Waveguide Terahertz Time-domain Spectroscopy Research Articles

Measurable lower limit of thin film conductivity with parallel plate waveguide terahertz time domain spectroscopy.

Parallel plate waveguide (PPWG) terahertz (THz) time domain spectroscopy (TDS) is a powerful tool to investigate the properties of thin and low conductive materials. In this Letter, we determine the lower limit of detection of the PPWG-THz-TDS approach. We provide a closed-form expression of the minimal measurable conductivity by the system. The experimental results of amorphous YBa2Cu3O7-δ films indicate that the factor limiting the spectroscopic modality is the waveguide device misalignment. On the other hand, the expression of the minimal detectable conductivity provides a clear scheme of optimization by increasing the waveguide length and therefore enhancing the sensitivity of the system.

Waveguide terahertz time-domain spectroscopy of ammonium nitrate polycrystalline films

We report temperature-dependent measurements of the terahertz (THz) vibrational spectrum of ammonium nitrate (AN) films and mixed potassium nitrate (KN)–ammonium nitrate films using waveguide THz time domain spectroscopy. The experiments were performed on polycrystalline films on the metal surface of a parallel plate waveguide. At cryogenic temperature and with frequency resolution as high as 7 GHz, our measurements produce a complex vibrational spectrum for AN, and show vibrational resonances not observed in previous far infrared and Raman measurements. We investigate potential interactions between AN and the metal surface by measuring THz spectra of films on aluminum, gold, and a gold surface coated with an organic self-assembled monolayer. Measurements are also performed on a deuterated AN film and indicate that the observed THz modes are due largely to the motion of the nitrate ions in the AN crystal. Finally, the effect of introducing small amounts of an impurity into the AN lattice is examined. We find that introduction of as little as 1%-2% by weight of potassium nitrate into the AN lattice causes line broadening of the THz modes, which is consistent with increased disorder introduced by the impurity.

Temperature dependent characterization of terahertz vibrations of explosives and related threat materials

Waveguide terahertz time-domain spectroscopy (THz-TDS) is used to characterize the temperature dependent vibrational properties of three threat-related materials: 4-amino-dinitrotoluene (4A-DNT), pentaerythritol tetranitrate (PETN), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). These materials are characterized as thin polycrystalline layers deposited in the 50 micron gap of a metal parallel plate waveguide. For each material waveguide THz-TDS at least partially resolves the underlying vibrational spectrum and reveals new features that have not been observed in previous free space measurements of these materials. Strong experimental evidence for a phase transformation is observed for 4A-DNT as the polycrystalline layer on the waveguide surface is cooled to near 200 K. For PETN a highly resolved spectrum containing eleven vibrational lines is observed at 11 K with full-width at half maximum linewidths ranging from 7 GHz to 40 GHz. Based on comparison to measurements in the literature, our PETN measurement suggests that it is possible to produce narrow linewidths from a polycrystalline layer that approach those from a single crystal. Finally, for HMX, a highly resolved vibrational spectrum is measured that is assigned to the metastable gamma polymorph.

Guided-wave terahertz spectroscopy of molecular solids [Invited

One of the outstanding problems of terahertz spectroscopy is the measurement of the underlying vibrational spectrum of a molecular solid, where individual vibrational transitions are often merged into broad absorption features by line-broadening processes. We address this problem using the technique of waveguide terahertz time-domain spectroscopy (THz-TDS), whereby a thin polycrystalline molecular film is contained within a single-mode metal parallel plate waveguide. Thin films of the molecular solids cyclotrimethylene trinitramine (RDX explosive) and 4-iodo-4′-nitrobiphenyl serve to demonstrate the ability of waveguide THz-TDS to resolve previously unseen complex underlying THz vibrational spectra at cryogenic temperatures, with linewidths as narrow as 7 GHz. With such narrow linewidths we are able to demonstrate the measurement of vibrational line-center frequencies to a precision of 1 GHz.

The underlying terahertz vibrational spectrum of explosives solids

Using waveguide terahertz time-domain spectroscopy, we demonstrate the measurement of the underlying terahertz vibrational spectrum of the explosive solids hexahydro-1,3,5-trinitro-1,3,5-triazine, and 2,4,6-trinitrotoluene. Each explosive is cast as a polycrystalline thin film with planar ordering on the inner surface of metal parallel plate waveguide. For measurements near 10K, each explosive reveals a complex spectrum of approximately 20 vibrational modes between 0.5 and 3.5THz. The explosive films are of sufficient quality to produce vibrational linewidths as much as an order of magnitude sharper compared to conventional terahertz measurements of corresponding pellet samples of explosives.

Narrow-Line Waveguide Terahertz Time-Domain Spectroscopy of Aspirin and Aspirin Precursors

Low frequency vibrational modes of pharmaceutical molecules are dependent on the molecule as a whole and can be used for identification purposes. However, conventional Fourier transform far-infrared spectroscopy (FT-IR) and terahertz time-domain spectroscopy (THz-TDS) often result in broad, overlapping features that are difficult to distinguish. The technique of waveguide THz-TDS has been recently developed, resulting in sharper spectral features. Waveguide THz-TDS consists of forming an ordered polycrystalline film on a metal plate and incorporating that plate in a parallel-plate waveguide, where the film is probed by THz radiation. The planar order of the film on the metal surface strongly reduces the inhomogeneous broadening, while cooling the waveguide to 77 K reduces the homogeneous broadening. This combination results in sharper absorption lines associated with the vibrational modes of the molecule. Here, this technique has been demonstrated with aspirin and its precursors, benzoic acid and salicylic acid, as well as the salicylic acid isomers 3- and 4-hydroxybenzoic acid. Linewidths as narrow as 20 GHz have been observed, rivaling single crystal measurements.

High-Resolution Waveguide THz Spectroscopy of Biological Molecules

Low-frequency vibrational modes of biological molecules consist of intramolecular modes, which are dependent on the molecule as a whole, as well as intermolecular modes, which arise from hydrogen-bonding interactions and van der Waals forces. Vibrational modes thus contain important information about conformation dynamics of biological molecules, and can also be used for identification purposes. However, conventional Fourier transform infrared spectroscopy and terahertz time-domain spectroscopy (THz-TDS) often result in broad, overlapping features that are difficult to distinguish. The technique of waveguide THz-TDS has been recently developed, resulting in sharper features. For this technique, an ordered polycrystalline film of the molecule is formed on a metal sample plate. This plate is incorporated into a metal parallel-plate waveguide and probed via waveguide THz-TDS. The planar order of the film reduces the inhomogeneous broadening, and cooling of the samples to 77K reduces the homogenous broadening. This combination results in the line-narrowing of THz vibrational modes, in some cases to an unprecedented degree. Here, this technique has been demonstrated with seven small biological molecules, thymine, deoxycytidine, adenosine, D-glucose, tryptophan, glycine, and L-alanine. The successful demonstration of this technique shows the possibilities and promise for future studies of internal vibrational modes of large biological molecules.

High-Resolution Waveguide Terahertz Spectroscopy of Partially Oriented Organic Polycrystalline Films

We have characterized the terahertz (THz) vibrational spectroscopy of organic polycrystalline thin films using the new experimental technique of waveguide terahertz time domain spectroscopy (waveguide THz-TDS). The organic materials used in this study are tetracyanoquinodimethane (TCNQ) and 1,3-dicyanobenzene (13DCB). For each material, a thin film is cast onto one of the inner surfaces of a metal parallel plate waveguide (PPWG), followed by measurement of the low-frequency vibrational spectrum using waveguide THz-TDS. The vibrational spectra of the waveguide films are compared to corresponding vibrational spectra of standard pellet samples made by dispersing the organic solid in transparent polyethylene. We show how the waveguide films produce significantly narrower THz vibrational line shapes and reveal additional spectral lines that are obscured by inhomogeneous broadening effects in the pellet samples. When TCNQ waveguide films are cooled to 77 K, vibrational line widths as sharp as 25-30 gigahertz (0.83-1.0 cm(-1)) at the full width at half-maximum are observed, which are among the narrowest far-infrared line widths measured for this material. The origin of the line-narrowing effect for the waveguide films is the suppression of inhomogeneous broadening due to the planar ordering of the film on the waveguide surface. The TCNQ waveguide films are further characterized using optical microscopic evaluation to understand how film morphology affects the THz vibrational spectrum. X-ray diffraction is used to determine the orientation of the polycrystalline TCNQ films on the PPWG surface and to qualitatively explain the different vibrational line strengths observed for the ordered waveguide film relative to the random pellet.

Reduced conductivity in the terahertz skin-depth layer of metals

The terahertz conductivities of plates of Cu and Al were measured to remain the same at 295 and 77K using waveguide terahertz time domain spectroscopy (THz-TDS). This result was true for a variety of commercial alloys and surface preparations. Consequently, carrier scattering by lattice defects within the 100nm THz skin depth is much larger than scattering by phonons at room temperature. However, an exception was found to be the THz skin-depth layer of an evaporated 300nm Al film in contact with a polished Si surface. For this interface Al layer, the conductivity increased by a factor of 4 when cooled to 77K.

Line narrowing of terahertz vibrational modes for organic thin polycrystalline films within a parallel plate waveguide

Waveguide terahertz time-domain spectroscopy is used to demonstrate the narrowing of vibrational lines for a thin polycrystalline film of 1,2-dicyanobenzene in a parallel plate metal waveguide. When compared to corresponding linewidths for 1,2-dicyanobenzene in a pellet sample at room temperature, the linewidths for the waveguide film are found to be significantly sharper. For measurements near 77K, a dramatic line narrowing is observed for the waveguide film, yielding linewidths as much as five times sharper than found in the pellet. These effects are attributed to much smaller inhomogeneous broadening in the waveguide film and result in a more informative spectrum.

Waveguide terahertz time-domain spectroscopy of nanometer water layers.

Waveguide terahertz time-domain spectroscopy is demonstrated to have the sensitivity to characterize nanometer-thick water layers on the surfaces of a parallel-plate metal waveguide. The measured far-infrared absorption and index of refraction of the 20-nm water layers are in reasonable self-consistent agreement with those of bulk water.