Optical Cycles Research Articles

44-fs pulse generation at 2.05 µm from a SESAM mode-locked Tm:GdScO3 laser.

Pulses as short as 44 fs (6 optical cycles) with a spectral width of 120 nm are generated from a mode-locked solid-state laser near 2 µm employing an orthorhombic Tm:GdScO3 perovskite crystal. The average power amounts to 188 mW at a repetition rate of ∼77.6 MHz. The strong inhomogeneous line broadening in GdScO3 suggests optimum conditions for few-optical-cycle pulse generation of rare-earth ion doped GdScO3 bulk lasers.

2-µm nonlinear post-compression for generating ∼100-MHz few-cycle laser pulses with watt-level average power.

We firstly report a high pulse repetition rate (101.4 MHz) nonlinear post-compression based on the normal dispersion fiber (NDF) operating in 2-µm wavelength region. With only one-stage NDF-based nonlinear pulse compressor, the 2-µm ultrafast laser pulses are compressed from ∼460 fs down to 70 fs, corresponding to ∼10.4 optical oscillation cycle. With two-stage nonlinear pulse compressor, the input ultrafast laser pulses are further compressed to 28.3 fs (∼4.3 optical oscillation cycle). In each case, the average power of the compressed 2-µm laser pulses exceeds 1 W, which is believed to be the highest average power never achieved at ∼100-MHz pulse repetition rate. The efficiencies of the one-stage and two-stage nonlinear pulse compressors are 64% and 47% respectively.

Terawatt-Class Few-Cycle Short-Wave Infrared Vortex Laser

As a driving source for many nonlinear vortex phenomena, such as the generation of isolated attosecond optical vortices, terahertz vortices, etc., terawatt-class few-cycle short-wave infrared vortex lasers are now attracting widespread attention. However, because the vortex characteristics of optical vortices are difficult to maintain in the amplification and compression stages, the generation of high-intensity few-cycle vortex lasers is still in the exploratory stage. In this article, we report 20-Hz, 18.6-mJ, 60-fs, and 1.45-μm infrared vortex lasers with 1, 2, and 3 topological charges successfully generated in an optical parametric chirped pulse amplification system. A clean intensity node at the beam center is observed and highly stable propagation in free space is demonstrated. Moreover, this high-energy vortex pulse is spectrally broadened in multiple thin plates and temporally compressed to 10.59 fs (2.2 optical cycles) with chirped mirrors, corresponding to a peak power of 1.08 TW, while highly preserving the vortex information. We believe that the generated high-energy few-cycle vortex laser has important applications from vortex optics to strong-field physics.

Single-stage GaSe OPCPA delivering high-energy few-cycle pulses at 11 µm wavelength – INVITED

The generation of sub-five optical cycle pulses centered at 11.2 µm wavelength with 50 µJ energy at a 1 kHz repetition rate is reported. A GaSe optical parametric chirped pulse amplifier (OPCPA) is driven by the residual 2.0 µm pump and 5 µm idler of a high-energy midwave-IR OPCPA. The latter serves as driver for hard X-ray generation and this makes the achieved fs longwave-IR pulses available for X-ray pump-probe experiments.

Table-top optical parametric chirped pulse amplifiers: past and present

The generation of power- and wavelength-scalable few optical cycle pulses remains one of the major challenges in modern laser physics. Over the past decade, the development of table-top optical parametric chirped pulse amplification-based systems was progressing at amazing speed, demonstrating excellent performance characteristics in terms of pulse duration, energy, peak power and repetition rate, which place them at the front line of modern ultrafast laser technology. At present, table-top optical parametric chirped pulse amplifiers comprise a unique class of ultrafast light sources, which currently amplify octave-spanning spectra and produce carrier-envelope phase-stable, few optical cycle pulses with multi-gigawatt to multi-terawatt peak powers and multi-watt average powers, with carrier wavelengths spanning a considerable range of the optical spectrum. This article gives an overview on the state of the art of table-top optical parametric chirped pulse amplifiers, addressing their relevant scientific and technological aspects, and provides a short outlook of practical applications in the growing field of ultrafast science.

Short mid-infrared watt-level all-fiber nonlinear pulse compressor above 100-MHz pulse repetition rate

Abstract We firstly report a 2-μm all-fiber nonlinear pulse compressor based on two pieces of normal dispersion fiber (NDF), which enables a high-power scaling ability of watt-level and a high pulse compression ratio of 13.7. With the NDF-based all-fiber nonlinear pulse compressor, the 450-fs laser pulses with a repetition rate of 101.4 MHz are compressed to 35.1 fs, corresponding to a 5.2 optical oscillation cycle at the 2-μm wavelength region. The output average power reaches 1.28 W, which is believed to be the highest value never achieved from the previous 2-μm all-fiber nonlinear pulse compressors with a high pulse repetition rate above 100 MHz. The dynamic evolution of the ultrafast pulse inside the all-fiber nonlinear pulse compressor is numerically analyzed, matching well with the experimental results.

Relativistic ab initio study on the spectroscopic and radiative properties of the lowest states and modeling of the optical cycles for the LiFr molecule

The LiFr diatomic represents a promising candidate for indirect laser cooling that has not yet been investigated not theoretically or experimentally. The potential energy curves of the ground and low-lying excited states of the LiFr heteronuclear alkali metal dimer are calculated using the Fock-space relativistic coupled cluster theory for the first time. A number of properties such as the electronic term energies, equilibrium internuclear distances, transition and permanent dipole moments, sequences of vibrational energies, harmonic vibrational frequencies, Franck–Condon factors, and radiative lifetimes (including bound and free transitions) are predicted. The probabilities of the two-step schemes (optical cycles) for the transfer process of the LiFr molecules from high excited vibrational states to the ground vibronic state are also predicted. The data obtained would be useful for laser cooling and spectral experiments with LiFr molecules.

Controlling Floquet states on ultrashort time scales

The advent of ultrafast laser science offers the unique opportunity to combine Floquet engineering with extreme time resolution, further pushing the optical control of matter into the petahertz domain. However, what is the shortest driving pulse for which Floquet states can be realised remains an unsolved matter, thus limiting the application of Floquet theory to pulses composed by many optical cycles. Here we ionized Ne atoms with few-femtosecond pulses of selected time duration and show that a Floquet state can be observed already with a driving field that lasts for only 10 cycles. For shorter pulses, down to 2 cycles, the finite lifetime of the driven state can still be explained using an analytical model based on Floquet theory. By demonstrating that the amplitude and number of Floquet-like sidebands in the photoelectron spectrum can be controlled not only with the driving laser pulse intensity and frequency, but also by its duration, our results add a new lever to the toolbox of Floquet engineering.

Vibrationally resolved optical excitations of the nitrogen-vacancy center in diamond

A comprehensive description of the optical cycle of spin defects in solids requires the understanding of the electronic and atomistic structure of states with different spin multiplicity, including singlet states which are particularly challenging from a theoretical standpoint. We present a general framework, based on spin-flip time-dependent density function theory, to determine the excited state potential energy surfaces of the many-body singlet states of spin defects; we then predict the vibrationally resolved absorption spectrum between singlet shelving states of a prototypical defect, the nitrogen-vacancy center in diamond. Our results, which are in very good agreement with experiments, provide an interpretation of the measured spectra and reveal the key role of specific phonons in determining absorption processes, and the notable influence of non-adiabatic interactions. The insights gained from our calculations may be useful in defining strategies to improve infrared-absorption-based magnetometry and optical pumping schemes. The theoretical framework developed here is general and applicable to a variety of other spin defects and materials.

Shift of the photoelectron momentum against the radiation pressure force in linearly polarized intense midinfrared laser fields

The propagation direction momentum component of direct photoelectrons emitted by ionization of an atomic target in an intense, linearly polarized, midinfrared laser field is analyzed. Within the dipole approximation, the average value of this component is zero. However, when nondipole corrections are included, it becomes nonzero. Applying the saddle-point approximation to compute the integral over the ionization times in the expression for the nondipole strong-field approximation differential ionization rate, we surprisingly find a negative momentum shift, corresponding to a shift against the radiation pressure force. Our analysis shows that there is a positive contribution originating from individual ionization pathways within one optical cycle. The interference of contributions from ionization pathways arising within the same optical cycle of the field (intracycle interference) causes an oscillatory behavior, which, crossing to negative values, induces the shift against the radiation pressure force.

Further Development of the Short‐Pulse Petawatt Laser: Trends, Technologies, and Bottlenecks

AbstractThe petawatt (PW) laser has experienced a rapid development in the past two decades, and tens of giant facilities have been constructed worldwide. After realizing 10–100 PW, it seems to be close to some sort of engineering limit but its focused peak intensity still is much lower than the Schwinger limit, and therefore some technology improvements or innovations become indispensable for further increasing the peak power as well as the focused peak intensity. By quick reviewing the development of the PW laser in history, it is shown that reducing the pulse duration to near a single optical cycle is a feasible (easy and cheap) choice for this purpose. Here, technologies for the optical‐cycle pulse generation, ultrabroadband amplification, and capability boosting that aim to provide possible approaches for optical‐cycle PW lasers are briefly reviewed and discussed. Meanwhile, key bottlenecks that challenge the current as well as the future short‐pulse PW lasers and their possible solutions are summarized and discussed. This technology review aims to provide a possible roadmap for the next‐stage development of the short‐pulse PW laser.

Attosecond electron-beam technology: a review of recent progress.

Electron microscopy and diffraction with ultrashort pulsed electron beams are capable of imaging transient phenomena with the combined ultrafast temporal and atomic-scale spatial resolutions. The emerging field of optical electron beam control allowed the manipulation of relativistic and sub-relativistic electron beams at the level of optical cycles. Specifically, it enabled the generation of electron beams in the form of attosecond pulse trains and individual attosecond pulses. In this review, we describe the basics of the attosecond electron beam control and overview the recent experimental progress. High-energy electron pulses of attosecond sub-optical cycle duration open up novel opportunities for space-time-resolved imaging of ultrafast chemical and physical processes, coherent photon generation, free electron quantum optics, electron-atom scattering with shaped wave packets and laser-driven particle acceleration. Graphical Abstract.

Role of Spin-Orbit Coupling in High-Order Harmonic Generation Revealed by Supercycle Rydberg Trajectories.

High-harmonic generation is typically thought of as a sub-laser-cycle process, with the electron's excursion in the continuum lasting a fraction of the optical cycle. However, it was recently suggested that long-lived Rydberg states can play a particularly important role in high harmonic generation by atoms driven by the combination of the counterrotating circularly polarized fundamental light field and its second harmonic. Here we report direct experimental evidence of very long and stable Rydberg trajectories contributing to high-harmonic generation in such fields. We track their dynamics inside the laser pulse using the spin-orbit evolution in the ionic core, utilizing the spin-orbit Larmor clock. We confirm their effect on harmonic emission both via microscopic simulations and by showing how this radiation can lead to a well-collimated macroscopic far-field signal. Our observations contrast sharply with the general view that long-lived Rydberg orbits should generate negligible contribution to the macroscopic far-field high harmonic response of the medium.

Carrier-envelope phase controlled dynamics of relativistic electron beams in a laser-wakefield accelerator

In laser-wakefield acceleration, an ultra-intense laser pulse is focused into an underdense plasma in order to accelerate electrons to relativistic velocities. In most cases, the pulses consist of multiple optical cycles and the interaction is well described in the framework of the ponderomotive force where only the envelope of the laser has to be considered. But when using single-cycle pulses, the ponderomotive approximation breaks down, and the actual waveform of the laser has to be taken into account. In this paper, we use near-single cycle laser pulses to drive a laser-wakefield accelerator. We observe variations of the electron beam pointing on the order of 10 mrad in the polarisation direction, as well as 30% variations of the beam charge, locked to the value of the controlled laser carrier-envelope phase, in both nitrogen and helium plasma. Those findings are explained through particle-in-cell simulations indicating that low-emittance, ultra-short electron bunches are periodically injected off-axis by the transversally oscillating bubble associated with the slipping carrier-envelope phase.

Improving electrochromic performance of panchromatic all-in-one devices by retarding interfacial molecular aggregation/degradation in anode electrode

In this work, a well-known hole conductor spiro-OMeTAD was introduced into viologen-based all-in-one electrochromic devices (ECDs) as anode material, which exhibited panchromatic switching between colorless and truly black with efficient performance, involving fast response speed (around 0.5 s for both coloring and bleaching processes), high optical contrast (over 90% throughout the whole visible region), and high coloration efficiency (450.26 cm 2 C −1 at 514 nm, 248.31 cm 2 C −1 at 607 nm and 315.51 cm 2 C −1 at 888 nm). Highly stability was achieved with less than 10% decay in optical contrast after 10000 cycles switching at 514 nm and 607 nm. Moreover, ECDs based on spiro-OMeTAD/HV had a wide working temperature window, and operated efficiently under both low-temperature below −40 °C and high-temperature up to 60 °C. Most importantly, the suppression of the interfacial molecular aggregation and degradation on anode electrode in viologen-based all-in-one electrochromic devices was demonstrated to be crucial for long-term switching stability for the first time. • spiro-OMeTAD was introduced into panchromatic all-in-one ECDs toward enhanced performance. • The present ECDs showed high optical contrast, fast switch and long cycle stability. • Anode interfacial molecular aggregation and degradation in all-in-one ECDs was reported for the first time.

Strong Field Double Ionization of Formaldehyde Investigated Using Momentum Resolved Covariance Imaging and Trajectory Surface Hopping.

We use covariance velocity map imaging of fragment ions from the strong field double ionization of formaldehyde in conjunction with trajectory surface hopping calculations to determine the ionization yields to different singlet and triplet states of the dication. The calculated kinetic energy release for trajectories initiated on different electronic states is compared with the experimental values based on momentum resolved covariance measurements. We determine the state resolved double ionization yields as a function of laser intensity and pulse duration down to 6 fs (two optical cycles).

Controlling electron recollision with combined linear and circular polarization

We theoretically investigate the non-sequential double ionization of Ar atoms in the combined fields of linearly polarized laser and circularly polarized laser through 3D semiclassical simulations. By partially overlapping the two time-delayed multicycle laser pulses, we construct an optical waveform whose polarization ellipticity increase slowly for consecutive optical cycles. This composite laser pulses with the time-dependent ellipticity can tunnel-ionize atoms and steer the first tunneling electron to recollision with the second bound electron through different trajectories, in which the recollision occurs with different return times of the first ionized electron. Through tuning delay time between the two laser pulses, the double ionization yields and recollision trajectories with different return times can be controlled. The time-dependent ellipticity with different delay time can enhance or suppress the probability of different return times. This work provides a scheme exploring electron dynamics in few optical cycle or even subcycle time scale in a multicycle laser field without having to be limited to near-single-cycle laser pulses.

Laser compression via fast-extending plasma gratings

A method is proposed for compressing laser pulses by fast-extending plasma gratings (FEPGs), which are created by ionizing a hypersonic wave generated by stimulated Brillouin scattering in a background gas. Ionized by a short laser pulse, the phonon forms a light-velocity FEPG to fully reflect a resonant pump laser. As the reflecting surface moves with the velocity of light, the reflected pulse is temporally overlapped and compressed. One- and two-dimensional fully kinetic particle-in-cell simulations with a laser wavelength of 1 µm show that in this regime, a pump pulse is compressed from 10–40 ps to 7–10 fs (i.e., a few optical cycles), with a two-dimensional transfer efficiency up to 60%. This method is a promising way to produce critical laser powers while avoiding several significant problems that arise in plasma-based compressors, including an unwanted linear stage, major plasma instabilities, and the need for seed preparation.

Few-cycle vacuum squeezing in nanophotonics.

One of the most fundamental quantum states of light is the squeezed vacuum, in which noise in one of the quadratures is less than the standard quantum noise limit. In nanophotonics, it remains challenging to generate, manipulate, and measure such a quantum state with the performance required for a wide range of scalable quantum information systems. Here, we report the development of a lithium niobate-based nanophotonic platform to demonstrate the generation and all-optical measurement of squeezed states on the same chip. The generated squeezed states span more than 25 terahertz of bandwidth supporting just a few optical cycles. The measured 4.9 decibels of squeezing surpass the requirements for a wide range of quantum information systems, demonstrating a practical path toward scalable ultrafast quantum nanophotonics.

Free‐Electron Tomography of Few‐Cycle Optical Waveforms

AbstractUltrashort light pulses are ubiquitous in modern research but the electromagnetic field of the optical cycles is usually not easy to obtain as a function of time. Field‐resolved pulse characterization requires either a nonlinear‐optical process or auxiliary sampling pulses that are shorter than the waveform under investigation and pulse metrology without at least one of these two prerequisites is often thought to be impossible. Here is reported how the optical field cycles of laser pulses can be characterized with field‐linear sensitivity and no ultrashort probe events. A free‐space electron beam is let to cross with the waveform of interest. Randomly arriving electrons interact by means of their elementary charge with the optical waveform in a linear‐optical way and reveal the optical cycles as the turning points in a time‐integrated deflection histogram. The sensitivity is only limited by the electron‐beam emittance and can reach the level of thermal radiation and vacuum fluctuations. Besides overturning a common belief in optical pulse metrology, the idea also provides practical perspectives for in situ characterization and optimization of optical waveforms in higher‐harmonics experiments, ultrafast transmission electron microscopes, laser‐driven particle accelerators, free‐electron lasers, or generally any experiments with waveform‐controlled laser pulses in a vacuum environment.