel. This allows relatively large external resistance. Note thatthe distance between the two electrodes can be as small as

Please cite this article as: P. Bai, et al., Physica B (2010), doi:10.101

200 nm. The transit time for the excited carriers to travel to thetwo electrodes can be o1 ps. This not only greatly reduces thechances for the recombination of electron–hole pairs, leading tohigh responsivity, but also allows the converter to operate at avery high speed. The rough estimation suggests that the converteris able to operate in THz frequency range.

4. Conclusion

We have proposed a method to enhance the performance ofelectronic circuits by using subwavelength plasmonic waveguidesas on-chip optical interconnects. We have transmitted data byusing MIM waveguides that can propagate plasmonic waves in across-section much smaller than the free-space wavelength. Aplasmonics-to-electronics converter has been designed to convertthe plasmonic waves into electric signals. Two nanorods, whichform a dipole nanoantenna, are used to couple the plasmonicwaves from the waveguide to the converter. The electromagneticnear-field around the nanorods, which are inside the active area ofthe converter, is greatly enhanced through the localized surfaceplasmon effects. The converter has ultra-compact dimensions,high responsivity, potential THz bandwidth, and very low powerdispersion. The nanoscale features of the plasmonic waveguideand the converter, comparable to those of the nanoelectroniccounterparts, are paving the way for the integration of optical andelectronic components on the same chip; and hence a tremendoussynergy could be reached by combining the advantages of bothphotonics and nanoelectronics.

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