Propagation effects in polarization-gated attosecond soft-X-ray pulse generation.

Accurate estimation of the duration of soft-x-ray pulses from high-harmonic generation (HHG) remains challenging given their higher photon energies and broad spectral bandwidth. The carrier-envelope-phase (CEP) dependence of generated soft-x-ray spectra is indicative of attosecond pulse generation, but advanced simulations are needed to infer the pulse duration from such data. Here, we employ macroscopic propagation simulations to reproduce experimental polarization-gated CEP-dependent soft-x-ray spectra. The simulations indicate chirped pulses, which we theoretically find to be compressible in hydrogen plasmas, suggesting this as a viable compression scheme for broadband soft-x-rays from HHG.

Few-cycle, mJ-level, mid-wave infrared pulses generated via post-compression of a chirped pulse amplifier.

Few-cycle pulses were generated by passing a beam from a cryogenically cooled Fe:ZnSe chirped-pulse amplifier (CPA) at a repetition rate of 400 Hz through a gas-filled hollow core fiber (HCF) followed by dispersion-compensating bulk CaF2. The krypton-filled fiber at 370 kPa yielded 1.14-mJ, 42-fs pulses centered at 4.07 µm, while the oxygen-filled fiber at 310 kPa delivered 0.78-mJ, 39-fs pulses spanning from 3 to 5.5 µm. This work is a step toward a high repetition rate mid-wave infrared driver of isolated attosecond keV x-ray pulses.

Generation of high-order harmonics and attosecond pulses in the water window via nonlinear propagation of a few-cycle laser pulse.

We theoretically and computationally study the generation of high-order harmonics in the water window from a semi-infinite gas cell where a few-cycle, carrier-envelope-phase-controlled 1.7-µm driving laser pulse undergoes nonlinear propagation via optical Kerr effect (self-focusing) and plasma defocusing. Our calculation shows that high harmonic signals are enhanced for extended propagation distances and furthermore, isolated attosecond pulses in the water window can be generated from the semi-infinite gas cell. This enhancement is attributed mainly to better phase matching for extended propagation distances achieved via nonlinear propagation and resulting intensity stabilization.

Cryogenically cooled Fe:ZnSe-based chirped pulse amplifier at 4.07†µm.

A femtosecond chirped pulse amplifier based on cryogenically cooled Fe:ZnSe was demonstrated at 333 Hz-33 times higher than previous results achieved at near-room-temperature. The long upper-state lifetime allows free-running, diode-pumped Er:YAG lasers to be used as pump lasers. 250-fs, 4.59-mJ pulses are produced with a center wavelength of 4.07 µm, which avoids strong atmospheric CO2 absorption that cuts on around 4.2 µm. It is therefore possible to operate the laser in ambient air with good beam quality. By focusing the 18-GW beam in air, harmonics up to the ninth order were observed indicating its potential for use in strong-field experimentation.

Efficient generation of femtosecond millijoule pulses at 3.1†µm.

3.2-mJ, 92-fs pulses centered at 3.1 µm are generated at a 1-kHz repetition rate through a tabletop optical parametric chirped pulse amplification (OPCPA) system based on ZnGeP2 crystals. Pumped by a 2-µm chirped pulse amplifier with a flat-top beam profile, the amplifier achieves a 16.5% overall efficiency, which, to the best of our knowledge, is the highest efficiency achieved by OPCPA at this wavelength. Harmonics up to the seventh order are observed after focusing the output in the air.

Monitoring Argon L-Shell Auger Decay Using 250-eV Attosecond X-ray Pulses.

Electron correlation describes the interaction between electrons in a multi-electron system. It plays an important role in determining the speed of relaxation of atoms and molecules excited by XUV/X-ray pulses, such as the argon decay rate. Most research on electron correlation has centered on the role of correlation in stationary states. A time-resolved experimental study of electron correlation is a grand challenge due to the required temporal resolution and photon energy. In this research, we investigated Auger decay in argon using 200-attosecond X-ray pulses reaching the carbon K-edge. At such a high photon energy, ionization occurs not only from the outer most levels (3s and 3p), but also from the 2p core shells. We have measured a lifetime of 4.9 fs of L-shell vacancies of argon in pump-probe experiments with a home-built high-resolution time-of-flight spectrometer.

Surface structure evolution and Raman response for multipulse, few-cycle, laser damaged ZnSe.

Multiple 11-fs infrared, few-cycle laser pulses were applied to a polycrystal ZnSe surface to study the evolution of surface damage morphologies. The polycrystalline grain boundaries seem to be the initiation site of surface damage and formation of ripples, which evolve as the result of many laser pulses at the same site. Scanning electron microscopy and atomic force microscopy (AFM) were applied to characterize the surface. The crystalline change and material phase transition were examined by confocal Raman spectroscopy. The thermal expansion coefficient increased slightly in the ablated zone compared to the non-ablated zone according to an AFM thermal tip test. The results show the growth and organization of surface ripples and the change of thermal properties as the number of irradiations at each site increases.

Single-shot intense few-cycle pulse interaction with polycrystalline ZnSe.

The interaction of high-intensity few-cycle laser pulses with solids opens a new area of fundamental light-material interaction research. The applied research extends from extreme nonlinearity in solids to the next-generation high laser light damage resistance optical design. In this Letter, 11 fs infrared, carrier-envelope-phase (CEP) stable, two-cycle laser pulses were applied to investigate the process of laser-material interaction on the ZnSe surface. A systematic study of a few-cycle pulse laser-induced damage threshold on ZnSe was performed for a single-pulse regime (1-on-1). Laser damage morphologies were carefully characterized. Our experiment demonstrated the very beginning of laser-induced structures on the ZnSe surface by using the shortest infrared few-cycle laser pulse currently available with a stable CEP.

Attosecond streaking phase retrieval with neural network.

A new method for retrieving the spectral phase of isolated attosecond X-ray pulses from streaking traces is explored. The neural network method shows the potential for nearly instantaneous attosecond streaking phase retrieval, without use of the central momentum approximation. A neural network is trained with computer generated data that contain statistical noise and shown to correctly retrieve the phase of both computer generated and experimental attosecond streaking traces.

Double optical gating for generating high flux isolated attosecond pulses in the soft X-ray regime.

Double optical gating (DOG) technique was implemented with a two-cycle, 1.7 µm driving field to generate isolated attosecond pulses in the 100-250 eV spectrum range. The strong ellipticity dependency of the high harmonics from the 1.7 µm driving field makes polarization based gating method very efficient. When a second harmonic (SH) field is introduced, complete gating can be achieved with less ionization from the leading edge of the driving field, which yields supercontinua with a pulse energy of 0.3 nJ. We perform an attosecond streaking measurement to confirm the generation of isolated attosecond pulses.

In-line Spectral Interferometry in Shortwave-Infrared Laser Filaments in Air.

We investigate the nonlinear propagation of intense, two-cycle, carrier-envelope phase (CEP) stable laser pulses at 1.7 µm center wavelength in air. We observe CEP-dependent spectral interference in the visible part of the forward-propagating white light generated on propagation. The effect is robust against large fluctuations of the input pulse energy. This robustness is enabled by rigid clamping of both the peak optical field and the phase of the propagating waveform, which has been revealed by numerical simulations. The CEP locking can enhance the yield of the CEP-dependent strong-field processes in gaseous media with long-wavelength drivers, while the observed spectral interference enables single-shot, stand-off CEP metrology in the atmosphere.

Attosecond chirp compensation in water window by plasma dispersion.

Compensating attosecond chirp (atto-chirp) of broadband high-order harmonic pulses in the water window region (282 to 533 eV) is a major challenge, due to the lack of natural materials that exhibit negative group velocity dispersion and low loss. Analysis shows that the amount of dispersion of fully ionized hydrogen plasma with suitable density-length product is sufficient to compensate the chirp of attosecond pulses with center photon energy above 300 eV. This is confirmed by numerical simulations based on the Strong Field Approximation.

Generation of 1 kHz, 2.3 mJ, 88 fs, 2.5 µm pulses from a Cr2+:ZnSe chirped pulse amplifier.

We demonstrate the generation of 2.3 mJ, 88 fs, 2.5 µm laser pulses at 1 kHz repetition rate from a three-stage chirped pulse amplifier employing Cr2+:ZnSe crystals as the active gain media. 5 µJ seed of the amplifier is obtained via intrapulse difference frequency generation in a bismuth triborate (BIBO) crystal from spectrally broadened Ti:Sapphire amplifier output. A multi-pass amplifier followed by two single-pass amplifiers pumped by Q-switched Ho:YAG lasers boost the pulse energy to 6.5 mJ, yielding 2.3 mJ, 88 fs pulses upon pulse compression. Our results show the highest peak power at 2.5 µm with 1 kHz repetition rate. Such a laser will be a powerful source for studying strong-field physics and extending high-harmonic generation towards the keV region.

Laser waveform control of extreme ultraviolet high harmonics from solids.

Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength. Our results provide the foundation for attosecond pulse metrology based on solid-state harmonics and a new approach to studying sub-cycle dynamics in solids.

High-energy two-cycle pulses at 3.2 µm by a broadband-pumped dual-chirped optical parametric amplification.

A design for efficient generation of mid-infrared pulses at 3.2 µm is presented, which is based on numerical simulations of the broadband-pumped dual-chirped optical parametric amplification (DC-OPA) in LiNbO3 doped with 5 mol.% MgO (MgO:LiNbO3). The broadband seed can be generated by difference frequency generation in KTA using spectrally-broadened Ti:Sapphire lasers. The broad DC-OPA phase-matching bandwidth-spanning from 2.4 µm to 4.0 µm-is achieved by chirping both the broadband Ti:Sapphire pump pulses and the seed pulses in such a way that the individual temporal slice of pump spectrum is able to phase match that of seed spectrum. This phase matching scheme allows the use of longer crystals without gain narrowing or loss of conversion efficiency. The theoretical conversion efficiency from the pump to the idler reaches 19.1 %, enabling generation of a few hundred mJ of mid-IR energy with an available large-aperture MgO:LiNbO3 crystal. Furthermore, the commercially available acousto-optic programmable dispersive filter (AOPDF) ensures compression of such a broad bandwidth down to 20 fs (two optical cycles at 3.2 µm).

High-efficiency optical parametric chirped-pulse amplifier in BiB3O6 for generation of 3 mJ, two-cycle, carrier-envelope-phase-stable pulses at 1.7 µm.

We produce a 3 mJ, two-cycle (11.4 fs), 1 kHz, carrier-envelope phase (CEP)-stable laser source at 1.7 µm via a three-stage Ti:sapphire-pumped optical parametric chirped-pulse amplifier in BiB3O6. We achieve a pump-to-signal conversion efficiency of 18% in the last stage, which is, to the best of our knowledge, the highest yet achieved for near-octave bandwidth amplification. A f-to-2f measurement shows a CEP instability of 165 mrad over 1 h. This is an ideal light source for generating isolated attosecond pulses in the soft x-ray region.

Fabricating nanostructures on fused silica using femtosecond infrared pulses combined with sub-nanojoule ultraviolet pulses.

Circular craters with diameters of 500 nm are fabricated on the surface of fused silica by femtosecond ultraviolet-infrared (UV-IR) pulse trains with 0.8 nJ UV pulse energy. UV damage thresholds at different IR energies and UV-IR delays are measured. Diameters and depths of the ablated craters can be modified by adding the IR pulse and varying the UV-IR delays. These results demonstrate the feasibility of nanomachining using short wavelength lasers with pulse energy far below normal damage thresholds.

Suppression of driving laser in high harmonic generation with a microchannel plate.

Separating the infrared driving laser from the extreme ultraviolet (XUV) pulses after high-order harmonic generation has been a long-standing difficulty. In this Letter, we propose and demonstrate that the driving laser can be blocked by simply installing a microchannel plate (MCP) into the beam line. In addition to its high damage threshold, the MCP filter also transmits photons over the entire XUV region. This paves the way for attosecond pulse generation with unprecedented bandwidth.

Femtosecond laser nanomachining initiated by ultraviolet multiphoton ionization.

We report on the experimental results of 300 nm features generated on fused silica using a near-infrared (IR) femtosecond laser pulse initiated by an ultraviolet (UV) pulse. With both pulses at a short (~60 fs) delay, the damage threshold of the UV pulse is only 10% of its normal value. Considerable reduction of UV damage threshold is observed when two pulses are at ± 1.3 ps delay. The damage feature size of the combined pulses is similar to that of a single UV pulse. A modified rate equation model with the consideration of defect states is used to help explain these results. This concept can be applied to shorter wavelengths, e.g. XUV and X-ray, with the required fluence below their normal threshold.

In situ calibration of an extreme ultraviolet spectrometer for attosecond transient absorption experiments.

We report a method for calibrating an extreme ultraviolet spectrometer based on a flat-field grazing incidence spherical grating in the energy range of 20-30 eV. By measuring absorption lines corresponding to singly excited states in helium atoms and autoionizing states in argon atoms, the photon energy of the detected light was determined. The spectral resolution of the spectrometer, 60 meV, was obtained by deconvolving the Fano resonance profile of argon autoionizing states from the measured absorption line profiles.