# Design and Fabrication of a Wavelength-Selective Near-Infrared Metasurface Emitter for a Thermophotovoltaic System

^{*}

## Abstract

**:**

_{2}-based metal–insulator–metal-structured metasurface for the thermal emitter of the thermophotovoltaic system was designed and fabricated. The proposed emitter was fabricated by applying the photolithography method. The fabricated emitter has high emissivity in the visible to near-infrared region and shows excellent wavelength selectivity. This spectral emissivity tendency agreed well with the result calculated by the finite-difference time-domain method. Additionally, the underlying mechanism of its emission was scrutinized. Study of the fabrication process and theoretical mechanisms of the emission, clarified in this research, will be fundamental to design the wavelength-selective thermal emitter.

## 1. Introduction

## 2. Computational Design and Experimental Process

_{2}was chosen as a dielectric spacer and W was chosen as the metallic part of the proposed metasurface. Although there is a mismatch in the thermal expansion coefficient between W and SiO

_{2}, a metasurface using these materials, which is stable up to 800 K, has been fabricated previously and reported [9]. Therefore, we assumed that our proposed MIM-structured metasurface was reasonable, to be utilized, for the TPV emitter, which requires a high operating temperature. Periods of the unit cell for the x and y directions were λ = 600 nm and the diameter of the W disks was w = 350 nm. The height of the W disks and thickness of the dielectric spacer were fixed to 50 and 100 nm, respectively. To compute the proposed structure’s optical characteristics, we employed the Lumerical FDTD software. Dielectric functions of SiO

_{2}and W were obtained from the tabulated data from Palik [13].

_{λ}) could be calculated from the Kirchhoff’s law (i.e., ${\epsilon}_{\lambda}=1-{R}_{\lambda}$, where ${R}_{\lambda}$ is the spectral reflectance). It can be seen that the higher emissivity spectrum matched the higher QE region.

_{2}films were sputtered on a Si plate; (b) coating the top SiO

_{2}film with a positive photoresist; (c) pattern transfer to the photoresist with the exposure method using an i-line stepper; (d) after the post exposure bake, the photoresist layer was developed to form a resist pattern; (e) W was sputtered on the sample; and (f) the rest of the photoresist was removed.

_{2}film were sputtered for 100 nm on the substrate. Then, a positive type photoresist was spin-coated on the top SiO

_{2}film and prebaked before exposure to ultraviolet (UV) light to form the resist pattern. Reticle with a 0.375 μm hole was used for the i-line stepper. Afterwards, the exposed photoresist was dissolved and resist patterns appearred. Finally, W was sputtered for 50 nm on the sample and the unnecessary W of the rest of the photoresist was removed by acetone.

## 3. Results and Discussion

_{2}film between the W disk and the bottom W plate. Furthermore, the electric field created a closed current loop, which created an enhanced magnetic field and thus formed MP. Therefore, the proposed emitter—which excites MP—could be said to have a strong emissivity peak at the highest QE wavelength region of the PV cell.

_{2}, respectively, and ${c}_{1,\mathrm{W}}=0.32$ is the numerical factor to consider the fringe effect or non-uniform charge distribution along the surface of the capacitor. Originally, the numerical factor is recommended to be used in the range between about 0.2 and 0.3 [15]. ${\delta}_{W}=\frac{\lambda}{2\pi {\kappa}_{w}}$is the effective penetration depth of W, where ${\kappa}_{W}$ is the extinction coefficient of W. The total impedance of this LC circuit model can be obtained as:

_{in}, was defined by using Planck’s spectral distribution of emissive power.

_{m}is defined in a manner similar to a classic derivation by Loferski [20].

_{sc}is the short-circuit current, and V

_{mp}is the voltage at maximum power. In the ideal case, we assumed that there was no heat loss for the input power, and the view factor from the emitter to the PV cell was equal for unity. Therefore, the efficiency of the TPV system was defined as:

## 4. Conclusion

_{2}-based metasurface emitter for the TPV system, which shows a high emissivity peak at the high QE wavelength region of the PV cell, was designed and fabricated. The designed metasurface was fabricated by applying the photolithography method and it was compared to FDTD simulation results. The fabricated metasurface showed high emissivity in the visible to near-infrared light region and indicated excellent wavelength-selectivity. Its emissivity tendency agreed reasonably well with the FDTD simulation. In addition, we clarified that the emissivity enhancement that is underpinning the emissivity peak at 1.7 μm originated from the excitation of MP by calculating the EM field and LC circuit model. It has been shown that the spectral absorption of the proposed emitter is nearly independent of the incident and polarization angles. This study will not only help to understand the mechanisms that can be used to tailor the emissivity enhancement, but will also facilitate the design and practical fabrication of nanostructures for applications in TPV systems.

## Author Contributions

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**(

**a**) Schematic of the proposed metasurface. (

**b**) Simulated spectral emissivity of the proposed emitter obtained from finite-difference time-domain (FDTD) simulation (blue dot) and the quantum efficiencies (QE) of the GaSb PV cell (orange dot) [14].

**Figure 2.**Schematic of the fabrication process of the proposed metasurface. (

**a**) Thin W and SiO

_{2}films were sputtered; (

**b**) photoresist coating; (

**c**) ultraviolet (UV) light exposure through a reticle; (

**d**) the photoresist layer was developed to form a resist pattern; (

**e**) the thin W layer was sputtered; (

**f**) lift-off (photoresist with the unnecessary W removed). Reproduced with permission from [8], published by OSA Publishing, 2017.

**Figure 3.**(

**a**) Spectral emissivity of the proposed emitter obtained from FDTD simulation (blue line) and the fabricated sample (red dot). (

**b**) Scanning electron microscopy (SEM) images of the fabricated metasurface from the top.

**Figure 4.**(

**a**) Electromagnetic (EM) field profiles of the proposed emitter at 1.7 μm, calculated by FDTD simulation. The color counter shows the logarithm of the normalized magnitude of the square of the y-component magnetic field and the vectors show the direction and magnitude of the electric field; (

**b**) equivalent LC circuit model.

**Figure 5.**Contour diagram of the emissivity of the proposed emitter obtained by FDTD simulation for (

**a**) Transverse Magnetic (TM) waves and (

**b**) Transverse Electric (TE) waves, in terms of wavelength and incident angle up to 30°.

**Figure 6.**(

**a**) Ideal emissivity spectra with wavelength ranges of 0.25 μm (case 1), 0.50 μm (case 2), and 0.75 μm (case 3). The peak position was located at the highest quantum efficiency region. (

**b**) Efficiency of the present TPV system with emissivity spectra based on the three cases, the measured and simulated emissivity, and blackbody.

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**MDPI and ACS Style**

Sakurai, A.; Matsuno, Y.
Design and Fabrication of a Wavelength-Selective Near-Infrared Metasurface Emitter for a Thermophotovoltaic System. *Micromachines* **2019**, *10*, 157.
https://doi.org/10.3390/mi10020157

**AMA Style**

Sakurai A, Matsuno Y.
Design and Fabrication of a Wavelength-Selective Near-Infrared Metasurface Emitter for a Thermophotovoltaic System. *Micromachines*. 2019; 10(2):157.
https://doi.org/10.3390/mi10020157

**Chicago/Turabian Style**

Sakurai, Atsushi, and Yuki Matsuno.
2019. "Design and Fabrication of a Wavelength-Selective Near-Infrared Metasurface Emitter for a Thermophotovoltaic System" *Micromachines* 10, no. 2: 157.
https://doi.org/10.3390/mi10020157