Surface plasmon resonance (SPR) sensors represent a suitable method for broadband acoustic pulse detection. The reflectivity and phase of a p-polarized laser beam incident on an optical device under SPR conditions are strongly dependent on ambient conditions that are changed by an acoustic wave. Depending on the order of layers, SPR sensors can be arranged in the Kretschmann or in the Otto configuration acting as a pressure or as a displacement sensor. The aim of this study was to compare both configurations and to find linear and sensitive conditions for the application. Numerical calculations were carried out varying the layer dimensions and the angle of incidence. The results of the experimental investigation on both configurations confirm the working principle.
© 2007 Optical Society of America
Optical devices based on the surface plasmon resonance (SPR) effect have been intensively investigated for biological and chemical sensing applications. In recent years great efforts have been made to increase the sensitivity. One of the promising approaches is the use of not only the amplitude, but also the phase of light reflected under SPR conditions [1–6].
Only few authors have investigated SPR sensors for acoustic pulse recording [7,8]. They could show that surface plasmons can be successfully used for absolute pressure measurements on a nanosecond time scale with a lateral resolution given by the probe beam diameter. Thermoelastically excited acoustic transients in liquids are generally broadband with low pressure amplitudes and require sensors with high temporal resolution and sensitivity. Both requirements are fulfilled by SPR sensors.
Depending on the order of layers, SPR sensors can be arranged in the Kretschmann or in the Otto configuration. Whereas the previous studies have exclusively employed the Kretschmann configuration for acoustic pulse measurements, in this study it is shown that the Otto configuration can be used as well. It offers the possibility to use a combination of amplitude and phase of the reflected p-polarized light in the attenuated total reflection (ATR) region to maximize the sensitivity for acoustic pulse recording. This is not possible in Kretschmann configuration because the acoustic wave entering the glass substrate causes an additional phase shift due to induced birefringence. This phase shift takes place in the bulk and not on the surface of the device and gives rise to a relatively low bandwidth acoustic signal that is not useful for the applications discussed here. The measurement of the phase requires some kind of an interferometer. In this study the two configurations are compared and two methods for analyzing the reflected light, via its amplitude and its phase, are investigated theoretically and experimentally for the detection of acoustic transients in liquids.
2. Theory of surface plasmon resonance sensors
Excitation of surface plasmon waves (SPWs) optically in a metallic layer is only possible by illuminating with p-polarized light, whose momentum is matched to that of SPWs . Only the p-polarized electric field vector has a driving force component to excite the electron gas of the metal. The matching condition is given by
where k 0 = 2π/λ is the vacuum wave vector of the incident light, nd is the refractive index (RI) of the dielectric medium (e.g. the glass prism) through which the light is incident at an angle θ. The dispersion relation of SPWs is
where ϵd and ϵm are wavelength dependent complex dielectric functions of the dielectric and metallic media respectively. When SPR occurs, the incident light is absorbed and converted into Joule heating of the metal layer accompanied by a strong attenuation of the reflected light in the total reflection region.
To model an SPR sensor in ATR mode makes it necessary to calculate the reflectance R and phase φ of the reflected light for a three layer system. For a TM (p-) polarized wave the calculation yields
where rkl is the reflection coefficient at the interface between media k and l, θk is the incident angle into the kth medium, nk is the refractive index and d is the thickness either of the metal film in Kretschmann or of the air gap in Otto arrangement (Fig. 1).
3. SPR sensors for acoustic wave detection
In Kretschmann configuration, the order of layers [Fig. 1(a)] is a metal layer sandwiched between (glass) substrate (from where the probe laser beam is incident) and liquid (from where the acoustic wave arrives). An acoustic wave shifts the incident angle where SPR occurs relative to the initial position due to changes of RI of the liquid in the range of the evanescent SPR field. The RI is directly linked to the pressure of the acoustic wave via the acousto-optic coupling coefficient in liquid (e.g. dn/dp=1.35×10-5Pa-1 in water). On this account in Kretschmann configuration the device acts as a pressure sensor [7,8]. Also the pressure wave entering the substrate causes a shift of the SPR angle via the elastooptic constants of the glass. However, these constants are at least an order of magnitude smaller than the corresponding value in water and have a negligible influence on the recorded signal [10,11]. The dimension of the evanescent field in the liquid determines the bandwidth of the SPR sensor. Since the evanescent field has typically a size on the order of the light wavelength, this results in a bandwidth in the range of GHz.
In the Otto configuration the order of layers [Fig. 1(b)] is glass substrate (from where the laser beam is incident), air gap, metal layer, glass or plastic substrate, and liquid (from where the acoustic wave arrives). This sensor acts as a displacement sensor, because the displacement of the metal/air interface during the reflection of an incoming acoustic wave is the main disturbing effect for the initial SPR conditions. This follows from the fact that the evanescent wave of the plasmon is confined to the metal layer and does not enter the adjacent substrate. The SPR conditions of the device are therefore not affected by any changes of the substrate RI caused by the pressure wave. The metal layer itself due to its even higher bulk modulus compared to glass can be expected to be unaffected in its optical constants by the pressure variations. In the Otto configuration there is a complex dependence of sensitivity on air gap thickness and incident angle of the laser beam, mainly because an incoming acoustic wave causes both an angular shift of the SPR peak and a change of its amplitude.
4. Sensitivity simulations
For the design and the application of SPR-sensors it is important to find out via theoretical, numerical calculations the conditions for the highest sensitivity with suitable linearity. The parameters are the layer dimensions and the angle of incidence. For all theoretical investigations silver was used as metal layer (εAg=-15.7+i1.0728), BK7 was the material of the glass prism (εP=2.3) and the wavelength of the probe laser beam was 632.8 nm.
4.1 Kretschmann configuration
Figure 2 shows reflectance and sensitivity versus angle of incidence for three chosen initial RI values of the surrounding liquid. The simulation was performed for a 50 nm silver layer that gives under optimal SPR matching conditions a reflectance value at the ATR minimum around zero. The position of the minimum shifts almost linearly with refractive index. The shape of the reflectance curves remains unchanged, only the SPR matching angle is shifted.
A simulation taking into account the RI change in glass reveals a slightly reduced shift of the SPR angle. However, this modification is compensated by the effect of a concomitant change of incident angle, in a similar way as it was described for a sensor based on Fresnel reflectance . This justifies the use of the water RI variation alone for the calculation of the sensitivity.
For the use as acoustic sensor the incident angle is set to a certain operating point (OP). For a given OP the sensitivity Sampl is defined as
where R0,opt is the reflectance at ambient pressure and ∂Ropt/∂p is the derivative of the reflectance with respect to the pressure at the chosen OP.
The highest sensitivity is obtained for an OP in the steeper slope of the ATR peak near the reflectance minimum. The disadvantage of choosing an OP with a higher sensitivity is a loss in linearity. Figure 2 shows the relative change, Ropt/R0,opt, as a function of p for different values of R0,opt . There is a more or less wide linear range around p=0. For practical application a compromise must be found with respect to sensitivity and linearity. The chosen OP marked in Fig. 2 promises a sensitivity value of 0.01 bar-1, which is a relative change of one percent of the recorded signal per 1 bar pressure amplitude.
4.2 Otto configuration
4.2.1 Amplitude measurement
In the Otto configuration the sensing principle is a variation of the air gap thickness d during the reflection of the acoustic wave at the metal/air interface. The sensitivity is derived from the dependence of the optical reflectance Ropt on air gap thickness d and angle of incidence θ of the probe laser beam. Figure 3 (left) shows this dependence for a 100 nm silver layer thickness displayed as an intensity plot of the reflectance function. The quite complicated behavior becomes apparent when comparing the slices taken out of the intensity plot [Fig. 3 (right)]. The reflectance and the sensitivity are strongly dependant on the initial air gap thickness. The sensitivity of the direct amplitude measurement of the reflected light to interface displacements is defined as
where R0,opt is the reflectance at initial air gap thickness and ∂Ropt/∂d is the derivative of the reflectance Ropt with respect to the air gap thickness. For the chosen OP, marked in Fig. 3, follows a sensitivity of about 0.013-1 nm or in other words, a displacement of one nanometer causes a signal change of about 1.3 percent.
4.2.2 Phase measurement
Not only the amplitude but also the phase of the p-polarized component of a laser beam reflected from a metal film under SPR conditions is dependent on ambient conditions that are changed by an acoustic wave. The s-polarized component is quite unaffected.
For this reason it is also plausible to use the phase shift between p- and s-polarized portions of the probe laser beam as sensing physical parameter for acoustic waves.
For phase sensitive measurements an interferometric setup is used where s- and p-polarized portions of the laser beam are recombined after the SPR device. In this setup the s-polarized component acts as a reference. The interferometer output function is given by
where RP, RS, φP, φS are the reflectance and phase of the two polarization components and β is the azimuth angle with respect to p-polarization of a polarizer used to adjust the intensities of the two orthogonally polarized portions of the incoming laser beam. Φ is an additional phase factor used to set the interferometer OP into a slope of the cosine function. Figure 4 (left) shows the simulated phase and sensitivity curves versus angle of incidence for the chosen simulation parameters: 600 nm air gap thickness, β = 10° and Φ = 330°. The slope of the phase curve becomes highest at the minimum of the reflectance curve. The sensitivity of the interferometric setup measuring the phase and amplitude variations is defined as
with I0,out the interferometer output at initial air gap thickness and ∂Iout/∂d the derivative of the interferometer output function with respect to the air gap thickness.
The linear range of the sensor arranged in Otto configuration is determined by a constant slope of the curves shown in [Fig. 4 (right)] for different values of R0,opt . In practice the sensitivity and linearity can be tuned with the angle of incidence of the probe laser beam. For the same chosen OP as for the amplitude measurement [Fig. 3 (right)], the sensitivity amounts to 0.022 nm-1, almost two times higher than the sensitivity of the direct amplitude measurement.
In summary the results of the simulations show that it is straightforward to find the optimal sensitivity of the SPR device in Kretschmann configuration, which acts as a pressure sensor. In Otto configuration the SPR device is sensitive to displacements, making it more complicated to optimize it for maximum sensitivity and linearity. A higher sensitivity at a given OP can be achieved with the interferometric phase measurement.
5. Experimental setups and working principle
In all experiments the acoustic transients were excited thermoelastically by irradiating an aqueous dye solution with a laser pulse from a frequency doubled Nd:YAG laser with a wavelength of 532 nm, a radiant exposure on the liquid surface of 0.13 J/cm2 and a pulse duration of 10 ns. The dye solution had an optical absorption coefficient of 100 cm-1. Under these conditions a pressure wave with an amplitude of about 7 bar is generated in the liquid. Figure 5 shows the experimental setup for amplitude measurements consisting of a polarized He-Ne laser, a neutral density filter for reducing the intensity, a polarizer to adjust the portions of p- and s-polarized light, a rotary table for setting the OP by changing the incident angle, a right angle prism with one side acting as a mirror and a SPR device placed on the prism. For the reflectance amplitude measurement only p-polarized light was used and the temporal signals were directly recorded with a 125MHz bandwidth photo detector.
For the phase measurement two different interferometric setups were used. In both setups the s-polarized component acts as an undisturbed reference. At the entrance of the interferometer both components have the same intensity level in order to achieve the highest modulation of the interferometric output. Figure 6(a) shows the sketch of the used common path interferometer (CPI) . It uses a half wave plate as a polarization dependent phase shifter, and an analyzer to get the two polarization components to interfere. The second phase measurement setup shown in Fig. 6(b) is a Mach-Zehnder interferometer (MZI) consisting of a polarizing beam splitter to separate the polarization components, a half wave plate to match the polarization state of the separated components, a phase shifter and a non polarizing beam splitter for recombination . The two generated output beams are conjugate interference signals, recorded with balanced photodiodes and amplified with an 80 MHz bandwidth differential amplifier.
In comparison the MZI eliminates common mode signal perturbations such as laser noise and doubles the output signal compared to single beam detection, whereas the CPI represents a quite easy setup with a high stability against outside influences.
In order to compare the experimental signals between different configurations and with the theoretical sensitivity values, all results are given as relative intensity changes, by dividing the recorded AC voltage signal by the DC level that corresponds to the undisturbed optical signal without pressure wave. In the case of the phase measurement with the MZI, where the undisturbed output of the differential amplifier is zero, this DC level was measured by blocking one of the interferometer beams. Furthermore, the DC voltage was the same for all experiments performed in a particular setup, enabling a direct comparison of signal to noise ratios in the amplitude and phase measurements.
6. Experimental results and discussion
Figure 7(a) shows the time resolved pressure signal recorded with the amplitude measurement setup and the arrangement of the used SPR sensor. The first pressure pulse arriving at the sensing area of the SPR-device is shown. From the shape and amplitude of this peak it is possible to determine various physical properties of the liquid, such as speed of sound, optical absorption coefficient and thermoelastic coupling parameter [13–15]. Prerequisites for such measurements are the wide linear sensitivity range and the high bandwidth offered by the SPR sensor. Figure 7(b) shows the time resolved displacement signals recorded with the amplitude and the CPI measurement setup. The applied sensor was arranged in Otto configuration. In comparison the phase measurement shows a nearly two times higher signal to noise ratio than the amplitude measurement. With the MZI measurement setup a further increase of S/N ratio by at least a factor of two is achieved due to subtraction of conjugate signals and the rejection of common mode signal perturbations such as laser noise (Fig. 8). The comparison of acoustic signals recorded with Kretschmann and Otto configuration is complicated due to different measured quantities (pressure and displacement) and layer dimensions. The sensitivity definition allows the comparison of sensitivity at different values of R0. S was always highest near a minimum of R. In practice, to take advantage of this fact, the incident laser beam intensity has to be increased to compensate the losses near the SPR minimum.
A comparison of the measured amplitudes with the predictions obtained from the theoretical sensitivity values shows that the experimental sensitivity in the Kretschmann configuration is quite similar to the theoretical one. For instance, with an estimated pressure amplitude of 7 bar the experimental sensitivity obtained from Fig. 7(a) is ∼0.004 bar-1. This corresponds to a theoretical sensitivity in the falling slope of the reflectance curve (Fig. 2), where the actual operating point was chosen. In Otto configuration, the experimental sensitivity is still on the same order of magnitude as the theoretical one but lower. The estimated displacement amplitude for the measurements is ≥10nm , which would imply a relative intensity change of ≥ 20% at the point of maximum theoretical sensitivity. The lower experimental value can be attributed to the imperfection of the real device. Factors limiting the sensitivity are the finite divergence of the probe beam  and most of all the fact that the surface of the substrate carrying the metal layer was neither perfectly flat nor perfectly parallel to the surface of the glass prism. In order to set the sensor at the optimum operating point the air gap thickness must be controlled to a fraction of the optical wavelength. This was not possible with our substrate that was a 4 mm thick piece of polycarbonate (PC). This material was chosen to achieve good acoustic coupling to water, but it has not very high optical quality. Choosing a glass substrate with optimum optical surface quality would allow higher sensitivities but would lead to acoustic reverberations within the glass layer.
We have compared the two possible arrangements of SPR devices for acoustic wave detection. The Kretschmann configuration already investigated in Ref. [7, 8] is sensitive to RI variations of the surrounding liquid in the range of the evanescent plasmon field and acts as a pressure sensor with high bandwidth and a wide linear working range. The simulation shows a quite simple dependence of the reflectance on RI and pressure, making it easy to find optimal conditions for the sensor in terms of sensitivity and linearity.
The Otto configuration acts as a displacement sensor of the metal/air interface during the reflection of the arriving acoustic wave. The simulated result shows a complicated dependence of the reflectivity on incident angle and initial air gap thickness. It could be shown that in addition to the amplitude, also the phase of the reflected light can be used for acoustic wave detection. The S/N ratio is two times higher by using amplitude and phase, instead of a pure amplitude measurement of the reflected laser light. This behavior predicted by sensitivity simulations is confirmed by experimental results. The achievable bandwidth is only limited by the detection electronics. The sensor has a linear working range of a few tens of nanometers. To achieve a further two times better sensitivity for the experimental investigations a MZI was used by detecting the conjugated output beams with a balanced photodetector.
The finding of this study is that both configurations can be used for acoustic wave recording, with the difference that the measured quantity of an SPR sensor arranged in Otto configuration is displacement, whereas it is pressure in Kretschmann configuration. The combined measurement of phase and amplitude of the reflected light is more sensitive, but only useable in combination with the Otto configuration for acoustic wave detection.
This work has been supported by the Austrian Science Fund (FWF), Proj. Nr. P18172-N02.
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