Single-cycle Terahertz Pulses Research Articles

(Invited) Single-Molecule Electroluminescence Triggered by Field-Driven Tunneling Electrons

Exciton dynamics governs versatile functions of molecules such as photon emission, photovoltaic and photo chemical reactions. For clarifying the dynamics of excitons in a molecule, an optical measurement with high spatiotemporal resolution is required because the excitons have short lifetimes and their behavior in the local region is important for optical properties. Luminescence spectroscopy based on a scanning tunneling microscope (STM) enable to investigate optical properties with sub-nanometer spatial resolution[1-2]. In this method, scanning tunneling luminescence (STL) spectroscopy, a molecule is excited by the tunneling current of an STM, which can be controlled with atomic spatial resolution. In contrast, the time resolution of STL spectroscopy is limited, because the tunnel current is controlled by electrical circuits. Therefore, this method cannot be used to track the picosecond dynamics of excitons. For overcoming the time-resolution of STL, we have combined a single-cycle terahertz (THz) pulse with STL spectroscopy. By coupling the THz pulse to a tip of an STM, the photoelectric field of THz pulse with sub picosecond time width is confined to the tip apex, which enables to manipulate the tunneling electrons with high spatiotemporal resolutions[4-9]. In this work, we succeeded in forming an exciton in a single molecule by using THz-field-driven current and clarifying the mechanism of the exciton formation.Single-cycle THz pulses were generated by using optical rectification of near infrared (NIR) laser pulses from a Yb fiber laser using a tilted-pulse-front configuration (Fig. 1a). The generated THz pulses were guided into the STM chamber and focused by using a lens near the tip. We prepared a clean Ag(111) surface and deposited NaCl films. Then, we deposited Pd phthalocyanine (PdPc) molecules on the NaCl/Ag(111) surface. Figure 1b shows THz-STL spectra of a PdPc on NaCl/Ag(111). Several sharp peaks appeared at around 1.9 eV in the red spectrum (with THz irradiation) and no peak appeared in the gray spectrum (without THz). The peak energy of the main peak is identical to the fluorescence peak of a PdPc measured by conventional STL[3], thus we concluded that the singlet excited state was formed by THz-field-driven tunneling. In this presentation, we would like to discuss the detailed mechanism for the exciton formation in a molecule by THz-field-driven electrons.[1] H. Imada et al., Nature 538, 364 (2016). [2] K. Kimura et al., Nature 570, 210 (2019). [3] S. Cao et al., Nat. Chem. 13, 766 (2021). [4] T. L. Cocker et al., Nat. Photon. 7, 620 (2013). [5] K. Yoshioka et al., Nat. Photon. 10, 762 (2016). [6] V. Jelic et al., Nature Physics 13, 591 (2017). [7] D. Peller et al., Nature 585, 58 (2020). [8] L. Wang 376, 401 (2022). [9] K. Kimura et al., ACS photon. 8, 982 (2021). Figure 1

Generation of extremely strong accelerating electric field by focusing radially polarized THz pulses with a paraboloid ring

Efficient acceleration of electrically charged particles by focused electromagnetic pulses requires a strong accelerating electric field as well as a sufficiently large interaction region. Single-cycle terahertz pulses are promising candidates for such applications due to their advantageous wavelength and available peak electric field at the MV/cm level. We propose and characterize a focusing optics for a possible electron vacuum accelerator. This consists of a reflaxicon for beam shaping and a ring-like segment of an on-axis parabolic mirror for tight focusing of the radially polarized THz pulse in order to reach a strong accelerating electric field. The electric field distribution in the focal region was determined by the Stratton–Chu vector diffraction method. Semi-analytical and purely analytical formulae were also derived, simplifying the computation procedure. By focusing radially polarized single-cycle terahertz pulses with 3mJ pulse energy, calculations predict accelerating electric field component in the order of 100MV/cm, well suitable for particle acceleration.

THz vs NIR laser-assisted atom probe tomography of LaB6 samples

Terahertz (THz) radiation with low-energy photons (meV) is used in a wide range of applications, such as microscopy, sensing, and spectroscopy. However, recently, high amplitude THz pulses of MV/cm have been generated and used for electron emission and ion evaporation from field emitters, opening up the possibility of using high amplitude THz pulses for material imaging by THz-assisted atom probe tomography (APT). In this work, we compare the APT analyses of lanthanum hexaboride (LaB6) samples using a femtosecond near-infrared laser with those obtained using high-amplitude single-cycle THz pulses. The atomic-scale characterization of stoichiometric LaB6 is challenging in laser-assisted APT due to the detection losses of boron ions. Here, we show that the THz radiation reduces the emission of molecular ions and multiple detection events, and it increases the charge state of the emitted ions. All these effects result in an improvement in boron detection. Furthermore, the emission dynamics of boron and lanthanum ions differ in their evaporation times when using THz radiation. This work emphasizes the ability of high-amplitude, single-cycle THz pulses to well control material analysis in APT, leading to better results on chemical composition. It also paves the way for the use of this radiation for material manipulation.

Time-resolved THz Stark spectroscopy of molecules in solution

For decades, it was considered all but impossible to perform Stark spectroscopy on molecules in a liquid solution, because their concomitant orientation to the applied electric field results in overwhelming background signals. A way out was to immobilize the solute molecules by freezing the solvent. While mitigating solute orientation, freezing removes the possibility to study molecules in liquid environments at ambient conditions. Here we demonstrate time-resolved THz Stark spectroscopy, utilizing intense single-cycle terahertz pulses as electric field source. At THz frequencies, solute molecules have no time to orient their dipole moments. Hence, dynamic Stark spectroscopy on the time scales of molecular vibrations or rotations in both non-polar and polar solvents at arbitrary temperatures is now possible. We verify THz Stark spectroscopy for two judiciously selected molecular systems and compare the results to conventional Stark spectroscopy and first principle calculations.

The spectrum broadening of a single-cycle terahertz pulse caused by phase self-modulation in a nonlinear medium and the spectrum of radiation generated in its field at tripled frequencies

When the number of oscillations in an optical pulse decreases, the emission spectrum generated at triple frequencies and the spectrum of the fundamental pulse, broadened due to phase self-modulation during propagation in a medium with cubic nonlinearity, begin to overlap. This work demonstrates that the overlap magnitude of the spectrum broadening for a single-period terahertz pulse, caused by the emission of triple frequencies and phase self-modulation of the fundamental pulse, is |s| = 0.85. The complex spectra are phase-shifted by p, which mutually weakens these nonlinear effects. For a single-period pulse, the attenuation coefficient is 7.7. The frequency-dependent inhomogeneity of mutual attenuation of nonlinear effects results in the absence of radiation at the triple frequency relative to the frequency of the maximum spectrum of the single-period wave in the nonlinear medium, while the maximum spectrum of the generated high-frequency radiation shifts to quadrupled frequencies. Thus, for terahertz waves with a small number of oscillations, new possibilities for controlling their parameters during nonlinear processes in optical media open up.

Hollow metal tubes for efficient electron manipulation using terahertz surface waves.

Compact electron sources have been instrumental in multidiscipline sciences including fundamental physics, oncology treatments, and advanced industries. Of particular interest is the terahertz-driven electron manipulation that holds great promise for an efficient high gradient of multi-GeV/m inside a regular dielectric-lined waveguide (DLW). The recent study relying on terahertz surface waves has demonstrated both high terahertz energy and improved coupling efficiency with the DLW. However, the large energy spread pertaining to the laser-induced electron pulse impedes the practical use of the system. Here, we propose a scheme for extending the idea of surface-wave-driven electron manipulation to mature electron sources such as commercial direct-current and radio-frequency electron guns. By using a simple hollow cylinder tube for electron transmission, we show that the electron energy modulation can reach up to 860 keV, or compress the electron pulse width to 15 fs using a 2.9 mJ single-cycle terahertz pulse. The trafficability of the hollow tube also allows for a cascade of the system, which is expected to pave the way for compact and highly efficient THz-driven electron sources.

Quantum control of flying doughnut terahertz pulses.

The ability to manipulate the multiple properties of light diversifies light-matter interaction and light-driven applications. Here, using quantum control, we introduce an approach that enables the amplitude, sign, and even configuration of the generated light fields to be manipulated in an all-optical manner. Following this approach, we demonstrate the generation of "flying doughnut" terahertz (THz) pulses. We show that the single-cycle THz pulse radiated from the dynamic ring current has an electric field structure that is azimuthally polarized and that the space- and time-resolved magnetic field has a strong, isolated longitudinal component. We apply the flying doughnut pulse for a spectroscopic measurement of the water vapor in ambient air. Pulses such as these will serve as unique probes for spectroscopy, imaging, telecommunications, and magnetic materials.

Terahertz Spatiotemporal Wave Synthesis in Random Systems.

Complex media have emerged as a powerful and robust framework to control light-matter interactions designed for task-specific optical functionalities. Studies on wavefront shaping through disordered systems have demonstrated optical wave manipulation capabilities beyond conventional optics, including aberration-free and subwavelength focusing. However, achieving arbitrary and simultaneous control over the spatial and temporal features of light remains challenging. In particular, no practical solution exists for field-level arbitrary spatiotemporal control of wave packets. A new paradigm shift has emerged in the terahertz frequency domain, offering methods for absolute time-domain measurements of the scattered electric field, enabling direct field-based wave synthesis. In this work, we report the experimental demonstration of field-level control of single-cycle terahertz pulses on arbitrary spatial points through complex disordered media.

Efficient and Continuous Carrier-Envelope Phase Control for Terahertz Lightwave-Driven Scanning Probe Microscopy.

The fundamental understanding of quantum dynamics in advanced materials requires precise characterization at the limit of spatiotemporal resolution. Ultrafast scanning tunneling microscopy is a powerful tool combining the benefits of picosecond time resolution provided by single-cycle terahertz (THz) pulses and atomic spatial resolution of a scanning tunneling microscope (STM). For the selective excitation of localized electronic states, the transient field profile must be tailored to the energetic structure of the system. Here, we present an advanced THz-STM setup combining multi-MHz repetition rates, strong THz near fields, and continuous carrier-envelope phase (CEP) control of the transient waveform. In particular, we employ frustrated total internal reflection as an efficient and cost-effective method for precise CEP control of single-cycle THz pulses with >60% field transmissivity, high pointing stability, and continuous phase shifting of up to 0.75 π in the far and near field. Efficient THz generation and dispersion management enable peak THz voltages at the tip-sample junction exceeding 20 V at 1 MHz and 1 V at 41 MHz. The system comprises two distinct THz generation arms, which facilitate individual pulse shaping and amplitude modulation. This unique feature enables the flexible implementation of various THz pump-probe schemes, thereby facilitating the study of electronic and excitonic excited-state propagation in nanostructures and low-dimensional materials systems. Scalability of the repetition rate up to 41 MHz, combined with a state-of-the-art low-temperature STM, paves the way toward the investigation of dynamical processes in atomic quantum systems at their native length and time scales.

Optimized terahertz pulse generation with chirped pump pulses from an echelon-based tilted-pulse-front (TPF) scheme.

We successfully demonstrated the generation of single-cycle terahertz (THz) pulses through tilted-pulse-front (TPF) pumping using a reflective echelon in a lithium niobate crystal. By optimizing the pump pulse duration using a chirp, we achieved a maximum pump-to-THz conversion efficiency of 0.39%. However, we observed that the saturation behavior began at a relatively low pump energy (0.37 mJ), corresponding to a pump intensity of 22 GW/cm2. To elucidate this behavior, we measured the near- and far-field THz beam profiles and found variations in their beam characteristics, such as the beam size, location, and divergence angle in the plane of the tilted pulse direction, with the pump energy (intensity). This nonlinear behavior is attributed to the reduced effective interaction length, which ultimately leads to the saturation of THz generation. The results obtained from our study suggest that it is feasible to develop an effective THz source using echelon-based TPF pumping while also considering the impact of nonlinear saturation effects.

Single-Cycle Terahertz Pulse Generation via Superimposed Diffraction With Cherenkov Effect

The ultrashort terahertz (THz) pulse with a broad frequency spectrum has important applications in THz spectroscopy and imaging. Previous THz-pulse generating methods are restricted by either low power or high cost. Here, we propose a novel scheme for generating ultrashort THz pulses, which uses a low-energy sheet electron beam to drive an inclined array of size-graded grooves. The diffraction radiation (DR) from each groove is successively generated and coherently superimposed at a specified direction via the Cherenkov effect. A single-cycle THz pulse with a spectrum band covering the region from 0.1 to 1 THz can be realized. In addition, using several arrays of grooves, a train of ultrashort THz pulses can be further obtained. The radiation power, which depends on the beam current, reaches as high as several kilowatts by using available electron beams. It affords an effective way of generating ultrashort THz pulses with broad application prospects.

Ultrafast Martensitic Phase Transition Driven by Intense Terahertz Pulses

We report on an ultrafast nonequilibrium phase transition with a strikingly long-lived martensitic anomaly driven by above-threshold single-cycle terahertz pulses with a peak field of more than 1 MV/cm. A nonthermal, terahertz-induced depletion of low-frequency conductivity in Nb 3 Sn indicates increased gap splitting of high-energy Γ 12 bands by removal of their degeneracies, which induces the martensitic phase above their equilibrium transition temperature. In contrast, optical pumping leads to a Γ 12 gap thermal melting. Such light-induced nonequilibrium martensitic phase exhibits a substantially enhanced critical temperature up to ∼100 K, i.e., more than twice the equilibrium temperature, and can be stabilized beyond technologically relevant, nanosecond time scales. Together with first-principle simulations, we identify a compelling terahertz tuning mechanism of structural order via Γ 12 phonons to achieve the ultrafast phase transition to a metastable electronic state out of equilibrium at high temperatures far exceeding those for equilibrium states.

Effect of antiferromagnetic order on a propagating single-cycle THz pulse

Employing polarization sensitive terahertz (THz) transmission spectroscopy, we explored how the waveform of initially single-cycle linearly polarized THz pulses changes upon propagation through a thick antiferromagnetic crystal of CoF2. The changes upon propagation through CoF2 are found to depend strongly on both the incoming polarization and temperature. In particular, the ellipticity and polarization rotation acquired by initially linearly polarized light are quantified and explained in terms of magnetic linear birefringence and dichroism. Although the magneto-optical effects are often considered to be relatively weak, our experiments reveal that the polarization of the THz pulse substantially changes along the pulse duration. The pulse shape is further complicated by features assigned to the formation of magnon-polaritons. The findings clearly show the importance of accounting for propagation effects in antiferromagnetic spintronics and magnonics.

Polarized light emission from graphene induced by terahertz pulses

Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was investigated experimentally and explained theoretically. We found that emitted photons are polarized predominantly perpendicular to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. Comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate a momentum isotropization time for electrons in graphene to be ~25 fs and roughly reconstruct the distribution function evolution in k-space.

Nonlinear terahertz Kerr effect in quasi-2D MnPS3

The quadratic electro-optic effect (Kerr effect) is shown to be efficiently induced by a terahertz (THz) electric field in the quasi-two-dimensional (2D) material MnPS3. The waveform of the THz-induced response practically follows the intensity of the employed nearly single-cycle THz pulse. While neither THz-induced absorption nor the linear electro-optical effect are observed, we demonstrate that the THz electric field induces the refraction coefficient anisotropy experienced by light at the wavelength of 800 nm. In our experiment, the anisotropy results in polarization rotation of the light and corresponds to a nonlinear refractive index of the sample of approximately 13.1 × 10−14 W−1 cm2. This promotes the quasi-2D MnPS3 as a promising candidate for practical application in future ultrafast electro-optical devices.

Terahertz-slicing - an all-optical synchronization for 4th generation light sources.

A conceptually new approach to synchronizing accelerator-based light sources and external laser systems is presented. The concept is based on utilizing a sufficiently intense accelerator-based single-cycle terahertz pulse to slice a thereby intrinsically synchronized femtosecond-level part of a longer picosecond laser pulse in an electro-optic crystal. A precise synchronization of the order of 10 fs is demonstrated, allowing for real-time lock-in amplifier signal demodulation. We demonstrate successful operation of the concept with three benchmark experiments using a 4th generation accelerator-based terahertz light source, i.e. (i) far-field terahertz time-domain spectroscopy, (ii) terahertz high harmonic generation spectroscopy, and (iii) terahertz scattering-type scanning near-field optical microscopy.

Probing Charge Transport in Sp 3-Functionalized Single-Walled Carbon Nanotubes with Terahertz Spectroscopy

The covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp 3-hybridized defects gives rise to tunable, narrowband emission in the near-infrared and increases their photoluminescence quantum yields [ACS Nano 2019, 13, 9259]. To enable their application in electrically driven light sources, a detailed understanding of the impact of sp 3 defects on charge transport within individual nanotubes and nanotube networks is required. Here, we investigate the ultrafast carrier dynamics in sp 3-functionalized, polymer-sorted (6,5) SWCNT dispersions and thin films using optical-pump terahertz-probe (OPTP) spectroscopy. OPTP spectroscopy relies on the photo-generation of charge carriers that are subsequently probed by a single-cycle terahertz pulse, rendering it a highly suitable technique for the determination of intrinsic charge transport properties (e.g., carrier mobility) in a quantitative and contact-free fashion. We observe a large transient photoconductivity of pristine and functionalized nanotubes that is dominated by the contribution of free charge carriers. The photoconductivity intensity and lifetime decrease upon functionalization, corroborating the impact of sp 3 defects on intra-nanotube charge transport. Through combination of (temperature-dependent) OPTP measurements of sp 3-functionalized SWCNT dispersions and films with electrical measurements of ambipolar, light-emitting field-effect transistors based on functionalized SWCNT networks [ACS Nano 2021, 15, 10451], we can separate the contributions of sp 3 defects (intra-nanotube) and nanotube junctions (inter-nanotube) to charge transport in such networks and distinguish the short- and long-range transport characteristics. Fig. 1: Ultrafast optical-pump terahertz-probe spectroscopy is a highly suitable technique to study the intrinsic charge transport properties of sp 3-functionalized (6,5) SWCNTs in dispersions and thin films. Figure 1

(Invited) Development of THz-Field-Driven Scanning Tunneling Luminescence Spectroscopy for Future Investigation of Exciton Dynamics

Exciton dynamics is responsible for the photophysical and photochemical functions, such as energy harvesting, light emission, and photosynthesis. In particular, energy conversions, transfers, and dissipations at interfaces and nanostructures govern the dynamics of excitons. Nevertheless, the spatial resolutions of conventional optical methods are limited to several hundred nanometers due to the diffraction limit of light. Luminescence measurements combined with scanning tunneling microscopy (STM) enable to investigate optical properties with sub-nanometer spatial resolution in various systems such as metals[1], semiconductors[2], 2D materials[3], and molecules[4-5]. In this method, the scanning tunneling luminescence (STL) spectroscopy, photons induced by the tunneling current of STM are detected. Since the tunneling current can be controlled with atomic spatial resolution, it is possible to investigate optical phenomena in localized regions. While STL spectroscopy is a powerful tool for investigating exciton dynamics with atomic-scale spatial resolution, the time resolution of STL spectroscopy is limited because it uses “steady-state” tunneling current for the excitation. Therefore, this method cannot be applicable to track the ultrafast exciton dynamics in real-time.For overcoming the limitation of time resolution of STM, a single-cycle terahertz (THz) electric field has recently been installed to drive tunneling electrons[6-9]. By coupling the THz pulse to an atomically sharp metal tip of STM, the electric field is confined to the tip, and it enables to manipulate the tunneling electrons with extremely high spatiotemporal resolution. This method, THz-STM, has been utilized to track the ultrafast carrier dynamics[6-8] and motion of a molecule[9] with sub-picosecond time resolution. In this work, we combined a photon detection system with THz-STM to establish THz-STL spectroscopy (Fig. 1a). We measured visible photon emission from the radiative decay of a localized plasmon as the first demonstration of THz-STL spectroscopy[10]. We anticipate that the THz-STL spectroscopy will open new avenues for “real-time” and “real-space” investigations of exciton dynamics in near future.In this work, we generated single-cycle THz pulses via optical rectification of laser pulses from a Yb fiber laser in a LiNbO3 prism using a tilted-pulse-front configuration (Fig. 1b). The generated THz pulses were guided into a low-temperature STM and focused to the STM junction (Fig. 1a). Figure 1c shows a current trace on Ag(111) surface, where the current induced by the THz pulses was measured as 2.65 pA. Based on the simulation of electron tunneling probability, the maximum magnitude of the voltage applied by the THz pulse was estimated to be ~6.5 V. Figure 1d shows STL spectra when the THz pulse was either introduced to the STM (red) or blocked (grey). Whereas no peak is seen in the grey spectrum, a broad peak ranging from 1.3 eV to 2.3 eV appears in the red spectrum, which is originated from the radiative decay of a plasmon localized in the gap.In this presentation, we would like to discuss the detailed mechanism of plasmon excitation by THz-field-driven electrons as well as the challenges of applying THz-STL to the real-time investigation of exciton dynamics in a molecule.[1] R. Berndt et al., Phys. Rev. Lett. 67, 3796 (1991). [2] J. K. Gimzewski et al., Z. Phys. B Condensed Matter 72, 497 (1988). [3] Schuler et al., Sci. Adv. 6, eabb5988 (2020). [4] H. Imada et al., Nature 538, 364 (2016). [5] K. Kimura et al., Nature 570, 210 (2019). [6] T. L. Cocker et al., Nat. Photon. 7, 620 (2013). [7] K. Yoshioka et al., Nat. Photon. 10, 762 (2016). [8] V. Jelic et al., Nature Physics 13, 591 (2017). [9] D. Peller et al., Nature 585, 58 (2020). [10] K. Kimura et al., ACS photon. 8, 982 (2021). Figure 1

Parameter sensitivities in tilted-pulse-front based terahertz setups and their implications for high-energy terahertz source design and optimization.

Despite the popularity and ubiquity of the tilted-pulse-front technique for single-cycle terahertz (THz) pulse generation, there is a deficit of experimental studies comprehensively mapping out the dependence of the performance on key setup parameters. The most critical parameters include the pulse-front tilt, the effective length of the pump pulse propagation within the crystal as well as effective length over which the THz beam interacts with the pump before it spatially walks off. Therefore, we investigate the impact of these parameters on the conversion efficiency and the shape of the THz beam via systematically scanning the 5D parameter space spanned by pump fluence, pulse-front-tilt, crystal-position (2D), and the pump size experimentally. We verify predictions so far only made by theory regarding the optimum interaction lengths and map out the impact of cascading on the THz radiation generation process. Furthermore, distortions imposed on the spatial THz beam profile for larger than optimum interaction lengths are observed. Finally, we identify the most sensitive parameters and, based on our findings, propose a robust optimization strategy for tilted-pulse-front THz setups. These findings are relevant for all THz strong-field applications in high demand of robust high-energy table-top single-cycle THz sources such as THz plasmonics, high-harmonic generation in solids as well as novel particle accelerators and beam manipulators.

Terahertz Kerr effect in a methylammonium lead bromide perovskite crystal

We have observed short-lived optical birefringence in a CH$_3$NH$_3$PbBr$_3$ single crystal induced by a powerful nearly single-cycle terahertz pulse. Apart from the instantaneous contribution that follows the profile of the squared pump electric field, the recorded anisotropy signal contains an exponential component decaying in $\sim$ 350 fs, underdamped oscillations at the frequency of $\sim$ 0.16 THz and an intermediate picosecond relaxation process with a Gaussian tail. We associate these three non-trivial features with, respectively, Kerr effect in the inorganic lattice, terahertz-induced transient alignment of CH$_3$NH$_3^+$ cations, and their coherent rotation excited by the terahertz pulse in a Raman process.