Far-infrared Laser Research Articles

Computational Analysis of Vibrational Spectra of Hydrogen Bonds in sII and sH Gas Hydrates.

The amount of energy in natural gas hydrates is thought to be equivalent to twice that of all other fossil fuels combined. However, economic and safe energy recovery has remained a challenge till now. To develop a novel method of breaking the hydrogen bonds (HBs) surrounding the trapped gas molecules, we investigated the vibrational spectra of the HBs of gas hydrates with structure types II and H. Two models of 576-atom propane-methane sII hydrate and 294-atom neohexane-methane sH hydrate were built. A first-principles density functional theory (DFT) method was employed using the CASTEP package. The simulated spectra were in good agreement with the experimental data. Compared with the partial phonon density of states of guest molecules, we confirmed that the experimental infrared absorption peak in the terahertz region mainly arose from HB vibrations. By removing the components of guest molecules, we found that the theory of two kinds of hydrogen bond vibrational modes applies. The use of a terahertz laser to enable resonance absorption of HBs (at about 6 THz, to be tested) may therefore lead to the rapid melting of clathrate ice and release of guest molecules.

Progress of Gamma Ray Irradiation Experiments on ITER Diagnostics from JADA

The Japan domestic agency (JADA) is responsible for procuring five diagnostic systems for ITER: the microfission chamber (MFC), poloidal polarimeter (PoPola), edge Thomson scattering (ETS), divertor impurity monitor (DIM), and divertor infrared thermography (IRTh) systems. Components of these systems that will be installed in high radiation zones must undergo radiation resistance tests to ensure reliability. The JADA diagnostic group has been conducting gamma ray irradiation experiments on diagnostic components at QST since 2018. The MFC is a neutron diagnostic system that uses uranium fission chambers, the mineral-insulated cables of which were evaluated for corrosion resistance. In-situ observations of the MFC preamplifier have been launched. The PoPola provides the plasma current profile by detecting the polarizations of far-infrared laser beams at their inlets and outlets. Irradiation tests confirmed the durability of PoPola piezo actuators. The ETS is a laser-aided diagnostic to measure electron temperature and density in plasmas from scattered spectra. Laser-induced damages to the optical elements caused by irradiation were investigated. The DIM is a spectroscopic system having a wavelength range of 200 nm to 1000 nm. The effects of irradiation on optical devices, metal mirrors, and radiation-resistant optical fibers were investigated. The IRTh is an infrared thermography system to observe the surface temperature of the divertor. The optical elements and electrical devices of the IRTh have undergone irradiation experiments. The progress of the gamma ray irradiation experiments on the ITER diagnostic systems from JADA are reported.

Design of optical transmission line of ITER poloidal polarimeter

ITER Poloidal Polarimeter is laser-aided diagnostics that measures the change of polarization of injected far-infrared laser beam in order to identify plasma current profile. PoPola consists of thirteen measurement chords, and each chord is comprised of a set of optical components. This study shows the design of optics in the ITER vacuum vessel and those in high radiation area outside the vacuum vessel. The design takes into account zero-offset configuration, the minimization of the neutron streaming, remote alignment, accessibility, manufacturability and radiation hardness. The laser power loss due to vignetting loss and absorption is evaluated, and the signal-to-noise ratio at a detector is estimated at 50,000. This study evaluates the acceptable errors of manufacturing and assembly of the optical transmission line on the condition that the demonstrated alignment method using the two steering mirrors outside the vacuum vessel is applied. The evaluation results show that the medium tolerance class of ISO 2768 is basically enough for manufacturing ITER poloidal polarimeter. Only for assembling the in-vessel mirrors to the mirror module, the mirror angles need to be carefully adjusted with accuracy of 0.1° Finally, radiation hardness of a steering mirror is presented. The steering mirror consists of an angle sensor and a piezo actuator. The angle sensors are developed by the authors and free from semiconductor and soft resin. The prototype test results show that angle sensor can measure the mirror angle in the range of ±2.3° with accuracy of 0.37%. As for the piezo actuator, the irradiation test was carried out. The test results show that the piezo actuator is able to operate after the neutron fluence of 3.5 × 1015 n/cm2 and the gamma dose of 0.99 MGy.

Relative phase locking of a terahertz laser system configured with a frequency comb and a single-mode laser

Stable operation is one of the most important requirements for a laser source for high-precision applications. Many efforts have been made to improve the stability of lasers by employing various techniques, e.g., electrical and/or optical injection and phase locking. However, these techniques normally involve complex experimental facilities. Therefore, an easy implementation of the stability evaluation of a laser is still challenging, especially for lasers emitting in the terahertz (THz) frequency range because the broadband photodetectors and mature locking techniques are limited. In this work, we propose a simple method, i.e., relative phase locking, to quickly evaluate the stability of THz lasers without a need of a THz local oscillator. The THz laser system consists of a THz quantum cascade laser (QCL) frequency comb and a single-mode QCL. Using the single-mode laser as a fast detector, heterodyne signals resulting from the beating between the single-mode laser and the comb laser are obtained. One of the heterodyne beating signals is selected and sent to a phase-locked loop (PLL) for implementing the relative phase locking. Two kinds of locks are performed by feeding the output error signal of the PLL, either to the comb laser or to the single-mode laser. By analyzing the current change and the corresponding frequency change of the PLL-controlled QCL in each phase-locking condition, we, in principle, are able to experimentally compare the stability of the emission frequency of the single-mode QCL (fs) and the carrier envelope offset frequency (fCEO) of the QCL comb. The experimental results reveal that the QCL comb with the repetition frequency injection locked demonstrates much higher stability than the single-mode laser. The work provides a simple heterodyne scheme for understanding the stability of THz lasers, which paves the way for the further locking of the lasers and their high-precision applications in the THz frequency range.

Ultrafast optical switching and data encoding on synthesized light fields.

Modern electronics are founded on switching the electrical signal by radio frequency electromagnetic fields on the nanosecond time scale, limiting the information processing to the gigahertz speed. Recently, optical switches have been demonstrated using terahertz and ultrafast laser pulses to control the electrical signal and enhance the switching speed to the picosecond and a few hundred femtoseconds time scale. Here, we exploit the reflectivity modulation of the fused silica dielectric system in a strong light field to demonstrate the optical switching (ON/OFF) with attosecond time resolution. Moreover, we present the capability of controlling the optical switching signal with complex synthesized fields of ultrashort laser pulses for data binary encoding. This work paves the way for establishing optical switches and light-based electronics with petahertz speeds, several orders of magnitude faster than the current semiconductor-based electronics, opening a new realm in information technology, optical communications, and photonic processor technologies.

Effect of laser irradiation on RF-sputtered antimony sulfide thin films

One of the drawbacks of Sb2S3 thin films is the lack of crystallinity if no thermal treatment is applied. The interest for the development of crystallinity is to improve their properties. Thus, laser irradiation could provide sufficient energy to improve the crystallinity in the above-indicated material. In this work, Radio Frequency (RF)-sputtered Sb2S3 thin films were irradiated with a continuous CO2 laser. Laser treated samples showed crystallization corresponding to the stibnite phase (Sb2S3). Furthermore, the morphology of the sample changed from a cracked surface (before irradiation) to a more dense/homogeneous film (after irradiation). The structural and morphological changes induced by laser irradiation tend to improve the optical and electrical properties aside the crystalline one. When the sample was treated with laser, an increase in optical and photoconductivity properties were detected. The results presented here demonstrate the possibility of improving the quality of Sb2S3 films when a far-infrared laser is applied.

Electrical topological defects induced by terahertz laser pulses

Recent experimental evidence and theoretical simulations have shown that electrical polarization can be tuned and even deterministically controlled in ferroelectrics by applying laser pulses having a frequency that is higher than that of the main polarization (soft) mode. Here, by using ab initio computations, we further discover that such pulses can also create nontrivial topological polar phases in bulk ferroelectric materials. Such a phenomenon is found to originate from nonthermal quenching process in which the applied intense ultrashort laser pulse first squeezes the ferroelectric phase into a cubic-like paraelectric phase at a moderate temperature and then to a renucleation process akin to the Kibble-Zurek mechanism creating nontrivial topological polar defects in the bulk ferroelectric ground-state phase. Our work therefore presents a novel and optical mean of creating topological defects in ferroelectrics.

Design of a megawatt superradiant terahertz source for FELiChEM

FELiChEM is an infrared free electron laser user facility built at the University of Science and Technology of China (USTC). It consists of two free electron laser oscillators which produce mid-infrared and far-infrared lasers covering the spectral range of 2–200 μm at current stage. In this paper, we propose a new operation mode of a megawatt (MW) terahertz (THz) superradiance free electron laser (FEL) for FELiChEM. Start-to-end (S2E) simulations have been carried out, in which the three-dimension particle-in-cell (PIC) code PUFFIN is used for the generation of the THz superradiance. Simulation results demonstrate that this superradiant FEL can provide 100–1000 μm laser with MW-class peak power from a 1.2 meter single-pass undulator when the electron energy is around 15 MeV.

Self-Induced Mode-Locking in Electrically Pumped Far-Infrared Random Lasers.

Mode locking, the self-starting synchronous oscillation of electromagnetic modes in a laser cavity, is the primary way to generate ultrashort light pulses. In random lasers, without a cavity, mode-locking, the nonlinear coupling amongst low spatially coherent random modes,can be activated via optical pumping, even without the emission of short pulses. Here, by exploiting the combination of the inherently giant third-order χ(3) nonlinearity of semiconductor heterostructure lasers and the nonlinear properties of graphene, the authors demonstrate mode-locking in surface-emitting electrically pumped random quantum cascade lasers at terahertz frequencies. This is achieved by either lithographically patterning a multilayer graphene film to define a surface random pattern of light scatterers, or by coupling on chip a saturable absorber graphene reflector. Intermode beatnote mapping unveils self-induced phase-coherence between naturally incoherent random modes. Self-mixing intermode spectroscopy reveals phase-locked random modes. This is an important milestone in the physics of disordered systems. It paves the way to the development of a new generation of miniaturized, electrically pumped mode-locked light sources, ideal for broadband spectroscopy, multicolor speckle-free imaging applications, and reservoir quantum computing.

Construction and tests of phosphor view screen station for monitoring transverse profile of electron beam at PCELL

Development of an accelerator-based light source for generating coherent terahertz (THz) and mid-infrared (MIR) free-electron laser (FEL) is ongoing at the PBP-CMU Electron Linac Laboratory (PCELL). For producing high quality radiation, suitable electron beam property is necessary. In particle accelerator technology, transverse beam distribution and profile can be measured by using several techniques. The beam monitoring using view screen station is one of popular and effective methods. At PCELL, we use stations consisting of phosphor screen equipped with CCD camera to monitor, measure and record the beam image along the accelerator system and beamlines. The phosphor screen was chosen due to its high spatial resolution. The phosphor film is uniformly coated on a thin aluminium plate. Its green emission when electrons hitting the screen is matched well with the respond of the CCD camera. The screen can be moved in and out from the vacuum chamber using the air pressurized linear actuator. This paper presents about design, construction and tests of the screen stations at PCELL. The resolution of measurements for beam transverse profile and emittance is also be discussed.

Design, construction, and measurement of electromagnetic steering magnet for 25-MeV electron accelerator system

Mid-infrared and terahertz free-electron laser (MIR/THz FEL) facility is under the construction at the PBP-CMU Electron Linac Laboratory (PCELL) in Chiang Mai University. For producing MIR-FEL, electron beams with an energy of 25 MeV accelerated by a radio-frequency linear accelerator (RF linac) are required. Transporting the 25-MeV electron beam from the linac to the MIR-FEL station requires small dipole magnets for steering the electron beams along the beamline. This work focuses on the design, construction, and measurement of an electromagnetic steering magnet for controlling the electron beam trajectory. A computer software, CST Studio Suite 2022 is used for the 3D magnetic field simulation of the magnet. The physical length of the magnet and the diameter of copper coil are decided based on the simulation and calculation results. After the construction, a hall probe with a measuring system controlled by a computer program is used to measure the magnetic field for comparison with the simulation results. From simulated and calculated results, the steering magnet with a length of 40 mm and the coil with a diameter of 1.06 mm was chosen. At an applied current of 1.8 A, this steering magnet can bend the 25-MeV electron beam with a bending angle of 13.7 mrad. The measured magnetic field is consistent with the simulated results. The advantage of this steering magnet is that it can be assembled on a square frame for steering the beam in both horizontal and vertical direction. It can also be used for the THz-FEL beamline, which requires the electron beam with an energy of 10-16 MeV.

Nonlinear focusing of terahertz laser beam using a layered superconductor

We theoretically study the propagation of a Terahertz (THz) Gaussian beam through a thin sample of layered superconductor. We consider the beam axis and the superconducting layers to be perpendicular to the sample interface, while the electric field in the beam is perpendicular to the layers. We show that, in such a geometry, the Josephson current between the superconducting layers supports lensing of the beam instead of divergence on the Rayleigh range. Moreover, due to the nonlinearity, the focal length and waist of the transmitted beam depend on the incident beam intensity. These dependences demonstrate nontrivial hysteresis behavior that can be observed in experiments with THz lasers.

Terahertz Ratchet Effect in Interdigitated HgTe Structures

The emergence of ratchet effects in two-dimensional materials is strongly correlated with the introduction of asymmetry into the system. In general, dual-grating-gate structures forming lateral asymmetric superlattices provide a suitable platform for studying this phenomenon. Here, we report on the observation of ratchet effects in $\mathrm{Hg}\mathrm{Te}$-based dual-grating-gate structures hosting different band-structure properties. Applying polarized terahertz laser radiation we detect linear and polarization-independent ratchets, as well as a radiation-helicity-driven circular ratchet effect. Studying the ratchet effect in devices made of quantum wells (QWs) of different thickness, we observe that the magnitude of the signal substantially increases with decreasing QW width with a maximum value for devices made of QWs of critical thickness hosting Dirac fermions. Furthermore, sweeping the gate voltage amplitude we observe sign-alternating oscillations for gate voltages corresponding to $p$-type conductivity. The amplitude of the oscillations is more than two orders of magnitude larger than the signal for $n$-type conducting QWs. The oscillations and the signal enhancement is shown to be caused by the complex valence band structure of $\mathrm{Hg}\mathrm{Te}$-based QWs. These peculiar features of the ratchet currents make these materials an ideal platform for the development of terahertz applications.

High-power and efficient orthogonally-polarized dual-wavelength Nd:YLF laser

We demonstrate an actively Q-switched orthogonally-polarized dual-wavelength laser at 1047.4 nm and 1053.4 nm with high average power and high efficiency. The gain competition between dual-wavelength lasers is effectively suppressed by using two Nd:YLF crystals as laser gain medium. Investigations concerning different resonator structures and pulse repetition frequency were conducted to achieve the best performance of this laser. Under the total incident pump power of 32.7 W, a maximum dual-wavelength laser output power of 13.4 W is obtained at the pulse repetition rate of 30 kHz, corresponding to an optical conversion efficiency of 41%. For 1047.4 nm and 1053.4 nm lasers, the maximum average output powers are as high as 7.6 W and 5.8 W, respectively, and they also demonstrate good output power stability and beam quality. This orthogonally-polarized dual-wavelength laser has significant practical applications in various fields, such as terahertz radiation generation, precision metrology, laser radar, and spectral analysis.

Effects of external perturbations on light absorption by light/heavy hole excitons in a semi-parabolic quantum well

The effects of electric, magnetic, and intense terahertz laser fields on optoelectronic properties are investigated in a GaAs/AlxGa1-xAs semi-parabolic quantum well. The binding energy, exciton light absorption coefficients, interband transition energies, and dipole elements in the matrix are studied within the structure of the parabolic band and effective-mass approximations. The transition energy difference between the ground-state electron energy level and the ground-state heavy hole (light hole) energy level and matrix elements are carried out as functions of external perturbations with a constant well size. The results exhibit that the exciton binding energies of heavy and light holes reduce as long as the laser dressing parameter is larger. The optical absorption of light hole exciton in the system is stronger than the heavy hole exciton. And the excitonic energies, transition energies, and absorption coefficients are much more sensitive and controlled by changing the external perturbations.

Temporal behavior of the high-power pulsed gas terahertz laser pumped by a fundamental mode TEA CO2 laser.

Optically pumped gas molecular terahertz (THz) lasers are promising for generating high-power and high-beam-quality coherent THz radiation. However, for pulsed gas THz lasers, the temporal behavior of the output THz pulse has rarely been investigated. In this study, the temporal behavior of a pulsed gas THz pumped by a fundamental-mode TEA CO2 laser has been presented for the first time both in simulation and experiment. A modified laser kinetics model based on the density matrix rate equation was used to simulate the temporal behavior and output pulse energy of a pulsed gas THz laser at different gas pressures. The results clearly show that the working gas pressure and pump pulse energy have critical influences on the output THz pulse shape. Three typical pulse shapes were obtained, and the THz pulse splitting caused by gain switching was quantitatively simulated and explained based on the laser dynamic process. Besides, with an incident pump pulse energy of 342 mJ, the maximum output THz pulse energy of 2.31 mJ was obtained at 385 µm, which corresponds to a photon conversion efficiency of approximately 56.1%, and to our knowledge, this is the highest efficiency for D2O gas THz laser. The experimental results agreed well with those of the numerical simulation for the entire working gas pressure range, indicating that our model is a powerful tool and paves the way for designing and optimizing high-power pulsed gas lasers.

Anisotropic photoelectric properties in GaN/AlN quantum dots under terahertz laser polarization

The linear and nonlinear anisotropic optical absorption coefficients(OACs) and refractive index changes(RICs) of GaN/AlN cubical quantum dots(CQDs) subjected to terahertz laser field(TLF) are investigated. The results show that under the polarization of TLF, with the increase of laser dressing parameters, the peak of OACs and RICs increases nonlinearly and have a redshift. More importantly, our work indicated that with the enhancement of TLF polarization, the redistribution of electron excited state wave function will lead to the “loss” of optical properties in the specific frequency band. The redistribution of electron ground state wave function has a significant modulation effect on the magnitude of OACs(RICs) peak. • The enhancement of the refractive index changes was observed at low frequencies. • The phenomenon of wave function exchange in laser field modulation is found. • The modulation of laser field on optical coefficient cannot cover the whole frequency band.

Studies of energetic particle transport induced by multiple Alfvén eigenmodes using neutron and escaping energetic particle diagnostics in Large Helical Device deuterium plasmas

Studies of energetic particle transport due to energetic-particle-driven Alfvénic instability have progressed using neutron and energetic particle diagnostics in Large Helical Device deuterium plasmas. Alfvénic instability excited by injecting an intensive neutral beam was observed by a magnetic probe and a far-infrared laser interferometer. The interferometer showed Alfvénic instability composed of three modes that existed from the core to the edge of the plasma. A comparison between the observed frequency and shear Alfvén spectra suggested that the mode activity was most likely classified as an Alfvénic avalanche. A neutron fluctuation detector and a fast ion loss detector indicated that Alfvénic instability induced transport and loss of co-going transit energetic ions. The dependence of the drop rate of the neutron signal on the Alfvénic instability amplitude showed that significant transport occurred. Significant transport might be induced by the large amplitude and radially extended multiple modes, as well as a large deviation of the energetic ion orbit from the flux surface.

Simulation of far-infrared HgTe/HgCdTe quantum-well vertical-cavity surface-emitting laser

We propose an original design for a far-infrared vertical-cavity surface-emitting laser based on a heterostructure with ten 4.7-nm HgTe / Hg0.3Cd0.7Te quantum wells grown on a [013] GaAs substrate. Its characteristics were calculated, and it was shown that the quality factor of the proposed resonator design can reach 248 at a wavelength of 23.25 μm, which is sufficient for the occurrence of laser generation at this wavelength at a temperature of 20 K and a concentration of nonequilibrium carriers of 1.4 × 1010 cm − 2. The estimate of the threshold pump intensity using CO2 laser radiation at a wavelength of 10.6 μm is 1 W / cm2, which makes it possible to expect continuous wave mode lasing. Such a low-threshold pump intensity should also lead to lasing at substantially higher temperatures.

Intense terahertz laser field induced electro-magneto-donor impurity associated photoionization cross-section in Gaussian quantum wires

Using expressions derived within the compact density matrix approach, the peaks of optical absorption and the changes of refractive index of a hydrogenic impurity in a GaAs/GaAlAs Gaussian quantum well wire are calculated taking into account the influence of static electric, magnetic and intense laser fields. The photoionization cross section, with normalized photon energy, is computed for different values of static electric field, magnetic field, and laser dressing parameter. The dipole moment matrix elements and the transition energy between ground and first excited state energy levels are computed as functions of quantum wire width and confinement potential depth, with and without external perturbations. Donor impurity binding energy is investigated in presence of the aforementioned electromagnetic probes, using parabolic band and effective mass approximations. The results show that the absorption coefficients depend on the transition energy difference between initial and final states involved as well as on the corresponding polarization response via dipole matrix elements. Discussed optical properties are field-sensitive and they can be tuned within the desired energy ranges using these external perturbations. • Peaks of optical absorption and changes of refractive index of a hydrogenic impurity in a GaAs/GaAlAs Gaussian quantum well wire are calculated. • They have been computed taking into account the influence of static electric, magnetic and intense laser fields. • Photoionization cross section, with normalized photon energy, is computed. • Donor impurity binding energy is investigated in the presence of aforementioned electromagnetic-probes, using parabolic band and effective mass approximations. • Discussed optical properties are field-sensitive and they can be tuned within the desired energy ranges using these external perturbations.