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Dielectric loaded waveguides (DLWs) driven by multicycle terahertz (THz) pulses hold great promise as compact linear accelerators (LINACs) due to their ability to sustain higher breakdown fields at THz frequencies compared to conventional RF components. Precise control of the THz pulse's phase and group velocities within the DLW can be achieved by adjusting the dimensions of the dielectric tube. Optimization of the DLW parameters has to take various factors into account like initial electron energy, THz pulse energy available, THz pulse width, DLW dimensions, etc. To maximize the final kinetic energy of the accelerated electrons, this study presents a comprehensive analytical/numerical guideline for cylindrical DLW LINACs. Additionally, graphic representations are introduced to visualize optimal designs for varying initial electron and THz pulse energies. The provided guideline figures enable designers to tailor the accelerator to specific requirements, paving the way for potential advancements in THz-driven particle acceleration and offering cost-effective alternatives to conventional RF accelerators for various scientific, industrial, and medical applications.

Microwaves generated by optical techniques have demonstrated unprecedentedly low noise and hold significance in various applications such as communication, radar, instrumentation, and metrology. To date, the purest microwave signals are generated using optical frequency division with femtosecond mode-locked lasers. However, many femtosecond laser combs have a radio frequency (RF) repetition rate in the hundreds of megahertz range, necessitating methods to translate the generated low-noise RF signal to the microwave domain. Benchtop pulse interleavers can multiply the pulse repetition rate, avoid saturation of photodetectors, and facilitate the generation of high-power low-noise microwave signals, which have to date only been demonstrated using optical fibers or free space optics. Here, we introduce a large-scale photonic integrated circuit-based interleaver, offering size reduction and enhanced stability. The all-on-chip interleaver attains a 64-fold multiplication of the repetition rate, directly translated from 216 MHz to 14 GHz in microwave Ku-Band. By overcoming photodetector saturation, the generated microwave power was improved by 36 dB, with a phase noise floor reduced by more than 10 folds to -160 dBc/Hz on the 14 GHz carrier. The device is based on a low-loss and high-density photonic integrated circuit fabricated by the photonic Damascene process. Six cascaded stages of Mach-Zehnder interferometers with optical delay lines up to 33 centimeters long are fully integrated into a compact footprint of 8.5 mmx1.7 mm. The lithographically defined precision of the optical waveguide path length enables the scaling up of the interleaved frequency to millimeter-wave bands, which is challenging the fiber-based counterparts. This interleaver has the potential to reduce the cost and footprint of mode-locked-laser-based microwave generation, allowing for field deployment.

Femtosecond laser pulses enable the synthesis of light across the electromagnetic spectrum and provide access to ultrafast phenomena in physics, biology, and chemistry. Chip-integration of femtosecond technology could revolutionize applications such as point-of-care diagnostics, bio-medical imaging, portable chemical sensing, or autonomous navigation. However, current chip-integrated pulse sources lack the required peak power and on-chip amplification of femtosecond pulses has been an unresolved challenge. Here, addressing this challenge, we report >50-fold amplification of 1 GHz-repetition-rate chirped femtosecond pulses in a CMOS-compatible photonic chip to 800 W peak power with 116 fs pulse duration. This power level is 2-3 orders of magnitude higher compared to those in previously demonstrated on-chip pulse sources and can provide the power needed to address key applications. To achieve this, detrimental nonlinear effects are mitigated through all-normal dispersion, large mode-area and rare-earth-doped gain waveguides. These results offer a pathway to chip-integrated femtosecond technology with peak power-levels characteristic of table-top sources., Comment: 15 pages, 7 figures

We demonstrate strong performance enhancement of an all polarization-maintaining fiber oscillator mode-locked using NPE in a linear self-stabilized fiber interferometer via suppression of cross-phase modulation (XPM). Numerical simulations reveal that XPM significantly affects the saturable absorber dynamics resulting in distortions of mode-locked steady-states. In the experiment, we construct an oscillator with XPM suppression, employing an intra-cavity YVO4 crystal, and compare its characteristics with a reference oscillator in standard configuration. It is shown, that XPM suppression not only lowers the mode-locking threshold by more than 40%, but further results in improved spectral pulse quality at the output ports and reduced nonlinear loss of the artificial saturable absorber.

Supercontinuum generation (SCG) is an important nonlinear optical process enabling broadband light sources for many applications, for which silicon nitride(Si3N4) has emerged as a leading on-chip platform.To achieve suitable group velocity dispersion and high confinement for broadband SCG the Si3N4 waveguide layer used is typically thick (>~700 nm), which can lead to high stress and cracks unless specialized processing steps are used. Here, we report on efficient octave spanning SCG in a thinner moderate-confinement 400-nm Si3N4 platform using a highly nonlinear tellurium oxide (TeO2) coating. An octave-spanning supercontinuum is achieved at a low peak power of 258 W using a 100-fs laser centered at 1565 nm. Our numerical simulations agree well with the experimental results showing an increase of waveguide's nonlinear parameter by a factor of 2.5 when coating the Si3N4 waveguide with TeO2 film. This work demonstrates highly efficient SCG via effective dispersion engineering and an enhanced nonlinearity in a CMOS-compatible hybrid TeO2-Si3N4 waveguides and a promising route to monolithically integrated nonlinear, linear, and active functionalities on a single silicon photonic chip., Comment: 4 pages, 3 figures

We present an approach for solving optimal Dirichlet boundary control problems of nonlinear optics by using deep learning. For computing high resolution approximations of the solution to the nonlinear wave model, we propose higher order space-time finite element methods in combination with collocation techniques. Thereby, $C^{l}$-regularity in time of the global discrete is ensured. The resulting simulation data is used to train solution operators that effectively leverage the higher regularity of the training data. The solution operator is represented by Fourier Neural Operators and Gated Recurrent Units and can be used as the forward solver in the optimal Dirichlet boundary control problem. The proposed algorithm is implemented and tested on modern high-performance computing platforms, with a focus on efficiency and scalability. The effectiveness of the approach is demonstrated on the problem of generating Terahertz radiation in periodically poled Lithium Niobate, where the neural network is used as the solver in the optimal control setting to optimize the parametrization of the optical input pulse and maximize the yield of $0.3\,$THz-frequency radiation. We exploit the periodic layering of the crystal to design the neural networks. The networks are trained to learn the propagation through one period of the layers. The recursive application of the network onto itself yields an approximation to the full problem. Our results indicate that the proposed method can achieve a significant speedup in computation time compared to classical methods. A comparison of our results to experimental data shows the potential to revolutionize the way we approach optimization problems in nonlinear optics., Comment: 43 pages, 11 figures, 7 tables. Submitted to Journal of Comptutational Physics

Power amplifier is becoming a critical component for integrated photonics as the integrated devices try to carve out a niche in the world of real-world applications of photonics. That is because the signal generated from an integrated device severely lacks in power which is due mainly to the small size which, although gives size and weight advantage, limits the energy storage capacity of an integrated device due to the small volume, causing it to rely on its bench-top counterpart for signal amplification downstream. Therefore, an integrated high-power signal booster can play a major role by replacing these large solid-state and fiber-based benchtop systems. For decades, large mode area (LMA) technology has played a disruptive role by increasing the signal power and energy by orders of magnitude in the fiber-based lasers and amplifiers. Thanks to the capability of LMA fiber to support significantly larger optical modes the energy storage and handling capability has significantly increased. Such an LMA device on an integrated platform can play an important role for high power applications. In this work, we demonstrate LMA waveguide based CMOS compatible watt-class power amplifier with an on-chip output power reaching ~ 1W within a footprint of ~4mm2.The power achieved is comparable and even surpasses many fiber-based amplifiers. We believe this work opens up opportunities for integrated photonics to find real world application on-par with its benchtop counterpart.

Attosecond science has demonstrated that electrons can be controlled on the sub-cycle time scale of an optical wave, paving the way toward optical frequency electronics. Using controlled few-cycle optical waveforms, the study of sub-cycle electron emission has enabled the generation of attosecond ultraviolet pulses and the control of attosecond currents inside of solids. However, these experiments rely on high-energy laser systems not suitable for integration with microcircuits. To move towards integrated optical frequency electronics, a system suitable for integration into microcircuits capable of generating detectable signals with low pulse energies is needed. While current from plasmonic nanoantenna emitters can be driven at optical frequencies, low charge yields have been a significant limitation. In this work we demonstrate that large-scale electrically-connected plasmonic nanoantenna networks, when driven in concert, enable a much higher charge yield sufficient for shot-to-shot carrier-envelope phase detection, which is a hallmark of the underlying sub-cycle processes. We use a tailored sub-2-cycle mid-infrared waveform of only tens of nanojoules of energy to drive in excess of 2000 carrier-envelope-phase-sensitive electrons from interconnected plasmonic nanoantenna arrays that we detect on a single-shot basis using conventional electronics. Our work shows that electronically integrated plasmonic nanoantennas are a viable approach to integrated optical frequency electronics. By engineering the nanoantennas to the particular use case, such as carrier-envelope phase detection, and optimizing the density and total amount, the output signals are fully controlled. This approach to optical frequency electronics will further enable many interesting applications, such as petahertz-bandwidth electric field sampling or the realization of logic gates operating at optical frequencies.

Control over intensity, shape, direction, and phase of coherent light is essential in numerous fields, reaching from gravitational wave astronomy over quantum metrology and ultrafast sciences to semi-conductor fabrication. Modern laser optics, however, frequently demands parameter regimes where either the wavelength or the optical power restricts control due to linear absorption, light-induced damage or optical nonlinearity. The properties of solid media, upon which most photonic control schemes rely, impose these limitations. We propose to circumvent these constraints using gaseous media tailored by high-intensity ultrasound waves. We demonstrate a first implementation of this approach by deflecting ultrashort laser pulses using ultrasound waves in ambient air, entirely omitting transmissive solid media. At optical peak powers of 20 GW exceeding previous limits of solid-based acousto-optic modulation by about three orders of magnitude, we reach a deflection efficiency greater than 50% while preserving excellent beam quality. Our approach is not limited to laser pulse deflection via acousto-optic modulation: gas-phase photonic schemes controlled by sonic waves can prospectively be translated to various optical methods, e.g., lenses or waveguides, rendering them effectively invulnerable against damage and opening up new spectral regions., Comment: 23 pages, including 9 pages of main text and 9 pages of Methods and Extended Data, 3 figures, 3 Extended Data figures, 1 Extended Data table

In this work, we identify and characterize intra-pulse intensity noise shaping by saturable absorbers applied in mode-locked lasers and ultra-low noise nonlinear fiber amplifiers. Reshaped intra-pulse intensity noise distributions are shown to be inevitably interconnected with self-amplitude modulation, the fundamental physical mechanism for initiation and stabilization of ultra-short pulses in the steady-state of a mode-locked laser. A theoretical model is used to describe the ultrafast saturation dynamics by an intra-pulse noise transfer function for widely-applied slow and fast saturable absorbers. For experimental verification of the theoretical results, spectrally-resolved relative intensity noise measurements are applied on chirped input pulses to enable the direct measurement of intra-pulse noise transfer functions using a versatile experimental platform. It is further demonstrated, how the characterized intra-pulse intensity noise distribution of ultrafast laser systems can be utilized for quantum-limited intensity noise suppression via tailored optical bandpass filtering.

The fast multipole method (FMM) has received growing attention in the beam physics simulation. In this study, we formulate an interpolation-based FMM for the computation of the relativistic space-charge field. Different to the quasi-electrostatic model, our FMM is formulated in the lab-frame and can be applied without the assistance of the Lorentz transformation. In particular, we derive a modified admissibility condition which can effectively control the interpolation error of the proposed FMM. The algorithms and their GPU parallelization are discussed in detail. A package containing serial and GPU-parallelized solvers is implemented in the Julia programming language. The GPU-parallelized solver can reach a speedup of more than a hundred compared to the execution on a single CPU core., Comment: 30 pages, 10 figures

We investigate the accurate full broadband simulation of complex nonlinear optical processes. A mathematical model and numerical simulation techniques in the time domain are developed to simulate complex nonlinear optical processes without the usual used slowly varying envelope approximation. We illustrate the accuracy by numerical simulations. Furthermore, they are used to elucidate THz generation in periodically poled Lithium Niobate (PPLN) including optical harmonic generation., Comment: Submitted to Optics Express

In this work, we investigate an approach to scale up the output pulse energy in an all polarization-maintaining 17 MHz Yb-doped fiber oscillator via implementation of 25 um core-diameter large-mode-area fibers. The artificial saturable absorber in form of a Kerr-type self-stabilized fiber-interferometer enables highly stable mode-locked steady-states in the soliton-like operation regime with 170 mW average output power and a total output pulse energy of ~10 nJ distributed between two output ports. An experimental parameter comparison with a reference oscillator made of 5.5 um core-sized standard fiber-components reveals an increase of pulse energy by a factor of 36 with simultaneously reduced intensity-noise in the high frequency range > 100 kHz.

The availability of electromagnetic pulses with controllable field waveform and extremely short duration, even below a single optical cycle, is imperative to fully harness strong-field processes and to gain insight into ultrafast light-driven mechanisms occurring in the attosecond time-domain. The recently demonstrated parametric waveform synthesis (PWS) introduces an energy-, power- and spectrum-scalable method to generate non-sinusoidal sub-cycle optical waveforms by coherently combining different phase-stable pulses attained via optical parametric amplifiers. Significant technological developments have been addressed to overcome the stability issues related to PWS and to obtain an effective and reliable waveform control system. Here we present the main ingredients enabling PWS technology. The design choices concerning the optical, mechanical and electronic setups are justified by analytical/numerical modeling and benchmarked by experimental observations. In its present incarnation, the PWS technology enables the generation of field-controllable mJ-level few-femtosecond pulses spanning the visible to infrared range., Comment: 34 pages

Inverse Compton Scattering (ICS) has gained much attention recently because of its promise for the development of table-top-size X-ray light sources. Precise and fast simulation is an indispensable tool for predicting the radiation property of a given machine design and to optimize its parameters. Instead of the conventional approach to compute radiation spectra which directly evaluates the discretized Fourier integral of the Li\'enard-Wiechert field given analytically (referred to as the frequency-domain method), this article focuses on an approach where the field is recorded along the observer time on a uniform time grid which is then used to compute the radiation spectra after completion of the simulation, referred to as the time-domain method. Besides the derivation and implementation details of the proposed method, we analyze possible parallelization schemes and compare the parallel performance of the proposed time-domain method with the frequency-domain method. We will characterize scenarios/conditions under which one method is expected to outperform the other., Comment: 18 pages with 13 figures

Space-charge effects are of great importance in particle accelerator physics. In the computational modeling, tree-based methods are increasingly used because of their effectiveness in handling non-uniform particle distributions and/or complex geometries. However, they are often formulated using an electrostatic force which is only a good approximation for low energy particle beams. For high energy, i.e., relativistic particle beams, the relativistic interaction kernel may need to be considered and the conventional treecode fails in this scenario. In this work, we formulate a treecode based on Lagrangian interpolation for computing the relativistic space-charge field. Two approaches are introduced to control the interpolation error. In the first approach, a modified admissibility condition is proposed for which the treecode can be used directly in the lab-frame. The second approach is based on the transformation of the particle beam to the rest-frame where the conventional admissibility condition can be used. Numerical simulation results using both methods will be compared and discussed., Comment: 27 pages with 7 figures

We demonstrate record conversion efficiencies of up to 0.9 percent for narrowband (1 percent relative bandwidth) THz generation by strongly-cascaded difference frequency generation. These results are achieved using a novel laser source, customized for high efficiencies, with two narrow spectral lines of variable separation and pulse duration (250 ps). THz generation in 5 percent MgO doped PPLN crystals of varying poling period was explored at cryogenic and room temperature operation as well as different crystal lengths. This work addresses an increasing demand for high-field THz pulses which has, up to now, been largely limited by low optical-to-THz conversion efficiencies.

Carrier-envelope phase (CEP) detection of ultrashort optical pulses and low-energy waveform field sampling have recently been demonstrated using direct time-domain methods that exploit optical-field photoemission from plasmonic nanoantennas. These devices make for compact and integratable solid-state detectors operating at optical frequency that work in ambient conditions and require minute pulse energies (picojoule-level). Applications include frequency-comb stabilization, visible to near-infrared time-domain spectroscopy, compact tools for attosecond science and metrology and, due to the high electronic switching speeds, petahertz-scale information processing. However, these devices have been driven by free-space optical waveforms and their implementation within integrated photonic platforms has yet to be demonstrated. In this work, we design and simulate fully-integrated plasmonic bow-tie nanoantennas coupled to a Si$_3$N$_4$-core waveguide for CEP detection. We find that when coupled to realistic on-chip, few-cycle supercontinuum sources, these devices are suitable for direct time-domain CEP detection within integrated photonic platforms. We estimate a signal-to-noise ratio of 30 dB at 50 kHz resolution bandwidth. We address technical details, such as the tuning of the nanoantennas plasmonic resonance and the waveform's CEP slippage in the waveguide. Moreover, we evaluate power losses due to absorption and scattering and we study the device sensitivity to pulse duration and pulse peak field intensity. Our results provide the basis for future design and fabrication of time-domain CEP detectors and allow for the development of fully-integrated attosecond science applications, frequency-comb stabilization and light-wave-based PHz electronics., Comment: 13 pages, 9 figures, submitted to Journal of Lightwave Technology

In this letter, we investigate a Yb-doped mode-locked fiber oscillator that uses coherent pulse division and recombination to avoid excessive nonlinear phase shifts. The mode-locking mechanism of the laser is based on the accumulation of a differential nonlinear phase between orthogonal polarization modes in the polarization-maintaining fiber segment. The inserted coherent pulse divider, based on YVO4-crystals rotated successively by 45{\deg}, enables stable and undistorted mode-locked steady-states. The output pulse energy is increased from 89 pJ in the non-divided operation by ~6.5 dB to more than 400 pJ with three divisions. Measurements of the amplitude-fluctuations reveal a simultaneous broadband reduction of up to ~9 dB in the frequency range from 10 kHz to 2MHz.

Attosecond science promises to reveal the most fundamental electronic dynamics occurring in matter and it can develop further by meeting two linked technological goals related to high-order harmonic sources: higher photon flux (permitting to measure low cross-section processes) and improved spectral tunability (allowing selectivity in addressing specific electronic transitions). New developments come through parametric waveform synthesis, which provides control over the shape of high-energy electric field transients, enabling the creation of highly-tunable isolated attosecond pulses via high-harmonic generation. Here we show that central energy, spectral bandwidth/shape and temporal duration of the attosecond pulses can be controlled by shaping the laser pulse waveform via two key parameters: the relative-phase between two halves of the multi-octave spanning optical spectrum, and the overall carrier-envelope phase. These results not only promise to expand the experimental possibilities in attosecond science, but also demonstrate coherent strong-field control of free-electron trajectories using tailored optical waveforms.

We propose a nonlinear fiber system for shot-noise limited, all-optical intensity-noise reduction and signal amplification. The mechanism is based on the accumulation of different nonlinear phase shifts between orthogonal polarization modes in a polarization-maintaining fiber amplifier in combination with an implemented sinusoidal transmission-function. The resulting correlation between the input intensity-fluctuations and the system transmission enables tunable intensity noise reduction of the input pulse train. In the experiment, the noise spectral density of a mode-locked oscillator is suppressed by up to ~20 dB to the theoretical shot-noise limit of the measurement at -151.3 dBc/Hz with simultaneous pulse amplification of 13.5dB., Comment: 4 pages, 6 pictures

Terahertz (THz)-based electron manipulation has recently been shown to hold tremendous promise as a technology for manipulating and driving the next-generation of compact ultrafast electron sources. Here, we demonstrate an ultrafast electron diffractometer with THz-driven pulse compression. The electron bunches from a conventional DC gun are compressed by a factor of 10 and reach a duration of ~180 fs (FWHM) with 10,000 electrons/pulse at a 1 kHz repetition rate. The resulting ultrafast electron source is used in a proof-of-principle experiment to probe the photoinduced dynamics of single-crystal silicon. The THz-compressed electron beams produce high-quality diffraction patterns and enable observation of the ultrafast structural dynamics with improved time resolution. These results validate the maturity of THz-driven ultrafast electron sources for use in precision applications., Comment: 16 pages, 5 figures. arXiv admin note: text overlap with arXiv:1910.06639

In this work, we investigate the steady-states of a fiber lasers mode-locked with a nonlinear amplifying loop-mirror that has an inherent amplitude noise-suppression mechanism. Due to the interaction of the sinusoidal transmission function with the fluctuating intracavity pulse amplitude we show that this mechanism may lead to a detectable difference in relative intensity noise at the reflected and transmitted output port under specific preconditions. We present systematic intensity noise measurements with a nonlinear fiber-based system that replicates a single roundtrip in the laser cavity. Experimental results and simulations clearly show a reduction of the intracavity amplitude fluctuations up to 4 dB for certain steady-states., Comment: 4 pages, 4 figures

Time-domain sampling of arbitrary electric fields with sub-cycle resolution enables a complete time-frequency analysis of a system's response to electromagnetic illumination. This provides access to dynamic information that is not provided by absorption spectra alone, and has recently been shown through measurements in the infrared that time-domain optical-field sampling offers significant improvements with regard to molecular sensitivity and limits of detection compared to traditional spectroscopic methods. Despite the many scientific and technological motivations, time-domain, optical-field sampling systems operating in the visible to near-infrared spectral regions are seldom accessible, requiring large driving pulse energies, and large laser amplifier systems, bulky apparatuses, and vacuum environments. Here, we demonstrate an all-on-chip, optoelectronic device capable of sampling arbitrary, low-energy, near-infrared waveforms under ambient conditions. Our solid-state integrated detector uses optical-field-driven electron emission from resonant nanoantennas to achieve petahertz-level switching speeds by generating on-chip attosecond electron bursts. These bursts are used to probe the electric field of weak optical transients. We demonstrated our devices by sampling the electric field of a ~5 fJ, broadband near-infrared ultrafast laser pulse using a ~50 pJ near-infrared driving pulse. Our sampling measurements recovered the weak optical transient as well as localized plasmonic dynamics of the emitting nanoantennas $in~situ$. This field-sampling device--with its compact footprint and low pulse-energy requirements--offers opportunities in a variety of applications, including: broadband time-domain spectroscopy in the molecular fingerprint region, time-domain analysis of nonlinear phenomena, and detailed studies of strong-field light-matter interactions.

Recent studies have demonstrated that the polarization states of high harmonics from solids can differ from those of the driving pulses. To gain insights on the microscopic origin of this behavior, we perform one-particle intraband-only calculations and reproduce some of the most striking observations. For instance, our calculations yield circularly polarized harmonics from elliptically polarized pulses that sensitively depend on the driving conditions. Furthermore, we perform experiments on ZnS and find partly similar characteristics as reported from silicon. Comparison to our intraband-only calculations shows reasonable qualitative agreement for a below-band-gap harmonic. We show that intraband dynamics predict depolarization effects for higher field strengths. For harmonics above the band gap, interband dynamics become important and the high-harmonic response to elliptical excitation looks systematically different. Our work proposes a method to distinguish between different high-harmonic generation mechanisms and it could pave the way to compact solid-state high-harmonic sources with controllable polarization states.

Integrated modelocked lasers with high power are of utmost importance for next generation optical systems that can be field-deployable and mass produced. Here we study fully integrated modelocked laser designs that have the potential to generate ultrashort, high power, and high quality pulses. We explore a large mode area laser for high power pulse generation and study the various mode-locking regimes of dispersion managed soliton pulses in net anomalous and net normal dispersion cavities. Furthermore, we study numerically and experimentally general properties and tunability of a fast integrated saturable absorber based on low loss silicon nitride nonlinear interferometer. We believe this work guides the exploration of the future integrated high power modelocked lasers.

Coherently-pumped (Kerr) solitons in an ideal optical microcavity are expected to undergo random quantum motion that determines fundamental performance limits in applications of soliton microcombs. Here, this diffusive motion and its impact on Kerr soliton timing jitter is studied experimentally. Typically hidden below technical noise contributions, the quantum limit is discerned by measuring counter-propagating solitons. Their relative motion features only weak interactions and also presents excellent common mode suppression of technical noise. This is in strong contrast to co-propagating solitons which are found to have relative timing jitter well below the quantum limit of a single soliton on account of strong mutual motion correlation. Good agreement is found between theory and experiment. The results establish the fundamental limits to timing jitter in soliton microcombs and provide new insights on multi-soliton physics

Coherent, broadband pulses of extreme ultraviolet (XUV) light provide a new and exciting tool for exploring attosecond electron dynamics. Using photoelectron streaking, interferometric spectrograms can be generated that contain a wealth of information about the phase properties of the photoionization process. If properly retrieved, this phase information reveals attosecond dynamics during photoelectron emission such as multielectron dynamics and resonance processes. However, until now, the full retrieval of the continuous electron wavepacket phase from isolated attosecond pulses has remained challenging. Here, after elucidating key approximations and limitations that hinder one from extracting the coherent electron wavepacket dynamics using available retrieval algorithms, we present a new method called Absolute Complex Dipole transmission matrix element reConstruction (ACDC). We apply the ACDC method to experimental spectrograms to resolve the phase and group delay difference between photoelectrons emitted from Ne and Ar. Our results reveal subtle dynamics in this group delay difference of photoelectrons emitted form Ar. These group delay dynamics were not resolvable with prior methods that were only able to extract phase information at discrete energy levels, emphasizing the importance of a complete and continuous phase retrieval technique such as ACDC. Here we also make this new ACDC retrieval algorithm available with appropriate citation in return.

High energy single- to few-cycle terahertz pulses enable the exploration of the frontier of science, such as electron acceleration, strong-field physics, and spectroscopy. One important method of generating such terahertz pulses is to use the tilted-pulse-front (TPF) technique. However, due to the non-collinear phase-matching, the large angular dispersion leads to a spatial and temporal break-up of the optical pump, limiting the terahertz generation efficiency and reducing the few-cycle character of the generated terahertz fields. To reduce the effects caused by angular dispersion, multiple schemes with discrete pulse-front-tilt have been suggested. We propose a terahertz generation scheme, where a spatio-temporally chirped (STC) pump pulse is utilized. With our 2D+1 numerical model, we perform a systematic comparative study of the conventional TPF scheme, three discrete TPF schemes, and the STC scheme. This model predicts smaller optimal interaction length and significantly smaller conversion efficiency compared to simple 1D models. We conclude that the STC scheme delivers spatially homogeneous few-cycle terahertz pulses with the highest conversion efficiency. Additionally, given a short interaction length, the discrete TPF schemes cannot outperform the continuous ones. In general, this work gives guidance to choose the most appropriate setup for a given terahertz experiment

We introduce a new regime of cascaded quadratic nonlinearities which result in a continuous red shift of the optical pump, analogous to a Raman shifting process rather than self-phase modulation. This is particularly relevant to terahertz generation, where a continuous red shift of the pump can resolve current issues such as dispersion management and laser-induced damage. We show that in the absence of absorption or dispersion, the presented Raman shifting method will result in optical-to-terahertz energy conversion efficiencies that approach $100\%$ which is not possible with conventional cascaded difference-frequency generation. Furthermore, we present designs of aperiodically poled structures which result in energy conversion efficiencies of $\approx 35\%$ even in the presence of dispersion and absorption.

Ultrafast light-matter interactions lead to optical-field-driven photocurrents with an attosecond-level temporal response. These photocurrents can be used to detect the carrier-envelope-phase (CEP) of short optical pulses, and could be utilized to create optical-frequency, petahertz (PHz) electronics for information processing. Despite recent reports on optical-field-driven photocurrents in various nanoscale solid-state materials, little has been done in examining the large-scale integration of these devices. In this work, we demonstrate enhanced, on-chip CEP detection via optical-field-driven photocurrent in a monolithic array of electrically-connected plasmonic bow-tie nanoantennas that are contained within an area of hundreds of square microns. The technique is scalable and could potentially be used for shot-to-shot CEP tagging applications requiring orders of magnitude less pulse energy compared to alternative ionization-based techniques. Our results open new avenues for compact time-domain, on-chip CEP detection, and inform the development of integrated circuits for PHz electronics as well as integrated platforms for attosecond and strong-field science.

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with mm-scale, segmented interaction regions that are ideal for acceleration of electrons in the sub- to few-MeV range where electron velocities vary significantly. However, in order to extend this technology to the multi-MeV range, investigation of approaches supporting longer interaction lengths is needed. Here, we demonstrate first steps in electron acceleration and manipulation using dielectrically-lined waveguides powered by temporally long, narrowband, multi-cycle THz pulses that co-propagate with the electrons. This geometry offers centimeter-scale single-stage interaction lengths and offers the opportunity to further increase interaction lengths by cascading acceleration stages that recycle the THz energy and rephase the interaction. We prove the feasibility of THz-energy recycling for the first time by demonstrating acceleration, compression and focusing in two sequential Al2O3-based dielectric capillary stages powered by the same multi-cycle THz pulse. Since the multi-cycle energy achievable using laser-based sources is currently a limiting factor for the maximum electron acceleration, THz energy recycling provides a key enabling factor for reaching relativistic energies with existing sources., Comment: 22 pages, 4 figures

The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single- to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especially when pushing for high optical-to-terahertz conversion efficiency. A 3D+1 (x,y,z,t) numerical model is necessary in order to fully investigate the terahertz generation problem in the tilted-pulse-front scheme. We compare in detail the differences between 1D+1, 2D+1 and 3D+1 models. The simulations show that the size of the optical beam in the pulse-front-tilt plane sensitively affects the spatio-temporal properties of the terahertz electric field. The terahertz electric field is found to have a strong spatial dependence such that a few-cycle pulse is only generated near the apex of the prism. The part of the beam farther from the apex contains a large fraction of the energy but has a waveform that deviates from a few-cycle. This strong spatial dependence must be accounted for when using the terahertz pulses for strong-field physics and carrier-envelope-phase sensitive experiments such as terahertz acceleration, coherent control of antiferromagnetic spin waves and terahertz high-harmonic generation., Comment: a typo of the minus sign and the ratio of f1/f2 is corrected

Ability to selectively enhance the amplitude and maintain high coherence of the supercontinuum signal with long pulses is gaining significance. In this work an extra degree of freedom afforded by varying the dispersion profile of a waveguide is utilized to selectively enhance supercontinuum. As much as 16 dB signal enhancement in the telecom window and 100 nm of wavelength extension is achieved with a cascaded waveguide, compared to a fixed dispersion waveguide. Waveguide tapering, in particular with increasing width, is determined to have a flatter and more coherent supercontinuum than a fixed dispersion waveguide when longer input pulses are used. Furthermore, due to the strong birefringence of an asymmetric silicon waveguide the supercontinuum signal is broadened by pumping simultaneously with both quasitransverse electric (TE) and quasi-transverse magnetic (TM) mode in the anomalous dispersion regime. Thus, by controlling the dispersion for the two modes selective signal generation is obtained. Such waveguides offer several advantages over optical fiber as the variation in dispersion can be controlled with greater flexibility in an integrated platform. This work paves the way forward for various applications in fields ranging from medicine to telecom where specific wavelength windows need to be targeted., Comment: 14 pages, 10 figures

We explore the dynamics of a system where input spectra in the optical domain with very disparate center frequencies are strongly coupled via highly phase-matched, cascaded second-order nonlinear processes driven by terahertz radiation. The only requirement is that one of the input spectra contain sufficient bandwidth to generate the phase-matched terahertz-frequency driver. The frequency separation between the input spectra can be more than ten times larger than the phase-matched terahertz frequency. A practical application of such a system where the cascading of a narrowband pump line centered at 1064 nm induced by a group of weaker seed lines centered about 1030 nm and separated by the phase-matched terahertz frequency is introduced. This approach is predicted to generate terahertz radiation with percent-level conversion efficiencies and millijoule-level pulse energies in cryogenically-cooled periodically poled lithium niobate. A model that solves for the nonlinear coupled interaction of terahertz and optical waves is employed. The calculations account for second and third-order nonlinearities, dispersion in the optical and terahertz domains as well as terahertz absorption. Ramifications of pulse formats on laser-induced damage are estimated by tracking the generated free-electron density. Strategies to mitigate laser-induced damage are also outlined.

We analytically show how a superposition of beamlets produces terahertz radiation with greater spatial homogeneity and efficiency compared to tilted-pulse-fronts generated by diffraction gratings. The advantages are particularly notable for large pump bandwiths and beam sizes, alluding to better performance in the presence of cascading effects and higher energy pumping. A theory of terahertz generation using a superposition of beamlets is developed. It is shown how such an arrangement produces a distortion free tilted-pulse-front. Closed form expressions for terahertz spectra and transients in three spatial dimensions are derived. Conditions for obtaining performance parity and bounds for optimal parameters are furnished.

Given the quasi-static nature of optical-field emission and the nontrivial dependence of the emission rate on the instantaneous electric field strength, the CEP-sensitive component of the emitted photocurrent is highly sensitive to the energy of the optical pulse, and should carry information about the underlying sub-cycle dynamics of electron emission. Here we examine CEP-sensitive photoemission from plasmonic gold nanoantennas excited with few-cycle optical pulses of increasing energy. We observe antiresonance-like features in the CEP-sensitive photocurrent; specifically, at a critical pulse energy, we observe a sharp dip in the magnitude of the CEP-sensitive photocurrent accompanied by a sudden shift of {\pi}-radians in the phase of the photocurrent. Using a quasi-static tunneling emission model, we find that these antiresonance-like features arise due to competition between electron emission from neighboring optical half-cycles, and that they are highly sensitive to the precise shape of the driving optical waveform at the surface of the emitter. As the underlying mechanisms that produce the antiresonance-like features are a general consequence of nonlinear, field-driven photoemission, the antiresonance-like features could be used to probe sub-optical-cycle, sub-femtosecond emission processes, not only from solid-state emitters, but also from gas-phase atoms and molecules. Beyond applications in the study of ultrafast, field-driven electron physics, an understanding of these antiresonance-like features will be critical to the development of novel photocathodes for future time-domain metrology and microscopy applications that demand both attosecond temporal and nanometer spatial resolution.

A 2-D spatio-temporal analysis of terahertz generation by optical rectification of tilted-pulse-fronts is presented. Closed form expressions of terahertz transients and spectra in two spatial dimensions are furnished in the undepleted limit. Importantly, the analysis incorporates spatio-temporal distortions of the optical pump pulse such as angular dispersion, group-velocity dispersion due to angular dispersion, spatial and temporal chirp as well as beam curvature. The importance of the radius of curvature to the tilt-angle and group-velocity dispersion due to angular dispersion to terahertz frequency, conversion efficiency and peak field is revealed.In particular, the deterioration of terahertz frequency, efficiency and field at large pump bandwidths and beam sizes is analytically shown.

We describe a robust system for laser-driven narrowband terahertz generation with high conversion efficiency in periodically poled Lithium Niobate (PPLN). In the multi-stage terahertz generation system, the pump pulse is recycled after each PPLN stage for further terahertz generation. By out-coupling the terahertz radiation generated in each stage, extra absorption is circumvented and effective interaction length is increased. The separation of the terahertz and optical pulses at each stage is accomplished by an appropriately designed out-coupler. To evaluate the proposed architecture, the governing 2-D coupled wave equations in a cylindrically symmetric geometry are numerically solved using the finite difference method. Compared to the 1-D calculation which cannot capture the self-focusing and diffraction effects, our 2-D numerical method captures the effects of difference frequency generation, self-phase modulation, self-focusing, beam diffraction, dispersion, and terahertz absorption. We found that the terahertz generation efficiency can be greatly enhanced by compensating the dispersion of the pump pulse after each stage. With a two-stage system, we predict the generation of a $17.6$ mJ terahertz pulse with total conversion efficiency $\eta_{\text{total}}=1.6\%$ at $0.3$ THz using a 1.1 J pump laser with a two-line spectrum centered at 1 $\mu$m. The generation efficiency of each stage is above $0.8\%$ with the out-coupling efficiencies above $93.0\%$.