Terahertz Emission Research Articles

Возбуждение терагерцевого излучения в p-n-гетероструктурах на основе a-Si:H/c-Si

The results of studying the generation of terahertz (THz) radiation in p-n-heterostructures based on a-Si:H//c-Si upon their photoexcitation by radiation from a femtosecond titanium-sapphire laser with a wavelength of 800 nm are presented. The properties of the observed THz radiation can be explained by excitation of fast photocurrent of nonequilibrium charge carriers created in the region of the potential barrier under femtosecond interband photoexcitation of the structure. The fast photocurrent, in turn, emits a THz electromagnetic wave. The waveforms and amplitude spectra of the observed THz radiation reflect the dynamics of photoexcited charge carriers in structures. In terms of intensity, the THz radiation observed from the studied p–n heterostructures based on a-Si:H//c-Si is comparable to the THz radiation generated in n-InAs crystals, which are widely used as emitters in systems of THz time-domain spectroscopy. Therefore, a-Si:H//c-Si p-n-heterostructures can be used as THz emitters for solving tasks of THz spectroscopy.

Probing Interface of Perovskite Oxide Using Surface-Specific Terahertz Spectroscopy

The surface/interface species in perovskite oxides play essential roles in many novel emergent physical phenomena and chemical processes. With low eigen-energies in the terahertz region, such species at buried interfaces remain poorly understood due to the lack of feasible surface-specific spectroscopic probes to resolve the resonances. Here, we show that polarized phonons and two-dimensional electron gas at the interface can be characterized using surface-specific nonlinear optical spectroscopy in the terahertz range. This technique uses intra-pulse difference frequency mixing process, which is allowed only at the surface/interface of a centrosymmetric medium. Submonolayer sensitivity can be achieved using the state-of-the-art detection scheme for the terahertz emission from the surface/interface. Through symmetry analysis and proper polarization selection, background-free Drude-like nonlinear response from the two-dimensional electron gas emerging at the LaAlO 3 /SrTiO 3 or Al 2 O 3 /SrTiO 3 interface was successfully observed. The surface/interface potential, which is a key parameter for SrTiO 3 -based interface superconductivity and photocatalysis, can now be determined optically in a nonvacuum environment via quantitative analysis on the phonon spectrum that was polarized by the surface field in the interfacial region. The interfacial species with resonant frequencies in the THz region revealed by our method provide more insights into the understanding of physical properties of complex oxides.

Terahertz Emission in Quantum Materials

The study of the interaction between terahertz (THz) radiation and quantum materials has been an active area of research due to its potential for understanding fundamental physics and the development of novel technologies. In this review, we focus on the time-dependent photocurrents, behind which are the up-to-date understanding of the physical processes. We provide the recent advancements in revealing the unique properties of quantum materials via the THz emission spectroscopy. Because the theoretical interpretation of some new experimental results is still evolving, this review is intended to inspire further research in this exciting and rapidly growing field.

NH3 laser THz emission under optical pumping by “long” (∼100 µs) CO2 laser pulses

Terahertz (THz) NH3 lasing with optical pumping by electron-beam-sustained discharge “long” (∼100 µs) CO2 laser pulses was obtained. The NH3 laser emission pulses and the “long” pulses of the CO2 pump laser were simultaneously measured with nanosecond response time. The NH3 lasing duration and its delay with respect to the pump pulse were measured for various CO2 laser pulse energies. For the CO2 laser pump line 9R(30), three wavelengths of 67.2, 83.8, and 88.9 µm were recorded. For the CO2 laser pump line 9R(16), only a single NH3 laser line with a wavelength of 90.4 µm was detected.

Dichlorinated Organic‐Salt Terahertz Sources for THz Spectroscopy

AbstractAlthough in terahertz (THz) source materials molecular anions significantly influence the performance of THz generation, only limited classes of molecular counter anions have been reported. Here, utilizing dichlorinated molecular anions in THz generators is reported for the first time, to the best of our knowledge. In these new crystals, two dichlorinated molecular anions with different molecular symmetries, asymmetric 3,4‐dichlorobenzenesulfonate (34DCS) and symmetric 3,5‐dichlorobenzenesulfonate (35DCS), are incorporated with a 2‐(4‐hydroxystyryl)‐1‐methylquinolinium (OHQ) cation possessing top‐level molecular optical nonlinearity. OHQ‐34DCS exhibits a strong nonlinear optical response, in contrast to OHQ‐35DCS. In OHQ‐34DCS crystals, the dichlorinated groups form strong halogen bonds (XBs) and hydrogen bonds (HBs), which are beneficial for suppressing molecular (phonon) vibrations. The optical‐to‐THz conversion efficiency of the OHQ‐34DCS crystals is extremely high, comparable to that of the benchmark organic THz generators. Moreover, the THz emission spectra from the OHQ‐34DCS crystals, compared to those of previously reported benchmark analogous crystals, are stronger modulated toward a flatter shape, but possess substantially reduced spectral dimples. Therefore, the introduction of dichlorinated molecular anions is an efficient approach for the design of highly efficient electro‐optic salt crystals as efficient broadband THz wave sources.

Nanoengineered Spintronic-Metasurface Terahertz Emitters Enable Beam Steering and Full Polarization Control.

The demand for emerging applications at the terahertz frequencies motivates the development of novel and multifunctional devices for the generation and manipulation of terahertz waves. In this work, we report the realization of multifunctional spintronic-metasurface emitters, which allow simultaneous beam-steering and full polarization control over a broadband terahertz beam. This is achieved through engineering individual meta-atoms with nanoscale magnetic heterostructures and, thus, implementing microscopical control over the laser-induced spin and charge dynamics. By arranging the spintronic meta-atoms in the metagrating geometry, the generated terahertz beam can be flexibly steered in space between different orders of diffraction. Furthermore, we demonstrate a simultaneous control over the terahertz polarization states at different emission angles and show that the two control capabilities are mutually independent of each other. The nanoengineered multifunctional terahertz emitter demonstrated in this work can provide a solution to the challenge associated with a growing variety of applications of terahertz technology.

Tunable polarized terahertz wave generation induced by spontaneous polarization-dependent ultrafast shift current from vertically grown ferroelectric SnS

Ferroelectric group-IV monochalcogenides $MX$ ($M=\mathrm{Sn}/\mathrm{Ge}$ and $X=\mathrm{S}/\mathrm{Se}$) are of particular importance for the new-generation photovoltaic devices due to their special in-plane spontaneous polarization resulting in a bulk photovoltaic effect. However, the spontaneous polarization and its related dynamical photoresponse under an ultrafast laser excitation are rarely studied via a noncontact all-optical method. Herein, the vertically grown SnS nanosheets have been synthesized by a physical vapor deposition (PVD) method and the corresponding spontaneous polarization is characterized via the terahertz (THz) emission spectroscopy. The primary THz emission mechanism is ascribed to the shift current, which is proved by the THz wave dependences on the pump light polarization angle and pump fluence. Interestingly, the shift current depends on the spontaneous polarization along the SnS nanosheet grown direction, according to the reversed polarity of the THz wave under opposite directional excitations. Furthermore, the amplitude and phase of the THz wave are modulated by the pump light polarization angle, resulting in the elliptical THz wave emission with tunable ellipticity and orientation angle. Our study has shed light on the shift current of ferroelectrics and controllable polarized THz wave generation, which provides application prospects for ferroelectric-based photovoltaics, nonlinear photonics, and THz polarized devices.

Unidirectional spintronic terahertz emitters with high efficiency.

Due to the high performance and low cost, spintronic terahertz emitters (STEs) have been a hot topic in the field of terahertz sources. However, most of the research focuses on the THz generation process and little attention has been paid to the control and modulation of the THz wave generated by the STE. In this Letter, a unidirectional spintronic terahertz emitter (USTE) integrating a common STE with a metal grating is proposed to manipulate the THz emission process. The dyadic Green's function method and finite element method are adopted to survey the characteristics of the USTE. Simulations show that the metal grating not only has a transmission larger than 97% in the optical band, but also has a higher reflectivity larger than 99% in the THz band. As a result, the USTE has a unidirectional THz emission along the direction of the pump beam with a larger than 4-fold enhancement in intensity. Moreover, the USTE has the capability of tuning the central frequency and THz wave steering by adjusting the distance and angle between the STE and the metal grating. We believe that this USTE can be used in THz wireless communications and holographic imaging, especially in the field of THz bio-sensing, which needs some resonance frequencies to sense.

Nonlinear mixing-based terahertz emission in inclined rippled density plasmas

We propose to investigate the THz field generation using nonlinear mixing mechanism of laser beat wave with inclined rippled density plasmas. Two laser pulses with frequencies (ω 1, ω 2) and wave vectors (k 1, k 2) co-propagate and resultant laser beat wave forms at beat frequency (ω 1–ω 2). Laser beat wave imparts a nonlinear force on the ambient electrons and pushes them outward with nonlinear velocity v NL. Coupling of induced density perturbation and nonlinear velocity v NL generates nonlinear currents at laser beat frequency that further generates electromagnetic field E (ω 1–ω 2) in terahertz (THz) range. In the present scheme, density ripples are introduced at an angle with respect to laser propagation and flat Gaussian index (f) is introduced in laser field profile that transform curved top of Gaussian field envelope into flat top field envelope. The combined effect of flat laser pulses with inclined density ripples in plasmas shows 10-fold enhancement in THz field amplitude when flat-Gaussian index (f) varies from 1 to 4. Also, the THz field intensifies when density ripples inclination increases upto a certain angle and then decreases.

Enhanced forward THz yield from the long DC-biased femtosecond filament

We observed huge, almost two orders of magnitude, increase in the energy of the forward terahertz (THz) emission by the long femtosecond filament if transverse electrostatic field is applied along its full length. This proves summation of emission both from different parts of a single filament and from multiple filaments. The observed THz radiation has a narrow angular shape with a maximum in the direction of laser radiation and a spectrum in the range of 0.05–0.3 THz with a maximum at about 0.1 THz. Numerical simulations based on the UPPE well explain the experimental findings.

Ultrafast Drift Current Terahertz Emission Amplification in the Monolayer WSe2/Si Heterostructure.

Two-dimensional transition metal dichalcogenides (TMDs) have great potential application for seamless on-chip integration due to their strong photon-electron-spin-valley coupling. However, the contact-free measurements of the valley-coupled photocurrent in TMDs is still challenging. Here, ultrafast terahertz emission spectroscopy is employed to investigate the photocurrent dynamics in monolayer WSe2, and an interface-induced drift current amplification is found in the WSe2/Si heterostructure. The amplification of terahertz emission comes from the photocurrent enlarged by band bending in the WSe2 and Si junction, and the amplification ratio increase further near the valley resonant transition of WSe2. In addition, the valley-momentum locked photocurrent in the WSe2/Si heterostructure reserves the same chirality with monolayer WSe2 at room temperature. These findings could provide a new method for manipulating valley-momentum locked photocurrent by photon helicity and open new avenues for TMD-based valley-polarized terahertz emission devices.

Development of a hardened THz energy meter for use on the kilojoule-scale, short-pulse OMEGA EP laser.

A highly adaptable and robust terahertz (THz) energy meter is designed and implemented to detect energetic THz pulses from high-intensity (>1018 W/cm2) laser-plasma interactions on the OMEGA EP. THz radiation from the laser driven target is detected by a shielded pyrometer. A second identical pyrometer is used for background subtraction. The detector can be configured to detect THz pulses in the 1mm to 30μm (0.3- to 10-THz) range and pulse energies from joules to microjoules via changes in filtration, aperture size, and position. Additional polarization selective filtration can also be used to determine the THz pulse polarization. The design incorporates significant radiation and electromagnetic pulse shielding to survive and operate within the OMEGA EP radiation environment. We describe the design, operational principle, calibration, and testing of the THz energy meter. The pyrometers were calibrated using a benchtop laser and show linear sensitivity to up to 1000 nJ of absorbed energy. The initial results from four OMEGA EP THz experiments detected up to ∼15μJ at the detector, which can correspond to hundreds of mJ depending on THz emission and reflection models.

Parallel generation and coding of a terahertz pulse train

The generation and coding of multi-cycle terahertz (THz) pulses offer interesting possibilities, such as frequency comb spectroscopy or ultra-fast packet communication. In contrast to the radio frequency domain, which has largely exploited packet communication, this research area is almost unexplored at THz frequencies. Indeed, because of the lack of fast modulation and detection methods at THz frequencies, current developments often rely on hybrid techniques mixing photonics and ultrafast electronics. Here, we present a method for the generation and modulation of a coded THz pulse train. Our scheme is based on the combination of a spintronic THz emitter (STE) with an echelon mirror and a digital micromirror device. This highly scalable configuration is capable of modulating a hundred or more THz pulses in parallel with sub-picosecond accuracy. Strikingly, the temporal resolution of our modulation scheme depends on geometric optics and not on a high-speed electronic device. Furthermore, our scheme confirms the ability of STEs to generate quasi-continuous THz pulses and offers a new photonic solution on dynamic THz pulse train control.

Terahertz Emission Spectroscopy of Ultrafast Coupled Spin and Charge Dynamics in Nanometer Ferromagnetic Heterostructures.

Due to its high sensitivity and because it does not rely on the magneto-optical response, terahertz (THz) emission spectroscopy has been used as a powerful time-resolved tool for investigating ultrafast demagnetization and spin current dynamics in nanometer-thick ferromagnetic (FM)/heavy metal (HM) heterostructures. Here, by changing the order of the conductive HM coating on the FM nanometer film, the dominant electric dipole contribution to the laser-induced THz radiation can be unraveled from the ultrafast magnetic dipole. Furthermore, to take charge equilibration into account, we separate the femtosecond laser-induced spin-to-charge converted current and the instantaneous discharging current within the illuminated area. The THz emission spectroscopy gives us direct information into the coupled spin and charge dynamics during the first moments of the light-matter interaction. Our results also open up new perspectives to manipulate and optimize the ultrafast charge current for promising high-performance and broadband THz radiation.

Rapid, noncontact, sensitive, and semiquantitative characterization of buffered hydrogen-fluoride-treated silicon wafer surfaces by terahertz emission spectroscopy

Advances in modern semiconductor integrated circuits have always demanded faster and more sensitive analytical methods on a large-scale wafer. The surface of wafers is fundamentally essential to start building circuits, and quantitative measures of the surface potential, defects, contamination, passivation quality, and uniformity are subject to inspection. The present study provides a new approach to access those by means of terahertz (THz) emission spectroscopy. Upon femtosecond laser illumination, THz radiation, which is sensitive to the surface electric fields of the wafer, is generated. Here, we systematically research the THz emission properties of silicon surfaces under different surface conditions, such as the initial surface with a native oxide layer, a fluorine-terminated surface, and a hydrogen-terminated surface. Meanwhile, a strong doping concentration dependence of the THz emission amplitude from the silicon surface has been revealed in different surface conditions, which implies a semiquantitative connection between the THz emission and the surface band bending with the surface dipoles. Laser-induced THz emission spectroscopy is a promising method for evaluating local surface properties on a wafer scale.

Generation and Control of Ultrafast Circular Photon Drag Current in Multilayer PtSe2 Revealed Via Terahertz Emission

AbstractThe generation and control of ultrafast photocurrents on the surface of transition‐metal dichalcogenides (TMDs) are essential for the development of advanced optoelectronic devices. A detailed understanding of the mechanism of ultrafast photocurrent generation is primarily needed to design functional devices. Here, it is reported that ultrafast photocurrents in multilayer PtSe2 are induced by the photon drag effect with high conversion efficiency. Particularly, under circularly polarized laser excitation, anisotropic in‐plane photon drag currents are generated, leading to the elliptically polarized terahertz (THz) radiation directly controlled by the helicity of the pump beam. Furthermore, a new model called “THz emission by the circular ac Hall effect” (TECacHE) is proposed to unveil the carrier transport process. It is observed that the origins of elliptically polarized THz emission controlled by helicity involve the pure circular photon drag current and the retardation induced by the Lorentz force. Besides, ultrafast ternary encoding is proposed and demonstrated by polarized THz emission. The results not only provide a new contactless detection model to study surface ultrafast photocurrents but also afford a fundamental investigation of the circular photon drag current in layered TMDs for THz telecommunication devices.

Polarization analysis of terahertz emission from Bi-2212 cross-whisker intrinsic Josephson junction devices and their refractive index

Polarization analyses of the terahertz (THz) emission from Bi2Sr2CaCu2O8+δ whisker crystals used for superconducting THz emitters were conducted. The THz emission mode was estimated by a simple polarization measurement, and a simulation study was conducted to examine the validity of the polarization analysis. The refractive index of whisker crystals revealed through the polarization analyses was greater than that of bulk single crystals and agreed well with our previous THz emission report. The simulation study suggested the complex plasma excitation mode of the THz emission, and an interpretation of the refractive index obtained in this study is provided.

Multifunctional terahertz microscopy for biochemical and chemical imaging and sensing

Laser-excited terahertz emission microscopy (LTEM) has exhibited great potential for studying the dynamic physical properties of various materials and device evaluation. In this study, an up-to-date version of LTEM, the terahertz chemical microscopy, was developed for biochemical and chemical imaging and sensing. By functionalizing a terahertz semiconductor emitter with an ion-sensitive membrane, a DNA aptamer, and a specific polymer, the change in the terahertz signal amplitude attributed to the surface electrical potential change was successfully detected. Accordingly, the measurement of calcium ions (Ca2+), stress biomarker cortisol, and 2, 4, 6-trinitrotoluene (TNT) explosive was achieved. Measured of charged Ca2+ was via the change in the electrical potential of the ion-sensitive membrane with ion accumulation. For non-charged cortisol and TNT measurements, the surface potential change was recorded by the conformational change of the negatively charged DNA aptamer bound to cortisol and the charge-transfer complex formation between TNT and polyethylenimine polymer, respectively. Moreover, the specificity of this sensing approach was demonstrated by molecular docking and measuring the interfering substances such as sodium ions, potassium ions, brain chemicals histamine and dopamine, and TNT analogues. The results showed that the developed multifunctional terahertz microscopy technique can be used for trace biochemical and chemical sensing via visualization of the terahertz amplitude distribution.

Terahertz Pulse Generation with Binary Phase Control in Nonlinear InAs Metasurface.

The effect of terahertz (THz) pulse generation has revolutionized broadband coherent spectroscopy and imaging at THz frequencies. However, THz pulses typically lack spatial structure, whereas structured beams are becoming essential for advanced spectroscopy applications. Nonlinear optical metasurfaces with nanoscale THz emitters can provide a solution by defining the beam structure at the generation stage. We develop a nonlinear InAs metasurface consisting of nanoscale optical resonators for simultaneous generation and structuring of THz beams. We find that THz pulse generation in the resonators is governed by optical rectification. It is more efficient than in ZnTe crystals, and it allows us to control the pulse polarity and amplitude, offering a platform for realizing binary-phase THz metasurfaces. To illustrate this capability, we demonstrate an InAs metalens, which simultaneously generates and focuses THz pulses. The control of spatiotemporal structure using nanoscale emitters opens doors for THz beam engineering and advanced spectroscopy and imaging applications.

Scaling of the terahertz emission from liquid water lines by plasma reshaping.

Liquids are proposed to be promising terahertz (THz) sources. However, the detected THz electric field is limited by the collection efficiency and saturation effect. A simplified simulation based on the interference of ponderomotive-force-induced dipoles indicates that, by reshaping the plasma, the THz radiation is concentrated in the collection direction. Experimentally, by using a cylindrical lens pair to form a line-shaped plasma in transverse section, the THz radiation is redirected, and the pump energy dependence follows a quadratic trend, indicating that the saturation effect is significantly weakened. As a result, the detected THz energy is enhanced by a factor of ∼5. This demonstration provides a simple but effective way of further scaling detectable THz signals from liquids.