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High sensitive triple nanocavity biosensor based on 2-D photonic crystal.


Photonic crystals (PhC) are the periodic dielectric structures, containing the array of air pores in the dielectric slab or dielectric rods in air host which have the light localizing capability. A forbidden frequency range that is called the photonic band gap (PBG) is one of the most important properties of these structures [14].Photonic crystal sensors have applications in a wide measurement range of physical properties such as temperature, pressure, refractive index, bio-sensing [5-8]. Defects in the photonic crystal which break the periodicity of the structure are important for sensing applications because they help for localizing light and promoting the interaction of the light beam [9].

Lately, photonic crystals biosensors have attracted intensive researches because ultra-compact size, high measurement sensitivity, flexibility in structural design, and more suitable for monolithic integration. The sensing mechanism is mostly classified in two main categories like homogeneous sensing and Surface sensing [10,11]. The PhC biosensor mechanism is based on homogeneous sensing because it senses the presence of an analyte by means of refractive index variation. With refractive index variations, the resonant wavelength of the structure will shift. The resonant wavelength shift scheme is preferred for sensing approach because the shift of the resonant wavelength leads to high sensitivity [12,13].

S. H. Kwon et al. proposed a photonic crystal chemical sensor based on a cavity [14]. Also Wang et al. designed a refractive index sensor for bio layers and chemical sensing, consisting of a micro cavity and two waveguides [15]. Hsiao and Lee investigated a photonic crystal nanoring resonator as the biochemical sensor based on the single defect hole [16]. In order to overcome the single purpose operation of the sensor and amount of detected target molecules in a special time, a three nanocavity biosensor is regarded. On the contrary of other structures which can be utilized for sensing bio-molecules with one specific refractive index in a certain time, our novel designed biosensor can be used for sensing three specific refractive indices in a certain time; so a triple nanocavity biosensor is constructed.

In this paper, a triple nanocavity biosensor based on the two-dimensional photonic crystal is proposed by introducing waveguides and nanocavities. Proposed biosensor can be used for sensing three specific refractive indices in a certain time, so a multi cavity biosensor is constructed. With a system, the analyte is induced to nanocavities in the two-dimensional photonic crystal. After injecting analyte into the sensing holes, the refractive index of the nanocavities will change, and the transmission spectra of the structure vary which can be measured to determine the properties of the analyte. The sensing characteristics such as the Q-factor, resonant wavelength, output power and sensitivity are investigated.

II. Designing Multi Cavity Biosensor:

The proposed biosensor structure is shown in Fig. 1. It consists of a hexagonal lattice of air holes with refractive index of 1 in a silicon slab which has a refractive index of 3.46. The distance between the two adjacent holes is 440 nm which is termed as the lattice constant and denoted by a. The device is made up of 25 x 25 air holes in X and Z directions with a radius of r= 150 nm. The line defect is introduced by removing a row of air holes, allowing guided modes to propagate within the photonic band gap. The wave guide is coupled to a resonant cavity which is used to select a single frequency from a pulse propagating through the main wave guide, and rerouted it to another wave guide. Nanocvities are created by changing the radius of the holes, i.e. Rc= 90nm, as shown in Fig. 1.

Three nanocavities which are filled with different refractive indices are used for trapping the special wavelength in proportion to the injected bio-molecule, as shown in Fig. 1. The nano-cavities sizes are optimized for achieving the high quality factor, high transmission efficiency, and reasonable sensitivity. The two-dimensional finite-difference time-domain (2-D FDTD) method and plane-wave expansion (PWE) approach are used for analyzing this sensor. The photonic band gap range for transverse electric (TE) modes for the periodic structure obtained by the PWE method is 0.24381 to 0.41527 as shown in Fig. 2. Whose corresponding wavelength range is between 1.059-1.804 [micro]m.

This is a forbidden frequency range which cannot propagate through the structure. But after introducing defects like refractive index variation or hole radius modifying which break the periodicity of the structure, an allowable frequency range is created that can propagate through the structure within the PBG.

III. Simulation Results of Optimized Design:

Simulation was done in the Opti FDTD software and the simulation results of the optimized structure were obtained using the 2D finite difference time domain (FDTD) method. A light signal was launched into the input port of the wave guide. The output signal was recorded by a power monitor at the output port. The output power was the normalized output power. The obtained output response was used to analyze the resonant wavelength, sensitivity and quality factor [17].

In the bio-sensing application, the device is utilized for measuring refractive index variations. The refractive index (RI) of the nanocavities has been changed; it means that the target molecules flow in a sensing hole. The presence of target molecules varies the ERI of the sensing hole. This sensing mechanism is based on homogenous sensing. So the output spectrum is changed. As a result, the spectral shift is afforded due to ERI variations of the defect.

Fig. 3 shows the normalized transmission spectrum of the biosensor in the absence of the analyte, at the resonant wavelength is 1565 nm while keeping the cavity position and Rc at their optimum value.

Fig. 4 (a) and (b) illustrate the electric filed pattern of the ON-resonance and OFF-resonance of the sensor at 1565 nm and 1500 nm, respectively. At the resonant wavelength [lambda] = 1565 nm, the electric field of the wave guide is fully coupled in the cavity and reaches the output port, whereas at the OFF-resonant wavelength [lambda] = 1500 nm, the signal is decoupled at the output port.

Increasing the refractive index in nanocavities can be effected by injecting an analyte into the sensing surface. Fig. 5 illustrates the electric filed pattern at the resonance wavelength 1652 nm.

The transmission spectra of triple cavity outputs are depicted in Fig. 6. According to Fig. 6, with increasing the refractive index of nanocavities, the transmission spectrum shifts to longer wavelengths. The sensor can be based on the resonant wavelength shift scheme. It is noticed that the resonant wavelength shifts around 44 nm for every 0.1 refractive index increase at the injection of analyte as shown in table.1. Thus, this structure is able to measure a wide range of refractive index variation.

According to this table1, [lambda]0 characterizes the central wavelength of each channel, and Al FWHM is the full width at the half-maximum (FWHM) of the output which indicates the narrow band biosensor behaviour. According to [lambda]0 and [DELTA][lambda] FWHM, the values of quality factors for each channel are calculated as ([lambda]0/[DELTA][lambda] FWHM) [18], and also [DELTA][lambda] is the resonant wavelength shift from the reference state (no analyte is filled within the holes) that shows the mass sensitivity. Sensitivity of a sensor is calculated by the ratio of shift in resonance wavelength to the change in refractive index ([DELTA][lambda]/[DELTA]n)[18].

Fig. 7 indicates the resonant wavelength with respect to the RI for the defect size of Rc= 90 nm. It is shown that with increment of the RI, the resonant wavelength shifts to longer wavelengths. Therefore, we can calculate the sensitivity that is defined as the wavelength shift per refractive index unit (Al/An) equal to 440 nm/RIU.


In this paper, we proposed a two dimensional photonic crystal triple cavity biosensor with hexagonal lattice of air holes in silicon slab. For overcome with single purpose performance of sensor, we design a multi-cavity biosensor. The bio sensing mechanism is based on the effective refractive index change of the sensing hole. By varying refractive index of nanocavity which is filled with analyte, the transmission spectrum shifts to longer wavelength and we can determine the concentration and properties of analyte. The quality factor of biosensor is over 412.4, and the sensitivity is obtained as 440 nm/RIU. Simulation results show that there is a quasi-linear dependency of resonance wavelength shift with respect to refractive index variations.


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(1) J.Divya, (2) A.Sivanantha Raja, (3) S. Selvendran.

(1,2) A.C. College of Engineering and Technology, Karaikudi, Tamil Nadu, India -630 003.

(3) KL University, Green Fields, Vaddeswaram, Guntur, A.P, India-522 502

Received 28 February 2017; Accepted 29 April 2017; Available online 2 May 2017

Address For Correspondence:

J. Divya, A.C. College of Engineering and Technology, Karaikudi, Tamil Nadu, India -630 003.

E-mail: divyaselva0605@gmail.com_

Caption: Fig. 1: Proposed biosensor structure

Caption: Fig. 2: Band diagram of the structure

Caption: Fig. 3: Normalized transmission spectrum of the biosensor at the resonance wavelength is 1565 nm.

Caption: Fig. 4: Electric field distribution of the sensor at (a) ON- resonance and (b) OFF-resonance

Caption: Fig. 5: Electric field distribution of the sensor at resonance wavelength 1652 nm.

Caption: Fig. 6: Normalized transmission spectra of resonance wavelength when the refractive index is varied.

Caption: Fig. 7: Resonant wavelength shift as a function of refractive index.
Table 1: Analysis Of Biosensor For Different Refractive Index

Refractive   Resonance wavelength   Wave length shift
index(n)     ([[lambda].sub.0]) nm  ([DELTA][lambda])nm

1            1565                   --
1.1          1607                   42
1.2          1652                   45

Refractive   Q
index(n)     Factor

1            395.7
1.1          412.5
1.2          429.0
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Author:Divya, J.; Raja, A. Sivanantha; Selvendran, S.
Publication:Advances in Natural and Applied Sciences
Date:May 1, 2017
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