jnp-14026 authors copy

Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/262975247

SensitivityenhancementinphotoniccrystalwaveguideplatformforrefractiveindexsensingapplicationsARTICLEinJOURNALOFNANOPHOTONICSJUNE2014ImpactFactor:1.45DOI:10.1117/1.JNP.8.083088

CITATIONS2

DOWNLOADS14

VIEWS56

3AUTHORS:

HemantDuttaCentralElectronicsEngineeringResearchIns5PUBLICATIONS6CITATIONS

SEEPROFILE

AmitKumarGoyalCentralElectronicsEngineeringResearchIns6PUBLICATIONS2CITATIONS

SEEPROFILE

SuchandanPalCSIR-CentralElectronicsEngineeringResear63PUBLICATIONS230CITATIONS

SEEPROFILE

Availablefrom:HemantDuttaRetrievedon:06August2015

Sensitivity enhancement in photoniccrystal waveguide platform forrefractive index sensing applications

Hemant Sankar DuttaAmit Kumar GoyalSuchandan Pal

Sensitivity enhancement in photonic crystal waveguideplatform for refractive index sensing applications

Hemant Sankar Dutta,a,* Amit Kumar Goyal,a,b and Suchandan Pala,baOpto-electronic Devices Group, Central Electronics Engineering Research Institute,

Pilani, 333 031 Rajasthan, IndiabAcademy of Scientific and Innovative Research, New Delhi 110001, India

Abstract. A photonic crystal (PhC) waveguide platform based on ring-shaped holes in a silicon-on-insulator substrate is proposed in order to realize a refractive index sensor with an improvedsensitivity. The three-dimensional finite-difference time domain method is used to analyze theproposed design. The sensitivity is estimated by measuring the shift in the upper band-edge ofthe output transmission spectrum. Sensitivity analysis of a conventionally designed PhC wave-guide, followed by modification of the structure, has been carried out for improving the sensitivityby introducing a row of holes that forms the line defect. Further improvement in sensitivity isobtained by replacing the defect row of holes by ring-shaped holes, which shows a significantlyhigh sensitivity along with considerable output signal strength. The optimized design showsa wavelength shift of 210 nm for a change in ambient refractive index from air (RI 1) to xylene(RI 1.5), corresponding to an average sensitivity of 420 nmRIU. 2014 Society of Photo-OpticalInstrumentation Engineers (SPIE) [DOI: 10.1117/1.JNP.8.083088]

Keywords: photonic crystal waveguide; ring-shaped hole, finite-difference time domain;refractive index sensing; sensitivity.

Paper 14026 received Mar. 7, 2014; revised manuscript received Apr. 23, 2014; accepted forpublication May 14, 2014; published online Jun. 3, 2014.

1 Introduction

Photonic crystals (PhCs) are a periodic dielectric material that exhibits a photonic band gap, arange of frequency where the light propagation is forbidden that can be used to manipulate andcontrol the properties of light.1 This unique property has been exploited by researchers andimplemented in many research applications like filters,2 splitters,3 optical resonators,4 high-Qfilters,5 lasers,6 fibers,7 and sensors.8 Among all of them, researchers have shown a higher degreeof interest in PhC technology for sensing applications due to their compactness and tremendoussensing potential. Holes etched to form the PhC structure in a high dielectric material can beinfiltrated with analytes, which change the refractive index (RI) of the holes and thus alter theoptical properties of PhC structures. The method of sensing involves the change in the poweroutput and the shifts in the wavelength. The PhCs with line defects formed by altering a row ofholes can be designed to obtain highly sensitive photonic crystal waveguide (PhCW)-based RIsensors.9 The development of a sensor design with high sensitivity is especially importantbecause it enhances the ability of the device to detect low concentrations as well as small changesin the concentration of the analytes.10

In this article, we proposed a PhCW structure based on ring-shaped holes optimized forRI sensing applications based on three-dimensional finite difference time domain (3-D FDTD)simulations. The device is considered in a silicon-on-insulator (SOI) substrate consisting ofa triangular array of holes etched in its silicon device layer to form a PhC structure. Thewaveguide sensor is formed by replacing the central row of holes in the -X direction andsubsequently optimized by incorporating ring-shaped holes in the defect line in order to achievethe best possible improvement in the sensitivity.

*Address all correspondence to: Hemant Sankar Dutta, E-mail: [email protected]

0091-3286/2014/$25.00 2014 SPIE

Journal of Nanophotonics 083088-1 Vol. 8, 2014

2 Theory and Principle

Analyte infiltration in PhCs changes the RI contrast, which changes the effective index of theslab and thus changes the output spectrum accordingly. In general, the shift in wavelength of aparticular reference level corresponding to the change in the output spectrum is directly related tothe filling factor, i.e., the area available for sensing.11 Since in PhCW structures, the light ismainly concentrated in the defect, the wavelength shift is more sensitive to the change in RInear the waveguide. With the increase in the defect air-filling factor (fdefect), the sensitivityenhances. The improvement in the fdefect can be obtained by replacing the row of defectholes with a row of ring-shaped defect holes. The band diagram of a PhCW structure with ahole-shaped line defect for fdefect of 30% is displayed in Fig. 1(a). The band diagram illustratesa forbidden band of frequencies, where transmission through the waveguide is prohibited. ThisPhCW forbidden band (0.27 to 0.32a) reduces the transmitted frequencies. With increasingfdefect the forbidden band increases, which reduces the transmission signal strength; hence,detection of the output signal becomes difficult. However, for the same fdefect, the ring-shapedline defect holes show an improvement in transmission. The band diagram of a PhCW with ring-shaped line defect hole with fdefect of 30% is shown in Fig. 1(b), which exhibits a forbidden bandfor a much narrower band of frequencies (0.27 to 0.29a) leading to an enhancement in trans-mission signal. Thus, the PhCW structure consisting of a ring-shaped line of defect holesincreases the transmission significantly and therefore provides a better platform to increasethe effective value of fdefect, which in turn improves the sensing potential of the PhCW sensor.

3 Design and Modeling

In this work, a 3-D FDTD method-based CrystalWave software tool (from Photon Design,Oxford, United Kingdom) has been used for analyzing the structures. A broadbandGaussian excitor with a central wavelength of 1550 nm has been used as a source for launchinglight into the waveguide. An output sensor placed at the end of the waveguide is used for deter-mining the output transmission spectrum relative to the reference sensor that is placed close tothe excitor.

The PhCW is considered on a SOI substrate with a 350 nm (0.7a) thick Si device layer(RI 3.45) and a BOX (buried oxide layer) (RI 1.45) of thickness 1500 nm. A triangulararray of holes with radius r 200 nm (0.4a) is considered to be periodically arranged inthe device layer with a lattice constant a 500 nm. For calculation of the bandgap, the effectiveindex method is used for two-dimensional band calculations using the band solver package of theCrystalWave software. The effective index of the slab is found to be 3.097 and the bandgap isestimated in the frequency range of 0.27 to 0.41a for TE-polarization. The PhCW is formed

Fig. 1 Band diagram of PhCW with f defect 30% for (a) hole-shaped line defect and (b) ring-shaped line defect waveguide. The two guided modes (i.e., even and odd) are visible. The yellowregion illustrates the PhCW forbidden band, where the waveguide transmission is prohibited.

Dutta, Goyal, and Pal: Sensitivity enhancement in photonic crystal waveguide platform. . .


by creating a line defect in the -X direction of the PhC structure, which consists of a row ofring-shaped holes, defined by its inner and outer radii Rin 135 nm ( 0.27a) andRout 225 nm ( 0.45a), respectively, as shown in Fig. 2. The simulated length of thePhCW is considered to be 28a. The performance of the PhCW-based RI sensor is estimatedby the sensitivity parameter, S, which is the ratio of the shift in the cut-off wavelength tothe change in the RI due to analyte infiltration and is expressed by nanometer/RIU.

4 Results and Discussion

The functional estimation of the PhCW for RI sensing is carried out using 3-D FDTDsimulations. The output spectrum thus obtained is used for calculating the upper band edgewavelength shift, which measures the sensitivity of the sensor. The preliminary analysis forobtaining the optimized design is done by supposing infiltration of de-ionized (DI) water(RI 1.33), which changes the RI of the holes and the cladding from 1 to 1.33. This incrementin the ambient index leads to a red shift in the upper band-edge of the output spectrum.

4.1 Refractive Index Sensing Characteristics of Photonic Crystal Waveguide

Initially, a conventional PhCW structure is considered, which is designed by removing a singlerow of holes in the -X of the PhC structure formed by periodically arranging holes of radiusr 200 nm with a lattice constant of 500 nm. Infiltration of water shifts the upper band edge by30 nm, corresponding to a shift of 90 nm/RIU. Creating a line defect by shrinking a row ofholes to a radius as shown in Fig. 3(a), the fdefect and thus the sensing area in the high-field regioncan be increased, which results in higher sensitivity.12 However, the increase in fdefect affects thesignal transmission [as shown in Fig. 3(b)] due to an increase in the PhCW forbidden gap making

Fig. 2 A schematic of the proposed PhCW sensor platform in a SOI substrate. (a) Top view and(b) cross-sectional view.

Fig. 3 (a) Schematic of PhCW formed by shrinking a row of holes to a radius r d (120 nm i.e.,f defect 20%) and (b) effect in transmission and sensitivity of PhCW with variations in f defect.



the determination of sensitivity difficult. Therefore, further improvement in sensitivity may notbe feasible by simply increasing the hole radius, rd, in a PhCW.

4.2 Sensitivity Enhancement in Photonic Crystal Waveguide

Broadening the defect holes increases the sensitivity; however, higher transmission losses occurbeyond a particular size of defect hole. The replacement of the row of defect holes with a row ofring-shaped defect holes shows a significantly high transmission for the same fdefect (30%) asshown in Fig. 4(a) for the noninfiltrated case. The high transmission of the ring-shaped linedefect increases the sensing potential of the PhCW. The sensing characteristics of the ring-shaped line defect PhCW is defined by its air region given by Rout Rin, which is directly relatedto the fdefect. The increment in Rout Rin results in an increased air defect filling factor fdefect andthus shows high sensitivity, as shown in Fig. 4(b). With the increment in the fdefect, the trans-mission decreases slightly, but there is a significant increment in the sensitivity. Thus, the PhCWwith ring-shaped defect holes provides a better platform for RI sensing applications than thehole-shaped line defect in the PhCW structure.

4.3 Sensing Characteristics of Optimized Ring-Shaped Line Defect PhotonicCrystal Waveguide

As is obvious from Fig. 4(b), the transmission level falls as the fdefect is increased. Although theincrease of the fdefect to about 51% leads to a very high sensitivity of about 500 nmRIU, itstransmission loss approximately reaches 6.5 dB. Therefore, the fdefect chosen for the optimizeddesign is 47%, which has a reasonably high sensitivity (420 nmRIU) along with a low trans-mission loss. The line defect is defined by a row of ring-shaped holes with Rout 225 nm andRin 135 nm and thus the air region is given by Rout Rin 90 nm.

Further simulations have been carried out for evaluating the effects in transmission andsensitivity by varying the lattice hole diameter and slab thickness for the optimized design.Figure 5(a) shows that the sensitivity increases almost linearly when decreasing the latticehole dimensions; however, the transmission decreases drastically with any divergence fromthe optimum parameters. The result of a variation of slab thickness on transmission and sensi-tivity is shown in Fig. 5(b). It can be easily concluded that the optimized design parameters showa good performance both in transmission and sensitivity.

The performance estimation of the design is carried out by infiltrating de-ionized water(RI 1.33), ethanol (RI 1.361), polyethylene glycol (RI 1.471), and xylene (RI 1.5).The simulated output spectrum and the shift in cut-off wavelength, cut-off (signal strength ofupper band-edge at 10 dB), for each sample are shown in Figs. 6(a) and 6(b), respectively.The slope shows a wavelength shift of 210 nm for a total RI change of 0.5 (RI 1.0 to1.5) thus estimating the value of sensitivity as 420 nmRIU.

Fig. 4 (a) Output transmission spectrum for performance analysis of hole-shaped line defect andring-shaped line defect PhCW with 30% f defect (b) Plot of transmission and sensitivity for variousdefect air filling factor (f defect).



5 Conclusion

A PhCW platform based on ring-shaped holes is presented for refractive index sensingapplications. The analysis shows that replacing the line defect by ring-shaped defect holesenhances the sensitivity and transmission significantly which will allow easy signal detectionof lower concentration analytes. The optimized design shows a wavelength shift of 210 nmwhen infiltrated with xylene. This infiltration changes the ambient RI from 1 to 1.5, leadingto a sensitivity of 420 nmRIU.

Acknowledgments

Authors are thankful to the director, CSIR-CEERI, Pilani for his continuous encouragement inthis work. Authors thank to all members of Optoelectronic Devices Group for their cooperation.Authors would like to acknowledge CSIR for sponsoring the PSC-0102 network project.

References

1. A. Khelif et al., Complete band gaps in two-dimensional phononic crystal slabs, Phys.Rev. E 74, 046610 (2006).

2. S. Fan et al., Channel drop filters in photonic crystals, Opt. Express 3(1), 411 (1998).

Fig. 5 Effects in transmission and sensitivity with variation in (a) lattice hole dimension and(b) slab thickness.

Fig. 6 (a) Output transmission spectrum for the proposed sensor after infiltration of DI water(RI 1.33), ethanol (RI 1.361), polyethylene glycol (RI 1.471), and xylene (RI 1.5).(b) Plot of shift in cut-off in the proposed sensor for infiltration of various samples and its slopemeasures a sensitivity of 420 nmRIU.



3. H. Chien, C. Chen, and P. Luan, Photonic crystal beam splitters, Opt. Commun. 259(2),873875 (2006).

4. M. Loncar et al., Low-threshold photonic crystal laser, Appl. Phys. Lett. 81(15),26802682 (2002).

5. S. Guo and S. Albin, Numerical techniques for excitation and analysis of defect modes inphotonic crystals, Opt. Express 11(9), 10801089 (2003).

6. A. J. Danner et al., Single mode photonic crystal vertical cavity lasers, Appl. Phys. Lett.88, 091114 (2006).

7. T. A. Birks et al., Dispersion compensation using single material fibres, IEEE PhotonicsTechnol. Lett. 11(6), 674676 (1999).

8. B. T. Tung et al., Strain sensitive effect in a triangular lattice photonic crystal hole-modifiednanocavity, IEEE Sensors J. 11(11), 26572663 (2011).

9. R. K. Shruti, S. Sinha, and R. Bhattacharyya, Photonic crystal slab waveguide-basedinfiltrated liquid sensors: design and analysis, J. Nanophotonics 5, 053505 (2011).

10. S. Chakravarty et al., Slow light engineering for high Q high sensitivity photonic crystalmicrocavity biosensors in silicon, Biosens. Bioelectron. 38(1), 170176, (2012).

11. H. S. Dutta and S. Pal, Design of a highly sensitive photonic crystal waveguide platformfor refractive index based biosensing, Opt. Quantum Electron. 45(9), 907917 (2013).

12. S. C. Buswell et al., Specific detection of proteins using photonic crystal waveguides,Opt. Exp. 16(20), 1594915957 (2008).

Hemant Sankar Dutta received a BE from RGPV, Bhopal, India, and an MTech from TezpurUniversity, India, in 2010 and 2013, respectively. He completed his MTech dissertation onthe study and design of photonic crystal based refractive index sensor platform for biosensingapplications at the Opto-electronic Devices Group, CSIR-CEERI, Pilani, India, in 2013. He iscurrently working as a senior project fellow in CSIR-CEERI and his research interest includesphotonic crystals, silicon photonics, waveguides, and sensors.

Amit Kumar Goyal received his BTech degree in electronics and communication engineeringfrom Lovely Professional University, Punjab, India. Then he joined CSIR-CEERI as a quick hirescientist. Now he is working towards his MTech degree in AcSIR. His research interests includeIII-V nitride semiconductors and photonic crystal based sensors.

Suchandan Pal received his BTech and MTech degrees from University of Calcutta, India.He joined CSIR-CEERI, Pilani, as a scientist in 1995. In 2004, he completed his PhD degreefrom City University, London. Since then, he has been involved with design and fabrication ofsilica-on-silicon based integrated optic devices and GaN material-based LED. He has authoredand coauthored more than 70 scientific and technical papers in various reputed internationaljournals and conferences.



jnp-14026 authors copy

Documents