Compact realization of all-attosecond pump-probe spectroscopy.

The ability to perform attosecond-pump attosecond-probe spectroscopy (APAPS) is a longstanding goal in ultrafast science. While first pioneering experiments demonstrated the feasibility of APAPS, the low repetition rates (10 to 120 Hz) and the large footprints of existing setups have so far hindered the widespread exploitation of APAPS. Here, we demonstrate two-color APAPS using a commercial laser system at 1 kHz, straightforward post-compression in a hollow-core fiber, and a compact high-harmonic generation (HHG) setup. The latter enables the generation of intense extreme-ultraviolet (XUV) pulses by using an out-of-focus HHG geometry and by exploiting a transient blueshift of the driving laser in the HHG medium. Near-isolated attosecond pulses are generated, as demonstrated by one-color and two-color XUV-pump XUV-probe experiments. Our concept allows selective pumping and probing on extremely short timescales in many laboratories and permits investigations of fundamental processes that are not accessible by other pump-probe techniques.

Phase-locking of time-delayed attosecond XUV pulse pairs.

We present a setup for the generation of phase-locked attosecond extreme ultraviolet (XUV) pulse pairs. The attosecond pulse pairs are generated by high harmonic generation (HHG) driven by two phase-locked near-infrared (NIR) pulses that are produced using an actively stabilized Mach-Zehnder interferometer compatible with near-single cycle pulses. The attosecond XUV pulses can be delayed over a range of 400 fs with a sub-10-as delay jitter. We validate the precision and the accuracy of the setup by XUV optical interferometry and by retrieving the energies of Rydberg states of helium in an XUV pump-NIR probe photoelectron spectroscopy experiment.

Experimental Control of Quantum-Mechanical Entanglement in an Attosecond Pump-Probe Experiment.

Entanglement is one of the most intriguing aspects of quantum mechanics and lies at the heart of the ongoing second quantum revolution, where it is a resource that is used in quantum key distribution, quantum computing, and quantum teleportation. We report experiments demonstrating the crucial role that entanglement plays in pump-probe experiments involving ionization, which are a hallmark of the novel research field of attosecond science. We demonstrate that the degree of entanglement in a bipartite ion + photoelectron system, and, as a consequence, the degree of vibrational coherence in the ion, can be controlled by tailoring the spectral properties of the attosecond extreme ultraviolet laser pulses that are used to create them.

27 W 2.1 µm OPCPA system for coherent soft X-ray generation operating at 10 kHz.

We developed a high power optical parametric chirped-pulse amplification (OPCPA) system at 2.1 µm harnessing a 500 W Yb:YAG thin disk laser as the only pump and signal generation source. The OPCPA system operates at 10 kHz with a single pulse energy of up to 2.7 mJ and pulse duration of 30 fs. The maximum average output power of 27 W sets a new record for an OPCPA system in the 2 µm wavelength region. The soft X-ray continuum generated through high harmonic generation with this driver laser can extend to around 0.55 keV, thus covering the entire water window (284 eV - 543 eV). With a repetition rate still enabling pump-probe experiments on solid samples, the system can be used for many applications.

Single-step fabrication of surface waveguides in fused silica with few-cycle laser pulses.

Direct laser writing of surface waveguides with ultrashort pulses is a crucial achievement towards all-laser manufacturing of photonic integrated circuits sensitive to their environment. In this Letter, few-cycle laser pulses (with a sub-10 fs duration) are used to produce subsurface waveguides in a non-doped, non-coated fused-silica substrate. The fabrication technique relies on laser-induced microdensification below the threshold for nanopore formation. The optical losses of the fabricated waveguides are governed by the optical properties of the superstrate. We have measured losses ranging from less than 0.1 dB/mm (air superstrate) up to 2.8 dB/mm when immersion oil is applied on top of the waveguide.

Continuous every-single-shot carrier-envelope phase measurement and control at 100 kHz.

With the emergence of high-repetition-rate few-cycle laser pulse amplifiers aimed at investigating ultrafast dynamics in atomic, molecular, and solid-state science, the need for ever faster carrier-envelope phase (CEP) detection and control has arisen. Here we demonstrate a high-speed, continuous, every-single-shot measurement and fast feedback scheme based on a stereo above-threshold ionization time-of-flight spectrometer capable of detecting the CEP and pulse duration at a repetition rate of up to 400 kHz. This scheme is applied to a 100 kHz optical parametric chirped pulse amplification few-cycle laser system, demonstrating improved CEP stabilization and allowing for CEP tagging.

Sub-4 fs laser pulses at high average power and high repetition rate from an all-solid-state setup.

The generation of high average power, carrier-envelope phase (CEP) stable, near-single-cycle pulses at a repetition rate of 100 kHz is demonstrated using an all solid-state setup. By exploiting self-phase modulation in thin quartz plates and air, the spectrum of intense pulses from a high-power, high repetition rate non-collinear optical parametric chirped pulse amplifier (NOPCPA) is extended to beyond one octave, and pulse compression down to 3.7 fs is achieved. The octave-spanning spectrum furthermore allows performing straightforward f-to-2f interferometry by frequency-doubling the long-wavelength part of the spectrum. Excellent CEP-stability is demonstrated for extended periods of time. A full spatio-spectral characterization of the compressed pulses shows only minor asymmetries between the two perpendicular beam axes. We believe that the completed system represents the first laser system satisfying all requirements for performing high repetition rate attosecond pump-probe experiments with fully correlated detection of all ions and electrons produced in the experiment.

CEP-stable few-cycle pulses with more than 190 µJ of energy at 100 kHz from a noncollinear optical parametric amplifier.

Noncollinear optical parametric amplifiers (NOPAs) have become the leading technique for the amplification of carrier-envelope phase (CEP)-stable, few-cycle pulses at high repetition rate and high average power. In this Letter, a NOPA operating at a repetition rate of 100 kHz delivering more than 24 W of average power before compression is reported. The amplified bandwidth supports sub-7 fs pulse durations and pulse compression close to the transform limit is realized. CEP stability after amplification is demonstrated. The system paves the way to attosecond pump-probe spectroscopy with electron-ion coincidence detection.

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 800 nm, 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.

Role of Intrapulse Coherence in Carrier-Envelope Phase Stabilization.

The concept of coherence is of fundamental importance for describing the physical characteristics of light and for evaluating the suitability for experimental application. In the case of pulsed laser sources, the pulse-to-pulse coherence is usually considered for a judgment of the compressibility of the pulse train. This type of coherence is often lost during propagation through a highly nonlinear medium, and pulses prove incompressible despite multioctave spectral coverage. Notwithstanding the apparent loss of interpulse coherence, however, supercontinua enable applications in precision frequency metrology that rely on coherence between different spectral components within a laser pulse. To judge the suitability of a light source for the latter application, we define an alternative criterion, which we term intrapulse coherence. This definition plays a limiting role in the carrier-envelope phase measurement and stabilization of ultrashort pulses. It is shown by numerical simulation and further corroborated by experimental data that filamentation-based supercontinuum generation may lead to a loss of intrapulse coherence despite near-perfect compressibility of the pulse train. This loss of coherence may severely limit active and passive carrier-envelope phase stabilization schemes and applications in optical high-field physics.

Strong-field ionization of clusters using two-cycle pulses at 1.8 µm.

The interaction of intense laser pulses with nanoscale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters (103 to 105 atoms) using two-cycle 1.8 µm laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3 keV is observed using laser pulses with a wavelength of 1.8 µm and an intensity of 1 × 1015 W/cm2, whereas only electrons below 500 eV are observed at 800 nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.

Spatio-temporal characterization of intense few-cycle 2 µm pulses.

We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2 µm. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500-650 µJ pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).

Carrier-envelope phase stability of hollow fibers used for high-energy few-cycle pulse generation.

We investigated the carrier-envelope phase (CEP) stability of hollow-fiber compression for high-energy few-cycle pulse generation. Saturation of the output pulse energy is observed at 0.6 mJ for a 260 µm inner-diameter, 1 m long fiber, statically filled with neon. The pressure is adjusted to achieve output spectra supporting sub-4-fs pulses. The maximum output pulse energy can be increased to 0.8 mJ by either differential pumping (DP) or circularly polarized input pulses. We observe the onset of an ionization-induced CEP instability, which saturates beyond input pulse energies of 1.25 mJ. There is no significant difference in the CEP stability with DP compared to static-fill.

Stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate.

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.

Lateral shearing interferometry of high-harmonic wavefronts.

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects.

Characterization of high-intensity sub-4-fs laser pulses using spatially encoded spectral shearing interferometry.

We report on the full amplitude and phase characterization of high-intensity few-cycle laser pulses generated in a single-stage hollow core fiber system with subsequent compression by ultrabroadband chirped mirrors. We use a spatially-encoded arrangement (SEA) spectral phase interferometry for direct electric field reconstruction (SPIDER) with spectral filters for ancilla generation to characterize the sub-4 fs pulses with spatial resolution.

Resolution of the relative phase ambiguity in spectral shearing interferometry of ultrashort pulses.

We show that multiple-shear spectral shearing interferometry can overcome the relative phase ambiguity of disjoint spectral components that is present in single-shear approaches. By upconverting the unknown pulse with spatially chirped ancillae, we achieve a shear-to-space mapping that can be acquired on an imaging spectrometer. A subset of this continuous range of shears can be chosen for robust and accurate phase retrieval using a multiple-shear algorithm.

Broadband astigmatism-free Czerny-Turner imaging spectrometer using spherical mirrors.

We describe the elimination of the astigmatism of a Czerny-Turner imaging spectrometer, built using spherical optics and a plane grating, over a broad spectral region. Starting with the principle of divergent illumination of the grating, which removes astigmatism at one chosen wavelength, we obtain design equations for the distance from the grating to the focusing mirror and the detector angle that remove the astigmatism to first order in wavelength. Experimentally, we demonstrate near diffraction-limited performance from 740 to 860 nm and over a 5 mm transverse spatial extent, while ray-tracing calculations show that barring finite-aperture and detector size limitations, this range extends from 640 to 900 nm and over 10 mm transversely. Our technique requires no additional optics and uses standard off-the-shelf components.

Improved ancilla preparation in spectral shearing interferometry for accurate ultrafast pulse characterization.

We report a version of spectral phase interferometry for direct electric field reconstruction (SPIDER), in which spectral filters are used to produce the quasi-monochromatic fields required for upconversion. The advantages of this approach include improved calibration accuracy, robustness for strongly chirped input pulses, simplicity, and compactness. We verify the technique experimentally by measuring the spectral chirp of a grating compressor using a spatially encoded arrangement (SEA-)SPIDER.

Ultrashort pulse characterization by spectral shearing interferometry with spatially chirped ancillae.

We report a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER), in which two spatially chirped ancilla fields are used to generate a spatially encoded SPIDER interferogram. We dub this new technique Spatially Encoded Arrangement for Chirped ARrangement for SPIDER (SEA-CAR-SPIDER). The single shot interferogram contains multiple shears, the spectral amplitude of the test pulse, and the reference phase, which is accurate for broadband pulses. The technique enables consistency checking through the simultaneous acquisition of multiple shears and offers a simple and precise calibration method. All calibration parameters--the shears, and the upconversionfrequency--can be accurately obtained from a single calibration trace.