Design and simulation of high sensitive photonic crystal waveguide sensor

Design and simulation of high sensitive photonic crystal waveguide sensor

G Model IJLEO-55217; No. of Pages 4 ARTICLE IN PRESS Optik xxx (2014) xxx–xxx Contents lists available at ScienceDirect Optik journal homepage: www...

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G Model IJLEO-55217; No. of Pages 4

ARTICLE IN PRESS Optik xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Optik journal homepage: www.elsevier.de/ijleo

Design and simulation of high sensitive photonic crystal waveguide sensor Amit Kumar Goyal a,b,∗ , Suchandan Pal a a b

Academy of Scientific and Innovative Research (AcSIR), India Opto-Electronic Devices Group, CSIR–Central Electronics Engineering Research Institute (CSIR–CEERI), Pilani 333031, Rajasthan, India

a r t i c l e

i n f o

Article history: Received 26 December 2013 Accepted 15 August 2014 Available online xxx Keywords: FDTD Photonic crystal waveguide Sensitivity Silicon-on-insulator

a b s t r a c t Photonic crystal waveguide, formed by local line defect in a perfect periodic structure, is generally used for sensing. In this paper, we have proposed a very high sensitive photonic crystal waveguide (PCW) based sensor in silicon-on-insulator (SOI) material after optimisation of etch-depth of the circular holes up to a finite depth underneath the buried oxide layer. Properties of the sensor are analysed by 3-D finite difference time domain (FDTD) method. Output transmission and sensitivity of the proposed sensor is analysed by varying the defect radius and etch depth. Optimised structure shows an average value of sensitivity as 386 nm/RIU over a range of refractive index of 1.0 and 1.5. © 2014 Elsevier GmbH. All rights reserved.

1. Introduction Photonic crystal structures, which are regarded as an optical analogue of semiconductors, has attracted considerable interest among researchers due to their ability of controlling and manipulating the flow of light or photon [1]. During last decade, a lot of attention is focused on to photonic crystals for various applications such as coupler [2], resonator [3], waveguide [4], fibre [5], filters [6] etc. Photonic crystal fibres (PCFs) have been demonstrated for sensing, antibodies [7] and DNA [8] whereas photonic crystal waveguides (PCWs) have been utilised for refractive index measurements [9], and other various sensing applications. A simple structure of photonic crystal is shown in Fig. 1(a), which is designed by placing circular array of holes on top of silicon layer, in a triangular lattice arrangement in an SOI substrate. Band gap of this photonic structure depends on the filling factor and the lattice constant of the structure. A photonic crystal waveguide (PCW) structure thereafter can be realised by creating a line defect on this structure simply by removing a single row of holes, as shown in Fig. 1(b), where light is guided through this waveguide effectively. Various types of compact photonic devices and components like coupled cavity, filter, resonator, directional coupler, splitter, etc., have been realised by utilising this structure. PCW based

∗ Corresponding author at: Opto-Electronic Devices Group, CSIR–Central Electronics Engineering Research Institute (CSIR–CEERI), Pilani 333031, Rajasthan, India. E-mail addresses: [email protected] (A.K. Goyal), [email protected] (S. Pal).

sensors have also been widely studied in past few years due to their high sensing abilities. Recently, a PCW sensor having sensitivity of 260 nm/RIU has been reported by Dutta and Pal [10], using photonic crystal waveguide platform for refractive index based bio-sensing. Bagci and Akaoglu [11] have reported sensitivity of 282.4 nm/RIU using selective infiltration in photonic crystal waveguide. Photonic crystal waveguide based liquid sensor of sensitivity 200 nm/RIU and 174.8 nm/RIU have also been demonstrated [12,13]. The above-mentioned sensitivity values are relatively low and therefore needs improvement in order to achieve its better performance with enhanced sensitivity and output signal strength. In this paper, we have proposed a simple PCW structure for sensor application in silicon-on-insulator (SOI) material with significantly high sensitivity by optimising the etch-depth of the circular holes up to a finite depth underneath the buried oxide layer. The structure is analysed by 3D FDTD method. Sensitivity of the sensor is estimated by infiltrating liquids of different refractive index and calculating corresponding shift in cut off wavelength. In order to achieve the best possible sensitivity, the structure is optimised by varying the defect hole diameter and etch depth. 2. Designed structure and working principle Photonic crystal is considered an optical analogue to semiconductor, which shows band gap for certain range of frequency. This band gap can be tailored by changing various parameters such as lattice constant, hole radius, and slab thickness etc. Due to its unique properties photonic crystal is used in various applications as sensors, filters and couplers etc. A simple structure of photonic

http://dx.doi.org/10.1016/j.ijleo.2014.08.174 0030-4026/© 2014 Elsevier GmbH. All rights reserved.

Please cite this article in press as: A.K. Goyal, S. Pal, Design and simulation of high sensitive photonic crystal waveguide sensor, Optik Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.08.174

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Transm ission

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RI = 1 RI = 1.33 RI = 1.5 RI = 1.7 RI = 2

1.45

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Wavelength (um) Fig. 3. Effect of changing refractive index of analytes on minimum wavelength of dip. Fig. 1. (a) Schematic of a simple structure of photonic crystal made of triangular array of holes in silicon (b) Photonic crystal waveguide made of removing a single row of holes.

Initially a photonic crystal waveguide structure (shown in Fig. 2) is designed by etching circular array of holes in triangular lattice arrangement over SOI substrate. The structure is designed to have TE band gap centred on ∼1.55 ␮m (1.38–1.74 ␮m). But an additional dip is obtained within the band gap range which is due to anti-crossing of different modes within formed waveguide structure [15]. To design this structure lattice constant is taken to be

0.42 ␮m with filling factor of 0.3 and thickness of slab is taken to be 0.260 ␮m. To measure sensitivity different analytes are infiltrated and corresponding shift in minimum wavelength of dip is analysed. Fig. 3 shows output response of designed sensor structure. By analysing the minimum wavelength shift in dip in accordance with change in refractive index a sensitivity of 100 nm/RIU is obtained. Although designed structure provides very simple mean to analyse sensitivity (by measuring minimum wavelength) it provides very low value of sensitivity. To enhance the sensitivity further a new photonic crystal waveguide based structure is designed using different parameters. For that, again circular air holes are etched in triangular lattice arrangement over a SOI substrate and response of structure is analysed using crystal wave 3D FDTD software. A waveguide is obtained by removing a single row of holes in  -K direction as shown in Fig. 4, which shows a TE band gap around 0.2855–0.408 a/ centred at 1.55 ␮m. The waveguide comprises silicon (n = 3.45) as guiding layer, silicon dioxide (n = 1.45) as lower cladding and air (n = 1) as upper cladding layer. The lattice constant is taken as 0.5 ␮m with a fill factor of 0.4 and thickness of the slab (h) is taken to be 0.260 ␮m. A broadband light source is applied at the input of the waveguide such that light effectively guides through the waveguide and is detected at the output by output sensor. To calculate the sensitivity of this unperturbed PCW structure, holes are infiltered with different analytes and corresponding cut off wavelength shift is measured. The cut off wavelength shifts by 11 nm as refractive index of the analytes changes from 1 to 1.2 giving a sensitivity (cutoff /n) of 55 nm/RIU. Since sensitivity mainly depends on the filling factor and effective refractive index near the waveguide, sensitivity can be further enhanced by varying

Fig. 2. Simple photonic crystal waveguide structure made in SOI wafer, lattice constant = 420 nm, holes diameter = 250 nm.

Fig. 4. Photonic crystal waveguide structure made in SOI wafer, lattice constant = 500 nm, holes diameter = 400 nm.

crystal is shown in Fig. 1(a), which is designed by placing circular array of holes on top of silicon layer, in a triangular lattice arrangement over a SOI substrate. Other optical devices can also be made by utilising this structure, such as a PCW is made by removing a single row of holes as shown in Fig. 1(b) where light is guided through this waveguide effectively. Sensing principle is based on measurement of cut-off wavelength shift by changing analytes refractive index because change in analytes refractive index will lead to change in effective index of the slab [14]. In order to sense different gases, this change in cut off wavelength is measured. Because sensitivity is normally given by S=

  =F n n

where  is change in cut-off wavelength, n is change in refractive index of gas, n is slabs refractive index,  is optical wavelength transmitted inside the photonic crystal and F is filling factor. 3. Simulation results and analysis

Please cite this article in press as: A.K. Goyal, S. Pal, Design and simulation of high sensitive photonic crystal waveguide sensor, Optik Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.08.174

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Fig. 5. Modified structure with some defects holes incorporated in waveguide.

Fig. 6. Measurement of transmission spectrum by varying defect diameter from 0.31 ␮m to 0.34 ␮m.

Fig. 7. Effect of varying defect diameter on transmission.

The sensitivity can be further enhanced by optimising the etching depth of circular hole in silicon guiding layer as shown in Fig. 9. Since in SOI substrate, circular holes are etched to the thickness of silicon guiding layer (here 0.260 ␮m) as shown in Fig. 9(a), to

Sensitivity 300

Sensitivity (nm/RIU)

various parameters of the region near the line defect [16], this is due to the fact that in that region the electromagnetic field is very intense. Therefore, some defects are incorporated in the line defect as shown in Fig. 5. These defects result in the increase of filling factor and decrease in effective refractive index near the waveguide, hence affecting the sensitivity. In Fig. 5, yellow atoms represent the defects which are incorporated to enhance sensitivity. The diameter of these defects is increased from 0.15 ␮m to 0.35 ␮m. Fig. 6 represents the transmission spectra at defect diameter 0.31, 0.32, 0.33, and 0.34 ␮m, which is analysed by 3D FDTD method. It is noteworthy to mention that increase in defect diameter leads to decrease in transmission which is due to increase in backscattering [11]. The effect of increasing defect diameter (from 0.15 ␮m to 0.35 ␮m) on transmission is shown in Fig. 7. From Fig. 7 it can be seen that, the transmission decreases beyond −10 dB point for defect diameters greater than 0.35 ␮m. Since to measure cut off wavelength of the structure, −10 dB point is taken as reference therefore, the defect diameter is increased only up to 0.33 ␮m where transmission is about −7 dB. The effect of this increased defect diameter on sensitivity is shown in Fig. 8. Since sensitivity increases with direct proportion of filling factor and inverse proportion of effective refractive index [17], increase in defect diameter will lead to increase in sensitivity. This is due to the fact that initially light was guiding through a high refractive index material, but as defect diameter increases, it leads to decrease in effective refractive index of waveguide resulting in increase in sensitivity. The defect diameter is increased from 0.15 ␮m to 0.33 ␮m which results in increase in sensitivity from 130 nm/RIU to 325 nm/RIU. The obtained sensitivity is much higher than previous reported values [10–13].

250

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100 0.15

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0.25

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Defect Radius (m) Fig. 8. Effect of varying defect diameter on sensitivity.

Fig. 9. Atoms etching profile in structure. (a) Etched up to silicon guiding layer. (b) Over etched to some oxide also.

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change in cut off wavelength is measured. Since plotted graph shows an almost linear relation between refractive index and cut off wavelength, its slope will provide sensitivity, which is obtained as 386 nm/RIU. 4. Conclusion In this paper a very high sensitive photonic crystal waveguide based sensor structure has been presented which is designed at wavelength 1.55 ␮m (mostly used wavelength in communication) and is used to sense different aqueous analytes. For sensing purpose, refractive index based sensing mechanism has been used, which is based on measurement of shift in cut off wavelength by corresponding shift in refractive index of the analytes. By carefully designing the structure a sensitivity of 386 m/RIU has been obtained, which is much higher than the previous designed structure. Fig. 10. Effect of varying etch depth (over etching in oxide) on sensitivity.

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

Fig. 11. Variation in cut off wavelength by changing refractive index.

enhance the sensitivity of the device, the holes are over etched up to certain depth inside the lower oxide layer (Fig. 9(b)). Fig. 10 represents the effect of hole etch depth on sensitivity. By over etching in the oxide layer (keeping lattice holes and defect holes diameter fixed at 0.4 ␮m and 0.33 ␮m), the sensitivity is further increased. This is because of the increase in guiding mode energy which is mentioned below. Normally in PCW light is confined by Bragg reflection in horizontal direction and by total internal reflection in vertical direction. When light propagates through the waveguide, very small amount of light radiates in surrounding cladding layer. This radiated energy has most of its component near the interface of core and cladding. Therefore, this radiated light can also be guided by etching the hole up to some depth in cladding. This is best represented by Fig. 10, which shows increase in sensitivity with increase in holes depth. It attains its highest value of 386 nm/RIU around 0.2 ␮m and becomes almost constant thereafter. Etching oxide beyond this depth will not affect the sensitivity further. Fig. 11 represents the shift in cut off wavelength in accordance to change in refractive index of the final structure (hole diameter 0.4 ␮m, defect hole diameter 0.33 ␮m, etch depth 0.2 ␮m). From Fig. 11 it can be analysed that as refractive index increases from 1 to 1.5, there is a corresponding red shift in cut off wavelength. By using analytes of different refractive index,

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Please cite this article in press as: A.K. Goyal, S. Pal, Design and simulation of high sensitive photonic crystal waveguide sensor, Optik Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2014.08.174