In recent times, terahertz (THz) technologies have been actively applied in many biomedical research work, including gastric cancer diagnosis. In order to provide an effective removal of tumor during surgery, it is necessary to clearly distinguish it from different membranes of the stomach. In this work, we reported an investigation of various normal and cancer fresh gastric tissues using terahertz time-domain spectroscopy in the reflection mode. Refractive index and absorption coefficient of moderately differentiated and poorly differentiated gastric adenocarcinomas, as well as both serosa and mucosa were obtained in the frequency range from 0.2 to 1 THz. All cancer tissues were distinguishable from normal ones. The influence of the morphology of the investigated tissues on the obtained optical properties is discussed. The obtained results demonstrated a potential of THz time-domain spectroscopy to discriminate a tumor from normal serous and mucous gastric membranes. Thus, this method might be applied to gastric cancer diagnosis.

Ultrafast photo-induced phase transitions occurring under the impact of femtosecond laser pulses provide versatile opportunities for switching solids between distinctly-different crystalline, electronic, and magnetic states and thus modify their functional properties in a significant way. In this paper, we report on the laser-induced spin reorientation and Verwey phase transitions in a single crystalline ferrimagnetic magnetite ${\mathrm{Fe}}_3{\mathrm{O}}_4$. Using femtosecond optical and magneto-optical pump-probe techniques, we define the range of the initial sample temperatures and laser fluences when partial or complete photo-induced phase transitions occur from a monoclinic insulating to a cubic metallic state with concomitant switching of magnetic anisotropy from the uniaxial to the cubic one. We thus reveal a connection between these phase transitions when driven by femtosecond laser pulses. Using transient linear and quadratic magneto-optical effects, we examine magnetization dynamics launched the switching of the magnetic anisotropy axis. We unveil the presence of the domains undergoing the laser-induced phase transitions even below the established threshold fluence for the transitions, as well as when the material is initially in the cubic phase. This is the manifestation of the first order of these both laser-induced phase transitions beyond the range of thermodynamic equilibrium.

The development of spintronic emitters of broadband terahertz (THz) pulses relies on designing heterostructures in which the processes of laser-driven spin current generation and subsequent spin-to-charge current conversion are the most efficient. The interface between the ferromagnetic and nonmagnetic layers in an emitter is a critical element. In this study, we experimentally examined single-cycle THz pulse generation from a laser-pulse-excited Pt/Co emitter with a 1.2-nm-thick composition-gradient interface between the Pt and Co and compared it with the emission from a conventional Pt/Co structure with an abrupt interface. We found that the gradient interface improved the efficiency of the optics-to-THz conversion by a factor of two in a wide range of optical fluences up to 3 mJ⋅cm^-2. This enhancement was caused by a pronounced increase in the transmittance of the laser-driven spin-polarized current through the gradient interface compared with the abrupt interface. Moreover, it was evident that such transmission deteriorated with the laser fluence owing to the spin accumulation effect.

During laser-induced phase transitions, fast transformations of electronic, atomic, and spin configurations often involve the emergence of hidden and metastable phases. Being inaccessible under any other stimuli, such phases are indispensable for unveiling mechanisms and controlling the transitions. We experimentally explore spin kinetics during the ultrafast first-order 90° spin-reorientation (SR) transition in a canted antiferromagnet Fe_{3}BO_{6}, and reveal that the transition is controlled by canting between the magnetic sublattices. Laser-induced perturbation of the Dzyaloshinskii-Moriya interaction results in a change of the intersublattice canting within the first picoseconds, bringing Fe_{3}BO_{6} to a hidden phase. Once this phase emerges, laser-induced heating activates precessional 90° spin switching. The combination of spin canting and heating controls the final spin configuration comprising coexisting initial and switched phases. The extended phase coexistence range is in striking contrast to the narrow SR transition in Fe_{3}BO_{6} induced by conventional heating.

In this paper, the samples of healthy and cancer tissues of the internal and external sides of a human stomach were studied by time-domain terahertz reflectometry in the frequency range from 0.2 to 1 THz. Since healthy and cancer tissues have different morphological structure, and also contain different amounts of water, their optical properties and spectral characteristics differ. It was found that the refractive index of internal side of stomach cancer tissue is greater than for the healthy one. The refractive index of the external side of stomach cancer tissue is less than for the healthy one. This effect should be taken into account for development of stomach cancer diagnosis.

We propose a modified approach to more detailed study of gyrotropic materials using terahertz (THz) ellipsometry method based on the magneto-optical Kerr effect (MOKE). This approach allows to obtain polarization properties and to calculate the permittivity tensor of materials which are reflective or opaque in THz frequency range. The method allows to measure any values of the diagonal and off-diagonal components of the permittivity tensor and can be used for materials with a strong magneto-optical response.