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Subject and Purpose. Currently, numerous ideas and different methods have been in growth for generating vortex beams — areas of the circular motion of the electromagnetic wave energy flow around the so-called phase singularity points caused by a violation of the wave front topological structure. The purpose of this work is to obtain analytical expressions describing the nonparaxial diffraction of wave modes of the waveguide resonator of a terahertz laser during the wave mode interaction with a spiral phase plate. The resulting vortex beams are examined for their physical features in free space propagation. Methods and Methodology. The Rayleigh-Sommerfeld vector theory is adopted to consider the propagation of vortex laser beams generated by wave modes of the quasi-optical waveguide cavity when interacting with a spiral phase plate in different diffraction zones. Results. For the first time, analytical expressions have been obtained to describe the nonparaxial diffraction of wave modes of the waveguide resonator of a terahertz laser, when resonator modes interact with a spiral phase plate at different topological charges, n. The physical features of the resulting vortex beams were studied in their free space propagation. It has been shown that a spiral phase plate modifies the structure of the linearly polarized EH₁₁ mode so that the original (n=0) intensity profile with the maximum energy at the center turns at n=1 and 2 into a ring-like donut shape with an energy hole in the center. The azimuthally polarized TE₀₁ mode has originally (n=0) a ring-shaped intensity. At n=1, this configuration changes to have the maximum intensity in the center. At n=2, it becomes annular again. In the process, the spherical phase front of the beam of the linearly polarized EH₁₁ mode becomes spiral and have one singularity point on the axis, whereas the phase structure of the azimuthally polarized TE₀₁ mode gains a region with two phase singularity points off the axis. Conclusions. The results of the study can effectively facilitate information transfer in high-speed THz communication systems. They can provide a real platform to perform tasks related to tomography, exploring properties of materials, detecting astrophysical sources, which makes them very promising in modern technologies.

Analytical expressions for the nonparaxial mode diffraction of a terahertz laser metal waveguide resonator are obtained. The study assumes interaction between the modes and a spiral phase plate, considering different topological charges (n). Also, using numerical modeling, the physical features of the emerging vortex beams as they propagate in free space are studied. The Rayleigh-Sommerfeld vector theory is employed to investigate the propagation of vortex laser beams in the Fresnel zone, excited by the modes of a metal waveguide quasi-optical resonator upon incidence on a spiral phase plate. In free space, the spiral phase plate for exciting TE11 mode from the profile with the intensity maximum in the center (n = 0) forms an asymmetric ring one with two maxima (n = 1, 2). For the exciting TE01 mode, the initial ring (n = 0) structure of the field intensity is transformed into a structure with a maximum radiation intensity in the center (n = 1), and later again into a ring (n = 2). The phase front of the beam for the Ey component of the linearly polarized along the y axis TE11 mode changes from spherical to spiral with one on-axis singularity point. In the phase profile of the transverse components of the azimuthally polarized TE01 mode, a region with two and three off-axis phase singularity points appears.

Physical features of spatial-energy characteristics of linearly polarized THz laser beams at sharp and moderate focusing in free space are analyzed both theoretically and experimentally. The Rayleigh-Sommerfeld vector theory is used to model propagation of laser beams excited by quasi-optical waveguide resonator modes in free space. Well-known methods of measuring spatial-energy characteristics of laser beams in the THz range are used in the experimental study. It is shown that the intensity of the total electric field of both combined TE0n + EH2n and EН-1n + EH3n modes (n = 1, 2, 3) in the focal region is defined by all three components and has a dip on the axis for both focusing types. The central maxima of the field of these modes become significantly shifted from the geometric foci of the lenses used as the order n of these modes increases. The EН-11 + EH31 mode has the smallest diameter (2.94λ) of the focal spot in the maximum intensity region at sharp focusing. The TE01 + EH21 mode has the smallest diameter (13.65λ) of the focal spot at moderate focusing. [ABSTRACT FROM AUTHOR]

Analytical expressions for the nonparaxial mode diffraction of a terahertz laser metal waveguide resonator are obtained. The study assumes interaction between the modes and a spiral phase plate, considering different topological charges (n). Also, using numerical modeling, the physical features of the emerging vortex beams as they propagate in free space are studied. The Rayleigh-Sommerfeld vector theory is employed to investigate the propagation of vortex laser beams in the Fresnel zone, excited by the modes of a metal waveguide quasi-optical resonator upon incidence on a spiral phase plate. In free space, the spiral phase plate for exciting TE11 mode from the profile with the intensity maximum in the center (n = 0) forms an asymmetric ring one with two maxima (n = 1, 2). For the exciting TE01 mode, the initial ring (n = 0) structure of the field intensity is transformed into a structure with a maximum radiation intensity in the center (n = 1), and later again into a ring (n = 2). The phase front of the beam for the Ey component of the linearly polarized along the y axis TE11 mode changes from spherical to spiral with one on-axis singularity point. In the phase profile of the transverse components of the azimuthally polarized TE01 mode, a region with two and three off-axis phase singularity points appears. [ABSTRACT FROM AUTHOR]

Subject and Purpose. The problems under consideration concern selection and focusing of higher-order modes in a waveguide-based dielectric laser. The purpose is to clarify the physics underlying the behavior of, and permitting control over, continuous terahertz-frequency laser beams of various spatial polarizations. Methods and Methodology. The mode parameters of the waveguide-based laser resonator involving an inhomogeneous phase-stepped mirror were calculated in a matrix technique. To analyze the propagation and focusing of the laser beams that can be excited in a variety of diffraction zones by the wave modes of a waveguide-based quasi-optical resonator, a vectorial Rayleigh–Sommerfeld theory was used. The pertinent experimental studies were performed with the use of known measurement methods suitable for the terahertz frequency range. Results. A method for selecting the higher-order EH12q-mode of a terahertz-range laser resonator has been suggested, substantiated theoretically and approbated in experiment. It envisages placing an additional element to perform control over the system’s modal structure, namely a (2.3…2.8) λ-wide groove on the surface of one of the resonator mirrors. This measure can significantly increase losses for all undesirable modes. At the same time, the losses for the higher EH12q-mode remain practically unchanged, which creates conditions for its predominant excitation. Theoretical and experimental studies of moderate and ‘sharp’ focusing in free space of higher-order modes with different spatial polarizations of a dielectric waveguide-based resonator have been carried out. Conclusion. As has been shown, the proposed phase-stepped mirror with a groove can effectively select the higher-order transverse modes that may be required. The linearly polarized EH12q-mode has maximum field intensity in the focal region of the lens employed. For azimuthally polarized TE02q- and TE03q-modes the central lobes, noticeably shifted from the focus of the lens, have a field maximum. An increase in the axial intensity is observed upon ‘sharp’ focusing in the field distribution of the radially polarized TM02q- and TM03q-modes. In this case their central lobes, like those of the higher TE0nq-modes, are noticeably shifted from the lens focus.

Subject and Purpose. In this paper, spatial power characteristics of laser beams with inhomogeneous spatial polarization are theoretically investigated depending on whether they have moderate or sharp focusing. The spotlight of the numerical simulation of laser beam focusing in the terahertz range is on the laser waveguide dielectric resonator modes coincident with waveguide eigenmodes. Symmetric and asymmetric modes with both spatially inhomogeneous azimuthal and radial polarizations and homogeneous linear polarization of the field are considered. Methods and Methodology. The study of the electric field components of laser radiation beams in their free space propagation employs Rayleigh-Sommerfeld integrals in nonparaxial approximation. The focusing lens action on the laser radiation is considered using the amplitude-phase correction function. The total intensity distributions of the resonator modes and their particular field components in the lens focal area are examined. Results. Wave beams with inhomogeneous spatial polarization of radiation are needed for meeting important fundamental and applied problems concerning terahertz electromagnetic wave interaction with matter and including diagnostics of material surface, thin films and biological objects, design and development of terahertz systems of information transmission and processing, terahertz communication systems, achievement of subwave resolution in terahertz tomography, etc. The literature on pulsed radiation beam focusing in the terahertz region is available, which cannot be said about continuous radiation beam focusing, data on the subject being practically absent. In the present work, physical features of moderate and sharp focusing of laser beams have been theoretically studied in the case of their excitation by modes of the resonator with a circular dielectric waveguide with different spatial polarizations of the field. The obtained results give a more in-depth view of the focusing features of terahertz laser beams. Conclusions. Physical features have been established for spatial power characteristics of radiation beams with different spatial polarizations of the field in the case of the excitation by modes of the laser cavity upon a circular dielectric waveguide in the terahertz range and with the regimes of moderate and sharp beam focusing in the free space considered.

The possibility for control of tightly focused laser beams excited by the resonator modes of a THz laser based on a hollow circular dielectric waveguide is studied theoretically and experimentally. The wavelength of the radiation under study is 0.4326 mm corresponding to the generation line of the optically pumped HCOOH‐molecule THz laser. The focusing system is implemented by a short‐focus lens covered by absorbing masks in the central region. The focal area parameters were controlled by changing the size of the masks. The electric field components of the laser modes are calculated using the Rayleigh‐Sommerfeld integral transformations under non‐paraxial approximation. The use of absorbing masks made it possible to reduce the beam diameter in the focal region and increase the depth of focus of the linearly polarized EH11‐mode and azimuthally polarized TE01‐mode as well. In this case, despite of the presence of an absorbing mask, the distribution of the total field intensity of the TE01‐mode retained its annular shape, and the field maximum of the EH11‐mode is observed at center of the focal spot. The results of this work can be implemented in medical diagnostics, high‐speed communications, and environmental monitoring. [ABSTRACT FROM AUTHOR]

Subject and ppurpose. In this paper, the spatial-energy characteristics of laser beams with inhomogeneous spatial polarization with their moderate and sharp focusing are theoretically investigated. In the numerical simulation of focusing of wave laser beams in the terahertz range, laser resonator modes based on a circular metal waveguide were used as the radiation under study. These modes coincide with the proper types of oscillations of such a waveguide. For the calculation, we selected symmetric and asymmetric modes with azimuthal, radial, and complex spatial polarization of the field. Methods and methodology. In the study of the propagation of the components of the electric field of laser radiation in free space, Rayleigh-Sommerfeld integrals were used in the nonparaxial approximation. The effect of the focusing lens on the radiation under study was taken into account using the function of amplitude-phase correction. The transverse distributions of the total field intensity and its individual components of the studied resonant modes in the region of the minimum size of the focal spot of focused radiation beams were studied. Results. Wave beams with inhomogeneous spatial polarization of radiation are necessary for solving important fundamental and applied problems. These tasks are associated with the interaction of electromagnetic waves of the terahertz range with matter. For example, diagnostics of the surface of materials, thin films, biological objects, THz transmission systems and information processing, the achievement of subwave resolution THz tomography, terahertz communication systems, etc. The results of studies of focusing features in the terahertz range are practically absent. A theoretical study was made of the physical features of moderate and sharp focusing of laser beams of radiation excited by the modes of a resonator with a circular metal waveguide with different spatial polarization of the field. The results obtained extend knowledge of the focusing features of terahertz laser beams. Conclusions. The physical features of the spatial-energy characteristics are established with moderate and sharp focusing in free space of radiation beams with different spatial polarization of the field. These beams were excited by laser resonator modes based on a circular metal waveguide in the terahertz range.

Highlights • A high efficient, high power gas THz laser based on L-shaped cavity is present. • THz output performance was investigated both in theory and experiment. • The optimum output coupling is 0.8 and the optimum gas pressure is 500 Pa. • The maximum THz pulse energy is 8.4 mJ under the pumping energy of 1.6 J. Abstract A high power gas terahertz laser based on L-shaped cavity was demonstrated. We, for the first time, investigated the output performances of D 2 O gas 385 μm THz laser by optimizing the output coupler transmittance of the L-shaped cavity both in theory and experiment. Under the pump energy of 1.6 J, the optimum output coupler transmittance was about 0.78 and the optimum gas pressure was about 500 Pa. Up to 8.4 mJ pulse energy at 385 μm was achieved at output coupler transmittance of 0.8, corresponding to a photon conversion efficiency of 43.7%. Pulse width of 120 ns, and beam quality factor M2 of 1.58 were obtained at the highest output energy. In addition, the experimental results are in agreement with the theoretical simulation results. [ABSTRACT FROM AUTHOR]

Recently, the concept of the terahertz bosonic cascade laser (BCL) based on a parabolic quantum well (PQW) embedded in a microcavity was proposed. We refine this proposal by suggesting transitions between indirect exciton (IX) states as a source of terahertz emission. We explicitly propose a structure containing a narrow-square QW and a wide-parabolic QW for the realisation of a bosonic cascade. Advantages of this type of structures are in large dipole matrix elements for terahertz transitions and in long exciton radiative lifetimes which are crucial for realisation of threshold and quantum efficiency BCLs. [ABSTRACT FROM AUTHOR]

We study the effects of microscopic dynamics of electrons on the intersubband optical absorption in terahertz lasers based on optically pumped quantum wells (THz-OPQW). The ensemble Monte Carlo method is used to include different electron scattering mechanisms in the calculation, so that the dependence of intersubband absorption lineshape on microscopic dynamics of electrons can be easily investigated. We find that electron-electron scattering is the dominant factor to determine the absorption linewidth. The real distributions of hot electrons are included to study the temporal changes of the spectra lineshape. Our findings show that the dependence of spectral lineshape on electron population essentially results from the electron scattering and the non-Fermi electron energy distribution. [ABSTRACT FROM AUTHOR]

Heavy water gas (DO gas) which owns special structure property, can generate terahertz radiation by optically pumping technology, and its 385 μm wavelength radiation can be widely used. In this research, on the base of semi-classical density matrix theory, we set up a three-level energy system as its theoretical model, a TEA-CO laser 9R (22) output line (λ = 9.26 μm) acted as pumping source, DO gas molecules were operating medium, the expressions of pumping absorption coefficient G and Terahertz signal gain coefficient G were deduced. It was shown that the gain of Terahertz signal was related with the energy-level parameters of operating molecules and some operating parameters of the Terahertz laser cavity, mainly including cavity length. By means of iteration method, the output power density of Terahertz pulse signal was calculated numerically. Changing the parameter of cavity length and keeping others steady, the relationship curve between the output power intensity (Is) of Terahertz pulse laser and the operating cavity length (L) was obtained. The curve showed that the power intensity (Is) increased with cavity length (L) in a certain range, but decreased when the length (L) exceeded some value because of the absorption effect, and there was an optimal cavity length for the highest output power. We used a grating tuned TEA-CO laser as pumping power and a sample tube of variable length in 70-160 cm as terahertz laser operating cavity to experiment. The results of theoretical calculation and experiment matched with each other, and it is helpful for miniaturizing terahertz laser volume to make it practical. [ABSTRACT FROM AUTHOR]

The resonant wavelength and mode radius of the fundamental modes supported by an oxide apertured terahertz vertical surface emitting laser are determined from Gaussian resonant theory and scalar variational method. The reflectivity of the Bragg mirror is calculated for the lowest modes and it decreases as the aperture size decreases. The aperture radius, thickness, and axial position in the cavity are shown to be an important factor for high efficiency vertical cavity surface emitting lasers and single mode operation. When the aperture size is much larger than the emitting wavelength, the optical loss is negligible. However, the optical loss strongly depends on the aperture size and thickness when aperture size is similar to or smaller than emitting wavelength. [Copyright &y& Elsevier]

• The impurity-related optical response of a disc-shaped quantum dot is investigated. • The impurity binding energy can be controlled by adjusting the intense laser field. • The peak position and magnitude of the optical parameters crucially depend on external fields. We report the optical properties of a laser-driven two-dimensional quantum dot with a hydrogenic impurity exposed to a magnetic field. The effective mass approximation is used to bring out the effect of an intense laser field, resulting in dressed Coulomb and confinement potentials, on the one-electron energy levels and wave functions. The results obtained suggest that the action of the laser field leads to crucial changes in the energy spectra of the system and the impurity binding energy. Furthermore, the position and the height of the resonant peaks of the absorption coefficients and refractive index changes depend strongly not only on the geometrical confinement strength but also on the magnetic field and laser radiation through the "laser-dressed" potential. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the transverse electromagnetic wave propagating in a two-dimensional electron gas under a static uniform magnetic field and excited by an intense laser field using the field theoretical canonical transformation and the self-consistent linear response approximation. [Copyright &y& Elsevier]