A high sensitive sensor is demonstrated by exploiting strong transverse magneto-optical Kerr effect on a ferromagnetic surface plasmon grating. The surface plasmon grating, made of a hybridized Au/Fe/Au layer, exhibits a very dispersive Kerr parameter variation near the surface plasmon polariton (SPP) wavelength via coherent scattering of the SPP on the grating structure. Interrogating this Kerr parameter can be utilized for detecting chemical or biological objects in a fluid medium. The experiment results show the minimal detectable mass concentration of sodium chloride in a saline solution is 4.27 × 10-3 %, corresponding to a refractive index change of 7.60 × 10−6 RIU. For an avidin-biotin interaction experiment, the sensitivity of avidin detection in PBS solution is 1.97 nM, which is limited by the index fluctuation of flowing media during measurement.
© 2014 Optical Society of America
SPPs are coupled oscillation of electromagnetic waves and plasma at the dielectric and metallic interface, which is widely applied in nanophotonic circuits , nano-amplifiers/lasers  and biosensors . Because SPPs are concentrated near the metallic surface with a large propagation constant, many active functions such as nonlinear optics , electro-optics  and magneto-optics (for both the Faraday effect and the Kerr effect)  are enhanced, by comparing the same effects presented in pure dielectric photonic devices. Recently, SPP-enhanced magneto-optical effects receive a lot of attention. They are utilized for implementing ultra-compact isolators  or circulators and are explored for scientific studies on the nonreciprocal optical activity . In this paper, we investigate the SPP-enhanced transverse magneto-optical Kerr effect (TMOKE) on a hybridized ferromagnetic plasmonic grating and utilize this effect to demonstrate a very sensitive sensor.
TMOKE is a type of magneto-optic (MO) effects characterizing the complex reflectivity from a surface subject to a magnetic field applied perpendicularly to the incident plane. TMOKE enhanced by SPPs on smooth or perforated ferromagnetic metals such as cobalt, nickel or iron has been studied. However, the imaginary part of permittivity among these metals is very large, leading to significant propagation loss of SPPs. On the other hand, the noble metals like gold or silver are ideal media to carry SPPs but the MO effect is very weak. Huge external magnetic field (typically exceeding several tesla) is required to present this effect. Although a nanostructure composed of a thin noble metal perforated with subwavelength slits on top of a smooth ferromagnetic dielectric was demonstrated with a strong MO effect  but low free carrier absorption loss, it is not suitable for a sensor application since only a small portion of optical field is overlapped with the detected medium. A hybridized grating sensor consisting of both the noble and ferromagnetic metals is proposed in this study. Previously, a TMOKE SPP biosensor on a Au/Fe/Au or Au/Co/Au planar layer  has been reported. Here, through coherent scattering of SPPs on the grating structure, the MO effect could be further magnified by exploiting slow light phenomenon near the Bragg resonant condition .
2. Structure and operation principle of the SPP TMOKE grating sensors
The device structure is shown in Fig. 1. The incident light is TM-polarized (p-wave) and illuminated on the grating with an incident angle θ. The grating structure, made of a hybridized layer Au/Fe/Au, is developed from the bottom SiO2 nano-slits through thin-film deposition. An in-plane magnetic field is applied with a direction perpendicular to the plane of incidence and parallel to the grating. The magnetization-dependent reflectivity is influenced by the ferromagnetic dielectric tensor on the Fe layer given by .
In addition to the ferromagnetic dielectric tensor, the coupling condition of the incident wave to the SPPs is also varied by the modulated magnetization. Consequently, the Kerr parameter could be increased if the SPPs are excited at the grating surface, as long as the x-component of the incident wave vector, as viewed from Fig. 1(a), complies with the following phase matching condition:
In Eq. (4), ks is the wave number of the SPPs, kIx is the propagation constant of the incident wave along the x-direction, Λ is the period of the grating and m is an integer number. Since the grating is a periodic structure, the SPPs behave like a Bloch wave so the magnetic field can be written by12].Fig. 2(a). The Kerr parameter, according to the definition in Eq. (3), could change swiftly across the SPP wavelength if the primary reflectance is near zero, the SPP coupling bandwidth is narrow, and the wavelength shift due to TMOKE is extensive. The dispersion curve of the Kerr parameter is sketched in Fig. 2(b).
One example of using Kerr parameter for bio-sensing is illustrated in Fig. 3. First, the Au-Fe-Au SPP grating is covered with antibodies immobilized on the surface. Then a specimen including antigens flows onto the chip, where the antigens are specifically interacted with the antibodies. In this case, the SPP wavelength varies, resulting in a shift of the dispersion curve of Kerr parameter, as displayed in Fig. 3(c). If we illuminate a laser beam with a fixed wavelength near the coupling condition to excite SPPs, the Kerr parameter varies dramatically with the captured antigens because of the very dispersive characteristic of Kerr parameter near the SPP wavelength. Conventional SPP grating sensors interrogate the change of reflectance, reflection spectrum or incident angle  with respect to surface adsorbate, while the TMOKE SPP grating sensor interrogates the Kerr parameter (Fig. 3(d)), which could be more sensitive. Moreover, since the Kerr parameter is in response to a modulated magnetic field, the signal-to-noise ratio increases if a lock-in measurement technique is applied.
To achieve high sensitivity, the dispersion curve of Kerr parameter has to be steep near the SPP wavelength. According to Eq. (3), the Au-Fe-Au grating should be designed with a strong TMOKE R(+M)-R(-M) but a small primary reflectivity. Here, we use the rigorous coupled-wave analysis (RCWA) method [14, 15] to analyze and optimize the dimension of the grating. The launched wavelength is assumed to be 773 nm with the TM polarization (p-wave), and the incident angle is 55.7ﾟ, which is the Brewster’s angle at the air/glass interface, since the laser beam has to pass through a glass slide on top of a packaged chip. This glass slide covers the chip to form a fluidic channel for delivering a liquid specimen. The optimized grating period and pitch width, determined from a pristine Au grating to have a minimal reflectance and a narrow SPP coupling band near 773 nm, are found to be 340 nm and 177 nm, respectively. Next, another structure analysis on the top Au and Fe thicknesses is carried out to maximize the extreme value of Kerr parameter variation. In general, a thick Fe layer near the surface is desired to have a large Kerr effect. However, strong absorption of Fe could impact light coupled to the SPPs, resulting in an increasing reflectance and a wide coupling wavelength band. The analyzed Kerr parameters versus different Au and Fe thicknesses are plotted in Fig. 4(b), where the optimized values for the Au and Fe thicknesses are 22.2 nm and 5.5 nm, respectively. The external magnetic field is assumed to be 50 mT. Under this condition, the Kerr parameter η has a maximal value of 18. For real devices, the Au and Fe thickness are chosen to be 22.3 nm and 8 nm, since thickness control of Fe deposition is less critical but a large Kerr effect is still maintained. In fact, the SPP wavelength and the reflectance are also influenced by the Au and Fe thicknesses accordingly, as displayed in Figs. 4(c) and 4(d). The color maps of wavelength and reflectance shown in these figures provide a guideline on fine tuning the launched wavelength in experiments.
3. Experimental results
The Au-Fe-Au grating was fabricated through a process described in Fig. 5. First, a SiO2 layer of 100 nm was grown by wet oxidation, patterned by e-beam lithography and subsequently etched to create periodic sub-wavelength slits on silicon. These slits were the mold to form the zigzag SPP grating. A thick Au film (100 nm) was deposited to cover the structure as the bottom layer to avoid light tunneling through. Then an 8 nm Fe and a 1 nm Pt layer were coated via magnetron sputtering. The Pt layer is to prevent Fe oxidation. Finally, a 22.3 nm Au film was stacked on top of the grating. The cross section of the fabricated Au-Fe-Au SPP grating is shown in Fig. 5. The sensing area is 25 mm2. The device was packaged by bonding a pre-defined polydimethylsiloxane (PDMS) fluidic channel top sealed by a glass slide.
The Kerr parameter was measured by illuminating a p-polarized tunable cw laser (wavelength varying from 765 to 781 nm) on the packaged device and recording the reflective light intensity. The device was filled with de-ionized (DI) water and the incident angle was adjusted to be near 55.7° with a minimal reflectance of 1.3%. A transverse AC magnetic field with an intensity of 50 mT, modulated at a frequency of 40 Hz, was applied on the device. The orientation of the grating, incident wave and magnetic field was configured according to Fig. 1(a). A locking-in amplifier was utilized to obtain the Kerr parameter and suppress the background and photodetector noise. The whole experimental setup is illustrated in Fig. 6. The measured Kerr parameter shows a similar dispersion curve as the simulated result except the value is about two orders of magnitude smaller. This discrepancy mainly results from a larger reflectance on the real packaged device. Despite the small value, a very steep variation of the Kerr parameter across the SPP wavelength is still observed. The slope Δη/Δλ is about 1.92 × 10−2 nm−1, as displayed in Fig. 6.
Monitoring the Kerr parameter of the Au-Fe-Au SPP grating can be utilized to real-time detect substance dispersed in solution. We examined 4 saline solutions with different mass concentrations of sodium chloride (NaCl), which are 0.05%, 0.1%, 0.5% and 1%. The measured Kerr parameter is shown in Fig. 7(a). First, we injected DI water and adjusted the incident angle until a reference level of the Karr parameter near zero was achieved. The launched wavelength was 773 nm. Then we subsequently tested the saline solutions with different concentrations. During each test, DI water was introduced to purge the residual solution inside the channel. The Kerr parameter versus the mass concentration of NaCl (ranging from 0.05% to 1%) features a linear curve shown in Fig. 7(b). The noise level of the Kerr parameter measurement is around 3.87 × 10−4, corresponding to a sensitivity of 4.27 × 10-3 % to detect NaCl mass concentration. This noise is mainly caused by the index fluctuation of flowing media. By relating the refractive index of saline solution as a function of NaCl concentration reported in previous study , the minimal detectable index variation is 7.60 × 10−6 RIU, which is better than conventional SPR sensors.
Avidin-biotin interaction examined through interrogating the Kerr parameter of the Au-Fe-Au SPP grating was demonstrated. First, the grating surface was being covered by a phosphate buffered saline (PBS) solution containing 30-μM biotin-labelled bovine serum albumin (bBSA) overnight, to assure the grating surface was completely coated with bBSA. After washing the surface by PBS for 3 times to remove excess bBSA, we slightly adjusted the incident angle of the laser beam until the Kerr parameter varied linearly within a wavelength range (from 772.5 nm to 774.5 nm), as shown in Fig. 8(a). Then another PBS solution containing avidin (50 nM) was injected to the device with a constant flow rate of 0.5 mL/min. A dynamic response of the Kerr parameter is displayed in Fig. 8(b). The Kerr parameter increases and reaches to a steady value of 6.29 × 10−3 after 40 min. Such long reaction time is caused by slow diffusion of avidin in the PBS flow, which can be improved if a microfluidic device is integrated. The noise level of the Kerr parameter is estimated to be 2.37 × 10−4, corresponding to a sensitivity of 1.97 nM. In fact, the increment of Kerr parameter upon the avidin introduced to the sensor chip is linear and proportional to the concentration of avidin. To reduce the experiment time, we examined the speed of Kerr parameter variation rather than the final steady value to quantitatively characterize the concentration of avidin in the PBS solution. Three different concentrations (50 nM, 100 nM and 200 nM, respectively) of advidin dissolved in PBS were subsequently introduced into the chip, and the Kerr parameter was monitored and plotted in Fig. 8(c). Between each replacement, the PBS solution was injected to clean the channel. The speed of Kerr parameter variation with respect to the avidin concentration was extracted and displayed in Fig. 8(d).
We design and demonstrate a TMOKE SPP sensor made of a hybridized Au/Fe/Au grating structure. The MO effect is strong due to the nature of a large propagation constant of SPPs and is further enhanced through coherent scattering of SPPs on the grating structure. By optimizing the design of the grating dimension, strong TMOKE is observed and features steep Kerr parameter dispersion across the SPP wavelength. Monitoring the Kerr parameter can be utilized for real-time detecting chemical or biological objects in a fluidic medium. The minimal detectable index change is 7.60 × 10−6 RIU. For an avidin-biotin interaction experiment, the sensitivity of detecting avidin in a PBS solution is 1.97 nM. The sensitivity is mainly limited by the noise due to index fluctuation of flowing medium. Compared with conventional SPR sensors interrogating the reflectance or reflectance spectrum, the TMOKE SPP grating sensor interrogates the Kerr parameter without the requirement of a complex optical modulation setup or a spectrometer, which is potential to develop a compact, non-contact optical detection scheme.
This research work was financially supported by MOST (Project ID: 102-2218-E-007-001 & 102-2633-M-007-002) in Taiwan.
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