There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Terahertz biophotonics as a tool for studies of dielectric and spectral properties of biological tissues and liquids

A previous study of the growth conditions has shown that single-phase BiFeO3 thin films can only be obtained in a narrow pressure-temperature window and that these films display a weak magnetic moment. Here we study in more detail the structure and the magnetism of single-phase BiFeO3 films by means of reciprocal space mapping, Raman spectroscopy and neutron diffraction. X-ray and Raman data suggest that the BiFeO3 structure is tetragonal for 70 nm-thick films and changes to monoclinic for 240 nm-thick films, thus remaining different from that of the bulk (rhombohedral) structure. In the 240 nm monoclinically distorted film neutron diffraction experiments allow the observation of a G-type antiferromagnetic order as in bulk single crystals. However, the satellite peaks associated with the long-wavelength cycloid present in bulk BiFeO3 are not observed. The relevance of these findings for the exploitation of the magnetoelectric properties of BiFeO3 is discussed.

Structural distortion and magnetism of BiFeO3 epitaxial thin films: a Raman spectroscopy and neutron diffraction study

Over the past decades, signicant attention has been paid to biomedical applications of terahertz (THz) technology. Nowadays, THz spectroscopy and imaging allow for solving numerous demanding problems in biological, medical, food, plant and pharmaceutical sciences. Among all biomedical applications, a label-free diagnosis of malignant and benign neoplasms represents one of the most attractive branches of THz technology. Despite the attractiveness, the methods of THz diagnosis are still far from medical practice. In this review, we consider the modern research results in THz diagnosis of malignant and benign neoplasms, along with the topical research and engineering problems, which restrain the THz technology translation to clinics. We start from analysis of the common models of THz-wave { tissue interactions and the eects of tissue exposure to THz waves. Then, we discuss existing modalities of THz spectroscopic and imaging systems, which either have been already applied in medical imaging, or hold strong potential. We summarize the earlier-reported and the original results of THz measurements of neoplasms with dierent nosology and localization. We pay attention to the origin of contrast between healthy and pathological tissues in the THz spectra and images and discuss the prospective of THz technology in non-invasive, minimally-invasive and intraoperative diagnosis, as well as in aiding histology. Finally, we review the challenging problems of THz diagnosis, as well as attempts to solve them, which should bring the THz technology much closer to the medical practice. This review allows one to objectively uncover benets and weaknesses of the THz technology in diagnosis of malignant and benign neoplasms.

https://iopscience.iop.org/article/10.1088/2040-8986/ab4dc3/pdf

The progress and perspectives of terahertz technology for diagnosis of neoplasms: a review

Spin caloritronics, origin and outlook

For many years, the terahertz frequency domain has been neglected compared to neighboring parts of the spectrum. However, the situation has changed with the development of backward-wave oscillator (BWO) and BWO-based spectrometers. Packaged (30-180 GHz) and unpackaged (0.18-1.50 THz) BWOs are now being produced. The 0.18-1.50 THz frequency range is of greatest interest for spectroscopic research. To work with unpackaged BWOs, a universal magnetic focusing system on a permanent magnet with a protective housing shielding the magnetic field was developed. The stable generation of radiation with a frequency of up to 2.1 THz without geometry change was achieved. A universal automated power-supply unit with high-speed BWO frequency radiation scanning and a high degree of BWO electronic security was also designed for generating unpackaged BWOs through a heating and high cathode voltage.

BWO Generators for Terahertz Dielectric Measurements

Reflection and transmission infrared spectra of BiFeO3 ceramic samples have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 5 to 1000 cm−1 at temperatures in the range from 10 to 500 K. New resonant modes (probably, magnetic in nature) with the eigenfrequencies decreasing with an increase in the temperature have been recorded in the range 10–30 cm−1 by IR spectroscopy for the first time. An additional absorption with a fairly large dielectric contribution has been revealed in the range 30–60 cm−1. It has been demonstrated that the corresponding oscillators couple with both the lowest frequency phonon mode and the magnetic subsystem.

Optical properties of BiFeO3 ceramics in the frequency range 0.3–30.0 THz

Using methods of infrared, terahertz, and submillimeter spectroscopy, experimental transmission and reflection spectra of α-lactose monohydrate have been obtained at room temperature in the frequency range from 5 to 5000 cm–1. The experimental spectra of dense pressed tablets and powdered samples have been analyzed in terms of the model of a classical oscillator. Certain parameters of absorption bands of the resonant and quasi-relaxation types are markers by which components can be selected in heterogeneous pharmacological and organic preparations using the methods of coherent submillimeter and pulsed terahertz spectroscopy.

Electrodynamical Characteristics of α-Lactose Monohydrate in the Terahertz Range

The reflection and transmission spectra of a MgO single crystal at frequencies of 10–1000 cm−1 in the temperature range 10–300 K have been measured using submillimeter and infrared Fourier spectros-copy. The evolution of anharmonic absorption with variations in temperature has been investigated in the frequency range below the frequency of the transverse optical phonon. The parameters of the dispersion model that adequately describes the lattice absorption of terahertz radiation in single-crystal magnesium oxide have been determined.

Multiphonon absorption in a MgO single crystal in the terahertz range

We developed an optical cryostat with a sample-rotation unit for polarization-sensitive measurement in terahertz (THz) and infrared (IR) ranges. The cryostat, in combination with two metal-grid polarizers, provides full control of mutual orientation of the sample’s crystallographic axes and the light polarization plane. Importantly, this control is realized in-situ , i.e., during the sample cooling–heating cycle. To demonstrate the abilities of the developed cryostat, we used it in combination with a laboratory-made THz time-domain spectrometer, for polarization-sensitive measurements of an orthoferrite (YFeO3) in the range of 5 to 50  cm  −  1. These measurements revealed strong angular dependence of the sample transmission. The developed cryostat is capable for solving numerous demanding problems of THz and IR spectroscopy in condensed matter physics and materials science, biophysics, chemical, and pharmaceutical sciences.

O. E. Porodinkov

Papers

BWO Generators for Terahertz Dielectric Measurements

Optical properties of BiFeO3 ceramics in the frequency range 0.3–30.0 THz

Electrodynamical Characteristics of α-Lactose Monohydrate in the Terahertz Range

Multiphonon absorption in a MgO single crystal in the terahertz range

Optical cryostat with sample rotating unit for polarization-sensitive terahertz and infrared spectroscopy