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We propose a mathematical model for holographic relief reconstruction of phase objects based on the analysis of diffraction patterns of pulsed terahertz radiation, recorded in a dispersive medium. Numerical simulation shows the influence of the dispersion medium on the process of phase and relief reconstruction: disregard of the dispersion due to the refractive index of the medium where terahertz radiation propagates causes an error in the reconstructed image. The normalized standard deviation amounts to 0.3 when the wavefront propagation distance is 10 mm, increasing to 0.45 at a distance of 70 mm.
Optical characteristics of polymerization products under variable degrees of neutralization of acrylic acid (AA) and cross-linking agent and at a fixed content of redox components have been studied by IR spectroscopy of frustrated total internal reflection and THz spectroscopy. The results of the analysis show that absorption of IR radiation and dispersion of THz radiation for the samples under study depend on the degree of AA neutralization, which predetermines the mechanism of structure formation of hydrogels, refractive index, and degree of their limited swelling. It is found that the dependences of the refractive index of dry compact hydrogels and the degree of limited swelling of ground polymer samples on the content of neutralizing agent in solutions of the reaction systems have a symbatic character.
We present the study on spatial distribution of the maximum of terahertz field amplitude in time domain when generated by a femtosecond filament. It is shown that as a result of the propagation of the terahertz field forms a spherical wave front, on the edge of which the maximum of amplitude has a temporary delay in contrary to its central part.
Orbital angular momentum (OAM) represents new informational degree of freedom for data encoding and multiplexing in fiber and free-space communications. OAM-carrying beams (also called vortex beams) were successfully used to increase the capacity of optical, millimetre-wave and radio frequency communication systems. However, the investigation of the OAM potential for the new generation high-speed terahertz communications is also of interest due to the unlimited demand of higher capacity in telecommunications. Here we present a simulation-based study of the propagating in non-dispersive medium broadband terahertz vortex beams generated by a spiral phase plate (SPP). The algorithm based on scalar diffraction theory was used to obtain the spatial amplitude and phase distributions of the vortex beam in the frequency range from 0.1 to 3 THz at the distances 20-80 mm from the SPP. The simulation results show that the amplitude and phase distributions without unwanted modulation are presented in the wavelengths ranges with centres on the wavelengths which are multiple to the SPP optical thickness. This fact may allow to create the high-capacity near-field communication link which combines OAM and wavelength-division multiplexing.
In this paper, we discuss the features of numerical simulation of propagation of the two-dimensional pulsed broadband terahertz (THz) fields. A set of separate spectral components used to describe diffraction of broadband THz pulses. To eliminate sampling theorem restriction in the numerical simulation of diffraction process for the complete spectral range, we use the representation of two-dimension fields as angular spectrum for the high-frequency spectral components and the convolution of initial fields with the impulse response for the low-frequency components. The peculiarities of diffraction of pulsed broadband terahertz radiation are manifested in the form of the dips in its spectrum, which is obtained at different points of the screen. The application of the given model for THz pulsed time-domain holography is demonstrated by the reconstruction of the image of the letter "K".
The unique properties of narrowband and broadband terahertz Bessel beams have led to a number of their applications in different fields, for example, for the depth of focusing and resolution enhancement in terahertz imaging. However, broadband terahertz Bessel beams can probably be also used for the diffraction minimization in the short-range broadband terahertz communications. For this purpose, the study of spatial-temporal dynamics of the broadband terahertz Bessel beams is needed. Here we present a simulation-based study of the propagating in non-dispersive medium broadband Bessel beams generated by a conical axicon lens. The algorithm based on scalar diffraction theory was used to obtain the spatial amplitude and phase distributions of the Bessel beam in the frequency range from 0.1 to 3 THz at the distances 10-200 mm from the axicon. Bessel beam field is studied for the different spectral components of the initial pulse. The simulation results show that for the given parameters of the axicon lens one can obtain the Gauss-Bessel beam generation in the spectral range from 0.1 to 3 THz. The length of non-diffraction propagation for a different spectral components was measured, and it was shown that for all spectral components of the initial pulse this length is about 130 mm.
We numerically investigated the influence of wavelength of terahertz radiation and size of scanning pinhole on the quality of reconstructed image by terahertz pulse time domain holography method. The improvement of the quality is achieved mainly by reducing the wavelength of the radiation, pinhole size and pinhole step (value of the pinhole displacement at each step) affect the spatial resolution of the reconstructed object and significantly affect the scanning time. However, pinhole does not significantly affect the quality only if pinhole size is not less than used wavelength.