Focused Terahertz Radiation Research Articles

AI-Powered Terahertz VLSI Testing Technology for Ensuring Hardware Security and Reliability

Hardware cybersecurity has become a key issue, especially for very large integrated circuits. If counterfeit, forged, or defective ICs present a significant threat to system reliability and security. The growing complexity of digital and mixed-signal systems makes it increasingly challenging yet vital to develop robust methods to assess and confirm the reliability and authenticity of ICs. We introduce a new terahertz testing method for non-destructive and unobtrusive identification of counterfeit, damaged, forged or defective ICs by measuring their response to incident terahertz and sub-terahertz radiation at the circuit pins and analyzing the response using artificial intelligence (AI). These responses create unique signatures for ICs. We generated 2D images by measuring the response on a selected pin of a radio frequency IC (RFIC) scanned by a focused terahertz radiation. By applying the data augmentation processes, we created a secure image data set to train the convolutional neural network (CNN) model. An unsecured image data set representing altered or damaged ICs was generated by modifying the original image data. The trained models identified secure devices with a ~94% accuracy.

ФОКУСИРОВКА ТЕРАГЕРЦОВОГО ИЗЛУЧЕНИЯ МЕЗОРАЗМЕРНОЙ КУБОИДНОЙ ЛИНЗОЙ ИЗ ИСКУССТВЕННОГО ДИЭЛЕКТРИКА

The aim of this work is to study a new dimensionless cuboid lens made of artificial dielectric for the terahertz wavelength range. The research method is mathematical modeling of the process. It is es-tablished that a lens made of anisotropic artificial dielectric along the optical axis with a characteristic size of 3λ is able to focus terahertz radiation at the calculated frequency of 0.15 THz into a focal spot with a transverse size equal to the diffraction limit. The focus area is located along the optical axis of the lens. The ability to focus radiation remains in a wide range of effective refractive index, at least from 1.4 to 1.7, while the value of the focal length does not change significantly.

Study of thin, achromatic diffractive structures to focus terahertz radiation on a detector

Thin and lightweight achromatic focusing elements with F-number close to 1 are desirable in many practical applications. We present the idea to use diffractive structures designed to work for the substantially increased THz frequency range. The paper analyses mono- and multi-focal lenses forming point-like foci as well as axicon and light sword optical elements focusing THz radiation into line segments located along the optical axis. We consider diffractive elements in a form of the first and the second order kinoforms having various thicknesses. Designed and fabricated elements were numerically and experimentally examined to verify their achromatic functioning. We present point spread functions (XY scans) and 2D energy maps (XZ scans) for different THz frequencies. Moreover, a diagram of chromatic aberration is created by registering energy distribution along the optical axis for different frequencies. The distance corresponding to the highest energy is chosen for each frequency. Therefore, we can compare broadband working of designed structures. The spherical lens coded as kinoform of the second order provides the best broadband functioning, however it is two times thicker than structures providing extended depth of focus (light sword and axicon) working with slightly smaller efficiency but being much thinner.

Formation of a Nanoscale Graphite Hydrosol under the Influence of Terahertz Laser Radiation

The present report describes a new phenomenon, namely the formation of a nanoscale hydrosol during short-term exposure of the surface of water by focused terahertz radiation of the Novosibirsk free-electron laser. The composition of the particles is completely determined by the material of the container used. Hydrosols are technologically convenient forms for scientific and industrial applications, which are characterized by uniform deposition and high catalytic activity used in medicine, optics, and as a suspension for sample preparation for chemical and elemental analysis. This article presents the results of our study of graphite particles formed in water, a suspension, which is in demand for industrial purposes. Commercially available high purity graphite (99.5%) of GE grade is used. The initial material and the resulting particles are studied using an atomic force and electron microscope with an elemental analyzer.

Dielectric Resonator Reflectarray as High-Efficiency Nonuniform Terahertz Metasurface

Advances in terahertz technology rely on the combination of novel materials and designs. As new devices are demonstrated to address the terahertz gap, the ability to perform high-efficiency beam control will be integral to making terahertz radiation a practical technology. Here, we use a metasurface composed of nonuniform dielectric resonator antennas on a ground plane to achieve efficient beam focusing at 1 THz. The dielectric resonators are made of high-resistivity silicon, which is a low-loss, nondispersive material for terahertz waves. The resonators operate around the resonance of the displacement current in the silicon, which is crucial to attaining high efficiency. The reflectarray’s capacity to focus terahertz radiation is experimentally verified, and hence by the principle of antenna reciprocity, it can also be employed as a terahertz collimator. The demonstrated device can therefore be deployed for high-gain terahertz antennas. Further measurements show that the loss of the reflectarray is negli...

Formation of Nanosized Metal Hydrosols under the Influence of Novosibirsk Terahertz free Electron Laser Radiation

The present report describes a new phenomenon - short-term exposure of water with focused terahertz radiation of free electron laser leads to the formation of nanoparticle suspension (hydrosol). Composition of the particles formed corresponds to the material of metal containers used in the experiment. Fractional and elemental composition of the particles was determined by atomic force (AFM), scanning electron microscopy with elemental analyzer (SEM EDAX), and the results of mass spectrometry with inductively coupled plasma (ICP-MS). Hydrosols are technologically convenient form, suitable for scientific and technological applications which require uniform deposition and high catalytic activity, they are useful in medicine, as well as the alternate method of sample preparation for chemical and elemental analysis.

Terahertz generation by mixing of two super-Gaussian laser beams in collisional plasma

Considering a realistic situation, where electron-neutral collisions persist in plasma, analytical calculations are carried out for the Terahertz (THz) radiation generation by beating of two Super-Gaussian (SG) lasers of index p. The competency of these lasers over Gaussian lasers is discussed in detail with respect to the effects of collision and beam width on the THz field amplitude and efficiency of the mechanism. A critical transverse distance of the peak of the THz field is defined that shows a dependence on the index of SG lasers. Although electron-neutral collisions and larger beam width lead to the drastic reduction in the THz field when the SG lasers are used in the plasma, the efficiency of the mechanism remains much larger than the case of Gaussian lasers. Moreover, the higher index SG lasers produce stronger and focused THz radiation.

Investigation of wide-aperture plasmonic detectors by a tightly focused terahertz beam

Terahertz response of wide-aperture plasmonic detectors is studied experimentally by using a focused terahertz radiation with frequencies 1.63, 1.89 and 2.55 THz. Two different types of plasmonic detector have been investigated: (i) field-effect transistor (FET) with a grating-gate of large (2×2 mm2) area and (ii) FET with a single gate and ohmic contacts forming a bow-tie antenna. By raster scanning the detector area by using a tightly focused terahertz beam, we studied the contribution of individual parts of the detector to the total detection response and we determined the effective area of the detector structures.

Breakdown-prone volume in terahertz wave beams

This study was motivated by the recently proposed concept of remote detection of concealed radioactive materials by a focused terahertz (THz) radiation [V. L. Granatstein and G. S. Nusinovich, J. Appl. Phys. 108, 063304 (2010)]. According to this concept, a high-power THz radiation should be focused in a small spot where the field intensity exceeds the breakdown threshold. In the presence of free electrons in such a breakdown-prone volume, a THz discharge will occur there. However, this volume should be so small that in the absence of ionizing sources in its vicinity the probability to have there any free electrons is low. Then, the increased breakdown rate in a series of THz pulses would indicate the presence of hidden radioactive materials in the vicinity of the focused spot. For this concept, it is important to accurately determine the breakdown-prone volume created by a focused THz radiation. This problem is analyzed in this paper, first, for the case of a single wave beam and, then, for the case of crossing wave beams of different polarizations. The problem is studied first ignoring the diffraction spread of wave beams in the vicinity of the focal plane and, then, with the account for the diffraction spreading. Then, relations between the THz wave power, the range of such a system and the breakdown-prone volume are analyzed. Finally, the effect of the atmospheric turbulence on propagation and focusing of THz wave beams in air is considered.

Features of the scattering of focused terahertz radiation from the probe of a terahertz near-field microscope

The scattering of pulsed terahertz radiation from metallic probes in the form of thin cylinders and cones with a small opening angle, which are used in apertureless terahertz near-field microscopes, has been investigated. The extrema of the waveform of pulsed terahertz radiation scattered from a free probe are linearly shifted with a change in the vertical position of the probe, and the spectral distribution is characterized by an inversely proportional frequency dependence. In the presence of a reflecting surface under the probe, when new excitation and detection directions appear, the spectrum of scattered terahertz radiation does not differ from the spectrum of the incident radiation. The experimental data are in mutual agreement with the theoretical results obtained within the model of the generation of diffraction edge waves at the interface of inhomogeneous excitation between the excitation region and shadow region.

Coupling into tapered metal parallel plate waveguides using a focused terahertz beam

Coupling of focused terahertz radiation into tapered metal parallel plate waveguides with sub-wavelength gap widths is presented. A line-focus terahertz time-domain spectroscopy setup with flexible foci is used to investigate coupling of broadband terahertz radiation from free-space into waveguides for different gap widths. Various waveguide lengths are compared in experiment and calculation in terms of occurring loss and divergence phenomena. Amplitude coupling ratios of 80% from free-space into the waveguides are obtained.

Focused terahertz radiation formed by coherently scattered surface plasmon polaritons from partially uncorrugated metal surfaces

We present that a focus of terahertz radiation can be tailored based on coherent scattering of surface plasmon polaritons (SPPs) from a partially defected metal corrugation based on numerical simulations. The introduction of teeth defects in the corrugation allows coupling of the guided SPPs with the radiation and the far-field behavior is tailored by the spatial arrangement of such defects. The proposed structures serve as a kind of planar lenses which are quite thin and inexpensive. Promising applications include interfacing lens antennas between terahertz plasmonic integrated circuits and the external free space, which make terahertz systems very compact and low-cost.

Metamaterial-based gradient index lens with strong focusing in the THz frequency range

The development of innovative terahertz (THz) imaging systems has recently moved in the focus of scientific efforts due to the ability to screen substances through textiles or plastics. The invention of THz imaging systems with high spatial resolution is of increasing interest for applications in the realms of quality control, spectroscopy in dusty environment and security inspections. To realize compact THz imaging systems with high spatial resolution it is necessary to develop lenses of minimized thickness that still allow one to focus THz radiation to small spot diameters with low optical aberrations. In addition, it would be desirable if the lenses offered adaptive control of their optical properties to optimize the performance of the imaging systems in the context of different applications. Here we present the design, fabrication and the measurement of the optical properties of spectrally broadband metamaterial-based gradient index (GRIN) lenses that allow one to focus THz radiation to a spot diameter of approximately one wavelength. Due to the subwavelength thickness and the high focusing strength the presented GRIN lenses are an important step towards compact THz imaging systems with high spatial resolution. Furthermore, the results open the path to a new class of adaptive THz optics by extension of the concept to tunable metamaterials.