Tunable indium tin oxide for metamaterial perfect absorbers and nonlinear devices
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Research Letter
Tunable indium tin oxide for metamaterial perfect absorbers and nonlinear devices Evan M. Smith, KBR, 2601 Mission Point Blvd, Beavercreek, OH 45431, USA Joshua R. Hendrickson, Justin W. Cleary, and Kevin Leedy, Air Force Research Laboratory, Sensors Directorate, Wright-Patterson Air Force Base, OH 45433, USA Junpeng Guo, Department of Electrical and Computer Engineering, University of Alabama in Huntsville, Huntsville, AL 35899, USA Shivashankar Vangala, Air Force Research Laboratory, Sensors Directorate, Wright-Patterson Air Force Base, OH 45433, USA Address all correspondence to Evan M. Smith at [email protected] (Received 11 June 2020; accepted 6 August 2020)
Abstract Indium tin oxide (ITO) has become a very useful plasmonic and nonlinear optical material because of its highly tunable electrical and optical properties and strong optical nonlinearity. In this work, the authors conducted detailed fabrication process studies by using high-temperature reactive sputtering to finely tune the optical properties of ITO thin films, particularly the epsilon-near-zero (ENZ) wavelength in the near and mid-IR spectrum. Sputtered ITO thin films are characterized by using spectroscopic ellipsometry, surface profilometry, Hall measurements, and 4-point probe testing. Additionally, the effect of post-deposition annealing of ITO films is also investigated.
Introduction Indium tin oxide (ITO) is a useful plasmonic optical material because of its widely tunable optical permittivity and low loss in the near-infrared (NIR) and mid-wave infrared (MWIR) spectral ranges. ITO has been investigated for a variety of applications such as wideband tunable absorption,[1–5] waveguide modulation,[6] beam steering,[7] sub-picosecond modulation,[8] and resonance splitting and time refraction.[9] Furthermore, strong nonlinear responses have been measured in ITO which demonstrate a near unity change in index of refraction under high intensity illumination,[10] as well as a large enhancement of second harmonic generation.[11,12] Many of these applications exploit the strong optical enhancement around the epsilon-near-zero (ENZ) wavelength, which is the wavelength at which the real part of the permittivity switches from a positive value to a negative value. As the imaginary part of the permittivity is also low at this wavelength, the complex index of refraction is minimized. Although most commercially produced ITO films have an ENZ wavelength between 1.2 and 1.3 μm,[10] the ENZ wavelength can be configured by changing the deposition and annealing parameters. Tuning the ENZ wavelength of ITO films allows this material to be integrated into plasmonic and photonic devices with specific functionality. For example, tuning the ENZ wavelength allows the material to perfectly match the wavelength of high-powered lasers necessary to exploit the high index changes observed. Tuning the ENZ wavelength of multilayer structures can affect the absorption wavelength and bandwidth. Multiple groups have demonstrated the ability
to tune the ENZ wavelengt
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