Martin S. Heimbeck

Terahertz digital holography using angular spectrum and dual wavelength reconstruction methods

Terahertz digital off-axis holography is demonstrated using a Mach-Zehnder interferometer with a highly coherent, frequency tunable, continuous wave terahertz source emitting around 0.7 THz and a single, spatially-scanned Schottky diode detector. The reconstruction of amplitude and phase objects is performed digitally using the angular spectrum method in conjunction with Fourier space filtering to reduce noise from the twin image and DC term. Phase unwrapping is achieved using the dual wavelength method, which offers an automated approach to overcome the 2π phase ambiguity. Potential applications for nondestructive test and evaluation of visually opaque dielectric and composite objects are discussed.

Terahertz Digital Holographic Imaging of Voids Within Visibly Opaque Dielectrics

Terahertz digital off-axis holography (THzDH) has been demonstrated as a non-destructive tool for imaging voids within visually opaque dielectrics. Using a raster scanning heterodyne detector, the imager captures lensless transmission holograms formed by the interaction of a highly coherent, monochromatic beam with 3-D printed structures. Digital hologram reconstructions from two structures were used to measure the imagers modulation transfer function and to show that terahertz digital holography can provide sub-millimeter resolution images of voids within visually opaque printed structures. As a demonstration we imaged embedded air- and lossy dielectric filled-voids whose refractive indices differ from the host material.

Terahertz interferometric synthetic aperture tomography for confocal imaging systems.

Terahertz (THz) interferometric synthetic aperture tomography (TISAT) for confocal imaging within extended objects is demonstrated by combining attributes of synthetic aperture radar and optical coherence tomography. Algorithms recently devised for interferometric synthetic aperture microscopy are adapted to account for the diffraction-and defocusing-induced spatially varying THz beam width characteristic of narrow depth of focus, high-resolution confocal imaging. A frequency-swept two-dimensional TISAT confocal imaging instrument rapidly achieves in-focus, diffraction-limited resolution over a depth 12 times larger than the instruments depth of focus in a manner that may be easily extended to three dimensions and greater depths.

Carbon nanotube fiber terahertz polarizer

Conventional, commercially available terahertz (THz) polarizers are made of uniformly and precisely spaced metallic wires. They are fragile and expensive, with performance characteristics highly reliant on wire diameters and spacings. Here, we report a simple and highly error-tolerant method for fabricating a freestanding THz polarizer with nearly ideal performance, reliant on the intrinsically one-dimensional character of conduction electrons in well-aligned carbon nanotubes (CNTs). The polarizer was constructed on a mechanical frame over which we manually wound acid-doped CNT fibers with ultrahigh electrical conductivity. We demonstrated that the polarizer has an extinction ratio of ∼−30 dB with a low insertion loss (<0.5 dB) throughout a frequency range of 0.2–1.1 THz. In addition, we used a THz ellipsometer to measure the Muller matrix of the CNT-fiber polarizer and found comparable attenuation to a commercial metallic wire-grid polarizer. Furthermore, based on the classical theory of light transmissi...

Design, simulation, and characterization of THz metamaterial absorber

In recent years a great amount of research has been focused on metamaterials, initially for fabrication of left-handed materials (LHM) for use in devices such as superlenses, or electromagnetic cloaking device. [1, 2]. Such devices have been developed and demonstrated in regimes from radio frequency all the way up to infrared and near optical frequencies [3–5]. Metamaterials can be characterized by electric permittivity, ε(ω), and magnetic permeability, μ(ω). By manipulating these properties, metamaterials can be engineered to exhibit un-natural phenomena such as negative index of refraction (n < 0). Furthermore, the electric and magnetic responses of metamaterials can be independently adjusted which provides a significant advantage over conventional materials to allow a convenient method to develop a wide range of devices. More recently, it has been shown that by careful adjustment of ε(ω) and μ(ω), near perfect electromagnetic absorbers can be realized [6, 7]. 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 may both be absorbed by a metamaterial structure. By independently adjusting ε(ω) and μ(ω) such that the effective impedance of the metamaterial is matched to that of free space (Zeff = Z0), the reflection at some resonance frequency, ω0, can be minimized.

Comparative reconstructions of THz spectroscopic imaging for non-destructive testing and biomedical imaging

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.

Transmissive quasi-optical Ronchi phase grating for terahertz frequencies.

A transmissive, square-wave Ronchi phase grating has been fabricated from the dielectric polytetrafluoroethylene to diffract an ~0.7 THz beam quasi-optically. When illuminated by a coherent, cw terahertz (THz) source, the spot separation of the ±1 diffractive orders and the diffraction efficiency were measured as a function of THz frequency and rotation angle. The grating performance depends sensitively on the refractive index, whose value can be measured with an accuracy limited by the fabrication precision. The use of these gratings for polarization-insensitive quasi-optical imaging and phased arrays is discussed.

Adaptive millimeter-wave synthetic aperture imaging for compressive sampling of sparse scenes.

We apply adaptive sensing techniques to the problem of locating sparse metallic scatterers using high-resolution, frequency modulated continuous wave W-band RADAR. Using a single detector, a frequency stepped source, and a lateral translation stage, inverse synthetic aperture RADAR reconstruction techniques are used to search for one or two wire scatterers within a specified range, while an adaptive algorithm determined successive sampling locations. The two-dimensional location of each scatterer is thereby identified with sub-wavelength accuracy in as few as 1/4 the number of lateral steps required for a simple raster scan. The implications of applying this approach to more complex scattering geometries are explored in light of the various assumptions made.

Characterization of an active metasurface using terahertz ellipsometry

Switchable metasurfaces fabricated on a doped epi-layer have become an important platform for developing techniques to control terahertz (THz) radiation, as a DC bias can modulate the transmission characteristics of the metasurface. To model and understand this performance in new device configurations accurately, a quantitative understanding of the bias-dependent surface characteristics is required. We perform THz variable angle spectroscopic ellipsometry on a switchable metasurface as a function of DC bias. By comparing these data with numerical simulations, we extract a model for the response of the metasurface at any bias value. Using this model, we predict a giant bias-induced phase modulation in a guided wave configuration. These predictions are in qualitative agreement with our measurements, offering a route to efficient modulation of THz signals.

Off-Axis Fresnel Digital Holography at Terahertz Frequencies

Off-axis holograms were acquired using a highly coherent source tunable over 0.3 - 1.0 THz. Fresnel and angular spectrum reconstruction methods are presented and compared. Unprecedented depth resolution of nearly lambda/300 is reported.