Dissertation Defense: Full Wave Electromagnetic Simulations of Terahertz Wire Grid Polarizers and Infrared Plasmonic Wire Gratings by John Cetnar, Engineering PhD Candidate

Wright State University Physics Department hosts a

Dissertation Defense

Friday, March 28, 2014
3:00pm in 401 Millett Hall

Full Wave Electromagnetic Simulations of Terahertz Wire Grid Polarizers and Infrared Plasmonic Wire Gratings

By John S. Cetnar, Wright State University Engineering PhD Candidate
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This dissertation is a study of the interaction of terahertz (THz) and long-wave infrared (LWIR) radiation with various periodic sub-wavelength metallic structures in free-space and on dielectric substrates. THz radiation is electromagnetic radiation that exists between the microwave and infrared (IR) regions of the electromagnetic spectrum. It has wavelengths that are shorter than microwaves but longer than IR. IR radiation exists below the visible region and LWIR occurs in the 6 – 15 μm wavelength range. There are many new and useful applications for both THz and LWIR radiation. Unfortunately, THz radiation is heavily attenuated by the Earth’s atmosphere. In addition, THz sources tend to have low output power so that THz radiation is difficult to generate, and also difficult to detect. LWIR is not as prone to atmospheric attenuation as THz radiation. Nevertheless, the detection of LWIR can be improved upon by increasing the signal-to-noise ratio, the detectivity, and the spectral selectivity of LWIR detector systems.
Light passing through periodic sub-wavelength metallic structures can exhibit extraordinary optical transmission (EOT). EOT is the greatly enhanced transmission of light through a conducting film on which sub-wavelength apertures or other features have been patterned in a regular repeating periodic structure. These periodically repeating conducting structures are known in the literature as surface-plasmonic structures. When EOT occurs, the amount of light transmitted through such structures is enhanced to well beyond what would be predicted by geometric optics. In addition, exceedingly high electromagnetic (EM) fields develop in the apertures and along the conducting surfaces of EOT structures. These enhanced fields may be used to improve the performance of a THz or LWIR detector through a significant reduction in its size while maintaining good external radiation coupling. Reduction in size lowers parasitic impedance, such as the capacitance of THz Schottky diodes, and lowers the dark current, the dominant noise mechanism in LWIR photon detector type devices.
Full-wave numerical simulations using the finite element method (FEM) were used to study the interaction of THz and LWIR radiation with one- and two-dimensional surface plasmonic EOT structures. This dissertation examines the numerical solutions to the Helmholtz wave equation for radiation interacting with plasmonic structures in both the THz and LWIR regions. The simulation results predict that both EOT and EM field enhancement will occur. In several cases, plasmonic structures designed from optimized FEM results have been fabricated and characterized. The experimental results confirm the simulation predictions qualitatively and quantitatively to within a few dB.

Selected Journal Publications/Conference Proceedings/Patent Applications

• Cetnar, John S., John R. Middendorf, and Elliott R. Brown. "Extraordinary optical transmission and extinction in a Terahertz
wire-grid polarizer." Applied Physics Letters 100, no. 23 (2012): 231912.

• Cetnar, J. S., John R. Middendorf, and Elliott R. Brown. "Finite-element simulation and design of a high-extinction-ratio THz wire-grid polarizer." In Aerospace and Electronics Conference (NAECON), 2012 IEEE National, pp. 20-23. IEEE, 2012.

• Cetnar, J. S., J. R. Middendorf, and E. R. Brown. "Effective fill-factor design results in extraordinary optical transmission in a THz wire-grid polarizer." In Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2013 38th International Conference on, pp. 1-2. IEEE, 2013.

• Middendorf, John R., John S. Cetnar, Jack L. Owsley, and Elliott. R. Brown, “Substrate-Based Wire-Grid Polarizers and
Beam-Splitters with High Extinction Ratios,” IEEE Trans. Terahertz Sci. Technol (accepted for publication, Mar 2014).

• Elliott R. Brown, John S. Cetnar, John R. Middendorf., Spoof-Surface-Plasmon-Coupled THz Components and Devices,
U. S. Provisional Patent Application No. 61/790,433. 15 March 2013.

• Elliott R. Brown, John S. Cetnar, John R. Middendorf., Structured Surface-Plasmon THz Components and Devices, U. S.
Provisional Patent Application No. 61/811,510. 12 April 2013.