a biosensor based on plasmonic wave excitation with di ractive...

Scientia Iranica F (2017) 24(6), 3441{3447

Sharif University of TechnologyScientia Iranica

Transactions F: Nanotechnologywww.scientiairanica.com

A biosensor based on plasmonic wave excitation withdi�ractive grating structure

V. Faramarzi, V. Ahmadi�, F. Ghane Golmohamadi and B. Fotouhi

Department of Electrical & Computer Engineering, Tarbiat Modares University, Tehran, Iran.

Received 10 July 2016; received in revised form 6 February 2017; accepted 13 June 2017

KEYWORDSPlasmonic device;Refractive index;Di�ractive grating;Surface plasmonresonant;Graphene;Biosensingapplication.

Abstract. We propose an active plasmonic device based on graphene. By using adi�ractive grating on silicon, highly con�ned plasmonic waves in monolayer graphene aree�ciently excited. The high electric �eld of the surface plasmons near the graphene layermakes them ideal for use in biosensors, where a very small change in refractive index shouldbe detected. A small change in the refractive index of surroundings medium induces a largechange in resonant wavelength. The spectral shift per refractive index unit (��0/RIU) ofthe sensor is considerable and exceeds 2000 nm/RIU with a narrow spectral line width of theplasmon resonances. The �gure of merit for this sensor is approximately equal to 14.2 andelectrical tuning can be achieved by adjusting the Fermi level in graphene. Therefore, thisgrating plasmonic structure is highly sensitive and can be used in high-resolution biosensingapplications.

© 2017 Sharif University of Technology. All rights reserved.

1. Introduction

Good electrical conductivity and mechanical strength,ultra-high mobility of massless fermions, relativelygood transparency, high thermal conductivity, and high exibility make graphene the best option for replace-ment or simultaneous use in the silicon world. Fromthe viewpoint of optics, graphene has linear dispersionrelationship, strong interaction with electromagnetic�eld, and high-speed response [1]. The distinctivecharacteristics of graphene make it suitable to pro-pose integrated active plasmonic devices with a widerange of wavelengths from near-infrared to THz [2,3].By using the active plasmons, con�ned light can bemanipulated and externally controlled in a structurewith subwavelength dimensions [4]. Plasmonic waves

*. Corresponding author. Tel/Fax: +98 21 82883368E-mail address: v [email protected] (V. Ahmadi)

doi: 10.24200/sci.2017.4253

can be strongly controlled with a broad adjustabilityrange because plasmonic properties change dramati-cally when the free carrier density in a semiconductoris adjusted by several orders of magnitude. The carriermobilities in conventional semiconductors are too lowat room temperature, and cannot support plasmonicwaves for acceptable propagation distances. However,Carrier mobility in graphene is high even at roomtemperature. Thus, plasmonic wave in the graphene�lm can be highly con�ned and propagate up to severalmicrometers [3]. In addition to broad adjustability, theelectric �eld of a plasmonic wave is highly con�nedand localized in the thin layer of graphene. Thehighly localized optical �eld makes strong light-matterinteractions and can be used to build Surface PlasmonResonance (SPR) biosensors with high sensitivity [5].

Surface Plasmon Resonance (SPR) biosensors areoptical sensors, which use surface plasmon waves toprobe the interactions between biomolecules and thesensor surface. The technique of Surface PlasmonResonant (SPR) is very sensitive to refractive index

3442 V. Faramarzi et al./Scientia Iranica, Transactions F: Nanotechnology 24 (2017) 3441{3447

variations in sensing medium. When the liquid samplemakes contact with the sensor surface, due to binding ofanalyte with a�nity antibodies, which are immobilizedon the sensor surface, the change in the plasmonresonant modes occurs. Therefore, material, which isin contact with the liquid sample (top layer of sensor),is in uential and important in terms of refractive indexand thickness of the adsorbed biolayer [6,7]. Recently,the graphene layer has been used for anchoring an-tibodies to the SPR sensor as new surface function-alization. The biomolecules are e�ciently adsorbedon the graphene layer surface, due to the �-stackinginteractions between graphene's hexagonal cells andthe carbon-based ring structures in biomolecules [8,9].Thus, the molecules get adsorbed on graphene surface,as the sample-containing analyte molecules come ontothe graphene surface. This results in the formation ofa layer of which the refractive index is higher than abu�er solution (water) and thus, it a�ects the resonantwavelength of plasmonic wave. As the performance ofSPR sensor depends on the biomolecular adsorption,the material type of upper surface which immobilizesthe biomolecules (antibodies) on the sensor surfaceplays an important role. In [10], we have proposed andanalyzed new plasmonic-based structures for sequenc-ing DNA molecules and detecting nanoscale objectsby altering the external electromagnetic radiation tostrong local �eld of the plasmon.

A key challenge in the plasmonic resonance sen-sors is coupling the surface plasmon with optical wavein free space, because plasmonic wave vector is muchlarger than free-space wave vector, which gives rise tothe momentum mismatch between light and surfaceplasmon [11]. By using di�erent coupler con�gurationssuch as prism couplers and grating couplers [12],the momentum of light and surface plasmon can bematched. In this work, we use rectangular di�ractivegrating to create plasmonic wave resonance in a mono-layer graphene �lm. These resonant modes couple inci-dent light to the Surface Plasmon Polariton (SPP) withmaximum coupling e�ciency and induce intense changein the optical transmission at the resonant frequency.The ability of resonant waveguide grating biosensors tomonitor changes of cell adhesion of live cells in real timeand label-free investigations of binding events makes itan attractive tool in medical application [13]. In [14-16], plasmonic biosensors based on the graphene havebeen investigated. In these con�gurations, the prismcoupler (Kretchmann con�guration) has been used toexcite the surface plasmon waves. However, di�ractiongrating structures are generally more compact, phys-ically lighter, and less costly than the prism-coupledcon�gurations. Therefore, for integrated biosensor de-sign, grating-coupled SPR may be more bene�cial as itallows the use of semiconductor materials as substrates.Another advantage of this coupling method over the

previously mentioned methods is good reproducibilityof coupling conditions since no further optical elementsare directly involved.

We show that the di�ractive grating device actsas a highly tunable biosensor that can be tuned witha small change in the Fermi level of graphene. Byadjusting the Fermi level of graphene and gratingperiods, this structure can be used to build extremelysensitive biosensor with broad range of wavelengthsfrom mid-infrared (mid-IR) to terahertz (THz).

2. Materials and methods

To excite SPP waves in the graphene, wave vector ofincident light and SSP have to match. The use ofoptical gratings can compensate for the wave vectormismatching. Here, we use a silicon di�ractive gratingunderneath the graphene, as shown in Figure 1, toexcite the surface plasmons.

The e�ective refractive index of plasmonic modeis high and de�ned as ne� = Re(�(!))=k0 in the mid-IR wavelength range, where �(!) is the in-plane wavevector in graphene. The grating period, T , which isrelated to frequency, !0, is determined by the phasematch equation [4,17]:

Re(�(!0))� !0

csin � =

2�T; (1)

where c is the speed of light, � is the incident angle,and !0=c is the vacuum wave vector, k0. Under thenormal-incidence wave condition (� = 0), a Guided-Wave Resonant (GWR) is excited when the gratingperiod, T , matches the period of the plasmonic wave(2�(Re(�(!)))). When the SPP waves are excited andpropagated in graphene, optical energy is dissipateddue to the ohmic loss in the graphene layer. Therefore,a notch can be seen in the transmission spectrumaround the resonant frequency, !0, as:

Figure 1. Schematics of a silicon di�ractive grating basedon plasmonic waves in the graphene layer.

V. Faramarzi et al./Scientia Iranica, Transactions F: Nanotechnology 24 (2017) 3441{3447 3443

!0 =

s2e2 � Ef

~2"0 ("r1 + "r2)T; (2)

where ~ is the reduced Planck constant, "0 is thevacuum permittivity, "r1 and "r2 are the dielectric con-stants of the materials above and below the graphene�lm, respectively, and Ef = ~vf (�n)1=2 is the Fermienergy level, where n is the carrier density and vf �106 m/s is the Fermi velocity in graphene [4,17]. Tosimulate the proposed structure, the �nite elementmethod is used. The graphene �lm is considered tobe monolayer with the thickness of t = 0:5 nm andcarrier mobility of � � 10000 cm2/(V.s). Moreover,the dielectric tensor components are "r11 = "r22 =2:5+i�=("0!t), where � is the conductivity of graphene,and the surface-normal component is set as "r33 = 2:5based on the dielectric constant of graphite.

The intraband conductivity of graphene at mid-IR frequencies is modelled with a semiclassical Drudemodel as:

�(!)=2e2

�~2 kBTk�ln�2� cosh

�Ef

2kBTk

��i

!+i��1 ;(3)

where Tk is temperature and kB is the Boltzmannconstant. Here, the e�ect from the interband transitionis neglected because the photon energy in the simulatedspectral range is always less than 2Ef [18].

3. Results and discussion

The evanescent �eld of the plasmonic wave, typicallyextending 30-100 nm into the antibody-analyte (bio-layer) and cover solution layer, is sensitive to two typesof ambient changes. The �rst one is altering the coversolution (a liquid sample that covers the sensing region)refractive index (n1). The second type is adsorption ofmolecules from a liquid sample on the surface of thesensing layer and formation of a thin adhesion layerwith a speci�c thickness and refractive index.

The simulated normal-incidence transmissionspectra with di�erent ambient refractive indices areshown in Figure 2. As shown in the �gure, there isa red shift at the resonant wavelength of spectra bychanging the refractive index surrounding the graphenelayer and di�ractive grating (n1). This change may becaused by changes in biomolecules concentration, whichin turn can produce a local change in the refractiveindex. According to the equation "r1 = (n1 � ik1)2,by increasing the refractive index of cover medium,the dielectric constant of the surrounding mediumincreases. The dielectric constant change in turn leadsto a change in the propagation constant of plasmonicwave and will eventually lead to red shift in SPPresonant wavelength.

Figure 2. Simulated normal-incidence transmissionspectra for di�erent refractive indices of sample mediumn1 for Ef = 0:64 eV and T = 150 nm.

Figure 2 illustrates simulated normal-incidencetransmission spectra for di�erent refractive indices ofsample medium (n1) for Ef = 0:64 eV and T =150 nm. It clearly shows that by a small variation inthe refractive index, the resonant frequency for whichtransmission is minimal varies. According to Eq. (2)and "r1 = (n1 � ik1)2 and considering that k1 = 0,it can be concluded that the resonant frequency (!0)is inversely proportional to cover medium refractiveindex (n1). Thus, as shown in Figure 3, the resonantwavelength has a linear relationship with the covermedium refractive index [19,20].

The spectral shift per Refractive Index Unit(RIU) change is proportional to the fraction of the�elds and energy density associated with the plasmonexcitation located in the sensing medium. Taking thewavelength interrogation approach, sensitivity is de-�ned as the shift of the resonant wavelength as a func-tion of the refractive index of the sample medium (S =�0/RIU). Another major factor in biosensing applica-tion is Figure Of Merit (FOM). FOM is considered asa useful parameter in verifying plasmonic biosensor. Itis de�ned as the ratio of the refractive index sensitivityto the resonant width (FOM=S(nm:RIU�1)��(nm)).

Figure 3. Resonant wavelength of the surface plasmonicwave versus refractive index of the biological sample.


In most biosensing mechanisms, Biomolecular Recogni-tion Element (BRE) is used to absorb presented analyteor biomolecule in aqueous solution [21]. In this work,we consider antibodies as BRE. By immobilization ofthe antibodies on the graphene surface, the analytescan be captured by antibodies and form a thin layeron the graphene surface, which induces a local increasein the refractive index near the graphene surface. Theschematic mechanism is shown in Figure 4. At theinitial stage, when the cover medium only includes theaqueous solution (water), the refractive index is equalto 1.332. When the BRE is added to the aqueoussolution, the refractive index of the cover mediumchanges negligibly. In the next stage, biomolecules arecaptured by the BRE and the cover medium refractiveindex increases.

Figure 5 shows simulated normal-incidence trans-mission spectra for three di�erent cases of mediumrefractive index around the grating structure: 1) air,2) cover solution-antibody, and 3) antibody-analyte

Figure 4. Principle of SPR biosensing; immobilization ofthe antibodies on the graphene surface to capture theanalytes.

Figure 5. Simulated normal-incidence transmissionspectra for three di�erent cases of the medium refractiveindex changes around the grating structure: 1) air, 2)cover solution-antibody, and 3) antibody-analyte adhesionlayer (Ef = 0:64 eV and T = 150 nm). The thickness ofadhesion biolayer is set to 80 nm.

Figure 6. Simulated normal-incidence transmissionspectra for di�erent thicknesses of adhesion layer forEf = 0:64 eV and T = 150 nm.

adhesion layer. As shown in Figure 5, before bindingthe analyte to antibody, the resonant wavelength is9510 nm and it is red-shifted to 10272 nm after bindingthe analyte, due to the increase in the propagationconstant of Surface Plasmon Wave (SPW) propagatingalong the graphene surface.

The evanescent �eld of SPW has a limited pene-tration depth into the graphene adjacent layers. Themagnitude of the change in the propagation constantand, consequently, resonant wavelength red-shift of anSPW depends on the refractive index changes (inducedby antibody-analyte binding on the graphene surface)and its distribution with respect to the pro�le of theSPW �eld and thickness of adhesion layer. As shownin Figure 6, when the refractive index changes arecaused by a binding event, which occurs at a speci�cdistance from the sensing surface (much smaller thanthe penetration depth of the SPW), the magnitude ofthe resonant wavelength red-shift is large because thewhole region of antibody-analyte binding event (thewhole thickness of biolayer) is within the extent of theSPW �eld. By increasing the thickness of adhesionlayer, the magnitude of shift in the resonant wavelengthis reduced because only a fraction of the thicknessof the adhesion layer is within the evanescent �eldof SPW and depth of the area in which the changeoccurs is limited (Figure 6). The resonant wavelengthchange in terms of adhesion layer thickness is shown inFigure 7.

This sensor has a high spectral shift ��0=�n witha narrow spectral line width of the plasmon resonance(�� 1412 nm). The FOM for this plasmonicbiosensor structure is approximately equal to 14.2 andthe sensitivity exceeds 2000 nm/RIU, which is largerthan most metal mid-IR resonant biosensor struc-tures [22,23]. Recently, a highly sensitive refractive in-dex sensor based on exciting localized Surface PlasmonPolaritons (SPPs) on graphene-coated nanowire arrayshas been studied [20]. The sensitivity of this device


Figure 7. Resonant wavelength of the SPW versusthickness of the adhesion layer.

Figure 8. Simulated normal-incidence transmissionspectra of monolayer graphene on silicon gratings withdi�erent Fermi energy levels in graphene when n1 = 1:4and T = 150 nm.

is high; however, fabrication process of the proposedstructure seems to be di�cult. Our proposed structurehas some advantages such as simple fabrication process,small size, and easy electrical tuning by applying a gatevoltage between the graphene layer and substrate.

Electrical tuning of the proposed plasmonic sen-sor can be achieved by adjusting the Fermi level ingraphene. Sharp notches from the plasmonic waveresonant can be seen. One can identify multiplenotches on each transmission spectrum, which resultfrom multiple plasmonic modes. As shown in Figure 8,with a small change in the Fermi level, a broad tuningrange can be achieved. One can see in Figure 8 thatthe resonant wavelength is blue-shifted as the Fermienergy level increases from 0.64 eV to 0.84 eV.

In Figure 9(a), the electric �eld pro�le of thefundamental mode in one grating period and for abiological medium with speci�c refractive index (n1 =1:4) is shown. The notches at the lower wavelengthare caused by higher order modes. In Figure 9(b), theelectrical �eld pro�le of the second-order mode is shown

Figure 9. Side-view of electrical �eld distribution for (a)the fundamental mode and (b) the second-order mode inone grating period.

where the E-�eld has a 4� phase shift in each gratingperiod.

4. Conclusions

We have presented a novel concept for a biologicalsensor using surface plasmon waves based on periodicdi�ractive grating structures. This device has a numberof interesting bene�ts. First, due to using periodicdi�ractive grating structures, the plasmonic wave ishighly con�ned in monolayer graphene �lms. Second,the device is highly sensitive to the binding events and,consequently, refractive index change of the biologicalmedium. Third, the device is highly tunable due tochange in the graphene Fermi level by applying a gatevoltage. The numerical simulations show that theproposed structure has a great potential to be usedin biosensing applications. The sensitivity for thisplasmonic biosensor structure exceeds 2000 nm/RIU,which is larger than those for most metal mid-IRresonant biosensor structures.

Acknowledgment

The authors acknowledge the partial support of thisproject by Iran Nanotechnology Initiative Council(INIC).

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Biographies

Vahid Ahmadi (M'86, SM'09) received the PhDdegree in Electronic Engineering from the Kyoto Uni-versity, Japan, in 1994. He is a Professor in ElectronicEngineering at Tarbiat Modares University (TMU),Tehran, Iran. He was the Head of the SemiconductorDepartment of Laser Research Centre, Tehran, during1994-2006, and the Head of the Electrical EngineeringDepartment of TMU during 2006-2008. He was also theTechnical and Scienti�c Chair of 13th Iranian Confer-ence on Optics and Photonics, 2007, and the generalChair of the 16th Iranian Conference on ElectricalEngineering (ICEE2008), 2008. He is the member ofthe Founders-Board of Optics and Photonics Societyof Iran and an editorial board member of InternationalJournal of Information & Communication TechnologyResearch (IJICTR). He is also the Editor-in-Chief ofModares Journal of Electrical Engineering. His currentresearch interests include quantum photonics devices,CNT and GNR based photonic devices, optical mod-ulator and ampli�ers, optical microresonator activeand passive devices, all optical switches, slow light,photonic crystals, and plasmonic and metamaterialdevices.

Vahid Faramarzi received the BSc degree in Elec-trical Engineering from Razi University, Kermanshah,Iran, in 2010, and MSc degree in Electrical Engi-neering from the Department of Electrical Engineeringat Sharif University of Technology, Tehran, Iran, in


2012. He is currently working toward the PhD degreein the same �eld with Tarbiat Modares University,Tehran, Iran. His current research interests include bio-photonics, optoelectronic devices, and graphene-basedsensors.

Farzin Ghane Golmohamadi received his MSc de-gree from Tarbiat Modares University, Tehran, Iran,in 2012. His research interest is optoelectronic de-vices used in medical applications. He is currentlyHead of the Research and Development Department

of MehrOptics Company.

Bashir Fotouhi was born in Sanandaj, Iran, in1988. He received MSc degree in Electrical Engineeringfrom Tarbiat Modares University (TMU), in 2012,and is currently working toward his PhD degree withTMU, since 2012. His professional interests focus onoptoelectronics, bio-photonics, graphene plasmonics,nanopore DNA sequencing, and pattern recognition.His current projects include graphene plasmonics andnanopore DNA sequencing.

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