In this paper, the phase detection properties of a grating-coupled surface plasmon resonance (GCSPR) sensor with a thin metal film on the grating structure has been studied by performing finite-difference time-domain simulation first. Both the metal film thickness and modulation height of the grating considerably affect the phase detection properties of GCSPR sensors. The manner in which the metal film thickness affects the phase curve in the grating-coupled configuration is slightly different from that in the conventional prism-coupled surface plasmon resonance (PCSPR) configuration. For experiment, an electro-optic heterodyne interferometer is used to perform phase detection of the GCSPR sensor and a refractive index resolution of 1.5 × 0−6 RIU are obtained. The results reveal that the phase detection sensitivity of the GCSPR sensor may be comparable to that of the PCSPR sensor.
© 2010 OSA
Surface plasmon resonance (SPR) is a phenomenon in which an electromagnetic wave propagates along the interface between a metal and a dielectric. It can be excited by an incident light wave under phase-matching conditions . The SPR is highly sensitive to small changes in the refractive index (RI) of the dielectric layer on the metal surface. This SPR property has been widely exploited to detect changes in the RI due to bio-chemical reactions on the metal surface . In most practical applications, the SPR can be excited by using a p-polarized incident light and two major coupling mechanisms, a prism or a grating coupler to implement the phase-matching condition. For a fix incident wavelength light wave, a small change in RI can be detected by measuring the intensity or phase retardation of the light reflected from the metal surface. For the conventional prism coupling SPR (PCSPR) sensor, the intensity and phase detection sensitivities based on the Kretschmann’s configuration were first compared by Nelson  and later discussed by other researchers [4–7]. In these reports, it was claimed that the phase detection sensitivity of PCSPR sensors could be better than its intensity detection sensitivity by two orders of magnitude. Recently, the validity of the widely claimed superiority of the phase detection mode was checked by Ran and Lipson . They concluded that phase detection mode only has a slightly higher sensitivity than intensity detection mode because phase detection is also based on light intensity measurement and both modes are limited by photon statics. However, if the power of the incident light is large, the sensitivity of the phase detection mode still can be much better than that of the intensity detection mode.
In the case of a grating-coupled SPR (GCSPR) sensor, most measurements are based on intensity detection [9,10]. Though the intensity detection sensitivity of the GCSPR sensor is less than that of the PCSPR sensor , GCSPR sensors can be fabricated at low-cost and in array-type [12,13]. However, it seems that neither an experiment nor a simulation based on phase detection has been reported so far for the GCSPR sensors. Recently, Sedoglavich devised a phase-polarization contrast method using a recordable compact disk (CD) grating substrate . This method is based on the fact that the interference between s- and p-polarized light provides increased intensity difference; hence, this method is not a direct phase measurement technique. In this study, we have numerically and experimentally confirmed that the phase detection sensitivity of the GCSPR sensor may be comparable to that of the PCSPR sensor. The GCSPR sensor may have great potential in practical sensor applications when it is operated in the phase detection mode.
The phase behaviors of the PCSPR sensor are highly dependent on the thickness of the metal film . The thickness determines not only the phase response curve but also detection sensitivity. An example plot of the reflectivity and phase of reflected light versus the incident angle for different metal film thicknesses of a PCSPR sensor is shown in Fig. 1 . The phase curves exhibit a different behavior for relatively thin and thick metal layers. For the group with relatively thin metal layers as shown in Fig. 1(a), the phase monotonously decreases as the incident angle increases, and the full dynamic range is larger. On the other hand, for the group with relatively thick layers as shown in Fig. 1(b), the phase is not monotonous as the incident angle increases and the full dynamic range is smaller. A phase jump transition from the relatively thin group to the relatively thick group is observed as the previous report . Near the resonance point, if the metal film thickness or structure is modified, the excited mode of the surface plamson wave propagating along the interface between metal and dielectric is changed, and so is the phase behavior . The mode switching causes the phase transition. Therefore, near the resonance point, the sensor is very sensitivity to the metal film thickness or structure modification. In both groups, the phase curve with the sharpest variation near the resonance point corresponds to the intensity curve with the narrowest full width at half magnitude (FWHM). At the point where the sharpest phase change occurs, the intensity of reflected light becomes small and this may constrict the utilization of high-sensitivity phase detection near this point. For the case of the GCSPR sensor, in addition to the metal film thickness, the modulation height of the grating also affects phase behaviors significantly.
2. Principle of the grating coupling SPR
A schematic of the GCSPR structure is as shown in Fig. 2 . The structure comprises a polymer with sinusoidal grating having period Λ and modulation height h, and a metal film having thickness d. A p-polarized light beam is incident on the structure at an angle of θ. The energy of the incident light can be coupled to excite the SPR on the metal grating structure when the wavenumber of the incident light matches to that of the surface plasmon. In this structure, the wave number of the diffracted light in the horizontal direction, denoted by kl, can be expressed as follows ,Eq. (1), the first and the second terms of the wave number kl correspond to the wave number of the incident light along the horizontal direction and the multiple diffraction order of the grating wavenumber 2π/Λ. The wavenumber of the incident light can be increased by the grating structure to match that of the surface plasmon. Under the phase-matching condition, the wave number of the incident light is equal to that of the surface plasmon as
In the above Eq. (1) and (2), we assume that the dispersion properties of the surface plasmon are not affected by the grating structure. However, if the modulation height h is increased, changes of the dispersion properties attributable to the sinusoidal perturbation of the surface become significant . To study the effects of the variation in the values of d and h on phase detection properties of the GCSPR sensor, a three-dimensional electromagnetic solver (EM Explorer) based on the finite-difference time-domain (FDTD) method has been used.
3. Simulation of the phase properties of the GCSPR sensor
Figure 3 shows a plot of the calculated intensity and phase of reflective light versus the incident angle for the GCSPR structure as shown in Fig. 2 with a fixed modulation height (h) and different metal film thickness (d). In this calculation, the device parameters are as follow: Λ = 833 nm, h = 70 nm, n g = 1.475 (grating polymer), and n Au = 0.166 + 3.15i (gold). A comparison of the results obtained from Fig. 3 with the results for the PCSPR structure, shown in Fig. 1, reveals that the metal film thicknesses of the GCSPR and the PCSPR sensors affect their phase properties in a different manner. That is, for the group with relatively thick metal layers, the phase monotonously decreases as the incident angle increases and has a large full dynamic range; however, for the group with relatively thin metal layers, the phase does not monotonously decrease as the incident angle increases, and the full dynamic range is small. In the monotonously decreasing case, the phase curve with the sharpest variation near the resonance point does NOT correspond to the intensity curve with the narrowest FWHM, which is different from the results obtained for the PCSPR sensor. This implies that though the intensity detection sensitivity of the GCSPR sensor is less than that of the PCSPR sensor, the phase detection sensitivity of the former may be comparable to that of the latter if the metal film thickness can be controlled to have optimal values.
For a fixed metal film thickness d = 70 nm, the calculation results of the intensity and phase curves with different modulation heights (h) are shown in Fig. 4 . The FWHM of the intensity curve becomes wider as the modulation height increases. This behavior had been investigated and same conclusion was obtained by Raether at very early stage . However, the behavior of the phase curve is quite different from that of the intensity curve. For practical device fabrication, although the modulation height of the grating can be controlled by adjusting the exposure time of the photoresistor using in the interferometric method, it iscomparatively easier to control the metal film thickness. In the case of a GCSPR sensor, if we define the optimal value of the metal film thickness is defined as the value at which the phase jump occurs, the optimal metal film thicknesses for four modulation heights, 70 nm, 60 nm, 50 nm, and 40 nm are calculated and shown in Fig. 5 . The results show that a grating structure with a low modulation height requires a relatively thick metal film to achieve high phase detection sensitivity.
4. Experimental results
In our experiment, only the effect of the metal film thickness on the phase detection properties was examined. The AFM image of the polymer grating structure fabricated by the nanoimprinting technology is shown in Fig. 6 and its grating pitch and the modulation height are about 833 nm and 70 nm, respectively. The nanoimprint master was commercial available grating with 1200 lines/mm from Edmund Optics. The elastomer stamp was cast from poly (dimethylsiloxane) (PDMS) (SYLGARD 184, Dow Corning), which was poured on the master grating. The polymer used for grating was a VU curable polymer (No.9046, Everwide, Taiwan). Finally, the gold was deposited on the grating by the thermal evaporation method. Four GCSPR sensors coating with different gold film thickness, from 35 nm to 50 nm in 5 nm steps, were prepared. The intensity and phase detection system used in the experiment is based on the electro-optic (EO) heterodyne interferometer as shown in Fig. 7 . In this system, the electro-optic modulator (EOM) is driven by a saw-toothed waveform. The incident light source is the HeNe laser with a wavelength of 632.8 nm. The magnitude and the phase readings of the lock-in amplifier corresponded to the reflected heterodyne light intensity and phase difference between p- and s-polarization components, respectively. Because the p-polarization light is highly absorbed by the sensor to excite SPR at the resonance angle, the input polarizer was set to be 10° with respect to the horizontal line so that the incident light beam have larger p-polarization component.
The four prepared GCSPR sensors were immersed in a small liquid (water) tank in turn to perform the intensity and the phase measurements with altering the incident light angle. The calculation and the measurement results are shown in Fig. 8 and Fig. 9 , respectively. Figure 8 (a) and Fig. 9 (a) show the intensity results and Fig. 8 (b) and Fig. 9 (b) show the phase results. In both phase results, the sensors with 40-nm-thick gold film exhibits the sharpest phase variation near the resonance angle. In the measurement results, the phase curve of the sensor with 35-nm-thick gold film is different from those of the sensors with other gold film thickness and its counterpart in the calculation results is 38-nm-thick. The resonance angle is near 50.5° in the simulation while is near 44° in this experiment and the dynamic ranges of the simulation phase curves are greater than those of the experiment phase curves. These discrepancies may attribute to the grating shape difference between the simulation model and the actual profile of the sensor.
Next, a sensitivity comparison between two sensors with 40-nm-thick and 45-nm-thick gold films were performed. They were used to sense different sucrose concentration changes. In the first experiment, the sucrose concentration of the liquid was sequentially increased from 0% to 0.175%, 0.338%, and 0.491% at three different timing points by dropping high concentration liquid into the tank. The RIs of the liquid at the sequential timing points were about 1.3332, 1.3334, and 1.3336, respectively. In this experiment, the sensor with 45-nm-thick gold film was used. In the second experiment, the similar process was carried out while the sucrose concentration of the liquid was only one half of the previous one and the sensor with 40-nm-thick gold film was used. Time evolutions of phase changes for two sensors are shown in Fig. 10 (a) . The phase detection sensitivity is related to characteristics of the light source and the photo-detector. Laser light sources usually have excellent phase noise. Photo-detector noises can be divided into three categories: shot noise, dark current noise, and thermal noise. More detail noise discussions for the SPR phase detection have been addressed in reference . In this experiment, the system noise may be estimated from a stable piece (t = 220 s to 280 s) of time evolution of phase change for the sensor with 45-nm-thick gold film, as shown in Fig. 10 (b). The slow phase variation may be due to the temperature fluctuation. Hence, the statistical error phase during this time interval is about 0.5 degree. Based on this observation, the RI resolutions of the sensors with 45-nm-thick gold film and 40-nm-thick gold film are estimated to be about 5 × 10−6 RIU and 1.5 × 0−6 RIU, respectively. This sensitivity can be further improved by using a low phase noise detection method like photo-elastic modulator (PEM)-based scheme with sinusoidal temporal phase modulation . If the phase noise of the detection system can be controlled under 0.01°, these two sensor sensitivities with 45-nm-thick gold film and 40-nm-thick gold film are estimated to be about 10−7 RIU and 3 × 0−8 RIU, respectively. The achieved sensitivity of the GCSPR senor with 40-nm-thick gold film is close to that of the PCSPR sensor reported by Kabashin .
The phase behavior of the GCSPR sensor depending on the metal film thickness and modulation height has been studies numerically. The results suggest that though the FWHM of the intensity resonance curve of the GCSPR sensor has been recognized as being greater than that of the PCSPR sensor, the phase slope of the GCSPR sensor can be the same as that of the PCSPR sensor if the metal film thickness is suitably controlled when the modulation height is fixed. An experiment also has been demonstrated to show high-sensitivity phase detection by using the GCSPR sensor. A grating structure with fixed modulation height can be fabricated on a GCSPR sensor by nanoimprinting process at a low-cost. Hence, the GCSPR sensor based on phase detection scheme may have great potential for biomedical-sensing applications in the future.
The authors gratefully acknowledge the financial support provided by the National Science Council, Taiwan, under Grant No. NSC97-2221-E-150- 020-MY3.
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