Broadband Electro-Absorption Modulators Design Based on Epsilon-Near-Zero Indium Tin Oxide

1077-260X (c) 2013 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEEpermission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JSTQE.2014.2375153, IEEE Journal of Selected Topics in Quantum Electronics

JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. XX, NO.XX, XX 20XX 1

Broadband Electro-Absorption Modulators DesignBased on Epsilon-Near-Zero Indium Tin Oxide

Hongwei Zhao∗, Yu Wang∗, Antonio Capretti, Luca Dal Negro and Jonathan Klamkin,Senior Member, IEEE

Abstract—In this paper, we propose a compact silicon (Si)electro-absorption modulator based on a slot waveguide withepsilon-near-zero indium tin oxide materials. In order to integratethe device with low-loss Si strip waveguides, both butt-couplingand evanescent-coupling schemes are investigated. For both cases,our electro-absorption modulator demonstrates a high extinctionratio and a low insertion loss over a wide optical bandwidth.

Index Terms—Integrated Optoelectronics, Optical Modulators,Plasmonics.

I. I NTRODUCTION

As an integration platform, silicon (Si) photonics candemonstrate single-chip, CMOS-compatible photonic inte-grated circuits for optical interconnect applications [1], [2], [3].Significant development has been carried out in realizing low-loss waveguides, germanium (Ge)-on-Si photodetectors, andSi optical modulators based on p-n junctions. ConventionalMach-Zenhder modulators, however, suffer from low effi-ciency, high insertion loss, and large footprint [4]. By utilizinga microring resonator, the footprint can be significantly re-duced, but such devices exhibit narrow optical bandwidth andthermal instability [5]. Electro-absorption modulators based ontensile-strained Ge quantum wells or bulk Ge/Si materials arepromising, however require sophisticated epitaxial growth [6],[7].

Recently, CMOS-compatible transparent conducting oxides(TCOs) [e.g. indium tin oxide (ITO), aluminum zinc oxide(AZO), and gallium zinc oxide (GZO)] have shown promisefor integrated electro-absorptionmodulators [8], [9], [10], [11],[12]. The permittivity of TCOs can be tuned by activelyadjusting the carrier density, therefore these materials respondto applied electric signals with absorption modulation. Themodulation speed is limited only by the RC delay. The ITOmodulator based on high-confined hybrid plasmon waveguides(HPW) has demonstrated an extinction ratio (ER) of 5 dB witha 5-µm device length at the wavelength of 1.31µm; and it canbe further improved to be 6.0dBµm−1 [13], [14]. When the

∗Hongwei Zhao and Yu Wang equally contributed to this work.This work was partially supported by the Boston University College of

Engineering Dean’s Catalyst Award and the AFOSR program “Emitters forHigh Density Information Processing using Photonic-Plasmonic Coupling inCoaxial Nanopillars” under Award No. FA9550-13-1-0011.

Hongwei Zhao, Yu Wang and Antonio Capretti are with the Department ofElectrical and Computer Engineering, Boston University, Boston, MA 02215,USA (e-mail:[email protected]; [email protected]; [email protected].)

Luca Dal Negro and Jonathan Klamkin are with the Department of Electri-cal and Computer Engineering, Boston University, Boston, MA02215, USA;and the Department of Material Science and Engineering, Boston University,Boston, MA 02215, USA; (e-mail:[email protected]; [email protected].)

Manuscript received October 01, 2014; revised November 14, 2014.

permittivity of the ITO material is tuned to be around zero,which is referred as the “epsilon-near-zero” (ENZ) state, thecorresponding absorption loss can be optimized, enabling ahigher extinction ratio. A previous reported electro-absorptionmodulator with “epsilon-near-zero” ITO has shown a 3-dBextinction ratio with a 27-µm long device at the wavelengthof 1.55µm [15]. In their work, the ITO layer along with a thinHfO2 layer are deposited on top of a silicon strip waveguide,which provides a small optical confinement inside the activeITO material thus limits the device performance.

In this work, we propose a high-confinement Si slot-waveguide modulator design based on engineered ENZ ITOmaterials. First, we characterize ITO films with differentcarrier concentrations and use the experimentally extractedparameters in electromagnetic simulations to design a mod-ulator structure. Then, we investigate both butt coupling andevanescent coupling of the proposed device with conventionalSi strip waveguides. In both coupling schemes, the com-pact electro-absorption modulators designs, with device length≤1.5 µm, demonstrate a high extinction ratio over a wideoptical bandwidth while maintaining relatively low insertionloss.

This paper is organized as follows. In section II, themeasured permittivity of the ITO samples and the correspond-ing “epsilon-near-zero” states are discussed. The absorptionstudy based on a 2D mode solver is presented in sectionIII. In section IV, we propose a butt-coupled scheme withpolarization rotators connecting the Si strip waveguides andthe ITO modulator. The conventional evanescent coupling isalso investigated. At last, section V concludes this work.

II. M ATERIAL SYNTHESIS AND CHARACTERIZATION

The permittivity of the ITO films is described by the Drudemodel [15]:

ε = εr + jεi = ε∞ −ω2p

ω(ω + jγ)(1)

where the plasma frequency,ωp, is given byω2p = Ne2/ε0m

∗,ε∞ is the high-frequency permittivity,γ is the electron scat-tering rate,N is the free-carrier concentration in the material,andm∗ is the effective mass of the electron. According to (1),the permittivity of ITO can be tuned by actively changing thecarrier concentration. With such an ITO layer embedded in adielectric waveguide, the light absorption can be modulatedaccordingly.

ITO samples were prepared by radio-frequency magnetronsputtering (Denton Discovery 18) on Si substrates from an ITO

1077−260X (c) 2013 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEEpermission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.



1 1.2 1.4 1.6 1.8 2−6

−4

−2

0

2

4

wavelength (µm)

εr (N

1)

εr (N

2)

εr (N

3)

εr (N

4)

(a)

1 1.2 1.4 1.6 1.8 20

0.5

1

1.5

2

2.5

wavelength (µm)

εi (N

1)

εi (N

2)

εi (N

3)

εi (N

4)

(b)

Fig. 1: Permittivity of ITO with different carrier concentrations (N1= 4.33× 1020cm−3, N2= 6.67× 1020cm−3,N3= 8.31× 1020cm−3 and N4= 9.58× 1020cm−3), measured by spectroscopic ellipsometry. The thickness of ITO in thetested samples is 35 nm. (a) the real part of the complex permittivity; (b) the imaginary part of the complex permittivity.

(99.99%) disc target of four inches in diameter. The ITO targetpower was 200 W, and post-deposition annealing treatmentswere applied to tune the optical dispersion of the films.The permittivity was directly measured using spectroscopicellipsometry (Woollam VASE) [17]. The measured permittivityof ITO with different carrier concentrations (extracted from themeasured plasma frequency), are shown in Fig. 1. For the ITOsamples with carrier concentrations fromN1 to N4, the ENZwavelengths, are 1920 nm, 1550 nm, 1390 nm and 1270 nm,respectively.

With the above measurements, we have demonstrated thatthe ENZ wavelength of ITO can be shifted into the nearinfrared regime with a carrier concentration of∼ 1020 cm−3.Recent works by E. Feigenbaum etc. [16] on the ITO materialhave experimentally demonstrated that the change of thecarrier concentration inside the thin ITO layer (∆N ) up to1022 cm−3 can be achieved by carrier accumulation at theITO/oxide interface under a few volts applied across themetal/oxide/ITO structure. Therefore, the permittivity of theITO layer would be tunable by adjusting the applied voltage.In the next section, we will use the measured permittivity ofITO to design electro-absorption modulators embedded withENZ ITO materials.

III. M ODE ANALYSIS AND MODULATOR DESIGN

Figure 2(a) shows the cross-section schematic of our pro-posed slot-waveguide modulator based on ITO. The structureconsists of a Si strip waveguide (of thicknesst1 = 220 nm) ona buried oxide layer, an active ITO layer as thin astITO = 10nm (as demonstrated by [14]), two thin silicon dioxide (SiO2)buffer layers (tb = 10 nm) and a poly-Si capping layer(t2 = 160 nm). The width of the modulator is 500 nm. Theoptical intensity of the TM modes is well confined in the slot,which effectively enhances the overlap of the optical modesand the ITO active medium. Some previous experimentalwork [13], [16] have already demonstrated that a suitablevoltage applied across the metal/SiO2/ITO structure allows totune the permittivity of the ITO layer by carrier accumulationat the ITO/SiO2 interface, thereby varying the optical loss

of the modulator structure. The carrier concentration layerat the ITO/SiO2 has been estimated to be 5±1 nm thick.Accordingly, we will assume that a 5-nm thick accumulationlayer can be electrically controlled in the ITO at the interfacewith SiO2, and that the permittivity can be tuned by an appliedvoltage.

The optical loss of the fundamental TM mode (αm) ofthe slot-waveguide modulator is primarily due to the free-

(a)

1 1.2 1.4 1.6 1.8 20

2

4

6

8

10

12

Wavelength (µm)

dBµm

−1

α

m (N

1)

αm (N

2)

αm (N

3)

αm (N

4)

(b)

Fig. 2: (a) Schematic of the modulator structure and theprofiles of the fundamental TM mode (λ = 1310 nm); (b)modulator absorption loss for different carrier concentrationsbased on a 2D FDTD mode solver analysis.




1.1 1.2 1.3 1.4 1.5 1.6 1.70

2

4

6

wavelength (µm)

dBµm

−1

SiO

2: 10nm

SiO2: 7nm

Si3N

4:10nm

Si3N

4: 7nm

1.1 1.2 1.3 1.4 1.5 1.6 1.70

5

10

wavelength (µm)

dBµm

−1

1.1 1.2 1.3 1.4 1.5 1.6 1.70

5

10

15

wavelength (µm)

dBµm

−1

1.1 1.2 1.3 1.4 1.5 1.6 1.7−5

0

5

10

15

20

wavelength (µm)

dBµm

−1

αm

(N1)

αm

(N4)−α

m (N

1)

αm

(N2)−α

m (N

1)

αm

(N3)−α

m (N

1)

Fig. 3: Modulator performance with different buffer materials and thicknesses.

carrier absorption in the active ITO layer. This loss can beapproximated by the product of the optical mode confinementfactor (Γ) and the material absorption of the bulk ITO (αb):

αm = Γ · αb (2)

whereαb = 2k0 ·Im[ε1/2]. The confinement factor,Γ, dependson |ε| and the waveguide structure (the active layer thickness,the buffer material and thickness) [15]. As the permittivityis tuned to induce absorption modulation, bothΓ and αb

are altered. A smaller|ε| can enhance the electric field (E)magnitude confined in the ITO layer due to the continuityof the normal component of the electric displacement field(D = εE). Therefore, it is important to engineer the ENZcondition so that a minimum|ε| is achieved for a state withhigh optical loss. As shown in Figure 2(a), the electricalintensity of the fundamental TM mode is confined in theSiO2/ITO/SiO2 slot with ITO of N1= 4.33× 1020cm−3 ,resulting in a small absorption loss (0.6dB/µm−1) at thewavelength of 1.31µm. When the carrier concentration istuned to beN4= 9.58× 1020cm−3, the ǫr of the ITO is nearzero. The electrical intensity is mostly confined only in theENZ ITO material, thus a high absorption loss (9.8dB/µm−1)is obtained at this state.

Utilizing the permittivity results of the ITO in sectionII, we employ a 2D mode solver to simulate the opticalloss of the fundamental TM mode (TM0) of the mod-ulator. Figure 2(b) shows the optical loss of theTM0

mode of the slot-waveguide structure with ITO carrier con-centration N1= 4.33× 1020cm−3, N2= 6.67× 1020cm−3,N3= 8.31× 1020cm−3, andN4= 9.58× 1020cm−3. Over theentire optical fiber communications band, the mode absorptionloss (αm) is fairly low for a carrier concentration ofN1, and isrelatively high for carrier concentrations ofN2 , N3, andN4.Therefore, we define the state with carrier concentrationN1

as the ON state. As shown in Fig. 2(b), the maximum opticalloss forN2, N3 andN4 are achieved at 1.58µm, 1.42µm,and 1.29µm, respectively. Then, we can optimize the OFFstate of the modulator for different wavelengths of operation.The performance of the electro-absorption modulator is thendetermined by the ratio of the optical loss in the ON state andOFF state where:

αON = αm(N1) (3)

∆α = αm(Ni)− αm(N1) (i = 2, 3, 4). (4)

For the slot-waveguide structure,Γ depends on the bufferlayer thickness and its refractive index. Therefore, we evaluatethe performance of the modulator with two different processcompatible buffer materials,SiO2 and silicon nitride (Si3N4),and two different buffer thicknesses, 10 nm and 7 nm. ThecorrespondingαON and∆α are plotted in Fig. 3. The modula-

1.1 1.2 1.3 1.4 1.5 1.6 1.70

10

20

30

40

50

wavelength (µm)

Γ (%

)

SiO

2: 10nm

SiO2: 7nm

Si3N

4:10nm

Si3N

4: 7nm

Fig. 4: The optical confinement factor (Γ) inside the ITO layerwith a carrier concentration ofN1= 4.33× 1020cm−3.




Fig. 5: 3D schematic of the slot-waveguide modulator (with7-nm thickSi3N4 buffers) butt coupled to Si strip waveguides.

tor with 10-nm thickSiO2 buffers demonstrates the minimumαON, which is only 0.55dBµm−1 at the wavelength of 1.31µm, and 1.68dBµm−1 at the wavelength of 1.55µm. Themaximum∆α is achieved with 7-nm thickSi3N4 buffers: 15.9dBµm−1 at 1.31µm (for carrier concentration increased fromN1 to N4) and 8.9dBµm−1 at 1.55µm (for carrier concen-tration increased fromN1 to N2). Figure 4 demonstrates theoptical confinement factor (Γ) inside the active ITO layer witha carrier concentration ofN1= 4.33× 1020cm−3. Generally,with the same thickness, the structure with theSi3N4 buffershows higherαON as well as higher∆α since it enables ahigher Γ. For a given buffer material, the 10-nm thicknessshows lowerαON and lower∆α due to a smallerΓ.

For a specific application and wavelength of interest, severalparameters such as the carrier concentration for the OFF state,and the material and thickness of the dielectric buffers shouldbe selected accordingly. For instance, if the target operatingwavelength is 1.31µm, then a modulator with 7-nm thickSi3N4 buffers and carrier concentration tuning fromN1 toN4 would be preferred. These conditions enable a maximum∆α while αON is only 0.9 dBµm−1 at this wavelength. Ifthe modulator is designed to work at the wavelength of 1.55µm, then maintaining a lowαON is an important considerationsince the absorption loss increases as the wavelength increases.Based on the previous experimental results and analysis in[13], [16], the bias voltage for our proposed modulator isestimated to be 2 - 4 volts, which depends on the oxidebuffer (its refractive indexnb and thicknesstb) and the carrierconcentration change (∆N ). In a future work, the requiredbias voltage will be studied through ellipsometry and opticaltransmission measurements after fabricating the proposedge-ometry and making contacts on the ITO and doped-Si layer.

IV. I NTEGRATION OF SLOT-WAVEGUIDE MODULATOR WITH

SI STRIP WAVEGUIDES

In this section, we present two methods to integrate theITO modulator with conventional Si strip waveguides. First,we propose a butt-coupled scheme with polarization rotatorsconnecting the TE-loaded Si waveguides and the TM-modeslot-waveguide modulator (1-µm device length) section. Thisscheme works well for a wide optical band: from 1.28µm to1.60µm by choosing suitable carrier concentrations. Second,the evanescent-coupling scheme is presented at the wavelengthof 1.31µm.

1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.650

2

4

6

8

10

12

14

Wavelength (µm)

dB

IL [N

1]

ER [N1 −> N

2]

ER [N1 −> N

3]

ER [N1 −> N

4]

Fig. 6: The modulator performance of the butt-coupled struc-ture (1.0-µm modulator length) with the carrier concentrationtuned fromN1 to N2, N1 to N3 andN1 to N4, respectively.

x (µm)

z (µ

m)

−10 −5 0 5 10

0

0.2

0.4ITO with N

1 at λ = 1310 nm

T = 81.4 %

(a)

x (µm)

z (µ

m)

−10 −5 0 5 10

0

0.2

0.4ITO with N

4 at λ = 1310 nm

T = 5.4 %

(b)

Fig. 7: The light propagating from the front end to the backend in the butt-couple scheme atλ = 1.31µm: (a) N = N1;(b) N = N4.

A. Butt-coupled scheme

For on-chip optical interconnects, the proposed compactand efficient electro-absorption modulator should be integratedwith Si strip waveguides in the Si-on-insulator (SOI) platformsince these waveguides exhibit low propagation and bendloss. Based on mode analysis, the optical loss of theTM0

mode of the slot-waveguide modulator is efficiently modulatedby tuning the carrier concentration in the active ITO layer.Therefore, it is critical to optimize the coupling efficiency (η)from the Si strip waveguides to theTM0 mode of the slot-waveguide modulator [18].

Figure 5 shows a 3D schematic diagram of the proposedslot-waveguide modulator butt coupled to low-loss Si stripwaveguides. Here, the dielectric buffers are 7-nm thickSi3N4




layers. In such a 3D stucture, the total propagation lossdepends on the modal loss (αm) and the coupling efficiency(η) from the strip waveguide to the slot waveguide. Themodulator performance is given by the insertion loss (IL) andthe extinction ratio (ER):

IL = 10log[Iin

Iout(N1)] (5)

ER = 10log[Iout(N1)

Iout(Ni)] (i = 2, 3, 4). (6)

At the input, the horizontal Si strip waveguide is 220 nm thickand 380 nm wide, which is designed for single mode operation.The fundamental TE mode (TE0) is launched in the horizontalSi strip waveguide, then converted to theTM0 mode through apolarization rotator [18], [19]. The polarization rotatorconsistsof two vertically stacked linear width tapers (9µm in length).The end of the polarization rotator connects to a vertical stripwaveguide (380 nm thick and 220 nm wide). The resultingTM0 mode of the vertical strip waveguide is then coupledto the TM0 mode of the slot-waveguide modulator. Themodulator lengthLm is only 1µm for this design. Followingpropagation in the modulation region, the light is coupled totheTM0 mode of a strip waveguide and then rotated back totheTE0 strip mode through the output rotator.

A 3D FDTD simulation tool is employed to further evaluatethe performance of the ITO-based Si modulators. Figure 6shows the modulator performance when the carrier concen-tration is tuned fromN1 to N2, N1 to N3 and N1 to N4,respectively. For a wavelength range from 1.53µm to 1.64µm, the ER is greater than 5 dB while the IL is less than 2.7 dBwhen the carrier concentration is tuned fromN1 toN2. At 1.55µm, the ER and IL are 5.8 dB and 2.0 dB, respectively. If the7-nm thickSi3N4 buffers are replaced by some 10-nm thickSiO2 buffers, the IL can be further reduced. When the carrierconcentration is tuned fromN1 to N4, the ER is greater than6.0 dB while the IL is less than 1.3 dB for a wavelength rangefrom 1.25µm to 1.42µm. Especially, at 1.31-µm wavelength,the IL is 0.93 dB, which is only slightly higher than the modeabsorption lossαm(N1) =0.86 dB. This indicates an efficientcoupling from theTE0 mode of the Si strip waveguide to theTM0 mode of the slot-waveguide modulator for the ON state.The corresponding ER here is 11.75 dB. Figure 7 shows thelight propagation from the front end to the back end (includingthe polarization rotators on both sides) at 1.31µm with carrierconcentrations ofN1 and N4, respectively. The normalizedtransmission at the output are 81.4% in Fig. 7(a) and 5.4%in Fig. 7(b).

B. Evanescent-coupled scheme

In the previous subsection, by incorporating polarizationrotators, the slot-waveguide modulator was connected withTE-loaded Si strip waveguides and other passive/active com-ponents that operate for TE-polarized light. Alternatively, theslot-waveguide modulator can be evanescently coupled to TM-loaded Si strip waveguides, therefore we investigate such astructure in this subsection.

As shown in Fig. 8, the ITO-based modulator layers are po-sitioned on top of a Si strip waveguide (220 nm thick and 400

Fig. 8: 3D schematic of the slot-waveguide modulator (with10-nm thick SiO2 buffers) evanescently coupled to Si stripwaveguides.

0 0.5 1 1.5 2 2.5 30

5

10

15

Modulator Length (µm)

dB

0 0.5 1 1.5 2 2.5 30

1

2

3

dB

ER [N1 −> N

4]

IL [N1]

(a)

1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65

0

2

4

6

8

10

12

Wavelength (µm)

dB

IL [N1]

ER [N1 −> N

4]

(b)

Fig. 9: (a) Extinction ratio (ER) and insertion loss (IL)versus modulator length (Lm) at 1.31-µm wavelength in theevanescent-coupled scheme; (b) modulator performance withLm = 1.5 µm for a wavelength range from 1.25µm to 1.65µm.

nm wide). Compared with butt coupling, evanescent couplingis less efficient, however, it simplifies the fabrication. Fig. 9(a)shows the performance of the slot-waveguide modulator with10-nm thickSiO2 buffers as a function of the device length(Lm) at the wavelength of 1.31µm. In this case, the carrierconcentration is tuned fromN1 to N4. When the device lengthincreases, the ER increases gradually while the IL experiencesperiodic variation. To achieve a high ER and a low IL, we setLm = 1.5 µm. For a wavelength of 1.31µm, the ER and ILare 10.1 dB and 1.5 dB, respectively. The performance of the




modulator withLm = 1.5µm over a broad wavelength rangeis shown in Fig. 9(b). The ER is greater than 6.0 dB whilethe IL is less than 1.7 dB for a wavelength ranging from 1.25µm to 1.39µm.

V. CONCLUSION

In this paper, a compact broadband modulator designbased on engineered ENZ ITO materials has been pre-sented. By assuming electrically tunable permittivity of theactive ITO layer between the states with carrier concen-trations of N1= 4.33× 1020cm−3, N2= 6.67× 1020cm−3,N3= 8.31× 1020cm−3 and N4= 9.58× 1020cm−3, the op-tical loss of the proposed slot-waveguide modulator has beenmodulated with high efficiency. We have also discussed twomethods that integrate our modulator with Si Strip waveguides.In the first method, using a butt-coupled scheme with a 1.0-µm device length, our modulator has demonstrated a high ERabove 6.0 dB and an IL below 1.3 dB for a wavelength rangingfrom 1.25µm to 1.42µm. By adjusting the carrier concentra-tion, the broadband modulation has been demonstrated whenthe wavelength is around 1.55µm. In the second method (i.e.,the evanescent coupling scheme with a 1.5-µm device length),an ER greater than 6.0 dB and an IL less than 1.7 dB havebeen achieved for a wavelength ranging from 1.25µm to 1.39µm.

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[19] J. Zhang, M. Yu, G. Lo and D. Kwong, “Silicon-waveguide-based modeevolution polarization rotator”, IEEE J. Sel. Topics Quantum Electron,vol. 16, no. 1, pp. 53–60, 2010.

Hongwei Zhao received the M.S. degree from theInstitute of Semiconductors, Chinese Academy ofSciences (Beijing, China) in 2011. Prior to that, shecompleted her undergraduate studies at HuazhongUniversity of Science and Technology (Wuhan,China), with a BEng in electronics in 2008. Sheis currently a research assistant in the IntegratedPhotonics Lab in the department of the electricaland computer engineering at Boston University. Herresearch is focused on the design and fabrication ofthe nanophotonic waveguide modulators.

Yu Wang received the M.S degree of Electro-Optics from University of Dayton (Dayton, Ohio)in 2012, and B.Eng degree in Optoelectronic in-formation from HuaZhong University of Scienceand Technology (WuHan, China) in 2010. He iscurrently working toward the Ph.D. degree in electri-cal engineering in the Department of Electrical andComputer Engineering and Photonics Center, BostonUniversity, Boston, MA. His research interests in-clude fabrication, simulation, structural, optical, andelectrical characterization of optoelectronic material,

nanoscale photonic structures based on transparent conductive oxides for lightemission devices.

Antonio Carpretti received his Laurea and LaureaSpecialistica (B.S. and M.S. degrees) cum laude inElectronic Engineering in 2007 and 2009 from Uni-versity of Naples Federico II, Italy. He was VisitingResearcher at the Boston University Department ofElectrical & Computer Engineering from 2011 to2012. In 2013 he received his PhD in Electrical En-gineering from University of Naples Federico II witha thesis on nonlinear plasmonics. He is currentlya Postdoctoral Associate at the Boston UniversityDepartment of Electrical& Computer Engineering.

His research interests include nano optics& photonics, superconductivity andcomputational electromagnetism.




Luca Dal Negro received both the Laurea inphysics, cum laude, in 1999 and the Ph.D. degreein semiconductor physics from the University ofTrento, Italy, in 2003. After his Ph.D. in 2003 hejoined the Massachusetts Institute of Technologyas a post-doctoral research associate. Since January2006 he has been a faculty member of the Depart-ment of Electrical and Computer Engineering andthe Photonics Center at Boston University, where heis currently tenured Associate Professor. His mainresearch interests focus on light-matter interaction

in aperiodic media, nanophotonics, complex plasmonic nanostructures, andcomputational electrodynamics. He was the recipient of the 2009 NationalScience Foundation CAREER Award, published more than 180 technicalarticles, and has been an invited speaker at numerous international conferencesand symposia.

Jonathan Klamkin received the B.S. degree inelectrical and computer engineering (ECE) fromCornell University in 2002, and the M.S. degree inECE and Ph.D. in materials from the University ofCalifornia Santa Barbara in 2004 and 2008, respec-tively. From 2008-2011 he was a member of theTechnical Staff in the Electro-Optical Materials andDevices Group at MIT Lincoln Laboratory. From2011-2013 he was an assistant professor at the Insti-tute of Communication, Information and PerceptionTechnologies (TeCIP), Scuola Superiore Sant’Anna,

Pisa, Italy, where he was the recipient of an Erasmus Mundus scholarshipand a Marie Curie fellowship, and served as the director of the IntegratedPhotonic Technologies Center. In 2013 he joined Boston University (BU) asan assistant professor in ECE and Materials Science and Engineering. He isalso affiliated with the BU Photonics Center and leads the Integrated PhotonicsGroup. Prof. Klamkin has served on the Technical Program Committeefor the Microwave Photonics Conference, the IEEE PhotonicsConference,Photonics in Switching, the Optical Fiber Communication Conference, andIntegrated Photonics Research, Silicon and Nanophotonics. He received bestpaper awards at the 2006 Conference on Optoelectronic and MicroelectronicMaterials and Devices and the 2007 Microwave Photonics Conference, and isthe recipient of a NASA Early Career Faculty Research Grant.Prof. Klamkinhas authored or coauthored 90 papers on photonic integratedcircuits, siliconphotonics, nanophotonics, microwave photonics, coherent receivers, high-power photodiodes, optical modulators, high-power lasers,widely-tunablelasers, and semiconductor optical amplifiers. He is a senior member of theIEEE and member of OSA.

Broadband Electro-Absorption Modulators Design Based on Epsilon-Near-Zero Indium Tin Oxide

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