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We investigate the spectroscopic response of Rayleigh anomaly in a dielectric grating before and after it is coated with a layer of aluminum. The metallic coating enabled effective prohibition of the diffractions into the substrate and suppressed the background of the optical extinction spectrum of Rayleigh anomaly. This enhanced significantly both the contrast and the amplitude of the sensor signal based on the spectral shift of Rayleigh-anomaly. Sensor measurements were performed on the glucose/water solutions with different concentrations, which show improved performance due to the enhancement of the spectral contrast.
© 2016 Optical Society of America
1. Introduction
Sensors based on metallic and dielectric nanostructures have attracted extensive research interests due to their rich physics and potentially practical applications in biology. Photonic and plasmonic resonance modes in these structures laid basis for exploring new sensing mechanisms. Diffraction anomalies have been observed by Wood in 1902 and described theoretically by Rayleigh in 1907 [1, 2 ]. The Rayleigh anomaly (RA) is generally observed as a sharp peak in the optical extinction spectrum, which corresponds to a diffraction tangential to the grating surface, leading to an abrupt change of the diffraction efficiency of a metallic grating [3, 4 ]. Apparently, the spectrum of RA may be tuned by changing the angle of incidence. Moreover, it is sensitive to the change in the environmental refractive index. Therefore, the RA feature of a diffraction grating may be utilized to achieve index sensors [5–13 ] and tunable optical filters [14,15 ].
Rayleigh anomaly has been observed in both transparent dielectric gratings [16] and nontransparent metallic gratings [4, 6, 8 ], which can be characterized by either transmissive or reflective spectroscopy, respectively. However, such diffraction processes behave with different features in these two types of gratings and supply us different functions in practical applications [9–13, 17 ]. Furthermore, due to its straightforward photophysical principles, RA has also been easily functionalized in integrated photonic devices to achieved miniaturized sensors [9,11 ]. Further advantages of index sensors based on RA may cover the following aspects: high sensitivity determined only by the period of the grating, excellently linear response of the spectral position to the change in the environmental refractive index, narrow bandwidth and large operation spectral range, sensitive response almost solely to the environmental change on the top surface of the grating, and easy achievement of the resonance mode by convenient tuning of the structural parameters of a grating.
Compared with sensor performances based on localized surface plasmon resonance (LSPR) in metallic nanostructures, the RA sensor is more sensitive to the change in the environmental refractive index if the sensitivity is characterized by the shift of the resonance spectrum. This is because LSPR has a broader resonance spectrum and generally a smaller spectral shift than RA, which leads to a lower figure-of-merit (FOM) value of the sensor device. However, RA is not suitable for the detection of molecules with low concentrations according to its basic principles. In contrast, LSPR can be utilized to detect biological or chemical molecules with very low concentrations through binding the molecules to the metal surface. Thus, these two categories of sensor devices are suitable for different applications. Furthermore, when RA is produced in a metallic grating, localized surface plasmon resonance may be introduced simultaneously, so that a dual-band or RA-LSPR-coupled sensor may be achieved [12,13, 21 ]. Such a mechanism cannot only improve the sensitivity of the sensor device, but may also compensate the RA sensors’ unsuitability for biomolecular detection. This is an important advantage of RA sensors using metallic gratings. Some recent reports on gold gratings deposited on fiber ends [11], double-layer gratings consisting of gold stripes for label-free detection of molecular binding events [12], and gold-mushroom-pillar arrays for refractive index sensing with high FOM values [13] demonstrated very valuable performances of metallic gratings for sensor applications.
In this paper, we investigate the mechanisms responsible for the strong background of the RA spectrum in transparent dielectric gratings. Multifold diffractions into both the transmission and reflection space are found to be the main processes that contribute to the unexpected enhancement of the spectral background. By coating the dielectric grating with aluminum, we succeeded in suppressing these diffraction processes into the transmission space. As a result, both the contrast and the intensity of the sensor signal based on RA spectrum were enhanced significantly, enabling excellent performance in sensing the refractive-index change in the glucose/water solutions with different concentrations.
2. Design and fabrication of the sensor device
The metallic gratings for sensor applications were achieved simply by depositing a layer of aluminum on the top of a dielectric grating produced by interference lithography. The fabrication procedures are illustrated schematically in Fig. 1 . Interference lithography was employed to fabricate a photoresist (PR) grating on a silica substrate coated with 200-nm-thick indium-tin-oxide (ITO), where a He-Cd laser at 325 nm is used as the UV light source and photoresist S1805 is used as the recording medium. Through adjusting the separation angle between the two UV laser beams, the exposure time, and the development processes, we were able to control structural parameters of the PR grating, as shown in Fig. 1(a). These parameters generally include period, modulation depth, and duty cycle of the grating structures. In this work, we tried to reduce the duty cycle (width of the grating lines over the grating period) of the grating, which favors production of aluminum nanolines with a small duty cycle and steep nanolines as shown in Fig. 1(b). Then, a 60-nm-thick Al layer is thermally evaporated on the surface of the photoresist grating at a speed of about 0.65 nm/sec. Thereby, the aluminum-coated grating is achieved as shown in Fig. 1(c). As the edge of the grating is steep, the Al film deposited on the ridges and grooves of the grating are thicker than the edge of the nanolines.
Fig. 1 (a)-(c) Fabrication of the aluminum (Al) coated photoresist (PR) grating: (a) Interference lithography using UV laser beams at 325 nm with a separation angle of α; (b) The fabricated PR grating on a silica substrate coated with a layer of indium tin oxide (ITO); (c) Aluminum deposition using thermal evaporation. (d) and (e): SEM and AFM images of the Al coated grating. Inset: an enlarged image for local view.
Figure 1(d) shows the scanning electron microscopic (SEM) image of the Al coated grating. The period of the grating is about 480 nm and the duty cycle is as small as 30%. The inset of Fig. 1(d) shows an enlarged view for a close look at a local area. The coating of Al film is found to be composed of Al nanoparticles over the whole surface of the grating. However, much larger Al particles can be observed on the top of the grating lines than on the edges and on the grating grooves, which are aggregated more tightly to form continuous Al nanolines. Figure 1(e) is the atomic force microscopic (AFM) image of the Al coated grating, showing a modulation depth of about 175 nm.
In this work, we focus on the photophysical response of the diffraction anomaly of such metallic grating structures and the mechanisms for improving the performance for refractive-index sensor applications.
3. Photophysics and contrast enhancement for Rayleigh anomaly as the sensor signal
3.1. Principles for the contrast enhancement of Rayleigh anomaly
Figures 2(a) and 2(b) shows the principles how the metal-coated grating enhances the photophysical response and the contrast of the Rayleigh-anomaly signal by comparing it with a transparent grating consisting of photoresist nanolines on a similar ITO glass substrate. For a transparent grating, as shown in Fig. 2(a), the first-order diffraction processes include those into the air ( + 1 and −1 orders) with a diffraction angle of θd at a wavelength of λ and into the substrate ( + 1' and −1' orders) with a diffraction angle of θ'd at a wavelength of λ', which are defined by the following two equations, respectively:
andwhere Λ is grating period, θi is the angle of incidence, n0 and nS are refractive indices of the environmental medium and the substrate made of silica, respectively.Fig. 2 (a) and (b): Schematic illustration of the diffraction processes in a transparent and a metallic coated grating, respectively. (c) The calculated optical electric field distribution for TE and TM polarizations at a wavelength of 614 nm. (d) The simulated optical extinction spectra of aluminum coated grating for TM (red) and TE (black) polarizations.
The Rayleigh anomaly corresponds to θd = 90° or λ = n0Λ(1 ± sinθi). A similar signal can be observed in the spectroscopic response at θ'd = sin−1(1/nS), which corresponds to the diffraction into the substrate and totally reflected by the substrate/environment interface also with λ = Λ(1 ± sinθi) when n0 = 1. It is clear that the diffractions into the substrates, which are defined as + 1' and −1' orders of diffraction, induce strong optical extinction to both the transmission and the reflection light beams. As a result, strong background is observed in the optical extinction spectrum. Thus, the spectral contrast of Rayleigh anomaly is largely reduced, implying large reduction in the signal contrast of the sensor device.
However, if the transparent grating is covered with a metal film, for example, it is deposited by a layer of aluminum, above problem may be overcome by prohibiting the diffractions into the substrate ( + 1' and −1' orders), as indicated by the dashed arrows in Fig. 2(b). Meanwhile, the + 1 and −1 orders of diffraction into air or into the environmental medium, as well as the reflection, are enhanced dramatically. In addition, benefited from localized surface plasmon resonance of the Al nanolines, the metalized grating possesses higher diffraction efficiency for the TM than the TE polarization (perpendicular and parallel to the grating lines, respectively) [18–20 ]. Such LSPR-enhanced optical response is not available in dielectric transparent grating. This can be verified by some theoretical analysis, as shown by the simulation results using finite difference time domain (FDTD) in Figs. 2(c) and 2(d). Figure 2(d) shows the optical extinction spectra for TM (red) and TE (black) polarizations in the reflection space. Figure 2(c) shows the optical electric field distribution at 618 nm, corresponding to the spectral peaks of the Rayleigh anomaly in Fig. 2(d). All simulations have been based on an incident angle of 16° and a grating period of 485 nm. For TM polarization, both the local field and the propagation mode of RA on the top surface of the grating have been largely enhanced by coating with aluminum, as compared with TE. Such an enhancement is more clearly reflected in the spectroscopic response, where the red spectrum has much larger amplitude than the black at the peak wavelength of 618 nm. Additionally, the relatively broader spectral feature peaked at about 484 nm results from LSPR of the aluminum nanolines. Thus, these simulation data confirm that LSPR of the structured aluminum film made large contribution to the enhanced diffraction efficiency for TM polarization.
Furthermore, for optical sensor applications of Rayleigh anomaly excited in diffraction gratings, liquid medium needs to flow through or fill up the environmental space, where a chamber is required to hold the device and seal the liquid samples, as depicted in our previous publications [22] and [23]. Such a design facilitates stable flow of the target solution over the grating structures with a homogeneous thickness, which will not introduce disturbance to the reflected and transmitted light beams. Filling the chamber with liquids reduces largely the difference between n0 and nS, leading to a dramatic reduction in the grating diffraction efficiency and consequently in the strength of the sensor signal defined by the spectral amplitude of Rayleigh anomaly. However, coating the dielectric grating with a metal film, we may maintain the diffraction efficiency, even if the grating is immersed in liquids. Therefore, the metal-coated grating structures shown in Fig. 2(b) may enable high-contrast and high-efficiency sensor devices based on diffraction anomaly. Thus, all of our above discussions and the following spectroscopic analysis are based on the comparisons of the metalized with the transparent grating structures. This intends to verify the necessity and importance for coating the transparent photoresist grating with aluminum.
3.2 Spectroscopic response
Before we show the sensor performance in section 4, we first compare the spectroscopic response of the grating before and after it was coated with aluminum for understanding the principles how the intensity and contrast of the RA spectrum were enhanced. Figure 3 shows the enhancement of the spectral contrast of Rayleigh anomaly through coating the grating by aluminum. The sample shown in Fig. 1 was used to measure the optical extinction spectra at an incident angle of 16°. Figures 3(a) and 3(b) correspond to the optical extinction spectra before and after the PR grating was coated with aluminum film, respectively. All of the spectra in this work were collected by a fiber spectrometer USB4000 Pro from Ocean Optics. A tungsten halogen lamp HL2000 from Ocean Optics was used as the white light source. The reflective optical extinction spectrum was calculated by -log10[I(λ)/I0(λ)], where I(λ) is the reflected spectrum by the grating and I0(λ) is that by the substrate.
Fig. 3 (a) and (b): the optical extinction spectra measured on the gratings shown in Fig. 1 before and after metallic coating, respectively, at an incident angle of 16°. The upward black and red arrows denote the spectral positions of the Rayleigh anomaly in air and in water respectively. The thick yellow arrows show the enhancement of the spectral contrast of Rayleigh anomaly through coating the grating by aluminum.
The black curves show the optical extinction spectra measured in air, where the upward black arrows indicate the Rayleigh anomaly features, which are located both at about 618 nm in both Figs. 3(a) and 3(b). In Fig. 3(a) the peak amplitude of the Rayleigh anomaly spectrum is as large as PB = 0.42, which is overlapped with a strong background in the spectral range from 400 to 600 nm. We define the contrast of the signal of Rayleigh anomaly by ϕB = ΔB/PB, where ΔB is defined in Fig. 3(a) and ϕB is measured to be about 32%. However, in Fig. 3(b) the Rayleigh anomaly is as large as PA = 0.75 at the peak, whereas, the background spectrum has been reduced to 0.15 at about 530 nm and ϕA is measured to be 72% as it is defined by ϕA = ΔA/PA. Thus, the measurement results in Fig. 3 verify that both the amplitude and the contrast of the spectrum of Rayleigh anomaly have been enhanced significantly simply after the photoresist grating was deposited with a layer of aluminum, so that we have PA≈1.8PB and ϕA = 2.25 ϕB.
Furthermore, we have also measured the spectroscopic response with the sample immersed in water. The red curves in Fig. 3(a) and 3(b) show the corresponding optical extinction spectra measured on the photoresist and aluminum-coated gratings, respectively. The upward red arrows indicate the red-shifted spectra of Rayleigh anomaly, which are located at about 775 nm in both Figs. 3(a) and 3(b), implying a spectral shift of Δλ = 775-618 = 157 nm. The response sensitivity can be calculated by Δλ/Δn≈157 nm/0.33 = 478 nm/RIU, which agrees very well with the grating periods and verifies the spectroscopic response of Rayleigh anomaly. Although both structures show similar sensitivity in the response to the change in the environmental refractive index, the signal intensity of Rayleigh anomaly for the aluminum-coated grating is more than 7 times as large as that for the photoresist grating, which can be observed apparently in the comparison between Figs. 3(a) and 3(b). It should be noted that when the photoresist transparent grating was immersed from air into water, the amplitude of the RA spectrum was reduced dramatically, as shown in Fig. 3(a). This can be understood by considering that immersing the device into water implies large increase in the environmental refractive index from 1 to about 1.333. This is equivalent to the reduction in the modulation depth of the grating, since the refractive-index difference between the grating and the environment was then much reduced. As a result, the diffraction efficiency of the grating and consequently the sensor signal determined by the RA spectrum were largely reduced.
Additionally, in Fig. 3(b) a spectral feature is observed at about 420 nm in the black spectrum, which shifts to longer than 500 nm in the red spectrum due to the modification by the water environment. This feature is actually a result of localized surface plasmon resonance of the continuous aluminum nanolines on the top of the photoresist grating, which has been investigated in our previous work [21].
4. Refractive-index sensing performance of the aluminum coated grating structures
Figure 4(a) shows schematically the geometry for the sensor measurement, where TM-polarized white-light beam is incident at an angle of θi and the environmental medium with refractive index n0 has been changed from air (n0 = 1) to water (n0 = 1.33) for comparison. The reflective optical extinction spectra have been measured at θi = 16, 18, and 20°. Two sets of measurement results with the device located in air (dashed) and in water (solid) are presented in Fig. 4(b). LSPR and Rayleigh anomaly features are indicated by arrows. According to Fig. 4(b), the best sensor performance with highest contrast and largest signal intensity was obtained at θi = 18°. The peak of the RA spectrum shifts from 628 to 793 nm as n0 was changed from 1 to 1.33, corresponding to a sensitivity value of 500 nm/RIU.
Fig. 4 Refractive-index sensing performance of the aluminum-coated grating when its environmental medium is changed from air to water at different angles of incidence. The black, red, and brown colors correspond to incident angles of 16 o, 18 o, and 20°, respectively. The dashed and solid curves correspond to the air and water environments. The spectral positions of Rayleigh anomaly and localized surface plasmon resonance of the aluminum nanolines in different environmental media are marked out by arrows.
The sensor measurements were performed on the glucose/water solutions with different concentrations 0%, 1%, 3%, 7%, to 10%, where a concentration of 0% implies the detection of pure water. The final sensor signal spectrum was calculated by σ(λ) = –log10[S(λ)/S0(λ)], where S(λ) and S0(λ) are reflection spectra with the chamber enclosing the sensor device filled with glucose/water solution and with pure water (0% concentration), respectively. The sensor measurement system is the same as that depicted in [22, 23 ]. All measurements have been performed at an incident angle of 18° for the white-light beam. Figure 5 shows plots of the sensor signal spectrum defined by σ(λ) at different concentrations of the glucose/water solutions, which is featured with a sharp dip at about 783 nm and a peak at 815 nm for the spectroscopic response of the Rayleigh anomaly. The amplitude of the sensor signal defined by the peak-to-dip difference of the optical extinction spectrum was measured to be 0.0265, 0.052, 0.1107, and 0.161 for a concentration of 1%, 3%, 7%, and 10%, respectively, which is plotted as a function of the concentration of the glucose/water concentration in the inset of Fig. 5. The sensor device shows excellent linear response. Furthermore, comparing these features with those distributed in the spectral range from 450 to 650 nm, which resulted from the LSPR of the aluminum nanolines, the sensor signals in the spectral range from 750 to 850 nm exhibit much enhancement both in the contrast and in the intensity. In particular, the enhanced sensor signal is located at the rising edge of the RA spectrum, which can be found by comparing Fig. 5 with Fig. 4(b) and indicated by the red arrows in Fig. 5. This verifies convincingly that the reduction of the background at shorter wavelengths than the RA peak or the enhancement of the contrast through suppressing the redundant diffraction processes has been utilized to enhance the sensitivity of the sensor device.
Fig. 5 Sensor measurements on glucose/water solutions with concentrations increased from 0% to 10%. The extinction spectra were calculated using the reflection spectrum through pure water (0% concentration) as the blank. Inset: amplitude of the sensor signal as a function of the concentration of the glucose/water signal, which is defined by the peak-to-valley difference of the extinction spectrum.
5. Conclusions
By coating a photoresist grating with a layer of aluminum, we achieved enhancement of the intensity and the contrast of the Rayleigh anomaly spectrum through prohibiting the diffractions into the substrate in transparent structures, where the spectral background was largely reduced and the large modulation depth of the grating structure was sustained. This enables significant improvement in the performance of the corresponding sensor device based on the spectroscopic response of Rayleigh anomaly.
Acknowledgments
We acknowledge the 973 program (2013CB922404), the National Natural Science Foundation of China (11274031, 11434016), and the Beijing Key Lab of Microstructure and Property of Advanced Materials for the support.
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