Digital holography and coherent imaging are explored in the terahertz frequency region at 0.480 Terahertz with highly coherent, frequency-tunable continuous wave sources. The long coherence length of microwave sources suggests that interferometric imaging techniques such as Fresnel off-axis holography can achieve sub-micrometer depth resolution of surfaces and through materials and structures that are transmissive in the terahertz spectral region. Research in this area will provide an important non-destructive imaging tool for the rapidly expanding field of additive manufacturing and composite fabrication. Unfortunately, imaging optics as they are used in confocal imaging and optics in general adversely affect the performance of terahertz imagers as their aperture size if comparable to the wavelength, which artificially limits image resolution according to the diffraction limit and can cause undesirable coherence effects in the image. We report on imaging methods that minimize the use of optics but use signal processing techniques that form images digitally from recorded holograms. This approach is directly applicable to focal plane arrays in contrast to confocal imaging modalities. Furthermore, the performance of image reconstruction at multiple aspect angles compiled into videos as user friendly inspection tool is investigated.
Imaging with electromagnetic radiation in the THz frequency regime, between 0.2 THz and 10 THz, has made
considerable progress in recent years due to the unique properties of THz radiation, such as being non-ionizing and
transparent through many materials. This makes THz imaging and sensing promising for a plethora of applications;
most notably for contraband detection and biomedical diagnostics. Though many methods of generation and detection
terahertz radiation exist, in this study we utilize Terahertz Time Domain Spectroscopy (THz TDS) and THz digital
holography using a coherent, tunable CW THz source. These methods enable access to both the amplitude and phase
information of the traveling THz waves. As a result of the direct time-resolved detection method of the THz electric
field, unique spectroscopic information about the objects traversed can be extracted from the measurements in addition
to being able to yield intensity imaging contrast. Utilizing such capabilities for THz based imaging can be useful for
both screening and diagnostic applications. In this work, we present the principles and applications of several
reconstruction algorithms applied to THz imaging and sensing. We demonstrate its ability to achieve multi-dimensional
imaging contrast of both soft tissues and concealed objects.
In recent years a great amount of research has been focused on metamaterials, initially for fabrication of left-handed
materials for use in devices such as superlenses or electromagnetic cloaking. Such devices have been developed and
demonstrated in regimes from the radio frequency all the way to infrared and near optical frequencies. More recently, it
has been shown that, by careful adjustment of the effective permittivity and permeability, near perfect electromagnetic
absorbers can be realized. High absorption occurs when transmission and reflection are simultaneously minimized. With
some clever tuning of the electric and magnetic responses, the electric and magnetic energy can therefore both be
absorbed by the same metamaterial structure.
In this work we present the design, simulation and characterization of a novel thin, flexible, polarization insensitive
metamaterial absorber. Finite-element simulation results show that this device achieves almost perfect absorption at THz
frequencies. Each unit cell of the absorber is made up of two metallic structures separated by a dielectric filler material.
The electric response can be tuned by adjusting the geometry of the top metallic electric ring resonator structure. We
demonstrate that a rotation about the axis of THz wave propagation at normal incidence does not change the absorption
or the resonance frequency by a significant amount. A value of absorption of 99.6 % at a resonance frequency of 0.84
THz can be achieved. We also demonstrate the characteristics of this absorber structure under various THz wave
incidence angles, with respect to both the incident electric and magnetic fields.
The standalone, portable Terahertz (THz) Imaging Profiler Array (TIPA) based on an Offner Relay design has been
constructed as a THz beam profiler and multispectral imager. It integrates a solid-state detector technology (Schottky
Diodes) that can be configured in an array to cover the frequency range from 0.60 to 0.90 THz. The reconfigurable 16
element Schottky diode detector array is utilized along with imaging and scanning mirror modules and system control
hardware and software to produce high spatial or temporal beam profiles of THz beams. Images of THz source profiles
are presented along with THz images of relevant targets. Potential applications are discussed.
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