Geometric phase shaping of terahertz vortex beams

Generation of terahertz spatiotemporal optical vortices with frequency-dependent orbital angular momentum

Optical vortices with longitudinal orbital angular momentum (OAM) as a powerful tool for particle control, imaging and communication have been greatly developed. We introduce a novel property of broadband terahertz (THz) pulse, represented by frequency-dependent OAM orientation in spatiotemporal domain with transverse and longitudinal OAM projection. We illustrate a frequency-dependent broadband THz spatiotemporal optical vortex (STOV) in plasma-based THz emission driven by cylindrical symmetry broken two-color vortex field. We detect the evolution of OAM by time-delayed 2D electro-optic sampling combined with Fourier transform. This THz optical vortices tunability in spatiotemporal domain expands a new way for studying STOV and plasma-based THz radiation.

THz Multi-Mode Q-Plate with a Fixed Rate of Change of the Optical Axis Using Form Birefringence

We report the design, fabrication and experimental validation of a THz all-dielectric multi-mode q-plate having a fixed rate of change of the optical axis. The device consists of space-variant birefringent slabs manufactured by 3D printing using melt-extruded Acrylonitrile Butadiene Styrene (ABS). The desired form birefringence is analytically evaluated and experimentally measured by the THz time domain spectroscopy technique. The manufactured q-plate design is characterized using a polarization-sensitive imaging setup. The full electric field spatial maps are acquired from the beam propagating through the q-plate. The device enables the realization of both radial and azimuthal vector beams at discrete frequency intervals by controlling the space-dependent orientation of the ordinary and extraordinary axes in the transverse plane with a multi-mode sequence.

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

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.

3D Printed Sub-Terahertz All-Dielectric Lens for Arbitrary Manipulation of Quasi-Nondiffractive Orbital Angular Momentum Waves

Terahertz (THz) vortex waves carrying orbital angular momentum (OAM) hold great potential in dealing with the capacity crunch in wireless high-speed communication systems. Nevertheless, it is quite a challenge for the widespread applications of OAM in the THz regime due to the beam divergence and stringent alignment requirement. To address this issue, an all-dielectric lens (ADL) is proposed for the arbitrary manipulation of quasi-nondiffractive THz OAM waves (QTOWs). On the basis of the concept of the optical conical lens and the multivorticity metasurface, the beam number, the topological charge (TC), and the deflection angle as well as the nondiffractive depth of the generated THz OAM waves are controllable. For proof-of-concept, two ADLs are 3D printed to create single and dual deflected QTOWs, respectively. Remarkably, measured by a THz imaging camera, the desired QTOWs with high mode purity are observed in predesigned directions with a nondiffractive depth predefined theoretically. The proposed designs and experiments, for the first time, verified that the QTOWs could be achieved with a nondiffractive range of 55.58λ_g (λ_g = wavelength at 140 GHz) and large deflection angles of 30° and 45°.

Fabricating THz spiral zone plate by high throughput femtosecond laser air filament direct writing

The sixth-generation wireless communication will exploit the radio band with frequencies higher than 90 GHz, reaching terahertz (THz) band, to achieve huge signal bandwidths. However, the cost-effective fabrication methods of the key components in THz band, which can compromise large scale, high precision, and high efficiency, remain great challenges at present. In this work, we have developed a high throughput fabrication method based on the femtosecond laser filament direct writing. The ability of fabricating large-scale THz elements with high precision and fast speed has been demonstrated by fabricating 100 × 100 mm² spiral zone plates (SZPs), which can convert the Gaussian THz beam into vortex beam. The performance of the obtained THz vortex beam is in good agreement with the theoretical predictions. The fabrication method reported here has promising applications in fabricating various kinds of THz elements on substrates with both flat and curved surfaces.

Effect of object thickness on ultrashort pulse diffraction

When calculated in the spectral domain, the propagation of an ultrashort optical pulse may suffer from inaccuracy due to the finite thickness of the object it diffracts on. Unlike monochromatic radiation, ultrashort pulse interaction with an object in the time domain depends on the pulse longitudinal coordinate. Here, we propose an algorithm to study the effect of the object thickness on ultrashort pulse diffraction on amplitude, phase, and three-dimensional highly scattering objects. The algorithm comprises a stepwise approach to simulating the diffraction of ultrashort pulses on apertures or scatterers having a finite thickness. We confirm the applicability of the approach and convergence of the result upon reducing the simulation step. We compare the simulation results obtained with traditionally calculated wavefields and the updated results obtained with the proposed approach. We reveal a discrepancy of about 7% for pulsed radiation with λ=800nm on a 1 mm thick object. Then, we demonstrate the dependence of this mismatch on the object thickness and show that for non-Gaussian vortex beams, this effect is even more pronounced. We reveal that spatiotemporal coupling effects depend on the pulse-object interaction simulation approach as well. The obtained results demonstrate that applicability of the single-layer representation of the simulated object strongly depends on its specific features, and inaccuracy of such an approach strongly depends on individual characteristics of the object.

Intensity modulated terahertz vortex wave generation in air plasma by two-color femtosecond laser pulses

We investigate the generation of broadband terahertz (THz) pulses with phase singularity from air plasmas created by fundamental and second harmonic laser pulses. We show that when the second harmonic beam carries a vortex charge, the THz beam acquires a vortex structure as well. A generic feature of this THz vortex is that the intensity is modulated along the azimuthal angle, which can be attributed to the spatially varying relative phase difference between the two pump harmonics. Fully space- and time-resolved numerical simulations reveal that transverse instabilities of the pump further affect the emitted THz field along nonlinear propagation, which may produce additional singularities resulting in a rich vortex structure. The predicted intensity modulation is experimentally demonstrated with a thermal camera, in excellent agreement with simulation results. The presence of phase singularities in the experiment is revealed by astigmatic transformation of the beam using a cylindrical mirror.

Terahertz vortex generation methods in rippled and vortex plasmas

Terahertz vortices have strong potential for many applications such as imaging and sensing in medicine, biomedical engineering, rotations of molecules, quantum condensation, optical tweezers, manipulation of electron beams, and communications. However, owing to recent developments, there has been less research about vortex generation in the terahertz domain. Due to the damaging limit and low conversion efficiency, a few schemes to generate terahertz vortices based on plasma have recently been reported. Generally, to excite the helicity of the terahertz vortices, two scenarios have been reported: one is transferring the orbital angular momentum from the plasma vortex to the emitted terahertz radiation, and the other is exciting the helicity of the terahertz vortices using twisted input lasers. This paper is a review of recent studies on terahertz vortex generation based on the rippled and vortex plasma substrata.

Generation of terahertz vortex pulses without any need of manipulation in the terahertz region

Converting a Gaussian mode to a vortex beam is much more inconvenient in the terahertz (THz) region than in the near-infrared (NIR) region due to underdevelopment of THz components and strong THz diffraction. This Letter reports the direct generation of THz vortex pulses by optical difference-frequency between two NIR chirped pulses with different topological charges (TCs). By designing a passive and transmissive device for a collinear NIR pulse pair with conjugated TCs, we have experimentally obtained stable THz vortex pulses with a TC value of 2 or -2. The process needs no THz components and so is flexible to be realized and has promising applications in the THz field.

Spatio-temporal and spatiospectral metrology of terahertz broadband uniformly topologically charged vortex beams

A comprehensive characterization of the diffraction properties of terahertz (THz) pulsed broadband vortex beams consisting of several electromagnetic field oscillations requires state-of-the-art techniques for studying the evolution of a wavefront as it propagates. For this purpose, we have applied the capabilities offered by THz pulse time domain holography. Accurate metrological study of pulsed single-period THz field propagation allowed us to reveal the spatio-temporal and spatiospectral couplings in broadband uniformly topologically charged vortex beams. Here, we reveal dynamics of such beam propagation in a free space as well as in the experiment with edge diffraction with 50% blocking of the beam focal waist. In this study, we compare the dynamics of freely propagating and edge-diffracted THz vortex. Despite the fact that in the amplitude representation one can observe the emergence of strong asymmetry, analysis of the spectral trajectory of the singular point at some distance from the obstacle and the visualization of phase distribution for individual spectral components testify to the conservation of transverse energy circulation. Similar to the edge diffraction of monochromatic optical vortices, it can be interpreted as self-reconstruction of vortex properties. The given term has not previously been used for the case of pulsed broadband THz beams, to the best of our knowledge.

Terahertz vortex beam generator based on a photopatterned large birefringence liquid crystal

A terahertz (THz) q-plate is proposed and demonstrated to generate THz vortex beams. It is composed of a large birefringence liquid crystal (LC) with spatially-varying director distribution sandwiched by two pieces of fused silica glass. A polarization-sensitive alignment agent is photopatterned to carry out the specific LC director distribution. THz vortex beams with different topological charges are characterized with a THz digital holographic imaging system. The intensity and phase distributions consistent with theoretical analyses are obtained. Besides, an eight-lobed intensity distribution is observed corresponding to the vertical polarization component of a cylindrical vector beam. This work may inspire novel THz applications.