An effective experiment scheme is proposed to generate the terahertz (THz) perfect optical vortex (POV) beams by diffractive elements at the frequency of 0.1THz. Two diffractive elements are designed and fabricated by 3D-printing to form the generation system. The ring radius of the generated beams is independent of the topological charge and positive linear relationship with the radial wave vector. By controlling the radial wave vector, the ring radius can be freely adjusted. The experiment results are shown to corroborate the numerical simulation ones. Such generated beams hold promise for developing the novel THz fiber communication systems.

Generating terahertz perfect optical vortex beams by diffractive elements.

Terahertz (THz) radiation has recently received a lot of interests in science and technology. The potential of orbital angular momentum (OAM) of light in the THz region is a novel idea for researchers with increasing applications in broadband communications. THz waveguides have rarely been reported to generate and guide OAM modes. In this paper, we propose for the first time hollow-core optical fibers consist of circular air-hole arrays for guiding THz OAM modes with the topological charge order up to l = 3. Using numerical simulations, two low-loss transmission windows over 0.47 – 0.65 THz (window1) and 0.95 – 1.20 THz (window2) are recognized to guide OAM modes. Results show that both windows of the THz fiber exhibit the flat near-zero dispersion as well as low confinement loss (∼ 10-3 dB/cm) with simultaneously high purity of the OAM modes (higher than 90%). Finally, two designs of THz optical fibers consist of one and two rings of air-hole arrays in their cladding are examined and it is shown that difference of the confinement loss and the modal purity between two designs are negligible. This implies that hollow-core optical fibers consist of one ring of air-hole array in their cladding are very promising for efficient transmission of OAM modes in the THz region.

Terahertz Hollow-Core Optical Fibers for Efficient Transmission of Orbital Angular Momentum Modes

We present a holographic detection module to measure the spatially resolved distribution of pulsed terahertz field in a single scan by a motorized translation stage, responsible for the time delay. All mounts of the optical elements of the module are easily reproduced by 3D printing and attached to the optical cage system. The latter greatly simplifies the measurement procedure, allowing the experimenter to move and adjust the detection system as a single device. The developed mounts are made universal and can be used in other setups. We have made 3D models available as open-source hardware. The module is based on an electro-optical detection scheme with wide-aperture ZnTe crystal, crossed polarizers, and a matrix photodetector. The validation of its operability was performed with two experiments to measure the spatial distribution of the unperturbed field from the generator and the vortex field formed by the spiral phase plate. Optical vortices with multiple topological charges of 2–4 were detected on spectral components in the range from 0.3 to 1.1 THz. In addition, we have detailed the alignment process of terahertz imaging systems. 

Open-source 3D-printed terahertz pulse time-domain holographic detection module

A sub-terahertz vortex dielectric waveguide was designed and fabricated in the cyclic olefin copolymer (TOPAS) compound. The annular index profile was engineered using a holey cladding to support operation from approximately 200–300 GHz. The vortex waveguide was tested at 280 GHz using an OAM-endowed Laguerre–Gaussian mode generated by a stepped spiral phase plate.

Design, fabrication, and demonstration of a dielectric vortex waveguide in the sub-terahertz region.

It has been experimentally shown that a rigid polyimide Kapton film of submillimeter thickness is sufficiently transparent in the frequency range of 1.2–3 THz and can be used for splitting surface plasmon polaritons (SPPs) propagating along a flat conducting surface. The transmittance T and reflectance R measured for SPPs excited by a free-electron laser radiation (λ=130µm) in “gold–ZnS layer–air” flat structures are in good agreement with the results of numerical simulations obtained using the rigorous coupled-wave analysis technique. Transmittance of the SPPs decreases slightly when the layer gets thicker. To minimize the SPP radiative losses emerging due to diffraction of the SPPs on the impedance discontinuity, the Kapton film should be placed normal to the guiding surface and brought in good contact with it (the gap must be less than λ/2). The performed experiments clearly demonstrated Kapton films to be very promising for SPP beam splitting in large-scale terahertz SPP devices.

Splitting a terahertz surface plasmon polariton beam using Kapton film

A holey cladding dielectric waveguide with square cross section is designed, simulated, fabricated and characterized. The TOPAS waveguide is designed to be single mode across the broad frequency range of 180 GHz to 360 GHz as shown by finite-difference time domain simulation and to robustly support simultaneous TE and TM mode propagation. The square fiber geometry is realized by pulling through a heat distribution made square by appropriate furnace design. The transmitted mode profile is imaged using a vector network analyzer with a pinhole at the receiver module. Good agreement between the measured mode distribution and the calculated mode distribution is demonstrated.

Square dielectric THz waveguides.

This paper presents the functional performance of a square holey dielectric waveguide that operates from 180 to 360 GHz for supporting high data rate communication systems. A 7 cm waveguide is excited using a microstrip patch antenna at 264 GHz and shows 30 dB improvement from the system noise floor and 12 dB improvement from a free space transmission in an uncalibrated measurement. This waveguide minimizes cross talk and allows for polarization division multiplexing supporting vertical and horizontal polarizations.

Functional Performance of a Millimeter Wave Square Holey Dielectric Waveguide

This paper shows an application of hybrid combination of agile Millimeter-wave and Optical waveguiding solutions with advanced CMOS, BiCMOS, SOI technologies will enable new possibilities for Chip-to-Chip communications with high data-rate and improved broadband RF performances. Such hybridization will benefit to wide range of applications relative to mobile communications with smart devices and systems: e.g., practical applications relative to autonomous vehicles including machine learning and cognitive signal processing. Requirements for unified RF, mm-Waves and Optical sensing are drawn toward optical MIMO and Phased-Array systems implementing 3D Chip-Package-PCB-Antenna Co-Integration technologies combined with smart data fusion with environmental awareness.

Nafiseh Aflakian

Papers

Square dielectric THz waveguides.

Design, fabrication, and demonstration of a dielectric vortex waveguide in the sub-terahertz region.

Functional Performance of a Millimeter Wave Square Holey Dielectric Waveguide

Photonics Chip-to-Chip Communication for Emerging Technologies: Requirements for Unified RF, Millimeter-Waves and Optical Sensing

Square holey cladding dielectric THz waveguides for chip-to-chip communication