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Abstract
Thermal emission control has been attracting increased attention in both fundamental science and many applications including infrared sensing, radiative cooling and thermophotovoltaics. In this paper, a tunable dual-band thermal emitter including phase-changing material Ge2Sb2Te5 (GST) is experimentally demonstrated. Two emission peak wavelengths are at 7.36 μm and 5.40 μm at amorphous phase, and can be continuously tuned to 10.01 μm and 7.56 μm while GST is tuned to crystalline phase. Compared with other dual-band metamaterial emitters, this tunable dual-band thermal emitter is only composed of an array of single-sized GST nanodisks (on a gold film), which can greatly simplify the design and manufacturing process, and pave the way towards dynamical thermal emission control.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared spectral region is important for chemical sensing since many chemical bonds exhibit unique absorption bands in this region. Nondispersive infrared (NDIR) technique is an effective technique for infrared sensing [1–5], which is based on the ratio of the chemical compound absorption at two wavelengths. In a typical NDIR sensing, one wavelength is selected in the absorption band of the target chemical compound (the active channel), and the other is in the non-absorbing region (the reference channel). Conventional NDIR sensors employ blackbody thermal emitters (such as micro-bulbs) as sources and the dual-band thermal emission necessary for the sensing is picked out from the broad-band source by two narrow-band filters. Since only a small portion of the thermal emission is utilized and most is discarded, the NDIR technique is fundamentally energy inefficient.
To solve this problem, the application of dual-band thermal emitters to NDIR sensing has been explored [6–8]. Compared with a conventional blackbody emitter, the power consumption of dual-band thermal emitters is reduced due to the suppression of unnecessary radiation. However, previous methods to achieve dual-band metamaterials are either based on resonators of different sizes forming a super-unit-cell [9–19], or by stacking multiple layers of resonators with different geometric dimensions separated by dielectric layers [20, 21], the design and fabrication process of which are rather complicated. Besides, these dual-band thermal emitters are not tunable, and cannot be used to for multi-chemical-compound sensing. Recently growing interests in metamaterial-based thermal emitters [22–35] and tunable thermal emitters [36–40] offer possibilities of achieving a tunable dual-band thermal emitter with simple structures. Tunable thermal emission using metal-insulator-metal (MIM) structure, where continuous Ge2Sb2Te5 (GST) film is selected as the middle layer, has been proposed [41]. The third-order magnetic mode is mixed with the anti-reflection mode, so the bandwidth of the peak is very broad and the magnetic mode cannot be controlled individually. In this work, pure magnetic resonances can be obtained by fabricating GST as nanodisks. Pure fundamental and third-order magnetic resonances can be used for tunable dual-band thermal emitters with potential application in the NDIR sensing.
In this paper, a tunable dual-band thermal emitter consisting of single-sized phase-changing material GST nanodisks on a gold film is experimentally demonstrated. The dual-band thermal emission is based on the fundamental and third-order magnetic resonances. The tunable thermal emission behavior is implemented by incorporating phase-changing material GST. Two thermal emission peaks can be continuously tuned from 7.36 μm and 5.40 μm to 10.01 μm and 7.56 μm while GST is tuned from amorphous to crystalline phase. The dual-band thermal emitter contains only an array of GST nanodisks in consistent disk size, which can greatly simplify the design and manufacturing process compared with other dual-band metamaterials. Spatially resolved thermal emission is further explored for transverse electric (TE) and transverse magnetic (TM) polarizations. The fundamental magnetic resonance is not sensitive to emission angles for TE polarization, while it redshifts with the increase of emission angles for TM polarization. The third-order magnetic resonance is not sensitive to emission angles for both polarization. Two second-order magnetic resonances can be excited only for TM polarization at oblique angles. One can be only excited over a narrow angular range and the other can be excited over a wide angular range. Such a tunable dual-band thermal emitter combined with phase-changing materials paves a new way to dynamically control of thermal emission.
2. Methods
2.1 GST-based thermal emitter fabrication
A 100-nm-thick Au film is deposited on a silicon substrate by magnetron sputtering. A 1.5-μm-thick photoresist (AR-P 5350) is then spun onto the Au film and baked for 5 minutes at 105 °C. The photoresist is exposed to define the nanohole array by photo lithography using a double sided mask aligner System (MA6 –BSA). The photoresist is then developed in 1:6 AR 300-26/ DI water followed by rinsing in DI water. After development, a 450-nm-thick GST film is then deposited onto the sample by magnetron sputtering. The thermal emitter is realized after liftoff by ultrasonic processing in acetone for 1 minutes.
2.2 Optical Measurement
The emitted spectra are measured by a Fourier transform infrared spectrometer (FTIR) with a room-temperature doped triglycine sulfate (DTGS) detector. The black soot is generally regarded as a perfect reference owing to its high wavelength-independent emissivity. Here the black soot reference is made by firing a rectangular stainless steel slice with a candle and its emissivity is assumed to be 0.97. The as-deposited GST alloy is at the amorphous phase and an annealing process at 180 °C on a hot plate is applied to get the crystalline GST in this paper. The thermal emitter is fixed on a heating stage with a temperature range of 0-300 °C. The baking temperature is read out from heating stage display, which is around 9 °C higher than temperature of sample surface measured by a thermocouple.
2.3 Numerical Simulations
Finite-difference time-domain method (FDTD Solutions v8.13, Lumerical) is used to compute the optical responses of the thermal emitter [42]. The relative permittivity of gold is obtained from Palik’s handbook [43]. According to Kirchhoff's law of thermal radiation, the thermal emissivity of the sample at different emission angles is equal to its absorptivity at corresponding incident angles, so the simulation is done by calculating absorptivity for this thermal emitter.
3. Results and discussion
The schematic of the tunable thermal emitter is depicted in Fig. 1(a), and Fig. 1(b) presents the corresponding scanning electron microscope (SEM) images. The thickness of the bottom gold film and the top GST nanodisks of the fabricated thermal emitter is 100 nm and 450 nm, respectively. The array periodicity is 5 μm, and the diameter of the nanodisks is 3 μm. The total dimensions of the nanostructure arrays are 10 × 10 mm2. The as-deposited GST is at the amorphous phase (termed as aGST). An annealing process above 180 °C on a hot plate is applied to get GST at the crystalline phase (termed as cGST). The experimental and simulated emissivities are obtained at normal direction. The electric field E, the magnetic field H and the wave vector k are along x, y and z direction, respectively, as shown in Fig. 1(a).
Fig. 1 (a) Schematic and (b) SEM images of the fabricated thermal emitter incorporating phase changing material GST. The thermal emitter is composed of the bottom gold film and the top GST nanodisk array.
The emissivities of the thermal emitter at normal incidence are investigated in experiments (Fig. 2(a)) and simulations (Fig. 2(b)). For experimental results, there are two resonances peaking at 7.36 μm and 5.40 μm for the aGST emitter (Fig. 2(a)). The emission peaks at 7.36 μm (with an emissivity of 0.61) and 5.40 μm (with an emissivity of 0.57) correspond to the fundamental (first-order) magnetic resonance and the third-order magnetic resonance, respectively. For the cGST emitter, the peaks of the fundamental magnetic resonance and the third-order magnetic resonance shift to 10.01 μm (with an emissivity of 0.41) and 7.56 μm (with an emissivity of 0.36), respectively (Fig. 2(a)). For simulated results, the aGST-based emitter has two resonances peaking at 7.43 μm (with an emissivity of 0.73) and 5.78 μm (with an emissivity of 0.97), as shown in Fig. 2(b). The cGST-based emitter has two resonances peaking at 10.47 μm (with an emissivity of 0.65) and 8.32 μm (with an emissivity of 0.37). The experimental peak emissivities are generally lower than the simulated ones for both aGST and cGST thermal emitters. This may be caused by the rough disk boundaries owing to the imperfection in fabrication and oxidation of GST during the annealing process [44]. The error in fitting experimental permittivities based on multi-coefficient models (MCMs) can also cause difference between the simulated and experimental results.
Fig. 2 (a) and (b) are experimental and simulated emissivities of the thermal emitter in the normal direction. The black and red lines are for the aGST and cGST samples, respectively. (c) A-D represent the peak wavelength of the aGST and cGST thermal emitters. The colormaps represent the normalized magnetic field intensities and the arrows represent the normalized electric-field vector distribution at corresponding wavelength.
The field patterns for the aGST and cGST thermal emitters are further investigated (A-D in Fig. 2(c)). The peak wavelengths of cGST thermal emitter are 10.47 μm, 8.32 μm, indicated by A, B, respectively, and the peak wavelengths of aGST thermal emitter are 7.43 μm, 5.78 μm, indicated by C, D, respectively. The four colormaps represent magnetic field intensities and the arrows represent the normalized electric-field vector distribution at corresponding wavelength. There are two resonant modes, the fundamental magnetic resonance (A and C in Fig. 2(c)) and the third-order magnetic resonance (B and D in Fig. 2(c)). The maximum magnetic field is located at the bottom of the GST nanodisks just above the gold plane. The electric-field vectors in the gold film and the GST nanodisks are opposite to each other, which generates a significant magnetic response.
Tunability of the dual-band thermal emitter is explored through the GST intermediate phases, which are controlled by different baking temperatures. The crystallization temperature of GST is around 180 °C and the emissivities are measured at a temperature step of 1 °C from 185 °C to 205 °C. The relative permittivity of GST at intermediate phases can be approximately calculated [45]. Simulated resonances are matched with the measured resonances by changing the crystallization fraction of GST from 0% to 100%.
In Fig. 3, both the fundamental magnetic resonance and the third-order magnetic resonance redshift as temperature grows, indicating that the real part of the refractive index of the GST spacer film increases. In Fig. 3(a), when the baking temperature increases from 100 °C to 203 °C, the fundamental magnetic resonance and the third-order magnetic resonance continuously redshift from 7.36 μm to 10.01 μm and from 5.40 μm to 7.56 μm, respectively. In Fig. 3(b), when the GST film is gradually crystallized (crystallization fraction from 0% to 100%), the fundamental magnetic resonance and the third-order magnetic resonance continuously redshift from 7.43 μm to 10.47 μm and from 5.78 μm to 8.32 μm, respectively. The bandwidths of the fundamental (first-order) magnetic resonance and the third-order magnetic resonance increase as GST crystallization fraction grows, as shown in Fig. 3(c), because the GST gets more and more lossy at high crystallization fraction. The emissivity of the third-order magnetic resonance decreases as crystallization fraction grows. With crystallization fraction increasing, the emissivity of the fundamental magnetic resonance increases first and then decreases, as shown in Fig. 3(d). The fundamental magnetic resonance is a resonant mode with non-optimal coupling at aGST phase, and goes through the optimal coupling with the highest emissivity at 30% crystallization. The peak emissivity of the fundamental and third magnetic modes decreases with crystallization fraction increasing from 30% to 100%. When GST changes from aGST to cGST, the imaginary part of GST increases. This leads to the larger loss in GST nanostructures at high crystallization fraction. Larger loss results in larger deviation of the fundamental and third magnetic modes from optimal coupling. Thus the peaks decrease as crystallization fraction increases. Higher temperature can be applied to the device to compensate the peak reduction.
Fig. 3 Tunability of the dual-band thermal emitter through the intermediate phases. (a) Experimental results of continuously tuning emissivities of the thermal emitter at different baking temperatures in the normal direction. (b) Simulated emissivities of the thermal emitter at corresponding crystallization fraction in the normal direction. Extracted (c) bandwidths and (d) peak emissivities of the 1st and 3rd order magnetic resonances versus crystallization fraction in the normal direction.
Spatially resolved thermal emission of aGST thermal emitter is further explored for TE polarization and TM polarization. For TE polarization, the emission peak wavelengths of the fundamental magnetic resonance and the third-order magnetic resonance are robust when emission angles increase for 0° to 5°, 10° and 20°, as shown in Figs. 4(a) and 4(c). It is verified that the fundamental and the third-order magnetic resonances are not sensitive to the emission angles, as shown in Fig. 4(e). For TM polarization, the relation between the emissivities and emission angles are more complicated. The second-order magnetic resonance can be excited at oblique angles. When the angle is 10°, two second-order magnetic resonances are excited with peak wavelengths located between first-order and third-order magnetic resonances, as shown in Fig. 4(b). The magnetic field is confined to the interface between GST nanodisks and bottom Au layers (A and B in Fig. 4(g)), signifying that a typical second-order magnetic resonance is generated. One of the second-order magnetic resonances can only be excited at specific incident angle and vanishes quickly when incident angle increases or decreases. This is the reason why there is only one second-order peak in the experimental results because the other second-order resonance is difficult to excite, as shown in Fig. 4(d). The peak wavelengths of the second-order magnetic resonance and the third-order magnetic resonance are robust for different incident angles, as shown in Fig. 4(f). However, the first-order magnetic resonance redshifts when the emission angle increases. The magnetic field of the first-order magnetic resonance at 10° and 20° emission angle is investigated (C and D in Fig. 4(g)). The magnetic field which is confined to the interface between GST nanodisks and bottom Au layers signify a typical first-order magnetic resonance.
Fig. 4 Simulated emission spectra of the thermal emitter for (a) TE and (b) TM polarization at different emission angles (0°, 5°, 10° and 20°). Experimental emission spectra of the thermal emitter for (c) TE and (d) TM polarization at different emission angles (0°, 5°, 10° and 20°). Simulated emission spectra as functions of emission angle and wavelength for (e) TE and (f) TM polarization. (g) A-D represent normalized magnetic field intensities for TM polarization at 10° and 20° emission angle indicated in Fig. 4(f).
4. Conclusions
In conclusion, a tunable dual-band thermal emitter is experimentally demonstrated. The dynamic thermal emission control is implemented by incorporating zero-static-power phase-changing material GST. The dual-band thermal emission is based on the fundamental magnetic resonance and third-order magnetic resonance. Two emission peak wavelengths are at 7.36 μm and 5.40 μm at amorphous phase, and can be continuously tuned to 10.01 μm and 7.56 μm while GST is tuned to crystalline phase. The dual-band thermal emitter contains single-sized nanodisks which can greatly simplify the design and manufacturing process. One limitation of this thermal emitter is low tuning speed using thermal stimulation. Some works have already demonstrated that the phase transitions of GST can also be induced by electrical [46] or optical stimulation [47, 48]. The typical switching time for electrical stimulation is nanoseconds. The typical switching time for laser pulses can be nanoseconds, or even improved to femtoseconds. Future works on electric excitation and optical stimulation with higher speed need to be explored.
The tunable dual-band thermal emitter paves a new way to many applications including chemical compound sensing [49, 50], radiative cooling [51–55] and thermophotovoltaics (TPV) [56–61]. This dual-band thermal emitter has potential to increase cooling efficiencies in radiative cooling. There are two atmospheric transparent windows (3 µm - 5 µm, 8 µm - 14 µm) in mid-infrared range. Most methods in radiative cooling is based on only one transparent window. Dual-band thermal emitters which have high thermal emissivities in both windows could present a higher cooling efficiency.
Funding
National Key Research and Development Program of China (2017YFA0205700); National Natural Science Foundation of China (61425023, 61575177, 61775194).
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