the diffractive optical elements based on planar high-resistivity silicon gratings are proposed to perform as a quarter waveplate as well as a half waveplate component in the selected band of THz frequencies. As proof of principle quarter waveplates we developed the Si-based quarter waveplate for 0.4 THz frequency demonstrating an operational bandwidth of up to 200 GHz. The finite difference time domain simulations were found in good agreement with THz time domain spectroscopy, frequency domain spectroscopy as well as vector network analyzer experiments measuring transmission amplitude and phase spectra in the range of 0.1-1.0 THz.

Terahertz frequency domain spectroscopy provides a superior resolution in the broad range from 50 GHz to 5 THz. However, temporal frequency drift and the standing wave pattern in the spectrometer, which often act simultaneously, produce intensive coherent noise in the transmittance spectra of the measured samples. The paper presents a spectrum processing method allowing to reduce the coherent noise and thus significantly enhance the accuracy of spectral data analysis. The idea of the method is to remove the small-period (0.25 GHz) oscillations in the standing wave pattern of the measured signal by applying the windowed Fourier filtering. The large-period (4 GHz) oscillations in the measured spectra are broadened and then used to compensate the frequency drift of the two consecutive measurements of the signal with the sample and without it. The proposed approach is tested on the measured spectra of metamaterial and silicon wafer. Its advantage over the classical method based on the averaging of adjacent data points is confirmed. Our work benefits to optimization of the frequency-domain terahertz systems and paves the way for fast and accurate analysis of spectral data.

Propagation of tangential electromagnetic modes in array of split-ring resonators deposited on a thin dielectric plate is analyzed by studying the Fano resonance and electromagnetically induced transparency, which appear in the transmission spectra due to the coupling of these modes with the third plasmonic mode of the split-ring resonators. We determine the spatial dependence of the electric and magnetic field enhancement in these systems, showing a noticeable difference between diffracted modes in free-standing resonators and guided modes in the presence of a dielectric plate. We find that in the case of a subwavelength dielectric thickness, the in-plane modes are guided by both interference and internal reflections of diffracted waves. Numerical simulation is confirmed by analytical modeling and experimental data.

Abstract This work demonstrates the first experimental observation of multiple Fano resonances in the terahertz range in a system based on an array of mirror-symmetric split-ring resonators deposited on low-loss and low-refractive index polytetrafluoroethylene (PTFE) substrate. For the first time, selective surface activation induced by laser technology has been used to deposit a copper layer on a PTFE substrate with the further application of standard mask lithography for metasurface manufacturing.

Parametric generation of oscillations and waves is a paradigm, which is known to be realized in various physical systems. Unique properties of quantum semiconductor superlattices allow us to investigate high-frequency phenomena induced by the Bragg reflections and negative differential velocity of the miniband electrons. Effects of parametric gain in the superlattices at different strengths of dissipation have been earlier discussed in a number of theoretical works, but their experimental demonstrations are so far absent. Here, we report on the first observation of the dissipative parametric generation in a subcritically doped GaAs/AlGaAs superlattice subjected to a dc bias and a microwave pump. We argue that the dissipative parametric mechanism originates from a periodic variation of the negative differential velocity. It enforces excitation of slow electrostatic waves in the superlattice that provide a significant enhancement of the gain coefficient. This work paves the way for a development of a miniature solid-state parametric generator of GHz-THz frequencies operating at room temperature.

This work demonstrates the first experimental observation of multiple Fano resonances in the terahertz range in a system based on an array of mirror-symmetric split-ring resonators deposited on low-loss and low-refractive index polytetrafluoroethylene (PTFE) substrate. For the first time, selective surface activation induced by laser technology has been used to deposit a copper layer on a PTFE substrate with the further application of standard mask lithography for metasurface manufacturing.

A cavityless dissipative parametric gain in quantum GaAs/AlGaAs superlattice is demonstrated and discussed. Using a waveguide-based setup under the biasing of dc and microwave pump, emission lines, corresponding to the simultaneous multiphoton processes in the superlattice, were observed. It was shown that the incident electromagnetic wave experiences transformation into a slow longitudinal electrostatic wave inside the superlattice, ensuring hence the gain levels exceeding 1000 cm(-1).

Metasurfaces are the special case of metamaterials when periodic structures are located in a plane. Here we consider Fano resonances found in them. Special attention is paid to the recently found such type resonance in a mirrored array of split-ring resonators. The resonance appears due to the direct interaction of the lattice and plasmonic modes. Its frequency can be altered by changing the period of the array. The high-quality factor of Fano resonance, around 100, has been evidenced experimentally. Possible applications of the Fano resonance are considered.

Terahertz (THz) imaging and spectroscopy set-ups require fine optical alignment or precise control of spatial mode profile. We demonstrate universal, convenient and easy-to-use imaging—resonant and broadband antenna coupled ultrasensitive titanium-based—dedicated to accurately adjust and control spatial mode profiles without additional focusing optical components of weak power THz sources. Versatile operation of the devices is shown using different kinds of THz—electronic multiplier sources, optical THz mixer-based frequency domain and femtosecond optoelectronic THz time-domain spectrometers as well as optically pumped molecular THz laser. Features of the microbolometers within 0.15–0.6 THz range are exposed and discussed, their ability to detect spatial mode profiles beyond the antennas resonances, up to 2.52 THz, are explored. Polarization-sensitive mode control possibilities are examined in details. The suitability of the resonant antenna-coupled microbolometers to resolve low-absorbing objects at 0.3 THz is revealed via direct, dark field and phase contrast imaging techniques as well.

Room temperature terahertz detection based on the transverse transport of hot electrons in symmetric bow-tie diode was demonstrated and studied in this work. More than an order of magnitude improvement was achieved in detector responsivity and noise equivalent power compared to the previously reported bow-tie diodes of the asymmetric design. High detection efficiency is based on the electric field enhancement in the active part of the symmetric diode, as well as on reducing the device resistance and improved impedance matching between the detector and integrated bow-tie antenna. Hydrodynamic modeling was verified by the ensemble Monte Carlo simulations. Ways to further improve the performance of the symmetric hot-carrier detector are discussed.

Terahertz (THz) homodyne and direct spectroscopic images of low-absorbing materials packaged between up to six layers of a cotton fabric are recorded at 0.3 and 0.6 THz at room temperature. More than two orders of magnitude higher dynamic range is revealed due to the detection in a homodyne scheme, which is realized using paper sheets as a phase-shifting mechanism. It is demonstrated that the homodyne approach can serve as a convenient imaging tool to identify and resolve objects manifesting low absorbance of THz radiation, such as paper tissue, nitrile, and low-density polyethylene concealed in a textile environment. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)

Terahertz (THz) imaging of low absorbing objects is evaluated both in direct and homodyne set-ups at 0.3 THz and 0.6 THz at room temperature. Dynamic range increase by more than two order of magnitude in homodyne detection scheme is shown. Phase shift in homodyne approach is realized with no moving optical elements by using paper sheets. It is shown that the homodyne detection scheme is a very promising for identification of objects exhibiting low absorption of THz radiation, for instance, paper tissue, nitrile and low-density polyethylene concealed in textile.

The scattering of waves by a periodic chain of series resonant LC circuits in a two-dimensional lattice of lumped elements is considered, that imitates the scattering of electromagnetic waves by a periodic lattice of split-ring resonators. Using the translation symmetry of this chain we transformed the scattering problem into the problem of wave propagation along the strip with a single LC circuit. This problem was solved analytically presenting the transmittance as a sum of partial transmittances, corresponding to the frequency mini-bands of the strip. It is shown that the transmittance as a function of the incident wave frequency demonstrates two types of resonances with different resonant frequency dependence on the distance between neighbouring LC circuits.

It is demonstrated that the direct interaction of plasmonic and lattice modes can lead to Fano-type resonance in a mirrored array of simple split ring resonators. The physics behind the effect is overlapping of the frequencies of the lowest lattice mode and the broadband plasmonic mode, which plays the role of a continuum, whereas the lattice mode manifests itself as a discrete state. The overlapping is achieved by mirror symmetric orientation of two adjacent split ring resonators, which increases the lattice period twice. We have revealed that a further increase in the period of the modified array leads to a shift of the Fano resonance to a lower frequency. High quality factor of Fano resonance, around 100, has been evidenced experimentally.

InGaAs-based bow-tie diodes for the terahertz (THz) range are found to be well suited for development of compact THz imaging systems. To further optimize design for sensitive and broadband THz detection, one of the major challenges remains: to understand the noise origin, influence of growth conditions and role of defects for device operation. We present a detailed study of photoreflectance, low-frequency noise characteristics and THz sensitivity of InGaAs bow-tie diodes. The diodes are fabricated from InGaAs wafers grown by molecular beam epitaxy (MBE) on semi-insulating InP substrate under different technological conditions. Photoreflectance spectra indicated the presence of strong built-in electric fields reaching up to 49 kV/cm. It was demonstrated that the spectral density of voltage fluctuations at room temperature was found to be proportional to 1/f, while at lower temperatures, 77⁻200 K, Lorentzian-type spectra dominate due to random telegraph signals caused by individual capture defects. Furthermore, varying bias voltage, we considered optimal conditions for device room temperature operation in the THz range with respect to signal-to-noise ratio. The THz detectors grown with beam equivalent pressure In/Ga ratio equal to 2.04 exhibit the minimal level of the low-frequency noise, while InGaAs layers grown with beam equivalent pressure In/Ga ratio equal to 2.06 are found to be well suited for fabrication of room temperature bow-tie THz detectors enabling sensitivity of 13 V/W and noise equivalent power (NEP) of 200 pW/√Hz at 0.6 THz due to strong built-in electric field effects.

Convenience in use of room-temperature terahertz (THz) imaging systems, reduction of their dimensions and presence of on-chip solutions remains one of prime interests for direct implementation aims. Solid-state-based solutions in miniaturization of spectroscopic THz imaging systems including novel semiconductor nanostructures bias-free emitters, diffractive THz optics components and their on-chip integration with THz detectors are discussed. In particular, pulsed optoelectronic terahertz emitter based on a δ-doped p-i-n-i GaAs/AlxGa1−xAs heterostructure was studied and it is demonstrated that the heterostructure can serve as efficient antenna- and bias-free surface emitter. Diffractive optics elements – Fresnel zone plates –with integrated band-pass filters were simulated employing Finite-difference time domain method. Structures were fabricated using the laser direct writing and investigated using electronic THz sources and an optically pumped terahertz laser. Advantages of on-chip integration of diffractive optics and bow-tie-shaped InGaAs-based terahertz detectors are revealed via detection enhancement. Bow-tie diodes properties in frequency scale and detection sensitivity are considered and compared for different materials. Homodyne detection and imaging of low-absorbing objects at 0.6 THz are demonstrated and discussed.

Finite-difference time-domain (FDTD) method is frequently used for calculation of transmission and reflection characteristics of periodic structures. We propose a hybrid FDTD technique for evaluation of the transmittance and reflectance spectra of periodic structures that significantly reduces the usage of computer memory and time of the calculation. In the considered case, calculation time and memory occupation were reduced up to 30% and 20%, respectively, compared to the standard FDTD procedure. Calculated results fit well in the experimentally measured data.

It is demonstrated that higher-order plasmonic modes in the split-ring resonators (SRRs) are strongly enhanced when SRRs are arranged in a densely spaced two-dimensional array. The mode enhancement results from the near-field electrical coupling between the adjacent resonators. The effect is most pronounced in narrow gap SRRs which allows to observe experimentally plasmon modes up to the seventh order. In the array of narrow gap SRRs, the fifth-order resonance demonstrates high Q-factor, high resonance strength and wide tunability which opens up attractive features for practical applications of planar SRR structures.

On-chip integration of secondary diffractive optics and bow-tie-shaped InGaAs-based terahertz detectors is presented. Zone plates were produced directly on the bottom surface of a 500 μm-thick semi-insulating InP substrate employing direct laser write technique. Integration of the bow-tie detector and the zone plate allows to enhance detection more than one order of magnitude at 0.76 THz. Good correlation between experimental data and 3D finite-difference time-domain simulation results is found. It is confirmed that observed detection enhancement is caused mainly by the focusing performance of the zone plate.

A terahertz (THz) imaging system based on narrow band microbolometer sensors (NBMS) and a novel diffractive lens was developed for spectroscopic microscopy applications. The frequency response characteristics of the THz antenna-coupled NBMS were determined employing Fourier transform spectroscopy. The NBMS was found to be a very sensitive frequency selective sensor which was used to develop a compact all-electronic system for multispectral THz measurements. This system was successfully applied for principal components analysis of optically opaque packed samples. A thin diffractive lens with a numerical aperture of 0.62 was proposed for the reduction of system dimensions. The THz imaging system enhanced with novel optics was used to image for the first time non-neoplastic and neoplastic human colon tissues with close to wavelength-limited spatial resolution at 584 GHz frequency. The results demonstrated the new potential of compact RT THz imaging systems in the fields of spectroscopic analysis of materials and medical diagnostics.