Ultrafast Terahertz Research Articles

Insulator–metal transition in VO2 film on sapphire studied by broadband dielectric spectroscopy

Vanadium dioxide () is a favorable material platform of modern optoelectronics, since it manifests the reversible temperature-induced insulator-metal transition (IMT) with an abrupt and rapid changes in the conductivity and optical properties. It makes possible applications of such a phase-change material in the ultra-fast optoelectronics and terahertz (THz) technology. Despite the considerable interest to this material, data on its broadband electrodynamic response in different states are still missing in the literature. This hampers the design and implementation of the -based devices. In this paper, we combine the Fourier-transform infrared (FTIR) spectroscopy, THz pulsed spectroscopy (TPS), and four-contact probe method to study the films prepared by magnetron sputtering on a c-cut sapphire substrate. Considering different temperatures of a substrate and pressures of atmosphere, we reconstruct complex dielectric permittivity of film in the frequency range of 0.2–150 THz, along with its static conductivity. The dielectric response is modeled using Lorentz and Drude kernels, which make possible splitting contributions from vibrational modes and free charge carriers to the total dynamic conductivity. By studying at different substrate temperatures and atmosphere pressures, we show that IMT appears to be pressure-dependent, which we attribute to the different thermostatic conditions of a sample. Finally, we estimate somewhat optimal thickness and temperature of the film in metallic phase for the THz optoelectronic applications. Our finding should be useful for further developments of the -based devices and technologies.

Terahertz Nanoscopy on Low-Dimensional Materials: Toward Ultrafast Physical Phenomena.

Low-dimensional materials (LDMs) with unique electromagnetic properties and diverse local phenomena have garnered significant interest, particularly for their low-energy responses within the terahertz (THz) range. Achieving deep subwavelength resolution, THz nanoscopy offers a promising route to investigate LDMs at the nanoscale. Steady-state THz nanoscopy has been demonstrated as a powerful tool for investigating light-matter interactions across boundaries and interfaces, enabling insights into physical phenomena such as localized collective oscillations, quantum confinement of quasiparticles, and metal-to-insulator phase transitions (MITs). However, tracking the ultrafast nonequilibrium dynamics of LDMs remains challenging. Ultrafast THz nanoscopy, with femtosecond temporal resolution, provides a direct pathway to investigate and manipulate the motion of, for example, charges, currents, and carriers at ultrashort time scales. In this review, we focus on recent advances in THz nanoscopy of LDMs, with particular emphasis on the ultrafast dynamics of light-matter interaction. We provide a concise overview of recent advances and suggest future research directions in this impactful field of interdisciplinary science.

Above curie temperature ultrafast terahertz emission and spin current generation in a two-dimensional superlattice (Fe3GeTe2/CrSb)3

Abstract The increasing demand for denser information storage and faster data processing has fuelled a keen interest in exploring spin currents up to terahertz (THz) frequencies. The emergent two-dimensional (2D) intrinsic magnetic materials constitute a novel and highly controllable platform to access such femtosecond spin dynamics at atomic layer thickness. However, 2D van der Waals magnets are limited by their Curie temperatures (usually at low temperatures) to exhibit the functioning. Here, in a 2D superlattice (Fe3GeTe2/CrSb)3, we demonstrate ultrafast laser-induced spin current generation and THz radiation at room temperature, overcoming the challenge of its Curie temperature being only 206 K. In tandem with time-resolved magneto-optical Kerr effect measurements and first-principle calculations, we further elucidate the origin of the spin currents—a laser-enhanced proximity effect manifested as a laser-induced reduction of interlayer distance and enhanced electron exchange interactions, which causes transient spin polarization in the heterostructure. Our findings present an innovative, magnetic-element-free route for generating ultrafast spin currents in the 2D limit, underscoring the significant potential of laser THz emission spectroscopy in investigating laser-induced extraordinary spin dynamics.

Photon-Induced Ultrafast Multitemporal Programming of Terahertz Metadevices.

Dynamic terahertz (THz) metasurface can feature modulated and multiplexed electromagnetic functionalities, important for wave-based computation, six-generation communications, and other applications. The versatile dynamic switching typically relies on a series of complex or incompatible multifield activations, with excessive system complexity, additional loss, slow modulation speed, and inertial time-varying properties, limiting more widespread applications. Here, a photon-induced ultrafast programmable THz metadevice is reoprted in time-frequency dimensions with polarization-decoupled temporal responses. By the pixelated design with multi-materials and triggering switches, the multimodal modulation transcends the constraints inherent in materials, enabling the ultrafast programmable temporal evolution. All the resonances can be independently programmed at the working band from 0.6 to 2 THz. The tri-temporal (with switching time of 1.25, 1, and 4.75ps) and bi-temporal (with switching time of 2.25 and 4.75ps) dynamic manipulations are performed by all-optical driven molecularization process of hybrid metasurfaces loaded with silicon (Si)and germanium (Ge) under different polarizations. Combining these features, the temporally programmed THz logic gates are last experimentally demonstrated, possessing basic operation of XNOR, NOR, and OR,as a proof-of-concept application. This reported light-driven programmable THz flat-optics allows ultrafast hybrid molecularization processes and new possibilities for miniaturized, flexible, multifunctional, and temporally programmable integrated devices.

Ultrafast terahertz conductivity in epitaxial graphene nanoribbons: an interplay between photoexcited and secondary hot carriers

Abstract Optical pump-terahertz probe spectroscopy has been used to investigate ultrafast photo-induced charge carrier transport in 3.4 µm wide graphene ribbons upon scaling the optical pump intensity. For low pump fluences, the deposited pump energy is rapidly redistributed through carrier–carrier scattering, producing secondary hot carriers: the picosecond THz photoconductivity then acquires a negative sign and scales linearly with an increasing pump fluence. At higher fluences, there are not enough equilibrium carriers able to accept the deposited energy, directly generated (excess) carriers start to contribute significantly to the photoconductivity with a positive sign leading to its saturation behavior. This leads to a non-monotonic variation of the carrier mobility and plasmonic resonance frequency as a function of the pump fluence and, at high fluences, to a balance between a decreasing carrier scattering time and an increasing Drude weight. In addition, a weak carrier localization observed for the polarization parallel to the ribbons at low pump fluences is progressively lifted upon increasing the pump fluence as a result of the rise of initial carrier temperature.

Light-driven electrodynamics and demagnetization in FenGeTe2 (n = 3, 5) thin films

Two-dimensional materials-based ultrafast spintronics are expected to surpass conventional data storage and manipulation technologies, that are now reaching their fundamental limits. The newly discovered van der Waals (VdW) magnets provide a new platform for ultrafast spintronics since their magnetic and electrical properties can be tuned by many external factors, such as strain, voltage, magnetic field, or light absorption for instance. Here, we report on the direct relationship between magnetic order and Terahertz (THz) electrodynamics in FenGeTe2 (n = 3, 5) (FGT) films after being illuminated by a femtosecond optical pulse, studying their ultrafast THz response as a function of the optical pump-THz probe temporal delay. In Fe5GeTe2, we find clear evidence that light-induced electronic excitations directly influence THz electrodynamics similarly to a demagnetization process, contrasting with the effects observed in Fe3GeTe2, which are characterized by a thermal energy transfer among electrons, magnons, and phonons. We address these effects as a function of the pump fluence and pump-probe delay, and by tuning the temperature across the magnetic ordering Curie temperature, highlighting the microscopic mechanisms describing the out-of-equilibrium evolution of the THz conductivity. Finally, we find evidence for the incoherent-coherent crossover predicted by the Kondo-Ising scenario in Fe3GeTe2 and successfully simulate its light-driven electrodynamics through a three-temperature model. As indicated by these results, FGT surpasses conventional metals in terms of modulating their properties using an optical lever.

The effect of magnetic nanofilm morphology on spintronic terahertz emission performance

Femtosecond laser-driven spintronic terahertz (THz) emitters based on magnetic nanofilms are poised to be the next-generation mainstream THz radiation devices due to their low cost, high performance, ultra-broadband, and easy integration. The radiation performance of spintronic THz emitters is related to the material characteristics, heterostructure interfaces, pump laser, and magnetic field intensity. Additionally, the THz emission performance is greatly reliant on the material surface morphology. Here, we employed ultrafast THz scattering-type scanning near-field optical microscopy with nanoscale spatial resolution to obtain the static THz scattering nano-imaging of ferromagnetic/antiferromagnetic heterostructures (W/Co20Fe60B20/IrMn3). We established the relationship between surface morphology and THz scattering intensity. Utilizing laser-induced THz emission technology, we achieve injection and detection of nanoscale ultrafast spin current without the external magnetic field. The strong consistency of the THz emission nanoscopy with the atomic force microscopy topography demonstrates that the sample surface morphology is critical to the THz radiation performance. This study serves as a valuable reference for the further optimization of spintronic THz emitters and promotes the development of high-performance, strong-field spintronic THz sources.

Nanometric Ge Films for Ultrafast Modulation of THz Waves with Flexible Metasurface

AbstractHarnessing confined light on a subwavelength scale within Fano metasurfaces enhances light‐matter interaction on a flexible, low‐loss substrate. This approach enables the integration of ultra‐thin semiconducting films for designing low‐power, ultrafast switchable terahertz, and optical meta devices. Here, an ultra‐thin, ∼λ/6000 germanium overlayers of 25nm or 50nm is thermally evaporated onto a flexible Fano metallic metasurface to achieve ultrafast optical modulation of terahertz radiation. A remarkable 85% modulation depth with an ultrafast speed of 400 GHz is obtained in the resonant transmission. These ultrathin, flexible, and ultrafast functional metasurfaces pave the way for developing flexible terahertz active meta devices based on nanometric scale germanium films, offering the additional advantage of compatibility with complementary metal‐oxide‐semiconductor (CMOS) technology in microelectronics.

Viscous terahertz photoconductivity of hydrodynamic electrons in graphene.

Light incident upon materials can induce changes in their electrical conductivity, a phenomenon referred to as photoresistance. In semiconductors, the photoresistance is negative, as light-induced promotion of electrons across the bandgap enhances the number of charge carriers participating in transport. In superconductors and normal metals, the photoresistance is positive because of the destruction of the superconducting state and enhanced momentum-relaxing scattering, respectively. Here we report a qualitative deviation from the standard behaviour in doped metallic graphene. We show that Dirac electrons exposed to continuous-wave terahertz (THz) radiation can be thermally decoupled from the lattice, which activates hydrodynamic electron transport. In this regime, the resistance of graphene constrictions experiences a decrease caused by the THz-driven superballistic flow of correlated electrons. We analyse the dependencies of the negative photoresistance on the carrier density, and the radiation power, and show that our superballistic devices operate as sensitive phonon-cooled bolometers and can thus offer, in principle, a picosecond-scale response time. Beyond their fundamental implications, our findings underscore the practicality of electron hydrodynamics in designing ultra-fast THz sensors and electron thermometers.

Compositional engineering of interfacial charge transfer in van der Waals heterostructures of graphene and transition metal dichalcogenides.

Owing to their unique optical and electronic properties, vertical van der Waals heterostructures (vdWHs) have attracted considerable attention in optoelectronic applications, such as photodetection, light harvesting, and light-emitting diodes. To fully harness these properties, it is crucial to understand the interfacial charge transfer (CT) and recombination dynamics across vdWHs. However, the effects of interfacial energetics and defect states on interfacial CT and recombination processes in graphene-transition metal dichalcogenide (Gr-TMD) vdWHs remain debated. Here, we investigate the interfacial CT dynamics in Gr-TMD vdWHs with different chemical compositions (W, Mo, S, and Se) and tunable interfacial energetics. We demonstrate, using ultrafast terahertz spectroscopy, that while the photo-induced electron transfer direction is universal with graphene donating electrons to TMDs, its efficiency is chalcogen-dependent: the CT efficiency of S atom-based vdWHs is 3-5 times higher than that of Se-based vdWHs thanks to the lower Schottky barrier present in S-based vdWHs. In contrast, the electron back transfer process from TMD to Gr, which defines the charge separation time, is transition metal-dependent and dominated by the mid-gap defect level of TMDs: W transition metal-based vdWHs possess extremely long charge separation, well beyond 1ns, which is significantly longer than Mo-based vdWHs with only 10s of ps charge separation. This difference can be traced to the much deeper mid-gap defect reported in W-based TMDs compared to Mo-based ones, resulting in modified energetics for the back electron transfer from the trapped states to graphene. Our results shed light on the role of interfacial energetics and defects by tailoring chemical compositions of TMDs on the interfacial CT and recombination dynamics in Gr-TMD vdWHs, which is pivotal for optimizing optoelectronic devices, particularly in the field of photodetection.

Distinguishing carrier transport and interfacial recombination at perovskite/transport-layer interfaces using ultrafast spectroscopy and numerical simulation

In perovskite solar cells, photovoltaic action is created by charge transport layers (CTLs) either side of the light-absorbing metal halide perovskite semiconductor. Hence, the rates for desirable charge extraction and unwanted interfacial recombination at the perovskite-CTL interfaces play a critical role for device efficiency. Here, the electrical properties of perovskite-CTL bilayer heterostructures are obtained using ultrafast terahertz and optical studies of the charge carrier dynamics after pulsed photoexcitation, combined with a physical model of charge carrier transport that includes the prominent Coulombic forces that arise after selective charge extraction into a CTL, and cross-interfacial recombination. The charge extraction velocity at the interface and the ambipolar diffusion coefficient within the perovskite are determined from the experimental decay profiles for heterostructures with three of the highest-performing CTLs, namely C60, PCBM and Spiro-OMeTAD. Definitive targets for the further improvement of devices are deduced: fullerenes deliver fast electron extraction, but suffer from a large rate constant for cross-interface recombination or hole extraction. Conversely, Spiro-OMeTAD exhibits slow hole extraction but does not increase the perovskite’s surface recombination rate, likely contributing to its success in solar cell devices. Published by the American Physical Society 2024

Polymorph‐Dependent Multi‐Level Supramolecular Self‐Assembly and Local Charge Transport of a Conjugated Polymer in Solution and Solid States

AbstractThe polymorphic behavior of conjugated polymers enables tunable optoelectronic properties, but their transport mechanism remains elusive due to the inherent complexity and uncontrollability of polymorphic self‐assembly behaviors and electronic processes at various length scales, alongside the ambiguous relationship between solution and solid states. Herein, precise control of multi‐level supramolecular self‐assembly of a polymorphic conjugated polymer, N‐PDPP4T‐HD with two distinct semi‐crystalline aggregated phases (β1 and β2) via solvent engineering is demonstrated. β1 forms 1D worm‐like nanostructures in solution, whereas β2 generates 2D nanoscale lamellar configuration, confirmed by experimental observation and molecular dynamic simulation. Such solution‐state features are inherited in the solid state (1D nanofibers for β1 and 2D granular‐like structures for β2). X‐ray characterizations reveal larger crystalline domains on the nanometer scale, reduced π‐stacking distance on the Ångstrom scale, and diminished paracrystallinity disorder for solid‐state β2. Going beyond conventional DC transistor characterizations, contact‐free ultrafast terahertz spectroscopy to unveil AC short‐range, intrinsic transport properties is employed. Longer charge carrier scattering time and thus intrinsic mobility of β2 result in threefold higher short‐range photoconductivity than β1. This work establishes the “solution structure – solid structure – local transport” relation in polymorphic conjugated polymers and provides new opportunities for high‐performance plastic electronic devices.

Phase-resolved measurement and control of ultrafast dynamics in terahertz electronic oscillators

As a key component for next-generation wireless communications (6 G and beyond), terahertz (THz) electronic oscillators are being actively developed. Precise and dynamic phase control of ultrafast THz waveforms is essential for high-speed beam steering and high-capacity data transmission. However, measurement and control of such ultrafast dynamic process is beyond the scope of electronics due to the limited bandwidth of the electronic equipment. Here we surpass this limit by applying photonic technology. Using a femtosecond laser, we generate offset-free THz pulses to phase-lock the electronic oscillators based on resonant tunneling diode. This enables us to perform phase-resolved measurement of the emitted THz electric field waveform in time-domain with sub-cycle time resolution. Ultrafast dynamic response such as anti-phase locking behaviour is observed, which is distinct from in-phase stimulated emission observed in laser oscillators. We also show that the dynamics follows the universal synchronization theory for limit cycle oscillators. This provides a basic guideline for dynamic phase control of THz electronic oscillators, enabling many key performance indicators to be achieved in the new era of 6 G and beyond.

Ultrafast terahertz Stark spectroscopy reveals the excited-state dipole moments of retinal in bacteriorhodopsin

The photoinduced all-trans to 13-cis isomerization of the retinal Schiff base represents the ultrafast first step in the reaction cycle of bacteriorhodopsin (BR). Extensive experimental and theoretical work has addressed excited-state dynamics and isomerization via a conical intersection with the ground state. In conflicting molecular pictures, the excited state potential energy surface has been modeled as a pure S[Formula: see text] state that intersects with the ground state, or in a 3-state picture involving the S[Formula: see text] and S[Formula: see text] states. Here, the photoexcited system passes two crossing regions to return to the ground state. The electric dipole moment of the Schiff base in the S[Formula: see text] and S[Formula: see text] state differs strongly and, thus, its measurement allows for assessing the character of the excited-state potential. We apply the method of ultrafast terahertz (THz) Stark spectroscopy to measure electric dipole changes of wild-type BR and a BR D85T mutant upon electronic excitation. A fully reversible transient broadening and spectral shift of electronic absorption is induced by a picosecond THz field of several megavolts/cm and mapped by a 120-fs optical probe pulse. For both BR variants, we derive a moderate electric dipole change of 5 [Formula: see text] 1 Debye, which is markedly smaller than predicted for a neat S[Formula: see text]-character of the excited state. In contrast, S[Formula: see text]-admixture and temporal averaging of excited-state dynamics over the probe pulse duration gives a dipole change in line with experiment. Our results support a picture of electronic and nuclear dynamics governed by the interaction of S[Formula: see text] and S[Formula: see text] states in a 3-state model.

Si-CMOS compatible epsilon-near-zero metamaterial for two-color ultrafast all-optical switching

Driven by the escalating demands of advanced technologies, developing integration strategies has kept pace with the realization of ultrafast components during the past two decades. Ultrafast all-optical switches enabled by artificial materials are considered at the forefront of the next generation of photonic integration for communications and high-volume data processing. Encouraged by these advancements, applications, and interest have increased toward all-optical switches based on epsilon-near-zero (ENZ) materials. However, some all-optical switches lack CMOS compatibility, require high energy activation, and are limited in switching speed and working wavelength. Here, we propose and demonstrate a multilayered ENZ metamaterial utilizing Si-compatible titanium nitride and indium-tin-oxide materials with two effective working wavelengths in the visible and near-infrared spectrum. This device enables switching time down to a few hundred femtoseconds utilizing minimal energy at the corresponding ENZ regions induced by intraband pumping. Our approach can enhance the adaptability of designing ENZ metamaterials for new hybrid integrated photonic components for low-power ultrafast all-optical terahertz modulation.

Integration of liquid crystal optical delay and mechanical stage optical delay for measurement of ultrafast autocorrelations and terahertz pulses

To improve the temporal resolution in an optical delay system that uses a conventional mechanical delay stage, we integrate an in-line liquid crystal (LC) wave retarder. Previous implementations of LC optical delay methods are limited due to the small temporal window provided. Using a conventional mechanical delay stage system in series with the LC wave retarder, the temporal window is lengthened. Additionally, the limitation on temporal resolution resulting from the minimum optical path alteration (resolution of 400 nm) of the conventionally used mechanical delay stage is reduced via the in-line wave retarder (resolution of 50 nm). Interferometric autocorrelation measurements are conducted at multiple laser emission frequencies (349, 357, 375, 394, and 405 THz) using the in-line LC and conventional mechanical delay stage systems. The in-line LC system is compared to the conventional mechanical delay stage system to determine the improvements in temporal resolution relating to maximum resolvable frequency. This work demonstrates that the integration of the in-line LC system can extend the maximum resolvable frequency from 375 to 3000 THz. The in-line LC system is also applied for measurement of terahertz pulses.

Optically Tunable Ultrafast Broadband Terahertz Polarimetric Device Using Nonvolatile Phase‐Change Material

AbstractActively tunable ultrafast broadband terahertz (THz) polarimetry using a reconfigurable phase‐change material holds great potentials and prospects for the achievement of next‐generation versatile integrated THz components and systems in a variety of THz applications. Here, an optically tunable ultrafast broadband THz polarimetric device (THz‐PoD) composed of a phase‐change material Ge2Sb2Te5 (GST) and a thin mica substrate is demonstrated. This proposed novel THz‐PoD is verified for a frequency range of 0.1–2.5 THz, exhibiting broadband and ultrafast determination of polarization states for linearly polarized THz waves at polarization angles from −90° to 90°. It is shown that optical excitation with ns pulses allows easy and efficient control of the polarimetric properties of such THz‐PoD. The essential role of the GST film in switching the phase transition between the amorphous and crystalline phases is emphasized by the theoretical investigation of the optically tunable ultrafast polarimetric mechanism of the device. This phase transition allows optically changing the THz‐PoD's properties by ns‐pulsed laser in a controlled way to achieve THz polarimetry for linearly polarized THz waves. The combined advantages of this strategy can open up a new and promising way for realizing versatile reconfigurable and integrated THz devices, which may further promote the development of novel THz systems and applications.

DC electrical conductivity measurements of warm dense matter using ultrafast THz radiation

We describe measurements of the DC electrical conductivity of warm dense matter using ultrafast terahertz (THz) pulses. THz fields are sufficiently slowly varying that they behave like DC fields on the timescale of electron–electron and electron–ion interactions and hence probe DC-like responses. Using a novel single-shot electro-optic sampling technique, the electrical conductivity of the laser-generated warm dense matter was determined with <1 ps temporal resolution. We present the details of the single-shot THz detection methodology as well as considerations for warm dense matter experiments. We, then, provide proof-of-concept studies on aluminum driven to the warm dense matter regime through isochoric heating and shock compression. Our results indicate a decrease in the conductivity when driven to warm dense matter conditions and provide a platform for future warm dense matter studies.

THz active modulation device by electric field turned modulation characteristics of titanium sulfide nanofilm device

AbstractAn effective approach to manipulate terahertz (THz) waves involves the utilization of a device structure. In this particular investigation, we have successfully fabricated a SnS2 nanofilm device using femtosecond laser direct ablation. The passive and active polarization‐sensitive characteristics of the SnS2 nanofilm device were examined within the THz frequency range, employing different line spaces of 500, 600, and 700 μm. The combination of the device effect and SnS2 absorption led to an enhanced passive polarization modulation. Moreover, when an external electric field was applied to the SnS2 nanofilm device along the device line, persistent switching and linear modulation of THz wave transmission were observed. These findings indicate that the integration of material properties and device structure in this SnS2 device renders it highly suitable for applications in ultrafast THz switches, filters, and modulation devices.

Giant Spin-Orbit Induced Magnon Nonreciprocity in Ultrathin Ferromagnets.

The propagation characteristics of fermionic and bosonic quasiparticles determine the fundamental transport properties of solids and are of great technological relevance for designing logic devices. In particular, nonreciprocity, which describes that a quasiparticle flows preferably along a certain direction of a symmetry path, is an essential requirement to realize logic architectures, e.g., switches, diodes, transistors, etc. Here we introduce a mechanism, which leads to giant nonreciprocity of ultrafast terahertz magnons in ferromagnetic films with a large spin-orbit coupling. The mechanism is based on the competition between the exchange and spin-orbit scattering. We anticipate that the effect can be used to excite nonreciprocal or even unidirectional magnons in a large class of ultrathin films and nanostructures grown on substrates with a large spin-orbit coupling.