We demonstrate the concept of fiber-based terahertz hook. The subwavelength photonic hook is obtained in the vicinity of a shaped fiber tip with asymmetric radiation. A 193 THz continuous-wave source, commonly used for medical imaging, has been required. Photonic hook with a lateral feature size less than the half-wavelength is achieved using a hemispherical shaped fiber tip with metallic mask. This breakthrough is carried out in ambient air by using a 2.58λ - diameter fiber with a shaped tip (λ is a radiation wavelength). A good correlation is observed between the computed intensity distribution of photonic hook and the tip sizes. Photonic hook generated with a shaped fiber tip, easier to manipulate, shows far-reaching benefits for potential applications such as ophthalmic laser surgery, super-resolution microscopy, photolithography, and material processing.
The mechanism of formation of a terahertz jet by a dielectric cuboid and a sphere surrounded by ideally conducting screens is considered. The maximum screen influence is observed when the screen is located near the alight cuboid base. The screen influence eases when the screen is shifting along the dielectric object. Power flux density localization area is almost completely shifted inside the object when the screen is situated in the center. The minimum influence is observed when screen placed in the shadow plane of the cuboid base. This effect caused by the screen influence on electric field component tangential to the side edges and thus on the power flux directed to the central axis of the object. The screen influence on the terahertz jet in spherical object has been compared. Value of the power flux density after passing the object is higher in a cuboid, but the focusing characteristics are better (appearing on shorter distances) in a sphere.
The aim of this investigation is to search for the possibility of terahertz (THz) spectroscopy in combination with dielectrophoresis for studying erythrocytes from patients with diffuse liver diseases for diagnostics and differentiation of liver fibrosis degrees. Seventy-nine men aged 33 to 67 years with diffuse liver pathology, mainly alcohol, viral, and mixed genesis with varying degrees of liver fibrosis, were included in the study with 30 men (31- to 64-year-old) without signs of pathology of internal organs and liver fibrosis (F0, first group). The study of suspensions of red blood cells was carried out by THz spectroscopy and dielectrophoresis. An increase in the degree of liver fibrosis was associated with an increase in the number of deformed cells prone to aggregation and destruction, with a reduced surface charge, thickened membranes with high electrical conductivity, low deformability on the background of high summarized viscosity, and rigidity indicators (p < 0.0001 to 0.05). Strong correlations of THz spectroscopy indices with electrical and viscoelastic parameters of red blood cells were obtained. The revealed possibilities of the study of blood and its cell components are very promising in the diagnosis and differentiation of the degrees of liver fibrosis.
A simple millimeter wave and terahertz (THz) receiver scheme that uses subwavelength focusing of electromagnetic beam on the point-contact detector area with waveguide dimensions is studied. A detection system with such an optical coupling scheme is implemented, where the signal to be detected is coupled to a detector through a mesoscale dielectric particle lens based on terajet effect. We have experimentally demonstrated an enhancement of the point-contact detector sensitivity higher than 6 dB and with 1.5 times decreasing of the noise equivalent power value. The results show that the proposed method could be applied to reduce the size and increase the sensitivity of various THz systems, including imaging, which would enable significant progress in different fields such as physics, medicine, biology, astronomy, etc.
During last several years it was shown, that an electromagnetic field can be made to curve after propagation through a simple dielectric mesoscale Janus particle of special shape, which adds a newfound degree of simplicity. This effect was discovered by I.V.Minin and O.V.Minin and termed ‘photonic hooks’– it is an unique electromagnetic self-bending subwavelength structured light beams configuration behind a mesoscale particle with a broken symmetry and differ from Airy-family beams. PH features the radius of curvature, which is about 2 times smaller than the electromagnetic wavelength - this is the smallest curvature radius of electromagnetic waves ever reported. The nature of a photonic hook is in dispersion of the phase velocity of the waves inside of particle, resulting in interference. Here, we report an experimental verification of the photonic hook effect in terahertz waveband.
The diffraction limit of electromagnetic waves restricts the formation of sub-wavelength spots. The feasibility to generate scattered beams of light with a high-intensity main lobe, a weak sub-diffracting waist, and a very low divergence angle, named Photonic nanojets, was demonstrated traditionally with spherical particles. Various practical applications require the creation of different types of photonic jets or electromagnetic streams with specific characteristics and properties. For instance, photonic jets can be applied to ease the coupling into the optical waveguides. In this case, photonic jets play the role of a coupling element similar to the lens, grating coupler or prism. To address this challenge, we study the Fresnel Zone Plate (FZP) of rings-like shape. We show that the Babinet principle can be applied for studying the complementary diffractive structures for the formation of near-field photonic jets on a facet of the optical waveguide. Using COMSOL Multiphysics, we built a model of the Fresnel Zone Plate structure based on rings and demonstrate the applicability of Babinet’s principle for the formation of photonic jets in the near-infrared.
The spectral and amplitude-frequency characteristics of a new pyroelectric detector based on thin tetraaminodiphenyl polycyclic polymer films with a thickness of <1 μm were studied in the electromagnetic radiation ranges of 0.4 to 10 and 300 to 3000 μm and at local wavelengths of 81 and 100 μm, respectively. It is shown that the volt–watt sensitivity of such a detector in the entire range is practically nonselective and is 2 to 10 times higher than the sensitivity of other pyroelectric detectors and the Golay cell. The bandwidth of the proposed pyrodetector was 330 to 500 Hz. The results showed good prospects of these sensors for fast ultrawideband spectroscopy, covering visible, infrared, terahertz, and millimeter wave ranges.
We introduce a novel phase-only diffractive optical element called chiral binary square axicon (CBSA). The CBSA is designed by linearly rotating the square half-period zones of the binary square axicon with respect to one another. A quadratic phase mask (QPM) is combined with the CBSA using modulo-2π phase addition technique to bring the far-field intensity pattern of CBSA at the focal plane of the QPM and to introduce quasiachromatic effects. The periodically rotated zones of CBSA produce a whirlpool phase profile and twisted intensity patterns at the focal plane of QPM. The degree of twisting seen in the intensity patterns is dependent upon the angular step size of rotation of the zones. The intensity pattern was found to rotate around the optical axis along the direction of propagation. The phase patterns of CBSA with different angles of zone rotation are displayed on a phase-only spatial light modulator, and the experimental results were found to match with the simulation results. To evaluate the optical trapping capabilities of CBSA, an optical trapping experiment was carried out and the optical fields generated by CBSA were used for trapping and rotating yeast cells.
In this paper, we present the step-index sapphire fiber, applied as a THz probe. The low THz attenuation of sapphire makes it attractive for fabrication of THz optical components. Moreover, it has a high refractive index in THz range, which guarantees a strong modal confinement in a fiber core. The advantages of the edge-defined film-fed growth (EFG) technique allow for fabrication of fibers with close-to-cylindrical shape, the length of 1 m and longer, and the subwavelength diameter of 150 − 400 μm. In order to improve the coupling efficiency, the fiber has polished flat ends. We apply the fabricated 300-μm-diameter sapphire fiber for the THz near-field scanning-probe microscopy. The spatial resolution of our experimental setup is defined by the fiber diameter, thus, it reaches ~ λ/4 for the radiation wavelength λ = 1200 μm. The obtained images of the test objects demonstrate the advanced resolution, which is close to the theoretical limit and beyond the Abbe diffraction limit.
Based on the developed electro-optical cell detection system using dielectrophoresis, both the electrical and viscoelastic behavior of erythrocytes was studied. Studies were conducted to assess the possibility of using hemorheological parameters as precursors of stroke recurrence. As a result, the main hemorheological parameters of erythrocytes, such as electrical and viscoelastic properties, hemostasis indices, were determined. It is shown that these parameters can serve as harbingers of a relapse of a stroke.
We demonstrate that to form a high-intensity THz terajet (field localization) within the graphene monolayer placed at the shadow surface of one wavelength-dimensions dielectric particle with refractive index near 2 the cubic shape is preferable in comparison with spherical one. A spherical particle is more magnetic than electric due to multipole mode distributions. A graphene monolayer was applied for a picosecond timescale modulation of THz wave by controlling of IR radiation for integrated ultrafast all-optical THz modulator. Application of biological objects to the shadow surface of the mesoparticle instead of graphene will allow studying the biophysical aspects of the interaction of the electromagnetic field with such biological objects.
We have developed a method of the terahertz (THz) solid immersion microscopy for the reflection-mode imaging of soft biological tissues. It relies on the use of the solid immersion lens (SIL), which employs the electromagnetic wave focusing into the evanescent-field volume (i.e. at a small distance behind the medium possessing high refractive index) and yields reduction in the dimensions of the THz beam caustic. We have assembled an experimental setup using a backward-wave oscillator, as a source of the continuous-wave THz radiation featuring λ= 500 μm, a Golay cell, as a detector of the THz wave intensity, and a THz SIL comprised of a wide-aperture aspherical singlet, a truncated sphere and a thin scanning windows. The truncated sphere and the scanning window are made of high-resistivity float-zone silicon and form a unitary optical element mounted in front of the object plane for the resolution enhancement. The truncated sphere is rigidly fixed, while the scanning window moves in lateral directions, allowing for handling and visualizing the soft tissues. We have applied the experimental setup for imaging of a razor blade to demonstrate the advanced 0:2λ resolution of the proposed imaging arrangement. Finally, we have performed imaging of sub-wavelength-scale tissue spheroids to highlight potential of the THz solid immersion microscopy in biology and medicine.
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