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Surface plasmon resonance based fiber optic refractive index sensor utilizing silicon layer: Effect of doping
Optics Communications 286 (2013) 171–175
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Surface plasmon resonance based fiber optic refractive index sensor utilizing silicon layer: Effect of doping Priya Bhatia, Banshi D. Gupta, n Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India
a r t i c l e i n f o
abstract
Article history: Received 18 July 2012 Received in revised form 23 August 2012 Accepted 25 August 2012 Available online 13 September 2012
We present an experimental study on the surface plasmon resonance (SPR) based fiber optic refractive index sensor utilizing a high index silicon layer between a metal layer and sensing medium using the wavelength interrogation mode of operation. Both n- and p-type silicon have been used. For the metal layer, silver and gold have been used. For a given metal, experimental results predict higher sensitivity of the sensor for the n-type silicon than for the p-type silicon layer. Further, for a given type of silicon, the sensitivity for the gold coated probe is higher than that of the silver coated probe. Numerically, the sensitivity of the n-type silicon with silver as the metal layer is approximately 1.39 times higher than that of the p-type silicon. In the case of gold as the metal layer, the sensitivity of the n-type silicon is approximately 1.50 times that of the p-type silicon. Since the refractive index of both p-type and n-type silicon is the same it appears that the majority charge carriers in silicon play an important role in the sensitivity of the surface plasmon resonance based sensor. The charge carriers are either affecting the field in the analyte region or may be somehow affecting the propagation constant of the surface plasmon wave which is solely due to oscillation of free electrons in the metal layer. In addition, the sensitivity of only the metal coated probes is found to lie between their p-type and n-type silicon coated probes. This suggests that the effect of charge carriers on sensitivity is more than the refractive index of the silicon layer. The effects of charge carriers in silicon, electrons and holes appears to be opposite. & 2012 Elsevier B.V. All rights reserved.
Keywords: Optical fiber Surface plasmons Sensor Silicon Sensitivity
1. Introduction The surface plasmon resonance (SPR) technique has acquired great attention of researchers in recent years for the development of various photonics/nanophotonics devices such as sensors, superluminescent diodes, etc. because of its wide applications [1–6]. Surface plasmons are transverse-magnetic (TM) electromagnetic waves generated as a result of coherent oscillations of charges at metal–dielectric interfaces [7,8]. The fields associated with them have maxima on the interface and decay exponentially in both the media. For the excitation of SPR, the wave vector of the incident wave should match with the wave vector of the surface plasmon wave. The basic route to excite surface plasmons is the Kretschmann configuration [9] which uses various types of high index prisms for getting phase matching conditions between the incident wave and the surface plasmon wave. In this configuration, the base of the prism is coated with a thin metal layer which is further surrounded by a dielectric layer. If a p-polarized light beam is incident on the prism base at an angle equal to or
n
Corresponding author. Tel.: þ91 112 659 1355; fax: þ 91 1126 5811 14. E-mail address: [email protected] (B.D. Gupta,).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.08.097
greater than the critical angle it gets totally internally reflected from the prism/metal interface producing an evanescent wave. If the wave vector of the evanescent wave is equal to the surface plasmon wave at the metal/dielectric interface then the photon energy of the incident beam is strongly absorbed by surface plasmons, resulting in a sharp dip in the reflectance spectrum (reflectivity vs. angle of incidence). The angular position of dip changes with the variation in optical constants of the dielectric medium. Beside giving good sensitivity, prism based Kretschmann SPR sensors have some limitations as they are not compact, in addition to having high cost and also cannot be used for remote sensing applications. These shortcomings can be overcome by using an optical fiber core in place of the high index prism for the excitation of surface plasmons because in optical fibers the propagation of light also takes place through total internal reflection. A number of SPR sensors utilizing optical fibers have been reported in literature for chemical and biomedical applications [10–12]. In a SPR based fiber optic sensor, fiber is made uncladded from the middle portion and a metal layer is coated on this unclad portion. Light from a polychromatic source is launched inside the fiber. The evanescent wave generated at the core–metal layer interface excites surface plasmons at the metal/ dielectric interface. The SPR spectrum is recorded at the other end
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P. Bhatia, B.D. Gupta / Optics Communications 286 (2013) 171–175
of the fiber with the help of a spectrometer. In the past few years, lot of modifications such as tapering, incorporation of long-period and fiber Bragg gratings have been carried out in the fiber optic SPR probe to improve performance parameters of the sensor [13–16]. In addition, a nanometer thick layer of silicon between metal and dielectric layers has been shown to increase the sensitivity of the sensor [17–21]. This enhancement in sensitivity is due to the increase in the electric field intensity at the silicon– dielectric interface. The silicon layer also has an additional advantage of protecting the metal layer from oxidation and hence improving the stability of the probe. In this paper we have studied the effect of dopants in silicon on the performance of the SPR based fiber optic sensor. We fabricated and characterized SPR probes having n-type and p-type silicon layer over the metal layer. Gold and silver have been used as SPR active metals. Experimental results show that the increase in the refractive index of the sensing (dielectric) medium shifts the resonance wavelength to higher wavelengths for both n-type and p-type silicon. However, the shift is more in the case of the n-type silicon as compared to the p-type silicon, implying higher sensitivity of the SPR sensor having an n-type silicon layer over the gold/silver layer. Further, the charge carriers in silicon play an important role in the sensitivity of the SPR sensor utilizing silicon layer.
2. Experiment For the fabrication of the SPR based fiber optic sensor, cladding was removed from a 1 cm length of the middle portion of the plastic clad silica fiber of numerical aperture 0.4 and core diameter 600 mm purchased from Fiberguide Industries. The unclad portion of the fiber was first cleaned with acetone and then by high tension ion bombardment in a vacuum chamber. After cleaning, the metal coating was carried out in a vacuum coating unit kept at 5 10 6 mbars of pressure by the thermal evaporation technique. Such a low pressure was achieved by first roughing the system for about 15 min, followed by baking for 20 min. In the baking position the heater was kept on for around 1 h after which the baffle valve was half opened. After 5 min, the valve was opened completely till the desired amount of vacuum was achieved. The vacuum chamber was continuously cooled by running water. Gradually and slowly the current was increased till we got the desired thickness on the digital thickness monitor. After this, the system was allowed to cool and the fibers were taken out of the vacuum chamber. The thickness of the silver layer coated was 40 nm while that of the gold layer was 50 nm. Before coating the gold layer the fiber core was coated with a chromium layer of about 2 nm thickness because chromium acts as an adhesive between the fiber core and the gold layer. For achieving uniform thickness of film over the core of the fiber, the fiber was rotated during coating. It may be noted that the imaginary part of the dielectric constant of silver is smaller than
that of gold and hence a thicker silver film can excite surface plasmons more efficiently than a gold film, but this will only reduce the width of the SPR curve or improve the detection accuracy of the sensor. As far as sensitivity of the SPR sensor is concerned, it is the real part of the dielectric constant that determines it. The real part of dielectric constants of metals is negative. The smaller the absolute value (without negative sign) of the real part, the greater the sensitivity. For a given wavelength, the absolute value of the real part of the dielectric constant of gold is smaller than that of silver. The thicknesses of gold and silver that have been chosen for coating and given above are the optimized thicknesses obtained for the best performance and reported in literature [22,23]. After coating with gold and silver, these fibers were coated with n-type and p-type silicon of 5 nm thickness using the same technique. The dopant used for the p-type silicon is boron while for the n-type silicon it is phosphorous, and the concentration of the charge carriers in each is of the order of 1015 per cubic centimeter. In all we fabricated following four different types of probes: gold-n-type silicon, gold-p-type silicon, silver-n-type silicon and silver-p-type silicon. These coated regions act as sensing regions of the SPR probe. Finally both the ends of the fibers of these probes were cleaved with a tungsten carbide cutter and cleaned by acetone before fixing into the glass flow cell. Liquid samples of different refractive indices ranging from 1.333 to 1.358 were prepared by dissolving suitable amounts of sucrose into de-ionized water. For measurement of the refractive index an Abbe’s refractometer having an accuracy of 0.001 in white light was used. The experimental setup of such a sensor is shown in Fig. 1. Each probe before using was fixed in the glass flow cell having the facility of inlet and outlet for liquid samples of different refractive indices. Unpolarized light from a tungsten– halogen lamp (AvaLight-HAL) was focused onto one of the ends of the fiber with the help of a microscope objective. The numerical aperture of the microscope objective was greater than that of the fiber for achieving maximum coupling of light in the fiber. The transmitted light was detected with the help of a spectrometer (AvaSpec-3648) at the other end of the fiber. The spectrometer was interfaced with the computer to record the transmitted spectrum of light. The wavelength corresponding to the minimum transmitted power, called resonance wavelength, is determined from the SPR spectrum. By measuring the resonance wavelength corresponding to a different refractive index of the sample around the probe we calculate the shift in resonance wavelength and hence the sensitivity of the sensor.
3. Results and discussion Fig. 2 shows SPR spectra for the SPR probe with a silver film of thickness 40 nm and a p-type silicon layer of thickness 5 nm over an unclad fiber for refractive indices of the solution around the probe varying from 1.333 to 1.358 in steps of 0.005. From this figure it is obvious that each refractive index has its own dip and hence
Fig. 1. Schematic of the experimental setup used to study the SPR spectra.
P. Bhatia, B.D. Gupta / Optics Communications 286 (2013) 171–175
Fig. 2. Surface plasmon resonance spectra of the fiber optic SPR probe for the silver and p-type silicon layer of 5 nm thickness for refractive indices of the sensing region ranging from 1.333 to 1.358.
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Fig. 4. Variation of resonance wavelength with the refractive index of the sensing layer for p- and n-type silicon layers of 5 nm thickness in the case of silver.
Table 1 Sensitivities of n- and p-type silicon based fiber optic SPR sensors in case of silver and gold metal layers.
Fig. 3. Surface plasmon resonance spectra of the fiber optic SPR probe for the gold and p-type silicon layer of 5 nm thickness for refractive indices of the sensing region ranging from 1.333 to 1.358.
resonance wavelength. As the refractive index of the sample solution increases there appears a redshift in the resonance wavelength. Similar kinds of SPR spectra for the 40 nm thick silver film and 5 nm thick n-type silicon layer were obtained in our previous study [20] where we experimentally studied the surface plasmon resonance based fiber optic refractive index sensor incorporating a high-index dielectric layer using the wavelength interrogation method. Silver and gold were used as SPR active metals followed by a high index dielectric layer of n-type silicon. The sensitivity of the sensor was found to increase with the thickness of the silicon layer. Comparison of SPR spectra of the p-type silicon with the n-type silicon predicts that, for a given refractive index of the sample, the resonance wavelength in case of the SPR sensor with the p-type silicon layer is lower than that in the case of n-type silicon layer. Fig. 3 shows the similar kind of SPR spectra for the probe with the 50 nm thick gold film and 5 nm p-type silicon layer. The SPR spectra are of similar type but the dips have been shifted towards higher wavelength. It may be noted that in Fig. 3 the normalized transmitted power at resonance wavelength decreases as the refractive index increases while in Fig. 2 it is not affected by the refractive index. No physical reason is there behind this difference. This is because in Fig. 3 all the SPR spectra are starting from normalized transmitted power 1.0 while in Fig. 2 all the SPR spectra have not been normalized to 1.0 at lower wavelengths. The normalization of each spectrum to the value 1.0 at lower
Metal
Silicon (5 nm)
Sensitivity (mm/RIU)
Silver
n-type p-type
3.3852 2.4282
Gold
n-type p-type
4.1102 2.7314
wavelength, in Fig. 2, can also give refractive index dependence of the normalized transmitted power at the resonance wavelength as observed in Fig. 3. However, this has not been carried out because the present study is utilizing a wavelength modulation scheme for which the knowledge of resonance wavelength and not the transmitted power is required and hence the results of the present study will not be affected. To compare the results for a given metal layer we have plotted, in Fig. 4, the variation of resonance wavelength with the refractive index of the liquid samples in case of silver for the n-type and p-type silicon. The error bars have been drawn taking into consideration the accuracies of the spectrometer and the refractometer. The variations are linear in both cases. It may be noted from the figure that the resonance wavelength is higher in case of the n-type silicon than in case of the p-type silicon with silver as the SPR material. Further, the slope of each curve gives the sensitivity of the sensor. The plot corresponding to the n-type silicon has a slope greater than that in the case of the p-type silicon. This implies that the SPR probe utilizing the n-type silicon layer has greater sensitivity than that utilizing the p-type silicon layer. The sensitivity values determined from the slope have been tabulated in Table 1. Fig. 5 shows the variation of resonance wavelength with the refractive index of the liquid samples in case of gold film with n-type and p-type silicon layers. The data for the n-type silicon and gold layer have been taken from our previous study [20]. In the case of gold, the variation of resonance wavelength with refractive index is the same as in the case of silver. Further the slope of the curve is greater for the n-type silicon layer than that for the p-type silicon layer. The values of sensitivity determined from these curves have also been tabulated in Table 1. It may be noted that, for a given silicon layer thickness, the sensitivity in the case of gold is always higher than that in the case of silver as reported in the previous study [20]. Therefore, we conclude from the table that the sensitivity is maximum for the gold–n-type silicon combination and minimum for the silver–p-type silicon combination.
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Fig. 5. Variation of resonance wavelength with the refractive index of the sensing layer for p- and n-type silicon layers of 5 nm thickness in the case of gold.
To understand the difference in sensitivity between n-type and p-type silicon for the same metal we tried to look for the change in refractive index of silicon on addition of dopants in intrinsic silicon. This is because a change in refractive index changes the resonance wavelength and also the sensitivity of the sensor. The concentration of charge carriers in the silicon used in the present study is of the order of 1015 cm 3. Further, for a given concentration of charge carriers, free holes are more effective in perturbing the refractive index than the free electrons [24]. The effect of holes on refractive index can be around four times of that due to free electrons. However, for the charge carrier concentration in the silicon which we have used and in the visible region, the perturbation in refractive index is at the fourth place of decimal which is negligible for giving the huge difference in the resonance wavelength as observed experimentally in the present study. Thus, this does not appear to be the reason behind the difference in resonance wavelengths obtained between the p-type and n-type silicon layers based SPR sensors. We also numerically calculated the dielectric functions for the n- and p-type silicon from the Drude relation given in Ref. [25]. The frequency dependent dielectric function can be written as
eðoÞ ¼ e1 o2p =½oðo þ igÞ where o ¼2pc/l is the angular frequency, in which c and l are the speed of light and wavelength in vacuum respectively. In the above equation, eN is the high-frequency limiting value of dielectric function that is approximately equal to 11.7 for silicon and is independent of the doping concentration and op is the plasma frequency and g is the scattering rate. Further,
op ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ne2 =mn e0
where N is the carrier concentration, e is the electron charge, mn is the carrier effective mass and e0 is the permittivity of free space. In addition, g ¼e/mnm, where m is the carrier mobility. The carrier effective mass mn is 0.27m0 for the n-type silicon and 0.37m0 for the p-type silicon, where m0 is the electron mass. Using the dielectric function values obtained from the above formula after substituting the values of various constants and N ¼1015 cm 3 we calculated SPR spectra for structures with n- and p-type silicon using the N-layer matrix method [26]. But no appreciable difference in the SPR spectra for n- and p-type silicon was obtained. Therefore, we concluded from this simulation that this is also not the reason behind such a large difference in the resonance wavelengths for n- and p-type silicon.
The difference in resonance wavelengths and sensitivity of n- and p-type silicon may be due to the difference in the charge carriers in silicon. Due to vacancies in the p-type silicon, the charge density at the metal–silicon interface will be higher and therefore its field probing the analyte will be sharp and hence will not be probing the analyte completely, resulting in lower sensitivity. While in case of the n-type silicon, due to same type of charge carriers, there will be low charge gathering at the metal–silicon boundary so the field will be shallower in the analyte and will be probing the analyte completely and hence the sensitivity will be higher in case of the n-type silicon as compared to the p-type case. The other reason can be that the charge carriers in silicon may be affecting the propagation constant of the surface plasmon wave which is solely due to the collective oscillations of free electrons in metal. The holes in silicon are perturbing the propagation constant more than the free electrons in the silicon. To make sure which reason is responsible for the difference in sensitivity of the p-type and n-type silicon based SPR sensor few more studies are required. To further investigate the reason, we have also carried out experiments on only silver and only gold coated probes. It is found that the sensitivity of the silver only case lies between the silver–p-type case and silver–n-type case. Same is the case for the gold only coated probe. This suggests that both the refractive index of silicon and the sign of the charge carriers are contributing to the changes in sensitivity of the sensor. The high refractive index of silicon increases the sensitivity as already reported in literature [17]. The smaller sensitivity in the case of p-type silicon implies that the positive charge carriers, holes, are decreasing the sensitivity. Its effect is more than that of the silicon refractive index. In the n-type silicon, sensitivity increases due to high refractive index of silicon as well as negative charge carriers, electrons, resulting in sensitivity greater than the only silver case. In other words, negative charge carriers are increasing the sensitivity while positive charge carriers in the silicon layer are decreasing it. Similar is the case with the gold film. It may also be noted that when thermal deposition in vacuum is carried out the composition of the evaporated film may change from the material before deposition. This may reduce the concentrations of phosphorous and boron in the n-type and p-type silicon coated films respectively due to differential pumping of phosphorous and boron with respect to silicon. However the films will still remain n-type and p-type silicon. Since we have observed a difference between results of two types of silicon it implies that the films are still of n-type and p-type silicon after deposition.
4. Conclusions In the present work, fabrication and characterization of a fiber optic SPR sensor utilizing p-type and n-type silicon layer have been carried out. It is observed that, for a given refractive index of the sample and the metal layer, the resonance wavelength depends on the charge carrier in the silicon. For holes, the resonance wavelength is smaller than for electrons. Further, the sensitivity is greater for electrons than for holes. The effect on sensitivity is attributed to the interaction of charge carriers in silicon with the surface electrons of the metal. It is therefore proposed to use n-type silicon for the enhancement of sensitivity of the SPR based sensor.
Acknowledgments The present work is partially supported by the Council of Scientific and Industrial Research (India). One of the authors, Priya Bhatia, is thankful to CSIR (India) for providing research fellowship.
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Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Surface plasmon resonance based fiber optic refractive index sensor utilizing silicon layer: Effect of doping Priya Bhatia, Banshi D. Gupta, n Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India
a r t i c l e i n f o
abstract
Article history: Received 18 July 2012 Received in revised form 23 August 2012 Accepted 25 August 2012 Available online 13 September 2012
We present an experimental study on the surface plasmon resonance (SPR) based fiber optic refractive index sensor utilizing a high index silicon layer between a metal layer and sensing medium using the wavelength interrogation mode of operation. Both n- and p-type silicon have been used. For the metal layer, silver and gold have been used. For a given metal, experimental results predict higher sensitivity of the sensor for the n-type silicon than for the p-type silicon layer. Further, for a given type of silicon, the sensitivity for the gold coated probe is higher than that of the silver coated probe. Numerically, the sensitivity of the n-type silicon with silver as the metal layer is approximately 1.39 times higher than that of the p-type silicon. In the case of gold as the metal layer, the sensitivity of the n-type silicon is approximately 1.50 times that of the p-type silicon. Since the refractive index of both p-type and n-type silicon is the same it appears that the majority charge carriers in silicon play an important role in the sensitivity of the surface plasmon resonance based sensor. The charge carriers are either affecting the field in the analyte region or may be somehow affecting the propagation constant of the surface plasmon wave which is solely due to oscillation of free electrons in the metal layer. In addition, the sensitivity of only the metal coated probes is found to lie between their p-type and n-type silicon coated probes. This suggests that the effect of charge carriers on sensitivity is more than the refractive index of the silicon layer. The effects of charge carriers in silicon, electrons and holes appears to be opposite. & 2012 Elsevier B.V. All rights reserved.
Keywords: Optical fiber Surface plasmons Sensor Silicon Sensitivity
1. Introduction The surface plasmon resonance (SPR) technique has acquired great attention of researchers in recent years for the development of various photonics/nanophotonics devices such as sensors, superluminescent diodes, etc. because of its wide applications [1–6]. Surface plasmons are transverse-magnetic (TM) electromagnetic waves generated as a result of coherent oscillations of charges at metal–dielectric interfaces [7,8]. The fields associated with them have maxima on the interface and decay exponentially in both the media. For the excitation of SPR, the wave vector of the incident wave should match with the wave vector of the surface plasmon wave. The basic route to excite surface plasmons is the Kretschmann configuration [9] which uses various types of high index prisms for getting phase matching conditions between the incident wave and the surface plasmon wave. In this configuration, the base of the prism is coated with a thin metal layer which is further surrounded by a dielectric layer. If a p-polarized light beam is incident on the prism base at an angle equal to or
n
Corresponding author. Tel.: þ91 112 659 1355; fax: þ 91 1126 5811 14. E-mail address: [email protected] (B.D. Gupta,).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.08.097
greater than the critical angle it gets totally internally reflected from the prism/metal interface producing an evanescent wave. If the wave vector of the evanescent wave is equal to the surface plasmon wave at the metal/dielectric interface then the photon energy of the incident beam is strongly absorbed by surface plasmons, resulting in a sharp dip in the reflectance spectrum (reflectivity vs. angle of incidence). The angular position of dip changes with the variation in optical constants of the dielectric medium. Beside giving good sensitivity, prism based Kretschmann SPR sensors have some limitations as they are not compact, in addition to having high cost and also cannot be used for remote sensing applications. These shortcomings can be overcome by using an optical fiber core in place of the high index prism for the excitation of surface plasmons because in optical fibers the propagation of light also takes place through total internal reflection. A number of SPR sensors utilizing optical fibers have been reported in literature for chemical and biomedical applications [10–12]. In a SPR based fiber optic sensor, fiber is made uncladded from the middle portion and a metal layer is coated on this unclad portion. Light from a polychromatic source is launched inside the fiber. The evanescent wave generated at the core–metal layer interface excites surface plasmons at the metal/ dielectric interface. The SPR spectrum is recorded at the other end
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P. Bhatia, B.D. Gupta / Optics Communications 286 (2013) 171–175
of the fiber with the help of a spectrometer. In the past few years, lot of modifications such as tapering, incorporation of long-period and fiber Bragg gratings have been carried out in the fiber optic SPR probe to improve performance parameters of the sensor [13–16]. In addition, a nanometer thick layer of silicon between metal and dielectric layers has been shown to increase the sensitivity of the sensor [17–21]. This enhancement in sensitivity is due to the increase in the electric field intensity at the silicon– dielectric interface. The silicon layer also has an additional advantage of protecting the metal layer from oxidation and hence improving the stability of the probe. In this paper we have studied the effect of dopants in silicon on the performance of the SPR based fiber optic sensor. We fabricated and characterized SPR probes having n-type and p-type silicon layer over the metal layer. Gold and silver have been used as SPR active metals. Experimental results show that the increase in the refractive index of the sensing (dielectric) medium shifts the resonance wavelength to higher wavelengths for both n-type and p-type silicon. However, the shift is more in the case of the n-type silicon as compared to the p-type silicon, implying higher sensitivity of the SPR sensor having an n-type silicon layer over the gold/silver layer. Further, the charge carriers in silicon play an important role in the sensitivity of the SPR sensor utilizing silicon layer.
2. Experiment For the fabrication of the SPR based fiber optic sensor, cladding was removed from a 1 cm length of the middle portion of the plastic clad silica fiber of numerical aperture 0.4 and core diameter 600 mm purchased from Fiberguide Industries. The unclad portion of the fiber was first cleaned with acetone and then by high tension ion bombardment in a vacuum chamber. After cleaning, the metal coating was carried out in a vacuum coating unit kept at 5 10 6 mbars of pressure by the thermal evaporation technique. Such a low pressure was achieved by first roughing the system for about 15 min, followed by baking for 20 min. In the baking position the heater was kept on for around 1 h after which the baffle valve was half opened. After 5 min, the valve was opened completely till the desired amount of vacuum was achieved. The vacuum chamber was continuously cooled by running water. Gradually and slowly the current was increased till we got the desired thickness on the digital thickness monitor. After this, the system was allowed to cool and the fibers were taken out of the vacuum chamber. The thickness of the silver layer coated was 40 nm while that of the gold layer was 50 nm. Before coating the gold layer the fiber core was coated with a chromium layer of about 2 nm thickness because chromium acts as an adhesive between the fiber core and the gold layer. For achieving uniform thickness of film over the core of the fiber, the fiber was rotated during coating. It may be noted that the imaginary part of the dielectric constant of silver is smaller than
that of gold and hence a thicker silver film can excite surface plasmons more efficiently than a gold film, but this will only reduce the width of the SPR curve or improve the detection accuracy of the sensor. As far as sensitivity of the SPR sensor is concerned, it is the real part of the dielectric constant that determines it. The real part of dielectric constants of metals is negative. The smaller the absolute value (without negative sign) of the real part, the greater the sensitivity. For a given wavelength, the absolute value of the real part of the dielectric constant of gold is smaller than that of silver. The thicknesses of gold and silver that have been chosen for coating and given above are the optimized thicknesses obtained for the best performance and reported in literature [22,23]. After coating with gold and silver, these fibers were coated with n-type and p-type silicon of 5 nm thickness using the same technique. The dopant used for the p-type silicon is boron while for the n-type silicon it is phosphorous, and the concentration of the charge carriers in each is of the order of 1015 per cubic centimeter. In all we fabricated following four different types of probes: gold-n-type silicon, gold-p-type silicon, silver-n-type silicon and silver-p-type silicon. These coated regions act as sensing regions of the SPR probe. Finally both the ends of the fibers of these probes were cleaved with a tungsten carbide cutter and cleaned by acetone before fixing into the glass flow cell. Liquid samples of different refractive indices ranging from 1.333 to 1.358 were prepared by dissolving suitable amounts of sucrose into de-ionized water. For measurement of the refractive index an Abbe’s refractometer having an accuracy of 0.001 in white light was used. The experimental setup of such a sensor is shown in Fig. 1. Each probe before using was fixed in the glass flow cell having the facility of inlet and outlet for liquid samples of different refractive indices. Unpolarized light from a tungsten– halogen lamp (AvaLight-HAL) was focused onto one of the ends of the fiber with the help of a microscope objective. The numerical aperture of the microscope objective was greater than that of the fiber for achieving maximum coupling of light in the fiber. The transmitted light was detected with the help of a spectrometer (AvaSpec-3648) at the other end of the fiber. The spectrometer was interfaced with the computer to record the transmitted spectrum of light. The wavelength corresponding to the minimum transmitted power, called resonance wavelength, is determined from the SPR spectrum. By measuring the resonance wavelength corresponding to a different refractive index of the sample around the probe we calculate the shift in resonance wavelength and hence the sensitivity of the sensor.
3. Results and discussion Fig. 2 shows SPR spectra for the SPR probe with a silver film of thickness 40 nm and a p-type silicon layer of thickness 5 nm over an unclad fiber for refractive indices of the solution around the probe varying from 1.333 to 1.358 in steps of 0.005. From this figure it is obvious that each refractive index has its own dip and hence
Fig. 1. Schematic of the experimental setup used to study the SPR spectra.
P. Bhatia, B.D. Gupta / Optics Communications 286 (2013) 171–175
Fig. 2. Surface plasmon resonance spectra of the fiber optic SPR probe for the silver and p-type silicon layer of 5 nm thickness for refractive indices of the sensing region ranging from 1.333 to 1.358.
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Fig. 4. Variation of resonance wavelength with the refractive index of the sensing layer for p- and n-type silicon layers of 5 nm thickness in the case of silver.
Table 1 Sensitivities of n- and p-type silicon based fiber optic SPR sensors in case of silver and gold metal layers.
Fig. 3. Surface plasmon resonance spectra of the fiber optic SPR probe for the gold and p-type silicon layer of 5 nm thickness for refractive indices of the sensing region ranging from 1.333 to 1.358.
resonance wavelength. As the refractive index of the sample solution increases there appears a redshift in the resonance wavelength. Similar kinds of SPR spectra for the 40 nm thick silver film and 5 nm thick n-type silicon layer were obtained in our previous study [20] where we experimentally studied the surface plasmon resonance based fiber optic refractive index sensor incorporating a high-index dielectric layer using the wavelength interrogation method. Silver and gold were used as SPR active metals followed by a high index dielectric layer of n-type silicon. The sensitivity of the sensor was found to increase with the thickness of the silicon layer. Comparison of SPR spectra of the p-type silicon with the n-type silicon predicts that, for a given refractive index of the sample, the resonance wavelength in case of the SPR sensor with the p-type silicon layer is lower than that in the case of n-type silicon layer. Fig. 3 shows the similar kind of SPR spectra for the probe with the 50 nm thick gold film and 5 nm p-type silicon layer. The SPR spectra are of similar type but the dips have been shifted towards higher wavelength. It may be noted that in Fig. 3 the normalized transmitted power at resonance wavelength decreases as the refractive index increases while in Fig. 2 it is not affected by the refractive index. No physical reason is there behind this difference. This is because in Fig. 3 all the SPR spectra are starting from normalized transmitted power 1.0 while in Fig. 2 all the SPR spectra have not been normalized to 1.0 at lower wavelengths. The normalization of each spectrum to the value 1.0 at lower
Metal
Silicon (5 nm)
Sensitivity (mm/RIU)
Silver
n-type p-type
3.3852 2.4282
Gold
n-type p-type
4.1102 2.7314
wavelength, in Fig. 2, can also give refractive index dependence of the normalized transmitted power at the resonance wavelength as observed in Fig. 3. However, this has not been carried out because the present study is utilizing a wavelength modulation scheme for which the knowledge of resonance wavelength and not the transmitted power is required and hence the results of the present study will not be affected. To compare the results for a given metal layer we have plotted, in Fig. 4, the variation of resonance wavelength with the refractive index of the liquid samples in case of silver for the n-type and p-type silicon. The error bars have been drawn taking into consideration the accuracies of the spectrometer and the refractometer. The variations are linear in both cases. It may be noted from the figure that the resonance wavelength is higher in case of the n-type silicon than in case of the p-type silicon with silver as the SPR material. Further, the slope of each curve gives the sensitivity of the sensor. The plot corresponding to the n-type silicon has a slope greater than that in the case of the p-type silicon. This implies that the SPR probe utilizing the n-type silicon layer has greater sensitivity than that utilizing the p-type silicon layer. The sensitivity values determined from the slope have been tabulated in Table 1. Fig. 5 shows the variation of resonance wavelength with the refractive index of the liquid samples in case of gold film with n-type and p-type silicon layers. The data for the n-type silicon and gold layer have been taken from our previous study [20]. In the case of gold, the variation of resonance wavelength with refractive index is the same as in the case of silver. Further the slope of the curve is greater for the n-type silicon layer than that for the p-type silicon layer. The values of sensitivity determined from these curves have also been tabulated in Table 1. It may be noted that, for a given silicon layer thickness, the sensitivity in the case of gold is always higher than that in the case of silver as reported in the previous study [20]. Therefore, we conclude from the table that the sensitivity is maximum for the gold–n-type silicon combination and minimum for the silver–p-type silicon combination.
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Fig. 5. Variation of resonance wavelength with the refractive index of the sensing layer for p- and n-type silicon layers of 5 nm thickness in the case of gold.
To understand the difference in sensitivity between n-type and p-type silicon for the same metal we tried to look for the change in refractive index of silicon on addition of dopants in intrinsic silicon. This is because a change in refractive index changes the resonance wavelength and also the sensitivity of the sensor. The concentration of charge carriers in the silicon used in the present study is of the order of 1015 cm 3. Further, for a given concentration of charge carriers, free holes are more effective in perturbing the refractive index than the free electrons [24]. The effect of holes on refractive index can be around four times of that due to free electrons. However, for the charge carrier concentration in the silicon which we have used and in the visible region, the perturbation in refractive index is at the fourth place of decimal which is negligible for giving the huge difference in the resonance wavelength as observed experimentally in the present study. Thus, this does not appear to be the reason behind the difference in resonance wavelengths obtained between the p-type and n-type silicon layers based SPR sensors. We also numerically calculated the dielectric functions for the n- and p-type silicon from the Drude relation given in Ref. [25]. The frequency dependent dielectric function can be written as
eðoÞ ¼ e1 o2p =½oðo þ igÞ where o ¼2pc/l is the angular frequency, in which c and l are the speed of light and wavelength in vacuum respectively. In the above equation, eN is the high-frequency limiting value of dielectric function that is approximately equal to 11.7 for silicon and is independent of the doping concentration and op is the plasma frequency and g is the scattering rate. Further,
op ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ne2 =mn e0
where N is the carrier concentration, e is the electron charge, mn is the carrier effective mass and e0 is the permittivity of free space. In addition, g ¼e/mnm, where m is the carrier mobility. The carrier effective mass mn is 0.27m0 for the n-type silicon and 0.37m0 for the p-type silicon, where m0 is the electron mass. Using the dielectric function values obtained from the above formula after substituting the values of various constants and N ¼1015 cm 3 we calculated SPR spectra for structures with n- and p-type silicon using the N-layer matrix method [26]. But no appreciable difference in the SPR spectra for n- and p-type silicon was obtained. Therefore, we concluded from this simulation that this is also not the reason behind such a large difference in the resonance wavelengths for n- and p-type silicon.
The difference in resonance wavelengths and sensitivity of n- and p-type silicon may be due to the difference in the charge carriers in silicon. Due to vacancies in the p-type silicon, the charge density at the metal–silicon interface will be higher and therefore its field probing the analyte will be sharp and hence will not be probing the analyte completely, resulting in lower sensitivity. While in case of the n-type silicon, due to same type of charge carriers, there will be low charge gathering at the metal–silicon boundary so the field will be shallower in the analyte and will be probing the analyte completely and hence the sensitivity will be higher in case of the n-type silicon as compared to the p-type case. The other reason can be that the charge carriers in silicon may be affecting the propagation constant of the surface plasmon wave which is solely due to the collective oscillations of free electrons in metal. The holes in silicon are perturbing the propagation constant more than the free electrons in the silicon. To make sure which reason is responsible for the difference in sensitivity of the p-type and n-type silicon based SPR sensor few more studies are required. To further investigate the reason, we have also carried out experiments on only silver and only gold coated probes. It is found that the sensitivity of the silver only case lies between the silver–p-type case and silver–n-type case. Same is the case for the gold only coated probe. This suggests that both the refractive index of silicon and the sign of the charge carriers are contributing to the changes in sensitivity of the sensor. The high refractive index of silicon increases the sensitivity as already reported in literature [17]. The smaller sensitivity in the case of p-type silicon implies that the positive charge carriers, holes, are decreasing the sensitivity. Its effect is more than that of the silicon refractive index. In the n-type silicon, sensitivity increases due to high refractive index of silicon as well as negative charge carriers, electrons, resulting in sensitivity greater than the only silver case. In other words, negative charge carriers are increasing the sensitivity while positive charge carriers in the silicon layer are decreasing it. Similar is the case with the gold film. It may also be noted that when thermal deposition in vacuum is carried out the composition of the evaporated film may change from the material before deposition. This may reduce the concentrations of phosphorous and boron in the n-type and p-type silicon coated films respectively due to differential pumping of phosphorous and boron with respect to silicon. However the films will still remain n-type and p-type silicon. Since we have observed a difference between results of two types of silicon it implies that the films are still of n-type and p-type silicon after deposition.
4. Conclusions In the present work, fabrication and characterization of a fiber optic SPR sensor utilizing p-type and n-type silicon layer have been carried out. It is observed that, for a given refractive index of the sample and the metal layer, the resonance wavelength depends on the charge carrier in the silicon. For holes, the resonance wavelength is smaller than for electrons. Further, the sensitivity is greater for electrons than for holes. The effect on sensitivity is attributed to the interaction of charge carriers in silicon with the surface electrons of the metal. It is therefore proposed to use n-type silicon for the enhancement of sensitivity of the SPR based sensor.
Acknowledgments The present work is partially supported by the Council of Scientific and Industrial Research (India). One of the authors, Priya Bhatia, is thankful to CSIR (India) for providing research fellowship.
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