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Highly sensitive long range surface plasmon based liquid sensor by shining the radially polarized light in bimetallic taper fiber structure
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
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Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Highly sensitive long range surface plasmon based liquid sensor by shining the radially polarized light in bimetallic taper fiber structure Nabamita Goswamia, Deepak Chaurasiab, Ardhendu Sahaa, a b
T
⁎
Department of Electrical Engineering, National Institute of Technology Agartala, Barjala, Jirania, Tripura (West), 799046, India Department of Electronics and Instrumentation Engineering, IET, Bundelkhand University, Jhansi, Uttar Pradesh, 284001, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Long range surface plasmon resonance Radially polarized light Bimetal tapered fiber Fiber optic liquid sensor
A new approach towards the enhancement of sensitivity in fiber optic liquid sensor is proposed, designed and simulated using the long range surface plasmon resonance technique shining the radially polarized light in bimetallic fiber structure, which comprises a fiber core coated with 40 nm and 10 nm thin Ag and Au layer respectively. The radially outward field distribution of the radially polarized light enables the multiple point plasmonic interaction on the whole fiber periphery which enhances the sensitivity as well as the dynamic range of refractive index sensing through the proposed sensor. A 13 times and 25 times better sensitivity has been estimated using NaCl and KCl solution as compared with the existing articles till date using p-polarized light. Minimum measurable concentration is 0.5 g/l and 1 g/l for NaCl and KCl solutions for a minimum change in output power of 30.04 μW and 25.60 μW respectively using InGaAs detector. With enhanced sensitivity and higher dynamic range of refractive index, this new idea explores the new avenues in the field of fibre optic liquid sensing applications.
1. Introduction In this new era, the phenomenon of surface plasmon resonance (SPR) has attracted a great deal of attention, owing to its immense practical applications in the field of molecular adsorption [1–3], immunoassary [4,5], data interpretation [6], second harmonic generation [7,8], optical parametric generation [9], spectroscopy [10,11], bio sensing [12–14], solar cells [15,16], genetic engineering [17–19], high contrast SPR microscopy or imaging [20], mass spectrometry (MS) [21], functional proteomic screening [22,23], characterization of enzyme inhibitors [24,25], immunosensing [26] etc. Here SPR will exist at a metal-dielectric interface which supports the charge density oscillations propagating through that interface [27,28]. In the presence of one metal-dielectric interface, SPR phenomenon will exist whereas with two metal–dielectric interfaces, long range surface plasmon (LRSP) resonance as well as short range surface plasmon (SRSP) resonance phenomena will exist, but by the optimal choice of the thicknesses and dielectric constants of the composite layers the LRSP phenomenon can be made predominant [29]. This LRSP phenomenon can also be implemented in the field of optical fiber sensors for monitoring the various entitinin hertile and inaccessible environment hazard where the chemical encoded informations is transmitted through the optical fiber between the detection system and the remote sensing sample [30]. These optical fibers are unaffected to other source of noise like electromagnetic disturbances with respect to the electric field [31]. Owing to the possibilities of miniaturization on account of its flexibility, light weight and small size, the optical
⁎
Corresponding author. E-mail addresses: nabamita08@rediffmail.com (N. Goswami), [email protected] (A. Saha).
https://doi.org/10.1016/j.ijleo.2019.163001 Received 31 May 2019; Accepted 23 June 2019 0030-4026/ © 2019 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
N. Goswami, et al.
fibers make more popular use in sensing applications. Generally in case of LRSP based fiber liquid sensors, method of fixed range of angle of incident and modulated wavelength is used where this LRSP based sensors is commonly fabricated using the KretschmannRaether geometry comprising a fiber core coated with metal layers and sensing layers respectively [32]. The response of such type of LRSP resonance based liquid sensors can be enhanced in presence of taper fiber structure where a powerful interaction between the evanescent wave and surface plasmon wave can be buildup [33,34]. Broadening the angular range of light propagation together with the change in incident angle, enhancement of the performance with respect to the sensitivity can be facilitated near the taper waist region [35]. Here, taper angle plays a vital role for estimating the sensitivity using the taper fiber structure whereas, it is well known that the observed sensitivity is more in case of double taped fiber structure as compared to normal fiber structure. With the increment of the tapered angle, number of reflection of a rays undergoes within the taper waist region will decrease. It is well known that only ppolarized light is responsible for the creation of LRSP where the electric field is along the incident plane and in the direction of propagation. In presence of radially polarized light (RPL), generation of LRSP is also possible as the field distribution is always ppolarized at the spatial circumference of the radially polarized light beam [36]. The radially polarized light establishes the compact focal spot and strong longitudinal field compound conversion of focal point [37] and therefore is useful for the application in the field of tip enhanced Raman spectroscopy [38–41]. The field oscillation in case of radially polarized light is always outward from the center of the beam having inhomogeneous spatial distribution of electric field. To the best of our knowledge several articles have been devoted for depicting the LRSP based taper fiber liquid sensor with varying refractive index within the sensing region illuminated with p-polarized light [42], but the liquid sensor using RPL in bimetallic tapered structure with better sensitivity has not yet been reported. Here the idea deals with the LRSP based fiber optic liquid sensor using radially polarized light with better sensitivity for the detection of NaCl and KCl solution having different concentrations. 2. Proposed scheme A theoretical approach towards the sensitivity realization of the LRSP based liquid sensor with bimetal coated tapered fiber structure using radially polarized light is proposed here. Fig.1 comprises a double taper fiber sensor probe having core diameter 300 μm and tapering angle of 0.707 rad with consecutive thin layers deposition of Ag-Au with thicknesses of 40 nm and 10 nm respectively over the taper waist region of 10 mm after the removal of clad from the optical fibre. The unclad portion is initially cleaned by acetone with high tension bombardment on vacuum chamber. An Ag layer of 40 nm thickness using vacuum coating unit is coated on the unclad portion of the taper fiber using thermal evaporation technique which is kept at 5 × 10−6 mbar of pressure. An additional layer of Au having thickness 10 nm is coated using the same technique to enhance the sensitivity of the sensor which is shown in Fig. 1. Presence of Au layer not only enhances the overall performance of the sensor by increasing the detection sensitivity as compared to single metal coated structure but also protects the Ag layer from the degradation through oxidization and other chemical effects. Moreover, with the combination of Ag and Au layer, highest dip in spectrum at resonance angle and largest shift in resonance curve with the change of refractive index can be achieved in case of multilayered SPR structure. Here NaCl and KCl solution serves the purpose of sensing layer within the taper waist region where the concentration of the NaCl and KCl solutions varies from 60 g/l–360 g/l. For accurate measurement, the solutions are created by dissolving the required amount of NaCl in 1 L of deionized water and KCl in distilled water. Using extra-cavity generation technique with radial polarization converter (RPC)
Fig. 1. Schematic arrangement of liquid sensor assembly of LRSP based Ag-Au coated tapered multi-mode fiber using radially polarized light. 2
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
N. Goswami, et al.
manufactured by ARCoptix, the generation of radially polarized broadband light source with varying wavelength is facilitated which serves the purpose of proposed source of fundamental in this article. For calculating the ultimate output power from the proposed configuration the following steps need to be considered – 3. Normalized transmitted output power for radially polarized light For a double taper structure coated with different layers within the taper waist region, the overall normalized transmitted output power at the output end of the fiber is mathematically expressed by [43,44]ϕ
ϕ
⎡ α ∫2 R Nref (θ) . P (θ). d (θ) + (1 − α ) ∫2 P (θ). d (θ) ⎤ ⎢ ⎥ ϕ1 ⎢ ϕ1 ⎥ ϕ 2 ⎢ ⎥ ∫ P (θ). d (θ) ⎢ ⎥ ϕ1 ⎢ ⎥ ⎣ ⎦
(1)
This relation prevails for the case of SPR excitation with radially polarized light for a taper fiber optic sensor, where P (θ) represents the incident power within the optical fiber at an angleθ andα is the effective intensity coefficient.
n 2. sin(θ). cos(θ) ⎤ dθ dP (θ) ∞ ⎡ 1 ⎢ (1 − n12 . cos2 (θ))2 ⎥ ⎦ ⎣
(2)
In case of taper fiber, if the radially polarized light is incident at an angle greater than the critical angle inside the taper waist region, then after multiple bounces in taper waist region, light is reflected from the configuration. Here ϕ1and ϕ2 are the angles of propagation of light over the taper waist region and the value of the effective intensity coefficient and α is 1 and ½ for radially polarized light and p- polarized light respectively [45,46]. Within the SPR sensing region of length L, the number of reflections of a ray undergoes is calculated by-
L ⎤ Nref (θ) = ⎡ ⎢ 2 ρ ( z )tan( Ω + θ) ⎥ ⎣ ⎦
(3)
Taking Ω as a taper angle (0.707 rad) and ρo , ρi as the radius of input and output end of the taper region, the exiting different taper profiles with varying taper radius with respect to z (at input end, z = 0 and output end z = L) are given by –
Linear:ρ (z ) = ρi −
z (ρ − ρ0 ) L i
(4)
Exponential linear:ρc (z ) = (ρi − ρ0 ) ⎡e ⎣
−z L
−
z −1 e ⎤ + ρ0 L ⎦
(5)
1
z 2 Parabolic:ρ (z ) = ⎡ρi2 − (ρi2 − ρ02 ) ⎤ L ⎣ ⎦
(6)
Within this proposed configuration as shown in Fig. 1, only at the resonance condition of LRSP the maximum interaction between the evanescent wave and surface plasmon wave will occur within the taper waist region which is analytically expressed by – 1
2 2 ⎡ 2Π ⎛ εm nSR 2Π ⎞ ⎤ n1 sin θ = Re ⎢ ⎜ 2 ⎟ ⎥ λ λ + ε n SR ⎠ ⎥ ⎢ ⎝ m ⎦ ⎣
(7)
here nSR , n1stands for the refractive indices of the sensing region and core respectively, εm is the dielectric constant of the metal layer andλ is the wavelength in nm at an incident angle θ . Here at resonance condition the propagation constant of the light incident at an angle θ is equal to the real part of the surface plasmon propagation constant. 4. Dispersion relation for NaCl and KCl solution It is well known that the refractive index of NaCl solution decreases with the variation of wavelength from 300 nm to 2500 nm and increases with the increment of the concentration of NaCl solution [47,48]. In case of transparent medium, the empirical relation between wavelength and refractive index can be expressed by Sellmier equation. Here, the wavelength and concentration dependent refractive index of NaCl also determined by the Sellmeier equation which is given below-
n (c, λ ) =
A0 +
A1 λ2 A λ2 A λ2 + 2 2 2 + 2 3 2 + A 4 c − A5 c 2 λ2 − l12 λ − l2 λ − l3
(8)
here A0 , A1, A2 , A3 , A 4 , A5 are the Sellmeier coefficients, c represents the concentration in gram per liter andλ denotes the wavelength in nanometer. The value of these Sellmeier coefficients are A0 = 0.385, A1 = 1.32, A2 = 1, A3 = 0.0244, A 4 =2.07 × 105, A5 = 1.75 × 10−7, l12 = 8.79 × 103, l 22 = 1.1 × 108 and l32 =6.09 × 104 [49]. The temperature, wavelength and concentration dependent dispersion equation for the KCl solution is as follows where with the increase in temperature the refractive index of the KCl 3
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
N. Goswami, et al.
Fig. 2. Refractive index vs. wavelength (nm) for NaCl solution with varying concentration from 60 g/l to 360 g/l.
solution decreases, but in case of the proposed configuration for accurate measurement the temperature is kept constant at 25 °C –
nkcl (λ, T , c ) =
1.26486 +
0.30523 × λ2 0.41620 × λ2 0.18870 × λ2 2.6200 × λ2 + 2 + 2 + 2 λ2 − (0.100)2 λ − (0.131)2 λ − (0.162)2 λ − (70.42)2
+ (1.6167 × 10−3) c − (4.0 × 10−7) c 2 − (1.1356 × 10−4)(T − 273.15) − (5.7 × 10−9)(T − 273.15)2
(9)
The variation of refractive index with the varying concentration from 60 g/l to 360 g/l and wavelength from 200 nm to 1200 nm for NaCl and KCl solution are shown in Figs. 2 and 3 respectively. Graphical representations clearly indicate that with the increase in concentration the refractive index of NaCl as well as KCl solution decreases. 5. Sensitivity analysis with respect to wavelength, intensity and concentration 1
The propagation constant of the surface plasmon changes significantly owing to the change in refractive index, ns = (εd ) 2 of the surrounding media which results in the modification of the SPR condition and as a result, changes in several properties of the transmitted light. The resultant sensitivity can therefore be defined by -
Sn, ζ =
δζ δns
(10)
Hereδζ , is the change in the transmitted light properties which can be defined by the intensity, resonance angle, phase and wavelength and δns is the variation of refractive index in surrounding medium. With these four transmitted light properties four interrogation techniques are available for SPR sensing using the proposed configuration. One is “intensity interrogation technique” defined by the variation in transmitted light intensity at fixed incident angle, second is “angular interrogation technique” accomplished by the change of resonance angle, third is “phase interrogation technique” defined by the phase difference between ppolarization and s-polarization states of the reflected light and last is “wavelength interrogation technique” expressed by the change of resonance wavelength at fixed incident angle. These four interrogation techniques are analytically expressed by [50,51]-
Sn =
δInorm → Intensity interrogation technique δns
(11)
Fig. 3. Refractive index vs. wavelength (nm) for KCl solution with varying concentration from 60 g/l to 360 g/l. 4
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
N. Goswami, et al.
Fig. 4. Normalized transmitted output power vs. wavelength with the variation in concentration of NaCl solution from 60 g/l to 360 g/l with ppolarized light.
Sn =
δθres → Angular interrogation technique δns
(12)
Sn =
δϕ0 → Phase interrogation technique δns
(13)
Sn =
δλres → Wavelength interrogation technique δns
(14)
Here Inorm is the change in output light intensity, θres stands for the resonance angle, ϕ0 represents the phase of the output light and λres defines the resonance wavelength. 6. Result and discussion The response features of the proposed LRSP based configuration using different concentrations of the NaCl solution and KCl solution throughout the sensing probe are presented in Figs. 4 and 5 respectively. Here the two solutions individually serve the purpose of sensing layer for the proposed configuration. As, with increasing concentrations, the refractive index decrease for both the solutions, so using the Eqs. 1,7 and 8 the transmitted output power also decreases which is clearly observable from Figs. 4 and 5. Now for incident radially polarized light the variation of transmitted output power for the proposed liquid sensor structure with change in wavelength with varying concentrations of NaCl and KCl solutions are shown in Figs. 6 and 7 respectively. Here also the concentrations of NaCl and KCl solutions are inversely related to the transmitted output power. As compared to p-polarized light, owing to the more plasmonic interaction throughout the periphery of the optical fiber for radially polarized light, an increase in transmitted output power has been discerned from the Fig. 6 and Fig. 7 for both the solutions, which directly denotes the increase in sensitivity of the proposed sensor structure with incident radially polarized light [52].
Fig. 5. Normalized transmitted output power vs. wavelength with the variation in concentration of KCl solution from 60 g/l to 360 g/l with ppolarized light. 5
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
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Fig. 6. Normalized transmitted output power vs. wavelength with the varying concentration of NaCl solution from 60 g/l to 360 g/l with radially polarized light.
Fig. 7. Normalized transmitted output power vs. wavelength with the varying concentration of KCl solution from 60 g/l to 360 g/l with radially polarized light.
7. Sensitivity analysis of the proposed structure Here sensitivity has been calculated using intensity interrogation technique for both radially and p-polarized light. Also by varying concentrations, the sensitivity for the proposed structure is observed and analysed through graphical representations. Sensor output power with varying refractive index for p-polarized light and radially polarized light for NaCl solution and KCl solution are shown in Figs. 8 and 9. Graphical representations clearly indicate that sensitivity has significantly been increased in case of radially polarized light as compared to p-polarized light. Here, with the proposed scheme, the sensitivity measurement for radially polarized light comparing with p–polarized light is analytically defined by-
Fig. 8. Sensor output power vs. refractive index for sensitivity analysis in presence of p-polarized light and radially polarized light for NaCl solution. 6
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
N. Goswami, et al.
Fig. 9. Sensor output power vs. refractive index of radially polarized light and p-polarized light for KCl solution.
⎛ Snr = ⎜ ⎜ ⎝
δIout δns radially − polarized − light δIout δns p − polarized − light
⎞ ⎟ = 1.41 ⎟ ⎠ Iin = const.
(15)
where Snr stands for the resultant sensitivity of the proposed structure with intensity interrogation technique which is 1.41 times for radially polarized light as compared to p-polarized light with the variation of refractive index of sensing layer. With the comparison to the existing articles reported till date, using radially polarized light a 13 times better sensitivity is achieved for NaCl solution and 25 times better sensitivity is discerned for KCl solution as compared to p-polarized light [53–55]. Thus the proposed structure deals with a new idea of liquid sensor which explores better sensitivity than the existing aricles of liquid sensors repoted till date. Now the response of the proposed configuration with respect to resonance wavelength and concentration for NaCl solution and KCl solution with radially polarized light are shown in Figs. 10 and 11. Figures clearly display that, when the concentrations are increased from 60 g/l to 360 g/l, the resonance wavelength decreases almost linearly which matches with the experimental results depicted by S. K. Mishra et. al. in the year 2013 [56]. Here the sensitivity analysis with respect to concentration using radially polarized light for NaCl and KCl solution is shown in Figs. 12 and 13 respectively. Graphical representations clearly indicate that for NaCl solution with the minimum change in concentration of 0.5 g/l the indicated minimum change of output power is 30.04 μW, which can be accurately detected by the optical power meter head of InGaAs (PD300-IRG). In case of KCl solution with the minimum change in concentration of 1 g/l the indicated minimum output power change is 25.60 μW, which can also be accurately detected by the optical power meter head of InGaAs (PD300-IRG). So, with the proposed idea a better sensitive liquid sensor using bimetallic taper fiber sensor and radially polarized light is discerned where the variation of concentration upto 0.5 g/l for NaCl solution and 1 g/l for KCl solution can be detected by measuring the transmitted output power using power meter head.
8. Conclusion A proposal towards the highly sensitive long range surface plasmon resonance based taper fiber optic liquid sensor has been explored where the liquid sensor probe is coating by an unclad core of an taper optical fiber with bimetal layers of silver and gold followed by liquid (NaCl and KCl solution) as sensing layers around the taper waist region. The refractive index of NaCl and KCl solution decreases with their respective increase in concentration from 60 g/l to 360 g/l using p-polarized light and radially polarized
Fig. 10. Resonance wavelength (nm) variation for different concentration (g/l) of NaCl solution from 60 g/l to 360 g/l with radially polarized light. 7
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
N. Goswami, et al.
Fig. 11. Resonance wavelength (nm) variation for change in concentration (g/l) of KCl solution from 60 g/l to 360 g/l for radially polarized light.
Fig. 12. Normalized output power vs. concentration (g/l) for sensitivity analysis using radially polarized light for NaCl solution.
Fig. 13. Normalized output power vs. concentration (g/l) for sensitivity analysis using radially polarized light for KCl solution.
light respectively. The sensitivity analysis is observed using intensity interrogation technique. And the response of the proposed configuration also been observed with respect to the wavelength and concentration of NaCl and KCL solution. To the best our knowledge several articles have been devoted in the field of surface plasmon based fiber optic liquid sensor. But with the proposed idea using radially polarized light for bimetallic tapered structure almost 13 times better sensitivity has been obtained than the existing articles reported till date using p-polarized light in case of NaCl solution and 25 times better sensitivity is discerned in case of KCl solution. The minimum detectible concentration investigated through the proposed sensing probe with radially polarized light is 0.5 g/l for NaCl solution for a minimum change in output power of 30.04 μW and for KCl solution, it is 1 g/l for a minimum change in output power of 25.60 μW. This sensitivity enhancement is owing to the radially outward field distribution of the radially polarized light around the fiber core to the each point of the periphery, which enables the multiple point plasmonic interaction on the whole fiber periphery, instead of single point plasmonic interaction using p-polarized light. Thus this article presents a unique LRSP based 8
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
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taper fiber sensing probe with the enhanced sensitivity to detect the refractive index variation as well as the higher dynamic range of refractive index for the NaCl and KCl solutions using the propagating radially polarized light. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56]
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9
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Highly sensitive long range surface plasmon based liquid sensor by shining the radially polarized light in bimetallic taper fiber structure Nabamita Goswamia, Deepak Chaurasiab, Ardhendu Sahaa, a b
T
⁎
Department of Electrical Engineering, National Institute of Technology Agartala, Barjala, Jirania, Tripura (West), 799046, India Department of Electronics and Instrumentation Engineering, IET, Bundelkhand University, Jhansi, Uttar Pradesh, 284001, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Long range surface plasmon resonance Radially polarized light Bimetal tapered fiber Fiber optic liquid sensor
A new approach towards the enhancement of sensitivity in fiber optic liquid sensor is proposed, designed and simulated using the long range surface plasmon resonance technique shining the radially polarized light in bimetallic fiber structure, which comprises a fiber core coated with 40 nm and 10 nm thin Ag and Au layer respectively. The radially outward field distribution of the radially polarized light enables the multiple point plasmonic interaction on the whole fiber periphery which enhances the sensitivity as well as the dynamic range of refractive index sensing through the proposed sensor. A 13 times and 25 times better sensitivity has been estimated using NaCl and KCl solution as compared with the existing articles till date using p-polarized light. Minimum measurable concentration is 0.5 g/l and 1 g/l for NaCl and KCl solutions for a minimum change in output power of 30.04 μW and 25.60 μW respectively using InGaAs detector. With enhanced sensitivity and higher dynamic range of refractive index, this new idea explores the new avenues in the field of fibre optic liquid sensing applications.
1. Introduction In this new era, the phenomenon of surface plasmon resonance (SPR) has attracted a great deal of attention, owing to its immense practical applications in the field of molecular adsorption [1–3], immunoassary [4,5], data interpretation [6], second harmonic generation [7,8], optical parametric generation [9], spectroscopy [10,11], bio sensing [12–14], solar cells [15,16], genetic engineering [17–19], high contrast SPR microscopy or imaging [20], mass spectrometry (MS) [21], functional proteomic screening [22,23], characterization of enzyme inhibitors [24,25], immunosensing [26] etc. Here SPR will exist at a metal-dielectric interface which supports the charge density oscillations propagating through that interface [27,28]. In the presence of one metal-dielectric interface, SPR phenomenon will exist whereas with two metal–dielectric interfaces, long range surface plasmon (LRSP) resonance as well as short range surface plasmon (SRSP) resonance phenomena will exist, but by the optimal choice of the thicknesses and dielectric constants of the composite layers the LRSP phenomenon can be made predominant [29]. This LRSP phenomenon can also be implemented in the field of optical fiber sensors for monitoring the various entitinin hertile and inaccessible environment hazard where the chemical encoded informations is transmitted through the optical fiber between the detection system and the remote sensing sample [30]. These optical fibers are unaffected to other source of noise like electromagnetic disturbances with respect to the electric field [31]. Owing to the possibilities of miniaturization on account of its flexibility, light weight and small size, the optical
⁎
Corresponding author. E-mail addresses: nabamita08@rediffmail.com (N. Goswami), [email protected] (A. Saha).
https://doi.org/10.1016/j.ijleo.2019.163001 Received 31 May 2019; Accepted 23 June 2019 0030-4026/ © 2019 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 193 (2019) 163001
N. Goswami, et al.
fibers make more popular use in sensing applications. Generally in case of LRSP based fiber liquid sensors, method of fixed range of angle of incident and modulated wavelength is used where this LRSP based sensors is commonly fabricated using the KretschmannRaether geometry comprising a fiber core coated with metal layers and sensing layers respectively [32]. The response of such type of LRSP resonance based liquid sensors can be enhanced in presence of taper fiber structure where a powerful interaction between the evanescent wave and surface plasmon wave can be buildup [33,34]. Broadening the angular range of light propagation together with the change in incident angle, enhancement of the performance with respect to the sensitivity can be facilitated near the taper waist region [35]. Here, taper angle plays a vital role for estimating the sensitivity using the taper fiber structure whereas, it is well known that the observed sensitivity is more in case of double taped fiber structure as compared to normal fiber structure. With the increment of the tapered angle, number of reflection of a rays undergoes within the taper waist region will decrease. It is well known that only ppolarized light is responsible for the creation of LRSP where the electric field is along the incident plane and in the direction of propagation. In presence of radially polarized light (RPL), generation of LRSP is also possible as the field distribution is always ppolarized at the spatial circumference of the radially polarized light beam [36]. The radially polarized light establishes the compact focal spot and strong longitudinal field compound conversion of focal point [37] and therefore is useful for the application in the field of tip enhanced Raman spectroscopy [38–41]. The field oscillation in case of radially polarized light is always outward from the center of the beam having inhomogeneous spatial distribution of electric field. To the best of our knowledge several articles have been devoted for depicting the LRSP based taper fiber liquid sensor with varying refractive index within the sensing region illuminated with p-polarized light [42], but the liquid sensor using RPL in bimetallic tapered structure with better sensitivity has not yet been reported. Here the idea deals with the LRSP based fiber optic liquid sensor using radially polarized light with better sensitivity for the detection of NaCl and KCl solution having different concentrations. 2. Proposed scheme A theoretical approach towards the sensitivity realization of the LRSP based liquid sensor with bimetal coated tapered fiber structure using radially polarized light is proposed here. Fig.1 comprises a double taper fiber sensor probe having core diameter 300 μm and tapering angle of 0.707 rad with consecutive thin layers deposition of Ag-Au with thicknesses of 40 nm and 10 nm respectively over the taper waist region of 10 mm after the removal of clad from the optical fibre. The unclad portion is initially cleaned by acetone with high tension bombardment on vacuum chamber. An Ag layer of 40 nm thickness using vacuum coating unit is coated on the unclad portion of the taper fiber using thermal evaporation technique which is kept at 5 × 10−6 mbar of pressure. An additional layer of Au having thickness 10 nm is coated using the same technique to enhance the sensitivity of the sensor which is shown in Fig. 1. Presence of Au layer not only enhances the overall performance of the sensor by increasing the detection sensitivity as compared to single metal coated structure but also protects the Ag layer from the degradation through oxidization and other chemical effects. Moreover, with the combination of Ag and Au layer, highest dip in spectrum at resonance angle and largest shift in resonance curve with the change of refractive index can be achieved in case of multilayered SPR structure. Here NaCl and KCl solution serves the purpose of sensing layer within the taper waist region where the concentration of the NaCl and KCl solutions varies from 60 g/l–360 g/l. For accurate measurement, the solutions are created by dissolving the required amount of NaCl in 1 L of deionized water and KCl in distilled water. Using extra-cavity generation technique with radial polarization converter (RPC)
Fig. 1. Schematic arrangement of liquid sensor assembly of LRSP based Ag-Au coated tapered multi-mode fiber using radially polarized light. 2
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manufactured by ARCoptix, the generation of radially polarized broadband light source with varying wavelength is facilitated which serves the purpose of proposed source of fundamental in this article. For calculating the ultimate output power from the proposed configuration the following steps need to be considered – 3. Normalized transmitted output power for radially polarized light For a double taper structure coated with different layers within the taper waist region, the overall normalized transmitted output power at the output end of the fiber is mathematically expressed by [43,44]ϕ
ϕ
⎡ α ∫2 R Nref (θ) . P (θ). d (θ) + (1 − α ) ∫2 P (θ). d (θ) ⎤ ⎢ ⎥ ϕ1 ⎢ ϕ1 ⎥ ϕ 2 ⎢ ⎥ ∫ P (θ). d (θ) ⎢ ⎥ ϕ1 ⎢ ⎥ ⎣ ⎦
(1)
This relation prevails for the case of SPR excitation with radially polarized light for a taper fiber optic sensor, where P (θ) represents the incident power within the optical fiber at an angleθ andα is the effective intensity coefficient.
n 2. sin(θ). cos(θ) ⎤ dθ dP (θ) ∞ ⎡ 1 ⎢ (1 − n12 . cos2 (θ))2 ⎥ ⎦ ⎣
(2)
In case of taper fiber, if the radially polarized light is incident at an angle greater than the critical angle inside the taper waist region, then after multiple bounces in taper waist region, light is reflected from the configuration. Here ϕ1and ϕ2 are the angles of propagation of light over the taper waist region and the value of the effective intensity coefficient and α is 1 and ½ for radially polarized light and p- polarized light respectively [45,46]. Within the SPR sensing region of length L, the number of reflections of a ray undergoes is calculated by-
L ⎤ Nref (θ) = ⎡ ⎢ 2 ρ ( z )tan( Ω + θ) ⎥ ⎣ ⎦
(3)
Taking Ω as a taper angle (0.707 rad) and ρo , ρi as the radius of input and output end of the taper region, the exiting different taper profiles with varying taper radius with respect to z (at input end, z = 0 and output end z = L) are given by –
Linear:ρ (z ) = ρi −
z (ρ − ρ0 ) L i
(4)
Exponential linear:ρc (z ) = (ρi − ρ0 ) ⎡e ⎣
−z L
−
z −1 e ⎤ + ρ0 L ⎦
(5)
1
z 2 Parabolic:ρ (z ) = ⎡ρi2 − (ρi2 − ρ02 ) ⎤ L ⎣ ⎦
(6)
Within this proposed configuration as shown in Fig. 1, only at the resonance condition of LRSP the maximum interaction between the evanescent wave and surface plasmon wave will occur within the taper waist region which is analytically expressed by – 1
2 2 ⎡ 2Π ⎛ εm nSR 2Π ⎞ ⎤ n1 sin θ = Re ⎢ ⎜ 2 ⎟ ⎥ λ λ + ε n SR ⎠ ⎥ ⎢ ⎝ m ⎦ ⎣
(7)
here nSR , n1stands for the refractive indices of the sensing region and core respectively, εm is the dielectric constant of the metal layer andλ is the wavelength in nm at an incident angle θ . Here at resonance condition the propagation constant of the light incident at an angle θ is equal to the real part of the surface plasmon propagation constant. 4. Dispersion relation for NaCl and KCl solution It is well known that the refractive index of NaCl solution decreases with the variation of wavelength from 300 nm to 2500 nm and increases with the increment of the concentration of NaCl solution [47,48]. In case of transparent medium, the empirical relation between wavelength and refractive index can be expressed by Sellmier equation. Here, the wavelength and concentration dependent refractive index of NaCl also determined by the Sellmeier equation which is given below-
n (c, λ ) =
A0 +
A1 λ2 A λ2 A λ2 + 2 2 2 + 2 3 2 + A 4 c − A5 c 2 λ2 − l12 λ − l2 λ − l3
(8)
here A0 , A1, A2 , A3 , A 4 , A5 are the Sellmeier coefficients, c represents the concentration in gram per liter andλ denotes the wavelength in nanometer. The value of these Sellmeier coefficients are A0 = 0.385, A1 = 1.32, A2 = 1, A3 = 0.0244, A 4 =2.07 × 105, A5 = 1.75 × 10−7, l12 = 8.79 × 103, l 22 = 1.1 × 108 and l32 =6.09 × 104 [49]. The temperature, wavelength and concentration dependent dispersion equation for the KCl solution is as follows where with the increase in temperature the refractive index of the KCl 3
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Fig. 2. Refractive index vs. wavelength (nm) for NaCl solution with varying concentration from 60 g/l to 360 g/l.
solution decreases, but in case of the proposed configuration for accurate measurement the temperature is kept constant at 25 °C –
nkcl (λ, T , c ) =
1.26486 +
0.30523 × λ2 0.41620 × λ2 0.18870 × λ2 2.6200 × λ2 + 2 + 2 + 2 λ2 − (0.100)2 λ − (0.131)2 λ − (0.162)2 λ − (70.42)2
+ (1.6167 × 10−3) c − (4.0 × 10−7) c 2 − (1.1356 × 10−4)(T − 273.15) − (5.7 × 10−9)(T − 273.15)2
(9)
The variation of refractive index with the varying concentration from 60 g/l to 360 g/l and wavelength from 200 nm to 1200 nm for NaCl and KCl solution are shown in Figs. 2 and 3 respectively. Graphical representations clearly indicate that with the increase in concentration the refractive index of NaCl as well as KCl solution decreases. 5. Sensitivity analysis with respect to wavelength, intensity and concentration 1
The propagation constant of the surface plasmon changes significantly owing to the change in refractive index, ns = (εd ) 2 of the surrounding media which results in the modification of the SPR condition and as a result, changes in several properties of the transmitted light. The resultant sensitivity can therefore be defined by -
Sn, ζ =
δζ δns
(10)
Hereδζ , is the change in the transmitted light properties which can be defined by the intensity, resonance angle, phase and wavelength and δns is the variation of refractive index in surrounding medium. With these four transmitted light properties four interrogation techniques are available for SPR sensing using the proposed configuration. One is “intensity interrogation technique” defined by the variation in transmitted light intensity at fixed incident angle, second is “angular interrogation technique” accomplished by the change of resonance angle, third is “phase interrogation technique” defined by the phase difference between ppolarization and s-polarization states of the reflected light and last is “wavelength interrogation technique” expressed by the change of resonance wavelength at fixed incident angle. These four interrogation techniques are analytically expressed by [50,51]-
Sn =
δInorm → Intensity interrogation technique δns
(11)
Fig. 3. Refractive index vs. wavelength (nm) for KCl solution with varying concentration from 60 g/l to 360 g/l. 4
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Fig. 4. Normalized transmitted output power vs. wavelength with the variation in concentration of NaCl solution from 60 g/l to 360 g/l with ppolarized light.
Sn =
δθres → Angular interrogation technique δns
(12)
Sn =
δϕ0 → Phase interrogation technique δns
(13)
Sn =
δλres → Wavelength interrogation technique δns
(14)
Here Inorm is the change in output light intensity, θres stands for the resonance angle, ϕ0 represents the phase of the output light and λres defines the resonance wavelength. 6. Result and discussion The response features of the proposed LRSP based configuration using different concentrations of the NaCl solution and KCl solution throughout the sensing probe are presented in Figs. 4 and 5 respectively. Here the two solutions individually serve the purpose of sensing layer for the proposed configuration. As, with increasing concentrations, the refractive index decrease for both the solutions, so using the Eqs. 1,7 and 8 the transmitted output power also decreases which is clearly observable from Figs. 4 and 5. Now for incident radially polarized light the variation of transmitted output power for the proposed liquid sensor structure with change in wavelength with varying concentrations of NaCl and KCl solutions are shown in Figs. 6 and 7 respectively. Here also the concentrations of NaCl and KCl solutions are inversely related to the transmitted output power. As compared to p-polarized light, owing to the more plasmonic interaction throughout the periphery of the optical fiber for radially polarized light, an increase in transmitted output power has been discerned from the Fig. 6 and Fig. 7 for both the solutions, which directly denotes the increase in sensitivity of the proposed sensor structure with incident radially polarized light [52].
Fig. 5. Normalized transmitted output power vs. wavelength with the variation in concentration of KCl solution from 60 g/l to 360 g/l with ppolarized light. 5
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Fig. 6. Normalized transmitted output power vs. wavelength with the varying concentration of NaCl solution from 60 g/l to 360 g/l with radially polarized light.
Fig. 7. Normalized transmitted output power vs. wavelength with the varying concentration of KCl solution from 60 g/l to 360 g/l with radially polarized light.
7. Sensitivity analysis of the proposed structure Here sensitivity has been calculated using intensity interrogation technique for both radially and p-polarized light. Also by varying concentrations, the sensitivity for the proposed structure is observed and analysed through graphical representations. Sensor output power with varying refractive index for p-polarized light and radially polarized light for NaCl solution and KCl solution are shown in Figs. 8 and 9. Graphical representations clearly indicate that sensitivity has significantly been increased in case of radially polarized light as compared to p-polarized light. Here, with the proposed scheme, the sensitivity measurement for radially polarized light comparing with p–polarized light is analytically defined by-
Fig. 8. Sensor output power vs. refractive index for sensitivity analysis in presence of p-polarized light and radially polarized light for NaCl solution. 6
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Fig. 9. Sensor output power vs. refractive index of radially polarized light and p-polarized light for KCl solution.
⎛ Snr = ⎜ ⎜ ⎝
δIout δns radially − polarized − light δIout δns p − polarized − light
⎞ ⎟ = 1.41 ⎟ ⎠ Iin = const.
(15)
where Snr stands for the resultant sensitivity of the proposed structure with intensity interrogation technique which is 1.41 times for radially polarized light as compared to p-polarized light with the variation of refractive index of sensing layer. With the comparison to the existing articles reported till date, using radially polarized light a 13 times better sensitivity is achieved for NaCl solution and 25 times better sensitivity is discerned for KCl solution as compared to p-polarized light [53–55]. Thus the proposed structure deals with a new idea of liquid sensor which explores better sensitivity than the existing aricles of liquid sensors repoted till date. Now the response of the proposed configuration with respect to resonance wavelength and concentration for NaCl solution and KCl solution with radially polarized light are shown in Figs. 10 and 11. Figures clearly display that, when the concentrations are increased from 60 g/l to 360 g/l, the resonance wavelength decreases almost linearly which matches with the experimental results depicted by S. K. Mishra et. al. in the year 2013 [56]. Here the sensitivity analysis with respect to concentration using radially polarized light for NaCl and KCl solution is shown in Figs. 12 and 13 respectively. Graphical representations clearly indicate that for NaCl solution with the minimum change in concentration of 0.5 g/l the indicated minimum change of output power is 30.04 μW, which can be accurately detected by the optical power meter head of InGaAs (PD300-IRG). In case of KCl solution with the minimum change in concentration of 1 g/l the indicated minimum output power change is 25.60 μW, which can also be accurately detected by the optical power meter head of InGaAs (PD300-IRG). So, with the proposed idea a better sensitive liquid sensor using bimetallic taper fiber sensor and radially polarized light is discerned where the variation of concentration upto 0.5 g/l for NaCl solution and 1 g/l for KCl solution can be detected by measuring the transmitted output power using power meter head.
8. Conclusion A proposal towards the highly sensitive long range surface plasmon resonance based taper fiber optic liquid sensor has been explored where the liquid sensor probe is coating by an unclad core of an taper optical fiber with bimetal layers of silver and gold followed by liquid (NaCl and KCl solution) as sensing layers around the taper waist region. The refractive index of NaCl and KCl solution decreases with their respective increase in concentration from 60 g/l to 360 g/l using p-polarized light and radially polarized
Fig. 10. Resonance wavelength (nm) variation for different concentration (g/l) of NaCl solution from 60 g/l to 360 g/l with radially polarized light. 7
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Fig. 11. Resonance wavelength (nm) variation for change in concentration (g/l) of KCl solution from 60 g/l to 360 g/l for radially polarized light.
Fig. 12. Normalized output power vs. concentration (g/l) for sensitivity analysis using radially polarized light for NaCl solution.
Fig. 13. Normalized output power vs. concentration (g/l) for sensitivity analysis using radially polarized light for KCl solution.
light respectively. The sensitivity analysis is observed using intensity interrogation technique. And the response of the proposed configuration also been observed with respect to the wavelength and concentration of NaCl and KCL solution. To the best our knowledge several articles have been devoted in the field of surface plasmon based fiber optic liquid sensor. But with the proposed idea using radially polarized light for bimetallic tapered structure almost 13 times better sensitivity has been obtained than the existing articles reported till date using p-polarized light in case of NaCl solution and 25 times better sensitivity is discerned in case of KCl solution. The minimum detectible concentration investigated through the proposed sensing probe with radially polarized light is 0.5 g/l for NaCl solution for a minimum change in output power of 30.04 μW and for KCl solution, it is 1 g/l for a minimum change in output power of 25.60 μW. This sensitivity enhancement is owing to the radially outward field distribution of the radially polarized light around the fiber core to the each point of the periphery, which enables the multiple point plasmonic interaction on the whole fiber periphery, instead of single point plasmonic interaction using p-polarized light. Thus this article presents a unique LRSP based 8
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taper fiber sensing probe with the enhanced sensitivity to detect the refractive index variation as well as the higher dynamic range of refractive index for the NaCl and KCl solutions using the propagating radially polarized light. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56]
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