THz Laser Research Articles

Lensless Fourier-Transform Terahertz Digital Holography for Full-Field Reflective Imaging

Continuous-wave terahertz digital holography (TDH) is a full-field lensless phase imaging approach usually with the coherent THz laser. It has the potential to be applied to nondestructive testing. In order to simplify the reconstruction and utilize the THz radiation with higher efficiency, a full-field reflective lensless Fourier-transform TDH (RLF-TDH) configuration is proposed with oblique illumination mode based on 2.52 THz radiation. A spherical reference beam is generated by a reflective concave mirror in order to reduce the loss of THz radiation, which is different from other configurations of the same kind. In the reconstruction process, the complex-amplitude image can be obtained by directly applying single Fourier transform to the digital hologram; thus, it is very possible to achieve real-time imaging. A tilted plane correction method is implemented to correct the anamorphism caused by the nonparallel planes between the object and recording plane. The profile information of the object can be measured from the unwrapped, aberration-free phase image. Two reflective gold-coated samples are adopted to demonstrate the validity of the RLF-TDH imaging system.

Selective tuning of order parameters of multiferroicBiFeO3in picoseconds using midinfrared terahertz laser pulses

Selectively tuning the properties of a material with multiple order parameters, such as polarization and magnetization, requires precise control of the crystal structure. Recently developed midinfrared THz laser techniques provide an efficient way to manipulate the structure of materials at an ultrafast rate. This paper presents a methodology to evaluate the possibility of picosecond multiple switching of order parameters in perovskite structure oxides using first-principles calculations. With multiferroic ${\mathrm{BiFeO}}_{3}$ as a model system, we demonstrate that the desired property can be tuned by an external laser pulse's perturbation with frequency 15.2 THz. The dynamics of phonon modes are illustrated, revealing that lasers enable switching of polarization and magnetization within 1 ps. The switching mechanisms can be attributed to the nonlinear coupling of the pumped infrared-active mode with the ferroelectric and antiferrodistortive modes. Our work provides an initiative for more detailed experimental investigations to explore light control orders and dynamic material design.

Effect of Razavy Potential Well Parameters on the Optical Rectification, Second, and Third Harmonic Generation Coefficients of Razavy Quantum Well in the Presence of Electric, Magnetic, and Thz Laser Fields

Effect of Razavy Potential Well Parameters on the Optical Rectification, Second, and Third Harmonic Generation Coefficients of Razavy Quantum Well in the Presence of Electric, Magnetic, and Thz Laser Fields

Terahertz exposure enhances neuronal synaptic transmission and oligodendrocyte differentiation in vitro

SummaryTerahertz (THz) frequency occupies a large portion of the electromagnetic spectrum that is between the infrared and microwave regions. Recent advances in THz application have stimulated interests regarding the biological effects within this frequency range. In the current study, we report that irradiation with a single-frequency THz laser on mice cortical neuron cultures increases excitatory synaptic transmission and neuronal firing activities. Microarray assay reveals gene expression dynamics after THz exposure, which is consistent with morphology and electrophysiology results. Besides, certain schedule of THz irradiation inhibits the proliferation of oligodendrocyte precursor cells (OPCs) and promotes OPC differentiation. Of note, the myelination process is enhanced after THz exposure. In summary, our observations suggest that THz irradiation can modulate the functions of different neuronal cells, with different sensitivity to THz. These results provide important understanding of the mechanisms that govern THz interactions with nervous systems and suggest THz wave as a new strategy for neuromodulation.

Terahertz monolayer metamaterials modulated by high power THz lasers

The Terahertz laser modulating processes of monolayers of spheres have been theoretically investigated by using density functional and electromagnetic scattering method. The introduced free energy including static energy, electronic excitations and atomic vibrations was calculated by the density functional method. Thus, the heat capacity, the coefficient of thermal expansion and dielectric constant of materials could be obtained at various temperatures. When the high power THz laser passes through a CdTe sphere, the temperature traces can be changed and calculated by setting any laser waveforms. At a special frequency point, the real parts of permittivity could exhibit an oscillation with double modulation frequency. Furthermore, it was found that the resonance of monolayers could be dynamically tuned, and the oscillation behaviors of transmission are consistent with the modulation waveform. Finally, the resonant frequencies at Terahertz regions could be shifted to a given frequency by adjusting the lattice constants of monolayers.

Phase change of Ge2Sb2Te5 under terahertz laser illumination

Ge2Sb2Te5 (GST) is the typical phase change material (PCM) that can reversibly transform between the amorphous (a) and crystalline (c) states. Because the optical properties are phase-dependent, GST has been widely used in various photonic applications, such as optical switches and non-volatile memories. Currently, the photonic applications of the GST PCM have been demonstrated by employing lasers in visible and infrared wavelengths. Extending the photonic applications into other wavelengths is much demanded. Here, we investigate the phase change of the GST material illuminated by using a terahertz quantum cascade laser emitting around 2.5 THz. A finite-element simulation is employed to study the temperature and transmission changes induced by terahertz irradiation. It reveals that the phase change can be achieved and the transmission is reduced by 30% when the GST material is irradiated by the 2.5 THz laser light. Furthermore, a z-scan technique experimentally presents the phase change behaviors of GST illuminated by the terahertz light, which is visually proved by transmission electron microscopy. Our work paves a way for the applications of GST materials in the terahertz optical components, e.g., modulators and switches.

Anderson pseudospin and Superradiant Superconductivity revisited

Using Anderson pseudospins (Anderson, 1958; Bardeen et al., 1957), I suggested in 2012 (Baskaran, 2012) superradiant superconductivity (SRSC) and related Dicke phenomena in bulk superconductors. It was a response to a report from Cavalleri’s lab (Kaiser et al., 2012; Fausti et al., 2011), of transient room temperature superconductivity in YBCO, when pumped by 20 THz laser pulses. In my minimal model, Anderson pseudospins in k-space in a bulk superconductor were coupled to single polariton mode. The model was adapted to the pseudogap normal state of YBCO to explain observed transient superconductivity. We also pointed out, possibility of other Dicke phenomena, (i) spontaneous superradiance above a critical polariton matter coupling, with no pumping, (ii) enhanced quantum entanglement near the critical coupling and (iii) superfluoresence. Here we revisit our early work. Recent discovery by Ebbesens group (Thomas et al., 2019) of large increase of superconducting Tc from 30 K to 45 K in K3C60 via specially designed plasmon–phonon-polariton mode, but with no external pumping seems to be in line with our early prediction. Relevance of SRSC to developments, including other superradiant matters and Higgs Spectroscopy is pointed out.

Linear and nonlinear optical responses in Kitaev spin liquids

We theoretically study THz-light-driven high-harmonic generation (HHG) in the spin-liquid states of the Kitaev honeycomb model with a magnetostriction coupling between spin and electric polarization. To compute the HHG spectra, we numerically solve the Lindblad equation, taking account of the dissipation effect. We find that isotropic Kitaev models possess a dynamical symmetry, which is broken by a static electric field, analogous to HHG in electron systems. We show that the HHG spectra exhibit characteristic continua of Majorana fermion excitations, and their broad peaks can be controlled by applying static electric or magnetic fields. In particular, the magnetic-field dependence of the HHG spectra drastically differs from those of usual ordered magnets. These results indicate that an intense THz laser provides a powerful tool to observe dynamic features of quantum spin liquids.

The ferroelectric photo ground state of SrTiO3: Cavity materials engineering.

Optical cavities confine light on a small region in space, which can result in a strong coupling of light with materials inside the cavity. This gives rise to new states where quantum fluctuations of light and matter can alter the properties of the material altogether. Here we demonstrate, based on first-principles calculations, that such light-matter coupling induces a change of the collective phase from quantum paraelectric to ferroelectric in the [Formula: see text] ground state, which has thus far only been achieved in out-of-equilibrium strongly excited conditions [X. Li et al., Science 364, 1079-1082 (2019) and T. F. Nova, A. S. Disa, M. Fechner, A. Cavalleri, Science 364, 1075-1079 (2019)]. This is a light-matter hybrid ground state which can only exist because of the coupling to the vacuum fluctuations of light, a photo ground state The phase transition is accompanied by changes in the crystal structure, showing that fundamental ground state properties of materials can be controlled via strong light-matter coupling. Such a control of quantum states enables the tailoring of materials properties or even the design of novel materials purely by exposing them to confined light.

Current induced by a tilted magnetic field in phosphorene under terahertz laser radiation

In this study, we investigate the cyclotron resonance effect in the first-order AC current, magnetic ratchet effect, and second harmonic generation in phosphorene in the presence of a steady tilted magnetic field and under THz laser radiation. We establish that various cyclotron resonances exist in the deduced currents based on the angular frequency of the incoming light. These resonances are dependent on the $\omega_{c_{+}}$ value, a function of the carrier charge, the perpendicular magnetic field, and the effective masses along the armchair or zigzag edges. We discuss the direction and the magnitude of the deduced currents for various radiation polarizations. We compare the results with a zero perpendicular magnetic field. Cyclotron resonance for the first order AC current occurs at $\omega= \pm \omega_{c_{+}}$. The deduced current declines if the perpendicular magnetic field is zero. Meanwhile, for the ratchet current, cyclotron resonances occur at $\omega= \pm \omega_{c_{+}}$, $\omega= \pm 2\omega_{c_{+}}$, and radiation helicity affects the deduced current for circularly polarized light. Cyclotron resonance for the second harmonic generation current occurs at $\omega= \pm \omega_{c_{+}}$, $\omega= \pm 2 \omega_{c_{+}}$ and $\omega= \pm \omega_{c_{+}}/2 $ and the current is stronger compared to the case with no perpendicular magnetic field. As the magnetic field rotates in the plane of anisotropic phosphorene, separate directions are predicted for the second harmonic generation--related current. It is noteworthy that the magnitude of the ratchet and second harmonic generation current are within the same range and comparable to the magnetic ratchet current in monolayer graphene, $\mu A / cm$.

Terahertz emission spectroscopy of GaN-based heterostructures

This study investigates the band bending profile of gallium-nitride-based heterostructures with respect to the change in the direction of the induced current in the out-of-plane plane direction, which gives the phase of the terahertz emission peak and can be detected by laser THz emission microscopy. We use a wavelength-tunable laser THz emission spectroscope to observe the transport of charges within the band bending regions, excitation of nearby photocarriers, and carrier scattering. We observe flip-flop peak-to-peak THz emission waveforms for a GaN heterostructure field-effect transistor (FET) compared to a strong THz emission radiated from an Al0.3Ga0.7N/GaN high-electron-mobility-transistor (HEMT). The flip-flop THz emission indicates the emission from various interfaces inside the FET structure, and intense THz emission from the HEMT structure indicates band-edge excitation. Our results provide a valuable perspective for characterizing complex heterostructures that provide insight into possible defects, carrier mobilities, and band bending of multilayer interface electronic devices.

Control of tightly focused laser beams in the THz range

AbstractThe 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.

Robust light beam diffractive shaping based on a kind of compact all-optical neural network.

A kind of compact all-optical learning-based neural network has been constructed and characterized for efficiently performing a robust layered diffractive shaping of laser beams. The data-driven control lightwave strategy demonstrates some particular advantages such as smart or intelligent light beam manipulation, optical data statistical inference and incident beam generalization. Based on the proposed method, several typical aberrated light fields can be effectively modulated into the desired fashion including the featured flat-top beams, an arrayed sub-beam arrangement and complex annular fringes compared with conventional GS-based DOEs. An actual THz laser is utilized to evaluate the effectiveness of the method developed.

THz laser generation on a hybrid surface plasmon in a HgCdTe-based structure

The possibility of amplifying a THz hybrid surface plasmon in a structure with an Hg0.82Cd0.18Te epitaxial film grown on a GaAs substrate and covered with a metal layer is investigated. It is shown that for a film thickness of 100 nm and a temperature of 80 K, the hybrid surface plasmon mode gain can be greater than external losses at a pump radiation intensity with a wavelength of 2.3 μm, exceeding 850 kW cm−2. Additional doping of the Hg0.82Cd0.18Te layer with a donor impurity having a concentration of 4 × 1017 cm−3 will lead to a 1.5-fold decrease in the threshold pump intensity.

Застосування градієнтних металоплівкових вихідних дзеркал у газорозрядних лазерах терагерцового діапазону

Subject and Purpose. The problem of output beam matching with the waveguide transmission line often arises when using terahertz lasers. The special quasi-optical devices that are used to combat the problem lead to radiation losses and additional material costs. The aim of this work is to develop appropriate output mirrors for THz lasers so as to make the output laser beam match the transmission line of a given diameter. Methods and Methodology. As part of the experimental research into the action of the output mirror configuration on the parameters of THz laser radiation, gradient metal-film mirrors are made like a transparent substrate with a thin metal layer on it. The layer thickness varies over the substrate surface, forming therewith a transparency spot with a given change in the transmittance in the middle of the mirror. The properties of the gradient metal-film output mirrors are examined as applied to a gas-discharge HCN laser at a wavelength of 337 μm. Results. The gradient metal-film mirrors enable obtaining laser beams of a diameter equal to the inner diameter of the relevant transmission line and with a proper transverse energy distribution. The efficiency of the transmission of laser radiation energy into the waveguide line increases, and so does the performance of the laser installation. Another positive point is that the enhanced transparency in the middle of the mirror raises efficiency of the active substance energy utilization and conveys focusing properties to the flat mirror, which reduces diffraction losses. Conclusions. The employment of gradient metal-film output mirrors makes it possible to raise efficiency of laser installations. The features of metal-film mirrors suggest broad potentials for their application and make reasonable their further research.

Power and efficiency enhancement of THz radiation in a plasma by dark hollow laser beams: Effects of magnetic field

The power and efficiency of terahertz radiation (THz) in a rippled density plasma by dark hollow laser beams are very small due to the collisional effect. We show that when a magnetic field is applied perpendicularly to the plasma, the THz radiation power and efficiency significantly enhance. Also, the magnetic field changes the THz radiation profile to a profile similar to a dark hollow laser beam. The mutual effect between the laser beams and THz radiation in plasma, which is important in high energy transfer between laser beams and THz radiation, is taken into account. We find that the maximum efficiency obtained in our scheme is more than four orders of magnitude greater than the efficiency in the absence of magnetic field.

Tunable terahertz gas laser based on a germanium spectrum splitter

A tunable terahertz gas laser is demonstrated with a germanium ealton served as the spectrum splitter. The germanium ealton is 1.2-mm thick and anti-reflection coated at 10.17 μm, giving good dichroic performance for infrared and far-infrared radiation. By tuning the incident angle of the germanium ealton, the transmittance of 95% at 9– 11 μm wavelengths and the reflectance of more than 70% at 180 – 500 μm wavelengths can be achieved simultaneously. Based on the germanium ealton, a tunable CH3F gas laser is presented when pumped by a transversely excited atmospheric CO2 laser. By tuning the pump lines and the incident angles, four THz laser wavelengths are obtained including 181 μm, 261 μm, 360 μm and 496 μm. The energy conversion efficiency is in the order of 10–3, which is comparable to those of typical efficient CH3F molecule lasers. The germanium ealton is anticipated to be an efficient dichroic element for terahertz gas lasers with different wavelengths.

Laser THz emission nanoscopy and THz nanoscopy

We present an experimental and theoretical comparison of two different scattering-type scanning near-field optical microscopy (s-SNOM) based techniques in the terahertz regime; nanoscale reflection-type terahertz time-domain spectroscopy (THz nanoscopy) and nanoscale laser terahertz emission microscopy, or laser terahertz emission nanoscopy (LTEN). We show that complementary information regarding a material's charge carriers can be gained from these techniques when employed back-to-back. For the specific case of THz nanoscopy and LTEN imaging performed on a lightly p-doped InAs sample, we were able to record waveforms with detector signal components demodulated up to the 6th and the 10th harmonic of the tip oscillation frequency, and measure a THz near-field confinement down to 11 nm. A computational approach for determining the spatial confinement of the enhanced electric field in the near-field region of the conductive probe is presented, which manifests an effective "tip sharpening" in the case of nanoscale LTEN due to the alternative geometry and optical nonlinearity of the THz generation mechanism. Finally, we demonstrate the utility of the finite dipole model (FDM) in predicting the broadband scattered THz electric field, and present the first use of this model for predicting a near-field response from LTEN.

Disentangling elastic and inelastic scattering pathways in the intersubband electron dynamics of n -type Ge/SiGe quantum fountains

$\mathit{n}$-type Ge/SiGe quantum wells have been suggested as a promising platform for the realization of a Si-compatible THz laser. Focusing on this material system, we have developed a numerical model to describe the intersubband carrier dynamics which restores the equilibrium after pulsed optical excitation in asymmetric coupled Ge/SiGe quantum wells. We take into account inelastic and elastic scattering processes and investigate different quantum-well geometries, doping densities, and excitation regimes. In this configuration space, we disentangle the effect on the overall dynamics of each scattering channel and provide intersubband relaxation times, finding larger values with respect to III-V based materials, thanks to the weaker electron-phonon coupling with respect to III-V compounds. Finally, the model is used to study and optimize the population inversion between the first- and second-excited subband levels and to assess its dependence on the lattice temperature, providing a sound theoretical framework to guide forthcoming experiments.

Photonic integrated circuits for terahertz source generation

The authors introduce four kinds of terahertz photonics components based on photonic integrated circuits (PICs). A PIC-based integrated optoelectronic synthesiser for THz communications is described, which can be tuned continuously over the range 0.254-2.723 THz using photomixing. A laterally-coupled dual-grating distributed feedback laser (DFB) diode integrated with an electroabsorption modulator is used to generate an 820 GHz beat signal. THz signal production is reported using a dual-wavelength DFB diode laser with a two-section phase-shifted sampled Bragg grating. Finally, a THz source at 640 GHz, based on a sampled grating distributed Bragg reflector semiconductor mode-locked laser diode, is reported offering superior reproducibility, controllability, and a wider operation range than other reported mode-locked THz laser diodes. Each of these sources is a monolithic construction emitting light at 1.5 μm. The light can be amplified in an erbium-doped fibre amplifier, delivered over silica optical fibre and used to generate THz radiation via a photodiode antenna or photoconductive antenna in a remote location.