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Abstract
When a one-dimensional (1D) metal array is coupled to a planar metal mirror with a dielectric gap, localized plasmon resonance is excited inside the gap at a specific polarization of light in free space. Herein, we report on the completely polarized, mid-infrared thermal radiation that is released from gap plasmon resonators with a nanometer-thick dielectric. We fabricated nanogap plasmon resonators with 1D Au or Ni array of various widths (w) using laser interference lithography. An atomic layer deposition process was used to introduce a 10 nm-thick alumina gap between a 1D metal array and a planar metal mirror. It was observed that only for the Au nanogap plasmon resonators, high-amplitude absorption peaks that were attributed to gap plasmon modes with different orders appeared at discrete wavelengths in a polarization-resolved spectrum. In addition, all the pronounced peaks were gradually redshifted with increasing w. At w = 1.2–1.6 µm, the fundamental gap plasmon mode was tuned to the main wavelengths (8–9 µm) of thermal radiation at room temperature (e.g., ∼300 K), which led to polarization-selective camouflage against standard infrared thermal imaging. The results of electromagnetic simulations quantitatively agreed with the measured absorbance spectra in both peak wavelength and amplitude. We believe that these experimental efforts towards achieving radiation/absorption spectra tailored at mid-infrared wavelengths will be further exploited in thermal-radiation harnessed energy devices, spectroscopic sensors, and radiative coolers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Thermal-radiation light sources such as carbon arc lamps and incandescent light bulbs are generally incoherent, randomly directional, and unpolarized. The spectrum of thermal radiation is substantially broad (e.g., λ = 3–30 µm at room temperature), relying strongly on the temperature of an object. However, efficaciously designed photonic structures facilitate the manipulation of the optical states of thermal radiation [1–8]. For example, thermal radiation is enhanced or suppressed at specific wavelengths based on the principles of photonic crystals [9–11], metamaterials [12–14], and plasmonics [15,16]. These photonic structures exhibit wavelength-dependent absorption based on optical resonance, thus creating modified thermal-radiation spectra according to Kirchhoff’s law of thermal radiation. Such spectrum engineering of thermal radiation enables the development of solar steamers [17,18], spectroscopic sensors [19], radiative coolers [20–22], and various techniques of thermal camouflage for military purposes [23–26]. For implementing thermal camouflage, the emissivity of structures must be tuned to an ambient value [23,24], or they should behave as a mirror [25] or as an absorber [26] at their thermal-radiation wavelengths.
To engineer emissivity/absorptivity of the mid-infrared thermal radiation spectrum at room temperature, metal nanostructures can be designed in three different ways: a micron-depth array (I) [27], nanometer-thick 1D array (II) [28], and 1D metal array-coupled nanogap resonator (III) [Fig. 1(a)]. Figure 1(b) shows how each Au nanostructure can squeeze a normally incident electromagnetic wave into a subwavelength-volume space at a resonant wavelength obtained by finite-difference time-domain (FDTD) simulations. To facilitate more quantitative comparisons, we simulated the absorption spectra for the same structures [Fig. 1(c)]. For the micron-depth Au array, the electric-field was localized through the needle-like trenches [left, Fig. 1(b)]. Thus, a pronounced absorption peak progressively swept a considerable range of the mid-infrared spectrum by an increase in the trench depth (d) while maintaining a near-unity absorbance [left, Fig. 1(c)]. Although this approach can facilitate a tunable and absorptive mode characteristic, the array requires high-aspect-ratio (> 10: 1), deep (d > 1 µm) trenches that cannot be practically fabricated [27]. In contrast, the nanometer-thick Au array is extremely compact (t < 10 nm) and effectively confines an electric field at both sides of the ultrathin Au bars [middle, Fig. 1(b)]. Although the dipolar plasmon resonance is redshifted by simply increasing the width of the metal bars (w) [middle, Fig. 1(c)], the absorbance spectrum is almost flattened at w > 1.5 µm. As a result, the tuning range is limited to approximately λ < 4 µm owing to the mitigated localization of the electric field at long wavelengths [28,29]. The localization of the electric field can be dramatically improved by employing a metal/dielectric/metal configuration [30] or by using an atomically-thick conductive material [31,32], i.e., graphene or hexagonal boron nitride encapsulating graphene is considered an alternative promising candidate for mid-infrared plasmonic materials. Based on the metal/dielectric/metal configuration, for example, Nielson et al. observed gap plasmon mediated high-quality-factor absorption peaks at red wavelengths [33]. In our simulation model, a 10 nm-thick alumina gap was used to excite gap plasmon modes at infrared wavelengths. The 1D metal array-coupled nanogap resonator, in which the electric field was tightly trapped inside the ultrathin alumina gap [right, Fig. 1(b)], yielded an appreciable absorption peak that was shifted to λ > 8 µm in the main room-temperature thermal radiation spectrum.
Fig. 1. (a) Schematic diagrams of the three primary metal nanostructures that generate polarization-dependent, mid-infrared plasmon resonance: a micron-depth array (I), a nanometer-thick array (II), and a 1D metal array-coupled nanogap resonator (III). (b) Profiles of the electric-field intensity acquired at the wavelengths identified by the asterisks in (c). They each correspond to the structures in (a): (p, d, s) = (2 µm, 1.1 µm, 100 nm) (I), (p, w, t) = (2 µm, 1.1 µm, 10 nm) (II), and (p, w, g, t) = (2 µm, 1.1 µm, 10 nm, 50 nm) (III). (c) Simulated absorbance spectra of the structures in (a) with progressively increasing d or w values. For each structure, s = 100 nm (I), t = 10 nm (II), and (g, t) = (10, 50) nm (III).
In this work, we studied one-dimensional (1D) metal array coupled gap plasmon resonators with a 10 nm-thick dielectric spacer to achieve polarized, mid-infrared thermal radiation. To determine the relationship between the electrical conductivity of the metal and the localization of the electric field, we compared the absorption spectra of Au, Ni, and TiN nanogap resonators by conducting FDTD simulations. For the Au structures, we probed the wavelength range of the absorption peaks at a variety of w values. We fabricated Au and Ni nanogap resonators in which laser interference lithography was used to define different 1D array of different w values. An atomic layer deposition (ALD) process was used to control the width of an alumina gap between a 1D metal array and a metal mirror at nanoscales. For fabricated Au and Ni samples, we obtained polarization-resolved absorbance spectra in the range of λ = 4–13 µm using a Fourier transform infrared (FTIR) spectrometer equipped with an integrating sphere. To confirm the polarization-selective feature of the thermal radiation, we imaged Au and Ni samples using a standard thermal camera with an external polarizer.
2. Results
2.1 Tuning of polarization-selective nanogap plasmon resonance
To investigate the nanogap plasmon resonance as a function of the type of metal, we obtained polarization-resolved absorbance spectra with 1D metal array and mirrors composed of Au, Ni, or TiN by conducting FDTD simulations [Fig. 2(a)]. For Au structures, (w, g, t) = (1.5 µm, 10 nm, 50 nm), and for Ni and TiN structures, (w, g, t) = (1.1 µm, 10 nm, 50 nm). Transverse-electric (TE) and -magnetic (TM) polarizations were defined as shown in the schematics of Fig. 2(a); TE (TM) polarization corresponded to an electric field oscillating parallel (perpendicular) to the translational direction of the 1D array. For the Au nanogap resonator, the TE-polarized spectrum exhibited pronounced absorbance peaks at specific mid-infrared wavelengths, whereas the TM-polarized spectrum was not structured, exhibiting near-zero absorbance. The Ni structure also exhibited similar polarization-dependent absorbance spectra, but the TE-polarized absorption peaks were relatively broadened. The sharpness (i.e., quality factor) of plasmon peaks was determined by the complex permittivity of the metal used, more technically by the collision coefficient in the Lorentz-Drude model. In comparison to both structures, the TiN structure exhibited featureless absorbance spectra for both TE and TM polarizations. TiN is considered a promising visible plasmonic material owing to its high-temperature stability [34]. Additionally, it exhibits a screened plasma wavelength at specific visible wavelengths, which can be tailored depending on deposition techniques [35], thereby enhancing light-matter interactions at those wavelengths [36]. To elucidate the absorption response as a function of the structure materials, we plotted the complex permittivity values (ε1 + iε2) of Au, Ni, and TiN materials at mid-infrared wavelengths, derived from Refs [37]. and [38] as shown in Fig. 2(b). In addition, we obtained the sheet resistance values of Au, Ni, and TiN films by using a four-point probe system [inset, Fig. 2(b)]. Both data supported the use of Au as an appropriate material for mid-infrared plasmonics because Au is a good electrical conductor that possesses numerous free electrons, thereby leading to extremely high ε2 values in the range of mid-infrared wavelengths [39]. Therefore, other metals with good electrical conductivity such as Ag and Cu can exhibit the polarization-dependent, structured absorbance spectra.
Fig. 2. (a) Simulated TE (upper) and TM (lower) absorbance spectra of nanogap plasmon resonators consisting of Au, Ni, or TiN. For Au structures, (p, w, g, t) = (2 µm, 1.5 µm, 10 nm, 50 nm), and for Ni and TiN structures, (p, w, g, t) = (2 µm, 1.1 µm, 10 nm, 50 nm). (b) Material dispersions of Au, Ni, and TiN used in the simulations, obtained from Refs [37]. (Au), (Ni), and [38] (TiN), respectively. The inset shows the sheet resistance of Au (10 nm), Ni (10 nm), and TiN (20 nm) films.
For Au nanogap resonators, we explored the role of each structural parameter (e.g., width, w, and gap size, g) by conducting FDTD simulations. Figure 3(a) shows the simulated absorbance at different w and g values, acquired at λ = 7 µm. The maximum absorbance was achieved at specific combinatorial sets of (w, g); a smaller g was associated with a smaller w, which was consistent with a previous theoretical work [40]. This relationship was understood by calculating the normalized momentum (i.e., the ratio between the wavevectors of the gap plasmon modes and light in free space) of the gap plasmon modes [inset, Fig. 3(a)]. When g was smaller than 20 nm, the normalized momentum dramatically increased owing to the augmented localization of the electric field [41]. Therefore, a reduced metal bar width helps to satisfy the phase-matching condition with light in free space.
Fig. 3. (a) Surface plots showing the TE absorbance (λ = 7 µm) of an Au nanogap resonator at a variety of g and w values. For all the simulated structures, t = 50 nm. The inset presents normalized momentum (left axis) and the maximum absorbance wavelength (right axis) at a variety of g values. (b) Surface plots of the TE absorbance spectra of an Au nanogap resonator at a variety of w values. For all the simulated structures, (g, t) = (10, 50) nm. The unfilled circles indicate the measured absorption peak wavelengths of the fabricated Au nanogap resonators in Fig. 5(a). (c) Profiles of the electric-field intensity obtained at λ = 7.6 µm (upper), 2.9 µm (middle), and 1.8 µm (bottom) for the same structure (w = 1.18 µm).
We probed the tunability of the gap plasmon modes depending on the g and w values. First, using a fixed width (w = 1.5 µm), we traced the maximum absorbance wavelength at a variety of g values [inset, Fig. 3(a)]. The maximum absorbance wavelength shifted from 10 to 6 µm when g was increased for values less than 20 nm. With a larger g, the maximum absorbance appeared at a longer wavelength because gap plasmon resonance generates electric dipoles between parallel metal plates because a smaller gap size induces a lower energy state [42,43]. Next, with a fixed gap size (g = 10 nm), we obtained absorbance spectra for a variety of w values [Fig. 3(b)]. Multiple bands that were identified as gap plasmon modes with different orders [Fig. 3(c)] appeared in the simulated surface plot. As shown in Fig. 1(c), the absorption peak gradually red-shifted with increasing w; for the first-order mode, the absorption peak wavelength shifted from 2 to 8 µm as w increased gradually from 0.4 to 2 µm. Overall, Au nanogap resonators with tailored structural parameters can excite high-amplitude absorption peaks that are tuned to mid-infrared, thermal-radiation wavelengths.
2.2 Observation of polarized thermal radiation
To demonstrate the polarized, mid-infrared thermal radiation investigated thus far in the simulations, we fabricated 1D nanogap resonators composed of Au or Ni. Mason et al. also adopted a similar Au/dielectric/Au configuration to achieve omnidirectional, polarized, high-amplitude thermal radiation. However, the dielectric gap had a thickness of over 100 nm and was composed of a mid-infrared absorptive material, both of which contributed to the thermal radiation feature [44]. Figure 4(a) shows schematic diagrams that illustrate the fabrication process for the 1D nanogap resonators. An ALD process was performed to introduce a 10 nm-thick alumina gap (g = 10 nm) on a 50 nm-thick planar metal mirror-coated infrared-transparent Si substrate. A laser interference lithography system was then used to define various 1D arrays of bars of different w values while their pitches were approximately twice larger than w values. Note that the pitch (i.e., filling fraction) affects only peak amplitude, not peak position. Finally, an e-beam evaporator was used to deposit a 50 nm-thick metal film (t = 50 nm) for the upper array. Figure 4(b) shows scanning electron microscopy (SEM) images of a fabricated 1D Au nanogap resonator. The ultrathin alumina layer clearly separated the upper Au 1D metal array from the underlying Au planar mirror [inset, Fig. 4(b)]. In addition, atomic force microscopy (AFM) imaging was performed to characterize the w and t values of the upper metal array [Fig. 4(c)]. Figure 4(d) shows the visible camera images of the fabricated Au and Ni samples; for the Au structures, w = 0.47 (I), 0.70 (II), 0.88 (III), 1.07 (IV), 1.23 (V), and 1.60 µm (VI), and for the Ni structures, w = 0.45 (I), 0.55 (II), 0.70 (III), 0.90 (IV), and 1.00 µm (V).
Fig. 4. (a) Schematic diagrams showing the fabrication process of 1D metal nanogap resonators. (b) SEM images of the top and cross-sectional views (inset) of a fabricated Au nanogap resonator. (c) AFM image of the fabricated structure in (b). (d) Visible camera images of fabricated Au (left) and Ni (right) nanogap resonators with different w values; for the Au structures, w = 0.47 (I), 0.70 (II), 0.88 (III), 1.07 (IV), 1.23 (V), and 1.60 µm (VI), and for the Ni structures, w = 0.45 (I), 0.55 (II), 0.70 (III), 0.90 (IV), and 1.00 µm (V). For all the fabricated structure, t = 50 nm. (e) Schematic diagram showing an experimental setup for polarimetric thermal camera imaging. (f) TE- and TM-polarized thermal images of the Au (upper) and Ni (lower) structures in (d).
For direct visualization of polarized thermal radiation, we performed polarimetric imaging using a polarizer-mounted thermal camera [Fig. 4(e)]. We obtained polarization-resolved thermal camera images of the Au and Ni samples in Fig. 4(d) at room temperature [Fig. 4(f)]. With increasing w, the Au samples exhibited a stronger intensity for TE polarization while retaining the minimum intensity at TM polarization. The w- and polarization-dependent thermal radiation was not observed for the Ni samples; the intensity was strong even at TM polarization. These observations are consistent with the simulated polarization-resolved absorbance spectra in Fig. 2(a).
To elucidate the origin of the intensity change in the Au samples compared to that in the Ni samples, we obtained mid-infrared (4–13 µm) absorbance spectra using an FTIR spectrometer equipped with an integrating sphere. Figures 5(a) and 5(b) show the TE-polarized absorption spectra of the Au and Ni samples with the spectral sensitivity curve of the thermal camera that was used. Although the peak amplitudes of the Au samples are slightly smaller than the experimental values of existing Au micron-depth array structures [27], their peak sharpness relatively improves; for a peak at λ ∼ 5.5 µm, the linewidths are 0.83 and 1.23 µm for our structure and the reference structure, respectively. Note that the amplitudes could be improved through the critical coupling that is achieved by choosing an appropriate combinatorial set of gap, width and filling fraction of metal bars [40]. In addition, the measured peak wavelengths derived from Figs. 5(a) and 5(b) are plotted as a function of w [circles, Fig. 5(c)].
Fig. 5. (a,b) Measured absorbance spectra of the Au (a) and Ni (b) samples in Fig. 4(d). The blue shaded area represents the spectral sensitivity curve of the thermal camera. (c) Measured and simulated absorption peak wavelengths of the Au and Ni samples as a function of w. The inset shows the cross-sectional TEM image of the fabricated Au sample. (d) Thermal camera images of an Au nanogap resonator with (w, g, t) = (1.60 µm, 10 nm, 50 nm), obtained at discrete polarizer angles (θ). The white dashed lines represent the boundaries of the sample containing the 1D array of metal bars. (e) Normalized thermal intensity of the Au sample in (d) as a function of the polarizer angle (θ).
For direct comparison, FDTD-simulated absorption spectra were obtained at TE polarization for the same structures, showing a good agreement with the measured data [lines, Fig. 5(c)]. A slight discrepancy between the measured and simulated data results mainly from the truncated shape of fabricated metal bars, as evident from a cross-sectional transmittance electron microscopy (TEM) image [inset, Fig. 5(c)]. For the simulation, a 1D array of metal bars with a rectangular cross section was introduced.
Overall, for an increase of w, the absorption peaks of both samples progressively shifted to longer wavelengths and were anchored at λ ∼ 10 µm due to the phonon-polariton resonance of the alumina spacer. The tuning range could be further extended by introducing the air spacer devoid of any dispersion features. Therefore, the absorption spectra could be more overlapped with the spectral sensitivity curve of the thermal camera. In comparison to the Ni samples, the Au samples generated pronounced absorption peaks, leading to the w-dependent intensity change in the upper panel of Fig. 4(f). Given that the Ni samples had non-zero background absorbance over the thermal camera’s sensitivity spectrum, their overall intensity was stronger compared to that of the Au samples [Fig. 4(f)].
Finally, we obtained polarimetric thermal images of an Au nanogap resonator with (w, g, t) = (1.60 µm, 10 nm, 50 nm) at several axis angles (θ) of a front-mounted polarizer [Fig. 5(d)]. The maximum intensity was acquired at θ = 0°, corresponding to the TE-polarized case. The intensity gradually dimmed with an increase in θ and the Au sample completely disappeared at θ = 90°. Note that the 1D array of metal bars was not formed along the facets of the sample; therefore, the underlying Si substrate with a relatively high thermal intensity was exposed along the facets. To quantify these images, the normalized intensity was plotted as a function of θ for the same sample [Fig. 5(e)]. The intensity was maximized at θ = 0° and minimized to a near-zero value at the orthogonal angle. This indicated that the Au sample released the completely polarized thermal radiation. Furthermore, these experimental findings suggest that for an Au nanogap resonator, the level of thermal radiation is controllable when paired with an external polarizer, enabling the implementation of thermal camouflage.
3. Conclusions
In summary, we demonstrated polarized, mid-infrared thermal radiation from 1D metal array-coupled nanogap plasmon resonators. By utilizing an ultrathin (10 nm) alumina gap, the wave vector magnitude of gap plasmon modes was dramatically augmented, leading to the creation of pronounced absorption peaks at mid-infrared wavelengths. We fabricated various Au and Ni nanogap resonators of different 1D metal bar widths using a laser interference lithography process. The measured polarization-resolved absorption spectra revealed that only Au samples yielded high-amplitude and tunable absorption peaks, depending on the 1D metal bar width. This result was strongly supported by FDTD simulations. Polarimetric thermal imaging experiments confirmed that completely polarized thermal radiation was released from the fabricated Au nanogap resonators. We believe that these theoretical and experimental efforts will be useful in developing thermal energy harvesting devices, radiative coolers, and thermal camouflage techniques.
Appendix
Fabrication of the plasmon nanogap resonators: An infrared-transparent Si substrate was cleaned using concentrated sulfuric acid solution and dehydrated at 150 ℃. A 50 nm-thick Au or Ni layer was deposited by an e-beam evaporation process. An ALD process was performed to introduce a 10 nm-thick alumina layer. The substrate was then spin-coated with a 250 nm-thick photoresist (Futurrex, NR9-250P). For the laser interference lithography process, an ultraviolet (λ = 325 nm) laser source (Kimmon, IK3301R-G) was used. Finally, a 50 nm-thick Au or Ni layer was deposited by an e-beam evaporation process and lifted-off in an acetone solution.
Mid-infrared spectroscopy: Measured absorbance spectra in Figs. 5(a) and 5(b) were acquired using an FTIR spectrometer (INVENIO R, Bruker) equipped with a gold-coated integrating sphere (A562-G/Q, Thorlabs).
Thermal polarimetric imaging: A standard thermal camera (FLIR, A655SC) was used to obtain the images shown in Figs. 4(f) and 5(d). A Cu plate was placed under the fabricated nanogap resonators as the minimum thermal radiation background. A ZnSe holographic wire grid polarizer (WP50H-Z, Thorlabs) was used to obtain the polarimetric images.
Structure characterization: An AFM system (XE150, Park systems) was used to measure the width (w) and depth (t) of the array of the metal bars. An ellipsometry system (Elli-SE-U, Ellipso Technology) was used to measure the thickness of the alumina gap.
Electromagnetic simulation: To simulate absorption spectra of the nanogap resonators, we conducted FDTD simulations using a commercial software (Lumerical, FDTD Solutions). The optical dispersion of Au and Ni were obtained from Ref [37]. In particular, the TiN dispersion at mid-infrared wavelengths (3–14 µm) was acquired by fitting a tabulated TiN dispersion at visible and near-infrared wavelengths (0.3–2 µm) to the Lorentz-Drude model [38]. A normally incident plane wave (3–13 µm) was used as a source and a flux monitor (frequency-domain power monitor) was located above the source to measure the reflectance. The nanogap was spatially resolved into the mesh of 1 nm in the x- and y- directions. A perfectly matched layer was used on the top and a perfect electric conductor located under the metal substrate.
Funding
Samsung (SRFC-IT1701-06).
Disclosures
The authors declare no conflicts of interest.
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