Highly intense monocycle terahertz vortex generation by utilizing a Tsurupica spiral phase plate

Generating Angular-Varying Time Delays of THz Pulses via Direct Space-to-Time Mapping of Metasurface Structures

We experimentally demonstrate the generation of double terahertz (THz) pulses with tailored angular-dependent time delays from a nonlinear metasurface excited by a near-infrared femtosecond pulse. The tailored temporal properties of the generated pulses emerge from a direct mapping of the nonlinear spatial response of the metasurface to the emitted THz temporal profile. We utilize the Pancharatnam-Berry phase to implement symmetric and antisymmetric metasurface configurations and show that the emitted patterns present spatiotemporal "X-shaped" profiles after collimation by a parabolic mirror, with angular-dependent pulse delays corresponding to the intended design. In addition, we show that the addition of polarization multiplexing presents the opportunity to achieve a full range of elliptical THz polarizations. Double pulse generation and spatiotemporal shaping of THz waves in general show potential for THz spectroscopy and molecular dynamics applications, particularly in pump-probe experiments.

Broadband Achromatic Metalens for Tunable Focused Vortex Beam Generation in the Near-Infrared Range

Vortex beams accompanied with orbital angular momentum have attracted significant attention in research fields due to their formidable capabilities in various crucial applications. However, conventional devices for generating vortex beams still suffer from bulky sizes, high cost, and confined performances. Metalens, as an advanced platform to arbitrarily control the optical waves, has promising prospects to address the predicament for conventional devices. Although great progress has been demonstrated in the applications of vortex beams, they are still confronted with fixed functionality after fabrication that severely hinders their application range. In this work, the phase-change material of Ge2Sb2Te5 (GST) is employed to design the meta-atoms to realize tunable optical responses. Moreover, the focused vortex beam can be accomplished by superimposing a helical phase and hyperbolic phase, and the chromatic aberrations in near-infrared (NIR) range can be corrected by introducing an additional phase compensation. And the design strategy is validated by two different metalenses (BAMTF-1 and BAMTF-2). The numerical results indicate that the chromatic aberrations for two metalens can be corrected in 1.33-1.60 μm covering the telecom range. Moreover, the average focusing efficiency of BAMTF-1 is 51.4%, and that of BAMTF-2 is 39.9%, indicating the favorable performances of designed BAMTF. More importantly, their average focal lengths have a relative tuning range of 38.82% and 33.17% by altering the crystallization ratio of GST, respectively. This work may provide a significant scheme for on-chip and tunable devices for NIR imaging and communication systems.

Sub-terahertz vortex beam generation using a spiral metal reflector

We demonstrate sub-terahertz vortex beam generation using a spiral metal reflector that can be used for both polarizations. A vortex beam is a ring-shaped beam that possesses sub-wavelength null in the center formed by angular phase variation. While the sub-terahertz vortex beams have gained increasing attention for a wide range of applications in sensing and communications, techniques for generating them are still accompanied by challenges. For example, the use of a phase plate, which is common in the optical regime, suffers from intrinsic losses of dielectric materials in the sub-terahertz regime. Moreover, holographic diffraction gratings, which could replace transmissive components, are inefficient and sensitive to the polarization. To reconcile these challenges, here we design a reflector type metal component with a spiral surface shape. We firstly derive a direct equation to design its shape. We then experimentally validate the design by mapping the radiation pattern of a vortex beam for the WR10 frequency band (75 to 110 GHz) in both of the orthogonal polarizations. The result confirms an inexpensive and versatile approach to generate a vortex beam in the sub-terahertz regime.

Selection rules for the orbital angular momentum of optically produced THz radiation

In this work, we theoretically study the transduction of orbital angular momentum (OAM) l for infrared pump lasers into the THz domain. In the case of optical rectification, the transduction of OAM occurs only through a spin-orbit interaction, with the selection rule on the OAM l=0 valid for any kind of polarization of the pump, which means that there is no transfer of OAM along the propagation axis. In difference frequency generation, the selection rule for the difference Δl between the OAM of the pump fields with linear or circular polarization is l=Δl, whereas l ranges from Δl-2 to Δl+2 in cases of both radial and azimuthal polarization. Moreover, for THz generation in the latter case, the high diffraction obtained with tightly focused pumps yields l tending to Δl±2, while l tends to zero in the opposite case of large pump beams.

Generation of high-quality terahertz OAM mode based on soft-aperture difference frequency generation

We demonstrate the generation of high-quality tunable terahertz (THz) vortices in an eigenmode by employing soft-aperture difference frequency generation of vortex and Gaussian modes. The generated THz vortex output exhibits a high-quality orbital angular momentum (OAM) mode with a topological charge of ℓ_THz = ±1 in a frequency range of 2-6 THz. The maximum average power of the THz vortex output obtained was ∼3.3 µW at 4 THz.

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.

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.

Focusing light with orbital angular momentum by circular array antenna

We experimentally demonstrated focusing of light with orbital angular momentum (OAM) using an 8-element circular array of linear antennas. A spiral phase plate was used to generate a vortex beam with an OAM of ħ in the terahertz (THz) frequency region. We used THz near-field microscope to directly measure the phase vortex. A beam profile with a center dark spot and 2π phase rotation was observed in the small center gap region of the circular array antenna after the vortex beam excitation. The beam size is reduced by a factor of 3.4 ± 0.2. Half-wave resonance of the antenna element is responsible for the focusing function, indicating the scalability of this method to other frequency regions. This method will enable deep subwavelength focusing of light with OAM and eliminate the obstacle for the observation of the dipole forbidden transition with finite OAM of the vortex beam.