491 research outputs found
One-dimensional terahertz imaging of surfactant-stabilized dodecane-brine emulsion
No full textTerahertz line-images of surfactant-stabilized dodecane(C 12 H 26 )-brine emulsions are obtained by translating the emulsified region through the focus of a terahertz time-domain spectrometer, capturing a time-domain waveform at each vertical position. From these images, relative dodecane content, emulsion size, and stability can be extracted to evaluate the efficacy of the surfactant in solvating the dodecane. In addition, the images provide insight into the dynamics of concentrated emulsions after mixing
Terahertz Spectroscopy Unambiguously Determines the Orientation of Guest Water Molecules in a Structurally Elusive Metal–Organic Framework
Get PDF: Porous materials, particularly metal-organic frameworks (MOFs), hold great promise for advanced applications. MIL-53(Al) is an exceptionally well-studied MOF that exhibits a phase transition upon guest capture─in this case, water─resulting in a dramatic change in the pore volume. Despite extensive studies, the structure of the water-loaded narrow-pore phase, MIL-53(Al)-np, remains controversial, particularly with respect to the positions of the adsorbed water molecules. We use terahertz spectroscopy, coupled with powder X-ray diffraction and density functional theory simulations, to unambiguously resolve this controversy. We show that the low-frequency (<100 cm-1) vibrational spectrum depends on weak long-range forces that are extremely sensitive to the orientation of the adsorbed water molecules. This enables definitively determining the correct structure of MIL-53(Al)-np while highlighting the extreme sensitivity of terahertz spectroscopy to bulk structure, suggesting its potential as a robust complement to X-ray diffraction for precise characterization of host-guest complexes
Terahertz Spectroscopy of Soft Condensed Matter: Single Methyl Branched n-Alkanes and Plastic-Crystals
No full textTerahertz time-domain spectroscopy has long been recognized as a valuable tool for characterizing the far-IR range (0.1 THz to 10 THz) properties of innumerable condensed phase materials. Soft condensed matter, a broad subcategory of condensed matter, encompasses thermally and/or mechanically deformable phases such as liquids, liquid-crystals, plastic-crystals, and polymers. With comparatively more internal degrees of freedom and weaker atomic/molecular interactions, complex phase behavior and dynamics often emerge in these materials, which generally have not been explored in the THz range. Here, temperature-dependent THz-TDS is applied to three soft condensed matter systems. Using a custom designed reflection geometry spectrometer and a novel variable path length liquid sample cell, the isotropic polarizabilities of liquid phase methyl-branched n-alkanes and their linear structural isomers are characterized and compared in the THz range. In addition, the plastic-crystal’s succinonitrile and camphor are studied using transmission geometry THz-TDS. The THz range properties and dynamics characterized in these studies, from the structure-dependent isotropic polarizabilities of prototypical liquid hydrocarbons to the complex disorder/order transitions and correlated molecular dynamics of plastic-crystals, provide valuable fundamental insight into the behavior of soft condensed matter systems
A terahertz two-wire waveguide with low bending loss
No full textWe present experimental and theoretical evidence for low loss terahertz propagation along a two-wire waveguide. We find that the mode pattern at the end of the waveguide resembles that of a dipole consistent, with the expected fundamental mode of the structure. We experimentally observe that the two-wire configuration can be used to efficiently excite the radial mode (Somerfield wave) of a single wire, by adiabatically separating the two wires into two single wires. We observe for the first time that a commercial 300-Ω TV twin-lead antenna cable can be used to propagate low loss THz radiation, guiding frequency components up to 0.2 THz even though they are designed only to operate to about 800 MHz. We find this to be the case even at considerable wire-bend angles
The propagation of single-cycle terahertz pulses in random media
No full textWe describe what are to our knowledge the first measurements of the propagation of coherent, single-cycle pulses of terahertz radiation in a scattering medium. We measure the propagation constants for pulses in a dense collection of spherical scatterers, and compare to the predictions of the quasi-crystalline approximation. Even though the fractional volume in our measurements exceeds the limit of validity of this model, we find that it still predicts certain features of the propagation with reasonable accuracy. By measuring the transmission as a function of the length L of the medium, we extract the scattering mean free path lambdasc (o) over a broad bandwidth. We observe variations in lambda sc ranging over nearly two orders of magnitude, and covering the entire thin sample regime from L/lambda sc << 1 to L/lambdasc ∼10
Applications of surface plasmon polaritons in terahertz spectral regime
Get PDFThis thesis presents the experimental work on the applications of surface plasmon polariton (SPP) in terahertz (THz) spectral range.
Apertureless near-field optical microscopy (ANSOM) has been widely used to study the localized SPP on various material surfaces. THz ANSOM technique was recently developed to combine the THz time-domain spectroscopy and the ANSOM technique to provide a near-field detection on the localized THz surface waves with improved spatial resolution and signal-noise ratio. We have studied the metal-insulator transition in vanadium dioxide (VO2) thin film using THz ANSOM. We observe a variation of the terahertz amplitude due to the phase transition induced by an applied voltage across the sample. The change of the terahertz signal is related to the abrupt change of the conductivity of the VO2 film at the metal-insulator transition. The subwavelength spatial resolution of this near-field microscopy makes it possible to detect signatures of metallic domains, which exist in the VO2 thin films in the vicinity of the phase transition.
We experimentally investigate the propagation of guided waves in finite-width parallel-plate waveguides (PPWGs) in the terahertz spectral range. We observe the propagation of SPPs in this guiding structure, instead of the fundamental transverse electromagnetic (TEM) mode. We find that the two-dimensional (2-D) energy confinement within the finite-width PPWG increases exponentially as the plate separation is reduced. We speculate that edge plasmons play an important role in the energy confinement in this open-structure waveguide. For comparison, the infinite-width PPWGs, the plates of which are much wider than the THz beam size, are also studied with several plate separations. The free-space beam diffraction produces a Gaussian profile along the unconfined direction. The unusual electric field profiles along the vertical direction, perpendicular to the plate are observed. The field enhancement near the metal surfaces are also explained by the SPPs coupled to the metal surfaces.
Based on the 2-D energy confinement in the finite-width PPWGs, we design the tapered slot waveguide by slowly tapering the plate width and slot gap. We first study the transverse component of the THz electric field, where a subwavelength 2-D energy confinement is observed. The output spot size strongly depends on the output facet size, where the slot gap and the tip width are in the same scale range. Subwavelength confinement is obtained, corresponding to lambda/4. Further confinement is limited by the spatial resolution of the detecting technique. To overcome this problem, we adapt the THz ASNOM setup to scattering-probe imaging technique, which has been proven to obtain deep subwavelength spatial resolution and great signal-noise ratio. Scattering-probe imaging setup measures the longitudinal component of the electric field of SPPs in the tapered slot waveguides. By slowly tapering the tip width and the slot gap, we squeeze a single-cycle THz pulse down to a size of 10 mum (lambda/260) by 18 mum (lambda/145), a mode area of only 2.6 x 10-5lambda2. We also observe a polarity reversal for the electric field between the guiding region near the upper and lower plates of the waveguide. This polarity flip is similar to that associated with the symmetric plasmon mode of slot waveguides
Terahertz imaging with compressive sensing
No full textMost existing terahertz imaging systems are generally limited by slow image acquisition due to mechanical raster scanning. Other systems using focal plane detector arrays can acquire images in real time, but are either too costly or limited by low sensitivity in the terahertz frequency range.
To design faster and more cost-effective terahertz imaging systems, the first part of this thesis proposes two new terahertz imaging schemes based on compressive sensing (CS). Both schemes can acquire amplitude and phase-contrast images efficiently with a single-pixel detector, thanks to the powerful CS algorithms which enable the reconstruction of N-by- N pixel images with much fewer than N2 measurements. The first CS Fourier imaging approach successfully reconstructs a 64x64 image of an object with pixel size 1.4 mm using a randomly chosen subset of the 4096 pixels which defines the image in the Fourier plane. Only about 12% of the pixels are required for reassembling the image of a selected object, equivalent to a 2/3 reduction in acquisition time. The second approach is single-pixel CS imaging, which uses a series of random masks for acquisition. Besides speeding up acquisition with a reduced number of measurements, the single-pixel system can further cut down acquisition time by electrical or optical spatial modulation of random patterns.
In order to switch between random patterns at high speed in the single-pixel imaging system, the second part of this thesis implements a multi-pixel electrical spatial modulator for terahertz beams using active terahertz metamaterials. The first generation of this device consists of a 4x4 pixel array, where each pixel is an array of sub-wavelength-sized split-ring resonator elements fabricated on a semiconductor substrate, and is independently controlled by applying an external voltage. The spatial modulator has a uniform modulation depth of around 40 percent across all pixels, and negligible crosstalk, at the resonant frequency. The second-generation spatial terahertz modulator, also based on metamaterials with a higher resolution (32x32), is under development. A FPGA-based circuit is designed to control the large number of modulator pixels. Once fully implemented, this second-generation device will enable fast terahertz imaging with both pulsed and continuous-wave terahertz sources
Space-time cross correlations of diffuse broadband terahertz waves
No full textWe describe observations of the amplitude and phase of an electric field diffusing through a three-dimensional random medium by using Terahertz Time-Domain Spectroscopy. These measurements are spatially resolved with a resolution smaller than the speckle spot size, and temporally resolved with a resolution better than one optical cycle. We propose a new correlation function---time-windowed correlation function---to explore the correlation among the measured waveforms. We have obtained information about individual scattering events experienced by the diffusing fields and how correlation fields evolve temporally. This represents a new method to characterize a multiply scattered wave and opens up new possibilities for imaging in biological media
Novel devices and systems for terahertz spectroscopy and imaging
No full textThis doctoral thesis documents my research on novel devices and systems for terahertz (THz) spectroscopy and imaging. The research is particularly focused on the manipulation of THz radiation, including subwavelength concentration and low-loss wave guiding.
One of the major obstacles for THz imaging is the poor spatial resolution due to the diffraction of the long-wavelength light source. To break this restriction, we build a THz near-field microscopy system by combining apertureless near-field scanning optical microscopy (ANSOM) with terahertz time-domain spectroscopy (THz-TDS). The experimental result indicates a sub-wavelength spatial resolution of about 10 micron. Abnormal frequency response of the ANSOM probe tip is observed, and a dipole antenna model is developed to explain the bandwidth reduction of the detected THz pulses. We also observe and characterize the THz wave propagation on the near-field probe in ANSOM. These studies not only demonstrate the feasibility of ANSOM in the THz frequency range, but also provide fundamental insights into the near-field microscopy in general, such as the broadband compatibility, the propagation effects and the antenna effects.
Motivated by our study of the propagation effects in THz ANSOM, we characterize the guided mode of THz pulses on a bare metal wire by directly measuring the spatial profile of electric field of the mode, and find that the wire structure can be used to guide THz waves with outstanding performance. This new broadband THz waveguide exhibits very small dispersion, extremely low attenuation and remarkable structural simplicity. These features make it especially suitable for use in THz sensing and imaging systems. The first THz endoscope is demonstrated based on metal wire waveguides. To improve the input coupling efficiency of such waveguides, we develop a photoconductive antenna with radial symmetry which can generate radially polarized THz radiation matching the waveguide mode. Through THz-TDS measurements and theoretical calculations, we study the dispersion relation of the surface waves on metal wires, which indicates the increasing importance of skin effects for surface waves in the THz frequency range
Multiple scattering of broadband terahertz pulses
No full textPropagation of single-cycle terahertz (THz) pulses through a random medium leads to dramatic amplitude and phase variations of the electric field because of multiple scattering. We present the first set of experiments that investigate the propagation of THz pulses through scattering media. The scattering of short pulses is a relevant subject to many communities in science and engineering, because the properties of multiply scattered or diffuse waves provide insights into the characteristics of the random medium. For example, the depolarization of diffuse waves has been used to form images of objects embedded in inhomogeneous media.
Most of the previous scattering experiments have used narrowband optical radiation where measurements are limited to time averaged intensities or autocorrelation quantities, which contain no phase information of the pulses. In the experiments presented here, a terahertz time-domain spectrometer (THz-TDS) is used. A THz-TDS propagates single-cycle sub-picosecond pulses with bandwidths of over 1 THz into free space. The THz-TDS is a unique tool to study such phenomena, because it provides access to both the intensity and phase of those pulses through direct measurement of the temporal electric field. Because of the broad bandwidth and linear phase of the pulses, it is possible to simultaneously study Rayleigh scattering and the short wavelength limit in a single measurement.
We study the diffusion of broadband single-cycle THz pulses by propagating the pulses through a highly scattering medium. Using the THz-TDS, time-domain measurements provide information on the statistics of both the amplitude and phase of the diffusive waves. We develop a theoretical description, suitable for broadband radiation, which accurately describes the experimental results. We measure the time evolution of the degree of polarization, and directly correlate it with the single-scattering regime in the time domain. Measurements of the evolution of the temporal phase of the radiation demonstrate that the average spectral content depends on the state of polarization. In the case of broadband radiation, this effect distinguishes photons that have been scattered only a few times from those that are propagating diffusively
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