Expression of Concern: Direct observation of the THz Kerr effect (TKE) in deionized, distilled and buffered (PBS) water.

Expression of concern for 'Direct observation of the THz Kerr effect (TKE) in deionized, distilled and buffered (PBS) water' by Andrzej Dobek et al., Phys. Chem. Chem. Phys., 2017, 19, 26749-26757, https://doi.org/10.1039/c7cp04061j.

Terahertz endoscopic imaging for colorectal cancer detection: Current status and future perspectives.

Terahertz (THz) imaging is progressing as a robust platform for myriad applications in the field of security, health, and material science. The THz regime, which comprises wavelengths spanning from microns to millimeters, is non-ionizing and has very low photon energy: Making it inherently safe for biological imaging. Colorectal cancer is one of the most common causes of death in the world, while the conventional screening and standard of care yet relies exclusively on the physician's experience. Researchers have been working on the development of a flexible THz endoscope, as a potential tool to aid in colorectal cancer screening. This involves building a single-channel THz endoscope, and profiling the THz response from colorectal tissue, and demonstrating endogenous contrast levels between normal and diseased tissue when imaging in reflection modality. The current level of contrast provided by the prototype THz endoscopic system represents a significant step towards clinical endoscopic application of THz technology for in-vivo colorectal cancer screening. The aim of this paper is to provide a short review of the recent advances in THz endoscopic technology and cancer imaging. In particular, the potential of single-channel THz endoscopic imaging for colonic cancer screening will be highlighted.

Direct observation of the THz Kerr effect (TKE) in deionized, distilled and buffered (PBS) water.

Nonlinear THz pump-optical probe (THz-OKE) measurements in deionized, distilled and buffered (PBS) water are reported. A laser system that produces pulses at 800 nm, 30 fs FWHM, at a repetition rate of 1 kHz and an energy of 6 µJ per pulse, was a source of the probe optical beam and of the pump beam of THz 2 ps pulses. The electric field strength inducing birefringence in water samples was ETHz = 1.35 × 106 V m-1. These samples were chosen in order to study the effect of ionic concentration on water behavior in the ultrafast time scale. Differentiation between ultrafast effects resulting from internal H2O properties and those resulting from H2O-ion interactions was analyzed. These two effects may be connected to a difference in the fluctuations of the network of intermolecular hydrogen bonds of water molecules in the presence and absence of ions in solution. The results indicate that such fluctuations significantly alter water birefringence amplitude and its dynamics.

Continuous-wave circular polarization terahertz imaging.

Biomedical applications of terahertz (THz) radiation are appealing because THz radiation is nonionizing and has the demonstrated ability to detect intrinsic contrasts between cancerous and normal tissue. A linear polarization-sensitive detection technique for tumor margin delineation has already been demonstrated; however, utilization of a circular polarization-sensitive detection technique has yet to be explored at THz frequencies. A reflective, continuous-wave THz imaging system capable of illuminating a target sample at 584 GHz with either linearly or circularly polarized radiation, and capable of collecting both cross- and copolarized signals remitted from the target, is implemented. To demonstrate the system's utility, a fresh ex vivo human skin tissue specimen containing nonmelanoma skin cancer was imaged. Both polarization-sensitive detection techniques showed contrast between tumor and normal skin tissue, although some differences in images were observed between the two techniques. Our results indicate that further investigation is required to explain the contrast mechanism, as well as to quantify the specificity and sensitivity of the circular polarization-sensitive detection technique.

Tunable millimeter and sub-millimeter spectral response of textile metamaterial via resonant states.

We report on a new textile metamaterial created by adding metal wires directly into the polymer yarn. Split-ring resonator-like extended states are created. Simulations revealed that the extended states can be easily tuned via the geometry. Measurements of the transmittance spectrum as a function of the polarization angle in the low terahertz range were also performed and these peaks were ascribed to a polarization-dependent resonator model. The fabrics are viable candidates for flexible and deformable gigahertz and terahertz-enabled metamaterials.

Imaging of ex vivo nonmelanoma skin cancers in the optical and terahertz spectral regions optical and terahertz skin cancers imaging.

We tested the hypothesis that polarization sensitive optical and terahertz imaging may be combined for accurate nonmelanoma skin cancer (NMSC) delineation. Nine NMSC specimens were imaged. 513 µm and 440 nm wavelengths were used for terahertz and optical imaging, respectively. Histopathology was processed for evaluation. Terahertz reflectance of NMSC was quantified. Our results demonstrate that cross-polarized terahertz images correctly identified location of the tumours, whereas cross-polarized and polarization difference optical images accurately presented morphological features. Cross-polarized terahertz images exhibited lower reflectivity values in cancer as compared to normal tissue. Combination of optical and terahertz imaging shows promise for intraoperative delineation of NMSC.

Characterization of bending loss in hollow flexible terahertz waveguides.

Attenuation characteristics of hollow, flexible, metal and metal/dielectric coated polycarbonate waveguides were investigated using an optically pumped far infrared (FIR) laser at 215 µm. The bending loss of silver coated polycarbonate waveguides were measured as a function of various bending angles, bending radii, and bore diameters. Minimal propagation losses of 1.77, 0.96 dB/m were achieved by coupling the lowest loss TE11 mode into the silver or gold coated waveguide, and HE11 mode into the silver/polystyrene coated waveguides respectively. The maximal bending loss was found to be less than 1 dB/m for waveguides of 2 to 4.1 mm bore diameters, with a 6.4 cm bend radius, and up to 150° bending angle. The investigation shows the preservation of single laser mode in smaller bore waveguides even at greater bending angles.

Continuous wave terahertz transmission imaging of nonmelanoma skin cancers.

BACKGROUND AND OBJECTIVE: Continuous wave terahertz imaging has the potential to offer a safe, noninvasive medical imaging modality for delineating human skin cancers. Terahertz pulse imaging (TPI) has already shown that there is contrast between basal cell carcinoma and normal skin. Continuous-wave imaging offers a simpler, lower cost alternative to TPI. The goal of this study was to investigate the feasibility of continuous wave terahertz imaging for delineating skin cancers by demonstrating contrast between cancerous and normal tissue in transmission mode. MATERIALS AND METHODS: Two CO(2) optically pumped far-infrared molecular gas lasers were used for illuminating the tissue at two frequencies, 1.39 and 1.63 THz. The transmitted signals were detected using a liquid Helium cooled Silicon bolometer. Fresh skin cancer specimens were obtained from Mohs surgeries. The samples were processed and imaged within 24 hours after surgery. During the imaging experiment the samples were kept in pH-balanced saline to prevent tissue dehydration. At both THz frequencies two-dimensional THz transmission images of nonmelanoma skin cancers were acquired with spatial resolution of 0.39 mm at 1.4 THz and 0.49 mm at 1.6 THz. For evaluation purposes, hematoxylin and eosin (H&E) histology was processed from the imaged tissue. RESULTS: A total of 10 specimens were imaged and it was determined that for both frequencies, the areas of decreased transmission in the THz image correlated well with cancerous areas in the histopathology. Two negative controls were also imaged. The difference in transmission between normal and cancerous tissue was found to be approximately 60% at both frequencies, which suggests that contrast between normal and cancerous tissue at these frequencies is dominated by differences in water content. CONCLUSIONS: Our results suggest that intraoperative delineation of nonmelanoma skin cancers using continuous-wave terahertz imaging is feasible.

Terahertz inverse synthetic aperture radar (ISAR) imaging with a quantum cascade laser transmitter.

A coherent transceiver using a THz quantum cascade (TQCL) laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz, the coherence length of the TQCL transmitter will allow coherent imaging over distances up to several hundred meters. Image data obtained with the system is presented.

Characterization of roughness parameters of metallic surfaces using terahertz reflection spectra.

This paper reports on the effect of random Gaussian roughness with rms roughness values of 5-20 microm on the terahertz reflection spectra of metallic aluminum surfaces using Fourier transform IR spectroscopy. By comparing experimental data with a theoretical model based on the Kirchhoff approximation, the rms roughness of a surface is accurately determined. The rms roughness determined by this method is in good agreement with the rms roughness measured using a stylus surface profilometer. In addition, we demonstrate that this method can be used to clearly resolve rough surfaces that differ in rms roughness by approximately 1 microm.

Frequency stabilization of a single mode terahertz quantum cascade laser to the kilohertz level.

A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH(2)DOH THz CO(2) optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t approximately sec) linewidth of the optically pumped laser showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability should be sufficient to enable the use of THz quantum cascade lasers as transmitters in short-range coherent transceivers.

Terahertz sideband-tuned quantum cascade laser radiation.

A compact, tunable, narrowband terahertz source was demonstrated by mixing a single longitudinal mode 2.408 THz, free running quantum cascade laser with a 2-20 GHz microwave sweeper in a conventional corner-cube-mounted Schottky diode. The sideband spectra were characterized with a Fourier transform spectrometer, and the radiation was tuned through several D(2)O rotational transitions to estimate the longer term (t > or = several sec) bandwidth of the source. A spectral resolution of 2 MHz in CW regime was observed.

Transformation of the multimode terahertz quantum cascade laser beam into a Gaussian, using a hollow dielectric waveguide.

We demonstrate that a short hollow dielectric tube can act as a dielectric waveguide and transform the multimode, highly diverging terahertz quantum cascade laser beam into the lowest order dielectric waveguide hybrid mode, EH(11), which then couples efficiently to the free-space Gaussian mode, TEM(00). This simple approach should enable terahertz quantum cascade lasers to be employed in applications where a spatially coherent beam is required.