Crystallization Caught in the Act with Terahertz Spectroscopy: Non-Classical Pathway for l-(+)-Tartaric Acid.

Crystal formation is a highly debated problem. This report shows that the crystallization of l-(+)-tartaric acid from water follows a non-classical path involving intermediate hydrated states. Analytical ultracentrifugation indicates solution clusters of the initial stages aggregate to form an early intermediate. Terahertz spectroscopy performed during water evaporation highlights a transient increase in the absorption during nucleation; this indicates the recurrence of water molecules that are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz, which can be assigned to a natural vibration of large hydrated aggregates, vanishes after the final crystal has formed. Furthermore, THz data reveal the vibration of nanosized clusters in the dilute solution indicated by analytical ultracentrifugation. Infrared spectroscopy and wide-angle X-ray scattering highlight that the intermediate is not a crystalline hydrate. These results demonstrate that nanoscopic intermediate units assemble to form the first solvent-free crystalline nuclei upon dehydration.

Terahertz near-field imaging of dielectric resonators.

As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.

Terahertz imaging modalities of ancient Egyptian mummified objects and of a naturally mummified rat.

During the last few years, terahertz (THz) imaging has been used to investigate artwork and historic artifacts. The application of THz imaging to mummy investigations is very attractive since it provides spectroscopic information over a broad frequency range and its radiation has proven to be harmless to human cells. However, compared with the current standard imaging methods in mummy imaging-X-ray and computed tomography (CT)--it remains a novel, emerging technique whose potential still needs to be fully evaluated. Here, ancient Egyptian mummified objects as well as a naturally mummified rat have been investigated by two different THz imaging systems: a broadband THz time domain imaging system and an electronic THz scanner. The obtained THz images are compared with conventional CT, X-ray, and magnetic resonance images. While the broadband THz time domain setup permits analyses of smaller samples, the electronic THz scanner allows the recording of data of thicker and larger samples at the expense of a limited spectral bandwidth. Terahertz imaging shows clear potential for mummy investigations, although currently CT imaging offers much higher spatial resolution. Furthermore, as commercial mobile THz scanners become available, THz imaging could be applied directly in museums or at excavation sites.

Double-layered nitrocellulose membrane sample holding technique for THz and FIR spectroscopic measurements.

In terahertz (THz) and far-infrared (FIR) spectroscopic measurements, weak absorption spectral features due to small quantities of test sample can be masked by undesirable etalon fringe artifacts caused by multiple reflections within a pellet or a rigid sample holder. A double-layered nitrocellulose (NC) membrane structure is proposed in this paper as an alternative holder for small quantities of either dry or wet pure (no added polyethylene powder) samples with significantly reduced etalon artifacts. Utilizing a THz time-domain spectroscopy system and a synchrotron source, we demonstrate the performance of the NC structure across the THz/FIR spectrum, benchmarking against pellets holding similarly small quantities of α-lactose powder either with or without different grades of polyethylene powder. With only pure samples to consider, scattering can be mitigated effectively in NC-derived spectra to reduce their baselines.

Metamaterial fibres for subdiffraction imaging and focusing at terahertz frequencies over optically long distances.

Using conventional materials, the resolution of focusing and imaging devices is limited by diffraction to about half the wavelength of light, as high spatial frequencies do not propagate in isotropic materials. Wire array metamaterials, because of their extreme anisotropy, can beat this limit; however, focusing with these has only been demonstrated up to microwave frequencies and using propagation over a few wavelengths only. Here we show that the principle can be scaled to frequencies orders of magnitudes higher and to considerably longer propagation lengths. We demonstrate imaging through straight and tapered wire arrays operating in the terahertz spectrum, with unprecedented propagation of near field information over hundreds of wavelengths and focusing down to 1/28 of the wavelength with a net increase in power density. Applications could include in vivo terahertz-endoscopes with resolution compatible with imaging individual cells.

Low-cost ultra-thin broadband terahertz beam-splitter.

A low-cost terahertz beam-splitter is fabricated using ultra-thin LDPE plastic sheeting coated with a conducting silver layer. The beam splitting ratio is determined as a function of the thickness of the silver layer--thus any required splitting ratio can be printed on demand with a suitable rapid prototyping technology. The low-cost aspect is a consequence of the fact that ultra-thin LDPE sheeting is readily obtainable, known more commonly as domestic plastic wrap or cling wrap. The proposed beam-splitter has numerous advantages over float zone silicon wafers commonly used within the terahertz frequency range. These advantages include low-cost, ease of handling, ultra-thin thickness, and any required beam splitting ratio can be readily fabricated. Furthermore, as the beam-splitter is ultra-thin, it presents low loss and does not suffer from Fabry-Pérot effects. Measurements performed on manufactured prototypes with different splitting ratios demonstrate a good agreement with our theoretical model in both P and S polarizations, exhibiting nearly frequency-independent splitting ratios in the terahertz frequency range.

Low-frequency spectroscopic analysis of monomeric and fibrillar lysozyme.

Terahertz time-domain spectroscopy (THz-TDS) and Fourier transform infrared (FT-IR) spectroscopy were used to generate far-infrared and low-frequency spectral measurements of monomeric lysozyme and lysozyme fibrils. The formation of lysozyme fibrils was verified by the Thioflavin T assay and transmission electron microscopy (TEM). It was evident in the FT-IR spectra that between 150 and 350 cm(-1) the two spectra diverge, with the lysozyme fibrils showing higher absorbance intensity than the monomeric form. The broad absorption phenomenon is likely due to light scattered from the fibrillar architecture of lysozyme fibrils as supported by simulation of Rayleigh light scattering. The lack of discrete phonon-like peaks suggest that far-infrared spectroscopy cannot detect vibrational modes between the highly ordered hydrogen-bonded beta-pleated sheets of the lysozyme subunit.

Chemical sensing and imaging with pulsed terahertz radiation.

Over the past decade, terahertz spectroscopy has evolved into a versatile tool for chemically selective sensing and imaging applications. In particular, the potential to coherently generate and detect short, and hence, broadband terahertz pulses led to the development of efficient and compact spectrometers for this interesting part of the electromagnetic spectrum, where common packaging materials are transparent and many chemical compounds show characteristic absorptions. Although early proof-of-principle demonstrations have shown the great potential of terahertz spectroscopy for sensing and imaging, the technology still often lacks the required sensitivity and suffers from its intrinsically poor spatial resolution. In this review we discuss the current potential of terahertz pulse spectroscopy and highlight recent technological advances geared towards both enhancing spectral sensitivity and increasing spatial resolution.

Terahertz spectroscopic differentiation of microstructures in protein gels.

We demonstrate that terahertz (THz) spectroscopy can be used to differentiate soft protein microstructures. Differentiation of soft microstructures in gels has to date been performed using optical imaging techniques (e.g. electron microscope), but a non-destructive differentiation tool is lacking. Particulate and fine-stranded (fibrillar) soft protein microstructures are of interest, particularly to medical researchers, because they form from naturally occurring proteins that are thought to be involved in several human diseases, such as Alzheimer's disease. In this study, globular beta-lactoglobulin structures with diameters of 2 microm, and fibrillar structures with diameters less than 0.03 microm are observed between 0.8 and 1.5 THz. Results show that the globular structures have a decline in THz transmission when compared to the fibrillar ones. The cause of this decline is possibly due to Rayleigh scattering from the globular microstructures.

THz porous fibers: design, fabrication and experimental characterization.

Porous fibers have been identified as a means of achieving low losses, low dispersion and high birefringence among THz polymer fibers. By exploiting optical fiber fabrication techniques, two types of THz polymer porous fibers--spider-web and rectangular porous fibers--with 57% and 65% porosity have been fabricated. The effective refractive index measured by terahertz time domain spectroscopy shows a good agreement between the theoretical and experimental results indicating a lower dispersion for THz porous fiber compared to THz microwires. A birefringence of 0.012 at 0.65 THz is also reported for rectangular porous fiber.

Sensing the hygroscopicity of polymer and copolymer materials using terahertz time-domain spectroscopy.

We present the hygroscopicity of polymer and copolymer materials in the low terahertz (THz) frequency range using a linear absorption model. We identify COC 6013 and COC 5013 as optimal THz window materials, possessing both low hygroscopicity and high transmission in the THz regime. The correct choice of window material is of significance for transmission THz spectroscopy and of particular interest for THz liquid spectroscopy.

Effects of formalin fixing on the terahertz properties of biological tissues.

We demonstrate how the terahertz properties of porcine adipose tissue and skeletal muscle are affected by formalin fixing. Terahertz radiation is sensitive to covalently cross-linked proteins and can be used to probe unique spectroscopic signatures. We study in detail the changes arising from different fixation times and see that formalin fixing reduces the refractive index and the absorption coefficient of the samples in the terahertz regime. These fundamental properties affect the time-domain terahertz response of the samples and determine the level of image contrast that can be achieved.

Material thickness optimization for transmission-mode terahertz time-domain spectroscopy.

The thickness of a sample material for a transmission-mode terahertz time-domain spectroscopy (THz-TDS) measurement is the subject of interest in this paper. A sample that is too thick or too thin can raise the problem of measurement uncertainty. Although greater thickness allows the terahertz radiation--or T-rays--to interact more with bulk material, the SNR rolls off with thickness due to signal attenuation. A sample that is too thin renders itself nearly invisible to T-rays, in such a way that the system can hardly sense the difference between the sample and a free space path. The optimal trade-off is analyzed and revealed in this paper, where our approach is to find the optimal thickness that results in the minimal uncertainty of measured optical constants. The derived model for optimal thickness is supported by the results from experiments performed with polyvinyl chloride (PVC), high-density polyethylene (HDPE), and lactose samples.

Porous fibers: a novel approach to low loss THz waveguides.

We propose a novel class of optical fiber with a porous transverse cross-section that is created by arranging sub-wavelength air-holes within the core of the fiber. These fibers can offer a combination of low transmission loss and high mode confinement in the THz regime by exploiting the enhancement of the guided mode field that occurs within these sub-wavelength holes. We evaluate the properties of these porous fibers and quantitatively compare their performance relative to that of a solid core air cladded fiber (microwire). For similar loss values, porous fibers enable improved light confinement and reduced distortion of a broadband pulse compared to microwires.

Dynamic range in terahertz time-domain transmission and reflection spectroscopy.

We present a quantitative method for identification of the dynamic range of the detectable absorption coefficient in the analysis of transmission terahertz (THz) time-domain spectroscopy data. In transmission measurements the largest detectable absorption coefficient is determined by the dynamic range of the THz signals, whereas in reflection measurements the largest detectable absorption coefficient is determined by the scan-to-scan reproducibility of the signal.