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Intensity tunable infrared broadband absorbers based on VO2 phase transition using planar layered thin films.

Kocer H, Butun S, Palacios E, Liu Z, Tongay S, Fu D, Wang K, Wu J, Aydin K - Sci Rep (2015)

Bottom Line: Here, we demonstrate a simple, lithography-free approach for obtaining a resonant and dynamically tunable broadband absorber based on vanadium dioxide (VO2) phase transition.Using planar layered thin film structures, where top layer is chosen to be an ultrathin (20 nm) VO2 film, we demonstrate broadband IR light absorption tuning (from ~90% to ~30% in measured absorption) over the entire mid-wavelength infrared spectrum.Broadband tunable absorbers can find applications in absorption filters, thermal emitters, thermophotovoltaics and sensing.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208, USA.

ABSTRACT
Plasmonic and metamaterial based nano/micro-structured materials enable spectrally selective resonant absorption, where the resonant bandwidth and absorption intensity can be engineered by controlling the size and geometry of nanostructures. Here, we demonstrate a simple, lithography-free approach for obtaining a resonant and dynamically tunable broadband absorber based on vanadium dioxide (VO2) phase transition. Using planar layered thin film structures, where top layer is chosen to be an ultrathin (20 nm) VO2 film, we demonstrate broadband IR light absorption tuning (from ~90% to ~30% in measured absorption) over the entire mid-wavelength infrared spectrum. Our numerical and experimental results indicate that the bandwidth of the absorption bands can be controlled by changing the dielectric spacer layer thickness. Broadband tunable absorbers can find applications in absorption filters, thermal emitters, thermophotovoltaics and sensing.

No MeSH data available.


Simulated electric field intensity and absorbed power density of 500 nm thick PMMA i-VO2 (black curves) and m-VO2 (red curves) at λ = 4 μm.20 nm-width red stripe is the VO2 layer, 500 nm-width blue stripe is the PMMA layer and 60 nm-width yellow stripe is the gold layer. (a) Electric field intensity inside the structure. (b) Absorbed power density along the 20 nm top VO2. (c) Absorbed power density along the 60 nm bottom Au.
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f4: Simulated electric field intensity and absorbed power density of 500 nm thick PMMA i-VO2 (black curves) and m-VO2 (red curves) at λ = 4 μm.20 nm-width red stripe is the VO2 layer, 500 nm-width blue stripe is the PMMA layer and 60 nm-width yellow stripe is the gold layer. (a) Electric field intensity inside the structure. (b) Absorbed power density along the 20 nm top VO2. (c) Absorbed power density along the 60 nm bottom Au.

Mentions: In Fig. 3, we plot simulated and measured absorption spectra of i-VO2 (black curves) and m-VO2 (red curves) for the PMMA thicknesses of 500 nm (Fig. 3(a,c)) and 700 nm (Fig. 3(b,d)). For the PMMA thicknesses of 500 nm, both simulated and measured absorption spectra of i-VO2 and m-VO2 coincide around the wavelength of 1.6 μm. Beyond this wavelength, absorption intensity was shown to be dynamically changing from the near perfect absorption (~90% for the simulated and measured absorption) to the low absorption levels (~10% for the simulated and ~30% for measured absorption) in the mid-wavelength infrared spectrum based on VO2 phase transition. When we increase the PMMA thickness to 700 nm, the coincidence wavelength shifts to around 2.2 μm. Again, beyond this wavelength similar dynamic tunability of the absorption intensity was observed. There is a deviation in the measured absorption spectrum of the i-VO2 case from the simulations. This might have been resulted from two facts. Firstly, index of the experimental 60 nm thick Au structure might be somewhat different than the bulk index of the Ref. 34. Secondly, the experimental PMMA index may exhibit slightly dispersive and lossy characteristics instead of the constant one, which was not considered in our simulations. Overall, we obtained remarkably good agreement between the simulations and measurements in terms of predicting the resonant dip positions and broadband intensity tunability with respect to the changing phase of the VO2. When VO2 is insulator, IR light passes through the thin VO2 layer and travels into the lossless PMMA spacer layer and bounces back from the optically thick Au layer. Therefore, i-VO2 structure has high reflection or in other words low absorption. On the other hand, when VO2 becomes metallic at high temperature, increased absorption of the structure may be considered physically reasonable due to the electric field confinement and enhancement within the Fabry-Perot type nanocavity structure. Even though VO2 layer itself is already a temperature-tunable absorber in the MWIR range, we claim that the PMMA and Au are used to localize the electric field and amplify the VO2 effect as shown in Supplementary Figure S2. We also note that measured absorption spectra have two small resonant peaks at the wavelengths of 3.4 μm and 4.2 μm. The first feature around 3.4 μm is caused by the molecular vibrational absorption of PMMA itself. It is only visible in i-VO2 case (black curves of Fig. 3(c,d)) because in m-VO2 case there is an order of magnitude less electric field penetration into the PMMA layer (Fig. 4). This reduces the vibrational mode amplitude to a level which is not visible in red curves of Fig. 3(c,d). The second feature at 4.2 μm is due to the absorption of atmospheric CO2, an artifact related with measurement conditions. Depending on CO2 levels in the room at the time of sample and the reference measurements, it may or may not show up in the measured spectra.


Intensity tunable infrared broadband absorbers based on VO2 phase transition using planar layered thin films.

Kocer H, Butun S, Palacios E, Liu Z, Tongay S, Fu D, Wang K, Wu J, Aydin K - Sci Rep (2015)

Simulated electric field intensity and absorbed power density of 500 nm thick PMMA i-VO2 (black curves) and m-VO2 (red curves) at λ = 4 μm.20 nm-width red stripe is the VO2 layer, 500 nm-width blue stripe is the PMMA layer and 60 nm-width yellow stripe is the gold layer. (a) Electric field intensity inside the structure. (b) Absorbed power density along the 20 nm top VO2. (c) Absorbed power density along the 60 nm bottom Au.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4543955&req=5

f4: Simulated electric field intensity and absorbed power density of 500 nm thick PMMA i-VO2 (black curves) and m-VO2 (red curves) at λ = 4 μm.20 nm-width red stripe is the VO2 layer, 500 nm-width blue stripe is the PMMA layer and 60 nm-width yellow stripe is the gold layer. (a) Electric field intensity inside the structure. (b) Absorbed power density along the 20 nm top VO2. (c) Absorbed power density along the 60 nm bottom Au.
Mentions: In Fig. 3, we plot simulated and measured absorption spectra of i-VO2 (black curves) and m-VO2 (red curves) for the PMMA thicknesses of 500 nm (Fig. 3(a,c)) and 700 nm (Fig. 3(b,d)). For the PMMA thicknesses of 500 nm, both simulated and measured absorption spectra of i-VO2 and m-VO2 coincide around the wavelength of 1.6 μm. Beyond this wavelength, absorption intensity was shown to be dynamically changing from the near perfect absorption (~90% for the simulated and measured absorption) to the low absorption levels (~10% for the simulated and ~30% for measured absorption) in the mid-wavelength infrared spectrum based on VO2 phase transition. When we increase the PMMA thickness to 700 nm, the coincidence wavelength shifts to around 2.2 μm. Again, beyond this wavelength similar dynamic tunability of the absorption intensity was observed. There is a deviation in the measured absorption spectrum of the i-VO2 case from the simulations. This might have been resulted from two facts. Firstly, index of the experimental 60 nm thick Au structure might be somewhat different than the bulk index of the Ref. 34. Secondly, the experimental PMMA index may exhibit slightly dispersive and lossy characteristics instead of the constant one, which was not considered in our simulations. Overall, we obtained remarkably good agreement between the simulations and measurements in terms of predicting the resonant dip positions and broadband intensity tunability with respect to the changing phase of the VO2. When VO2 is insulator, IR light passes through the thin VO2 layer and travels into the lossless PMMA spacer layer and bounces back from the optically thick Au layer. Therefore, i-VO2 structure has high reflection or in other words low absorption. On the other hand, when VO2 becomes metallic at high temperature, increased absorption of the structure may be considered physically reasonable due to the electric field confinement and enhancement within the Fabry-Perot type nanocavity structure. Even though VO2 layer itself is already a temperature-tunable absorber in the MWIR range, we claim that the PMMA and Au are used to localize the electric field and amplify the VO2 effect as shown in Supplementary Figure S2. We also note that measured absorption spectra have two small resonant peaks at the wavelengths of 3.4 μm and 4.2 μm. The first feature around 3.4 μm is caused by the molecular vibrational absorption of PMMA itself. It is only visible in i-VO2 case (black curves of Fig. 3(c,d)) because in m-VO2 case there is an order of magnitude less electric field penetration into the PMMA layer (Fig. 4). This reduces the vibrational mode amplitude to a level which is not visible in red curves of Fig. 3(c,d). The second feature at 4.2 μm is due to the absorption of atmospheric CO2, an artifact related with measurement conditions. Depending on CO2 levels in the room at the time of sample and the reference measurements, it may or may not show up in the measured spectra.

Bottom Line: Here, we demonstrate a simple, lithography-free approach for obtaining a resonant and dynamically tunable broadband absorber based on vanadium dioxide (VO2) phase transition.Using planar layered thin film structures, where top layer is chosen to be an ultrathin (20 nm) VO2 film, we demonstrate broadband IR light absorption tuning (from ~90% to ~30% in measured absorption) over the entire mid-wavelength infrared spectrum.Broadband tunable absorbers can find applications in absorption filters, thermal emitters, thermophotovoltaics and sensing.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208, USA.

ABSTRACT
Plasmonic and metamaterial based nano/micro-structured materials enable spectrally selective resonant absorption, where the resonant bandwidth and absorption intensity can be engineered by controlling the size and geometry of nanostructures. Here, we demonstrate a simple, lithography-free approach for obtaining a resonant and dynamically tunable broadband absorber based on vanadium dioxide (VO2) phase transition. Using planar layered thin film structures, where top layer is chosen to be an ultrathin (20 nm) VO2 film, we demonstrate broadband IR light absorption tuning (from ~90% to ~30% in measured absorption) over the entire mid-wavelength infrared spectrum. Our numerical and experimental results indicate that the bandwidth of the absorption bands can be controlled by changing the dielectric spacer layer thickness. Broadband tunable absorbers can find applications in absorption filters, thermal emitters, thermophotovoltaics and sensing.

No MeSH data available.