A nanostructured Fabry-Perot interferometer - OSA Publishing

A nanostructured Fabry-Perot interferometer Tianhua Zhang,1 Zhongcheng Gong,1 Rebecca Giorno,2 and Long Que1 1

Institute for Micromanufacturing, Louisiana Tech University, Ruston 71272, Louisiana, USA 2 School of Biological Sciences, Louisiana Tech University, Ruston 71272, Louisiana, USA [email protected]

Abstract: A polymer-based micromachined Fabry-Perot interferometer (µFPI) with embedded nanostructures in its cavity, called nanostructuredFPI, is reported. The nanostructures inside the cavity are a layer of Aucoated nanopores. As a refractive-index sensitive optical sensor, it offers the following advantages over a traditional µFPI for label-free biosensing applications, including increased sensing surface area, extended penetration depth of the excitation light and amplified optical transducing signals. For a nanostructured-FPI with nanopore size of 50 nm in diameter and the gap size of FPI cavity of 50 µm, measurements find that it has ~20 times improvement in free spectral range (FSR), ~2 times improvement in finesse and ~4 times improvement in contrast of optical transducing signals over a traditional µFPI even without any device performance optimization. Several chemicals have also been evaluated using this device. Fourier transform has been performed on the measured optical signals to facilitate the analysis of the transducing signals. ©2010 Optical Society of America OCIS codes: (130.6010) Sensors; (050.2230) Fabry-Perot; (230.3990) Micro-optical devices.

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Received 13 Jul 2010; revised 22 Aug 2010; accepted 31 Aug 2010; published 8 Sep 2010

13 September 2010 / Vol. 18, No. 19 / OPTICS EXPRESS 20282

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1. Introduction Being a very efficient and cost-effective technique for medical, environmental monitoring and biology research applications, label-free biosensing is becoming increasingly important [1]. There are three main transducing mechanisms for label-free biosensors, which include electrical, mechanical and optical responses [1]. Electrical transduction is realized by carbon nanotubes (CNTs) or nanowires (NWs) where the electrical conductance of the CNTs or NWs changes upon the binding between the receptors immobilized on them and the targets. Mechanical transduction is achieved by a MEMS cantilever where the binding between an antigentic target to an antibody immobilized on a cantilever surface, resulting in its bending and resonant frequency shifting due to the changes of surface stress in the cantilever. The major label-free techniques by optical means are propagation surface plasmon resonance (PSPR), Raman spectroscopy, localized SPR (L-SPR), liquid core ring resonator technology and photonic crystal nanostructures [2–6]. All of these technologies have shown excellent detection sensitivity and detection-of-limit (DoL). As a refractive-index sensitive optical sensor, the transducing signal of a Fabry-Perot interferometer (FPI) varies upon the changes of the effective refractive index in its cavity. This property makes a FPI an ideal technical platform for label-free biosensing. For the past decades, Fabry-Perot interferometers (FPIs) have been miniaturized for a variety of applications including chemical sensing, biosensing, gas sensing, ultrasonic sensing, optical communication, optical modulation, nanofluidics dynamics analysis and spectral endoscope optical imaging [7–21]. Usually micromachined FPIs (µFPIs) are fabricated from silicon, polysilicon, silicon nitride, silicon oxide thin film or other semiconductor materials and are often operated by a laser source. In order to further simplify the operational procedure and lower the cost, recently a white-light source operated polymer-based µFPI has been developed for biochemical sensing by our group [19–21]. The traditional µFPI shows very good sensitivity and performance for biochemical sensing applications. For instance, experiments found that DoL of a traditional µFPI sensor is between 50 ng/ml and 5 ng/ml for the rabbit Immunoglobulin G (IgG) with Protein A serving as probes [20]. However, for biochemical sensing, a traditional µFPI has the following intrinsic limitations [20]: (i) limited sensing area due to the small surface area of the planar plate of a µFPI, which means that available binding sites for biomolecules are limited; (ii) limited penetration depth of the excitation light in the sensing area, namely the optical sensitive range/area is essentially limited only on the surface of the planar plate of a µFPI; and (iii) limited intensity of the transducing signal due to the optical power losses of the reflected or transmitted light from the FPI at the interface of air and the FPI plates.

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In order to address these issues of a conventional µFPI as well as amplify its optical transducing signal and enhance its sensitivity, we report a novel polymer-based nanostructured-µFPI device with embedded gold-coated nanostructures in its cavity. 2. Device description, operation, design and fabrication A schematic of 2 × 2 nanostructured-µFPI devices on a single chip is illustrated in Fig. 1 (a). Each device consists of a PDMS plate, an Au-coated nanopore layer and a glass plate. The Au-coated nanopore layer is anchored inside the FPI cavity. The average size of nanopores before Au coating is ~50 nm in diameter and the gap size of the µFP cavity is 50 µm. Note that for some nanostructured-µFPI devices, the nanopore layer is not Au coated for comparison in some experiments. The embedded nanopores inside the FPI cavity are used for increasing the sensing surface area, namely increasing the binding sites for biomolecules. The sensing surface area includes the top, bottom surfaces of the nanopore layer and the sidewalls of the nanopores. The thickness of the nanopore layer is 3 µm, therefore, the penetration depth of the light can be 3 µm. In other words, the optical sensitive range has been extended to 3 µm from the top surface of the nanopore layer. The Au thin film that is coated on the nanopore layer and the sidewalls of the nanopores is used for enhancing the optical signal intensity.

Fig. 1. (a) Schematic of 2 × 2 nanostructured-FPI devices with embedded Au-coated nanopores inside their FPI cavities; (b) Cross-sectional sketch of a nanostructured-FPI and its operational principle: the reflected light is monitored as the transducing signal.

The basic operation principle of the device is described in Fig. 1 (b). Upon entering the cavity and the nanopores between the two plates, the broadband light source undergoes multiple internal reflections and interferences inside the cavity and nanopores. As a result, modulated transducing signals will be generated as transmitted or reflected interference fringes from the nanostructured-FPI. For this device, the reflected signals serve as the sensing signals. If the effective refractive index inside the FPI cavity and nanpores changes due to the presence of different chemicals or biochemicals, the reflected interference fringes shift. The fabrication procedure of the device is described as follows. The nanopore structures, anodic aluminum oxide (AAO), are fabricated using a standard two-step anodization process [25]. Thereafter, a PDMS microfluidic chip is bound with the nanopore layer. The detailed process flow is shown in Fig. 2. For comparison, the nanopore layers of some devices are coated with Au thin film and some are not. More specifically, the fabricated AAO layer has arrayed pores with size of ~50 nm in diameter. 5Å Cr was deposited onto AAO layer as an adhesive layer before the Au thin film was deposited by sputtering. Au thicknesses of 20 Å, 50 Å, 100 Å are used in our experiments. A SEM image of the AAO layer is given in Fig. 3(a). Second, a 50 µm thick SU8 mold of the device is formed on a silicon substrate using optical lithography. PDMS is casted on the mold, followed by 1.5 hour curing at the temperature of 65 °C. Finally, the PDMS layer and the nanopore-layer are bonded together to complete the device fabrication after oxygen plasma treatment. The input and output wells are

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made in the PDMS layer thereafter for the delivery of the chemical samples to the nanostructured-FPI devices. The optical micrograph of a device is given in Fig. 3 (b).

Fig. 2. Fabrication process flow of the nanostructured FPI device (a) start from polished Aluminum; (b) nanopore (AAO) formed using 2-step anodization process [25]; (c) gold film sputtered on AAO; (d)-(e) PDMS microfluidic channel formation using SU8 mold with soft lithography process; (f) bind PDMS microfluidic channel with the gold thin film coated AAO.

For comparison, close-up SEM images of a bare AAO layer and an Au-coated AAO layer are given in Fig. 3(c) and Fig. 3(d), respectively. As shown, the Au thin film on AAO layer clearly consists of Au nanoparticles with size in the range of 10-20 nm. It should be noted that depending on specific applications, the nanopore size can be readily tuned during the fabrication process from several ten nanometers to several hundred nanometers (e.g., 100-200 nm) in diameter [25]. Hence the nanopores are big enough and can provide sufficient volumes and surface areas for biomolecular interaction inside them.

Fig. 3. (a) Scanning electron microscopy (SEM) image of the nanopore structures inside the FPI cavity; (b) Photo of a fabricated nanostructured FPI sensor shown with a 5-cent coin; (c) close-up SEM image of a bare AAO layer; (d) close-up SEM image of a AAO layer coated with 50 Å Au thin film. Its surface, which consists of Au nano-particles with typical size of 1020 nm, is much rougher than that of a bare AAO layer. Insets of (c-d) are low magnification SEM images of AAO substrate with arrayed nanopores.

The testing setup is based on a fiber-optic spectrometer, similar to what we reported before [19–21]. The custom-designed optical fiber probe delivers the white light to the device and also receives the reflected transducing signals from the nanostructured-FPI device, which are then coupled to a spectrometer.

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3. Experimental results and discussions Figure 4 gives the measured reflected optical signals from a traditional µFPI with an Aucoated planar plate, a nanostructured-FPI without Au thin film coating and a nanostructuredFPI with Au thin film coating. It shows clearly that the output signal intensity, sensitivity and contrast can be amplified for a nanostructured-FPI device with Au thin film coating. The measured average free spectral range (FSR) of a traditional µFPI with 50 µm cavity gap size is about ~3 nm with a finesse of ~1.4, while the average FSR of a nanostructured-FPI with an Au-coated nanopore layer is ~58 nm with a finesse of ~2.8. The peak contrast (visibility) of a traditional µFPI is ~0.14, while the peak contrast (visibility) of a nanostructured-FPI with an Au-coated nanopore layer is ~0.52 and that of a nanostructure-FPI without Au thin film coating is ~0.10. Note that experiments find that Au-coated planar plate of a traditional µFPI does increase the intensity of the reflected signals, but the FSR, finesse and visibility of its transducing signals remain essentially unchanged compared to the one without Au coating. All these figures-of-merit (FOM) indicate that the performance of a nanostructured-FPI with an Au-coated nanopore layer is much better than that of a traditional µFPI and a nanostructured-FPI without an Au-coated nanopore layer. A rigorous model (e.g., a model based on finite-difference time-domain (FDTD) analysis) is needed for understanding the mechanism of the signal enhancement of a nanostructured-FPI with an Au-coated nanopore layer. However, based upon previous research [4,22–24], the optical signal enhancement may result mainly from the L-SPR effect of the Au nanoparticles on AAO layer as shown in Fig. 3(d) [4,22–24] and the increased reflectance due to the coated Au thin film. The contribution to optical signal enhancement by L-SPR effect from Au nanoparticles has been indirectly confirmed by Raman signal enhancement experiments (e.g., enhancement factor up to 106) using similar Au-coated GaN nanopore structures [24].

Fig. 4. Measured output optical signals from a traditional-FPI with Au thin film coated on the planar plate, a nanostructured-FPI with and without Au thin film coating, showing significant signal intensity, resolution and contrast enhancement for the one with Au thin film coating. Air is in the FPI cavity.

The effect of different thicknesses of Au thin films on the signals has also been examined. Figure 5 (a) gives the measured results for different thicknesses (20Å, 50Å, 100Å) of Au, which show essentially the same signal intensity and contrast enhancement. The interference fringes shift with the changes of Au thicknesses in tens of angstroms, indicating tremendous sensitivity of the nanostructured-FPI devices. Given the typical size of a small single molecule is in the range of several nanometers, therefore this device might achieve single layer of single molecule detection. The nanostructured-FPI device has also been used to detect some chemicals including IPA, methanol and water. As expected, a clear shift of the interference fringes has been observed in Fig. 5(b) due to their different refractive indices. In Table 1, their average fringe peak shifts relative to air inside the FPI cavity are summarized. The average shift is obtained by averaging the shift of the fringe peaks between the 400 nm to 1100 nm

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spectrum window. Given the resolution of the HR4000 Spectrometer (Ocean Optics, Inc.) is 0.02 nm, the measurement error for the fringe peak shift is ± 0.01 nm. It should be noted that if the nanopores are blocked due to thicker Au coating, the interference fringes will disappear as observed in our experiments, confirming that the nanopores play an important role in the formation of interference fringes. In order to simplify the analysis of the transducing signals, Fourier transform (FT) has been performed on the measured optical signals. It is well known that for a FPI device, the wavelength (λ) of the peak maxima in the interference spectrum is given by: mλ = 2nL, where m is the spectral fringe order, n is the effective refractive index of nanostructure layer and its contents in the cavity, L is the geometric thickness of nanostructure layer and FPI cavity gap. 2nL represents the effective optical thickness (EOT) and can be obtained from the FT of measured spectrum in Fig. 5 (a) and 5 (b), respectively [26]. As a result, only a single peak is presented for each case as shown in Fig. 5 (c) and 5 (d), in which the X-axis coordinate of the peak is the EOT [26]. The changes of EOT for different chemicals relative to air inside the FPI cavity are summarized in Table 1.

Fig. 5. (a) Measured output optical signals from nanostructured-FPIs with different Au thicknesses, showing similar signal intensity and contrast enhancement. The interference fringes shift up to 8-17 nm with the Au thicknesses change just in the range of tens of angstroms, indicating its great sensitivity; (b) Measured output optical signals of different chemical samples on a nanostructured-FPI device, showing clear shift of the interference fringes. The coated Au thickness is 50 Å; (c) The plot for FT of the figure (a), the EOT changes for Au film with different thickness; (d) The plot for FT of figure (b), clear changes of the EOT are obtained for different chemicals. Table 1. Fringe peak shift and EOT change relative to air for different samples Material Water IPA Methanol

Average interference peak shift relative to air (400 nm-1100 nm) 24.94 ± 0.01nm 37.09 ± 0.01 nm 22.67 ± 0.01 nm

Calculated EOT change relative to air (nm) 517.8 nm 776.7 nm 258.9 nm

5. Conclusion In summary, a polymer-based nanostructured-FPI device has been developed and its performance evaluated. The nanostructured-FPI device offers some unique properties such as increased sensing area, extended penetration depth of the excitation light and the tremendous #131586 - $15.00 USD

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amplification of the optical transducing signals. It is anticipated that this new type of nanostructured-FPI device can offer an ultrasensitive (e.g., femtomole) technical platform for label-free biosensing. In addition, due to the easiness of large scale fabrication of the nanostructured-FPI devices on a single chip, this technique might benefit the multiplexing biosensing tremendously. The simple, cost-effective and disposable nature of this type of sensor is attractive for rapid point-of-care and field biodetection applications as well. Finally Fourier transform of the measured spectrum can simplify the data analysis since only one single peak presents in its corresponding plot for different samples, whose X-axis coordinate is the EOT of the nanostructure layer and the gap size of the FPI cavity. Acknowledgement This research is supported in part by grants of NSF CAREER Award 2009 and NSF 2008 PFUND-110. The authors thank the technical support from the technical staffs at Institute for Micromanufacturing at Louisiana Tech University.

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