Ultrafast pump-probe spectroscopy via chirped-pulse up-conversion with dispersion compensation.

In this study, ultrafast transient signals were detected on a single-shot basis using chirped-pulse up-conversion spectroscopy with dispersion compensation. Unlike in the conventional time-encoding technique using chirped pulses, distortion of the ultrafast waveform was reduced by applying dispersion compensation to the chirped probe pulses and using sum-frequency generation with the chirped readout pulses. The method was applied to terahertz time-domain spectroscopy and near-infrared pump-probe spectroscopy, providing ultrafast observations with an improved temporal resolution comparable to the transform-limited pulse durations. Terahertz waveforms, Kerr rotation signals, and phonon-polariton oscillations were measured accurately with no significant waveform distortion, thereby showing the proposed scheme to be promising for single-shot pump-probe spectroscopy in a wide range of spectroscopic applications.

Observation of interplay between phonon chirality and electronic band topology.

The recently demonstrated chiral modes of lattice motion carry angular momentum and therefore directly couple to magnetic fields. Notably, their magnetic moments are predicted to be strongly influenced by electronic contributions. Here, we have studied the magnetic response of transverse optical phonons in a set of Pb1-xSnxTe films, which is a topological crystalline insulator for x > 0.32 and has a ferroelectric transition at an x-dependent critical temperature. Polarization-dependent terahertz magnetospectroscopy measurements revealed Zeeman splittings and diamagnetic shifts, demonstrating a large phonon magnetic moment. Films in the topological phase exhibited phonon magnetic moment values that were larger than those in the topologically trivial samples by two orders of magnitude. Furthermore, the sign of the effective phonon g-factor was opposite in the two phases, a signature of the topological transition according to our model. These results strongly indicate the existence of interplay between the magnetic properties of chiral phonons and the topology of the electronic band structure.

Pulse-to-pulse detection of terahertz radiation emitted from the femtosecond laser ablation process.

Determining the dynamics of electrons and ions emitted from a target material during laser ablation is crucial for desirable control of laser processing. However, these dynamics are still challenging to understand because of a lack of ubiquitous spectroscopic tools to observe tangled-up dynamics appearing at ultrafast timescales. Here by harnessing highly sensitive single-shot terahertz time-domain spectroscopy using an echelon mirror, we investigate pulse-to-pulse temporal profile of terahertz radiation generated from the material surface. We clearly found that the carrier-envelope phase and the electric field amplitude of the terahertz waveform systematically vary between the pre- and post-ablation depending on the laser fluence and irradiated pulse numbers. Our results provide a stepping-stone towards perception of Coulomb explosion occurring throughout the laser ablation process, which is indispensable for future laser processing applications.

K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2.

In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.

Magnetic Control of Soft Chiral Phonons in PbTe.

PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25 T, the phonon mode splits into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies display Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are the result of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Thus, our Letter reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.

Ultrastrong magnon-magnon coupling dominated by antiresonant interactions.

Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations owing to antiresonant terms in the Hamiltonian. However, such predictions have not been realized because antiresonant interactions are typically negligible compared to resonant interactions in light-matter systems. Here we report an unusual, ultrastrongly coupled matter-matter system of magnons that is analytically described by a unique Hamiltonian in which the relative importance of resonant and antiresonant interactions can be easily tuned and the latter can be made vastly dominant. We found a regime where vacuum Bloch-Siegert shifts, the hallmark of antiresonant interactions, greatly exceed analogous frequency shifts from resonant interactions. Further, we theoretically explored the system's ground state and calculated up to 5.9 dB of quantum fluctuation suppression. These observations demonstrate that magnonic systems provide an ideal platform for exploring exotic quantum vacuum phenomena predicted in ultrastrongly coupled light-matter systems.

High-throughput terahertz spectral line imaging using an echelon mirror.

This work demonstrates terahertz (THz) line imaging that acquires broadband spectral information by combining echelon-based single-shot THz spectroscopy with high-sensitivity phase-offset electrooptic detection. An approximately 40 dB signal-to-noise ratio is obtained for a THz spectrum from a single line of the camera, with a detection bandwidth up to 2 THz at the peak electric-field strength of 1.2 kV/cm. The spatial resolution of the image is confirmed to be diffraction limited for each spectral component of the THz wave. We use the system to image sugar tablets by quickly scanning the sample, which illustrates the capacity of the proposed spectral line imaging system for high-throughput applications.

Observation of Photoinduced Terahertz Gain in GaAs Quantum Wells: Evidence for Radiative Two-Exciton-to-Biexciton Scattering.

We have observed photoinduced negative optical conductivity, or gain, in the terahertz frequency range in a GaAs multiple-quantum-well structure in a strong perpendicular magnetic field at low temperatures. The gain is narrow band: it appears as a sharp peak (linewidth <0.45 meV) whose frequency shifts with applied magnetic field. The gain has a circular-polarization selection rule: a strong line is observed for hole-cyclotron-resonance-active polarization. Furthermore, the gain appears only when the exciton 1s state is populated, which rules out intraexcitonic transitions to be its origin. Based on these observations, we propose a possible process in which the stimulated emission of a terahertz photon occurs while two free excitons scatter into one biexciton in an energy and angular-momentum conserving manner.

Spatially resolved spectral phase interferometry with an isolated attosecond pulse.

We demonstrate spatially resolved supercontinuum spectral phase interferometry with an isolated attosecond pulse (IAP). The measured spatial-spectral interferogram over the broadband region indicates a high degree of IAP coherence in both spatial and spectral domains. In addition, the spectral-delay interferogram shows periodic temporal oscillations over the full IAP continuous spectrum, which indicates high temporal coherence. The supercontinuum spectral phase interferometry with broadband IAP will contribute to exploring spatiotemporal dispersive electronic dynamics through phase-based spectroscopy in the future.

Macroscopic Ionic Flow in a Superionic Conductor Na

Ionic motion significantly contributes to conductivity in devices such as memory, switches, and rechargeable batteries. In our work, we experimentally demonstrate that intense terahertz electric-field transients can be used to manipulate ions in a superionic conductor, namely Na^{+} ß-alumina. The cations trapped in the local potential minima are accelerated using single-cycle terahertz pulses, thereby inducing a macroscopic current flow on a subpicosecond timescale. Our results clearly show that single-cycle terahertz pulses can be used to significantly modulate the nature of superionic conductors and could possibly serve as a basic tool for application in future electronic devices.

Subcycle mid-infrared coherent transients at 4 MHz repetition rate applicable to light-wave-driven scanning tunneling microscopy.

We produce subcycle mid-infrared (MIR) pulses at a 4 MHz repetition rate via the optical rectification (OR) of sub-10 fs near-infrared pulses delivered by an optical parametric chirped pulse amplifier. The coherent MIR pulses generated in a GaSe crystal under an ultrabroadband phase-matching condition contain only 0.58-0.85 oscillation cycles within the full width at half-maximum of the intensity envelope. The use of OR enables excellent phase stability of 56 mrad over 5.6 h, which is confirmed by field-resolved detection using electro-optic sampling. An electromagnetic simulation using a finite integration technique reveals that the peak field strength can easily exceed 10 V/nm owing to the field enhancement resulting from focusing MIR pulses onto a tunnel junction.

Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode versus the Thermal Mode.

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

An echelon-based single shot optical and terahertz Kerr effect spectrometer.

We report on the design and performance of an echelon-based single shot visible/near-infrared spectrometer with adequate sensitivity to measure the nonlinear optical and terahertz Kerr effects in neat molecular liquids at room temperature. Useful molecular information spanning tens of picoseconds can be measured in just a few milliseconds, and the signal-to-noise performance scales favorably with respect to the standard stage scan technique. These results demonstrate the viability of stage-free nonlinear Kerr effect measurements and provide a route for improvements to the speed of future multidimensional Kerr effect studies.

High precision frequency measurement of terahertz waves using optical combs from a Mach-Zehnder-modulator-based flat comb generator.

Using a frequency-tunable optical comb generated from a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and a nonlinear optical fiber, we demonstrated a frequency measurement of continuous terahertz wave sources with the frequency of 0.1 and 0.6 THz by an electro-optic sampling method. We clearly observed beat signals between the terahertz source and the optical two-tone extracted from the optical comb, allowing us to determine the absolute frequency. Owing to the wide comb spacing of the MZ-FCG, this method has a high potential for the high-speed measurement of the frequency of terahertz wave sources.

Fast-frame single-shot pump-probe spectroscopy with chirped-fiber Bragg gratings.

To acquire single-shot pump-probe waveforms for each laser pulse at a high repetition rate and high signal-to-noise ratio, we combined the photonic time-stretch technique and time-encoding method using a chirped-fiber Bragg grating (CFBG) and a grating-pair pulse compressor. By changing the pre-chirping of the probe pulse, a variable time window of the pump-probe traces from 1.4 to 17 ps was demonstrated. The use of a CFBG improved the signal-to-noise ratio of the waveforms by minimizing the loss of probe pulses due to the transmission through a long fiber. These techniques are promising, for example, in applications in multi-timescale pump-probe spectroscopy of irreversible phenomena.

Simultaneous acquisition of complex transmittance and birefringence with two counter-rotating, circularly polarized THz pulses.

Here we demonstrate simultaneous measurements of the complex transmittance and birefringence using left- and right-handed circularly polarized terahertz (THz) pulses. We change the polarization of the THz pulses periodically by modulating the polarization of the pump pulses directed onto a ZnTe (111) crystal, and we convert linear to circular polarization using a broadband THz quarter-wave retarder. By integrating the alternating-emission system with the polarization-sensitive terahertz time-domain spectrometer, we are able to obtain the electric-field vector of the transmitted terahertz pulses for both the left- and right-handed circular polarizations. Utilizing this technique, we are able to measure simultaneously the frequency-dependent complex refractive indices (real and imaginary parts) and the orientations of the slow and fast axes of birefringent materials, a quartz disc and a barium borate crystal, in a single temporal sweep.

Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields.

Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the atomic-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip-sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription-in situ tailoring of single-cycle THz near fields in a tunnel junction-will offer unprecedented control of electrons for ultrafast atomic-scale electronics and metrology.

Multi-petahertz electron interference in Cr:Al2O3 solid-state material.

Lightwave-field-induced ultrafast electric dipole oscillation is promising for realizing petahertz (1015 Hz: PHz) signal processing in the future. In building the ultrahigh-clock-rate logic operation system, one of the major challenges will be petahertz electron manipulation accompanied with multiple frequencies. Here we study multi-petahertz interference with electronic dipole oscillations in alumina with chromium dopant (Cr:Al2O3). An intense near-infrared lightwave-field induces multiple electric inter-band polarizations, which are characterized by Fourier transform extreme ultraviolet attosecond spectroscopy. The interference results from the superposition state of periodic dipole oscillations of 667 to 383 attosecond (frequency of 1.5 to 2.6 PHz) measured by direct time-dependent spectroscopy and consists of various modulations on attosecond time scale through individual electron dephasing times of the Cr donor-like and Al2O3 conduction band states. The results indicate the possible manipulation of petahertz interference signal with multiple dipole oscillations using material band engineering and such a control will contribute to the study of ultrahigh-speed signal operation.

High-Acquisition-Rate Single-Shot Pump-Probe Measurements Using Time-Stretching Method.

Recent advances of ultrafast spectroscopy allow the capture of an entire ultrafast signal waveform in a single probe shot, which greatly reduces the measurement time and opens the door for the spectroscopy of unrepeatable phenomena. However, most single-shot detection schemes rely on two-dimensional detectors, which limit the repetition rate of the measurement and can hinder real-time visualization and manipulation of signal waveforms. Here, we demonstrate a new method to circumvent these difficulties and to greatly simplify the detection setup by using a long, single-mode optical fiber and a fast photodiode. Initially, a probe pulse is linearly chirped (the optical frequency varies linearly across the pulse in time), and the temporal profile of an ultrafast signal is then encoded in the probe spectrum. The probe pulse and encoded temporal dynamics are further chirped to nanosecond time scales using the dispersion in the optical fiber, thus, slowing down the ultrafast signal to time scales easily recorded with fast detectors and high-bandwidth electronics. We apply this method to three distinct ultrafast experiments: investigating the power dependence of the Kerr signal in LiNbO3, observing an irreversible transmission change of a phase change material, and capturing terahertz waveforms.

Single-shot terahertz time-domain spectroscopy in pulsed high magnetic fields.

We have developed a single-shot terahertz time-domain spectrometer to perform optical-pump/terahertz-probe experiments in pulsed, high magnetic fields up to 30 T. The single-shot detection scheme for measuring a terahertz waveform incorporates a reflective echelon to create time-delayed beamlets across the intensity profile of the optical gate beam before it spatially and temporally overlaps with the terahertz radiation in a ZnTe detection crystal. After imaging the gate beam onto a camera, we can retrieve the terahertz time-domain waveform by analyzing the resulting image. To demonstrate the utility of our technique, we measured cyclotron resonance absorption of optically excited carriers in the terahertz frequency range in intrinsic silicon at high magnetic fields, with results that agree well with published values.