We present an improved optical sensor based on surface plasmon resonance and phase detection. The sensor incorporates a surface plasmon resonance (SPR) device and a total internal reflection (TIR) device. In addition, a quarter-wave-plate (QWP) is placed in front of and behind the sensor. This gives rise to the optimization of the response curve and then the sensitivity. Theoretical simulations have been developed and verified by experimental results. With this new design, we obtain a sensitivity-tunable optical sensor whose resolving ability of refractive index is 1×10-6 RI.
©2004 Optical Society of America
The optical sensors based on the detection of binding of analyses to thin receptor films on to the sensor surface have been studied intensively. Several methods have been employed to monitor the excitation of SPR by measuring the reflection coefficient from the sensor interface [1–3]. These intensity-based techniques suffer from the fluctuation of intensity in light sources and the relatively small-reflected coefficient from the sensor surface. Higher detection sensitivity is always desirable for improving sensing performance. Fortunately, it has been found that the phase can change much more abruptly than the intensity as the refractive index or thickness of a binding layer on the surface has been changed . Several methods have been employed to monitor the excitation of SPR by measuring the phase change from the sensor interface [5–9]. Recently, a sensor based on the combination of SPR and heterodyne interferometry with extremely high sensitivity and low-noise has been proposed . However, due to some reasons the high-sensitivity sensors highlighted above are still not good enough for the measurements of physiological concentrations for most biomolecules. For example, the optimum thickness of metal film of the SPR devices is difficult to be achieved and the immobilized layers on the metal surface will reduce the sensitivity, too.
In this paper, we present an improved optical sensor based on surface plasmon resonance and phase detection. A herterodyne, collinear light beam with TE and TM polarizations is used to impinge onto the sensor surface in an effort to greatly reduce noises resulting from the environment. With a SPR device, the sensor further contains a total internal reflection (TIR) device used not only to enhance the relative phase shift between the TE and TM waves but also to keep the output beam anti-parallel to the input beam. This structure prevents the photo-detector from rotating when the sensor is scanned (rotated) with respect to the incident angle. A quarter-wave-plate is placed in front of and behind the sensor. With an appropriate orientation for each of the fast-axis of the QWPs, the residual TM wave, which has been strongly absorbed by the SPR device, is redistributed equally into each of the two elliptical-polarized output waves. This gives rise to the optimization of the response curve and the sensitivity.
A schematic diagram of the designed optical sensor is shown in Fig. 1. A polarizing beam splitter (PBS1) and two acousto-opto-modulators (AOMs) are used to produce two linearly orthogonally polarized beams from a linearly polarized laser output, where the two beams have a frequency difference of 60 kHz. These two beams, with the frequencies ω 1 and ω 2, respectively, are merged into one by a second polarizing beam splitter (PBS2). The merged single beam is further imparted into two identical beams by a beam splitter (BS), these are, the measurement beam and the reference beam. The reference beam is interfered after an analyzer (Pol1) and then is converted into an electrical signal by a photo-detector (PhD1), and it serves as the reference signal. The measurement beam, passing through the first QWP, oriented at 45° relative to the x-axis (shown in Fig. 2), impinges onto the SPR device. The reflected beam from the SPR device is further reflected at a glass/air interface where the TIR occurs. After passing through the second QWP, the measurement beam is interfered at a second analyzer (Pol2) whose fast axis is at 45° relative to the x-axis. It is converted into an electrical signal by a photo-detector (PhD2) and serves as the measurement signal. Fig. 2 is the zoom-in Fig. of the part enclosed with dashed line in Fig. 1.
Conventionally, the horizontally polarized wave that is transmitted by the PBS2 is denoted the TM wave and the vertically polarized wave that is reflected by the PBS2 is denoted the TE wave. If the TM and the TE waves after the analyzer, Pol1, have real amplitudes E po and E so and the frequencies ω 1 and ω 2, respectively, the reference signal after the photo-detector (PhD1) is simply obtained by the interference of the TM and TE waves as
where Δω represents the beat frequency between the TM and TE waves. However, the measurement signal after the photo-detector (PhD2) has a phase-shift compared to the reference signal, which is due to undergoing different path and passing different components such as the QWPs, the SPR, the ATR, and the Pol2. The phase-shift varies accordantly with the binding of analyses to thin receptor films on to the sensor surface. By detecting the phase-shift, we can obtain the amount of absorption of biological molecules. This is the measurement principle of our system. The measurement signal can be obtained by the aids of Jone’s matrix. The matrices that correspond to the measurement path for the optical devices are as follows:
where JAOM, JQWP1, JSPR, JTIR, JQWP2, and Jpol are Jone’s matrices for the acosto-opto-modulator, the first QWP, the SPR device, the TIR device, the second QWP, and the second polarizer, respectively. rp (SPR) and rs(SPR) are the respective coefficients of TM and TE waves in a layered SPR device, respectively. rp(TIR) and rs(TIR) are the complex reflective coefficients of TM and TE waves for the TIR device, respectively, and can be obtained simply by the Fresnel equations. At the output end of the second polarizer (pol2), the electric field is obtained by doing inner product on Jone’s matrices encountered along the measurement path in the reversed order.
The interfered signal is then obtained by multiplying the electric field and its conjugate part.
where Idc is the direct-current (dc) part and I is the alternative-current (ac) part. The ac part is proportional to the product of E so and E po. The real-time phase-shift Δϕ reflects the changes of amount of the binding of analyses to thin receptor films on to the sensor surface and can be obtained simply by an electronic phase meter. In the experiment, a commercial dual-phase lock-in amplifier was used (not shown in figures).
3. Experimental results and discussion
We first performed simulations of the set-up as shown in Fig. 1 according to equations (3) and (4). The dc part in equation (4) was discarded. For demonstrating the sensitivity-tunable ability, we intended to chose a SPR device with 5 nm thick of titanium-film (Ti-film) and 35 nm thick of gold-film (Au-film), which significantly deviated from the optimum condition (i.e. 5 nm of Ti-film +42 nm of Au-film). The Ti-film, with permittivity -3.84+12.15i, was used as an adhesive between the glass substrate, with refractive index 1.515, and the Au-film, with permittivity -12+1.26i. Simulation results are shown in Fig. 3. Line-a is the response curve without the two QWPs; line-b through line-f are those with the first QWP oriented as shown in Fig. 2 and the second QWP oriented at the azimuth angle of 20°, 22.5°, 24°, 26°, and 28°, respectively.
To verify the validation of the system, a glass substrate coated with 5 nm Ti-film and 40 nm Au-film was employed as an SPR device. Pure water was injected into a flow-cell on the Au-film surface, and the phase-shift between the TE and TM waves was recorded by a looking amplifier (not shown). Fig. 4 shows the results. Similar to those in Fig. 3, the solid line-a is the response curve without the two QWPs; line-b through line-g are the curves with the second QWP oriented at the azimuth angle of 20°, 22.5°, 25°, 26°, 27.5°, and 30°, respectively. These experimental results confirm that the response curve, and hence the sensitivity, is tunable.
Checking the results, we see that the point of the extreme slope for each response curve in the case with two QWPs is always smaller than that in the case without the two QWPs. Smaller incident angles permit less ellipticity of the interaction spot, which is more convenient for measurement. Steeper slopes of response curves provide more phase-shift for unit change of the incident angles, resulting in higher sensitivity. From the results shown in Figs. 3 and 4, we can, in principle, obtain sensitivity as high as we want. As the azimuth angle of the second QWP is lower than 25°, the response curves all go down sharply at around 69° of the incident angle. These curves correspond to the case in which the coated films are thinner than the optimum film thickness. In contrast, the response curves all go up when the azimuth angle is larger than 26°; this corresponds to the case in which the coated films are thicker than the optimum film thickness. Consequently, tuning the azimuth angle of the second QWP results in changing the effective thickness of the coated film. Response curves with steeper slope will yield higher sensitivity but limited dynamic range. For sensors in which both sensitivity and dynamic range are equally important, the slope of the response curve should be carefully adjusted.
In order to know the real-time measurement ability of the sensor, we compared three methods, the intensity modulation, the traditional phase modulation, and the sensitivity-tunable phase modulation (which is the method that we adopt). All the three methods were implemented on the setup shown in Fig. 1. In the intensity modulation, the reference output was discarded and only the intensities of TE and TM waves along the measurement path were recorded. Of course, the two QWPs and the polarizer had been moved away from the path. In the phase modulation, only the two QWPs were moved away from the path. We chose 1% and 2% methanol solutions as the measurement sample. They differ in the refractive index of 2×10-4. In the beginning, the 1% sample was delivered to the flow-cell through a fluid delivery system (not shown); at a latter time, the sample was changed to the 2% methanol and kept flowing for about half an hour; finally, the fluid was changed back to the original 1% sample. These processes were applied to each modulation method. Measurement results of the three methods are shown in Fig. 5. Curve-a from the intensity modulation reveals the worst signal-to-noise ratio (SNR). Curve-b is from the traditional phase modulation method. Because the Au-film was deviated from the optimum thickness, we got a small phase-shift (0.6°) for the change of the sample from 1% to 2%. Curve-c is from our system, which reveals a relatively lager phase-shift (17°)
To estimate the resolving power of the refractive index (ΔRI) of each modulation, we can divide each curve’s standard deviation (Sd) into the unit change of phase-shift (or voltage) per refractive index (i.e. Δϕ/Δn or ΔV/Δn):
The results of Fig. 5 indicate that the resolving power of the refractive index for intensity modulation, phase modulation, and our system are 3×10-5 RI, 3×10-5 RI, and 1×10-6 RI, respectively. Further improvement on the resolving ability can be obtained by reducing the standard deviation and optimizing the thickness of Au-film.
A sensitivity-tunable optical sensor based on surface plasmon resonance and phase detection is developed. A pair of quarter-wave-plates (QWPs) is employed in the sensor to equally redistribute the strongly absorbed TM wave, and then to optimize the response curve and hence the sensitivity. With this new design, we can always get the highest sensitivity regardless of weather the sensors are manufactured in perfect conditions or not. Preliminary resolving ability of the refractive index for our system is 1×10-6 RI. Further improvement on the resolving ability can be obtained by reducing the standard deviation and optimizing the thickness of Au-film.
This work was supported by the National Science Council, Taipei, Taiwan, under the grand no. NSC-92-2215-E-007-004-.
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