Phase-locked Pulses Research Articles

Fast spectroscopic imaging using extreme ultraviolet interferometry

Extreme ultraviolet pulses as generated by high harmonic generation (HHG) are a powerful tool for both time-resolved spectroscopy and coherent diffractive imaging. However, the integration of spectroscopy and microscopy to harness the unique broadband spectra provided by HHG is hardly explored due to the challenge to decouple spectroscopic and microscopic information. Here, we present an interferometric approach to this problem that combines Fourier transform spectroscopy (FTS) with Fourier transform holography (FTH). This is made possible by the generation of phase-locked pulses using a pair of HHG sources. Crucially, in our geometry the number of interferometric measurements required is at most equal to the number of high-harmonics in the illumination, and can be further reduced by incorporating prior knowledge about the structure of the FTH sample. Compared to conventional FTS, this approach achieves over an order of magnitude increase in acquisition speed for full spectro-microscopic data, and furthermore allows high-resolution computational imaging.

Direct observation of entangled electronic-nuclear wave packets

We present momentum resolved covariance measurements of entangled electronic-nuclear wave packets created and probed with octave spanning phaselocked ultrafast pulses. We launch vibrational wave packets on multiple electronic states via multiphoton absorption, and probe these wave packets via strong field double ionization using a second phaselocked pulse. Momentum resolved covariance mapping of the fragment ions highlights the nuclear motion, while measurements of the yield as a function of the relative phase between pump and probe pulses highlight the electronic coherence. The combined measurements allow us to directly visualize the entanglement between the electronic and nuclear degrees of freedom and follow the evolution of the complete wavefunction. Published by the American Physical Society 2024

Eigen-function division multiplexed coherent optical transmission in time domain by using higher-order Hermite-Gaussian pulses.

We describe a new multiplexing technique and its application to demultiplexing in the time domain by using higher-order Hermite-Gaussian (HG) pulses, which are solutions of the Schrödinger equation. We call this technique eigen-function division multiplexing (EDM). This method enables us to further increase the total transmission capacity by superimposing many different HG pulses in the same time slot. This technique is different from a conventional optical time domain multiplexing (OTDM) technique using interleaving, where one pulse exists only in one time slot. The transmitted EDM HG pulses can be demultiplexed by adopting the time-domain orthogonality of the HG pulses (eigen-function orthogonality). The information carried by the mth-order HG pulse (HGm pulse) can be coherently detected by a photo detector, where photo-mixing with a phase-locked HGm pulse generated by a local oscillator can realize demultiplexing. The overlap integral with a different HG pulse becomes zero due to the time domain orthogonality. First, we show numerically that such a new EDM transmission scheme in the time domain is possible. We then show experimentally that we could successfully carry out an EDM HG coherent pulse transmission with four different HG pulses (HG0, HG1, HG2, and HG3), where we report a 400∼480 Gbit/s (10 Gbaud x 4 eigen-functions x 2 pol-mux.) 32∼64 QAM EDM transmission over 300∼450 km.

Measuring the motion of the eardrum using Digital Image Correlation

The human eardrum is a translucent curved membrane and is in direct contact with the middle ear bones. Studying this complicated biomechanical system remains challenging. Digital image correlation (DIC) is an optical technique that allows measuring the shape and displacement of curved surfaces in 3D. In this work, we discuss the advantages and disadvantages of using DIC to study the eardrum at frequencies around 1 kHz. Special attention is given to the application of speckle patterns required to perform DIC on the translucent eardrum. Additionally, we detail an alternative approach to high-speed cameras: phase-locked light pulses. Using light pulses synced to the phase of the sound, regular DIC cameras can be used at their full resolution. This enables a high spatial resolution to thoroughly investigate the motion of the eardrum at the relevant frequencies.

Destabilization of spin-Peierls phase via a charge-spin modulated Floquet state induced by intramolecular vibrational excitation

The electronic state control using a periodic light field is one of the central subjects in photophysics. In molecular solids, intramolecular vibrations sometimes couple to intermolecular electron transfer, thus modulating electron and spin densities of each molecule. Here, we show that in a quasi-one-dimensional molecular solid K-tetracyanoquinodimethane (TCNQ) in which TCNQ molecules are dimerized by the spin-Peierls mechanism, an intramolecular vibrational excitation with a phase-locked mid-infrared pulse induces a charge-spin modulated Floquet state, which destabilizes the spin-Peierls phase. By detecting reflectivity changes of the intramolecular transition band along the mid-infrared electric field with 6.6-fs probe pulses, we detected high-frequency oscillations reflecting electron- and spin-density modulations synchronized with intramolecular vibrations. More significantly, we observed an oscillation of ~110 cm−1 due to a dimeric mode driven by a decrease in spin-Peierls dimerization. This dimerization reduction was confirmed by measuring transient reflectivity changes of the Mott-gap transition band. These results demonstrate the effectiveness of intramolecular vibrational excitation as a method for Floquet engineering in molecular solids.

Vacuum-ultraviolet dispersive wave emission driven by phase-locked pulse pairs in a gas-filled hollow-core fiber: a numerical study

Vacuum-ultraviolet dispersive wave emission driven by phase-locked pulse pairs in a gas-filled hollow-core fiber: a numerical study

Rapid scan white light two-dimensional electronic spectroscopy with 100kHz shot-to-shot detection.

We demonstrate an approach to two-dimensional electronic spectroscopy (2DES) that combines the benefits of shot-to-shot detection at high-repetition rates with the simplicity of a broadband white light continuum input and conventional optical elements to generate phase-locked pump pulse pairs. We demonstrate this through mutual synchronization between the laser repetition rate, the acousto-optical deflector, the pump delay stage, and the CCD line camera, which allows for rapid scanning of pump optical delay synchronously with the laser repetition rate, while the delay stage is moved at a constant velocity. The resulting shot-to-shot detection scheme is repetition rate scalable and only limited by the CCD line rate and the maximum stage velocity. Using this approach, we demonstrate the measurement of an averaged 2DES absorptive spectrum in as much as 1.2s of continuous sample exposure per 2D spectrum. We achieve a signal-to-noise ratio of 6.8 for optical densities down to 0.05 with 11.6s of averaging at 100kHz laser repetition rate. Combining rapid scanning of mechanical delay lines with shot-to-shot detection as demonstrated here provides a viable alternative to acousto-optic pulse shaping approaches that is repetition-rate scalable, has comparable throughput and sensitivity, and minimizes sample exposure per 2D spectrum with promising micro-spectroscopy applications.

Terahertz Néel spin-orbit torques drive nonlinear magnon dynamics in antiferromagnetic Mn2Au

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In materials such as Mn2Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by Néel spin-orbit torques (NSOTs). However, these torques have not yet been observed on ultrafast time scales. Here, we excite Mn2Au thin films with phase-locked single-cycle terahertz electromagnetic pulses and monitor the spin response with femtosecond magneto-optic probes. We observe signals whose symmetry, dynamics, terahertz-field scaling and dependence on sample structure are fully consistent with a uniform in-plane antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance of (150 ± 50) cm2 A−1 s−1. At incident terahertz electric fields above 500 kV cm−1, we find pronounced nonlinear dynamics with massive Néel-vector deflections by as much as 30°. Our data are in excellent agreement with a micromagnetic model. It indicates that fully coherent Néel-vector switching by 90° within 1 ps is within close reach.

Dynamical aspects of excitonic Floquet states generated by a phase-locked mid-infrared pulse in a one-dimensional Mott insulator

A periodic electric field of light applied on a solid is predicted to generate coupled states of the light electric fields and electronic system called photon-dressed Floquet states. Previous studies of those Floquet states have focused on time-averaged energy-level structures. Here, we report time-dependent responses of Floquet states of excitons generated by a mid-infrared (MIR) pulse excitation in a prototypical one-dimensional (1D) Mott insulator, a chlorine-bridged nickel-chain compound, [Ni(chxn)2Cl](NO3)2 (chxn = cyclohexanediamine). Sub-cycle reflection spectroscopy on this compound using a phase-locked MIR pump pulse and an ultrashort visible probe pulse with the temporal width of ∼7 fs revealed that large and ultrafast reflectivity changes occur along the electric field of the MIR pulse; the reflectivity change reached approximately 50% of the original value around the exciton absorption peak. It comprised a high-frequency oscillation at twice the frequency of the MIR pulse and a low-frequency component following the intensity envelope of the MIR pulse, which showed different probe-energy dependences. Simulations considering one-photon-allowed and one-photon-forbidden excitons reproduced the temporal and spectral characteristics of both the high-frequency oscillation and low-frequency component. These simulations demonstrated that all responses originated from the quantum interferences of the linear reflection process and nonlinear light-scattering processes owing to the excitonic Floquet states characteristic of 1D Mott insulators. The present results lead to the developments of Floquet engineering, and demonstrate the possibility of rapidly controlling the intensity of visible or near-IR pulse by varying the phase of MIR electric fields, which will be utilized for ultrafast optical switching devices.

Coherent control of the orbital occupation driving the insulator-to-metal Mott transition in V2O3

Managing light-matter interactions on timescales faster than the loss of electronic coherence is key for achieving full quantum control of the final products in solid-solid transformations. In this Letter, we demonstrate coherent optical control of the orbital occupation that determines the insulator-to-metal transition in the prototypical Mott insulator ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$. Selective excitation of a specific interband transition with two phase-locked light pulses manipulates the occupation of the correlated bands in a way that depends on the coherent evolution of the photoinduced superposition of states. A comparison between experimental results and numerical solutions of the optical Bloch equations provides an electronic coherence time on the order of 5 fs. Temperature-dependent experiments suggest that the electronic coherence time is enhanced in the vicinity of the insulator-to-metal transition critical temperature, thus highlighting the role of fluctuations in determining the electronic coherence. These results open different routes to selectively switch the functionalities of quantum materials and coherently control solid-solid electronic transformations.

Amplification of electron-mediated spin currents by stimulated spin pumping

Stimulated emission is a process during which an atomic system gives away energy to create a coherent photon. It is fundamental to the operation of the optical amplifier. Here we propose two mechanisms for amplifying AC spin currents in a solid-state magnetic medium by a stimulated spin pumping process. The first is synchronous and consists of phase-locked pulses that perturb a precessing magnetic moment. The second is asynchronous and is driven by DC spin currents. The amplification relies on a non-adiabatic interaction taking place in a ferromagnetic medium in which the magnetic moment emits spin angular momentum in the form of spin current before equilibrating with the environment. The pumped spin current amplifies or absorbs the injected AC spin current mimicking the operation of the optical gain medium as readily seen from the gain saturation profiles. The mechanisms we propose are a first step towards a realistic spin current amplifier.

(Invited, Digital Presentation) Photocurrent Detection of Cooperative Exciton Quantum Interference in Nanocrystal Thin Films

Colloidal semiconductor quantum dots (QDs) are excellent materials for studying the photophysics of exciton complexes such as biexcitons, triexcitons, and charged excitons (trions). Dynamics of exciton complexes usually determines the performance of optoelectronic devices [1]. For example, trions reduce the optical gain threshold in QD lasers and multiexcitons increase photon-to-current conversion efficiencies via carrier multiplication processes in QD solar cells. In addition, unconventional properties of multiexcitons emerge in coherent processes in QD systems. We recently discovered the high-frequency coherent oscillations with integer multiples of the exciton resonance frequency, which is called multiexciton harmonic quantum coherence [2,3]. The multiexciton coherent properties have been investigated using optical methods. However, to clarify their roles in optoelectronic devices, it is necessary to conduct the electrical detection of multiexciton coherence.Here, we performed photocurrent detection of exciton quantum interference signals in QD thin films. The samples used in this study were closely packed PbS QD thin films. The QD films were sandwiched between the electron and hole transport layers to extract photogenerated carriers. Multiexcitons were generated by phase-locked femtosecond laser pulses, and then their photocurrent quantum interference signals were measured by using a quantum interference technique [4]. The photocurrent interference signal in the weak excitation shows a single sinusoidal oscillation originating from single excitons, while the interference signal changes to the profile involving multiple sinusoidal oscillations with increasing excitation intensity. This means that the multiexciton quantum coherence exhibiting harmonic oscillations is successfully detected in a photocurrent technique [5]. Furthermore, the amplitudes of harmonic quantum coherent signals in coupled QDs are significantly larger than those in isolated QDs. We clarified that the enhancement of the amplitudes is caused by cooperative processes in coupled QDs, where excitons in adjacent QDs interact with each other through their inter-QD coherent coupling. This cooperative effect can provide a new way to use inter-QD coherent coupling in advanced optoelectronic applications, e.g., amplifiers of coherent signals.Part of this work was supported by JSPS KAKENHI (JP19H05465 and JP22H01990) and JST CREST (JPMJCR21B4).

Mode-locking induced by coherent driving in fiber lasers.

Mode-locking is a broad concept that encompasses different processes enabling short optical pulse formation in lasers. It typically requires an intracavity mechanism that discriminates between single and collective mode lasing, which can be complex and sometimes adds noise. Moreover, known mode-locking schemes do not guarantee phase stability of the carrier wave. Here, we theoretically propose that injecting a detuned signal seamlessly leads to mode-locking in fiber lasers. We show that phase-locked pulses, akin to cavity solitons, exist in a wide range of parameters. In that regime the laser behaves as a passive resonator due to the non-instantaneous gain saturation.

Passively phase-locked Er:fiber source of single-cycle pulses in the near infrared with electro-optic timing modulation for field-resolved electron control.

A single-cycle light source in the near infrared is demonstrated enabling sensitive applications of ultrafast optical field control of electronic transport. The compact Er:fiber system generates passively phase-locked pulses with broadband spectra covering 150 THz to 350 THz at a duration of 4.2 fs and 40 MHz repetition rate. A second output arm is equipped with an electro-optic modulator (EOM) that switches the arrival time of the pulses by 700 ps at arbitrary frequencies up to 20 MHz, enabling timing modulation of the pump pulse without changing the average intensity. As a benchmark demonstration, we investigate the carrier relaxation dynamics in low-temperature-grown InGaAs (LT-InGaAs) using quantum interference currents (QuICs).

Scalable high-repetition-rate sub-half-cycle terahertz pulses from spatially indirect interband transitions

Intense phase-locked terahertz (THz) pulses are the bedrock of THz lightwave electronics, where the carrier field creates a transient bias to control electrons on sub-cycle time scales. Key applications such as THz scanning tunnelling microscopy or electronic devices operating at optical clock rates call for ultimately short, almost unipolar waveforms, at megahertz (MHz) repetition rates. Here, we present a flexible and scalable scheme for the generation of strong phase-locked THz pulses based on shift currents in type-II-aligned epitaxial semiconductor heterostructures. The measured THz waveforms exhibit only 0.45 optical cycles at their centre frequency within the full width at half maximum of the intensity envelope, peak fields above 1.1 kV cm−1 and spectral components up to the mid-infrared, at a repetition rate of 4 MHz. The only positive half-cycle of this waveform exceeds all negative half-cycles by almost four times, which is unexpected from shift currents alone. Our detailed analysis reveals that local charging dynamics induces the pronounced positive THz-emission peak as electrons and holes approach charge neutrality after separation by the optical pump pulse, also enabling ultrabroadband operation. Our unipolar emitters mark a milestone for flexibly scalable, next-generation high-repetition-rate sources of intense and strongly asymmetric electric field transients.

Proposal to generate a pair of intense independently tunable attosecond pulses from undulator radiation

We propose and numerically evaluate two schemes to generate a pair of extreme-ultraviolet monocycle pulses with gigawatt-level peak power, whose time delay and central wavelengths can be precisely controlled. The methods are based on coherent emission of radiation by an ultrarelativistic electron beam with a current profile given by a chirped sinusoid, which is generated through the interaction with a conventional broadband laser. The possibility to produce phase-locked attosecond pulses with independently tunable properties in the extreme-ultraviolet spectral region has the potential to significantly advance studies of charge dynamics in molecules of biological interests.

A perfect X-ray beam splitter and its applications to time-domain interferometry and quantum optics exploiting free-electron lasers

X-ray free-electron lasers (FELs) deliver ultrabright X-ray pulses, but not the sequences of phase-coherent pulses required for time-domain interferometry and control of quantum states. For conventional split-and-delay schemes to produce such sequences, the challenge stems from extreme stability requirements when splitting Ångstrom wavelength beams, where the tiniest path-length differences introduce phase jitter. We describe an FEL mode based on selective electron-bunch degradation and transverse beam shaping in the accelerator, combined with a self-seeded photon emission scheme. Instead of splitting the photon pulses after their generation by the FEL, we split the electron bunch in the accelerator, prior to photon generation, to obtain phase-locked X-ray pulses with subfemtosecond duration. Time-domain interferometry becomes possible, enabling the concomitant program of classical and quantum optics experiments with X-rays. The scheme leads to scientific benefits of cutting-edge FELs with attosecond and/or high-repetition rate capabilities, ranging from the X-ray analog of Fourier transform infrared spectroscopy to damage-free measurements.

Achieving sub-diffraction spatial resolution using combined Fourier transform spectroscopy and nonlinear optical microscopy.

Fourier transform nonlinear optical microscopy is used to perform nonlinear spectroscopy of single gold nanorods in an imaging platform, which enables sub-diffraction spatial resolution. The nonlinear optical signal is detected as a function of the time delay between two phase-locked pulses, forming an interferogram that can be used to retrieve the resonant response of the nanoparticles. Detection of the nonlinear signal through a microscopy platform enables wide-field hyperspectral imaging of the longitudinal plasmon resonances in individual gold nanorods. Super-resolution capabilities are demonstrated by distinguishing multiple nanorods that are co-located within the optical diffraction limit and are spatially separated by only tens of nanometers. The positions and resonance energies obtained through Fourier transform nonlinear optical microscopy agree with the relative positions and aspect ratios deduced from electron microscopy.

Full characterization of a phase-locked DUV double pulse generated in an all-reflective shaping setup working under grazing incidence in a broad spectral range

Controlling the temporal and spectral properties of ultrashort laser pulses in the visible and near-infrared spectral range by means of a femtosecond pulse-shaping device is a powerful tool with many applications in ultrafast spectroscopy. A major and successful concept is known as the 4f design, which has a symmetric zero-dispersion-compressor geometry. Most 4f pulse shapers rely on using transmissive optics in their beam path limiting the operational wavelength ranges. In the present contribution, we use an all-reflective shaping setup to generate a phase-locked 266 nm double pulse to benchmark its performance in the limit of short wavelengths. The setup comprises the complete spectral amplitude and phase diagnostics for quantitative analysis of the pulse properties before and after the shaper using the technique of frequency-resolved optical gating. The measured time–frequency spectra are in good agreement with optical simulations. The geometry and hardware of the device including the optical components are designed, such that all harmonics of the deep UV pulses travel the same path, giving the instrument the ability to work with soft X-ray pulses, under vacuum conditions, down to the few-nanometer wavelength scale.

A versatile two-colour pulse generation setup with active feedback phase-locking

We present a technique for generating phase-locked two-colour pulses with controllable electric field waveform, whereby the polarization states of the two frequency components, as well as their intensities, relative phase and pulse delay can be completely controlled. To characterize the phase stability and the waveform of the output two-colour pulse in situ, we measured the momentum spectra of protons from dissociative ionization of the hydrogen molecule for different combinations of polarizations.