Fundamental limits of light-wave sensing for non-contact respiration monitoring.

An experimental testbed was constructed to rigorously assess the fundamental limits of light-wave sensing-an economic, non-contact vitals monitoring approach previously reported. We improve the testbed using lock-in amplification and demonstrate that a photodetector and a commonplace array of infrared LEDs are sufficient to detect respiratory motion and quantify respiration rate up to 2.5 meters away.

Artificial dielectric beam-scanning prism for the terahertz region.

We design and fabricate an artificial dielectric prism that can steer a terahertz beam in space and experimentally investigate its behavior. The artificial dielectric medium consists of a uniformly spaced stack of metal plates, electromagnetically equivalent to an array of parallel-plate waveguides operating in tandem. At an operating frequency of 0.

Low Strength Magnetic Fields Serve as a Cue for Foraging Honey Bees but Prior Experience is More Indicative of Choice.

Species of migrating insects use magnetic fields as a navigational tool that is independent of current weather conditions and non-migrating species have been shown to discriminate anomalies in magnetic field from the earth's baseline. Honey bee discrimination of magnetic field has been studied in the context of associative learning, physiology, and whole hive responses. This article uses a combination of free-flight and laboratory studies to determine how small fluctuations from Earth's magnetic field affect honey bee (Apis mellifera L.

Compensating Atmospheric Channel Dispersion for Terahertz Wireless Communication.

We report and demonstrate for the first time a method to compensate atmospheric group velocity dispersion of terahertz pulses. In ultra-wideband or impulse radio terahertz wireless communication, the atmosphere reshapes terahertz pulses via group velocity dispersion, a result of the frequency-dependent refractivity of air. Without correction, this can significantly degrade the achievable data transmission rate.

Controllable broadband asymmetric transmission of terahertz wave based on Dirac semimetals.

We present a dynamic metamaterial based on Dirac semimetals and capable of realizing broadband and tunable asymmetric transmission in the terahertz region. The Dirac semimetal resonators have a chiral structure patterned with double-T resonators that results in partial polarization conversion of waves incident upon the material, leading to asymmetric transmission across a wide frequency range. We show how the gradual shift of the semimetal Fermi energy permits a method of control over the asymmetric total transmission.

Remote NO gas sensing by enhanced 910-m propagation of THz pulses.

We modified our 910-m long path THz system to increase the signal-to-noise ratio (S/N) with a nanostructure plasmonic THz transmitter (Tx) chip and a seven-mirror array reflector with 1 m diameter. When the THz pulse propagates the 910-m distance in the atmosphere, the S/N is up to 1170:1, which made the THz pulse measurable at a high water vapor density (WVD) of up to 25.2 g/m.

Independently tunable dual-band perfect absorber based on graphene at mid-infrared frequencies.

We design a dual-band absorber formed by combining two cross-shaped metallic resonators of different sizes within a super-unit-cell arranged in mirror symmetry. Simulations indicate that absorption efficiencies greater than 99% can be achieved at two different frequencies under normal incidence. We employ a design scheme with graphene integration, which allows independent tuning of individual absorption frequencies by electrostatically changing the Fermi energy of the graphene layer.

Active metasurface terahertz deflector with phase discontinuities.

Metasurfaces provide great flexibility in tailoring light beams and reveal unprecedented prospects on novel functional components. However, techniques to dynamically control and manipulate the properties of metasurfaces are lagging behind. Here, for the first time to our knowledge, we present an active wave deflector made from a metasurface with phase discontinuities.

Limitation in thin-film sensing with transmission-mode terahertz time-domain spectroscopy.

Thin-film sensing with a film thickness much less than a wavelength is an important challenge in conventional transmission-mode terahertz time-domain spectroscopy (THz-TDS). Since the interaction length between terahertz waves and a sample film is short, a small change in the transmitted signal compared with the reference is considerably obscured by system uncertainties. In this article, several possible thin-film measurement procedures are carefully investigated.

A broadband planar terahertz metamaterial with nested structure.

We demonstrate the broadening of fundamental resonance in terahertz metamaterial by successive insertion of metal rings in the original unit cell of a split ring resonator (SRR) forming an inter connected nested structure. With the subsequent addition of each inner ring, the fundamental resonance mode shows gradual broadening and blue shift. For a total of four rings in the structure the resonance linewidth is enhanced by a factor of four and the blue shift is as large as 316 GHz.

Tailored resonator coupling for modifying the terahertz metamaterial response.

We experimentally and numerically study the nature of coupling between laterally paired terahertz metamaterial split-ring resonators. Coupling is shown to modify the inductive-capacitive (LC) resonances resulting in either red or blue-shifting. Results indicate that tuning of the electric and magnetic coupling parameters may be accomplished not by changing the orientation or density of SRRs, but by a design modification at the unit cell level.

Tuning the resonance in high-temperature superconducting terahertz metamaterials.

In this Letter, we present resonance properties in terahertz metamaterials consisting of a split-ring resonator array made from high-temperature superconducting films. By varying the temperature, we observe efficient metamaterial resonance switching and frequency tuning. The results are well reproduced by numerical simulations of metamaterial resonance using the experimentally measured complex conductivity of the superconducting film.

Orientation dependent far-infrared terahertz absorptions in single crystal pentaerythritol tetranitrate (PETN) using terahertz time-domain spectroscopy.

Terahertz time-domain spectroscopy (THZ-TDS) has been used to measure the absorption spectra in the range 7-100 cm(-1) (0.2-3 THz) of single crystal pentaerythritol tetranitrate (PETN). Absorption was measured in transmission mode as a function of incident polarization with the incident and transmitted wave vectors oriented along the crystallographic directions [100], <10(a/c)(2)>, and <110>.

Antireflection coating using metamaterials and identification of its mechanism.

We present a novel approach of antireflection coating using metamaterials. It dramatically reduces the reflection and greatly enhances the transmission near a specifically designed frequency over a wide range of incidence angles for both transverse magnetic and transverse electric polarizations. A classical interference mechanism is identified through analytical derivations and numerical simulations.

Large dynamic resonance transition between surface plasmon and localized surface plasmon modes.

We present resonant terahertz transmission in a composite plasmonic film comprised of an array of subwavelength metallic patches and semiconductor holes. A large dynamic transition between a dipolar localized surface plasmon mode and a surface plasmon resonance near 0.8 THz is observed under near infrared optical excitation.

Metamaterials for THz polarimetric devices.

We present experimental and numerical investigations of planar terahertz metamaterial structures designed to interact with the state of polarization. The dependence of metamaterial resonances on polarization results in unique amplitude and phase characteristics of the terahertz transmission, providing the basis for polarimetric terahertz devices. We highlight some potential applications for polarimetric devices and present simulations of a terahertz quarter-wave plate and a polarizing terahertz beam splitter.

Effect of metal permittivity on resonant properties of terahertz metamaterials.

We investigate the effect of metal permittivity on resonant transmission of metamaterials by terahertz time-domain spectroscopy. Our experimental results on double split-ring resonators made from different metals confirm the recent numerical simulations [Phys. Rev.

Electronic control of extraordinary terahertz transmission through subwavelength metal hole arrays.

We describe the electronic control of extraordinary terahertz transmission through subwavelength metal hole arrays fabricated on doped semiconductor substrates. The hybrid metal-semiconductor forms a Schottky diode structure, where the active depletion region modifies the substrate conductivity in real-time by applying an external voltage bias. This enables effective control of the resonance enhanced terahertz transmission.

Optically thin terahertz metamaterials.

Resonant properties of optically thin metamaterials are studied by terahertz time-domain spectroscopy. Both the lower energy inductor-capacitor (LC) and the higher energy dipole resonances of the planar double split-ring resonators (SRRs) exhibit characteristic evolution with various sub-skin-depth thicknesses of the constituent Pb film. The signature of the LC resonance begins to emerge at a critical thickness near 0.

Thin-film sensing with planar terahertz metamaterials: sensitivity and limitations.

The limiting effects of varying the thickness of a dielectric overlayer on planar double split-ring resonator (SRR) arrays are studied by terahertz time-domain spectroscopy. Uniform dielectric overlayers from 100 nm to 16 mum thick are deposited onto fixed SRR arrays in order to shift the resonance frequency of the electric response. We discuss the bounds of resonance shifting and emphasize the resulting limitations for SRR-based sensing.

Complementary planar terahertz metamaterials.

Planar electric split ring resonator (eSRR) metamaterials and their corresponding inverse structures are designed and characterized computationally and experimentally utilizing finite element modeling and THz time domain spectroscopy. A complementary response is observed in transmission. Specifically, for the eSRRs a decrease in transmission is observed at resonance whereas the inverse structures display an increase in transmission.