Recent developments in femtosecond laser technology have enabled the generation of a nearly monocyclic strong terahertz (THz) pulse with an amplitude greater than 100 kV cm−1. Such a THz pulse can be used to control not only the elementary excitations in solids such as phonons, magnons, and excitons but also the electronic phases. To achieve ultrafast phase control in the sub-ps time domain with a THz pulse, correlated electronic materials that demonstrate electronic phase transitions without large structural changes induced by external stimuli, such as by temperature, pressure, and light can be used. In this paper, we review recent studies on electronic phase controls using a nearly monocyclic THz pulse in organic molecular compounds with correlated electron systems, TTF-CA (TTF: tetrathiafulvalene and CA: p-chloranil), α-(ET)2I3 (ET: bis(ethylenedithio)tetrathiafulvalene), and κ-(ET)2Cu[N(CN)2]Br. TTF-CA undergoes a neutral-to-ionic phase transition as the temperature decreases. It demonstrates an electronic-type ferroelectricity in the ionic phase, in which ferroelectric polarization is generated from intermolecular charge transfers across the neutral-to-ionic phase transition. THz-pulse pump second-harmonic-generation probe and optical-reflectivity probe measurements show that ferroelectric polarization in the ionic phase can be rapidly modulated by a THz pulse via charge transfers induced by an electric field. In α-(ET)2I3, rapid polarization modulation by a THz pulse was also achieved in the ferroelectric charge-order phase. Detailed analyses of reflectivity changes induced with THz electric fields revealed that the ferroelectric polarization originated from intermolecular charge transfers and was oriented diagonally to the crystal axes. These results demonstrate that the ferroelectricity of this compound was electronic, similar to that of the ionic phase of TTF-CA. In the para-electric neutral phase of TTF-CA, a macroscopic polarization was generated by a THz pulse via the dynamics induced by an electric field on microscopic ionic domains. In κ-(ET)2Cu[N(CN)2]Br, a transition from a Mott insulator to a metal by a THz pulse was demonstrated by observing Drude-like low-energy spectral weights induced by the electric field. A THz pulse induced doublon−holon pair production by quantum tunnelling processes, which collapsed the original Mott gap in under a picosecond. These results suggest that strong THz-pulse irradiation is an effective approach for the ultrafast control of electronic phases in correlated electron materials.
Special Issue on ultrafast spectroscopy: fundamentals
Guest Editors
Keith Nelson, Massachusetts Institute of Technology, USA
Alfred Leitenstorfer, University of Konstanz, Germany
Koichiro Tanaka, University of Kyoto, Japan
As part of our celebrations of the 50th anniversary of the Journal of Physics series, we have launched this joint special issue on ultrafast spectroscopy which is being published in our sister titles, Journal of Physics B: Atomic Molecular and Optical Physics and Journal of Physics D: Applied Physics.
Papers for the special issue on ultrafast spectroscopy: applications will appear here or you can scroll down to view them directly at the bottom of this page.
Scope
Progress in the generation of electromagnetic waves ranging from the long wave infrared (LWIR) to the Terahertz (THz) wavelength range with full control over the electric field and with extremely strong field strength ranging up to the multi-GV/m scale, in recent years, has lead to the exploration of strong-field effects with long wavelength pulses in physics, chemistry, biology and the engineering sciences. Thus the time is right to publish a peer-reviewed collection of contributions concerning the state-of-the-art in: LWIR-THz generation using laser based and accelerator based approaches; the use of strong LWIR-THz fields to steer the motion of free electrons, drive rotations of molecules, vibrations of crystal lattices, the precession of spins and to explore transient states of matter. We hope in producing this special edition of the Journal of Physics B: Atomic, Molecular and Optical Physics, that we may help further a challenging mission and ongoing intellectual adventure: the harnessing of strong fields in a yet rather unexplored frequency range.
The special issue "Ultrafast Spectroscopy: fundamentals" seeks to provide an overview of some of the most important developments in the field, while at the same time indicating applications of these new developments.
Topics that this special issue is looking to cover, include:
Editorial
Topical Review
Special Issue Papers
Contributions of the pre-ionized H2 (PI-H2) and ionized 
subsystems of the two-electron H2 system to its high-order harmonic generation in eight-cycle sin2-like ultrafast intense laser pulses are calculated and analyzed based on the solution of the time-dependent Schrödinger equation for the one-dimensional two-electronic H2 system with fixed nuclei. The laser pulses have λ = 390 and 532 nm wavelengths and I = 1 × 1014, 5 × 1014, 1 × 1015 and 5 × 1015 W cm−2 intensities. It is found that at the two lower intensities, the PI-H2 subsystem dominantly produces the HHG spectra. However, at the two higher intensities, both PI-H2 and ionized 
subsystems contribute comparably to the HHG spectra. In the 
subsystem, the symmetry of the populations of 
(I) and 
(II) regions (left and right regions of 
subsystem) is broken by increasing the laser intensity. Complex patterns and even harmonics also appear at these two higher intensities. For instance, at 1 × 1015 W cm−2 intensity and λ = 532 nm wavelength, the even harmonics are appeared near cutoff region. Interestingly, at 5 × 1015 W cm−2 intensity and λ = 390 nm wavelength, the even harmonics replaced by the odd harmonics with red shift. At λ = 390 and 532 nm wavelengths and I = 1 × 1015 intensity, the two-electron cutoffs corresponding to nonsequential double-recombination with maximum return kinetic energy of 4.70Up are detected. The HHG spectra of the whole H2 system obtained with and without nuclear dynamics treated classically are approximately similar. However, at 1 × 1015 W cm−2 intensity and λ = 532 nm wavelength, if we take into account nuclear dynamics, the even harmonics which are appeared near cutoff region, replaced by the odd harmonics with blue shift.
Multicycle THz pulse generation by optical rectification in GaP semiconductor nonlinear material is investigated by numerical simulations. It is shown that GaP can be an efficient and versatile source with up to about 8% conversion efficiency and a tuning range from 0.1 THz to about 7 THz. Contact-grating technology for pulse-front tilt can ensure an excellent focusability and scaling the THz pulse energy beyond 1 mJ. Shapeable infrared pump pulses with a constant intensity-modulation period can be delivered for example by a flexible and efficient dual-chirped optical parametric amplifier. Potential applications include linear and nonlinear THz spectroscopy and THz-driven acceleration of electrons.
The acceleration of single electrons and electron bunches by focused THz pulse pairs has been investigated by numerical simulations. The effect of the choice of the beam waist radius, the carrier-envelope phase, and the propagation direction of the THz pulses on the energy of the accelerated electrons was investigated. The acceleration of electron bunches from rest up to 150 keV was predicted using single-cycle THz pulses with 1 mJ energy and a central frequency in the 0.1 THz to 3.0 THz range. The post-acceleration of electrons by pairs of focused THz pulses has also been proposed.
After the first introduction of ultrafast electron guns for acceleration of particles using single-cycle electro-magnetic pulses, the basic structure has gained increasing interest as promising solutions for high gradient compact electron guns. The significant benefit of using transient ultrashort pulses in this acceleration scheme opens a realistic path towards gigavolt-per-meter acceleration gradients. In this paper, we present an optimized design strategy for these electron guns. The goal is to estimate the THz energy needed for an optimum device to accelerate electrons at rest to a certain energy using materials that endure a pre-determined maximum electric field. We start with designing a gun delivering 400 keV electron beam energy and discuss different techniques to enhance the performance. Throughout this design process, it is implicitly shown that the concept of single-cycle ultrafast electron guns can apply THz beams with energies in the level of 100–400 μJ to accelerate electrons, which is the state-of-the-art technology in THz radiation sources. Subsequently, upgrading the design to an 800 keV device is outlined, to demonstrate the eligibility of this concept to perform as linac injectors in compact accelerator facilities.
We aim to generate high-intensity terahertz (THz) electric fields and study nonlinear phenomena in GaAs and graphene to investigate their applications. To obtain a high-efficiency intense THz field, we employ the tilted pump-pulse front technique using a LiNbO3 crystal. With this technique, we obtain a THz field strength of over 300 kV cm−1. We investigate the hot-carrier dynamics in n- and p-type GaAs driven by high-field THz pulses. Although both samples show similar carrier concentrations, the nonlinear THz responses show different trends. Owing to hot-carrier generation, intervalley scattering is dominant in n-type GaAs, and intervalence band scattering is the main cause in p-type GaAs. In addition, we study the hot-carrier dynamics in graphene with the grain-size dependency. Although graphene has the same Fermi level regardless of the grain size, the THz responses are different for large- and small-grained graphene: charged impurity scattering in large-grained graphene and defect scattering in small-grained graphene. From these results, our study provides insights into high-speed electronics applications.
We present the generation of THz radiation by focusing ultrafast laser pulses with three incommensurate wavelengths to form a plasma. The three colors include 800 nm and the variable IR signal and idler outputs from an optical parametric amplifier. We observe that stable THz is generated when all three colors are present, with a peak-to-peak field strength of ∼200 kV cm−1 and a relatively broad, smooth spectrum extending out to 6 THz, without any strong dependence on the selection of signal and idler IR wavelengths (in the range from 1300 to 2000 nm). We confirm that three colors are indeed needed, and we present plasma current modeling that corroborates our observations.
After optical pumping, strong single cycle THz pulses are used to probe and manipulate excitons in bulk germanium. For strong THz fields a significant broadening and bleaching of the 1s–2p THz absorption peak is observed. The experimental results are analyzed using a microscopic many-body theory attributing the observations to a shortening of the excitonic state lifetime and eventual exciton ionization. Simultaneously, the ac THz Stark effect leads to a shift in the 1s–2p transition energy.
We demonstrate optical rectification of 1 μm pulses with a duration of 20 fs, a repetition-rate of 78 MHz and an average power of 5.5 W, in a 2 mm thick GaP crystal. The spectrum of the resulting far-infrared pulses is centered at 1.5 THz and extends to 5 THz at −50 dB intensity. In the absence of resonant absorption of GaP in this range, the spectrum has a well-behaved shape, facilitating spectroscopic applications. In the context of the recent rapid evolution of high-power Yb-based femtosecond laser systems, these results show a viable route towards sources of THz pulses combining broad bandwidth, high average power and a smooth spectral shape.
We have demonstrated transient charge localization effects with a driving high-frequency field of 7 fs, 1.5-cycle near-infrared light in correlated organic conductors. In a layered organic conductor α-(BEDT-TTF)2I3 (BEDT-TTF: bis[ethylenedithio]-tetrathiafulvalene), a transient short-range charge order (CO) state is induced in a metallic phase. In contrast to such drastic change in the electronic state from the metal to the transient CO in α-(BEDT-TTF)2I3, dynamics of a field-induced reduction of a transfer integral are captured as a red-shift of the plasma-like reflectivity edge in a quasi-one-dimensional organic conductor (TMTTF)2AsF6 (TMTTF: tetramethyltetrathiafulvalene). These studies on the field-induced charge localization have been motivated by the theory of dynamical localization on the basis of tight-binding models with no electron correlation under a strong continuous field. However, the results of pump–probe transient reflectivity measurements using nearly single-cycle 7 fs, 11 MV cm−1 pulses and the theoretical studies which are presented in this review indicate that the pulsed field contributes to the similar phenomenon with the help of a characteristic lattice structure and Coulomb repulsion.
A relativistic electron source is proposed, driven by the wakefield of an intense terahertz (THz) pulse in low-density gas plasma. In contrast to the optical and near-infrared regimes, the low (3.5 THz) frequency and the long (λT = 85.6 μm) wavelength of the THz pulse offers distinct advantages, such as the 
-scaling of the electron ponderomotive energy. Two-dimension-in-space and three-dimension-in-velocity particle-in-cell simulation results show that relativistic electrons of ∼1 MeV energy and high charge can be generated by an intense THz pulse at kilohertz repetition rate from a gas plasma target. These results may lead to a new regime of applications, such as ultrafast electron diffraction or high-repetition-rate gamma ray sources for materials characterization or medical radiography, which would benefit from lower energy (1–10 MeV) but higher repetition rate (∼1 kHz) sources of relativistic electrons.
We use finite element simulations in both the frequency and the time-domain to study the terahertz resonance characteristics of a metamaterial (MM) comprising a spiral connected to a straight arm. The MM acts as a RLC circuit whose resonance frequency can be precisely tuned by varying the characteristic geometrical parameters of the spiral: inner and outer radius, width and number of turns. We provide a simple analytical model that uses these geometrical parameters as input to give accurate estimates of the resonance frequency. Finite element simulations show that linearly polarized terahertz radiation efficiently couples to the MM thanks to the straight arm, inducing a current in the spiral, which in turn induces a resonant magnetic field enhancement at the center of the spiral. We observe a large (approximately 40 times) and uniform (over an area of ∼10 μm2) enhancement of the magnetic field for narrowband terahertz radiation with frequency matching the resonance frequency of the MM. When a broadband, single-cycle terahertz pulse propagates towards the MM, the peak magnetic field of the resulting band-passed waveform still maintains a six-fold enhancement compared to the peak impinging field. Using existing laser-based terahertz sources, our MM design allows to generate magnetic fields of the order of 2 T over a time scale of several picoseconds, enabling the investigation of nonlinear ultrafast spin dynamics in table-top experiments. Furthermore, our MM can be implemented to generate intense near-field narrowband, multi-cycle electromagnetic fields to study generic ultrafast resonant terahertz dynamics in condensed matter.
Articles for the special issue on ultrafast spectroscopy: applications on Journal of Physics D: Applied Physics
To understand the mechanics of cellular packing of two-dimensional (2D) materials, we perform systematic molecular dynamics simulations and theoretical analysis to investigate the packing of a flexible circular sheet in a spherical vesicle and the 2D packing problem of a strip in a cylindrical vesicle. Depending on the system dimensions and the bending rigidity ratio between the confined sheet and the vesicle membrane, a variety of packing morphologies are observed, including a conical shape, a shape of three-fold symmetry, a cylindrically curved shape, an axisymmetrically buckled shape, as well as the initial circular shape. A set of buckling analyses lead to phase diagrams of the packing morphologies of the encapsulated sheets. These results may have important implications on the mechanism of intracellular packing and toxicity of 2D materials.
Improved control over the electromagnetic properties of metals on a nanoscale is crucial for the development of next-generation nanoelectronics and plasmonic devices. Harnessing the terahertz (THz)-electric-field-induced nonlinearity for the motion of electrons is a promising method of manipulating the local electromagnetic properties of metals, while avoiding undesirable thermal effects and electronic transitions. In this review, we demonstrate the manipulation of electron delocalization in ultrathin gold (Au) films with nanostructures, by intense THz electric-field transients. On increasing the electric-field strength of the THz pulses, the transmittance in the THz-frequency region abruptly decreases around the percolation threshold. The observed THz-electric-field-induced nonlinearity is analysed, based on the Drude-Smith model. The results suggest that ultrafast electron delocalization occurs by electron tunnelling across the narrow insulating bridge between the Au nanostructures, without material breakdown. In order to quantitatively discuss the tunnelling process, we perform scanning tunnelling microscopy with carrier-envelope phase (CEP)-controlled single-cycle THz electric fields. By applying CEP-controlled THz electric fields to the 1 nm nanogap between a metal nanotip and graphite sample, many electrons could be coherently driven through the quantum tunnelling process, either from the nanotip to the sample or vice versa. The presented concept, namely, electron tunnelling mediated by CEP-controlled single-cycle THz electric fields, can facilitate the development of nanoscale electron manipulation, applicable to next-generation ultrafast nanoelectronics and plasmonic devices.
We measure the conductivity spectra of thin films comprising bundled single-walled carbon nanotubes (CNTs) of different average lengths in the frequency range 0.3–1000 THz and temperature interval 10–530 K. The observed temperature-induced changes in the terahertz conductivity spectra are shown to depend strongly on the average CNT length, with a conductivity around 1 THz that increases/decreases as the temperature increases for short/long tubes. This behaviour originates from the temperature dependence of the electron scattering rate, which we obtain from Drude fits of the measured conductivity in the range 0.3–2 THz for 10 μm length CNTs. This increasing scattering rate with temperature results in a subsequent broadening of the observed THz conductivity peak at higher temperatures and a shift to lower frequencies for increasing CNT length. Finally, we show that the change in conductivity with temperature depends not only on tube length, but also varies with tube density. We record the effective conductivities of composite films comprising mixtures of WS2 nanotubes and CNTs versus CNT density for frequencies in the range 0.3–1 THz, finding that the conductivity increases/decreases for low/high density films as the temperature increases. This effect arises due to the density dependence of the effective length of conducting pathways in the composite films, which again leads to a shift and temperature dependent broadening of the THz conductivity peak.
Self-organized layers of anodic TiO2 nanotubes were investigated using time-resolved terahertz spectroscopy in the steady state and upon photoexcitation. The interpretation of the conductivity spectra is based on the response of confined charges calculated by the Monte-Carlo method and on the evaluated distribution of the probing terahertz electric field in the heterogeneous structure. We show that the charge motion perpendicular to the nanotube axis is confined on ~10 nm scale, and that the charge mobility inside these confinement areas is comparable to that observed in a bulk anatase crystal. The electrical connectivity between individual nanotubes assessed from the terahertz spectra qualitatively correlates with the geometry observed in SEM images. The measured transient terahertz transmission spectra feature an apparent resonance; we demonstrate that it is not a signature of a new low-energy excitation but a geometrical effect of Fabry–Pérot interferences in the photoexcited slab.
In this work, by using terahertz time-domain spectroscopy (THz-TDS), we investigate the temperature range of the SRT, and the resonant frequency and relaxation time of the THz spin waves in SmxDy1−xFeO3 single crystal. We show that the resonant frequency of the FM mode (measured at 40 K) increases linearly with the Sm dopant concentration within the range from x = 0.5 to 0.7. The temperature- and dopant-induced changes of the magnetic anisotropy of Fe3+ ions are accessed by the resonant frequency shifts. Upon cooling, the lifetime of oscillations in the AFM mode increases exponentially and can be subtly tuned by varying the Sm dopant. These results lead to an improved understanding of dopant-tuned spin wave dynamics and magnetoanisotropy parameter in rare-earth orthoferrites.
We present femtosecond optical pump-terahertz probe studies on the ultrafast dynamical processes of photo-generated charge carriers in silicon photovoltaic cells with various nanostructured surfaces and doping densities. The pump-probe measurements provide direct insight on the lifetime of photo-generated carriers, frequency-dependent complex dielectric response along with photoconductivity of silicon photovoltaic cells excited by optical pump pulses. A lifetime of photo-generated carriers of tens of nanosecond is identified from the time-dependent pump-induced attenuation of the terahertz transmission. In addition, we find a large value of the imaginary part of the dielectric function and of the real part of the photoconductivity in silicon photovoltaic cells with micron length nanowires. We attribute these findings to the result of defect-enhanced electron–photon interactions. Moreover, doping densities of phosphorous impurities in silicon photovoltaic cells are also quantified using the Drude–Smith model with our measured frequency-dependent complex photoconductivities.
We report the broadband terahertz (THz) radiation in the metallic ferromagnetic (FM) heterostructures, upon irradiation of a femtosecond laser pulse at room temperature. The origin of THz generation from FM heterostructures can be interpreted using two terms: the transient demagnetization (a local modification of spin order of the FM metal) and electric-dipole radiation resulting from a non-local spin current pulses. Here, we show that the THz emission is dominated by the photo-excited transient charge current, which is converted from the spin current with inverse spin Hall effect. We tailor the metallic heterostructures with different non-magnetic thin layer (Pd or Ru) and FM materials (CoFeB or CoFe), to shape the THz transients. Moreover, we find that a saturation effect of THz radiation for CoFeB/Pd is less compared to CoFeB/Ru. THz emission spectroscopy can be used to qualitatively visualize the spin accumulation in the heterostructures.
We investigate the broadband dielectric properties of vertically aligned, multi-wall carbon nanotubes (VACNT), over both the terahertz (THz) and mid-infrared spectral ranges. The nominally undoped, metallic VACNT samples are probed at normal incidence, i.e. the response is predominantly due to polarisation perpendicular to the CNT axis. A detailed comparison of various conductivity models and previously reported results is presented for the non-Drude behaviour we observe in the conventional THz range (up to 2.5 THz). Extension to the mid-infrared range reveals an absorption peak at 
, reminiscent of that observed in single-wall CNT, only there it arises for polarisation parallel to the CNT axis. To account for the observed resonance here, we apply a Bergman-type effective-medium theory, based on first-principles' electromagnetic simulations for the perpendicular polarisation including both the intra- and inter-tube response, which can reproduce the observed spectrum if one assumes a much higher plasma frequency and scattering rate than that reflected in the low-frequency spectra, and proposes an explanation for the non-Drude behaviour at low-frequencies.
We demonstrate a metamaterial exhibiting a frequency-tunable response in the terahertz domain, controlled by a bias electric field. The active part of the metamaterial consists of a periodic metallic pattern deposited on a thin epitaxially strained strontium titanate film. The role of the metallic structure is two-fold: it gives rise to the metamaterial resonance and it enables applying an electric bias to the strontium titanate layer. The strained film exhibits a pronounced dependence of its permittivity on the bias, which exerts a strong influence on the resonance. Specifically, the resonance of our structure occurs near 0.5 THz and, upon applying a bias voltage of 55 V, a relative tunability of the resonance frequency of 19% was achieved at room temperature.
We study THz-driven spin dynamics in thin CoPt films with perpendicular magnetic anisotropy. Femtosecond magneto-optical Kerr effect measurements show that demagnetization amplitude of about 
can be achieved with a peak THz electric field of 300 kV cm−1, and a corresponding peak magnetic field of 0.1 T. The effect is more than an order of magnitude larger than observed in samples with easy-plane anisotropy irradiated with the same field strength. We also utilize finite-element simulations to design a meta-material structure that can enhance the THz magnetic field by more than an order of magnitude, over an area of several tens of square micrometers. Magnetic fields exceeding 1 Tesla, generated in such meta-materials with the available laser-based THz sources, are expected to produce full magnetization reversal via ultrafast ballistic precession driven by the THz radiation. Our results demonstrate the possibility of table-top ultrafast magnetization reversal induced by THz radiation.
We demonstrate that electromagnons can be used to directly probe the nature of a phase transition between magnetically ordered phases in an improper ferroelectric. The antiferromagnetic/paraelectric to antiferromagnetic/ferroelectric phase transition in Cu1−xZnxO (x = 0,0.05) alloys was tracked via the electromagnon response using terahertz time-domain spectroscopy, on heating and cooling through the phase transition. The transition was found to exhibit thermal hysteresis, confirming its first-order nature, and to broaden under the influence of spin-disorder upon Zn substitution. The energy of the electromagnon increases upon alloying, as a result of the non-magnetic ions modifying the magnetic interactions that give rise to the multiferroic phase and electromagnons. We describe our findings in the context of recent theoretical work that examined improper ferroelectricity and electromagnons in CuO from phenomenological and first-principles approaches.
We investigated the interaction between an intense terahertz (THz) pulse and excitons in bulk GaAs by using THz pump near-infrared (NIR) optical probe spectroscopy. We observed a clear spectral oscillation in the NIR transient absorption spectra at low temperature, which is interpreted as the THz pump-induced perturbed free induction decay (PFID) of the excitonic interband polarization. We performed a numerical simulation based on a microscopic theory and identified that the observed PFID signal originates from the THz field-induced ionization of excitons. Using a real-space representation of the excitonic wave function, we visualized how the ionization of an exciton proceeds under the intense single-cycle THz electric field. We also calculated the nonlinear susceptibility with the lowest-order perturbation theory assuming a weak THz pump, which showed a similar spectral feature with that obtained by the full treatment to field-induced ionization process. This coincidence is attributed to the fact that 1s-excitonic interband polarization is modified predominantly through interactions with the p-wave component of the excitonic wave function. A simple phenomenological expression of the PFID signal is presented to discuss effects of the THz pump pulse duration on the spectral oscillation.
Recent advancements of accelerator technology enable the generation of carrier-envelope-phase stable THz pulses with high fields at adjustable high repetition rates. The appropriate choice of THz radiator allows generation of narrow-band, spectrally dense, multicycle THz transients of tunable THz frequency which are ideally suited to selectively excite low-energy excitations such as magnons or phonons. They also allow one to study the frequency dependence of nonresonant THz-field interactions with various order parameters with high dynamic range. In this paper, we discuss the future prospects of this new type of THz light source for studying the coherent control of magnetic order based on recent results.
The ultrashort laser excitation of Co/Pt magnetic heterostructures can effectively generate spin and charge currents at the interfaces between magnetic and nonmagnetic layers. The direction of these photocurrents can be controlled by the helicity of the circularly polarized laser light and an external magnetic field. Here, we employ THz time-domain spectroscopy to investigate further the role of interfaces in these photo-galvanic phenomena. In particular, the effects of either Cu or ZnO interlayers on the photocurrents in Co/X/Pt (X = Cu, ZnO) have been studied by varying the thickness of the interlayers up to 5 nm. The results are discussed in terms of spin-diffusion phenomena and interfacial spin–orbit torque.
We proposed an approach to tailor the mode interference effect in plasmon-induced transparency (PIT) metamaterials. Through introducing an extra coupling mode using an asymmetric structure configuration at terahertz (THz) frequencies, the well-known single-transparency-window PIT can be switched to dual-transparency-window PIT. Proof-of-concept subwavelength structures were fabricated and experimentally characterized. The measured results are in good agreement with the simulations, and well support our theoretical analysis. The presented research delivers a novel approach toward developing subwavelength devices with varies functionalities, such as ultra-slow group velocities, longitudinal pulse compression and light storage in the THz regime, which can also be extended to other spectral regimes.
Terahertz pulses are a direct and general probe of ultrafast spin dynamics in insulating antiferromagnets (AFM). This is shown by using optical-pump, THz-probe spectroscopy to directly track AFM spin dynamics in the hexagonal multiferroic HoMnO3 and the orthorhombic multiferroic TbMnO3. Our studies show that despite the different structural and spin orders in these materials, THz pulses can unambiguously resolve spin dynamics after optical photoexcitation. We believe that this approach is quite general and can be applied to a broad range of materials with different AFM spin alignments, providing a novel non-contact approach for probing AFM order with femtosecond temporal resolution.
Terahertz excitation spectroscopy was used for the determination of energy separation between the main (Γ) and subsidiary (L and X) conduction band valleys of GaAs1−xBix. The samples used in this study were 1 µm–1.5 µm thick bismide layers grown by Molecular Beam Epitaxy on GaAs substrates. They contained up to 8% of bismuth as determined by high resolution x-ray diffraction (HR-XRD) and reciprocal space mapping (RSM), taking into account the layer relaxation. It was found that both subsidiary conduction band valleys at L and X points of the Brillouin zone move away from the conduction band minimum at rates of 18 meV/%Bi and 25 meV/%Bi, respectively, with increasing Bi content in the alloy.