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The concept of an integrated nanoplasmonic sensor implemented on a silicon substrate is presented. Developed experimental setup based on rotation of linearly polarized light provides intensity detection between two orthogonal polarizations of a He-Ne laser beam. This optical configuration yields to a sensitivity improvement and noise reduction, resulting in a resolution of 4x10−5 Refractive Index Units. Proposed methodology is promising for the application in portable nanoplasmonic multisensing and imaging.
©2011 Optical Society of America
1. Introduction
Despite the importance of accurate and rapid diagnosis in biomedical, available techniques are expensive, have limited ability to differentiate between multiple pathogens, are slow, and have a poor detection threshold. The ideal diagnostic device would need to be a sensitive, accurate, cost-effective and portable point-of-care detection system. Since the conditions of plasmon excitation are extremely sensitive to the refractive index (RI) and the thickness of the thin surface films, Surface Plasmon Resonance (SPR) [1] biosensing is now considered as a leading technology for real-time detection and studies of biological binding events [2]. Significant effort is now undertaken on the application of localized plasmons for biosensing where unique optical properties of nanoplasmonic structures on the Si substrate [3] can allow the development of novel promising sensor designs and architectures [4,5], impossible to achieve with conventional glass-based technology.
In this article we present the concept of cost-effective miniaturized integrated nanoplasmonic biosensing device on Si where enhanced light transmission through nanohole array due to photon-plasmon coupling with normal incident light is combined with the detecting properties of a semiconductor substrate. Experimental set-up where rotation of linearly polarized light allows application of self-referencing noise reduction on the asymmetrical nanohole array structures was developed and tested.
2. Basic ideas and approach
Two dimensional (2D) nanohole arrays built on gold films have been shown to exhibit extraordinary transmission of light for wavelengths that excite surface plasmon at the surface of the metallic layer. The wavelength that satisfies the condition of resonant excitation of the surface plasmon at normal incidence is given by [6]:
where m and n are integer relative to the diffraction order, ax and ay are the periodicities of the array in the x and y axes, and na and εm are the refractive index and the dielectric constant of the surrounding medium and of the metal, respectively.Usually the wavelength corresponding to the excitation of localized plasmons is used as the sensing parameter for the spectral nanoplasmonic devices. Here, we propose an intensity interrogation plasmonic sensing where fixed wavelength corresponds to the point of maximum slope of spectral curve (Fig. 1a .) and all transmitted light is captured by an optically active substrate playing the role of a photodetector. Then, any modification of the medium refractive index close to the sensing surface will result in a shift of the spectral curve and the sensor will detect changes in the transmitted intensity. The proposed approach provides a complete integration of the sensing transducer with the detector. These very simple and novel designs for nanoplasmonic biosensing could offer a cost-effective alternative to the bulk SPR system while providing sufficient sensitivity in a portable configuration.
Fig. 1 Integrated nanoplasmonic biosensor designs with (a) symmetrical and (b) asymmetrical nanohole arrays structure.
Sensitivity, dynamic range and overall performance of plasmonic intensity sensors depend on many parameters: stability of the monochromatic light source and optical set-up, detector sensitivity, nanoplasmonic structure size and its fabrication precision. In this work, we propose to use 2D structural properties of the planar nanoplasmonic systems fabricated directly on the detecting semiconductor substrate to increase the sensitivity and signal-to-noise ratio of the intensity interrogation method. For example, asymmetrical nanohole plasmonic structure with different periods in orthogonal directions [7] provides shifted spectral dependences with an intersection point that could be optimized to the working light wavelength (Fig. 1b.). Experimental set-up measuring a signal difference between orthogonal polarizations will provide a detected signal due to RI changes almost two times larger comparing to the standard intensity method (Fig. 1a.). Also, we expect that this self-referenced approach will effectively eliminate any experimental noises from external sources, including light source intensity fluctuations, detection system noises, absorption and scattering when transmitting through measuring liquids. Experimental realization of the optical set-up where rotation of linearly polarized light allows differential measurements of orthogonal polarization with only one photodetector is shown in the next chapter.
3. Experimental setup and instrumental methodology
3.1 Optical set-up
To test asymmetric nanoplasmonic structures, the experimental set-up illustrated in Fig. 2 was developed. A 5mW stabilized He-Ne laser operating at a wavelength of 632.8 nm was used as the light source. Spatially filtered, light passes through a polarizer to the Photo-Elastic Modulator (PEM, Hinds Instruments) and the quarter-wave plate at 45° to produce a linearly polarized 90deg. or 180deg. rotating light. Range of rotation depends on the PEM modulation depth MPEM. (Fig. 2a). This light is then transmitted through the asymmetric gold nanostructure, thus exciting alternatively the different surface plasmons corresponding to the orthogonal direction in nanohole plasmonic structure with frequency 50 kHz or 100 kHz. Detection with a lock-in amplifier allows to measure the amplitude difference between two orthogonal polarizations on the frequency of modulation. Specially designed open and flow-injection measurement cell was used for tests in liquids.
Fig. 2 (a) Experimental set-up. (b) Detector responses at different analyzer angular positions and at two PEM modulation depths: 90deg. and 180deg. Dotted lines correspond to the signal in absence of analyzer.
Changes of light polarization state in the proposed scheme can be considered using Jones transformation matrix method. Here we imply that after the polarizer, light is linearly polarized with a P-component over the plane XZ and an S-component over the YZ plane: (see Fig. 2a for axis definition). The corresponding Jones matrices are given by:
where JQWP is a matrix for the quarter waveplate, JPEM corresponds to the PEM, where phase modulation φ = MPEMsin(ωt) with MPEM as PEM modulation amplitude. 50 kHz modulation of the transmitted light is produced at MPEM = 90deg. and 100 kHz correspondingly at MPEM = 180deg. Discarding all phase effects that may be introduced by the plasmonic nanostructure, we can write sample Jones matrix as Jsample. The total light intensity transmitted by the nanostructure is then given by I = E.Ē, with E = JsampleJQWPJPEMEini. After computing all the matrixes, we obtainThe alternative part of the intensity depends on the difference between the transmission coefficients for S and P polarization (Fig. 2b), providing for asymmetric nanostructure a self-referencing method with only one detector. This feature greatly improves the biosensing sensitivity. We can also use this setup with symmetric nanohole array structures by placing an analyzer (Fig. 2a.) before the sample to produce a temporally PEM-modulated light falling on the detector. In this case, the intensity right after the plasmonic structure is given by depending on the chosen polarization.
3.2 Integrated Silicon photodetector
Various silicon based photodetectors have been developed such as Schottky, heterojunction, p-i-n, avalanche and metal-oxide-semiconductor (MOS) structures [8]. Depending upon the process and materials used, these photodetectors can be optimized for any part of the spectrum from ultraviolet to infrared. In this article we propose to detect the transmitted light directly with the silicon substrate that works as a simplified silicon based photodetector implemented on the metal-oxide-semiconductor (MOS) structure where the front contact is formed by the nanoplasmonic structure (Fig. 3a ). The device operates under a reverse-biased at −3V to give a wide depletion range. Assuming that the carriers are photogenerated only in the depletion region, the resulting photocurrent under illumination with our modulation scheme is [9]:
where Iph is the photocurrent, Sill is the surface under illumination, αox and αSi are respectively the absorption coefficient of oxide and silicon, dox is the thickness of the oxide film, Xd is the thickness of the depletion region, ρ is the reflection coefficient at the Si/SiO2 interface, and I is the intensity of light passing through the nanoplasmonic structure given by Eq. (2). More precise and realistic calculations could be made to take into account the carriers generated in the bulk, but this simplified model is sufficient to give a general understanding of the phenomenon. The resulting alternative current, proportional to the incident modulated light is registered by the external electronic scheme with a lock-in amplifier. (Fig. 2b).Fig. 3 a) Si-based nanoplasmonic device schematics and SEM image of the nanoholes array; b) Responses of the system for different ethanol concentrations for one polarization and rotating polarization.
4. Results and Discussion
To provide the electrical stability of the MOS integrated photodetector (Fig. 3a) in the liquids and to fix the operational ”window” of 4mm2 for sensing, a SiO2 and Si3N4 double dielectric layer was employed. The Si3N4 thickness is 72nm, whereas for SiO2, it is 1µm outside the window and 38nm inside. A thin aluminum film was deposited on the backside to provide an ohmic electrical contact to the structure. The 2mm2 gold 100 nm thin film over a 5nm Titanium adhesion layer was deposited by E-Beam through the mask over the silicon structure. Two functions are applied to this metal film, to work as a counter electrode in the photocurrent detection and to serve as a substrate for nanoplasmonic structure fabrication.
The intensity method sensitivity depends on the sharpness of the extraordinary transmission peak and initial measurement point position. As shown on Fig. 1, for symmetrical nanohole arrays or for one polarization sensing method, working wavelength must be equal to the position of the maximum deviation point. 2D asymmetrical structure with different spectra in the orthogonal polarizations provides best sensitivity for wavelength that match perfectly the spectral curves intersection point. By tuning the light source wavelength we can optimize initial experimental conditions. As a proof of principle, we used stabilized He-Ne laser at a fixed wavelength of 632.8 nm and optimized the system by adapting the periods of nanohole arrays. Using Eq. (1) and estimating the red shift due to the interference between the resonant state (the surface plasmon) and a continuum of state (light scattered into the hole) that is typical of the fano-type resonance [10], we have obtained nanoholes arrays period for the biosensing in liquids. The array period for symmetrical nanostructure is 420nm and for asymmetrical one is 380nm x 420mn. For experimental tests, nanoholes arrays with sizes 150x150µm were produced by Focused Ion Beam (FIB) using a 30 kV acceleration voltage and 30 pA ion current (Fig. 3a). Similar structures but on the transparent substrate (glass BK7) were also fabricated to test experimental set-up and to verify our theoretical predictions.
Spectral characteristics of the nanoplasmonic structures fabricated in the thin metal films usually depend on the used substrate. We suppose that in our case spectral dependences will be similar for glass and silicon-based samples due to the fact that for rather thick metal films substrate influence is limited to the gold/substrate plasmon resonance condition [11]. When fabricating nanoplasmonic structure directly on the silicon surface, electro-optical semiconductor properties could be used to change resonance condition on both interfaces.
Measured transmission spectra of plasmonic nanostructure on the glass substrate for two orthogonal polarizations in water and ethanol are presented on Fig. 3b. Obtained red shift of the resonance wavelength is 9nm and corresponds to the increase in the refractive index between water and ethanol of 2.5 x 10−2 RIU [12]. Note that the initial intersection point A for spectral curves in water coincides with the used He-Ne laser light. If necessary, slight deviation could be adjusted experimentally by fine sample tilting along one axe. It changes the correspondent component of the incident light wavevector, therefore changing resonant condition for surface plasmon excitation and minimizing initial signal. Modification of tested medium RI leads to an increase in transmitted intensity for one polarization A->B and correspondingly to the decrease for the other one A->C and the intensity difference between the two components will be detected by proposed experimental method. This approach benefits from several features in comparison to the intensity based symmetrical nanohole arrays sensing system. First, a greater change in intensity to be measured (about two-fold) from a modification of refractive index (Fig. 3b). Also, the problem of uncorrelated intensity fluctuations is solved by using the difference of the two polarization components, which rejects the common fluctuations of the signal. This results in an improved sensitivity and resolution of the sensing system.
Experimental evaluation of the proposed approach was performed on the asymmetric and symmetric nanoplasmonic Si-based structures. The intensity difference between the two polarizations was recorded as a function of time with different concentrations of ethanol successively added to water. The obtained results are presented in Fig. 4 where, for example, 0.7% mix of ethanol in water corresponds to a 1.75x10−4 RIU change, allowing for a detection sensitivity of 4 x 10−5 RIU. In order to demonstrate the improvement accomplished by the rotating polarization method with asymmetric structure, this result was compared with the signal coming from one polarization and symmetric structure, as shown in Fig. 4. The sensitivity obtained from the single polarization is lower: 1 x 10−4 RIU, clearly demonstrating the advantage of the self-referencing detection method.
Fig. 4 Responses of the system for different ethanol concentrations for one polarization on the symmetric structure and rotating polarization on the asymmetric structure.
The proposed Si-based sensing platform enables the application of microfabrication methods for MEMS, microfluidic systems and multi-channel arrays on the Si chip. The appropriate photodetector performance can be achieved by controlling the semiconductor type and the doping level. Optimization of metal thickness and surface quality, geometry and periodicity of nanostructures will improve the nanoplasmonic sensor sensitivity. The possibility of multisensing or even imaging where different sensing spots are separated by using light amplitude or polarization modulation also could be considered. Additionally, by exploiting the ability to integrate sensing spots with analog and digital processing, new types of complementary metal oxide semiconductor (CMOS) imaging devices with sensing nanoplasmonic components can be created for chemical reaction monitoring and biological testing. Such integration further reduces sensing system power and size and enables the implementation of new sensor functionalities.
5. Conclusions
We introduced a novel methodology and instrumentation to build miniaturized Si-based nanoplasmonic biosensing platform using the 2D plasmonic properties of metallic nanostructures designed on the integrated MOS photodetector. Sensitivity of 4 x 10−5 RIU is sufficient for portable application and is obtained on the simplest implementation of Si-based photodetector, which efficiency could be greatly improved by using novel CMOS technologies. It is anticipated that in combination with stable monochromatic laser source and self-referenced detection this platform offers major advantages of miniaturization, multisensing and cost reduction over conventional SPR systems.
Acknowledgments
The authors acknowledge the financial contribution from the Natural Science and Engineering Research Council of Canada, NanoQuébec, and Canadian Institute for Photonics Innovations.
References and links
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