Ultrafast Pulse Characterization Research Articles

Carrier-envelope-phase characterization of ultrafast mid-infrared laser pulses through harmonic generation and interference in argon

The propagation of an intense, femtosecond, mid-infrared laser pulse in a gaseous medium results in the efficient generation of spectrally overlapping low-order harmonics, whose optical carrier phases are linked to the carrier-envelope phase (CEP) of the mid-infrared driver pulse. Random peak-power fluctuations of the driver pulses, converted to the fluctuations of the nonlinear phases, acquired by the pulses on propagation, cause this phase correlation to smear out. We show that this seemingly irreversible loss of phase can be recovered, and that the complete information needed for the phase correction is contained in the harmonic spectra itself. The optical phases of the intense driver pulse and its harmonics, as fragile as they appear to be against even weak disturbances, evolve deterministically during highly nonlinear propagation through the extended ionization region.

A Space‐Time Knife‐Edge in Epsilon‐Near‐Zero Films for Ultrafast Pulse Characterization

AbstractEpsilon‐near‐zero (ENZ) materials have shown strong refractive nonlinearities that can be fast in an absolute sense. While continuing to advance fundamental science, such as time varying interactions, the community is still searching for an application that can effectively make use of the strong index modulation offered. Here, the effect of strong space‐time index modulation in ENZ materials is combined with the beam deflection technique to introduce a new approach to optical pulse characterization that is termed a space‐time knife edge. It is shown that in this approach, temporal and spatial information of a Gaussian beam can be extracted with only two time resolved measurements. The approach achieves this without phase‐matching requirements (<1 µm thick) and can achieve a high signal to noise ratio by combining the system with lock‐in detection, facilitating the measurement of weak refractive index changes (Δn 10−5) for low intensity beams. Thus, the space‐time knife edge can offer a new avenue for ultrafast light measurement and demonstrates a use case of ENZ materials. In support of this, temporal dynamics for refractive index changes in non‐colinear experiments opening avenues are outlined for better theoretical understanding of both the spatial and temporal dynamics of emerging ENZ films.

Phase-matched third-harmonic generation in silicon nitride waveguides.

Third-harmonic generation (THG) in silicon nitride waveguides is an ideal source of coherent visible light, suited for ultrafast pulse characterization, telecom signal monitoring and self-referenced comb generation due to its relatively large nonlinear susceptibility and CMOS compatibility. We demonstrate third-harmonic generation in silicon nitride waveguides where a fundamental transverse mode at 1,596 nm is phase-matched to a TM02 mode at 532 nm, confirmed by the far-field image. We experimentally measure the waveguide width-dependent phase-matched wavelength with a peak-power-normalized conversion efficiency of 5.78 × 10-7 %/W2 over a 660-μm-long interaction length.

Random quasi-phase-matching for pulse characterization from the near to the long wavelength infrared.

Experiments requiring ultrafast laser pulses require a full characterization of the electric field to glean meaning from the experimental data. Such characterization typically requires a separate parametric optical process. As the central wavelength range of new sources continues to increase so too does the need for nonlinear crystals suited for characterizing these wavelengths. Here we report on the use of poly-crystalline zinc selenide as a universal nonlinear crystal in the frequency resolved optical gating characterization technique from the near to long-wavelength infrared. Due to its property of random quasi-phase-matching it's capable of phase matching second-harmonic and sum-frequency generation of ultra-broadband pulses in the near and long wavelength infrared, while being crystal orientation independent. With the majority of ultra-fast laser sources being in this span of wavelengths, this work demonstrates a greatly simplified approach towards ultra-fast pulse characterization spanning from the near to the long-wavelength infrared. To our knowledge there is no single optical technique capable of such flexible capabilities.

Variable electro-optic shearing interferometry for ultrafast single-photon-level pulse characterization.

Despite the multitude of available methods, the characterization of ultrafast pulses remains a challenging endeavor, especially at the single-photon level. We introduce a pulse characterization scheme that maps the magnitude of its short-time Fourier transform. Contrary to many well-known solutions it does not require nonlinear effects and is therefore suitable for single-photon-level measurements. Our method is based on introducing a series of controlled time and frequency shifts, where the latter is performed via an electro-optic modulator allowing a fully-electronic experimental control. We characterized the full spectral and temporal width of a classical and single-photon-level pulse and successfully tested the applicability of the reconstruction algorithm of the spectral phase and amplitude. The method can be extended by implementing a phase-sensitive measurement and is naturally well-suited to partially-incoherent light.

Near-zero-index ultra-fast pulse characterization

Transparent conducting oxides exhibit giant optical nonlinearities in the near-infrared window where their linear index approaches zero. Despite the magnitude and speed of these nonlinearities, a “killer” optical application for these compounds has yet to be found. Because of the absorptive nature of the typically used intraband transitions, out-of-plane configurations with short optical paths should be considered. In this direction, we propose an alternative frequency-resolved optical gating scheme for the characterization of ultra-fast optical pulses that exploits near-zero-index aluminium zinc oxide thin films. Besides the technological advantages in terms of manufacturability and cost, our system outperforms commercial modules in key metrics, such as operational bandwidth, sensitivity, and robustness. The performance enhancement comes with the additional benefit of simultaneous self-phase-matched second and third harmonic generation. Because of the fundamental importance of novel methodologies to characterise ultra-fast events, our solution could be of fundamental use for numerous research labs and industries.

Online single-shot characterization of ultrafast pulses from high-gain free-electron lasers

X-ray free-electron lasers (FELs) provide cutting-edge tools for fundamental researches to study nature down to the atomic level at a time-scale that fits this resolution. A precise knowledge of temporal information of FEL pulses is the central issue for its applications. Here we proposed and demonstrated a novel method to determine the FEL temporal profiles online. This robust method, designed for ultrafast FELs, allows researchers to acquire real-time longitudinal profiles of FEL pulses as well as their arrive times with respect to the external optical laser with a resolution better than 6 fs. Based on this method, we can also directly measure various properties of FEL pulses and correlations between them online. This helps us to further understand the FEL lasing processes and realize the generation of stable, nearly fully coherent soft X-ray laser pulses at the Shanghai Soft X-ray FEL facility. This method will enhance the experimental opportunities for ultrafast science in various areas.

Linear chirp instability analysis for ultrafast pulse metrology

Pulse train instabilities have often given rise to confusion and misinterpretation in ultrafast pulse characterization measurements. Most prominently known as the coherent artifact, a partially mode-locked laser with a non-periodic waveform may still produce an autocorrelation that has often been misinterpreted as indication of a coherent pulse train. Some modern pulse characterization methods easily miss the presence of a coherent artifact, too. Here, we address the particularly difficult situation of a pulse train with chirp-only instability. This instability is shown to be virtually invisible to autocorrelation measurements, but can be detected with frequency-resolved optical gating, spectral phase interferometry for direct electric-field reconstruction, and dispersion scan. Our findings clearly show that great care is necessary to rule out a chirp instability in lasers with an unclear mode-locking mechanism and in compression experiments in the single-cycle regime. Among all dynamical pulse train instabilities analyzed so far, this instability appears to be the best-hidden incoherence and is most difficult to detect.

Phenomenological model for mixed multiphoton absorption in bialkali photocathode and application to ultrafast pulse characterization

We propose a phenomenological model composed of concise formulas to describe mixed multiphoton absorption (MPA), revealing its quasi-exponent-function in relation to intensity, complicated spectral features, and polarization dependence, and then demonstrate it perfectly by experiments on a bialkali-cathode photomultiplier-tube (PMT) single-photon detector. The dichroism parameter is obtained based on polarization dependent visibility. Accomplishing MPA-based autocorrelation, we manifest that the peak-to-baseline contrast ratio is determined by the mixture exponent. After extracting the pure two-photon absorption, we obtain the ultrafast pulse width which agrees well with that of the standard autocorrelator. Furthermore, we put forward the mutual-correlation scheme and acquire accurately the time jitter of a femtosecond-pulse train. Our results pave the way for analyzing high-order complex MPAs and characterization of ultrafast pulses with super accuracy by a widely used bialkali PMT.

A pulsewidth measurement technology based on carbon-nanotube saturable absorber

We demonstrate a proof-of-concept saturable absorption based pulsewidth measurement (SAPM) by exploring the intensity dependent nonlinear transmission (i.e., saturable absorption) of low-dimensional material (LDM) carbon nanotubes. A minimum pulse energy of 75 fJ is experimentally detected with an average-power-peak-power product (Pav⋅Ppk) of 5.44×10-7 W2 near 1550 nm. A minimum detectable pulse energy of 10 fJ with a Pav⋅Ppk of 1.3×10-9 W2 is estimated with further optimization. The nanometer-level thickness and femtosecond-level decay time of LDMs allow ultrafast light interaction on a very small footprint, which potentially supports chip-scale characterization of ultrafast pulses with minimum distortion.

Spectrally resolved wavefront characterization of broadband ultrafast high-harmonic pulses

We demonstrate a sensor that measures wavefronts of multiple extreme ultraviolet wavelengths simultaneously. By incorporating transmission gratings into the apertures of a Hartmann mask, we can record wavefront information for series of discrete harmonics from a high-harmonic generation source in a single camera exposure, without the need for scanning parts. Wavefronts of up to nine high harmonics at 25-49 nm wavelength are retrieved, and ultrafast spatiotemporal couplings can be detected.

Ultrafast pulse characterization method is updated and optimized for picosecond applications

Frequency-resolved optical gating, a pulse characterization typically available only to femtosecond systems, is adapted for single-shot characterization of high-intensity picosecond lasers.

Double Blind Ultrafast Pulse Characterization by Mixed Frequency Generation in a Gold Antenna

Ultrafast pulse characterization requires the analysis of correlation functions generated by frequency mixing of optical pulses in a nonlinear medium. In this work, we use a gold optical nanoantenna to generate simultaneously Four Wave Mixing and Sum Frequency Generation across the tuning range of a Ti:Sapphire and Optical Parametric Oscillator (OPO) system. Since metal nanoparticles create remarkably strong nonlinear responses for their size without the need for phase matching, this allows us to simultaneously characterize the unknown OPO pulse and its pump pulse using a single spectrogram. The nonlinear mixing is efficient enough to retrieve pulses with energies in the picojoule range.

Streak camera imaging of single photons at telecom wavelength

Streak cameras are powerful tools for temporal characterization of ultrafast light pulses, even at the single-photon level. However, the low signal-to-noise ratio in the infrared range prevents measurements on weak light sources in the telecom regime. We present an approach to circumvent this problem, utilizing an up-conversion process in periodically poled waveguides in Lithium Niobate. We convert single photons from a parametric down-conversion source in order to reach the point of maximum detection efficiency of commercially available streak cameras. We explore phase-matching configurations to apply the up-conversion scheme in real-world applications.

Characterization of ultrafast free-electron laser pulses using extreme-ultraviolet transient gratings.

The characterization of the time structure of ultrafast photon pulses in the extreme-ultraviolet (EUV) and soft X-ray spectral ranges is of high relevance for a number of scientific applications and photon diagnostics. Such measurements can be performed following different strategies and often require large setups and rather high pulse energies. Here, high-quality measurements carried out by exploiting the transient grating process, i.e. a third-order non-linear process sensitive to the time-overlap between two crossed EUV pulses, is reported. From such measurements it is possible to obtain information on both the second-order intensity autocorrelation function and on the coherence length of the pulses. It was found that the pulse energy density needed to carry out such measurements on solid state samples can be as low as a few mJ cm-2. Furthermore, the possibility to control the arrival time of the crossed pulses independently might permit the development of a number of coherent spectroscopies in the EUV and soft X-ray regime, such as, for example, photon echo and two-dimensional spectroscopy.

Third-harmonic interferometric frequency-resolved optical gating

The ability to generate third-harmonic light with a broad phase-matching bandwidth at any dielectric interface offers a promising tool for the characterization of ultrafast laser pulses. Third-harmonic, interferometric frequency-resolved optical gating (TH-iFROG) exploits this nonlinearity. The TH-iFROG traces contain an abundance of information, but their complicated analytical form poses a problem for standard FROG algorithms. Here we solve the pulse retrieval problem with an evolutionary algorithm known as differential evolution. The algorithm processes a TH-iFROG trace, extracting three novel types of FROG traces that are subsequently used for pulse retrieval. The analytical forms of these FROG traces are also presented. Built-in error correction mechanisms and the large amount of redundant data make the pulse retrieval robust against measurement noise and systematic errors. The technique is demonstrated via the characterization of unamplified few-cycle pulses using a simple fused silica window as the nonlinear medium. Comparison to a reference measurement made with a commercial pulse characterization device shows excellent agreement between the measured complex electric fields.

Ultrashort Free-Electron Laser X-ray Pulses

For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up concomitant challenges for their characterization on a suitable time scale. In this review paper we describe progress and results of recent work on ultrafast pulse characterization at soft and hard X-ray free-electron lasers. We report on different approaches to laser-assisted time-domain measurements, with specific focus on single-shot characterization of ultrashort X-ray pulses from self-amplified spontaneous emission-based and seeded free-electron lasers. The method relying on the sideband measurement of X-ray electron ionization in the presence of a dressing optical laser field is described first. When the X-ray pulse duration is shorter than half the oscillation period of the streaking field, few-femtosecond characterization becomes feasible via linear streaking spectroscopy. Finally, using terahertz fields alleviates the issue of arrival time jitter between streaking laser and X-ray pulse, but compromises the achievable temporal resolution. Possible solutions to these remaining challenges for single-shot, full time–energy characterization of X-ray free-electron laser pulses are proposed in the outlook at the end of the review.

Interferometric time-domain ptychography for ultrafast pulse characterization.

A novel pulse characterization method is presented, favorably combining interferometric frequency-resolved optical gating (FROG) and time-domain ptychography. This new variant is named ptychographic-interferometric frequency-resolved optical gating (πFROG). The measurement device is simple, bearing similarity to standard second-harmonic FROG, yet with a collinear beam geometry and an added bandpass filter in one of the correlator arms. The collinear beam geometry allows tight focusing and circumvents possible geometrical distortion effects of noncollinear methods, making πFROG especially suitable for the characterization of unamplified few-cycle pulses. Moreover, the direction-of-time ambiguity afflicting most second-order FROG variants is eliminated. Possible group delay dispersion of pulses leads to a characteristic tilt in the πFROG traces, allowing the detection of uncompensated dispersion without a retrieval. Using nanojoule, three-cycle pulses at 800nm, the πFROG method is tested, and the results are compared with spectral phase interferometry for direct electric field reconstruction measurements. Measured pulse durations agree within a fraction of a femtosecond. As a further test, the πFROG measurements are repeated with added group delay dispersion, and found to accurately reproduce the dispersion computed with Sellmeier equations.

Ultrafast measurements of optical spectral coherence by single-shot time-stretch interferometry.

The palette of laser technology has significantly been enriched by the innovations in ultrafast optical pulse generation. Our knowledge of the complex pulse dynamics, which is often highly nonlinear and stochastic in nature, is however limited by the scarcity of technologies that can measure fast variation/fluctuation of the spectral phase (or coherence) and amplitude in real-time, continuously. To achieve this goal, we demonstrate ultrafast interferometry enabled by optical time-stretch for real- time spectral coherence characterization with microsecond-resolution. Accessing the single-shot interferograms continuously, it further reveals the degree of second-order coherence, defined by the cross-spectral density function, at high speed-a capability absent in any existing spectroscopic measurement tools. As the technique can simultaneously measure both the high-speed variations of spectrally resolved coherence and intensity, time-stretch interferometry could create a new arena for ultrafast pulse characterization, especially favorable for probing and understanding the non-repetitive or stochastic dynamics in real-time.

Wavelength Corrected Ultrafast Pulse Characterization Using Frequency Resolved Optical Gating

Frequency Resolved Optical Gating (FROG) has been used extensively for ultrafast pulse characterization. While the reconstructed electric field can be uniquely determined from the FROG trace, there is no real time calibration of wavelength dependent nonlinearity that affects the FROG trace in the reconstruction algorithm. This modifies the measured FROG trace which ultimately affects the reconstructed electric field. In this paper, we proposed an extension to the FROG setup through an optical spectrum analyser (OSA) for real time calibration. Through the addition of an OSA, the nonlinearity can be compensated to get the original FROG trace which allows for the accurate reconstruction of the ultrafast pulse.