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

Analyzing the effect of doping concentration in split-well resonant-phonon terahertz quantum cascade lasers

The effect of doping concentration on the temperature performance of the novel split-well resonant-phonon (SWRP) terahertz quantum-cascade laser (THz QCL) scheme supporting a clean 4-level system design was analyzed using non-equilibrium Green's functions (NEGF) calculations. Experimental research showed that increasing the doping concentration in these designs led to better results compared to the split-well direct-phonon (SWDP) design, which has a larger overlap between its active laser states and the doping profile. However, further improvement in the temperature performance was expected, which led us to assume there was an increased gain and line broadening when increasing the doping concentration despite the reduced overlap between the doped region and the active laser states. Through simulations based on NEGF calculations we were able to study the contribution of the different scattering mechanisms on the performance of these devices. We concluded that the main mechanism affecting the lasers' temperature performance is electron-electron (e-e) scattering, which largely contributes to gain and line broadening. Interestingly, this scattering mechanism is independent of the doping location, making efforts to reduce overlap between the doped region and the active laser states less effective. Optimization of the e-e scattering thus could be reached only by fine tuning of the doping density in the devices. By uncovering the subtle relationship between doping density and e-e scattering strength, our study not only provides a comprehensive understanding of the underlying physics but also offers a strategic pathway for overcoming current limitations. This work is significant not only for its implications on specific devices but also for its potential to drive advancements in the entire THz QCL field, demonstrating the crucial role of e-e scattering in limiting temperature performance and providing essential knowledge for pushing THz QCLs to new temperature heights.

Rapid readout of terahertz orbital angular momentum beams using atom-based imaging

We demonstrate the rapid readout of terahertz orbital angular momentum (OAM) beams using an atomic-vapor-based imaging technique. OAM modes with both azimuthal and radial indices are created using phase-only transmission plates. The beams undergo terahertz-to-optical conversion in an atomic vapor, before being imaged in the far field using an optical CCD camera. In addition to the spatial intensity profile, we also observe the self-interferogram of the beams by imaging through a tilted lens, allowing the sign and magnitude of the azimuthal index to be read out directly. Using this technique, we can reliably read out the OAM mode of low-intensity beams with high fidelity in 10 ms. Such a demonstration is expected to have far-reaching consequences for proposed applications of terahertz OAM beams in communications and microscopy.

Generation of soft annular beams with high uniformity, low ring width increment, and a smooth edge

In this paper, soft-edge toroidal amplitude filter (STAF) and soft-edge toroidal complex amplitude filter (STCAF) are designed according to the principle that soft-edge structures can eliminate diffraction. Based on the Mach-Zehnder interference principle, a double optical path compound interference modulation method that can generate soft annular beams is proposed by using STAF and STCAF. The 1/e² radius and peak-to-average ratio (PAR) were used to evaluate the ring width and uniformity of the annular beam. Compared with the annular beams generated by STAF and hard-edge toroidal amplitude filter (HTAF), it can be found that the soft annular beam generated by this proposed method has the advantages of high uniformity, small ring width increment, and smooth edges. By analyzing the influence of the number and height of the sawtooth structures on the annular beam propagation performance, the relationship between the PAR and the structure parameters of the STAF was established. Moreover, three kinds of toroidal filters were designed by lithography processing, and an experimental system was built to generate the soft annular beam. In the experiment, the average value of the ring width increment of the soft annular beam is 0.0125, the PAR is less than 1.5, and the root mean square error of the PAR curve is 0.0865, which indicates that the soft annular beam maintains high uniformity during propagation.

A Nonlinear Radio-Photon Conversion Device

The article analyzes existing materials and structures with quadratic-nonlinear optical properties that can be used to generate a difference frequency in the terahertz and sub-terahertz frequency ranges. The principle of constructing a nonlinear optical-radio converter, based on an optical focon (a focusing cone), is proposed. Based on the assumption that this focon can be implemented from the metal-organic framework (MOF), we propose a technique for modeling its parameters. The mathematical model of the process of propagation and nonlinear interaction of waves inside the focon is based on a simplification of the nonlinear wave equation. Within the framework of the developed model, the following parameters are approximately determined: the 3D gradient of the linear refractive index and the function determining the geometric profile of the focon, which provide a few-mode-based generation of the difference frequency. The achieved theoretical efficiency of radio frequency generation is at least 1%; the proposed device provides a guiding structure for both optical and radio signals in contrast to the known solutions.

Propagation and Transformation of Vortexes in Linear and Nonlinear Radio-Photon Systems

The article is devoted to issues related to the propagation and transformation of vortexes in the optical range of frequency. Within the framework of the traditional and modified model of slowly varying envelope approximation (SVEA), the process of converting vortex beams of the optical domain into vortex beams of the terahertz radio range based on nonlinear generation of a difference frequency in a medium with a second-order susceptibility is considered. The modified SVEA splits a slowly varying amplitude into two factors, which makes it possible to more accurately describe the three-wave mixing process. The theoretical substantiation of the rule of vortex beams topological charges conversion is given—the topological charge of the output radio-vortex beam is equal to the difference between the topological charges of the input optical vortex beams. A numerical simulation model of the processes under consideration has been implemented and analyzed.

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.

Broadband high-resolution terahertz single-pixel imaging

We report a simple single-pixel imaging system with a low mean squared error in the entire terahertz frequency region (3-13 THz) that employs a thin metallic ring with a series of directly perforated random masks and a subpixel mask digitization technique. This imaging system produces high pixel resolution reconstructed images, up to 1200 × 1200 pixels, and imaging area of 32 × 32 mm². It can be extended to develop advanced imaging systems in the near-ultraviolet to terahertz region.

Second-harmonic generation of asymmetric Bessel-Gaussian beams carrying orbital angular momentum

Nonlinear processes of laser beams carrying orbital angular momentum (OAM) offer a means to generate new wavelengths and to manipulate OAM charge numbers. We demonstrate the second-harmonic generation (SHG) of asymmetric Bessel-Gaussian (BG) beams carrying OAM of both integer and fractional charge numbers. Experimental results show a good one-to-one correspondence of the charge numbers and compliance with the OAM conservation law. The SHG conversion process and efficiency with different combined charge numbers are also discussed.