Experimental comparison of terahertz and infrared data signal attenuation in dust clouds.

In order to study and compare propagation features of terahertz (THz) links with infrared (IR) links under different weather conditions such as turbulence, fog, and dust particles, THz and IR free space communication links at 625 GHz carrier frequency and 1.5 µm wavelength, respectively, with a maximum data rate of 2.5 Gb/s have been developed. After propagating through the same channel perturbation caused by dust, attenuation of the carrier frequencies by dust as well as scintillation effects on both channels are analyzed by measuring the power attenuation and bit error rates. Attenuation by the presence of dust degrades the IR channel but exhibits almost no measurable impact on the THz signal. Numerical simulations of THz attenuation with different dust concentrations are presented and agree with the measured results.

Experimental comparison of performance degradation from terahertz and infrared wireless links in fog.

We describe a lab setup for analyzing impairments of terahertz (THz) and infrared (IR) free space links caused by local refraction index changes in the signal's propagation paths that could be induced by turbulence, particles, humidity, etc. A THz signal comprising a 2.5 Gb/s data load modulated on a carrier at 625 GHz, is launched through a weather emulating chamber, detected, and its performance analyzed. An IR beam at 1.5 um wavelength carrying the same data load is superposed with the THz beam, propagating through the same weather conditions and also performance analyzed. We modulate the IR channel with a usual non-return-to-zero (NRZ) format but use duobinary coding for driving our THz source, which enables signaling at high data rate and higher output power. As both beams pass through the same channel perturbations and as their degradations are recorded simultaneously we can simultaneously compare the weather impact on both. We investigate scintillation and fog attenuation effects for the THz and IR signals by measuring bit error rates (BER), signal power, and phase front distortions.

Two-dimensional interferometric and synthetic aperture imaging with a hybrid terahertz/millimeter wave system.

We have developed an interferometric synthetic aperture incoherent imaging system at 94 GHz, in which a high-power electronic millimeter wave source (Gunn Oscillator) is integrated with a continuous-wave terahertz (THz) photomixing detection system to achieve a high signal-to-noise ratio. Imaging of a point source located 10?m away from the detector array is presented. Two-dimensional THz reflective images of an extended object with different shapes are reconstructed with only four detectors by use of rotational synthesis.

Terahertz response of microfluidic-jetted three-dimensional flexible metamaterials.

We demonstrate the fabrication and characterization of three-dimensional (3D) metamaterials in the terahertz (THz) range using the microfluidic-jetted technique. This technique has proven a convenient technique to fabricate metamaterial structures at the micrometer scale. The metamaterials are fabricated using dodecanethiol functionalized gold nanoparticles on flexible polyimide substrates. The metamaterials consist of alternate layers of single split-ring resonator and microstrip arrays that are stacked to form a 3D metamaterial medium. The fabricated metamaterials, with lattice sizes of 180 microm, are characterized using THz time-domain spectroscopy within 0.1 to 2 THz in the transmission mode. Numerical simulation is performed to calculate the effective metamaterials parameter.

Video-rate terahertz interferometric and synthetic aperture imaging.

Experimental results from a video-rate terahertz interferometric imaging system are presented. The source emits continuous narrow bandwidth radiation at 0.1 THz. The 2D image of a point source is reconstructed at a rate of 16 ms per frame with a four-element detector array. The image resolution and quality are affected by the number of detectors, the configuration of the detection array, and how well the baselines are calibrated. Details of the hardware system and video-rate terahertz image are presented.

Application of terahertz Gouy phase shift from curved surfaces for estimation of crop yield.

The application of terahertz time-domain spectroscopy (THz-TDS) and imaging to differentiate flat and curved surfaces in reflection via the THz Gouy phase shift is demonstrated. For a THz-TDS system that is aligned for reflection from a flat surface, the presence of a curved surface displaces the image focal plane from the detector plane, resulting in a Gouy phase shift. The potential of utilizing this configuration for estimating the number and size of curved objects is discussed with particular emphasis on agriculture crop yield estimates.

Rapid-phase modulation of terahertz radiation for high-speed terahertz imaging and spectroscopy.

Rapid voltage-controlled phase modulation of cw terahertz (THz) radiation is demonstrated. By transmitting an infrared beam through a lithium niobate phase modulator the phase of the THz radiation, which is generated by the photomixing of two infrared beams, can be directly modulated through a 2pi phase shift. The 100 kHz modulation rate that is demonstrated with this technique is approximately 3 orders of magnitude faster than what can be achieved by mechanical scanning.

Nondestructive evaluation of cork enclosures using terahertz/millimeter wave spectroscopy and imaging.

Natural cork enclosures, due to their cell structure, composition, and low moisture are fairly transparent to terahertz (THz) and millimeter waves enabling nondestructive evaluation of the cork's surface and interior. It is shown that the attenuation coefficient of the defect-free cork can be modeled with a Mie scattering model in the weakly scattering limit. Contrast in the THz images is a result of enhanced scattering of THz radiation by defects or voids as well as variations in the cork cell structure. The presence of voids, defects, and changes in grain structure can be determined with roughly 100-300 microm resolution.

Terahertz interferometric and synthetic aperture imaging.

The stand-off imaging properties of a terahertz (THz) interferometric array are examined. For this application, the imaged object is in the near-field region limit of the imaging array. In this region, spherical and circular array architectures can compensate for near-field distortions and increase the field of view and depth of focus. Imaging of THz point sources is emphasized to demonstrate the imaging method and to compare theoretical predictions to experimental performance.

Determining thickness independently from optical constants by use of ultrafast light.

We show that the application of ultrafast techniques, especially terahertz time-domain spectroscopy, allows simultaneous measurements of material thickness and optical constants from transmission measurements, by analyzing not only the phase difference between the main terahertz pulse through the medium but also the subsequent multireflection pulse (an echo) from the medium. Such a method provides a fast and precise characterization of the optical properties and can extract thickness information and hence other optical constants in a broad bandwidth. It may have applications in science and engineering such as in situ film thickness and quality monitoring, optical constants measurement, medical imaging, noninvasive detection, and remote sensing.

Polarized light reflection from strained sinusoidal surfaces.

We propose optical polarization imaging as a minimally invasive technique for measuring the mechanical properties of plastics and soft tissues through their change in reflectance properties with applied strain or force. We suggest that changes in surface roughness are responsible for the linear reflectivity changes with applied stretch or strain. Several aspects of this model are tested, including the dependence on the angle of incidence, the change in scattering and absorption coefficients with strain, and the lateral spatial resolution. The application of the technique to multilayer structures such as skin and competing optical effects such as laser speckle are discussed.

Measurement of skin stretch via light reflection.

A noninvasive technique for measuring the stretch of skin is described. The technique utilizes changes in the reflectivity of polarized light intensity as a monitor of skin stretch. Measurements of in vitro pigskin and in vivo human skin show that the reflectivity of polarized light intensity increases linearly with stretch. The changes in diffusive reflectivity properties of skin result from the alterations that take place in the roughness across the thickness of the skin layers due to stretch. Conceptually, as the roughness of a layer decreases with stretch, a smoother reflecting media is produced, resulting in a proportional increase in the specular reflection. Results can be easily extended to a real-time stretch analysis of large tissue areas that would be applicable for mapping the stretch of skin.

Terahertz near-field imaging.

A near-field probe is described that enables high spatial resolution imaging with terahertz (THz) pulses. The spatial resolution capabilities of the system lie in the range of few microns and we demonstrate a resolution of 7 microm using broad-banded THz pulses with an intensity maximum near 0.5 THz. We present a study of the performance of the near-field probes in the collection mode configuration and discuss some image properties.