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A transmitted-type guided-mode resonance (GMR) sensor is presented for using an electro-optic heterodyne interferometer to tune phase detection sensitivity. The GMR grating waveguide structure is fabricated using a low-cost nanoimprinting SiO2 sol-gel process and sputtering TiO2 film. The phase properties of the GMR sensor are numerically investigated to verify its phase detection capability in a heterodyne interferometer. The phase curves for both transmitted- and reflected-type GMR sensors are experimentally obtained and compared. We conclude that the transmitted-type GMR sensor is more feasible for tuning phase detection sensitivity by rotating the analyzer in the electro-optic heterodyne interferometer. In our experiment, we achieved the GMR sensor phase detection sensitivity as high as 1.8 × 10−7 RIU.
© 2014 Optical Society of America
1. Introduction
A guided-mode resonant (GMR) grating waveguide structure can maximally reflect incident light when the incident wave is phase matched to the leaky waveguide mode [1]. Because the GMR reflective peak is very sensitive to the refractive index of the film on the structure surface, it has been used for biosensor and optical component applications [2, 3].
For typical GMR biosensors, when the biolayer is attached to the sensor’ grating surface, the resonance peak wavelength changes and a spectrometer is usually used to detect the peak wavelength shift [4, 5]. The shift value is proportional to the reflective index as well as thickness of the attached biolayer and can be used as an indicator of biochemical reactions on the grating surface [6]. In addition to the wavelength-resolved method, the incident angle or angular-resolved method has been introduced using a motorized stage [7]. Both wavelength- and angular-resolved approaches are based on the intensity or amplitude detection scheme.
For phase detection with GMR devices, a reflected-type sensor with wavelength scanning has been theoretically discussed [8]. The phase properties of a GMR device have been used to implement a wave plate [9]. Furthermore, GMR sensors based on ellipsometry have also been proposed to convert the phase change to intensity [10].
The phase detection sensitivity of a surface plasmon resonance (SPR) sensor could be better than its intensity detection sensitivity by two orders of magnitude [11]. In our previous work, we showed that the phase detection sensitivity of a SPR sensor could be tuned and enhanced by rotating an analyzer in an electro-optic heterodyne interferometer [12]. Because the phase properties of a transmitted-type GMR device are similar to that of a reflected-type SPR device, in this study, we apply this simple phase sensitivity tuning method to the transmitted-type GMR sensor, which was fabricated by a low-cost nanoimprinting process. Our experimental results show that the GMR sensor detection sensitivity can be easily tuned.
2. Guided-Mode Resonance Sensor Structure and Simulation
Our GMR sensor structure is shown in Fig. 1. First, the SiO2 grating structure was fabricated by a nanoimprinting sol-gel process [13]. The master mold was a commercially available holographic grating with 1800 lines/mm form Edmund Optics. Then, the TiO2 film as a waveguide layer was coated on the grating surface by a sputtering process. Therefore, the grating period Λ is about 555 nm. The incident laser wavelength λ is 685 nm. The incident angle is θ and direction is from the bottom substrate to the top grating surface. The TiO2 film thickness d is 400 nm. Based on the average atomic force microscopy (AFM) measurement results, the depth h is 75nm.
According to the simulation results, when the depth is in the 50 nm to 80 nm range, the magnitude and phase curves do not show large discrepancies. Figure 2 shows the simulation results of the transmittance and phase curves of the p-wave and s-wave by scanning the incident angle θ. In this simulation, the refractive indexes of the glass substrate, SiO2 grating layer, and TiO2 film are ng = 1.515, ns = 1.500, and nt = 2.298, respectively. The sensor surface is assumed to be water (testing liquid) (nw = 1.330). Under these conditions, for the p-wave, the resonance incident angle is about 33.2°. Its corresponding resonance phase changes dramatically and has largest slope at this resonance angle. In contrast, for the s-wave, no obvious resonance is found so it can be used as a reference in heterodyne interferometry. These transmittance and phase curves are similar to reflectivity and phase curves of SPR sensors [14]. Therefore, the phase detection method for the reflected-type SPR sensors can be also applied to the transmitted-type GMR sensors. For comparison, the same simulation results of the reflected p-wave and s-wave are shown in Fig. 3.The phase curve behavior of the reflected p-wave is different from that of the transmitted p-wave.
Fig. 2 Simulation results of transmittance (a) magnitude and (b) phase curves of transmitted p-wave and s-wave by scanning the incident angleθ.
Fig. 3 Simulation results of reflectivity (a) magnitude and (b) phase curves of reflected p-wave and s-wave by scanning the incident angleθ.
3. Measurement system and results
The electro-optic heterodyne interferometer measurement system is shown in Fig. 4.The heterodyne interferometry using an electro-optic modulator has been proposed in a SPR measurement [15]. In this system, phase readings of the lock-in amplifier are relative to the phase difference between p-wave and s-wave. More detail about its operational principle can be found in our previous work [12]. The only different between our previous experiment and this experiment is the rotational stage. In our previous work, a complex θ-2θ motorized rotation stage was required for the reflected-type measurement. Two rotational stages are stacked in the θ-2θ system. One stage is used for rotating the SPR sensor device to change the incident angle (θ-stage), while the other stage is used for rotating the detector and the analyzer to follow the reflected beam angle (2θ-stage). Unlike the previous experiment, only one rotational stage is required for the transmitted-type GMR sensor measurement. The detector and the analyzer can be stationary during the measurement. This advantage may reduce cost and some mechanical noise caused by heavy load on the stage.
Fig. 4 Electro-optic heterodyne interferometer system for phase detection of the transmitted-type GMR sensor
For the heterodyne interferometer measurement system, the phase change detected by the lock-in amplifier can be modified from our previous work [12]. Assuming the input laser power to be the normalized value, the light electric-field signal received by the photo-detector (PD) is
Where tp (ϕp) and ts (ϕs) are the transmission coefficient magnitudes (phases) of the p-wave and s-wave, respectively; and α is the analyzer rotation angle with respect to the horizontal axis. In Eq. (1), the phase retardation Γ between the s- and p-polarization light caused by the electro-optic phase modulator (EOM) is not included. The function generator (FG) outputs the timing waveform to the lock-in amplifier (LIA) as a phase reference signal and the saw-toothed waveform to the high-voltage amplifier to drive the EOM. The real and image parts in Eq. (1) isThen, the phase difference of the heterodyne signal converted by the PD is asFrom Eq. (3), the phase change ΔΦ depends on the transmission coefficient, including magnitude and phase, as well as the analyzer rotation angle α.For comparison, we first describe the reflected-type GMR sensor measurement results. The reflectance curves of the p-wave and s-wave measured by scanning the incident angleθ are shown in Fig. 5.The results show that only the p-wave has a resonance angle, which is consistent with the simulation results shown in Fig. 2. If no absorption in the GMR sensors is assumed, the transmittance curve can be easily obtained from the reflectance curve.
The measurement results of the reflected-type GMR sensor phase change curves are shown in Fig. 6(a) for five different analyzer rotation angles: 5, 10°, 15°, 20°, and 25°. The slopes of the phase curves near the resonance angle change from negative to positive values when the analyzer angle is increased from 15° to 20°. Furthermore, based on the simulated reflectance and phase curves in Fig. 3, as the analyzer angle α changes, the incident angle with the steepest phase curve slope also changes. Rotating analyzer should not change the resonance angle. This phenomenon causes serious inconvenience because when we rotate the analyzer to tune the detection sensitivity, the incident angle also must be altered to be the value with steepest slope. Therefore, the reflected-type GMR sensors is not suitable for tunable phase detection sensitivity in the heterodyne interferometer system.
Fig. 6 Measured phase change curves for (a) the reflected-type and (b) the transmitted-type GMR sensors for five different analyzer rotation angles: α = 5°, 10°, 15°, 20°, and 25°.
In contrast, the same measurement results of the transmitted-type GMR sensors, based on the simulated reflectance and phase curves in Fig. 2, are shown in Fig. 6(b). As the analyzer angle α changes, the incident angle with the steepest phase curve slope is kept and equal to the resonance angle. Unlike the reflected-type results, this is more convenience because we can rotate the analyzer to have different phase detection sensitivity. This tunable sensitivity allows us to choose either high sensitivity or high dynamic range operation modes based on the sensing requirements. Therefore, the tunable phase detection method is more suitable for the transmitted-type GMR sensors. Moreover, a phase jump occurs when the analyzer is rotated from −60° and −65° in our experiment. Therefore, the steepest slope occurs when the analyzer rotation angle falls within this interval. In other words, the highest phase detection sensitivity can be achieved in this interval.
In the final experiment, we tune and compare the phase detection sensitivity of the transmitted-type GMR sensor for five different analyzer rotation angles: α = −50°, −55°, −60°, −65°. The sensor was biased at the resonance angle to sense different sucrose concentration changes. The sucrose concentration of the test liquid was sequentially increased from 0% to 0.714% and 1.25% at two different timing points by dropping high concentration sucrose liquid into the test tank. The refractive index of testing sucrose liquid at the sequential timing points was about 1.3333, 1.3338, and 1.3345, and the phase value of the lock-in amplifier was recorded every two seconds.
Figure 7 shows the all measurement results. The results in Fig. 7(a) correspond to the phase curves with a negative slope near the resonance angle. When the sucrose concentration increases, the phase curve shifts to right, and the phase change increases. In contrast, the results in Fig. 7(b) correspond to the phase curves with a positive slope near the resonance angle; therefore, when the sucrose concentration increases, the phase change decreases. Furthermore, the linearly dynamic range of these phase curves with positive slope is about 60° ( ± 30°); while that of the curve with negative slope is about 180° ( ± 90°). Among these results, the best detection sensitivity is obtained when α = −60°. Assuming that the statistical error of the lock-in amplifier is 0.01°, this optimal analyzer rotation angle can achieve a detection sensitivity of 1.8 × 10−7RIU.
Fig. 7 Measured phase change curves of sucrose concentration test for analyzer rotation angle of (a) α = −50°, −55°, −60° as well as (b) α = −65° and −70°.
4. Conclusion
The tunable phase detection sensitivity of a transmitted-type GMR sensor using an electro-optic heterodyne interferometer is proposed and experimentally measured. This system only requires a rotation stage; the photo-detector and analyzer can be stationary. The GMR grating structure is successfully fabricated by a low-cost nanoimprinting sol-gel process. The detection sensitivity can be tuned by rotating the analyzer angle. The phase curves of both reflected- and transmitted-type GMR sensors are experimentally compared. The results show the transmitted-type GMR sensor is more convenient for tuning the sensitivity by rotating the analyzer angle. In our experiment, the best sensitivity that can be achieved is 1.8 × 10−7 RIU.
Acknowledgments
The authors gratefully acknowledge the financial support provided by the National Science Council, Taiwan, under Grant No. NSC101-2221-E-150- 007-MY3.
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