1. Introduction

Refractive index sensing has received considerable recent attentions for their applications in biological and chemical fields [1–3]. Particularly, refractive index sensing can be used for label-free monitoring of bio-molecular interactions on surfaces [4–6], which does not require the sample to be marked with fluorescent dyes but rather relies on the detection of tiny refractive index changes due to bonding events, e.g., with antibodies or antigenes. Miniaturization of refractive index sensors is of particular interest for realizing ultracompact lab-on-a-chip applications with dense arrays of functionalized spots for multiplexed sensing, that may lead to portable, low cost and low power devices [7, 8]. Among various technologies to implement the on-chip index sensing functionality, planar waveguide devices have an inherent advantage of being convenient to realize miniaturization [9–11].

Planar optical waveguides are comprised of an optically transparent guiding layer with a refractive index that is higher than the substrate layers. By careful selection of the guiding material, optical sensors based on planar waveguides are considered to be a very promising technique due to its high sensitivity and its potential for integration with sample delivery and detection systems to achieve simultaneous detection of complex environmental and medical samples. Among them, silicon photonics is a promising approach for miniaturization of integrated photonic circuits because, due to the high index contrast between silicon and air, light is effectively confined within the silicon core. High-refractive-index silicon nanophotonic sensor platforms in various extensions have been developed, with the advantages of high sensitivity and measurement in the presence of the sample without any rinsing [12–14]. Recently, we have proposed an optical index sensor by integrating a circular-hole defect with an etched diffraction grating (EDG) spectrometer based on amorphous silicon photonic platforms [15, 16]. In the present paper, we show that much higher refractive index sensitivity can be obtained by using two air circular-holes with appropriate interval. In addition, some efforts have been made to characterize the index sensitivities of various intervals between the two circular-holes.

2. Fabrication and measurement

Figure 1 presents some schematic illustrations and some pictures of the fabricated device of the current refractive index sensor by integrating two air circular-holes with an EDG spectrometer. As shown, the proposed chip consists of a fiber coupler, an EDG, a sensing area with two air circular-holes and arrayed photodetectors heterogeneously integrated on top of the output silicon waveguides. Light is coupled into the chip from the fiber coupler. An input single mode fiber is aligned and glued on the top of the fiber coupler, using UV-curable glue. An amplified spontaneous emission (ASE) source gives a broadband unpolarized light. This unpolarized light is butt-coupled to the single mode fiber through a focusing gradient index lens. The light is guided through the input waveguide and crosses the sensing area with two air circular-holes. Next, the light is guided to the EDG spectrometer which separates and focuses the light into different wavelength channels. Finally, InGaAs photodetectors can be integrated on top of the output waveguides to measure the optical power in the corresponding channel. The chip is protected by an area with SU-8, a photo-definable polymer and a rectangular contact window is opened on top of the sensing area. When a measurement is carried out, the analyte will directly be dropped into the sensing box.

 

Fig. 1 The Schematic configuration and fabricated pictures of proposed optical sensor by integrating two air circular-holes with an EDG spectrometer.

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The EDG spectrometer based on a Rowland mounting is illustrated in Fig. 1. The field propagating from an input waveguide to the free propagation region is diffracted by each grating facet. It is then refocused onto an imaging curve and guided into the corresponding output waveguides according to the wavelengths. To improve the diffraction efficiency, we use the total internal reflection facets for each grating groove (see the photograph at the lower-right corner of Fig. 1).

In the present paper, two sensing chips, with one and two circular-holes respectively, were fabricated, while other parameters were the same for both devices: the central wavelength is 1562 nm; the refractive indexes of silica buffer layer and α-Si:H core layer are 1.46 and 3.58, respectively; the incident angle is 45 degrees; the diffraction order is 10; the channel number is 33; the wavelength interval is 0.5 nm; and the interval of output waveguides is 1μm. For the chip design, the thickness of the silicon nanowire waveguide is fixed as h = 250nm, and the width w = 500nm is chosen, which lies in the single mode region of the waveguide.

As the beginning step of the whole device fabrication, a 5 μm silica buffer layer and a 250 nm α-Si:H core layer are successively deposited on a silicon wafer. Then a process of pattern generation will be carried out using lithography technology with high resolution and accuracy. In this paper, the electron beam lithography based on negative resists is employed due to its low running cost and ability to push the resolution further down to 50nm. In Fig. 1, the convex structure is the silicon core layer, and the concave part is the silica buffer (i.e., the corresponding silicon core was etched). The two circular-hole defects in the sensing chip (see Fig. 1) were fabricated based on the thermal annealing at temperatures above deposition temperatures. The more detailed fabrication and measurement process was previously described by us [15, 16].

To verify the validity of the sensing chips, the refractive index of the mixture of sunflower oil methyl ester (SuME) and diesel oil was measured. The pure SuME and diesel have a refractive index of 1.455 and 1.46, respectively. Therefore, the index of the mixture sample is on the rise as the ratio of the diesel increases. When the sensing area in Fig. 1 was filled with the pure diesel oil, the scattering loss at different wavelengths for both chips was measured (see Fig. 2). Note that, for the characterization of the effect from the circular-hole defects, another device without any defect in the spectrometer using the same parameters was also measured, and then the transmission loss in Fig. 2 was normalized by subtracting both measured results. In this way, the normalized spectrum removed the waveguide propagation and the grating diffraction losses, and the resonant scattering loss from the analyte was more clearly illustrated. We have measured the thermo-optic coefficient of the EDG spectrometer based on the α-Si:H platform, and a ~90 pm/°C (dλ/dT) red shift of the output wavelength can be observed as the temperature increases. In the present paper, all measurements are carried out in a superclean lab with a 25°C stable room temperature.

 

Fig. 2 Measured normalized transmission loss at 33 channels for two different SuME concentrations in a diesel oil mixture for the chip using only one circular-hole defect (a) and using two circular-holes with 1.22 μm interval (b). The resonant scattering wavelength varies as the blend level of the SuME in a diesel oil mixture increases for both chips (c).

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Figures 2(a) and 2(b) show the detected transmission loss at 33 channels of the EDG spectrometer for the two different blend levels using the chip with only one (~0.9568 μm diameter) and two circular-hole defects (~0.9872 and 0.9795 μm diameters, and ~1.22 μm interval), respectively. As can be observed from Figs. 2(a) and 2(b), the resonant scattering peak linearly shifts towards the short wavelengths for both chips as the blend level of the SuME increases. However, the chip using two air circular-holes with appropriate interval can contribute to a narrower scattering peak, a larger peak loss, and a larger wavelength shift for the same change of the blend level. Figure 2(c) shows that the normalized resonant scattering wavelength varies as the blend level increases for both chips. From this figure, one can see that the spectral position of the resonant scattering peak shifts linearly with the blend level. Based on the measured refractive index of the pure SuME and diesel, one can easily obtain the sensitivity of both sensing chips. The results show that the chip with double holes can give much higher sensitivity (~17422 nm/RIU) than that with only one hole (~9653 nm/RIU). When the transmission spectrum of the device with two holes is measured with a commercial optical spectrum analyzer (OSA), we obtain the standard deviation of the noise on the signal to ~32 pm by averaging it with a 3-minute window, which corresponds to a detection limit of the device as high as ~1.84 × 10⁻⁶ RIU. The value of the device with only one hole is equal to 3.8 × 10⁻⁶ RIU [15].

3. Numerical result and discussion

A fast analysis method has previously been proposed by us, to analyze the effect of circular-hole defects on sensing applications, using modified Green's tensor, which is a solution to a point source of the wave equation [15]. Based on the same method, the influence of the interval between two air circular-holes on the sensing application can be characterized. The light in Fig. 1 is incident along the line between two centres of two holes. Therefore, in the numerical model, the symmetry axis of the input waveguide is overlapped with the two centres of the two circular-holes. Figure 3 shows the normalized resonant scattering loss for the chip using two air circular-holes with 1μm diameter versus the incident wavelength and the interval when the sample with a 1.46 refractive index was filled with. From the figure, one sees that a remarkable oscillation of the loss-peak happens as the interval increases. The light with some special wavelengthw can induce resonant scattering with both circular-hole defects. The final multiple resonance behavior in Fig. 3 might result from the competition between the two resonant mechanisms. Physically, we can consider the multiple resonant scattering between two holes as a special optical bistable phenomenon so that a slight change of the interval can considerably influence the resonance spectrum. In addition, both the position and the spectral width of the resonant scattering peak slightly vary versus the interval. This figure also shows that there occur more resonant peaks for the same defects in the calculated wavelength range, which means that the sensing chip has a limited free spectral range.

 

Fig. 3 The normalized scattering loss for two circular-hole defects with 1 μm diameter versus the incident wavelength and the interval between their centers. The refractive index of the filled analyte is 1.46.

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Using the above system, we also calculated the full width at half maximum (FWHM) at different intervals and the corresponding sensitivity for sensing applications (see Fig. 4). A clear sinusoidal pattern with a period approximately equaling to the twice defect diameter can be seen from this figure as the interval increases corresponding to constructive and destructive interference. As expected, the smaller the FWHM (i.e., the higher the Q-factor) is, the higher the sensitivity can be obtained for the sensing applications. For the present defect diameter, the highest sensitivity can attain ~17865 nm/RIU. However, since the resonant peak is not stable for the present double-hole system (see Fig. 3), the present sensing chip has an extreme tolerable requirement of the interval between two holes. A slight shift of the length of the interval from the designed value may make the resonant peak more difficult to be detected than the chip using only one circular-hole, although the double-hole chip can contribute to a higher sensitivity.

 

Fig. 4 The Calculated FWHW and sensitivity vary as the interval between two circular-holes with 1 μm diameter increases.

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4. Conclusions

It was shown that the sensing chip using two circular-hole defects with appropriate interval integrated with an EDG spectrometer is much more sensitive to the refractive index change of filled samples compare with that using only one defect. The property of the interval between the two defects was effectively characterized for the sensing applications. It was demonstrated that a clear oscillation of the resonant peak occurs as the interval increases, which means the resonant behavior is very sensitive to the interval. The numerical results showed that the curve of the sensitivity versus the interval exhibits a clear sinusoidal pattern with a period approximately equaling to the twice defect diameter. By using the best interval between two air circular holes with 1μm diameter, one can obtain sensitivity up to 17865 nm/RIU in theory. However, the detectability of the resonant peaks for the double-hole chip is sometimes worse than that for one-hole chip in terms of the interval due to the erratic multiple resonances.

The present planar chip is suitable for a wide range of optical sensing applications, and is also superior to a conventional index sensor especially for a low refractive index change.