In this paper, the optical properties of a plasmonic nanoantenna array have been investigated. The proposed plasmonic structure presents omnidirectional resonance properties, such as omnidirectional reflection dip and omnidirectional emission peak. In addition, the reflection and emission of the plasmonic nanoantenna array with various metal/insulator/metal cavity thicknesses are theoretically and experimentally investigated. The simulation reveals a fair agreement with the experimental results.
© 2014 Optical Society of America
Surface plasmon polaritons (SPPs) are collective charge oscillations along metal/dielectric interfaces . The unique properties of SPPs have been applied to diverse applications, such as photonics [2,3], medical drug delivering [4,5] and bio-sensing [6,7]. The middle-infrared (mid-IR) spectral range is of critical importance for thermal imaging, sensing of gases and aerosols, and environmental monitoring. Unfortunately, very few radiation sources exist in this range with sufficient power and they are mostly in the development phase. Unfortunately, radiation sources in the mid-IR range are still insufficient. Plasmonic thermal emitters (PTEs) based on periodically structured metallic multilayers provide tunable, narrow-band radiation, much narrower than from a black-body at the same heating temperature. Besides PTEs, several groups have demonstrated spectral control of emittance by means of subwavelength structures such as one-dimensional SiC grating [8,9], SiC nanoanttena , and photonic crystals (PhC) [11–13]. One of the most promising venues is based on plasmonic and metamaterial effects : the plasmonic metamaterials provide the adjustability of spectral design that can boost thermal emission at the desired wavelength, up to the level of an ideal blackbody. It has been shown that the resonance mode arising from the SP coupling between the adjacent metallic structures can be thermally excited from a heated PTE . PTEs have become a promising candidate as a near-IR, mid-IR and even THz radiation  source owing to their versatile resonance properties. For example, extremely high temporal coherent thermal emission of PTEs, through the use of the resonance of surface phonon  and delocalized SP , have been demonstrated. However, the resonance condition based on surface phonons and/or delocalized SPs often leads to a sensitive response to the observing angle . Besides narrow band resonance emission, broadband thermal emission exceeding the blackbody limit in the near-field based on hyperbolic metamaterials has also been theoretically proposed . Broadband thermal emission based on the resonance of Fabry-Perot-like LSP has also been experimentally demonstrated . To extend the application of PTEs, single , dual  and triple  resonance thermal emission of PTEs has been engineered and realized.
In this paper, we use a plasmonic nanoantenna array as an omnidirectional thermal emitter. Both of the reflection and emission properties as a function of MIM cavity thickness are investigated. The simulation presents a fair agreement with the experimental results.
2. Description of the omnidirectional plasmonic nanoantenna array
The schematic of the nanoantenna array is shown in Fig. 1. It is made of an optically thick silver film acting as a semi-infinite mirror, a thin dielectric layer (SiO2) forming the gap of the antenna of thickness tw, and an upper metallic nanowire. The plasmonic structure consists of a dielectric layer, acting as a load, sandwiched by two metallic structures which can be also treated as a metal/insulator/metal (MIM) cavity array. The periodicity and the nanoscale structures provide additional momentum to couple the free-space light into SP. The blue line represents the field distribution of the SP mode. As the two adjacent metals are sufficiently close, the two metallic structures are coupled. The resonance properties dramatically depend on the geometry of the whole MIM cavity which plays the role of the optical antenna and converts free-space optical radiation into SP localized within a subwavelength scale. The plasmonic nanoantenna array is fabricated by using conventional lithography and lift-off technique. Here, the pitch, width and thickness of the nanowire are fixed to be Λg = 3000nm, wAg = 1500nm and tAg = 100nm, respectively. Samples with different thickness of SiO2 are then made.
The optical properties of the structure are calculated by using the Rigorous Coupled Wave Analysis (RCWA) as described in [24,25]. The input light is TM (or TE) polarized in which the magnetic (electric) field is parallel to the grating grooves (i.e., parallel to the y-axis). The dispersive complex dielectric constants of SiO2 and Ag are taken from . Here, we consider the primary resonance to arise from a LSP which in turn is due to a standing wave of plasmons trapped below a metal patch. The Hy field distribution of the considered resonance mode is simulated by using RCWA. The considered resonance mode is a fundamental Fabry-Perot (FP) one causing from the MIM SP wave gets reflected off the two edges of the metal patch. The resonance mode within the MIM cavity is also shown in Fig. 1. The resonances not are not coupled to each other, as the metal patches are sufficiently separated from each other.
3. Thickness-dependence resonance properties of the plasmonic nanoantenna array
Figure 2(a) shows the reflection spectrum of the plasmonic nanoantenna array. Black, red, blue, green and pink lines represent tw = 0nm, tw = 20nm, tw = 50nm, tw = 80nm, and tw = 140nm, respectively. The other geometric parameters are: Λg = 3μm, tAg = 0.1μm and WAg = 1.5μm. The reflection of the resonant modes of the plasmonic nanoantenna is studied in the 3μm-12μm wavelength range using an angle-resulted Fourier transform infrared spectroscopy (FTIR). The incident light is TM-polarized (magnetic field parallel to the wires) with an incident angle of 15°. The reflection signal (R) was normalized to the reflection of a flat Ag mirror.
As shown in Fig. 2(a), there is no resonance mode that can be observed for tw = 0nm, i.e. the structure is a surface-relief Ag grating. For tw = 20nm, a LSP mode at λ = 7.45μm can be observed within the tiny MIM cavities with a size of λ2/1625. The MIM cavities play the role of a FP resonator for the slow plasmonic wave propagating along the x-direction and reflected at the terminations of the wire. The FP resonant mode inside the MIM cavity is a fundamental FP one with no node. As tw increases, the resonance mode shifts to a shorter wavelength. It has been theoretically reported that the propagation constant and effective index inside the MIM cavity increases with a decreasing thickness of the MIM cavity [27,28]. The high effective index of the mode is due to the strong coupling between the thin metal wire and the Ag mirror. The optical path inside the MIM cavity decreases for an increasing tw. As a result, the resonance wavelength shifts to shorter wavelength. It can be also found that there is a resonance mode at 11.8μm for tw = 20nm. In a similar manner to the resonance mode at 7.45μm, it shifts to a shorter wavelength with an increasing tw.
Figure 2(b) shows the simulation and experimental results of the resonance wavelength as a function of thickness. The experimental results show fair agreement with the simulation results. The resonance wavelength dramatically changes for a relatively small tw. As tw > 100nm, the variance of the resonance wavelength can barely be observed.
The Hy field distribution of the three resonance modes for tw = 20nm are shown in Figs. 2(c), 2(d) and 2(e). It can be seen that the resonance mode for λ = 7.45μm is a fundamental FP resonance mode. The resonance mode for 11.8μm is also a fundamental FP resonance mode but with a weak local field enhancement. This resonance wavelength is close to the SiO2 phonon vibration absorption peak. Therefore, the refractive index at λ = 11.8μm (n = 2.14) is much higher than that at λ = 7.45μm (n = 1.2). Therefore, the MIM cavity can support another fundamental FP mode with a longer wavelength.
Figure 3 shows the omnidirectional property of the nanoantenna array. The black solid line is the SP dispersion for Ag/Air interface. The red, blue, green, and pink symbols represent tw = 20nm, tw = 50nm, tw = 80nm, and tw = 140nm, respectively. It can be seen that the resonant wavelength remain almost the same for tw = 20nm and tw = 50nm for an incident angle varying from 15° to 65° with a step of 5°. In the area of the left hand side of the SP dispersion line, the resonance wavelength for tw = 140nm slight shifts to a longer wavelength as the incident angle approaches the SP dispersion line, while in the area of the right hand side of the SP dispersion line, the resonance wavelength for tw = 140nm slightly shifts to a shorter wavelength as the incident angle approaches the SP dispersion line. As the resonance wavelength and the incident angle satisfy the SP resonance condition, the grating-coupled SP at the Ag grating/Air interface with a high local density of state couples out the LSP inside the MIM cavity. The resonance condition near the SP dispersion line is thus altered.
According to the Kirchhoff’s law, the emissivity is equal to absorptivity. The previous works have been shown that the thermal emission peak is coincide to the SP dispersion  and surface phonon dispersion . It is found that the absorption peak at specific incident angle leads to an emission peak at the same collection angle. These significant results imply that the Kirchhoff’s law can be extend to be: the emissivity of a material at specific energy (wavelength) and momentum (collection angle) is equal to its absorptivity at specific energy and momentum (incident angle). Based on this concept, our previous work  showed that the emission spectrum can be well predicted based on the angle-resolved reflectance spectrum. Therefore, we conclude that the angle-independent resonance properties shown in the Fig. 3 can contribute to angle-independent thermal emission. The thermal emission, based on high density of state at the band edge of PhC  and the dispersion of the grating-coupled SP, presents relatively higher quality-value than that of the MIM cavity resonance. This makes the thermal emission of the PhC band edge and the grating-coupled SP present high temporal coherence. However, the resonance of the PhC band edge and the grating-coupled SP highly depends on the momentum matching condition. Thereby, the resonance can only be observed at a specific angle. This extremely high angle-dependent property is especially obvious for the thermal emission from the grating-coupled SP resonance. This characteristic limits their applications and overall efficiency. In this study, the proposed plasmonic nanoantenna arrays present an omnidirectional resonance property. The resonance thermal emission can be observed over a wide observing angle. This kind of characteristic is more convenient as an IR illumination source.
4. Resonant emissivity
For the measurement of the emissivity of a gray body, an ideal blackbody emitter is usually needed as a reference. Here, a self-reference method is applied. First, the TE-polarized thermal radiation of the PTE was measured as a reference using an FTIR spectrometer. The thermal radiation from the heated PTE was collected using a concave mirror with a numerical aperture of 0.1. In the measuring spectral range, there is no resonance mode for TE-polarized light. Thereby, the ideal emissivity spectrum of the TE-polarized light can be theoretically evaluated. In this paper, the ideal emissivity of TE-polarized light is calculated using the RCWA method. The absorptance of different incident angle is averaged. Therefore, the calculated emissivity is angle-averaged. Then, the TM-polarized thermal emission was measured and normalized to the TE-polarized thermal radiation. The normalized results can be seen as the enhancement of the thermal emission. The enhancement of the thermal emission at λ = 5.8μm for tw = 50nm is 12.3. Multiplying the normalized thermal radiation spectrum by the theoretical TE-polarized emissivity, one can obtain the TM-polarized emissivity spectrum as shown in Fig. 4(a). The emissivity spectrum is measured from the PTE heated at a temperature of 250°C. Black, red, blue and green solid lines represent the TM-polarized emissivity of the plasmonic nanoantenna array with tw = 20nm, tw = 50nm, tw = 80nm and tw = 140nm. The emissivity for tw = 0nm is not shown here because there is no resonance mode.
As shown in Fig. 4(a), the resonance wavelength gradually shifts to a shorter wavelength as tw increases. Under the off-resonance condition, the plasmonic nanoantenna array shows a low emissivity, around 2% within the investigated spectral range, which is consistent with the inherent properties of a Ag plate. It can be seen that the trend of emissivity climbs up and then declines for an increasing tw. For tw = 50nm, the maximum of the TM emissivity is 0.19 and the enhancement is 12.3. Additionally, the emissivity is an angle-averaged one. Therefore, the sharp resonance peak reported in [8,17] is not found owing that the angle-dependent SP mode was merged with the angle-independent SP mode after the average over solid angles.
Figure 4(b) shows the maximum of the TM emissivity for different tw. Also shown in Fig. 4(b) is the Hy field enhancement at resonance wavelength for different tw. The field enhancement is defined as the maximum of the localized magnetic field normalized to the input magnetic field. The input field is TM-polarized. Therefore, only y-component of the magnetic field, Hy, is shown. The resonance mode is a fundamental FP-like mode, as shown in Fig. 1. It can be seen that the trend of the Hy field enhancement as a function of tw is similar to that of emissivity. The Hy field enhancement (450 folds) shows a maximum for tw = 50nm. As tw > 50nm, the coupling strength between the two metallic structure decreases leading to a decreasing Hy field enhancement. As tw < 50nm, the MIM cavity becomes lossy. The Hy field enhancement thus decreases.
In this paper, the optical properties of a plasmonic nanoantenna array have been investigated. The proposed plasmonic structure supports omnidirectional LSP resonance, leading to an omnidirectional reflection dip and omnidirectional emission peak. It is shown that the omnidirectional properties will be perturbed as the resonance condition gets close to the grating-coupled SP dispersion line. In addition, the reflection and emission of the plasmonic nanoantenna array with various MIM cavity thicknesses are theoretically and experimentally investigated. It is shown that the resonance wavelength shifts to a longer wavelength for a decreasing MIM cavity thickness owing to the strong coupling between the two adjacent metallic structures. The resonance condition simulated by using RCWA shows a fair agreement with the experimental results.
The authors are grateful for the financial support of this research received from the National Science Council of Taiwan, R.O.C. under grant number NSC 101-2221-E-259-024-MY3 and NSC 102-2120-M-259-002-.
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