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Infiltrated photonic crystal cavity as a highly sensitive platform for glucose concentration detection
Optics Communications 384 (2017) 93–100
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Infiltrated photonic crystal cavity as a highly sensitive platform for glucose concentration detection ⁎
Safia Arafaa, , Mohamed Bouchemata, Touraya Bouchemata, Ahlem Benmerkhia, Abdesselam Hocinib a Laboratoire Micro-Systèmes et Instrumentation, Département d'électronique, Université des frères Mentouri, Constantine 1, route d’Aïn el Bey, Constantine 25000, Algeria b Laboratoire d’Analyse des Signaux et Systèmes, Département d’Electronique, Faculté de Technologie, Université Mohamed boudiaf de M'sila, BP.166, Route Ichebilia, M'sila 28000, Algeria
A R T I C L E I N F O
A BS T RAC T
Keywords: Optical biosensor Photonic crystal Cavity-coupled waveguide High sensitivity Glucose concentration
A Bio-sensing platform based on an infiltrated photonic crystal ring shaped holes cavity-coupled waveguide system is proposed for glucose concentration detection. Considering silicon-on-insulator (SOI) technology, it has been demonstrated that the ring shaped holes configuration provides an excellent optical confinement within the cavity region, which further enhances the light-matter interactions at the precise location of the analyte medium. Thus, the sensitivity and the quality factor (Q) can be significantly improved. The transmission characteristics of light in the biosensor under different refractive indices that correspond to the change in the analyte glucose concentration are analyzed by performing finite-difference time-domain (FDTD) simulations. Accordingly, an improved sensitivity of 462 nm/RIU and a Q factor as high as 1.11х105 have been achieved, resulting in a detection limit of 3.03х10−6 RIU. Such combination of attributes makes the designed structure a promising element for performing label-free biosensing in medical diagnosis and environmental monitoring.
1. Introduction Optical sensors that provide instantaneous detection and quantification of biological analytes have emerged as a field of great interest due to their promising characteristics such as safety in an inflammable and explosive environment, immunity to electromagnetic interference, rapid response speed and the remote on-line sensing capability. The most well exploited techniques for optical sensing are based on the principle of surface plasmon resonance (SPR) [1,2], colorimetric resonances, and interferometry methods [3]. Among those optical resonance technologies, responsive photonic crystal (PhC) based sensors have attracted substantial attention because of their miniaturized size, their high spectral sensitivity, minimal sample preparation without fluorescence labeling and the possibility of integrating MEMS (Micro-Electro-Mechanical Systems) [4]. These highly ordered devices can be fabricated using microelectronic fabrication techniques, and can be easily integrated with microelectronics, microfluidics [5,6] and other kinds of photonic devices. A typical planar photonic crystal (PhC) consists of cylindrical air holes with defined lattice constant in a thin silicon membrane. PhC sensors provide strong light confinement within the analyte itself due to the photonic band-gap effect [7].
⁎
Light can be concentrated in a very small volume, leading to a large light-matter interaction [8]. This phenomenon makes the sensor highly sensitive to small refractive index (RI) variations that are produced by biological species immobilization on the PhC pore walls. The ideal sensing technology should be highly selective and appropriately sensitive to the biological analyte concentrations. Hence, different techniques and setup configurations are constantly being developed and optimized to increase the detection performances. Therefore, a proper PhC design for biosensing is an essential task which should be carefully handled to obtain the required sensing properties. Various designs and configurations of biosensing devices have been proposed and performed using different types of PhC structures such as microcavities [9–14], ring resonators [15] waveguides [16–19], slot waveguides [20,21], heterostructures [22]. They all exploit extremely sensitive resonant conditions for guided modes in the PhC with respect to RI changes in the ambient medium. In practical applications, due to the difficulties in coupling light into the PhC resonator-only system, the waveguide-resonator system is usually preferred. Incorporating with PhC waveguide, coupled microcavities [23–26] provide several advantages in terms of compactness, high sensitivity and quality (Q) factor, easy extension to sensor arrays and
Corresponding author. E-mail address: arafa.safi[email protected] (S. Arafa).
http://dx.doi.org/10.1016/j.optcom.2016.10.019 Received 10 June 2016; Received in revised form 21 September 2016; Accepted 10 October 2016 0030-4018/ © 2016 Elsevier B.V. All rights reserved.
Optics Communications 384 (2017) 93–100
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Assuming harmonic time dependence for the magnetic field with (angular) frequency ω. H(r, t) = H(r)e−iωt , we obtain the master equation for the magnetic field:
the capability of parallel measurement [27–29]. Examples of such designs include that of Dorfner et al. [30], who experimentally demonstrated a PhC channel drop filter for fluid sensing applications with a Q-factor of 3000 and a sensitivity of 155 nm/RIU, as well as that of Zhou et al. [31] who have proposed the integration of high transmittance H2 nanocavity and broadband waveguide, the obtained results revealed a RI sensitivity of 131.70 nm/RIU with a Q factor of about 3000. Further, Liu et al. [32] proposed an experimental model of an optical PhC sensor based on a channel-drop configuration, where simulation and experimental results showed a good agreement with RI sensitivity of 153 nm/RIU. It is argued that higher Q factors will contribute to detect a very small shift in the resonance wavelengths. Therefore, maximizing the Q factor of the resonator, will reduce the impact of noise on the determination of the resonance wavelength [33], this would further enhance the sensitivity and accurate the detection limit. However, for sensing applications, the quality factor is not the only crucial parameter to gain higher sensitivities. The detection performances can also be enhanced by conducting improvements in the sensor setup such as temperature stabilization, coupling enhancement and the topology optimization of device geometry (the shape, the size of the holes and the thickness of the Si layer) which further enhance the intensity within the detection area. The alteration of the cavity geometric parameters offers a great structural freedom to tune the sensor optical properties [34,35]. Some critical issues such as achieving high refractive index sensitivity or enhancing light coupling into PhC structures must still be overcome. In this paper, we report the design and the simulation of an integrated PhC biosensor that is potentially used for glucose concentration detection. Considering silicon-on-insulator (SOI) technology, the designed structure is formed by two waveguides and one cavity system. The device sensitivity is determined by the magnitude of lightmatter interaction. Therefore, to achieve higher sensitivities, the optical resonant field needs to be strongly localized and overlapping with the analyte. In this context and in order to enhance the electric field intensity and hence the light-matter interactions within the cavity sensing area, the ring shaped holes cavity configuration is adopted. Results obtained by performing FDTD simulations indicate that by adjusting the ring's width as well as the number of functionalized holes around the cavity sensing area, the sensitivity and the quality factor can be greatly improved. Moreover, we demonstrated that the resonant wavelength mode shifts its spectral position following a linear behaviour when a glucose concentration ranging between 0% and 60% is applied. For the glucose detection, a sensitivity of 462.61 nm/ RIU and a detection limit of 3.03х10−6 RIU are observed.
⎛ 1 ⎞ ⎛ ω ⎞2 ∇×⎜ ∇ × H(r⃗) ⎟=⎜ ⎟ H(r ⃗ ) ⎝ ɛ(r⃗) ⎠ ⎝c⎠
where c = 1/ μ 0ɛ0 is the speed of light in vacuum. To obtain numerical solutions for these equations, the finite difference time domain (FDTD) method is the best suitable [36]. This method has been one of the most commonly used for simulating pulse propagation through photonic crystal devices [37,38]. After introducing a known initial field and sources into the computational domain, time development of the electric and magnetic field in the structure can be calculated in discrete time steps [39]. The FDTD simulations in this work have been carried out using RSoft software. In order to study the dispersion diagram and the modal distribution of the field through PhC structure, the plane-wave expansion method (PWE) is applied [40,41]. Using this method the solutions of Eq. (4) are expanded in a truncated basis of plane waves. These plane waves are expressed as a Fourier expansion series in k-space (reciprocal space), which facilitates the computation.
H (r ⃗ ) =
1 = ɛ(r ⃗ )
→ ⎯
∑ η (G ).exp
→ ⎯ (iG . r ⃗ )
⇀ ⎯ G
(6)
where and η (G ) represents the Fourier transform of the inverse of ɛ(r ⃗ ). It is defined by:
→ ⎯ 1 η (G ) = Ω
∫cell
→ ⎯ 1 .exp(−iG . r ⃗ ) dr ⃗ ɛ(r ⃗ )
(7)
Ω designates the unit surface cell. For 2D-PhC structure and for the TE polarization the eigen value equation is given by:
∑
⎛→ ⎯ ⎯ ⎞ ⎛→ ⎯ ⎞ ω2 → → ⎯ → ⎯ → ⎯ → ⎯ → ⎯ k +G k +G′ η ⎜G −G′⎟ h1 ⎜G′⎟= 2 h1 (G ) ⎠ ⎝ ⎠ c ⎝
(8)
The Fourier coefficients of the inverse of ɛ(r ⃗ ) for ring-shaped holes are given by [42,43]:
⎧ → ⎯ 1 π ⎛1 1⎞ ⎪ + ⎜ − ⎟ (R 2 −Rin 2 ), G =0 ɛb Ω ⎝ ɛa ɛb ⎠ out ⎯→ ⎪ η (G )=⎨ → ⎯ ⎪ 2π ⎛ 1 1 ⎞ ⎪ ΩG ⎜⎝ ɛa − ɛb ⎟⎠ [Rout J1 (G (Rout ))−Rin J1 (G (Rin ))], G ≠0 ⎩
(1) (2)
(9)
where J1 is the Bessel function of the first kind, Rin and Rout are the inner and the outer radius of air rings, respectively. ɛa =1 is the dielectric constant of air rings and ɛb is the silicon permittivity. For PhC air holes case Rin=0 and Rout=r, where r is the air holes radius. In this study, the dispersion diagram is computed numerically using BandSOLVE software [44,45]. Fully integrated into the RSoft Component Design Suite, BandSOLVE is ideal for producing band structures for PhC bandgap structures. The simulation engine is based on an advanced optimized implementation of the PWE technique for periodic structures.
μ is the permeability which is equals to µ0 as the considered material is non-magnetic and ε represents the permittivity which is usually written as ε=ε0ε(r), where ε0 is the permittivity of vacuum and ε(r) is the relative permittivity of the material. Eq. (3) is obtained by solving Eq. (1) and inserting it into the time derivative of Eq. (2)
⎛ 1 ⎞ ∂2 ∇×⎜ ∇ × H(r, t) ⎟=− 2 [μ 0H(r, t)] ⎝ ɛ(r) ⎠ ∂t
(5)
⎯⎯⎯→
Mathematically, light propagation in photonic crystals is evaluated by solving Maxwell's equations in a mixed dielectric medium, expressed in Eqs. (1) and (2) E and H are the electric and magnetic field intensities.
∂ [ɛɛ0 (r)E(r, t)] ∂t
→ ⎯
→ ⎯ where G is the reciprocal lattice vector and k⃗ is a wave vector in the first brillouin zone. N=1 or 2 and eˆ1, eˆ 2 are orthogonal unit vectors ⎯ ⃗ → perpendicular to (k+G i). Because of the periodicity of the dielectric constant for PhC ⎯ → → ⎯ ɛ(r ⃗ )=ɛ(r ⃗+R ) with respect to the real space lattice vector R , Bloch's theorem can also be applied to expand it in terms of plane waves:
→ ⎯ G′
∇ × H(r, t)=−
→ ⎯
⃗ ∑ h(Gi).eˆN.exp.((ik+G i). r⃗)
⎯⎯⎯⎯⎯⎯→ Gi,N
2. Modeling and theory
∂ ∇ × E(r, t)=− [μ 0H(r, t)] ∂t
(4)
(3) 94
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matched layers (PML) conditions have been considered in the calculations to ensure no back reflection in the limit of the analyzed region. In simulation process, a TE polarized Gaussian optical pulse, covering the whole frequency-range-of interest, is launched at the input port to excite the cavity modes. A power monitor was placed at the end of the output waveguide to measure transmitted signal. For improving the simulation accuracy, FDTD analysis was carried out with a grid size of 0.01. The spectral response of the cavity without the sample presence is depicted in Fig. 2(a). When the PhC cavity is in resonance situation, a sharp peak with a Lorentzian line shape appears inside the complete PhC bandgap in TE polarization. In this case the designed cavity presents two resonant modes at wavelengths of 1.3424 nm and 1373.7 nm, respectively. The corresponding quality factor (Q) values are then calculated using the 2D finite-difference time domain (FDTD) method, combined with fast harmonic analysis. To quantitatively well analyze the sensing properties of the structure, the resonant mode with λ=1373.7 nm is chosen to observe the resonant wavelength shift as it presents the highest Q factor of 7.06 х103. Fig. 2(b) shows the light propagation profile through the cavity in the x-y plane, it can be clearly seen that the optical field is well confined in the cavity sensing area. Therefore, the properties of wide band and strong light confinement have been well applied in the structure design. This causes the cavity to be very sensitive to refractive index variations due to the photon lifetime enhanced and the large degree of lightmatter interaction.
2.1. Structure design In this work, the used 2D-PhC structure consists of triangular lattice of air holes etched in silicon (Si) slab (refractive index nsi=3.45). The radius of the bulk air holes is r=0.37a where a is the PhC lattice constant (a=530 nm), the thickness of the PhC slab is set as h=230 nm. Silicon dioxide (SiO2) layer with a thickness of 1500 nm is used as a bottom to support the Si slab. The low index SiO2 layer lying beneath the high index Si slab helps to confine light within the cavity core, preventing optical losses into the lower substrate, the light confinement in the vertical direction is then ensured by total internal reflection. In order to reduce the computational efforts needed for full 3D calculations, the PhC is replaced by a 2D system with the background dielectric medium having the effective refractive index of 2.838, which corresponds to the effective index of the fundamental guided TE mode in a 230 nm thick silicon slab on silicon dioxide at a wavelength of 1550 nm. For the following numerical simulations, we applied the effective index approach [46] with a combination of the 2D-PWE and the 2DFDTD methods from the RSoft software package. The dispersion properties of the regular PhC structure have been analyzed using the 2D-PWE method of BandSOLVE software. For the TE polarization, the photonic band gap (PBG) extends from ω1=0.278 (a/λ) to ω2=0.4 (a/λ) corresponding to the broad wavelength range of 1323.7–1908.1 nm. Therefore, the effective working wavelength range is extensively sufficient to meet the sensing demand where the resonant mode produces a wide wavelength shift. Fig. 1 shows the schematic side sectional view of the designed biosensor. The device consists of two waveguide couplers and one cavity. The two waveguides are obtained by removing one row of air holes in the x direction. They are used to couple light in and out of the PhC cavity. The designed cavity consisted of ring-shaped holes etched in the Si layer. It is necessary to note that the ring-shaped holes configuration does not only provide more flexibility in terms of designing new structures compared to circular shaped ones, but also enhance the optical field confinement and hence light-fluid interactions within the ringed low-dielectric material area [18,19]. The ring shapedholes parameters are defined by their inner and outer radius Rin=126.5 nm (0.23a) and Rout=247.5 nm (0.45a) respectively, thus the ringed air region is given by Rout−Rin=121 nm. The whole cavity system was separated from the input and output waveguides by three PhC holes. Details on ring shaped holes fabrication are given in [47]. Two-dimensional finite-difference time-domain (2D-FDTD) method was performed to estimate the device functionality. Perfectly
2.2. Sensing properties of the designed structure In order to illustrate the operating principal of the PhC cavity based RI biosensor, a series of FDTD simulations have been conducted. The preliminary sensing analysis is done by supposing the local infiltration of de-ionized (DI) water in the cavity sensing area, this corresponds to the refractive index (RI) change of the rings-shaped holes from 1 (air) to 1.33 (DI-water). For a local infiltration a precise technique based on micro-infiltration technology via hollow submicron size pipettes has been experimentally demonstrated [48]. This technique allows a controlled liquid deposition inside a desired PhC hole, while not affecting the adjacent ones. The optical detection properties of the designed structure have been quantitatively estimated by the sensitivity parameter S (Δλ/Δn), which is defined as the ratio of the shift in the wavelength (Δλ) to the change in the RI due to analyte infiltration (Δn). According to the obtained
Fig. 1. (a) Schematic side sectional view of the proposed PhC biosensor structure in SOI substrate. (b) Schematic diagram of the proposed ring shaped photonic crystal cavity coupled to an input and output PhC waveguides.
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Fig. 2. (a) The transmission spectra of the designed device before the sample infiltration for Rin=0.23a. (b) The electric field distribution through the cavity structure in the x–y plane (The color bar indicates the normalized optical field intensity ranging from −1 to 1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sharp decrease in the Q factor value. The calculated RI sensitivity for the inner radius Rin modified as a function of RI change is shown in Fig. 4(b). It can be noted that by increasing the Rin value which corresponds to the decrease of the ringed sensing area, the overlap between the optical modes and the detecting target decreases gradually which adversely affect the RI sensitivity. In view of the fact that, the Q factor was remarkably enhanced by the increase of the inner radius value while the RI sensitivity was gradually declined, it has been necessary to choose a trade-off between the Q factor and the sensitivity. The maximum product of Q factor and sensitivity can therefore be obtained when the inner radius Rin equals to 0.255a. Through the discussion about high Q factor and sensitivity S, the label-free biosensor is designed for Rin=0.255a. Fig. 5(a) and (b) shows the schematic view of the improved biosensor design and the fundamental quasi-TE mode propagation through the defect cavity before and after water infiltration, respectively. Considering water absorption at telecom wavelength range, the total Q factor was about 1.0172х105 at the resonant wavelength of 1558.1 nm. Compared to recent reported works [31,34,35], the Q factor and the RI sensitivity values have been significantly improved using ring shaped holes cavity design.
results the output resonant wavelength is red-shifted due to the increase in the ambient RI of the sensing area, which confirms the analyte identification. As seen in Fig. 3, a change in the RI of Δn=0.33 between air and water results in a spectral red-shift of 140.7 nm which corresponds to a RI sensitivity of 426.36 nm/RIU (refractive index unit). To further investigate and optimize the Q factor of the proposed biosensor, the width of the ring-shaped holes in the PhC cavity structure is altered by adjusting the inner radius (Rin). We fixed the value of the outer radius (Rout) at 0.45a and the inner radius (Rin) is varied from 0.23a to 0.28a with an increment of 0.01a. The variation tendencies of the Q factor and the resonance wavelengths according to Rin change are plotted in Fig. 4(a). It can be seen that the resonance wavelength shifts towards higher values due to the increase of high dielectric material in the cavity region. Similarly, the quality factor increases as the inner ring radius (Rin) increases from 0.23a to 0.28a and reaches its maxima of 1.36х105 at the resonant mode located at 1470.9 nm for Rin=0.27a, followed by a sharp decrease. The increase in the inner radius value corresponds to the decrease of the annular air area, when the Rin is more than 0.27a the rings area becomes critically thin. Consequently, the light confinement within the cavity area decreases and the optical leakages become more significant, which negatively affect the photon lifetime within the cavity area causing a
2.3. Functionalized holes number discussion It is essential to note that for biosensing or chemical detection applications, the magnitude of the resonant wavelength shift is dependent on the combination of many factors such as the number of functionalized holes and the effective refractive index change of targets. We assume that the defect ring-shaped holes and the surrounding holes are completely filled with the analyte (DI-Water). When the detection event occurs the refractive index will change due to the detection targets infiltrated into the air holes around the resonant cavity. To study the dependence of the sensitivity on the number of functionalized holes, we varied the number of holes being functionalized (N) around the resonant cavity in the PhC biosensor illustrated in Fig. 6. (Indicated by the blue circular holes). The transmission spectra corresponding to the different infiltration cases are depicted in Fig. 7(a). As can be seen in this figure, the resonant wavelength shift gets larger as the number of functionalized holes increases. This is due to the reduced effective refractive index between the infiltrated holes and the semiconductor membrane [49,50], as a result, the sensitivity becomes higher. The highest
Fig. 3. The resonant wavelength shift according to water infiltration (green line) within the cavity sensing area for Rin=0.23a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. (a) The Q factor and the resonant wavelength variations according to the change of the inner radius (Rin) from 0.23a to 028a. (b) The calculated refractive index sensitivity (S) as function of the inner radius change (Rin).
Fig. 5. (a) Schematic diagram of the optimized PhC biosensor structure based ring shaped holes cavity, the blue rings refers to the sensing area filled with DI-water. (b) The resonant wavelength shift (Δλ) according to water infiltration within the cavity sensing area for Rin=0.255a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Schematic diagram of the biosensor structure indicating the number of functionalized holes around the resonant cavity; (a) 4 functionalized holes, (b) 6 functionalized holes, (c) 14 functionalized holes, (d) 20 functionalized holes, (e) 54 functionalized holes, (f) all air holes are used as functionalized holes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
can explain the choice of the local infiltration instead of the total filling. The optimal Q factor and sensitivity as a function of the change in the number of functionalized holes (N) that corresponds to the different local infiltration cases are shown in Fig. 7(b). As indicated
sensitivity value of 688.18 nm/RIU has been obtained for the total infiltration, where all the air holes around the cavity are infiltrated. However, the corresponding Q factor and the transmission efficiency (shown by the brown line in Fig. 7(a)) are in their lower values, which 97
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Fig. 7. (a) Transmission spectra with different number of functionalized circular holes (N), (b) Quality factor and refractive index sensitivity (S) variations as a function of the number of functionalized circular holes (N).
Fig. 8. (a) Schematic diagram of the optimal PhC biosensor structure, the blue shapes refers to the sensing area filled with glucose solution. (b) The electric field distribution through the cavity structure in the x–y plane (The color bar indicates the normalized optical field intensity ranging from −1 to 1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Refractive index (RI), resonant wavelength (λ), Q factor and sensitivity (S) as function of different glucose concentrations (C). C (g/L)
RI
λ (nm)
Q factor
S (nm/RIU)
0 10 20 30 40 50 60
1.33230545 1.33349435 1.33468325 1.33587215 1.33706105 1.33824995 1.33943885
1569.9 1570.4 1571.0 1571.5 1572.0 1572.6 1573.1
1.087х105 1.099х105 1.112х105 1.125х105 1.137х105 1.142х105 1.164х105
420.55 462.61 448.59 441.58 454.20 448.59
2.4. Sensitivity dependence on the glucose concentration Through the design and discussion about the use of the structure based PhC-RSH cavity as a RI biosensor in the above section, we theoretically demonstrate that this structure can be well used as an optofluidic biosensor for glucose concentration measurement (Fig. 8(a)). Therefore, to investigate the relationship between the refractive index sensitivity and the glucose concentration, a refractive index range was selected to match the glucose concentration samples ranging from 0 to 60 g/l. The RI of glucose solution can be calculated by [51]:
Fig. 9. Resonant wavelength changes of the designed biosensor as function as a function of changes in the refractive index corresponding to the seven infiltration states of glucose concentrations.
in this figure, the sensitivity is enhanced by increasing the number of functionalized holes, whereas the Q factor is declined. Therefore, it has been necessary to choose a trade-off between Q factor and RI sensitivity. Accordingly, the optimal Q factor of 1.064х105 and the improved sensitivity of 408.49 nm/RIU are both achieved for N=4.
n = 0. 00011889C + 1. 33230545
98
(10)
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Table 2 Comparison of the proposed biosensor with various similar PhC designs. References
Sensing structure
Sensitivity (nm/RIU)
Q factor
detection limit (RIU)
[24] [31] [35] [53] [54] In this work
RI RI RI RI RI RI
330 131.70 160 330 293 462.61
3.82х103 2.96х103 107 8.2х106 950 1.112х105
_ _ 8.75 х10−5 1.24 х10−5 _ 3.03х10−6
biosensor formed by two waveguide and one microcavity biosensor consists of H2-type nanocavity and broadband W1 waveguide sensor based on a ring–slot cavity coupled to an input and output waveguides sensor consists of ring resonant cavity coupled to an in and out line defect PhC structure. sensor consists of ring cavity coupled to an optofluidic slow-light waveguide in a PhC platform. biosensor based ring-shaped holes cavity coupled to an input and output waveguides
manufacture. As they are composed of dielectric rods emerged and centered in air holes, a very precise alignment is required for positioning process. In order to overcome these problems, more controlled manufacturing techniques have been developed. Using electron beam lithography (EBL), Säynätjoki et al. [47] demonstrated that a thin ringed area as narrow as 60 nm patterned above 240 nm thick silicon slab could be successfully realized. Avoiding the challenging EBL alignment, Zhou [55] established a novel method based on atomic layer deposition (ALD) and sacrificial etching. This technique could achieve alignment accuracy down to the atomic level. Based on the arguments previously developed, the practical implementation of the designed structure may be quite possible. The overall results demonstrate that the proposed device is a promising building block for performing monolithic integration of high label-free multiplexed detection for biosensing applications.
where C is the glucose concentration (g/L) and n is the glucose solution RI. The intensity field distribution profile in the x–y plane of the resonant mode located at λ=1569.9 nm for the optimal biosensor structure is represented in Fig. 8(b). Due to the photon lifetime enhanced and the large degree of light-matter interaction, a significant amount of light amplification in the cavity sensing area is clearly observed. This causes the cavity to be very sensitive to small refractive index changes. The spectral response of the cavity as a function of changes in the refractive index corresponding to the seven infiltration states of glucose concentrations is shown in Fig. 9. The resonance peak located at 1569.9 nm (a/λ=0.3376) was identified as a reference mode that corresponds to 0 g/L of glucose concentration. It is noted that, as the glucose concentration increases, the cavity mode redshifts due to the increase in the cavity ambient refractive index from 1.33230545 to 1.33943885. Simulation results are illustrated in Table 1. It can be clearly seen that the shifts in resonant wavelengths have been obtained even for the slightest change in the sample RI, hence different glucose concentrations in the infiltrated sample could be well detected. It can also be noted that the Q factor value increases relative to its initial value before the samples infiltration. This is due to the increase in the ambient refractive index, which reduces the PhC contrast [50]. Therefore, we can conclude that the glucose biosensor based PhC-RSH cavity presents high precision and it is very sensitive to the solution RI variations. A 0.18% RIU change in solution RI may lead to the resonance peak shifting nearly 1.1 nm, resulting in RI sensitivity as high as 462.21 nm/RIU and in a Q factor of 1.112х105. Moreover, high biosensor performance requires also a low detection limit (D), which is inversely proportional to Q and S. the detection limit of refractive index change can be expressed as follows [33,52]:
D=
λ0 10QS
3. Conclusion In summary, a label-free photonic crystal based biosensor has been presented and theoretically investigated using FDTD method. The device structure consisted of a ring shaped holes cavity system, coupled to an input and output waveguides in 2D-PhC silicon layer with a triangular lattice of air holes. In conventional photonic crystal cavities and other devices such as ring resonators, most of light is confined to the waveguide dielectric structure, and only the evanescent tail of the optical mode meets the analytes of interest. In contrast, for the PhCRSH cavity case, it has been demonstrated that a change in RI of the ring’s contents, which corresponds to the increase in analyte glucose concentration, is felt by the majority of the modal field, altering strongly the resonant wavelength of the system. According to simulation results this field overlaps is reflected by a sensitivity of 462.61 nm/ RIU and a quality factor as high as 1.112х105, resulting in a detection limit of less than 3.03х10−6 RIU. These features open opportunities for the PhC-RSH cavity system as a fundamental building block for performing label-free multiplexed detection that can be widely used for biosensing detection and environmental monitoring.
(11)
where λ0 represents the resonant wavelength, which is equal to 1571 nm, S is the RI sensitivity and Q means the quality factor. Therefore, based on the above results the calculated detection limit of the proposed biosensor is about 3.03х10−6. Table 2 displays the comparisons among the proposed biosensor based PhC-RSH cavity and several previously reported PhC biosensors. Sensitivity and Q factor values of these designs in addition to their corresponding detection limit are also summarized. As shown in Table 2, the proposed biosensor exhibited significantly higher sensitivity due to the increase in light matter interaction within the sensing area. Additionally, the Q factor value and the detection limit of the designed device were favorably comparable to the reported works. However, the accuracy of the proposed glucose biosensor and the biochemical sensor reported in reference [53] was estimated by the biosensor figure of merit (FOM=SQ/λ0) where S, Q and λ0 represent the sensitivity, quality factor and resonant wavelength, respectively. Based on the above results, the proposed biosensor exhibits the high FOM value of 3.27х104. From the experimental point of view, the difficulties that can be encountered during the process of this structure lies in the rings
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Infiltrated photonic crystal cavity as a highly sensitive platform for glucose concentration detection ⁎
Safia Arafaa, , Mohamed Bouchemata, Touraya Bouchemata, Ahlem Benmerkhia, Abdesselam Hocinib a Laboratoire Micro-Systèmes et Instrumentation, Département d'électronique, Université des frères Mentouri, Constantine 1, route d’Aïn el Bey, Constantine 25000, Algeria b Laboratoire d’Analyse des Signaux et Systèmes, Département d’Electronique, Faculté de Technologie, Université Mohamed boudiaf de M'sila, BP.166, Route Ichebilia, M'sila 28000, Algeria
A R T I C L E I N F O
A BS T RAC T
Keywords: Optical biosensor Photonic crystal Cavity-coupled waveguide High sensitivity Glucose concentration
A Bio-sensing platform based on an infiltrated photonic crystal ring shaped holes cavity-coupled waveguide system is proposed for glucose concentration detection. Considering silicon-on-insulator (SOI) technology, it has been demonstrated that the ring shaped holes configuration provides an excellent optical confinement within the cavity region, which further enhances the light-matter interactions at the precise location of the analyte medium. Thus, the sensitivity and the quality factor (Q) can be significantly improved. The transmission characteristics of light in the biosensor under different refractive indices that correspond to the change in the analyte glucose concentration are analyzed by performing finite-difference time-domain (FDTD) simulations. Accordingly, an improved sensitivity of 462 nm/RIU and a Q factor as high as 1.11х105 have been achieved, resulting in a detection limit of 3.03х10−6 RIU. Such combination of attributes makes the designed structure a promising element for performing label-free biosensing in medical diagnosis and environmental monitoring.
1. Introduction Optical sensors that provide instantaneous detection and quantification of biological analytes have emerged as a field of great interest due to their promising characteristics such as safety in an inflammable and explosive environment, immunity to electromagnetic interference, rapid response speed and the remote on-line sensing capability. The most well exploited techniques for optical sensing are based on the principle of surface plasmon resonance (SPR) [1,2], colorimetric resonances, and interferometry methods [3]. Among those optical resonance technologies, responsive photonic crystal (PhC) based sensors have attracted substantial attention because of their miniaturized size, their high spectral sensitivity, minimal sample preparation without fluorescence labeling and the possibility of integrating MEMS (Micro-Electro-Mechanical Systems) [4]. These highly ordered devices can be fabricated using microelectronic fabrication techniques, and can be easily integrated with microelectronics, microfluidics [5,6] and other kinds of photonic devices. A typical planar photonic crystal (PhC) consists of cylindrical air holes with defined lattice constant in a thin silicon membrane. PhC sensors provide strong light confinement within the analyte itself due to the photonic band-gap effect [7].
⁎
Light can be concentrated in a very small volume, leading to a large light-matter interaction [8]. This phenomenon makes the sensor highly sensitive to small refractive index (RI) variations that are produced by biological species immobilization on the PhC pore walls. The ideal sensing technology should be highly selective and appropriately sensitive to the biological analyte concentrations. Hence, different techniques and setup configurations are constantly being developed and optimized to increase the detection performances. Therefore, a proper PhC design for biosensing is an essential task which should be carefully handled to obtain the required sensing properties. Various designs and configurations of biosensing devices have been proposed and performed using different types of PhC structures such as microcavities [9–14], ring resonators [15] waveguides [16–19], slot waveguides [20,21], heterostructures [22]. They all exploit extremely sensitive resonant conditions for guided modes in the PhC with respect to RI changes in the ambient medium. In practical applications, due to the difficulties in coupling light into the PhC resonator-only system, the waveguide-resonator system is usually preferred. Incorporating with PhC waveguide, coupled microcavities [23–26] provide several advantages in terms of compactness, high sensitivity and quality (Q) factor, easy extension to sensor arrays and
Corresponding author. E-mail address: arafa.safi[email protected] (S. Arafa).
http://dx.doi.org/10.1016/j.optcom.2016.10.019 Received 10 June 2016; Received in revised form 21 September 2016; Accepted 10 October 2016 0030-4018/ © 2016 Elsevier B.V. All rights reserved.
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Assuming harmonic time dependence for the magnetic field with (angular) frequency ω. H(r, t) = H(r)e−iωt , we obtain the master equation for the magnetic field:
the capability of parallel measurement [27–29]. Examples of such designs include that of Dorfner et al. [30], who experimentally demonstrated a PhC channel drop filter for fluid sensing applications with a Q-factor of 3000 and a sensitivity of 155 nm/RIU, as well as that of Zhou et al. [31] who have proposed the integration of high transmittance H2 nanocavity and broadband waveguide, the obtained results revealed a RI sensitivity of 131.70 nm/RIU with a Q factor of about 3000. Further, Liu et al. [32] proposed an experimental model of an optical PhC sensor based on a channel-drop configuration, where simulation and experimental results showed a good agreement with RI sensitivity of 153 nm/RIU. It is argued that higher Q factors will contribute to detect a very small shift in the resonance wavelengths. Therefore, maximizing the Q factor of the resonator, will reduce the impact of noise on the determination of the resonance wavelength [33], this would further enhance the sensitivity and accurate the detection limit. However, for sensing applications, the quality factor is not the only crucial parameter to gain higher sensitivities. The detection performances can also be enhanced by conducting improvements in the sensor setup such as temperature stabilization, coupling enhancement and the topology optimization of device geometry (the shape, the size of the holes and the thickness of the Si layer) which further enhance the intensity within the detection area. The alteration of the cavity geometric parameters offers a great structural freedom to tune the sensor optical properties [34,35]. Some critical issues such as achieving high refractive index sensitivity or enhancing light coupling into PhC structures must still be overcome. In this paper, we report the design and the simulation of an integrated PhC biosensor that is potentially used for glucose concentration detection. Considering silicon-on-insulator (SOI) technology, the designed structure is formed by two waveguides and one cavity system. The device sensitivity is determined by the magnitude of lightmatter interaction. Therefore, to achieve higher sensitivities, the optical resonant field needs to be strongly localized and overlapping with the analyte. In this context and in order to enhance the electric field intensity and hence the light-matter interactions within the cavity sensing area, the ring shaped holes cavity configuration is adopted. Results obtained by performing FDTD simulations indicate that by adjusting the ring's width as well as the number of functionalized holes around the cavity sensing area, the sensitivity and the quality factor can be greatly improved. Moreover, we demonstrated that the resonant wavelength mode shifts its spectral position following a linear behaviour when a glucose concentration ranging between 0% and 60% is applied. For the glucose detection, a sensitivity of 462.61 nm/ RIU and a detection limit of 3.03х10−6 RIU are observed.
⎛ 1 ⎞ ⎛ ω ⎞2 ∇×⎜ ∇ × H(r⃗) ⎟=⎜ ⎟ H(r ⃗ ) ⎝ ɛ(r⃗) ⎠ ⎝c⎠
where c = 1/ μ 0ɛ0 is the speed of light in vacuum. To obtain numerical solutions for these equations, the finite difference time domain (FDTD) method is the best suitable [36]. This method has been one of the most commonly used for simulating pulse propagation through photonic crystal devices [37,38]. After introducing a known initial field and sources into the computational domain, time development of the electric and magnetic field in the structure can be calculated in discrete time steps [39]. The FDTD simulations in this work have been carried out using RSoft software. In order to study the dispersion diagram and the modal distribution of the field through PhC structure, the plane-wave expansion method (PWE) is applied [40,41]. Using this method the solutions of Eq. (4) are expanded in a truncated basis of plane waves. These plane waves are expressed as a Fourier expansion series in k-space (reciprocal space), which facilitates the computation.
H (r ⃗ ) =
1 = ɛ(r ⃗ )
→ ⎯
∑ η (G ).exp
→ ⎯ (iG . r ⃗ )
⇀ ⎯ G
(6)
where and η (G ) represents the Fourier transform of the inverse of ɛ(r ⃗ ). It is defined by:
→ ⎯ 1 η (G ) = Ω
∫cell
→ ⎯ 1 .exp(−iG . r ⃗ ) dr ⃗ ɛ(r ⃗ )
(7)
Ω designates the unit surface cell. For 2D-PhC structure and for the TE polarization the eigen value equation is given by:
∑
⎛→ ⎯ ⎯ ⎞ ⎛→ ⎯ ⎞ ω2 → → ⎯ → ⎯ → ⎯ → ⎯ → ⎯ k +G k +G′ η ⎜G −G′⎟ h1 ⎜G′⎟= 2 h1 (G ) ⎠ ⎝ ⎠ c ⎝
(8)
The Fourier coefficients of the inverse of ɛ(r ⃗ ) for ring-shaped holes are given by [42,43]:
⎧ → ⎯ 1 π ⎛1 1⎞ ⎪ + ⎜ − ⎟ (R 2 −Rin 2 ), G =0 ɛb Ω ⎝ ɛa ɛb ⎠ out ⎯→ ⎪ η (G )=⎨ → ⎯ ⎪ 2π ⎛ 1 1 ⎞ ⎪ ΩG ⎜⎝ ɛa − ɛb ⎟⎠ [Rout J1 (G (Rout ))−Rin J1 (G (Rin ))], G ≠0 ⎩
(1) (2)
(9)
where J1 is the Bessel function of the first kind, Rin and Rout are the inner and the outer radius of air rings, respectively. ɛa =1 is the dielectric constant of air rings and ɛb is the silicon permittivity. For PhC air holes case Rin=0 and Rout=r, where r is the air holes radius. In this study, the dispersion diagram is computed numerically using BandSOLVE software [44,45]. Fully integrated into the RSoft Component Design Suite, BandSOLVE is ideal for producing band structures for PhC bandgap structures. The simulation engine is based on an advanced optimized implementation of the PWE technique for periodic structures.
μ is the permeability which is equals to µ0 as the considered material is non-magnetic and ε represents the permittivity which is usually written as ε=ε0ε(r), where ε0 is the permittivity of vacuum and ε(r) is the relative permittivity of the material. Eq. (3) is obtained by solving Eq. (1) and inserting it into the time derivative of Eq. (2)
⎛ 1 ⎞ ∂2 ∇×⎜ ∇ × H(r, t) ⎟=− 2 [μ 0H(r, t)] ⎝ ɛ(r) ⎠ ∂t
(5)
⎯⎯⎯→
Mathematically, light propagation in photonic crystals is evaluated by solving Maxwell's equations in a mixed dielectric medium, expressed in Eqs. (1) and (2) E and H are the electric and magnetic field intensities.
∂ [ɛɛ0 (r)E(r, t)] ∂t
→ ⎯
→ ⎯ where G is the reciprocal lattice vector and k⃗ is a wave vector in the first brillouin zone. N=1 or 2 and eˆ1, eˆ 2 are orthogonal unit vectors ⎯ ⃗ → perpendicular to (k+G i). Because of the periodicity of the dielectric constant for PhC ⎯ → → ⎯ ɛ(r ⃗ )=ɛ(r ⃗+R ) with respect to the real space lattice vector R , Bloch's theorem can also be applied to expand it in terms of plane waves:
→ ⎯ G′
∇ × H(r, t)=−
→ ⎯
⃗ ∑ h(Gi).eˆN.exp.((ik+G i). r⃗)
⎯⎯⎯⎯⎯⎯→ Gi,N
2. Modeling and theory
∂ ∇ × E(r, t)=− [μ 0H(r, t)] ∂t
(4)
(3) 94
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matched layers (PML) conditions have been considered in the calculations to ensure no back reflection in the limit of the analyzed region. In simulation process, a TE polarized Gaussian optical pulse, covering the whole frequency-range-of interest, is launched at the input port to excite the cavity modes. A power monitor was placed at the end of the output waveguide to measure transmitted signal. For improving the simulation accuracy, FDTD analysis was carried out with a grid size of 0.01. The spectral response of the cavity without the sample presence is depicted in Fig. 2(a). When the PhC cavity is in resonance situation, a sharp peak with a Lorentzian line shape appears inside the complete PhC bandgap in TE polarization. In this case the designed cavity presents two resonant modes at wavelengths of 1.3424 nm and 1373.7 nm, respectively. The corresponding quality factor (Q) values are then calculated using the 2D finite-difference time domain (FDTD) method, combined with fast harmonic analysis. To quantitatively well analyze the sensing properties of the structure, the resonant mode with λ=1373.7 nm is chosen to observe the resonant wavelength shift as it presents the highest Q factor of 7.06 х103. Fig. 2(b) shows the light propagation profile through the cavity in the x-y plane, it can be clearly seen that the optical field is well confined in the cavity sensing area. Therefore, the properties of wide band and strong light confinement have been well applied in the structure design. This causes the cavity to be very sensitive to refractive index variations due to the photon lifetime enhanced and the large degree of lightmatter interaction.
2.1. Structure design In this work, the used 2D-PhC structure consists of triangular lattice of air holes etched in silicon (Si) slab (refractive index nsi=3.45). The radius of the bulk air holes is r=0.37a where a is the PhC lattice constant (a=530 nm), the thickness of the PhC slab is set as h=230 nm. Silicon dioxide (SiO2) layer with a thickness of 1500 nm is used as a bottom to support the Si slab. The low index SiO2 layer lying beneath the high index Si slab helps to confine light within the cavity core, preventing optical losses into the lower substrate, the light confinement in the vertical direction is then ensured by total internal reflection. In order to reduce the computational efforts needed for full 3D calculations, the PhC is replaced by a 2D system with the background dielectric medium having the effective refractive index of 2.838, which corresponds to the effective index of the fundamental guided TE mode in a 230 nm thick silicon slab on silicon dioxide at a wavelength of 1550 nm. For the following numerical simulations, we applied the effective index approach [46] with a combination of the 2D-PWE and the 2DFDTD methods from the RSoft software package. The dispersion properties of the regular PhC structure have been analyzed using the 2D-PWE method of BandSOLVE software. For the TE polarization, the photonic band gap (PBG) extends from ω1=0.278 (a/λ) to ω2=0.4 (a/λ) corresponding to the broad wavelength range of 1323.7–1908.1 nm. Therefore, the effective working wavelength range is extensively sufficient to meet the sensing demand where the resonant mode produces a wide wavelength shift. Fig. 1 shows the schematic side sectional view of the designed biosensor. The device consists of two waveguide couplers and one cavity. The two waveguides are obtained by removing one row of air holes in the x direction. They are used to couple light in and out of the PhC cavity. The designed cavity consisted of ring-shaped holes etched in the Si layer. It is necessary to note that the ring-shaped holes configuration does not only provide more flexibility in terms of designing new structures compared to circular shaped ones, but also enhance the optical field confinement and hence light-fluid interactions within the ringed low-dielectric material area [18,19]. The ring shapedholes parameters are defined by their inner and outer radius Rin=126.5 nm (0.23a) and Rout=247.5 nm (0.45a) respectively, thus the ringed air region is given by Rout−Rin=121 nm. The whole cavity system was separated from the input and output waveguides by three PhC holes. Details on ring shaped holes fabrication are given in [47]. Two-dimensional finite-difference time-domain (2D-FDTD) method was performed to estimate the device functionality. Perfectly
2.2. Sensing properties of the designed structure In order to illustrate the operating principal of the PhC cavity based RI biosensor, a series of FDTD simulations have been conducted. The preliminary sensing analysis is done by supposing the local infiltration of de-ionized (DI) water in the cavity sensing area, this corresponds to the refractive index (RI) change of the rings-shaped holes from 1 (air) to 1.33 (DI-water). For a local infiltration a precise technique based on micro-infiltration technology via hollow submicron size pipettes has been experimentally demonstrated [48]. This technique allows a controlled liquid deposition inside a desired PhC hole, while not affecting the adjacent ones. The optical detection properties of the designed structure have been quantitatively estimated by the sensitivity parameter S (Δλ/Δn), which is defined as the ratio of the shift in the wavelength (Δλ) to the change in the RI due to analyte infiltration (Δn). According to the obtained
Fig. 1. (a) Schematic side sectional view of the proposed PhC biosensor structure in SOI substrate. (b) Schematic diagram of the proposed ring shaped photonic crystal cavity coupled to an input and output PhC waveguides.
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Fig. 2. (a) The transmission spectra of the designed device before the sample infiltration for Rin=0.23a. (b) The electric field distribution through the cavity structure in the x–y plane (The color bar indicates the normalized optical field intensity ranging from −1 to 1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sharp decrease in the Q factor value. The calculated RI sensitivity for the inner radius Rin modified as a function of RI change is shown in Fig. 4(b). It can be noted that by increasing the Rin value which corresponds to the decrease of the ringed sensing area, the overlap between the optical modes and the detecting target decreases gradually which adversely affect the RI sensitivity. In view of the fact that, the Q factor was remarkably enhanced by the increase of the inner radius value while the RI sensitivity was gradually declined, it has been necessary to choose a trade-off between the Q factor and the sensitivity. The maximum product of Q factor and sensitivity can therefore be obtained when the inner radius Rin equals to 0.255a. Through the discussion about high Q factor and sensitivity S, the label-free biosensor is designed for Rin=0.255a. Fig. 5(a) and (b) shows the schematic view of the improved biosensor design and the fundamental quasi-TE mode propagation through the defect cavity before and after water infiltration, respectively. Considering water absorption at telecom wavelength range, the total Q factor was about 1.0172х105 at the resonant wavelength of 1558.1 nm. Compared to recent reported works [31,34,35], the Q factor and the RI sensitivity values have been significantly improved using ring shaped holes cavity design.
results the output resonant wavelength is red-shifted due to the increase in the ambient RI of the sensing area, which confirms the analyte identification. As seen in Fig. 3, a change in the RI of Δn=0.33 between air and water results in a spectral red-shift of 140.7 nm which corresponds to a RI sensitivity of 426.36 nm/RIU (refractive index unit). To further investigate and optimize the Q factor of the proposed biosensor, the width of the ring-shaped holes in the PhC cavity structure is altered by adjusting the inner radius (Rin). We fixed the value of the outer radius (Rout) at 0.45a and the inner radius (Rin) is varied from 0.23a to 0.28a with an increment of 0.01a. The variation tendencies of the Q factor and the resonance wavelengths according to Rin change are plotted in Fig. 4(a). It can be seen that the resonance wavelength shifts towards higher values due to the increase of high dielectric material in the cavity region. Similarly, the quality factor increases as the inner ring radius (Rin) increases from 0.23a to 0.28a and reaches its maxima of 1.36х105 at the resonant mode located at 1470.9 nm for Rin=0.27a, followed by a sharp decrease. The increase in the inner radius value corresponds to the decrease of the annular air area, when the Rin is more than 0.27a the rings area becomes critically thin. Consequently, the light confinement within the cavity area decreases and the optical leakages become more significant, which negatively affect the photon lifetime within the cavity area causing a
2.3. Functionalized holes number discussion It is essential to note that for biosensing or chemical detection applications, the magnitude of the resonant wavelength shift is dependent on the combination of many factors such as the number of functionalized holes and the effective refractive index change of targets. We assume that the defect ring-shaped holes and the surrounding holes are completely filled with the analyte (DI-Water). When the detection event occurs the refractive index will change due to the detection targets infiltrated into the air holes around the resonant cavity. To study the dependence of the sensitivity on the number of functionalized holes, we varied the number of holes being functionalized (N) around the resonant cavity in the PhC biosensor illustrated in Fig. 6. (Indicated by the blue circular holes). The transmission spectra corresponding to the different infiltration cases are depicted in Fig. 7(a). As can be seen in this figure, the resonant wavelength shift gets larger as the number of functionalized holes increases. This is due to the reduced effective refractive index between the infiltrated holes and the semiconductor membrane [49,50], as a result, the sensitivity becomes higher. The highest
Fig. 3. The resonant wavelength shift according to water infiltration (green line) within the cavity sensing area for Rin=0.23a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. (a) The Q factor and the resonant wavelength variations according to the change of the inner radius (Rin) from 0.23a to 028a. (b) The calculated refractive index sensitivity (S) as function of the inner radius change (Rin).
Fig. 5. (a) Schematic diagram of the optimized PhC biosensor structure based ring shaped holes cavity, the blue rings refers to the sensing area filled with DI-water. (b) The resonant wavelength shift (Δλ) according to water infiltration within the cavity sensing area for Rin=0.255a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Schematic diagram of the biosensor structure indicating the number of functionalized holes around the resonant cavity; (a) 4 functionalized holes, (b) 6 functionalized holes, (c) 14 functionalized holes, (d) 20 functionalized holes, (e) 54 functionalized holes, (f) all air holes are used as functionalized holes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
can explain the choice of the local infiltration instead of the total filling. The optimal Q factor and sensitivity as a function of the change in the number of functionalized holes (N) that corresponds to the different local infiltration cases are shown in Fig. 7(b). As indicated
sensitivity value of 688.18 nm/RIU has been obtained for the total infiltration, where all the air holes around the cavity are infiltrated. However, the corresponding Q factor and the transmission efficiency (shown by the brown line in Fig. 7(a)) are in their lower values, which 97
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Fig. 7. (a) Transmission spectra with different number of functionalized circular holes (N), (b) Quality factor and refractive index sensitivity (S) variations as a function of the number of functionalized circular holes (N).
Fig. 8. (a) Schematic diagram of the optimal PhC biosensor structure, the blue shapes refers to the sensing area filled with glucose solution. (b) The electric field distribution through the cavity structure in the x–y plane (The color bar indicates the normalized optical field intensity ranging from −1 to 1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Refractive index (RI), resonant wavelength (λ), Q factor and sensitivity (S) as function of different glucose concentrations (C). C (g/L)
RI
λ (nm)
Q factor
S (nm/RIU)
0 10 20 30 40 50 60
1.33230545 1.33349435 1.33468325 1.33587215 1.33706105 1.33824995 1.33943885
1569.9 1570.4 1571.0 1571.5 1572.0 1572.6 1573.1
1.087х105 1.099х105 1.112х105 1.125х105 1.137х105 1.142х105 1.164х105
420.55 462.61 448.59 441.58 454.20 448.59
2.4. Sensitivity dependence on the glucose concentration Through the design and discussion about the use of the structure based PhC-RSH cavity as a RI biosensor in the above section, we theoretically demonstrate that this structure can be well used as an optofluidic biosensor for glucose concentration measurement (Fig. 8(a)). Therefore, to investigate the relationship between the refractive index sensitivity and the glucose concentration, a refractive index range was selected to match the glucose concentration samples ranging from 0 to 60 g/l. The RI of glucose solution can be calculated by [51]:
Fig. 9. Resonant wavelength changes of the designed biosensor as function as a function of changes in the refractive index corresponding to the seven infiltration states of glucose concentrations.
in this figure, the sensitivity is enhanced by increasing the number of functionalized holes, whereas the Q factor is declined. Therefore, it has been necessary to choose a trade-off between Q factor and RI sensitivity. Accordingly, the optimal Q factor of 1.064х105 and the improved sensitivity of 408.49 nm/RIU are both achieved for N=4.
n = 0. 00011889C + 1. 33230545
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Table 2 Comparison of the proposed biosensor with various similar PhC designs. References
Sensing structure
Sensitivity (nm/RIU)
Q factor
detection limit (RIU)
[24] [31] [35] [53] [54] In this work
RI RI RI RI RI RI
330 131.70 160 330 293 462.61
3.82х103 2.96х103 107 8.2х106 950 1.112х105
_ _ 8.75 х10−5 1.24 х10−5 _ 3.03х10−6
biosensor formed by two waveguide and one microcavity biosensor consists of H2-type nanocavity and broadband W1 waveguide sensor based on a ring–slot cavity coupled to an input and output waveguides sensor consists of ring resonant cavity coupled to an in and out line defect PhC structure. sensor consists of ring cavity coupled to an optofluidic slow-light waveguide in a PhC platform. biosensor based ring-shaped holes cavity coupled to an input and output waveguides
manufacture. As they are composed of dielectric rods emerged and centered in air holes, a very precise alignment is required for positioning process. In order to overcome these problems, more controlled manufacturing techniques have been developed. Using electron beam lithography (EBL), Säynätjoki et al. [47] demonstrated that a thin ringed area as narrow as 60 nm patterned above 240 nm thick silicon slab could be successfully realized. Avoiding the challenging EBL alignment, Zhou [55] established a novel method based on atomic layer deposition (ALD) and sacrificial etching. This technique could achieve alignment accuracy down to the atomic level. Based on the arguments previously developed, the practical implementation of the designed structure may be quite possible. The overall results demonstrate that the proposed device is a promising building block for performing monolithic integration of high label-free multiplexed detection for biosensing applications.
where C is the glucose concentration (g/L) and n is the glucose solution RI. The intensity field distribution profile in the x–y plane of the resonant mode located at λ=1569.9 nm for the optimal biosensor structure is represented in Fig. 8(b). Due to the photon lifetime enhanced and the large degree of light-matter interaction, a significant amount of light amplification in the cavity sensing area is clearly observed. This causes the cavity to be very sensitive to small refractive index changes. The spectral response of the cavity as a function of changes in the refractive index corresponding to the seven infiltration states of glucose concentrations is shown in Fig. 9. The resonance peak located at 1569.9 nm (a/λ=0.3376) was identified as a reference mode that corresponds to 0 g/L of glucose concentration. It is noted that, as the glucose concentration increases, the cavity mode redshifts due to the increase in the cavity ambient refractive index from 1.33230545 to 1.33943885. Simulation results are illustrated in Table 1. It can be clearly seen that the shifts in resonant wavelengths have been obtained even for the slightest change in the sample RI, hence different glucose concentrations in the infiltrated sample could be well detected. It can also be noted that the Q factor value increases relative to its initial value before the samples infiltration. This is due to the increase in the ambient refractive index, which reduces the PhC contrast [50]. Therefore, we can conclude that the glucose biosensor based PhC-RSH cavity presents high precision and it is very sensitive to the solution RI variations. A 0.18% RIU change in solution RI may lead to the resonance peak shifting nearly 1.1 nm, resulting in RI sensitivity as high as 462.21 nm/RIU and in a Q factor of 1.112х105. Moreover, high biosensor performance requires also a low detection limit (D), which is inversely proportional to Q and S. the detection limit of refractive index change can be expressed as follows [33,52]:
D=
λ0 10QS
3. Conclusion In summary, a label-free photonic crystal based biosensor has been presented and theoretically investigated using FDTD method. The device structure consisted of a ring shaped holes cavity system, coupled to an input and output waveguides in 2D-PhC silicon layer with a triangular lattice of air holes. In conventional photonic crystal cavities and other devices such as ring resonators, most of light is confined to the waveguide dielectric structure, and only the evanescent tail of the optical mode meets the analytes of interest. In contrast, for the PhCRSH cavity case, it has been demonstrated that a change in RI of the ring’s contents, which corresponds to the increase in analyte glucose concentration, is felt by the majority of the modal field, altering strongly the resonant wavelength of the system. According to simulation results this field overlaps is reflected by a sensitivity of 462.61 nm/ RIU and a quality factor as high as 1.112х105, resulting in a detection limit of less than 3.03х10−6 RIU. These features open opportunities for the PhC-RSH cavity system as a fundamental building block for performing label-free multiplexed detection that can be widely used for biosensing detection and environmental monitoring.
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where λ0 represents the resonant wavelength, which is equal to 1571 nm, S is the RI sensitivity and Q means the quality factor. Therefore, based on the above results the calculated detection limit of the proposed biosensor is about 3.03х10−6. Table 2 displays the comparisons among the proposed biosensor based PhC-RSH cavity and several previously reported PhC biosensors. Sensitivity and Q factor values of these designs in addition to their corresponding detection limit are also summarized. As shown in Table 2, the proposed biosensor exhibited significantly higher sensitivity due to the increase in light matter interaction within the sensing area. Additionally, the Q factor value and the detection limit of the designed device were favorably comparable to the reported works. However, the accuracy of the proposed glucose biosensor and the biochemical sensor reported in reference [53] was estimated by the biosensor figure of merit (FOM=SQ/λ0) where S, Q and λ0 represent the sensitivity, quality factor and resonant wavelength, respectively. Based on the above results, the proposed biosensor exhibits the high FOM value of 3.27х104. From the experimental point of view, the difficulties that can be encountered during the process of this structure lies in the rings
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