We propose a biocompatible hybrid photonic platform incorporating a 3D silk inverse opal (SIO) crystal and a 2D plasmonic crystal formed on the top surface of the SIO. This hybrid photonic-plasmonic crystal (HPPC) structure simultaneously exhibits both an extraordinary transmission and a pseudo-photonic band-gap in its transmission spectrum. We demonstrate the use of the HPPC as a refractive index (RI) sensor. By performing a multispectral analysis to analyze the RI value, a sensitivity of 200,000 nm·Δ%T/RIU (refractive index unit) is achieved.
© 2013 OSA
Label free photonic sensors are of great interest because of their ability to analyze biomolecules with high sensitivity and in the absence of marker molecules . The most conventional sensors are based on surface plasmon polariton phenomena, whereby a prism is used to couple light to excite a surface plasmonic resonance (SPR) mode on a flat metal film . The sensor, however, is composed of a bulky optical set-up, so it is difficult to integrate this system into small devices for rapid measurements of mass-limited samples . Recent work shows that micro/nano photonic structures, such as photonic crystals (PhCs) and SPR-based metal nanostructures, can be used as label free sensors via compact and simple optical measurements [4–6]. An additional degree of utility is provided by using biocompatible materials as the material constituents to extend sensor applications to in vivo and in vitro experimentation [7,8]. Silk, the natural protein extracted from the Bombyx mori caterpillar, is an attractive material for applications in biophotonics due to its biocompatibility and unique mechanical and optical characteristics . Enzymes and drugs can be incorporated into the silk matrix under an all aqueous and mild processing workflow . Additionally, a series of studies on micro- and nanofabrication have introduced the use of silk films a platform for bioapplications [11,12].
PhCs and SPR structures were used independently to demonstrate micro/nano photonic devices with useful capability and functionality [13,14]. Recently, hybrid structures integrating PhC and SPR structures have been investigated with the expectation of extending device utility by leveraging the interplay between these phenomena [15–18]. However most of these studies have focused on creating and modifying the EOT, so a monolayer of closed packed submicron spheres or a two-dimensional PhC coated with thin metal films have been investigated. In such a structure, the metal coating changes the properties of the 2D PhC because modes of the PhC are directly affected by the metal layer. An approach which allows integrating the 3D PhC and 2D plasmonic crystal in the same device was demonstrated, and revealed the co-existence of the pseudo photonic band-gap (pseudo-PBG) found in the 3D PhC and the EOT in the same device. Motivated by the scheme, in this study we report a silk-based hybrid photonic-plasmonic crystal (HPPC) structure. We characterize the HPPC structure and prove that the coexistence of PhC and EOT in this protein-based optical device can play a significant role to improve the sensitivity of the structure to changes in the index of refraction, with promising implications for sensing applications. The hybrid structure is composed of a silk inverse opal (SIO) and a thin silver (Ag) film deposited on the structured surface of the inverse opal surface. The structure was fabricated using a template method . In the transmission spectra, the silk-HPPC simultaneously exhibits both a pseudo-PBG and an extraordinary transmission (EOT). The SIO improves the response by the EOT as well as by exhibiting a spectral response caused by the shift in the photonic bandgap. Finite difference time domain (FDTD) simulations were performed to confirm the existence of the EOT of our HPPC structure. The response of the HPPC was experimentally evaluated by modulating the index contrast of the device with different refractive index fluids.
Figure 1 shows the schematic diagram of the HPPC structure consisting of a 3D SIO and a 2D plasmonic crystal. We used a PMMA-nanosphere (1% concentration dispersed in water, Phosphorex Inc., Fall River MA) opal with a diameter of 350-nm stacked on a silicon wafer as a template. A 70-nm-thick Ag film was subsequently deposited on the template thus coating one side of the photonic crystal. This process creates the plasmonic crystal by forming an array of nanoscale holes. Silk solution was then cast onto the metal-coated template yielding a free standing film. Details for preparing silk solution are described in . Finally, the PMMA nanospheres were removed by exposure to acetone. Figure 2(a) and 2(b) show scanning electron microscope (SEM) images that display the hexagonal array of voids in the metal layer and the ordered voids in the SIO. The SEM image of the SIO shows a slight contraction of the lattice constant that appears to have become smaller (300-nm) than the diameter of PMMA nanospheres. This is due to the prolonged exposure to acetone that induces contraction of the silk film . Green structural color corresponding to the pseudo PBG of the SIO appears in the finalized sample as shown in Fig. 2(c).
The spectral response of the transmission measurement was examined to analyze the photonic response of the HPPC. In order to address this we conducted the transmission measurement of the bare SIO and the Ag-coated SIO (HPPC) in the visible and near-IR regions of the spectrum using a VIS/NIR fiber optic spectrometer (USB-2000, Ocean Optics). White light was propagated through the fiber and illuminated the HPPCs. The transmitted signal was coupled into the same fiber tip and went to the spectrometer. The distance between the fiber tip and sample was fixed at 500 μm. The reference signal was collected using an aluminum mirror. Figure 3(a) shows the transmission spectrum of the bare SIO. A dip at the wavelength of 542 nm corresponding to the PBG was clearly detected. According to the Bragg-Snell formula , the PBG position can be estimated throughEq. (1). To investigate the response of the plasmonic crystal the HPPC was infiltrated with fluid having index of refraction matched to the index of refraction of silk (e.g. n = 1.54). By eliminating the RI contrast between the silk and air, the HPPC can be considered simply as a plasmonic crystal composed of the Ag-cap array. The measurement performed on the infiltrated HPPC shows the EOT peak induced by the silver nanohole array at 470 nm (Fig. 3(b)). The periodic metal structure creates spatially coherent plasmonic modes that induce a varied and complex response comprising Bloch wave SPR modes and Wood’s anomalies . The incident beam approaching the Ag film is diffracted by the 2D hexagonal grating in a series of SPR diffraction orders that can be described with the momentum conservation law :Figure 3(c) shows the measured spectrum (green line) of the HPPC in air. By measuring the spectral response of the HPPC device, both EOT and PBG signatures were simultaneously detected. This measurement was also compared with the linear superposition (black line) of the spectra taken from the photonic (Fig. 3(a)) and the plasmonic crystal (Fig. 3(b)). This comparison, shown in Fig. 3(c), illustrates that the response of the HPPC can be understood by considering the combined contributions of the photonic and plasmonic functions.
Three dimensional finite-difference time-domain (FDTD) simulations using MEEP were used to analyze the transmission and the electromagnetic field distribution in the plasmonic crystal . It is worth noting that the SIO has a smaller lattice constant than the plasmonic crystal because of slight contraction during processing, thus the unit cell in the simulation using the periodic boundary condition is an approximation that does not keep this mismatch into account. Figure 3(d) shows the simulated transmission spectra of the plasmonic crystal. An especially strong transmission peak is found at λ = 470 nm, which is in agreement with the EOT observed experimentally. The field profile supported at 470-nm corresponding to the EOT revealed high localization at the Ag/silk interface consistent with the results obtained from diffraction theory as shown in Fig. 3(e).
The spectral response is strongly influenced by changes in the refractive index of the surrounding environment. To characterize this, we infiltrated the HPPC structure with index matching fluids (1.30 to 1.38 with 0.02 differences. Figure 4(a) shows the acquired transmission spectra for the HPPC as a function of different refractive indices. The measurement reveals two types of modes: red-shifting transmission-dips (Fig. 4(b)) around 650 nm that originate from a refractive index contrast modulation of the PBG of the SIO and increasing intensity of the EOT peaks at 470 nm. The EOT resonant wavelength is unaffected by changes in the analyte given the high field localization at the Ag/silk interface. However, changes in the analyte’s index of refraction affect the transmitted intensity of the EOT, which was found to increase with increasing refractive index. The scattering loss of incident light is reduced due to the lower index contrast between the SIO and the analyte thereby increasing the intensity of the transmission spectra (as shown in ). The combination of intensity increase of the EOT peak and wavelength shifts in the photonic bandgap affects the sensitivity of the device to refractive index changes.
Commonly used RI sensors typically monitor the response of a resonant spectral signature [24–26]. However, a single peak analysis would not fully recapitulate the spectral response shown in our structure given the shifts and intensity-changes of the multiple resonant peaks that concomitantly occur in the HPPC. We adopted a multispectral analysis approach to account for this complex response with the ultimate intention of determining the sensitivity of the structure [27,28]. The integrated response (R) over all wavelengths, including positive and negative differences in transmission spectra, can be defined by
In Eq. (3), Tanalyte is the observed transmission from the HPPC when it is infiltrated with an index matching fluid, and Tbase is the transmission for the HPPC when the lowest RI fluid (e.g. n = 1.30) is used. The integrand of Eq. (3) is the normalized difference in transmission relative to the base solution. The response R is directly related to the sensitivity of the device and would be actually improved by increasing the investigated bandwidth to cover resonances as many as the spectrometer allows, although there is a limitation that Tbase should have enough signal-intensity to be detected by the spectrometer with low signal-to-noise level since the extremely weak signal would induce large fluctuation in the integrand .
Figure 4(c) shows the normalized transmission differences, obtained from the experimental transmission spectra. Note that the intensity changes at the EOT and the band-gap shifts induce strong transmission differences. The response R was integrated over a wavelength range from 350 to 850 nm, which covers the band of the white light source and the spectrometer. We can define the device sensitivity as the slope of the integrated response as a function of the RI of the analyte (Fig. 4(d)) and achieve the a sensitivity of 200,000 nm·Δ%T/RIU (refractive index unit) which is an order of magnitude higher reported values from quasi-3D plasmonic crystal and nano-slit array in spite of the fact that our integrated wavelength-region was comparatively small [27, 29]. The simulated value for the quasi-3D plasmonic crystal was found to be 175,000 nm·Δ%T/RIU, which approached our experimental result despite the non-ideal simulation parameters (e.g. infinite structure, plane-wave, and polarized wave) . The response of the device found in this experimental analysis was governed by the interplay between the SIO and the plasmonic crystal. The sensitivity of the device can be improved by extending the measured wavelength-region to cover more than the EOT resonances and the pseudo-PBGs.
In summary, we presented a silk-based HPPC structure and its characterization. The complex spectral response in transmission is a result of the interplay between the 2D plasmonic crystal and the 3D pseudo-PBG material, making the structure suited to multispectral RI sensing. The coexistence of the EOT and the band-gap shift in the structure were experimentally and theoretically identified. The sensitivity obtained from the HPPC was found to be an order of magnitude higher than a comparable device based solely on a plasmonic crystal. Additional improvements and utility can be envisioned to improve the sensitivity by expanding the interrogation bandwidth and by choosing EOT modes that are more sensitive to the RI change in wavelength and intensity. Furthermore, the HPPC can be cost-effectively fabricated and the properties of silk enable facile functionalization of the structure with biological dopants ultimately adding further utility to this approach by taking advantage of the favorable biochemical properties of this all-protein material platform.
The authors gratefully acknowledge support from DARPA (MTO) and from the Army Research Office.
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