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Abstract
Metasurfaces are investigated intensively for biophotonics applications due to their resonant wavelength flexibly tuned in the near infrared region specially matching biological tissues. Here, we present numerically a metasurface structure combining dielectric resonance with surface plasmon mode of a metal plane, which is a perfect absorber with a narrow linewidth 10 nm wide and quality factor 120 in the near infrared regime. As a sensor, its bulk sensitivity and bulk figure of merit reach respectively 840 nm/RIU and 84/RIU, while its surface sensitivity and surface figure of merit are respectively 1 and 0.1/nm. For different types of adsorbate layers with the same thickness of 8 nm, its surface sensitivity and figure of merit are respectively 32.3 and 3.2/RIU. The enhanced electric field is concentrated on top of dielectric patch ends and in the patch ends simultaneously. Results show that the presented structure has high surface (and bulk) sensing capability in sensing applications due to its narrow linewidth and deep modulation depth. This could pave a new route toward dielectric-metal metasurface in biosensing applications, such as early disease detections and designs of neural stem cell sensing platforms.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, metasurfaces [1–30], which are two dimensional array formed by subwavelength periodic nanostructures of dielectric and metal, have been extensively investigated, due to their unique properties, such as light localization below the diffraction limit [8–12], scattering and absorption enhancement [2, 9–15, 31–37]. For biophotonics applications, their surface plasmon resonance can easily be tuned in the range of light wavelength from 750 to 1400 nm, which matches the range of the therapeutic light through biological tissues [38]. As we know, the structure of plasmonic perfect absorber has been investigated in sensing applications due to its large modulation depth [12–14]. In designing of the plamonics-perfect-absorber structure, metal-dielectric-metal structure (MDM) has been intensively investigated, which includes a central dielectric layer with a periodic structured metallic surface on topside and a metal plane at the bottom, and operated as refractive index sensors [2, 12, 39, 40]. However, these structures have linewidth (or full width of half maximum, FWHM) far larger than tens of nanometers, due to significant conductor loss of metals, which hampers realizations of sensor designs with high performance. Additionally, metal-dielectric interface’s thermal boundary resistance leads to unexpected temperature increase and consequent thermal expansion of nanostructures with nanogaps. In the experiment needing cryogenic environment, the expansion deters transport of potential molecules under light incidence of laser [41]. Mitigating the detrimental effect of metal loss, several avenues have been suggested [13, 39, 40], such as adopting gain media to compensate for the losses [42, 43], designing complex nanostructures for modes hybridization [39, 40, 44, 45], and synthesizing alternative low-loss plasmonic materials [46, 47]. However, these operations usually accompany with more complex configurations or synthesizing procedures [45, 48].
Recently, all-dielectric metasurfaces [15–30] have been shown theoretically and experimentally for magnetic and electric resonances, which are with narrow linewidth due to their free of Ohmic loss, though they are not fabricated easily and simply, as well as integrated easily into plasmonic, electronic and photonic devices on the same chip. In addition, researchers have realized perfect absorber structures using dielectric materials in the terahertz range [49–51]. For inevitable loss in metallic nanostructures, perfect absorber or total absorption is advantage of the loss and enhances the modulation depth of sensor which is beneficial for detection the minute variations of surrounding dielectric [12]. Therefore plasmonic perfect absorber works as a promising sensor for detecting refractive index change of solutions [12]. Subsequently, acheiving a perfect absorption at resonant frequency is key for high figure of merit [12]. Due to the perfect absorption features of metal nanostructure and transmission or reflective spectrum with narrow bandwidth of all-dielectric metasurface, researchers have taken attention to the benefit of combination of the both structures [52, 53]. Such as in [53], Callawaert et al. presented dielectric disk array on silver film to obtain the narrowband absorber in the visible range. Here, we investigate numerically a dielectric-metal hybrid metasurface structure consisting of a dielectric patch array and a metal plane, as well as its sensing performance in the infrared range. The FWHM and peak of the absorption spectrum are respectively 10 nm and nearly 1, due to the combination between resonant property of the dielectric patch array and surface plasmonic properties of the metal, the former’s narrow linewidth and the latter’s inevitable loss. This structure can open a new avenue for refractive index sensing applications in the infrared regime, especially bio-molecular detections. It also has potential in applications of substrates for sensing activities of differentiation and proliferation of neural stem cells [38, 54].
2. Results and discussions
2.1. Model and methods
The investigated structure of dielectric patch array backed by a metal plane (DAM) is shown in Fig. 1. It consists of a dielectric patch array on the topside and a metal plane at the bottom, which is situated on top of other dielectric substrate, such as glass. Structural parameters include periods in the x and y directions px and py, dielectric patch length in the x and y directions wx and wy, dielectric patch thickness LD, metal plane thickness LM.
Fig. 1 Schematic of dielectric patch array backed by a metal plane structure. Structural parameters: LD, LM, px, py, wx, wy. “DIC” and “DAM” are abbreviations of “dielectric” and “dielectric array metal”, respectively.
In simulations, we use 3-dimension finite element methods [55]. The dielectric of nanopatch array is set as SiO2, of which refractive index is selected as 1.46, while the metal plane is made of aluminum of which refractive index is from the reference [56]. The maximum mesh in the x, y and z directions are set as 10 nm, 10 nm, and 2 nm, respectively. To save the calculating space, we set periodic conditions in the x and y directions to consider periodic arrangement of the metasurface. The incident polarized light is normal to the structured surface (represented by kz), and electric field and magnetic field are along the x and y directions (Ex and Hy), respectively, as shown in Fig. 1.
2.2. Absorption properties
Reflective (R), transmission (T), and absorption (A) spectra of the presented DAM structure are shown in Fig. 2(a). For the thickness of the metal plane is larger than its skin thickness in the near-infrared range and transmission of the presented structure is nearly zero, the absorption A = 1 − R − T ≈ 1 − R, which shows R directly demonstrates A. There is a sharp dip in the reflective spectrum and the nearly total absorption at the resonant position, which utilizes the inevitable loss of the metal and is known as perfect absorption at the resonant wavelength [12]. In Fig. 2(a), we find that FWHM is 10 nm with quality factor (Q = λ/Δλ) [53] 120, which can bring much higher sensing performance than that the counterpart of the MDM structure brings. To investigate features of the absorption spectra influenced by the incident angle, we calculate the reflective spectra at oblique incidence and results are shown by Fig. 2(b). It can be seen in Fig. 2(b) that the resonance depth is much larger at the normal incidence than that at the oblique incidence. Besides, the resonance splits much more and the linewidth of resonance becomes broader while the angle of incidence is much lager. Most of the essential features of the reflective spectra can be associated with the incident light coupling to surface plasmon modes. The splits indicate coupling of incident light via scattering to both aluminium/air and aluminium/silicon surface plasmon polariton modes [57–59]. The quantitative analysis for the surface plasmon polariton dispersions of the presented DAM structure is not included in this work, but it would be interesting to explore.
Fig. 2 (a) Reflective (R), transmission (T), and absorption (A) spectra of the presented DAM structure. (b) Calculated refective spectra with varying incident angle from 0° to 40°, the horizontal axis represents magnitude of the x component of the wave vector which is normalized by π/p, and the vertical axis represents light wavelength. Parameters: wx = wy = 0.8µm, px = py = 1.1µm, LM = 1µm, LD = 0.25 µm.
For a nanostructure, patterns of electric and magnetic fields are fundamental to understand physical mechanism of the perfect absorption at the resonant wavelength and narrow linewidth of its spectra. We calculate patterns of electric and magnetic fields of the presented DAM structure at the resonant wavelength. In Fig. 3, the colors of blue and red indicate respectively minimum and maximum values of the fields’ magnitudes. Figures 3(a) and 3(d) schematically show that one part of electric field is trapped in the SiO2 patch, while another part is concentrated at the ends on topside surface of the dielectric patch and extended deeply into the surrounding medium at the resonant wavelength. It is different from MDM structures of which the enhanced electric field of the gap resonance mode is mostly concentrated in the cavity between the top structured metal surface and the bottom metal plane [12, 40]. Here, the maxima of the electric field intensity in the ends and at the top of dielectric nanopatch indicate electric dipoles interacting with their images in the metal plane, resulting in magnetic resonance along y-axis, as shown in Figs. 3(b) and 3(e). In other words, the magnetic dipole originates from circular displacement currents excited by dielectric patch and its image in the metal plane. The results show that the satisfaction of wavevector matching condition combining the surface plasmon polariton mode on the metallic surface, surface lattice resonance mode of the path array, and the guided mode in the dielectric cube [57–59]. We can also say that a guided mode is supported by the dielectric cube array [60], while magnetic field distributions have features of coupling between magnetic dipole resonance [12] and surface lattice resonance [40, 61].
Fig. 3 Electric field patterns (a) (d), magnetic field patterns (b) (e), and power loss patterns (c) (f) of the presented DAM structure at the resonance wavelength of 1.2641 µm. The sampling planes are both at z = 0 for (a) and (b), while it is at z = −2 nm for (c). The sampling planes are all at y = 0 for (d), (e), and (f). Parameters: wx = wy = 0.8 µm, px = py = 1.1 µm, LM = 1 µm, LD = 0.25 µm. Metal and dielectric are aluminum and SiO2, respectively. In the color scale, blue and red indicate minimum and maximum values of fields’ magnitudes, respectively.
To investigate effects induced by the loss of the metal plane and the dielectric cube array, we calculate the power loss of the presented DAM structure. Figure 3(c) shows that the power loss is distributed outside of the dielectric cube and in the extreme thin domain that is close to the interface between the dielectric patch and the metal plane, as shown in Fig. 3(f). It shows that the intrinsic loss of metal can not be avoided so that and the power loss has such kind of distribution. It can be explained by the fact that the low loss of metal and free loss of dielectric patch array excite both surface plasmon polariton modes in flat metal surface and guided modes in dielectric patch array [62, 63], which are different from the gap resonance of the metal-dielectric-metal structure. Here, the presented DAM structure without the top metal-dielectric interface can reduce the substantial temperature increase and thermal expansion of nanostructures [41]. It should be noted that the linewidth of absorption is far narrower than metal-dielectric-metal structures due to free loss of dielectric patch array and the low loss of aluminium in the infrared range. Therefore, the narrow linewidth of 10 nm can be explained by the guided mode and the surface lattice resonance mode of the dielectric patch array combining the surface polasmon polariton mode of the metal plane, while the metal leads to the perfect absorption at the resonant wavelength due to its intrinsic loss in the infrared regime. The electric field of the presented DAM structure concentrates apparently on the topside surface ends of the dielectric patch array and is distributed more extensively than that of MDM structures. This feature facilitates detecting molecules in much larger sensing volumes.
2.3. Sensing properties and performance
The performance of absorption of the investigated DAM structure is tuned by the geometric parameters [53]. From the electric field distributions in Figs. 3(a) and 3(d), we can find one proportion of the electric field concentrated is on the topside surface ends of the dielectric patch, which is easily accessible to the surrounding medium, while the other proportion is concentrated at the two ends inside the dielectric patch. Therefore the presented DAM structure has the desirable near-field characteristics for biosensing applications. It can be operated as refractive index sensor in the near infrared range. This is a new route for applications of dielectric-metal hybridization metasurfaces [53] towards sensing chemical and biomedical molecules, and detecting chemical reactions at nanoscale dimensions. For evaluating sensing performance, we define factors of sensitivity and figure of merit (FOM) [3, 12, 13] as the following:
Here, “i” is either “bulk” or “surface”. Δλ is the shift of resonant wavelength due to change of refractive index in the surrounding medium Δn or adsorbate layer thickness on the surface of nanostructure Δl. While the device is immersed into solution with refractive index from 1.350 to 1.355, which can be realized through changing concentration of solutions in experiments, such as glucose solution [3, 12, 40]. The resonant wavelength has a redshift 4.2 nm, as shown in Figs. 4(a) and 4(b). As we know, the overall sensing performance of a plasmonic nanostructure is typically characterized by the factor FOM. According to Eqs. (1) (3) [3, 12, 13], we calculate the bulk refractive index sensitivity Sbulk = 840 nm/RIU and FOMbulk = 84/RIU for the presented DAM structure, where RIU is unit of refractive index and 1/RIU is unit of the corresponding figure of merit [12]. According to Eqs. (2)–(3), Ssurface = 1, and FOMsurface ≈ 0.1, of which unit of surface sensitivity is 1, and unit of the corresponding figure of merit is 1/nm. The sensing performance is better than those in [12, 39, 40].
Fig. 4 (a) Reflective spectra (b) Reflective dips and resonant wavelength with different refractive index of the surrounding environment. (c) Reflective spectra of the DAM structure with and without an adsorbed thin protein layer of 8-nm thickness, of which the value of refractive index is 1.2, 1.6, and 1.7, respectively. (d) Reflectivity dip and resonant wavelength as a function of the adsorbate-layer thickness, while the reflective index of the adsorbed layer is 1.4. Parameters: px = py = 1.1 µm, wx = wy = 0.8 µm, LM = 1 µm, and LD = 250 nm.
Due to the distributions of electric field, there are two advantages for sensing applications. The first advantage is that the sensor is selective for only a certain kind of molecules through functionalizing the external surface of dielectric patch array. Such as in biomolecular sensing applications, the structured surface is functionalized with the appropriate binding molecules, such as antibody which binds preferentially to the target analyte, causing a plasmonic property change of the device. Therefore, the magnitude of the property change can be used to determine the concentration of the molecules. The second advantage is using dielectric patch with microchannel to utilize the inside localized field in the ends of the dielectric patch [4, 13], which facilitates realization of micro- or nano-fluidic devices.
While the functionalized dielectric patch surface is adsorbing molecules to create a thin protein layer, the total adsorbate thickness reaches 8 nm [3]. In experiments, thickness at nanometer scales is characterized by the ellipsometer [26]. Due to the presence of molecules are adsorbed to the dielectric patch array surface which is in the confined region of the metasurface, change of the reflective spectrum can be probed by measuring the shift of the sharp spectrum feature. Under conditions that the adsorbate thickness remains the value of 8 nm and the structure is located in air, and refractive index values of the adsorbate are 1.2, 1.6, and 1.7, respectively, the reflect spectra are shown in Fig. 4(c). It is shown in Fig. 4(c) that the reflective spectrum has a redshift comparing with the bared DAM, as well as absorption is larger than that of the bared structure, i.e. the resonant wavelength is changed from (λR = 1.2700 µm, Rdip = 0.054) to (λR = 1.2844 µm, Rdip = 0.046).
To investigate sensing performance of the sensor for different adsorbate layers on the presented DAM structure, we define two factors, which are different from those in [3, 10, 12, 39, 40], as the following:
Here, Δλ and Δn are relatively shift of wavelength and variation of refractive index due to different types of adsorbate layers with different refractive indexes and the same thickness. Eqs. (4)–(5) access the sensing performance of the presented DAM structure while the adsorbate on its surface is different and with the thickness. Unit of this surface sensitivity is 1, and the corresponding surface sensitivity is 1/nm. According to Eqs. (4)–Eq. (5), we obtain the result S′surface = 32.3 and FOM′surface = 3.2/nm under conditions that the adsorbate thickness is 8 nm and its refractive index is respectively 1.2, 1.6 and 1.7. In addition, increasing the thickness of the adsorbate layer of protein with the refractive index 1.4, the resonant wavelength has redshift and the absorption remains perfect, as shown by Fig. 4(d), the resonant wavelength varies from 1.2828 µm to 1.3471 µm. These features show that the DAM structure is highly suitable for biosensing applications. The sharp resonance feature facilitates detection of minute shift of resonant wavelength so that sensitivity of sensor is increased. We can get plasmonic absorber with narrower bandwidth through optimizing the structure and selecting the preferable dielectric materials to increase the FOM further [10]. The bandwidth can also be narrower through canceling the back metal plane and designing much complex nanostructure [26]. The narrow bandwith is key to achieve sensor device with large figure of merit using a single wavelength light source [12]. Though there are not optimization of the structural parameters and selecting preferable material for the presented DAM structure in this work, but it would be valuable to investigate [46, 47].
3. Conclusion
In conclusion, we present numerically a simple and flexible hybrid dielectric-metal metasurface structure consisting of a dielectric patch array and a metal plane, which is a perfect absorber in the infrared range and has exceptional sensing capabilities for its extremely sharp spectrum. The key lies in the combination of the dielectric nanostructure with the metal plane, of which the dielectric-array-guided mode is with narrow line width due to low loss of the dielectric medium, while surface plasmon polariton mode of the metal plane leads to large modulation depth due to the inevitable loss of the metal material. Therefore the combination considerably enhances its sensing performance. It can be explained by coupling of the guided mode, surface plasmon polariton mode, and surface lattice resonance mode. While the strucuture is operated as a refractive index sensor, its bulk sensitivity and FOMbulk are respectively 840 nm/RIU and 84/RIU, while its surface sensitivity and FOMsurface are respectively 1 and 0.1/nm. For different types of adsorbate layers with thickness of 8 nm and different refractive indexes, its surface sensitivity and figure of merit reach respectively 32.3 and 3.2/RIU. The results indicate that the presented dielectric-metal metasurface combines the advantage of narrow bandwidth of all-dielectric-metasurfaces and the inevitable loss of the metal materials. This structure has potential in the fields of biotechnology, medical diagnostics, or pharmacology including biomolecule detection, as well as real-time monitoring of chemical reactions or molecular kinetics.
Funding
The Research Project of Henan Provincial Department of Science and Technology (Grant No. 182102210122), the National Natural Science Foundation of China (Grant No. 61475191), the Major Basic Research Project of Natural Science Foundation of Shaanxi Province (Grant No. 2017ZDJC – 27), and the Research Project of Xinxiang Medical University (Grant No. 505212).
Acknowledgments
Authors thank Dr. Juntang Lin for helpful discussions about preparing the manuscript.
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