Abstract-Sensitivity limits of millimeter-wave photonic radiometers based on efficient electro-optic upconverters

Gabriel Santamaría Botello, Florian Sedlmeir, Alfredo Rueda, Kerlos Atia Abdalmalak, Elliott R. Brown, Gerd Leuchs, Sascha Preu, Daniel Segovia-Vargas, Dmitry V. Strekalov, Luis Enrique García Muñoz,  Harald G. L. Schwefel,

Fig. 1. Coupling of millimeter-wave and optical radiation to the WGM resonator. The microwave coupling region (enclosed by the dashed curve) is considered small and lossless. Input and output modes in the waveguide and the resonator are defined with normalized field amplitudes 𝑎𝑖(𝜈)$a_i(\nu )$ such that their squared norm equals the power spectral density of the mode. The coupling system can be viewed as a four-port network whose scattering matrix is characterized by the waveguide’s reflection coefficient 𝑟=𝑎2/𝑎1$r=a_2/a_1$ and waveguide-resonator transmission coefficient 𝑡=𝑎3/𝑎1$t=a_3/a_1$. The cavity’s reflection coefficient 𝑟′=𝑎3/𝑎4$r^{\prime }=a_3/a_4$ and cavity-waveguide transmission coefficient 𝑡′=𝑎2/𝑎4$t^{\prime }=a_2/a_4$ have the same magnitudes as 𝑟$r$ and 𝑡$t$, respectively, but a different phase in general

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-10-1210

Conventional ultra-high sensitivity detectors in the millimeter-wave range are usually cooled as their own thermal noise at room temperature would mask the weak received radiation. The need for cryogenic systems increases the cost and complexity of the instruments, hindering the development of, among others, airborne and space applications. In this work, the nonlinear parametric upconversion of millimeter-wave radiation to the optical domain inside high-quality (𝑄$Q$) lithium niobate whispering-gallery mode (WGM) resonators is proposed for ultra-low noise detection. We experimentally demonstrate coherent upconversion of millimeter-wave signals to a 1550 nm telecom carrier, with a photon conversion efficiency surpassing the state-of-the-art by 2 orders of magnitude. Moreover, a theoretical model shows that the thermal equilibrium of counterpropagating WGMs is broken by overcoupling the millimeter-wave WGM, effectively cooling the upconverted mode and allowing ultra-low noise detection. By theoretically estimating the sensitivity of a correlation radiometer based on the presented scheme, it is found that room-temperature radiometers with better sensitivity than state-of-the-art high-electron-mobility transistor (HEMT)-based radiometers can be designed. This detection paradigm can be used to develop room-temperature instrumentation for radio astronomy, earth observation, planetary missions, and imaging systems.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz vibrational signature of bacterial spores arising from nanostructure decorated endospore surface

Debopam Datta,   Michael A. Stroscio,   Mitra Dutta,   Weidong Zhang,   Elliott R. Brown,

https://onlinelibrary.wiley.com/doi/full/10.1002/jbio.201700398

This theoretical effort is the first to explore the possible hypothesis that terahertz optical activity of Bacillus spores arises from normal vibrational modes of spore coat subcomponents in the terahertz frequency range. Bacterial strains like Bacillus and Clostridium form spores with a hardened coating made of peptidoglycan to protect its genetic material in harsh conditions. In recent years, electron microscopy and atomic force microscopy has revealed that bacterial spore surfaces are decorated with nanocylinders and honeycomb nanostructures. In this article, a simple elastic continuum model is used to describe the vibration of these nanocylinders mainly in Bacillus subtilis, which also leads to the conclusion that the terahertz signature of these spores arises from the vibration of these nanostructures. Three vibrating modes: radial/longitudinal, torsional and flexural, have been identified and discussed for the nanocylinders. The effect of bound water, which shifts the vibration frequency, is also discussed. The peptidoglycan molecule consists of polar and charged amino acids; hence, the sporal surface local vibrations interact strongly with the terahertz radiation.

Abstract-THz Imaging System for in vivo Human Cornea

Sunshiny Sung, Skyler Selvin, Neha Bajwa,  Somporn Chantra,   Bryan Nowroozi,  James Garritano, Jacob Goell, Alexander D. Li,   Sophie X. Deng,  Elliott R. Brown,  Warren S. Grundfest,  Zachary D. Taylor

http://ieeexplore.ieee.org/document/8167336/

Terahertz (THz) imaging of corneal tissue water content (CTWC) is a proposed method for early, accurate detection and study of corneal diseases. Despite promising results from ex vivo and in vivo cornea studies, interpretation of the reflectivity data is confounded by the contact between corneal tissue and rigid dielectric window used to flatten the imaging field. This work develops a novel imaging system and image reconstruction methods specifically for nearly spherical targets such as human cornea. A prototype system was constructed using a 650-GHz multiplier source and Schottky diode detector. Resolution and imaging field strength measurement from characterization targets correlate well with those predicted by the quasioptical theory and physical optics analysis. Imaging experiments with corneal phantoms and ex vivo corneas demonstrate the hydration sensitivity of the imaging system and reliable measurement of CTWC. We present successful acquisition of noncontact THz images of in vivo human cornea, and discuss strategies for optimizing the imaging system design for clinical use.

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

John S. Cetnar, Shivashankar Vangala, Weidong Zhang, Carl Pfeiffer, Elliott R. Brown, and Junpeng Guo

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-5-955&origin=search

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$s$-polarization transmittance of less than −3  dB$-3\mathrm{dB}$ 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.

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

John S. Cetnar, Shivashankar Vangala, Weidong Zhang, Carl Pfeiffer, Elliott R. Brown, and Junpeng Guo

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-5-955

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$s$-polarization transmittance of less than −3  dB$-3\mathrm{dB}$ 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.

Abstract-Terahertz metal grid polarizer with bridges on quartz substrate

John S. Cetnar

Air Force Research Lab. (United States)

Junpeng Guo

The Univ. of Alabama in Huntsville (United States)

Elliott R. Brown

Wright State Univ. (United States)

Proc. SPIE 9547, Plasmonics: Metallic Nanostructures and Their Optical Properties XIII, 95471O (August 28, 2015); doi:10.1117/12.2185114

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2432846

The metal wire-grid polarizer is a venerable device that is used on radiation throughout the electromagnetic spectrum. It usually consists of a 1D-periodic array of subwavelength metallic wires in free space or mounted on a low-loss dielectric substrate, the plane of the grid being oriented perpendicular to the propagation direction. Herein is presented a new structure, a subwavelength wire-grid polarizer for the terahertz region that acts not only as a wideband polarizer but also as a transparent electrode. This function is achieved by the addition of periodically placed metallic bridges that connect the parallel metal wires of the polarizer. The bridges allow for the uniform distribution of an electrostatic potential over all wires while maintaining the polarizing functionality of the metal wire grid polarizer.

Full-wave electromagnetic simulations were performed on the device. The transmittance was computed in both perpendicular polarization and parallel polarization from 100 to 4000 GHz, and the extinction ratio was calculated across the same range. Furthermore, fill-factor studies were performed to understand how device performance is affected by varying slot width and bridge length, as well as bridge offset. The simulation results showed extraordinary optical transmission through the device for perpendicular polarization, creating excellent transmittance and extinction ratios over the frequency range. The perpendicular polarization transmittance and extinction ratio at 1 THz was calculated to be -1 dB and -36 dB respectively. Meanwhile, the bridges allow the device to behave like a DC electrode.
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