John S. Cetnar

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size.

Extraordinary optical transmission and extinction in a Terahertz wire-grid polarizer

A THz wire grid polarizer is simulated and demonstrated consisting of 40-μm periodic aluminum strips mounted on a polycarbonate substrate with a variable metal-to-gap ratio. Full-wave numerical simulations were performed from 100 GHz to 550 GHz predicting that the transmission in perpendicular (parallel) polarization is much higher (lower) than that predicted by geometric optics, leading to a very high extinction ratio of ∼60 dB between 100 and 550 GHz when the gaps become very small (<5 μm). This behavior is confirmed qualitatively in experiments between 100 and 530 GHz where extinction ratios exceeding 40 dB are achieved. These results are explained physically as an electromagnetic concentration effect in the gaps consistent with plasmonic-like behavior. The effect depends critically on gap width and weakly on frequency.

High Fill-Factor Substrate-Based Wire-Grid Polarizers With High Extinction Ratios

Low-cost, substrate-based, millimeter-wave-to-THz wire-grid polarizers have been fabricated on crystalline-quartz substrates using planar processing techniques. The polarizers achieve high extinction ratios (at least 60 dB) with a single layer, while maintaining low insertion loss (a few decibels) by taking advantage of previously under-utilized “spoof” surface-plasmon effects. Full-wave finite-element simulations done with High Frequency Structure Simulator and experiments both show that metal fill-factors upwards of 90% greatly improve polarizer extinction ratio. The extinction ratio of high fill-factor polarizers exceeds that of a commercial free-standing wire grid by up to 20 dB at normal incidence.

High extinction ratio terahertz wire-grid polarizers with connecting bridges on quartz substrates.

A terahertz (THz) wire-grid polarizer with metallic bridges on a quartz substrate was simulated, fabricated, and tested. The device functions as a wide-band polarizer to incident THz radiation. In addition, the metallic bridges permit the device to function as a transparent electrode when a DC bias is applied to it. Three design variations of the polarizer with bridges and a polarizer without bridges were studied. Results show the devices with bridges have average s-polarization transmittance of less than -3  dB and average extinction ratios of approximately 40 dB across a frequency range of 220-990 GHz and thus are comparable to a polarizer without bridges.

Finite-element simulation and design of a high-extinction-ratio THz wire-grid polarizer

A THz wire grid polarizer was simulated, designed, and demonstrated. The polarizer consists of 40-micron periodic aluminum strips mounted on a polycarbonate substrate. Finite element numerical simulations were performed from 100 GHz to 550 GHz. The results of these simulations predicted that the transmission in perpendicular polarization would be much higher than that predicted by geometric optics, leading to a very high extinction ratio of ~ 60 dB at high fill factors (~ 90%). This behavior was qualitatively demonstrated in experiments between 100 and 530 GHz where extinction ratios exceeding 40 dB were achieved. These results are explained physically as an electromagnetic field concentration effect in the gaps characteristic of plasmonic-like behavior. The effect is strongly dependent on gap width and weakly dependent on frequency.

MMIC technology for spectroscopy applications

Millimeter-wave monolithic integrated circuit (MMIC) technology is now widely recognized as a key to many modern applications in safety and security, ranging from near and far-field imaging and sensing to non-invasive material inspection. In this paper, we apply our state-of-the-art MMIC technology to the analysis of gaseous media by spectroscopic techniques. The paper presents recent developments of amplifying and frequency-translating MMICs based on metamorphic HEMT technology and their application to the spectroscopic analysis of the frequency range from 250 to 330 GHz, including the important absorption line of water around 321 GHz.