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Attosecond chronoscopy has revealed small but measurable delays in photoionization, characterized by the ejection of an electron on absorption of a single photon. Ionization-delay measurements in atomic targets provide a wealth of information about the timing of the photoelectric effect, resonances, electron correlations and transport. However, extending this approach to molecules presents challenges, such as identifying the correct ionization channels and the effect of the anisotropic molecular landscape on the measured delays. Here, we measure ionization delays from ethyl iodide around a giant dipole resonance. By using the theoretical value for the iodine atom as a reference, we disentangle the contribution from the functional ethyl group, which is responsible for the characteristic chemical reactivity of a molecule. We find a substantial additional delay caused by the presence of a functional group, which encodes the effect of the molecular potential on the departing electron. Such information is inaccessible to the conventional approach of measuring photoionization cross-sections. The results establish ionization-delay measurements as a valuable tool in investigating the electronic properties of molecules. Ionization delays from ethyl iodide around a giant dipole resonance are measured by attosecond streaking spectroscopy. Using theoretical knowledge of the iodine atom as a reference, the contribution of the functional ethyl group can be obtained.

The strong absorption of liquid water in the infrared (IR) molecular fingerprint region constitutes a challenge for applications of vibrational spectroscopy in chemistry, biology, and medicine. While high-power IR laser sources enable the penetration of ever thicker aqueous samples, thereby mitigating the detrimental effects of strong attenuation on detection sensitivity, a basic advantage of heterodyne-measurement-based methods has—to the best of our knowledge—not been harnessed in broadband IR measurements to date. Here, employing field-resolved spectroscopy (FRS), we demonstrate in theory and experiment fundamental advantages of techniques whose signal-to-noise ratio (SNR) scales linearly with the electric field over those whose SNR scales linearly with radiation intensity, including conventional Fourier-transform infrared (FTIR) and direct absorption spectroscopy. Field-scaling brings about two major improvements. First, it squares the measurement dynamic range. Second, we show that the optimum interaction length with samples for SNR-maximized measurements is twice the value usually considered to be optimum for FTIR devices. In order to take full advantage of these properties, the measurement must not be significantly affected by technical noise, such as intensity fluctuations, which are common for high-power sources. Recently, it has been shown that subcycle, nonlinear gating of the molecular fingerprint signal renders FRS robust against intensity noise. Here, we quantitatively demonstrate this advantage of FRS for thick aqueous samples. We report sub-μg/mL detection sensitivities for transmission path lengths up to 80 μm and a limit of detection in the lower μg/mL range for transmission paths as long as 200 μm.

The proper functioning of living systems and physiological phenotypes depends on molecular composition. Yet simultaneous quantitative detection of a wide variety of molecules remains a challenge1–8. Here we show how broadband optical coherence opens up opportunities for fingerprinting complex molecular ensembles in their natural environment. Vibrationally excited molecules emit a coherent electric field following few-cycle infrared laser excitation9–12, and this field is specific to the sample’s molecular composition. Employing electro-optic sampling10,12–15, we directly measure this global molecular fingerprint down to field strengths 107 times weaker than that of the excitation. This enables transillumination of intact living systems with thicknesses of the order of 0.1 millimetres, permitting broadband infrared spectroscopic probing of human cells and plant leaves. In a proof-of-concept analysis of human blood serum, temporal isolation of the infrared electric-field fingerprint from its excitation along with its sampling with attosecond timing precision results in detection sensitivity of submicrograms per millilitre of blood serum and a detectable dynamic range of molecular concentration exceeding 105. This technique promises improved molecular sensitivity and molecular coverage for probing complex, real-world biological and medical settings. A vibrational spectroscopy technique that measures the electric field emitted from organic molecules following infrared illumination allows their molecular fingerprints to be separated from the excitation background, even in complex biological samples.

The necessity to precisely determine the delay between ultrashort light pulses is ubiquitous in optical metrology. For pulses originating from the same source, optical-interference-based delay tracking using a monochromatic laser affords precisions of a few attoseconds [1] . However, if the pulses originate from separate modelocked oscillators with detuned repetition frequencies – such as widely employed in dual-frequency-comb spectroscopy [2] and electronically controlled optical sampling (ECOPS) [3] – interferometric delay tracking faces two challenges: (i) the optical phase typically fluctuates independently of the repetition frequency, requiring independent control, and (ii) the interference signal is confined to the time window of the overlap, usually < 1ps.

Isolated attosecond pulses (IAPs) produced through laser-driven high-harmonic generation (HHG) hold promise for unprecedented insight into physical, chemical, and biological processes via attosecond x-ray diffraction and spectroscopy with tabletop sources. Efficient scaling of HHG towards x-ray energies, however, has been hampered by ionization-induced plasma generation impeding the coherent buildup of high-harmonic radiation. Recently, it has been shown that these limitations can be overcome in the so-called “overdriven regime” where ionization loss and plasma dispersion strongly modify the driving laser pulse over small distances, albeit without demonstrating IAPs. Here, we report on experiments contrasting the generation of IAPs at 80 eV in argon with neon via attosecond streaking measurements. Comparing our experimental results to numerical simulations, we conclude that IAPs in argon are generated in the overdriven regime. We introduce a simple expression that fully describes the HHG dipole phase-mismatch contribution, specifically the effect of the blueshift of the driving laser. Furthermore, we present a method to numerically calculate the transient HHG phase mismatch, which allows us to demonstrate the accuracy of the introduced phase-mismatch expression. Finally, we perform simulations for different gases and wavelengths and show that including the full HHG dipole phase-mismatch contribution is important for understanding HHG with long-wavelength, few-cycle laser pulses in high-pressure gas targets, which are currently being employed for scaling isolated attosecond pulse generation beyond extreme ultraviolet (XUV) towards soft-x-ray photon energies.

Electro-optic sampling (EOS) has established itself as one of the main techniques for coherent THz spectroscopy. In recent years, EOS has been pushed towards recording infrared or even optical waveforms, opening up new possibilities for spectroscopy in these spectral regions. In this contribution, we demonstrate the potential of electric-field-resolved detection via EOS for broadband, mid-infrared molecular vibrational spectroscopy. We show that field-resolved spectroscopy can achieve detection sensitivities orders of magnitude higher than state-of-the art Fourier-transform infrared spectroscopy. This is achieved by high-quantum-efficiency non-linear temporal “piercing” and, therefore, isolation of the molecular signal from the orders-of-magnitude stronger impulsive excitation. Thereby limitations due to detector dynamic range and source excitation noise can be avoided. This promises a new level of molecular sensitivity and molecular coverage for probing complex biological samples.

Broadband dispersive mirrors operating in the mid-infrared spectral range of 6.5-11.5 μm are developed for the first time, to the best of our knowledge. The mirrors comprise Ge and YbF

A gold-coated silicon grating has been developed, enabling efficient spatial separation of broadband mid-infrared (MIR) beams with wavelengths >5 mm from collinearly propagating, broadband, high-power light in the near-infrared (NIR) spectral range (centered at 2 mm). The optic provides spectral filtering at high powers in a thermally robust and chromatic-dispersion-free manner such as that necessary for coherent MIR radiation sources based on parametric frequency downconversion of femtosecond NIR lasers. The suppression of a >20 W average-power, 2 mm driving pulse train by three orders of magnitude, while retaining high reflectivity of the broadband MIR beam, is presented.

Precise delay control is crucial for ultrafast optical pump probe experiments. For example to study attosecond electron dynamics, time-delay control down to a few attoseconds is required [1,2]. But also applications with longer time scales like THz-time domain spectroscopy benefit greatly from a high position accuracy [3,4].

Molecular vibrational spectroscopies provide chemically-specific information on complex samples. The label-free nature of these techniques renders them highly attractive for studies of biological processes and medical diagnosis [1,2]. Among these methods, the direct, broadband interrogation of molecular vibrations at their fundamental frequencies in the infrared (IR) molecular fingerprint region, profits from large interaction cross-sections, potentially affording a unique combination of detection sensitivity and molecular coverage. However, the strong absorption of (liquid) water in this spectral range has so far severely limited the applicability of IR vibrational spectroscopy (and microscopy) in transmission geometry. In fact, in most table-top setups, the transmission path length has to be limited to < 10 μm, or attenuated total reflection techniques with even smaller penetration depth are applied. Alternatively, the sample can be dried — however this strongly alters it. For larger thicknesses, the current approach is to use high-brightness sources like quantum cascade lasers, although their applicability is limited due to their narrowband emission and modest intensity stability [3].

Precise measurement and control of the frequency and phase of the electric-field carrier of ultrashort light pulses underneath their intensity envelope is instrumental to the study of dynamic processes unfolding on time scales shorter than the oscillation period of light, as well as for precision-spectroscopy measurements of quantum transitions. Among the techniques employed for the generation of trains of waveform-stable laser pulses, intra-pulse difference-frequency generation (IPDFG) is widely favored due to its passive carrier-envelope-phase (CEP) stabilization [1,2]. While in theory the CEP stability of IPDFG should be pristine, so far the lowest values reported for CEP noise measurements for trains of pulses generated via IPDFG lie in the range of a few tens of mrad [3], with — presumably — in most cases the metrology being the limitation rather than the IPDFG process itself.

The development of high-power, ultrafast coherent light sources in the mid-infrared (MIR) spectral region is subject to intensive research, due to the promise of improving both time- and frequency-domain spectroscopic methods, in particular for background-free field-resolved spectroscopy (FRS) [1,2]. In FRS, the molecular vibrational response of a sample upon impulsive, resonant excitation is measured in the time domain, e.g., via electro-optic sampling (EOS). Recent efforts have identified frequency down-conversion via intra-pulse difference frequency generation (IPDFG) as a viable route for producing high-power, waveform-stable MIR pulses for FRS [2,3]. With 2-μm driving sources, broader bandwidths [4,5] and higher efficiencies [5] can be achieved compared to 1-μm and 1.5-μm, in part due to a wider variety of nonlinear crystals with a high nonlinearity. Here, we present IPDFG driven by a 2-μm, thulium-fibre chirped-pulse amplification (CPA) system. This source uniquely combines broad bandwidth (6–18 μm), high average power (0.5 W), waveform stability, and a high repetition rate (50 MHz). In addition, we demonstrate for the first time EOS of the MIR using a short, high-power 2-μm sampling pulse.

The advancement of attosecond chronoscopy has made it possible to reveal ultrashort time dynamics of photoionization [1]. Ionization delay measurements in atomic targets provide a wealth of information about the timing of the photoelectric effect [2], resonances, electron correlations and transport. The extension of this approach to molecules, however, presents great challenges. In addition to the difficulty of identifying correct ionization channels, it is hard to disentangle the role of the anisotropic molecular landscape from the delays inherent to the excitation process itself. Here, we present the measurements of ionization delays from ethyl iodide around the 4d giant dipole resonance of iodine. We employ attosecond streaking spectroscopy, which enables to disentangle the contribution to the delay from the functional ethyl group, being responsible for the characteristic chemical reactivity of the molecule. An attosecond extreme ultraviolet (XUV) pulse ionizes the molecule around the energy of the giant resonance and the released electron is exposed to the ponderomotive force of a synchronized near-infrared (NIR) field, which yields a streaking spectrogram (see figure). Comparative phase analysis of the spectrograms corresponding to iodine 4d and neon 2p emission permits extracting overall photoemission delays for ethyl iodide. The data is recorded for multiple photon energies around the iodine 4d resonance and compared to classical Wigner propagation [3] and quantum scattering [4] calculations. Here the outgoing electron, produced via inner shell ionization of the iodine atom in ethyl iodide, and thereby hardly influenced by the molecular potential during the birth process, acquires the necessary information about the influence of the functional ethyl group during its propagation. We find significant delay contributions that can distinguish between different functional groups, providing a sensitive probe of the local molecular environment [5]. This would stimulate to perform further angle resolved measurements in molecules to probe the potential landscape in three dimension.

Electro-optic sampling (EOS) allows for field-sensitive detection of optical waveforms in a broad spectral range, extending from the far-infrared (THz) [1, 2] all the way to the near-infrared [3]. Most recently, EOS has been employed for highly sensitive spectroscopy of liquids [4] and gases [5] in the mid-infrared (MIR) molecular fingerprint region, overcoming the notorious limitations of broadband detectors in this spectral range and providing a glance at the potential of this coherent detection scheme for molecular spectroscopy.

We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6-18 μm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 μm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-μm pulses in GaSe.

We report on a novel source for generating high-energy, subcycle pulses based on a Yb thin-disk laser., The development of high-energy, high-power, multi-octave light transients is currently the subject of intense research driven by emerging applications in attosecond spectroscopy and coherent control. We report on a phase-stable, multi-octave source based on a Yb:YAG amplifier for light transient generation. We demonstrate the amplification of a two-octave spectrum to 25 μJ of energy in two broadband amplification channels and their temporal compression to 6 and 18 fs at 1 and 2 μm, respectively. In this scheme, due to the intrinsic temporal synchronization between the pump and seed pulses, the temporal jitter is restricted to long-term drift. We show that the intrinsic stability of the synthesizer allows subcycle detection of an electric field at 0.15 PHz. The complex electric field of the 0.15-PHz pulses and their free induction decay after interaction with water molecules are resolved by electro-optic sampling over 2 ps. The scheme is scalable in peak and average power.

Traditionally, infrared molecular spectroscopy has been performed with frequency-domain measurement techniques. Recent experiments have exploited the outstanding temporal coherence of state-of-the-art femtosecond lasers to overcome long-standing sensitivity and dynamic range limitations of these traditional techniques, with time-domain measurements. Here, we show how state-of-the-art 2-µm femtosecond technology provides (i) Watt-level infrared sources covering the entire molecular fingerprint region, with a spectral brightness exceeding even that of synchrotrons, (ii) background-free, high-sensitivity and high-dynamic range time-domain detection of molecular vibrations via electro-optical sampling with (iii) attosecond temporal accuracy. These advances herald a new regime for time-, frequency- and space-resolved molecular vibrational metrology.

We introduce the concept of broadband near-infrared molecular fieldoscopy. In this scheme, molecules are excited by femtosecond pulses in near-infrared spectral range and the complex electric field of their free induction decay is directly measured by means of electro-optic sampling. Few-cycle pulses centered at 2 µm and 1 µm are generated from a 5 kHz, Yb:YAG regenerative amplifier and employed for femtosecond excitation and electro-optic sampling, respectively. We chose water in an acetic acid solvent to demonstrate the first proof of principle measurement with the novel technique. The complex electric field of the combination bond of water molecules at 1930 nm at different molecular concentrations is detected and presented. We show the detection sensitivity of our time- domain technique is comparable to conventional specral-domain techniques. However, by employing a laser frontend with higher repetition rates, the detection sensitivity can be drastically enhanced. To the best of our knowledge, this is the first detection of the complex electric field of the molecular response in near-infrared spectral range. The new method holds promise for high-resolution overtone spectroscopy and microscopy with unparalleled sensitivity and specificity over the entire molecular fingerprint region.

In this work, the concept of near-infrared molecular fieldoscopy is introduced and bench-marking measurements are presented. The new method allows for high-resolution overtone spectroscopy and microscopy with an unparalleled sensitivity and specificity over the entire molecular fingerprint region.

Petahertz-bandwidth metrology is demonstrated in the measurement of nonlinear polarization in silica. Recent years have seen an increased interest in light–matter interactions in solid-state systems at ultrafast timescales. Ferenc Krausz and colleagues study the nonlinear polarization of silica in response to intense infrared light fields with a spectroscopy method in the attosecond time range. The method makes it possible to unravel details of the reversible and irreversible energy exchange between infrared light and electrons and points to the feasibility of using light-based switching techniques for signal processing in solid-state devices above 100 terahertz. Electric-field-induced charge separation (polarization) is the most fundamental manifestation of the interaction of light with matter and a phenomenon of great technological relevance. Nonlinear optical polarization1,2 produces coherent radiation in spectral ranges inaccessible by lasers and constitutes the key to ultimate-speed signal manipulation. Terahertz techniques3,4,5,6,7,8 have provided experimental access to this important observable up to frequencies of several terahertz9,10,11,12,13. Here we demonstrate that attosecond metrology14 extends the resolution to petahertz frequencies of visible light. Attosecond polarization spectroscopy allows measurement of the response of the electronic system of silica to strong (more than one volt per angstrom) few-cycle optical (about 750 nanometres) fields. Our proof-of-concept study provides time-resolved insight into the attosecond nonlinear polarization and the light–matter energy transfer dynamics behind the optical Kerr effect and multi-photon absorption. Timing the nonlinear polarization relative to the driving laser electric field with sub-30-attosecond accuracy yields direct quantitative access to both the reversible and irreversible energy exchange between visible–infrared light and electrons. Quantitative determination of dissipation within a signal manipulation cycle of only a few femtoseconds duration (by measurement and ab initio calculation) reveals the feasibility of dielectric optical switching at clock rates above 100 terahertz. The observed sub-femtosecond rise of energy transfer from the field to the material (for a peak electric field strength exceeding 2.5 volts per angstrom) in turn indicates the viability of petahertz-bandwidth metrology with a solid-state device.

New journal of physics 20(3), 033008 (2018). doi:10.1088/1367-2630/aab548, Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques., Published by IOP, [London]

A compact source that generates sub-two-cycle-duration pulses with an average power of 0.1 W spanning 6.8–16.4 μm combines the properties of power scalability, high repetition rate and phase coherence for the first time in this spectral region. Powerful coherent light with a spectrum spanning the mid-infrared (MIR) spectral range is crucial for a number of applications in natural as well as life sciences, but so far has only been available from large-scale synchrotron sources1. Here we present a compact apparatus that generates pulses with a sub-two-cycle duration and with an average power of 0.1 W and a spectral coverage of 6.8–16.4 μm (at −30 dB). The demonstrated source combines, for the first time in this spectral region, a high power, a high repetition rate and phase coherence. The MIR pulses emerge via difference-frequency generation (DFG) driven by the nonlinearly compressed pulses of a Kerr-lens mode-locked ytterbium-doped yttrium–aluminium–garnet (Yb:YAG) thin-disc oscillator. The resultant 100 MHz MIR pulse train is hundreds to thousands of times more powerful than state-of-the-art frequency combs that emit in this range2,3,4, and offers a high dynamic range for spectroscopy in the molecular fingerprint region4,5,6,7 and an ideal prerequisite for hyperspectral imaging8 as well as for the time-domain coherent control of vibrational dynamics9,10,11.

Excess relative intensity noise (RIN) constitutes one of the major limitations of most spectroscopic methods involving lasers. Here, we present an active RIN suppression scheme for a coherent mid-infrared (MIR) light source (8.4-11 µm), based on intra-pulse difference frequency generation (DFG). Three different stabilization concepts that rely on modulating the intensity of the driving near-infrared (NIR) pulse train with an acousto-optic modulator are investigated and compared. By using the wings of the NIR spectrum to generate the error signal, a RIN suppression of the MIR pulse train of up to a factor of 20 was achieved in the band between 1 Hz and 100 kHz, resulting in a total integrated RIN of 0.07%.

Broadband dispersive mirrors operating in the mid-infrared spectral range of 6.5–11.5 μm are developed for the first time, to the best of our knowledge. The mirrors comprise Ge and YbF3 layers, which have not been used before for manufacturing of multilayer dispersive optics. The design and production processes are described; mechanical stresses of the coatings are estimated based on experimental data; and spectral and phase properties of the produced mirrors are measured. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4 mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse.

Precise delay control is of paramount importance in optical pump-probe measurements. Here, we report on a high-precision delay tracking technique for mechanical scanning measurements in a Mach-Zehnder interferometer configuration. The setup employs a 1.55-µm continuous-wave laser beam propagating along the interferometer arms. Sinusoidal phase modulation at 30 MHz, and demodulation of the interference signal at the fundamental frequency and its second harmonic, enables delay tracking with sampling rates of up to 10 MHz. At an interferometer arm length of 1 m, root-mean-square error values of the relative delay tracking below 10 attoseconds for both stationary and mechanically scanned (0.2 mm/s) operation are demonstrated. By averaging several scans, a precision of the delay determination better than 1 as is reached. We demonstrate this performance with a mechanical chopper periodically interrupting one of the interferometer arms, which opens the door to the combination of high-sensitivity lock-in detection with (sub-)attosecond-precision relative delay determination.

Short-wavelength free-electron lasers are now well established as essential and unrivalled sources of ultrabright coherent X-ray radiation. One of the key characteristics of these intense X-ray pulses is their expected few-femtosecond duration. No measurement has succeeded so far in directly determining the temporal structure or even the duration of these ultrashort pulses in the few-femtosecond range. Here, by deploying the so-called streaking spectroscopy technique at the Linac Coherent Light Source, we demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. This method is independent of photon energy, decoupled from machine parameters, and provides an upper bound on the X-ray pulse duration. We measured the duration of the shortest X-ray pulses currently available to be on average no longer than 4.4 fs. Analysing the pulse substructure indicates a small percentage of the free-electron laser pulses consisting of individual high-intensity spikes to be on the order of hundreds of attoseconds. Using a spectroscopy streaking technique at LCLS (Linac Coherent Light Source), researchers demonstrate temporal characterization of X-ray pulses with sub-femtosecond resolution.

Highly nonlinear, but reversible changes in the refractive index of SiO 2 induced by strong near-infrared light fields are probed with wave-cycle resolved optical reflectivity and attosecond streaking characterization of the transmitted field.

This work reports on the attosecond real-time observation of the electron processes [1] underlying the ability of ultrastrong few-cycle laser pulses to turn a dielectric solid from an insulating into a conducting state [2].

The ultrafast reversibility of changes to the electronic structure and electric polarizability of a dielectric with the electric field of a laser pulse, demonstrated here, offers the potential for petahertz-bandwidth optical signal manipulation. Two studies published in this issue highlight the potential for ultrafast signal manipulation in dielectrics using optical fields. When it comes to electrical signal processing, semiconductors have become the materials of choice. However, insulators such as dielectrics could be attractive alternatives: they have a fast response in principle, but usually have extremely low conductivity at low electric fields and break down in large fields. The electronic properties of dielectrics can be controlled with few-cycle laser pulses that permit damage-free exposure of dielectrics to high electric fields. Agustin Schiffrin et al. demonstrate that strong optical laser fields with controlled few-cycle waveforms can reversibly transform a dielectric insulator into a conductor within the optical period (within one femtosecond). Martin Schultze et al. address the crucial issue of ultrafast reversibility, demonstrating that the dielectric can be repeatedly switched 'on' and 'off' with light fields, without degradation. The control of the electric and optical properties of semiconductors with microwave fields forms the basis of modern electronics, information processing and optical communications. The extension of such control to optical frequencies calls for wideband materials such as dielectrics, which require strong electric fields to alter their physical properties1,2,3,4,5. Few-cycle laser pulses permit damage-free exposure of dielectrics to electric fields of several volts per angstrom6 and significant modifications in their electronic system6,7,8,9,10,11,12,13. Fields of such strength and temporal confinement can turn a dielectric from an insulating state to a conducting state within the optical period14. However, to extend electric signal control and processing to light frequencies depends on the feasibility of reversing these effects approximately as fast as they can be induced. Here we study the underlying electron processes with sub-femtosecond solid-state spectroscopy, which reveals the feasibility of manipulating the electronic structure and electric polarizability of a dielectric reversibly with the electric field of light. We irradiate a dielectric (fused silica) with a waveform-controlled near-infrared few-cycle light field of several volts per angstrom and probe changes in extreme-ultraviolet absorptivity and near-infrared reflectivity on a timescale of approximately a hundred attoseconds to a few femtoseconds. The field-induced changes follow, in a highly nonlinear fashion, the turn-on and turn-off behaviour of the driving field, in agreement with the predictions of a quantum mechanical model. The ultrafast reversibility of the effects implies that the physical properties of a dielectric can be controlled with the electric field of light, offering the potential for petahertz-bandwidth signal manipulation.

We demonstrate the generation of waveform-controlled laser pulses with 1 mJ pulse energy and a full-width-half-maximum duration of ∼4 fs, therefore lasting less than two cycles of the electric field oscillating at their carrier frequency. The laser source is carrier-envelope-phase stabilized and used as the backbone of a kHz repetition rate source of high-harmonic continua with unprecedented flux at photon energies between 100 and 200 eV (corresponding to a wavelength range between 12–6 nm respectively). In combination we use these tools for the complete temporal characterization of the laser pulses via attosecond streaking spectroscopy.

We demonstrate generation and measurement of intense deep-ultraviolet light pulses with a duration of approximately 2.8 fs (FWHM of the intensity envelope) and a wavelength distribution between 230 and 290 nm. They emerge via direct frequency upconversion of sub-4 fs laser pulses of a carrier wavelength of approximately 750 nm focused into an Ne-filled, quasi-static gas cell. Dispersion-free, third-order autocorrelation measurements provide access to their temporal intensity profile.

We demonstrate generation of coherent microjoule-scale, low-order harmonic supercontinua in the deep and vacuum ultraviolet (4-9 eV), resulting from the nonlinear transformations of near-single-cycle laser pulses in a gas cell. We show theoretically that their formation is connected to a novel nonlinear regime, holding promise for the generation of powerful deep-UV and vacuum ultraviolet subfemtosecond pulses. Our work opens the route to pump-probe spectroscopy of subfemtosecond-scale valence-shell phenomena in atoms, molecules, and condensed matter.

In recent years, the development of high-power, near-infrared (NIR) femtosecond lasers has enabled a new class of broadband mid-infrared (MIR) radiation sources that combine the spectral coverage of the molecular fingerprint region, a brilliance exceeding even that of 3rd-generation synchrotrons, and outstanding temporal coherence [1–3]. One of the challenges that arises in the development of these sources is the need to separate a tens-of-Watts NIR beam from the broadband MIR beam arising out of a parametric process such as intra-pulse difference frequency generation (IPDFG) in a suitable nonlinear crystal. Conventional coated optics face the issues of adding significant amount of dispersion to a temporally well-compressed pulse and the linear and nonlinear absorption restrict the usable NIR power in many MIR-transparent materials, such as germanium.

Broadband mid-infrared spectroscopy of biofluids carries great potential for biological and biomedical applications, as it provides fast, reliable and label-free access to the molecular composition of the sample [1]. When applied to human blood serum, a range of molecular contents can be quantified [2] and specific changes in the absorption spectra, driven by diseases (e.g. cancer) can be identified and used for diagnostic purposes [3]. One remaining challenge is the complexity of human serum: physiological phenotypes are driven by minor changes in concentration of thousands of different molecules. At the same time, although many low-abundance molecules are very informative for disease detection, these are often not detectable with conventional Fourier-Transform Infrared (FTIR) spectroscopy and quantum-cascade laser (QCL) based approaches due to a lack of sensitivity and specificity.

Vibrational spectroscopy in the mid-infrared (MIR) molecular fingerprint region (∼2–25 μm) provides information on the molecular composition, structure and conformation, affording tremendous potential for advances in fields ranging from fundamental research over security and environmental applications to biology and medicine. Traditionally, broadband measurements in this spectral region are carried out in the frequency domain. For each spectral element, the absorption of a sample is determined from the attenuation of the source intensity when placing the sample in the beam path. This brings about two severe limitations for the detection of small absorptions (and of small absorption differences). First, intensity noise of the source directly affects the ability of the spectrometer to detect intensity changes induced by small changes in the absorption. Second, the detection dynamic range necessary to simultaneously resolve the full power of the source and small absorption changes restricts power scaling and ultimately limits the smallest detectable change induced by an absorption.

Attosecond polarization spectroscopy is a new experimental technique resolving the nonlinear polarization and energy-transfer induced by visible few-cycle strong fields in solids. It reveals the intensity dependent response time of the system with attosecond resolution.

Deploying the so-called ‘Streaking Spectroscopy’ technique at LCLS, we demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. Analyzing the substructure indicates pulse durations on the order of hundreds of attoseconds.

Mid-infrared (MIR) vibrational spectroscopy is a versatile tool for identifying organic compounds (OC) in gases or liquids in a label-free manner due to their unique molecular fingerprint, promoting a large variety of applications in biology, chemistry, medicine or environmental monitoring. Although new technologies like quantum-cascade lasers are rapidly evolving, the workhorse in this field is still Fourier-transform infrared (FTIR) spectrometry based on thermal sources. Without pre-treatment of the sample and/or enrichment of the OC of interest, those techniques offer a limit of detection (LOD) for compounds in aqueous environments at ppm levels [1]. Pushing this limit to ppb levels is commonly hindered by the lack of broadband, stable and bright MIR sources as well as of low-noise, high-dynamic-range MIR detectors.