1. Introduction

Controlling thermal radiation using artificially resonant structures have recently attracted widely attention [1–3]. Versatile applications based on thermal emitter have been demonstrated, for example, mid-infrared sensing [4], thermophotovoltaics [5] and even radiative cooling [6, 7]. The initial works in controlled thermal radiation areas were based on either grating anomaly [8–10] or photonic crystal structure (PhC) [11, 12]. Based on the grating anomaly, one can obtain an extremely sharp thermal radiation, nevertheless, only at specific observation angle. Engineering the bandgap of PhCs, one can obtain much less dispersive thermal radiation than that based on grating anomaly. Thanks to the fascinating initial works, a series of researches have been demonstrated to make the thermal emitter more practical. For instance, people use metamaterial structure to demonstrate low dispersive thermal radiation [13]. Furthermore, both the PhCs and multilayer metamaterials suffer from challenge in fabrication process. Thermal emitters based on metasurfaces have been proposed owing to its relative simple structure [14–16].

The compositions of the metamaterials as well as metasurfaces for thermal emitters usually are metal and lossless dielectric. As plasmonic modes are arisen, the resonant collective electrons lead to Ohmic loss which corresponds to a resonant absorption in spectrum. According to the well-known Kirchhoff's law of thermal radiation [17], the absorptivity of a material is equal to its emissivity in thermodynamic equilibrium. Therefore, engineering the resonant plasmonic modes can thus manipulate the absorption as well the emission properties.

Light sources with a spectral range at 9-11 μm corresponding to the radiation peak of human is important for health and disease sensing. Usually, people use CO₂ laser as a light source at this spectral range. SiO₂ and MgF₂ are the most commonly used dielectric materials as parts of metamaterial structures. However, these materials suffer from phonon absorption at 9-11 μm. Therefore, the thermal emitters at 9-11 μm based on plasmonic metamaterials and metasurfaces are seldom investigated.

In this paper, a thermal emitter with an emission wavelength at 10 μm is demonstrated. The phono absorption of SiO₂ is enhanced via plasmonic induced magnetic resonance. It is found that the far field distribution of the proposed emitter is Lambertian which will be useful for IR sensing application.

2. Schematic of the 1D metasurface

The schematic of the investigated 1D metasurface thermal emitter is shown in Fig. 1. The structure consists of a SiO₂ layer sandwiched by 1D Ag wire array and Ag thin film. The thickness of SiO₂ layer and Ag wire is denoted by T_SiO2 and T_Ag, respectively. The periodicity of the structure is denoted by Λ_g. The width of the Ag wire is denoted by W_Ag. Owing that the resonance of the gap plasmon is designed to be with resonance wavelength in mid-infrared (MIR) region. The plasmonic structure can be fabricated by standard photolithography and lift-off procedure. E-beam lithography is not needed. First, a 100 nm Ag layer and SiO₂ layer was sequentially deposited onto a silicon substrate using sputtering deposition. Conventionally, a well-defined nano-gap between two adjacent metallic structures, for example, bow-tie and dimer structures, relies on precisely E-beam lithography. Using the sputtering deposition technology, the gap between the two metallic structures can be well defined. This makes us able to investigate the property of the gap-plasmon resonance more stable. Usually a thinner T_SiO2 usually leads to a higher field enhancement. To investigate the dependence of the device properties on the thickness of SiO₂ layers, we fabricated 3 different samples with T_SiO2 = 50 nm, 100nm and 150nm, respectively. After SiO₂ deposition, a photo-resist (PR) was spin-coated on the SiO₂ layer. The design structure was transferred onto PR layer using a mask aligner. After UV light exposure, the samples was developed and coated by 100 nm of Ag using sputtering deposition. A metallic structure was transferred onto the SiO₂ layer and PR was removed using standard lift-off process.

 

Fig. 1 Side-view diagram of the investigated 1D metasurface. The thickness of Ag wire and SiO₂ layer is denoted by T_Ag and T_SiO2, respectively. The width of Ag wire is denoted by W_Ag. The incident angle is θ_in.

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The thickness of the Ag wire is kept to be T_Ag = 100nm. Under such a condition, the silt is too thin to support surface plasmon (SP) standing wave within it that usually plays part of role of extraordinary transmission [18]. The grating period Λ_g and the width of the Ag wires W_Ag are fixed to be 3μm and 1.5 μm, respectively. Therefore, the TE-polarized light is cut-off at the investigating spectral range from 7 μm to 14 μm. The total area of the fabricated samples is fixed to be 5mm X 5mm which is much larger than the probe beam.

3. Thickness-dependence resonance properties of the thermal emitter

First, as shown in Fig. 2(a), reflection spectra of the fabricated samples are measured using Fourier transform infrared spectroscopy (FTIR) under an oblique incident angle of 12°. The incident light is TM-polarized (electric vector is perpendicular to the grating grooves). Black, red, and blue solid lines represent the sample with T_SiO2 = 50 nm, T_SiO2 = 100 nm, and T_SiO2 = 150 nm, respectively. As one can seen that there is a clear resonance peak at λ = 6.2μm for T_SiO2 = 50 nm. This mode is assigned to be a fundamental Fabry-Perot (FP) gap-plasmon mode inside the metal/insulator/metal (MIM) cavity formed by the two adjacent metallic structures. At this wavelength, the absorption coefficient of SiO₂ is ignorable. The absorption mostly comes of the Ohmic heating inducing from the gap-plasmon resonance. Spectral scaling can be simply achieved by tuning the cavity length or the gap thickness of the MIM cavity structures. As shown in here, by tuning the gap thickness T_SiO2, we are able to manipulate the gap-plasmon resonance wavelength. Increasing the gap thickness shifts the gap-plasmon modes toward shorter wavelengths. This is because that the coupling between the two adjacent metallic structures is weaker under a thicker T_SiO2. This results in a decreasing effective index inside the MIM cavity as an increasing T_SiO2 [19]. It is shown that the gap-plasmon resonance wavelength blueshifts from 6.2 μm to 5.2 μm as T_SiO2 sweep from 50 nm to 150nm. For just 100nm difference in thickness can lead to 1μm wavelength shift.

 

Fig. 2 (a) Reflection spectrum of 1D thermal emitter. Black, red and blue solid lines indicate the thermal emitter with T_SiO2 = 50 nm, T_SiO2 = 100 nm, and T_SiO2 = 150 nm, respectively. The other geometry parameters of the structure are W_Ag = 1.5μm, T_Ag = 100nm, Λ_g = 3μm. (b) H_y field distribution at 10.2 μm.

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It can be also found a reflection dip at 10.2 μm which is close to the phonon absorption band of SiO₂ [20]. Although the deposited SiO₂ film is amorphous, we believe that the dielectric constant of monocrystalline SiO₂ refer from ref [20]. still offers fair agreement with amorphous one. As shown in the Fig. 2, the peak wavelength remains almost the same as an increasing T_SiO2. The plasmonic structure enhances the phonon absorption of SiO₂. Nevertheless, the peak wavelength is decided by the inherent material absorption of SiO₂. The y-component of the magnetic field (H_y) at 10.2 μm is shown in Fig. 2(b). It is shown that the localized field is weakly confined in the MIM cavity. The local field enhancement is weak because of the absorption of SiO₂. In the following, we will utilize the enhanced absorption to realize a thermal emitter with an emission wavelength at 10 μm.

4. Thermal emitter with Lambertian distribution

Since controllable thermal emission from structured materials has been proposed, it arises a lot of research interests. The initial research is based on grating structures that can support surface waves. The dissipation of the surface waves leads to a sharp thermal emission. However, it suffers from huge dispersion owing to the nature of grating anomaly. This phenomenon limits it practical application. Figure 3 shows the measured emissivity of our propose structure. The sample is heated with a temperature of 250°C in a vacuum chamber. The emissivity is at the resonance wavelength of 10.2 μm measured by using FTIR. As shown in Fig. 3, the emissivity at an observation angle of 12° can be as high as 0.43. For a Ag flat surface, it emissivity is about 0.05 at 10 μm. It can be found that the emissivity gradually deceases as an increasing observation angle. For reference, a Lambert's cosine curve is illustrated (blue solid line). Lambert's cosine law says that the radiant intensity observed from an ideal diffusely reflecting radiator is directly proportional to the cosine of the observation angle. A source with a Lambertian distribution is convenient for many applications. It is shown that the emissivity at 10.2 μm (black square) has a good agreement with the Lambert's cosine curve. As shown in Fig. 2(b), the localized field at 10.2 μm is weak because of the absorption of SiO₂. For such as weak local field enhancement, the thermal emission of the structure at 10 μm can be seemed as a planar uniform light source. Therefore, the angle-distribution at 10.2 μm shows a good agreement with the Lambertian distribution. The emissivity at 6.2 μm (red circle) is also shown in Fig. 3. At the same geometric condition, the local field enhancement for the resonance mode at 6.2 μm is larger than 100. Therefore, it is not so similar to Lambertian distribution compared to that at 10.2 μm.

 

Fig. 3 Emissivity as a function of observation angle. The blue solid line shows the ideal Lambertian distribution. The black square and red circle symbols indicates the experimental data at 10.2 μm and 6.2 μm, respectively. The emissivity is at the resonance wavelength of 10.2 μm. The geometry parameters of the structure are T_SiO2 = 100 nm, W_Ag = 1.5μm, T_Ag = 100nm, Λ_g = 3μm.

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Figure 4 shows the thermal enhancement of the plasmonic thermal emitter heated at 250°C in a vacuum chamber. The enhancement is defined as the thermal emission of TM-polarized light normalizing to TE-polarized one. The thermal radiation of TE polarized light is similar to the thermal emission from a Ag flat surface. Therefore, we use the TE-polarized emission as reference so that we can avoid the alignment sample of reference samples. Conventionally, for measuring the emissivity of a gray body, an ideal blackbody emitter is usually needed as a reference. It is challenge to quantifiably compare the thermal radiation of each sample. Here, we avoid to deal with the absolute radiation energy but to measure the plasmon-assisted emission enhancement. The thermal emission of the proposed thermal emitters with a series of thickness, T_SiO2 = 50 nm, T_SiO2 = 100 nm, and T_SiO2 = 150 nm, is collected using a Au coated concave mirror with a numerical aperture of 0.1. First, the TM-polarized and TE-polarized (electric vector is parallel to the grating grooves) thermal radiations of the metasurfaces are measured using an FTIR spectrometer and a wire grid polarizer. Then the TM-polarized emittance is normalized to the TE-polarized one which is named emission enhancement. This is a self-reference method. By this method, the error induced from the optical alignment tolerance can be easily eliminated. As shown in Fig. 4, we can obtain a very clear emission peak at the photon absorption of the SiO₂. The emittance enhancement can be as high as 5.8 folds for T_SiO2 = 100 nm. We believe that this can be as an efficient MIR polarized light source.

 

Fig. 4 Emittance enhancement of heated metasurfaces with a temperature of 250°C. Black, red, and blue solid lines represent the metasurfaces w the thermal emitter with T_SiO2 = 50 nm, T_SiO2 = 100 nm, and T_SiO2 = 150 nm, respectively. The other geometry parameters of the structure are W_Ag = 1.5μm, T_Ag = 100nm, Λ_g = 3μm.

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5. Conclusion

In summary, a narrow band thermal emission at 10 μm based on plamsonic enhanced absorption is demonstrated. The proposed structure is a 1D metasurface consisting of a weakly loss dielectric sandwiching by a Ag grating and Ag film. 5.8-folds enhancement of emissivity is achieved by optimizing the thickness of the loss dielectric, SiO₂. The proposed metasurface structure provides magnetic resonance mode that enhances the phonon absorption of SiO₂. It is found that the emissivity of the proposed metasurface shows a good agreement with the Lambert’s cosine law. This type of thermal emitter will be useful for IR sensing applications.