Intense Terahertz Electric Field Research Articles

Room temperature persisting surface charge carriers driven by intense terahertz electric fields in a topological insulator Bi2Se3

Topologically protected surface current is highly promising for next-generation low-dissipation and disorder-tolerant quantum electronics and computing. Yet, electric transport from the co-existing bulk state dominates the responses of the Dirac surface state, especially at elevated temperatures relevant to technological applications. Here, we present an approach that convincingly showcases the generation, disentanglement, and precise control of enduring surface charge carriers on a topological insulator, Bi2Se3, with high bulk conductivity, all achieved at room temperature. By using pump–probe modulation spectroscopy under ultrabroadband driving tunable from 4 meV to 1.55 eV, we show the terahertz (THz) field-induced surface carriers by discovering their initial temporal responses dominant over high density trivial bulk carriers. Strikingly, the response of the induced surface carrier responses persists for more than ∼5 ps and is enhanced by reducing pump photon energy. The dynamics and lifetime of the distinct surface response manifest themselves as the enhanced THz pump-induced THz transmission, which directly correlates with the transient negative THz conductivity. Increasing the THz driving field reduces the induced surface carrier lifetime and identifies, particularly, an optimal pump field of Es ∼ 224 kV cm−1 for generating the dominant surface response relative to the bulk. This surface carrier dominant regime is suppressed by a joint effect of enhanced surface-bulk scattering and a more rapid saturation of surface excitation compared to the bulk that sets in above Es. The controllability of room temperature topologically surface carriers through pump photon energy offer compelling possibilities for extending this approach to other topological complex materials.

Efficient Mott insulator-metal transition by an intense terahertz electric field pulse via quantum tunneling

When a semiconductor is subjected to a strong electric field, carriers are generated via quantum tunneling; this is termed as dielectric breakdown. Thus, using a terahertz pulse to drive the dielectric breakdown in Mott insulators, which exhibit variations in their electronic structures under carrier doping, a filling-controlled transition can be induced in the subpicosecond time scale. However, to generate carriers via quantum tunneling in a material with a band gap in the visible or near-infrared regions, an electric field pulse significantly exceeding $1 \mathrm{MV} {\mathrm{cm}}^{\ensuremath{-}1}$ is necessary. In this paper, using an organic molecular compound, bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane, which is a typical one-dimensional (1D) Mott insulator with a Mott gap of 0.7 eV, we aimed at realizing carrier generation and metallization via a strong electric field component of a terahertz pulse enhanced with an organic nonlinear optical crystal up to $2.8 \mathrm{MV} {\mathrm{cm}}^{\ensuremath{-}1}$. Even after the terahertz electric field decays, the reflectivity change caused by the terahertz pulse remains; this is different from the case involving the use of weaker electric fields. More importantly, this response indicates a threshold behavior against the electric field amplitude, which is characteristic of carrier generation via the quantum tunneling process. Furthermore, transient reflectivity spectra across the mid-infrared region could be reproduced well by numerical simulations using the Drude model, in which inhomogeneous carrier distributions are considered. The observed Drude response of the doublons and holons was ascribed to the spin-charge separation characteristic of 1D strongly correlated electron systems. We also demonstrate that the energy efficiency of such carrier generation by the terahertz pulse excitation is at least five times greater than that when using photoexcitation beyond the Mott gap. This indicates that excitation with the strong terahertz pulse is more effective for carrier doping in solids; thus, the proposed method is expected to be widely applicable for the electronic-state control of various correlated electron materials in which chemical carrier doping is currently difficult.

Terahertz pulse induced second harmonic generation and Kerr effect in the quantum paraelectric KTaO3

We present a joint experimental and computational study of second harmonic generation and Kerr effect induced by intense terahertz electric fields in the quantum paraelectric material ${\mathrm{KTaO}}_{3}$. The single-cycle terahertz (THz) electric pulses induce the free induction decay of the polar soft phonon mode and a ferroelectric polarization response that lasts for several picoseconds in both experiment and computation. We observe a much longer-lasting (43-ps) transient Kerr effect that follows the THz excitation. We explain the disparity in the relaxation times between the THz-induced second harmonic generation and the Kerr effect by the presence of the polar nanoregions intrinsic to ${\mathrm{KTaO}}_{3}$. The origin of the slow THz Kerr effect dynamics is not presently understood. We speculate that it may be associated with the rotational motion of polar nanoregions due to the material's proximity to the ferroelectric phase transition.

Terahertz Kerr Effect of Liquids

In recent years, tremendous advancements have been made in various technologies such as far-infrared, low-frequency Raman, and two-dimensional (2D) Raman terahertz (THz) spectroscopies. A coherent method has emerged from numerous experimental and theoretical investigations of molecular dynamics in liquids by comparing linear and non-linear spectroscopic techniques. Intermolecular hydrogen bond vibration, molecular reorientation motion, and interaction between molecule/ionic solute and hydrogen bonds have been demonstrated to occur in the THz region, which are closely related to their physical/chemical properties and structural dynamics. However, precise probing of various modes of motion is difficult because of the complexity of the collective and cooperative motion of molecules and spectral overlap of related modes. With the development of THz science and technology, current state-of-the-art THz sources can generate pulsed electric fields with peak intensities of the order of microvolts per centimeter (MV/cm). Such strong fields enable the use of THz waves as the light source for non-linear polarization of the medium and in turn leads to the development of the emerging THz Kerr effect (TKE) technique. Many low-frequency molecular motions, such as the collective directional motion of molecules and cooperative motion under the constraint of weak intermolecular interactions, are resonantly excited by an intense THz electric field. Thus, the TKE technique provides an interesting prospect for investigating low-frequency dynamics of different media. In view of this, this paper first summarizes the research work on TKE spectroscopy by taking a solid material without low-frequency molecular motions as an example. Starting from the principle of TKE technology and its application in investigating the properties of solid matter, we have explored the low-frequency molecular dynamics of liquid water and aqueous solutions using TKE. Liquid water is a core of life and possesses many extraordinary physical and biochemical properties. The hydrogen bond network plays a crucial role in these properties and is the main reason for its various kinetic and thermodynamic properties, which differ from those of other liquids. However, the structure of the hydrogen bond network between water and solutes is not well known. Therefore, evaluating the hydrogen bond-related kinetic properties of liquid water is important.

Ultrafast Terahertz Self-Induced Absorption and Phase Modulation on a Graphene-Based Thin Film Absorber

Interaction of intense terahertz (THz) waves with graphene based modulation devices holds great potential for the optoelectronic applications of the future. Here, we present a thin film graphene-based THz perfect absorbing device whose absorption and phase characteristics can be modulated through THz self-actions in the subps time scale. The device consists of a single-layer graphene placed on an ionic liquid substrate, back-plated by a metallic back-reflector, with the graphene doping level mediated through electrostatic gating. We experimentally record an absorption modulation of more than 3 orders of magnitude from the initial perfect absorption state, when the device is illuminated with THz field strengths in the range of 102 to 654 kV/cm. Furthermore, an absolute phase modulation of 130° is recorded. Detailed theoretical analysis indicates that the origin of the THz nonlinear response is the THz-induced heating of the graphene’s carriers that leads to a reduction of its conductivity and, consequently, to reduced absorption of the THz radiation. Our analysis also maps the temporal dynamics of the THz-induced temperature elevation of the graphene’s carriers, illustrating the ultrafast, subps nature of the overall process. These results can find applications in future dynamically controlled flat optics and spatiotemporal shaping of intense THz electric fields.

Terahertz field-induced ferroelectricity in quantum paraelectric SrTiO3.

"Hidden phases" are metastable collective states of matter that are typically not accessible on equilibrium phase diagrams. These phases can host exotic properties in otherwise conventional materials and hence may enable novel functionality and applications, but their discovery and access are still in early stages. Using intense terahertz electric field excitation, we found that an ultrafast phase transition into a hidden ferroelectric phase can be dynamically induced in quantum paraelectric strontium titanate (SrTiO3). The induced lowering in crystal symmetry yields substantial changes in the phonon excitation spectra. Our results demonstrate collective coherent control over material structure, in which a single-cycle field drives ions along the microscopic pathway leading directly to their locations in a new crystalline phase on an ultrafast time scale.

Terahertz-induced acceleration of massive Dirac electrons in semimetal bismuth.

Dirac-like electrons in solid state have been of great interest since they exhibit many peculiar physical behaviors analogous to relativistic mechanics. Among them, carriers in graphene and surface states of topological insulators are known to behave as massless Dirac fermions with a conical band structure in the two-dimensional momentum space, whereas electrons in semimetal bismuth (Bi) are expected to behave as massive Dirac-like fermions in the three-dimensional momentum space, whose dynamics is of particular interest in comparison with that of the massless Dirac fermions. Here, we demonstrate that an intense terahertz electric field transient accelerates the massive Dirac-like fermions in Bi from classical Newtonian to the relativistic regime; the electrons are accelerated approaching the effective “speed of light” with the “relativistic” beta β = 0.89 along the asymptotic linear band structure. As a result, the effective electron mass is enhanced by a factor of 2.4.

Terahertz-field-induced nonlinear electron delocalization in Au nanostructures.

Improved control over the electromagnetic properties of metal nanostructures is indispensable for the development of next-generation integrated nanocircuits and plasmonic devices. The use of terahertz (THz)-field-induced nonlinearity is a promising approach to controlling local electromagnetic properties. Here, we demonstrate how intense THz electric fields can be used to modulate electron delocalization in percolated gold (Au) nanostructures on a picosecond time scale. We prepared both isolated and percolated Au nanostructures deposited on high resistivity Si(100) substrates. With increasing the applied THz electric fields, large opacity in the THz transmission spectra takes place in the percolated nanostructures; the maximum THz-field-induced transmittance difference, 50% more, is reached just above the percolation threshold thickness. Fitting the experimental data to a Drude-Smith model, we found furthermore that the localization parameter and the damping constant strongly depend on the applied THz-field strength. These results show that ultrafast nonlinear electron delocalization proceeds via strong electric field of THz pulses; the intense THz electric field modulates the backscattering rate of localized electrons and induces electron tunneling between Au nanostructures across the narrow insulating bridges without any material breakdown.

Intense Terahertz Pulse-Induced Nonlinear Responses in Carbon Nanotubes

By using intense terahertz(THz) monocycle pulses, nonlinear light-matter interaction in aligned semiconducting single-walled carbon nanotubes(SWNTs) embedded in a polymer film was investigated. THz electric-field-induced ultrafast Stark effect of one-dimensional excitons in SWNTs was observed at room temperature, suggesting the potential functionality of SWNTs for high speed electro-optic devices operating at telecom wavelength with a THz bandwidth. When the peak electric field amplitude exceeds 200 kV/cm, the generation of excitons by the THz pump becomes prominent. The mechanism is described by the above-gap excitation of electrons and holes in SWNTs due to the impact excitation process induced by the intense THz electric field.

Terahertz harmonic generation in miniband semiconductor superlattices

We have numerically calculated the time-dependent response of a miniband semiconductor superlattice under an intense terahertz electric field, using the balance equation approach. The harmonics of the average electron velocity are carefully studied with the change of the static electric field, while fixing the amplitude and the frequency of the terahertz field. The numerical results indicate that the resonant peaks show up at the static electric field corresponding to the Bloch oscillation frequency ωΒ = mω. The calculated power of second harmonics as a function of static electric field is in good agreement with the experimental results.

Nonlinear optics of semiconductors under an intense terahertz field

A theory for nonlinear optics of semiconductors in the presence of an intense terahertz electric field is constructed based on the double-line Feynman diagrams, in which the nonperturbative effect of the intense terahertz field is fully taken into account through using the Floquet states as propagating lines in the Feynman diagrams.

Cyclotron-Stark-phonon-photon resonances in semiconductor superlattices under terahertz irradiation

High-field transport of electrons in semiconductor superlattices under intense parallel magnetic and time-dependent electric fields is investigated within a quantum-kinetic approach. Intra-collisional field effects of all fields are taken into account. In a biased superlattice subject to an intense radiation field of terahertz frequency, cyclotron-Stark-phonon-photon current resonances are predicted to occur for particular ratios of the Bloch , cyclotron , and terahertz frequencies . Under strong irradiation, pronounced maxima appear at resonance positions given by , where l, n, m are integers and the phonon frequency. Electro-phonon resonances are enhanced by a magnetic field but strongly weakened by an intense terahertz electric field.

Resonant generation of terahertz optical sidebands from confined magnetoexcitons

We have probed the internal structure and nonlinear response of magnetoexcitons in GaAs/AlGaAs quantum wells by resonantly driving one- and two-photon internal transitions with intense terahertz electric fields. Strong near-bandgap emission lines, or optical sidebands, appear at frequencies ω NIR ± 2 nω THz, where ω NIR is the interband exciton-creation frequency, ω THz is the frequency of the driving field, and n is an integer. The intensity of the sidebands exhibits pronounced enhancement when ω THz coincides with transitions between magnetically tuned energy levels in the excitons, providing new and accurate information on the internal dynamics of excitons.

Balance equations for electron transport in an arbitrary energy band driven by an intense terahertz field. Application to superlattice miniband transport

We suggest a balance-equation approach to hot-electron transport in a single arbitrary energy band subject to an intense radiation field of terahertz (THz) frequency, including all the multiphoton emission and absorption processes and taking account of realistic scatterings due to impurities and phonons. This approach, which allows one to calculate THz-driving, time-averaging transport based on a set of time-independent equations, provides a convenient method to study the effect of an intense THz electric field on carrier transport in a nonparabolic energy band. As an example, these fully three-dimensional, acceleration- and energy-balance equations are applied to the discussion of superlattice miniband transport at lattice temperature T=77 and 300 K driven by the THz radiation field of varying strengths. It is shown that the current through a dc biased miniband superlattice is greatly reduced by the irradiation of an intense THz electric field.

Resonant Terahertz Optical Sideband Generation from Confined Magnetoexcitons

We have probed the internal structure and nonlinear response of magnetoexcitons in GaAs/AlGaAs quantum wells by resonantly driving one- and two-photon internal transitions with intense terahertz electric fields. Strong near-band-gap emission lines, or optical sidebands, appear at frequencies ${\ensuremath{\omega}}_{\mathrm{NIR}}\ifmmode\pm\else\textpm\fi{}2n{\ensuremath{\omega}}_{\mathrm{THz}}$, where ${\ensuremath{\omega}}_{\mathrm{NIR}}$ is the interband exciton-creation frequency, ${\ensuremath{\omega}}_{\mathrm{THz}}$ is the frequency of the driving field, and $n$ is an integer. The intensity of the sidebands exhibits pronounced enhancement when ${\ensuremath{\omega}}_{\mathrm{THz}}$ coincides with transitions between magnetically tuned energy levels in the excitons, providing new and accurate information on the internal dynamics of excitons.

Virtual states, dynamic localization, absolute negative conductance and stimulated multiphoton emission in semiconductor superlattices

We report the observation of dynamic localization, absolute negative conductance (ANC) and multiphoton stimulated emission in sequential resonant tunnelling semiconductor superlattice bow-tie antenna coupled to intense terahertz electric fields. Perhaps the most remarkable observation is that with increasing terahertz field strength the conductance near zero d.c. bias decreases towards zero and then becomes negative. The results presented here compare favourably with a model in which virtual states, familiar from nonlinear optics, take a role similar to the unperturbed quantum well states.

Photon assisted transport through semiconductor quantum structures in intense terahertz electric fields

Quantum transport in resonant tunneling diodes, sequential resonant tunneling superlattices and miniband superlattices in the presence of intense terahertz electric fields is marked by new channels opened by the absorption or emission of one or more terahertz photons. In triple barrier resonant tunneling diodes, new transport channels supported by the absorption or stimulated emission of up to three terahertz photons are observed. In sequential resonant tunneling superlattices, dynamic localization is accompanied by absolute negative resistance. Transport in miniband resonances with controlled by coherent tunneling through several barriers and quantum wells, reveals multi-photon resonances with Bloch oscillation. Photon-assisted transport in these semiconductor quantum structures bears a strong analogy to quasi-particle tunneling and the AC Josephson effect in superconducting junctions and opens the arena of superconducting electronics to semiconductor systems. These experiments are made possible by the UCSB free-electron lasers that deliver kilowatts of tunable radiation from 120 GHz to 4.8 THz.

Multiphoton-assisted tunneling, dynamic localization and absolute negative conductance

We report the observation of Absolute Negative Conductance (ANC), multiphoton stimulated emission and dynamical localization in sequential resonant tunneling semiconductor superlattices driven by intense terahertz electric fields. With increasing terahertz field strength the conductance near zero dc bias decreases towards zero and then becomes negative. This is accompanied by new steps and plateaus that are attributed to multiphoton-assisted resonant tunneling between ground states of neighboring quantum wells accompanied by the stimulated emission of a photon.

Dynamic localization, absolute negative conductance, and stimulated, multiphoton emission in sequential resonant tunneling semiconductor superlattices.

We report the first observation of absolute negative conductance and multiphoton stimulated emission in sequential resonant tunneling semiconductor superlattices driven by intense terahertz electric fields. With increasing terahertz field strength the conductance near zero dc bias decreases towards zero and then becomes negative. This is accompanied by new steps and plateaus that are attributed to multiphoton-assisted resonant tunneling between ground states of neighboring quantum wells accompanied by the stimulated emission of a photon.

Photon-mediated sequential resonant tunneling in intense terahertz electric fields.

We have measured the current-voltage (I-V) characteristic of semiconductor superlattices in the presence of intense terahertz electric fields produced by free-electron lasers. The nonlinear I-V curves exhibit new structure that we attribute to photon-mediated sequential resonant tunneling. This tunneling process consists of well to well sequential tunneling into photon sidebands induced by the terahertz electric fields.