Sunday, November 18, 2018

Abstract-Prism coupling of high-Q terahertz whispering-gallery-modes over two octaves from 0.2 THz to 1.1 THz

Dominik Walter Vogt, Angus Harvey Jones, Harald G. L. Schwefel, and Rainer Leonhardt

Fig. 1 (a) Schematic of the CW THz spectroscopy system with fiber coupled photo-conductive antennas (PCAs) using two-inch diameter s-p THz lenses to focus the THz radiation onto the base of the HRFZ-Si prism. The length of the base of the prism is about 13 mm. The spherical HRFZ-Si WGMR is mounted on a 3D manual translation stage, and the position is observed using two microscope cameras. (b) and (c) show the corresponding top and side view, respectively, of the 8 mm HRFZ-Si sphere next to the HRFZ-Si prism. Please note that (c) is focused on the prism surface.

We report on prism coupling of high-quality (high-Q) terahertz (THz) whispering-gallery modes (WGMs) in spherical high resistivity float zone grown silicon (HRFZ-Si) resonators over two octaves from 0.2 THz to 1.1 THz. The WGMs are excited using a HRFZ-Si prism and show unprecedented quality factors of up to 2.2 × 104. A detailed discussion of the phase-and mode-matching criteria of the prism coupling scheme implemented in the continuous wave THz spectroscopy system is presented. The results provide numerous op
portunities for passive ultra-broadband high-Q devices operating in the THz frequency range.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Saturday, November 17, 2018

Abstract-Strong Coupling of Epsilon-Near-Zero Phonon Polaritons in Polar Dielectric Heterostructures

We report the first observation of epsilon near zero (ENZ) phonon polaritons in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP) supported by the adjacent SiC substrate. Employing a strong coupling model for the analysis of the dispersion and electric field distribution in these hybridized modes, we show that they share the most prominent features of the two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly propagative character and deeply sub-wavelength light confinement can be utilized as building blocks for future infrared and terahertz (THz) nanophotonic integration and communication devices.

Abstract-High-power terahertz emission from a plasma penetrated by counterstreaming different-size electron beams

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V. V. Annenkov, E. A. Berendeev,  I. V. Timofeev,  E. P. Volchok
A part of the simulation box near its left edge.

It is found that multi-cycle pulses of high-power coherent terahertz radiation can be efficiently generated in a plasma by counterstreaming long-pulse electron beams driving potential plasma waves via the two-stream instability. Instead of the well-known three-wave interaction processes between oblique beam-driven modes, we propose to generate electromagnetic radiation near the doubled plasma frequency due to the novel and much more efficient mechanism based on the head-on collision of longitudinal plasma waves with mismatching potential profiles. It is shown that this radiation mechanism can be implemented experimentally either by the collision of low-density electron beams with different transverse sizes or by the counter injection of denser equal-size beams unstable against filamentation perturbations. Particle-in-cell simulations for kiloampere electron beams capable of focusing into millimeter-scale sizes demonstrate the possibility to reach the high efficiency of beams-to-THz power conversion (3%–7%), which opens the path to gigawatt-class THz sources with a narrow spectral line.

Regulatory Challenges for the New Frontier of Medical Imaging: Terahertz Spectrum

Emerging medical imaging technologies being developed for the terahertz spectrum may face regulatory hurdles from an unexpected Federal agency: the Federal Communications Commission (FCC). While the Food and Drug Administration (FDA) has jurisdiction over medical devices with respect to safety and effectiveness – including premarket approval, inspection of manufacturing facilities, and monitoring of post-market adverse events – the FCC has jurisdiction over certain technical and spectrum use requirements for medical devices that operate on radiofrequency (RF) spectrum. Medical device developers need to be aware of the current state of play at the FCC in these areas.

Terahertz Imaging

Medical imaging technologies can operate on a variety of frequency bands. In the terahertz frequency range, the waves are extremely short, smaller than the size of a cell. When these radio waves are sent through tissue, the signal passes through the tissue and creates “scattering.” (Atoms are “shaken,” but not broken or destroyed.) Imaging in the terahertz frequencies can reveal what is occurring at the cellular level, providing the ability to distinguish between cells that have cancer or not, for example, or between different types of cells such as fatty tissue or liver tissue. Obviously, these are promising diagnostic methods that could enhance the detection and treatment of pathological conditions, such as skin cancer.

The Basics of FCC Regulation of Medical Devices

The FCC regulates several aspects of wireless medical devices. These include: 1) issuing test licenses to designers and manufacturers to test devices prior to approval; 2) issuing grants of equipment authorizations, or otherwise providing mechanisms for allowing device use; 3) setting out rules for the use of frequencies designated for medical devices; 4) establishing new frequencies for use by wireless medical devices; and 5) imposing requirements for ascertaining and complying with limits on human exposure to RF energy.
The spectrum managed by the FCC is commonly used for communications, such as cellphones, Wi-Fi and radio, with FCC currently regulating the frequency range from 3 kHz to 3000 GHz. (Frequency refers to the wavelength of the radio waves passing through the air. Radio waves can range in size from 100 meters at the lower frequencies to decimillimeters at the highest frequencies. The size of the radio wave determines the use of a frequency range.) Terahertz spectrum falls at the top end of the regulated frequency range, just below light waves on the electromagnetic spectrum – specifically, 300-3000 GHz.

The FCC Provides for Only Limited Operations on Terahertz Spectrum

The FCC does not have service rules in place to allow for operations in the terahertz bands. In fact, FCC presently only has service rules (i.e., rules for use) up to 95 GHz, and only has spectrum allocated (i.e., planned out for use) up to 275 GHz. Because spectrum is a finite resource, the FCC must plan its use to ensure that multiple users can co-exist.
The FCC currently has an open rulemaking proceeding that proposes to establish new rules for operations beyond the 95 GHz range, but only up to 275 GHz (just below the terahertz range). The only proposal that the FCC has made thus far with regard to use of the terahertz range is to provide for testing licenses in that range, with the possibility of allowing very limited sales of new technologies.
Absent service rules, medical imaging devices operating in the terahertz frequencies may only be marketed if they can be categorized as “Industrial Scientific and Medical (ISM)” equipment. And even then, the FCC employs a “case-by-case” evaluation of each device, which means that designers and manufacturers cannot design to specific technical rules, but instead must wait until the FCC evaluates their product before knowing whether they will receive FCC authority to market the device. This process obviously creates a good deal of regulatory uncertainty. Moreover, it does not apply to devices that have a communications function, such as Bluetooth or Wi-Fi used to send data collected by the device to a smartphone or router.
Europe and Japan are both ahead of the U.S. in terms of planning for the use of terahertz frequencies. Until FCC modifies its rules, and proposes new rules for the terahertz range, the effect will be to discourage capital formation for developers of medical imaging technologies that could operate in those frequencies. Advocacy is needed to press the FCC to clarify and establish rules to allow for medical imaging devices that operate in the terahertz ranges. Providing feedback on the current FCC proposal should be the start of this effort, but certainly not the end of it.

Friday, November 16, 2018

Abstract-Boosting terahertz-radiation power with two-color circularly polarized midinfrared laser pulses

V. A. Tulsky, M. Baghery, U. Saalmann, and S. V. Popruzhenko


A way to considerably enhance terahertz radiation, emitted in the interaction of intense midinfrared laser pulses with atomic gases, in both the total energy and the electric-field amplitude is suggested. The scheme is based on the application of a two-color field consisting of a strong circularly polarized midinfrared pulse with wavelengths of 1.64μm and its linearly or circularly polarized second harmonic of lower intensity. By combining the strong-field approximation for the ionization of a single atom with particle-in-cell simulations of the collective dynamics of the generated plasma, it is shown that the application of such two-color circularly polarized laser pulses may lead to an order-of-magnitude increase in the energy emitted in the terahertz frequency domain as well as in a considerable enhancement in the maximal electric field of the terahertz pulse. Our results support recently reported experimental and numerical findings.
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Abstract-A polarization-insensitive broadband terahertz absorber with a multilayer structure

Hai-Feng Zhang, Jia-Xuan Liu, Jing Yang, Hao Zhang,Hai-Ming Li,
Fig. 3. The magnetic field distributions for the different resonance frequencies, (a) 4

In this paper, a polarization-insensitive broadband terahertz absorber (PBTA) is presented and demonstrated, which can realize a polarization-insensitive, and broadband perfect absorption in the terahertz regime. By simulation, the polarization-insensitive broadband absorption is over 90%, which runs from 4.904THz to 6.632THz (the relative bandwidth is 29.96%), and the obtained absorption remains a good absorption performance with a wide incident angle for both TE and TM waves. The surface current distributions, power loss densities, electric and magnetic field distributions of such an absorber are investigated to figure out the physical mechanism of such a PBTA. The effects structure parameters on the absorption performance are also studied, which will be a guiding to realize a PBTA. The simulated results show that the proposed multilayer structure can help to design a PBTA.