Polarization-state-resolved high-harmonic spectroscopy of solids.

Attosecond metrology sensitive to sub-optical-cycle electronic and structural dynamics is opening up new avenues for ultrafast spectroscopy of condensed matter. Using intense lightwaves to precisely control the fast carrier dynamics in crystals holds great promise for next-generation petahertz electronics and devices. The carrier dynamics can produce high-order harmonics of the driving field extending up into the extreme-ultraviolet region. Here, we introduce polarization-state-resolved high-harmonic spectroscopy of solids, which provides deeper insights into both electronic and structural sub-cycle dynamics. Performing high-harmonic generation measurements from silicon and quartz, we demonstrate that the polarization states of the harmonics are not only determined by crystal symmetries, but can be dynamically controlled, as a consequence of the intertwined interband and intraband electronic dynamics. We exploit this symmetry-dynamics duality to efficiently generate coherent circularly polarized harmonics from elliptically polarized pulses. Our experimental results are supported by ab-initio simulations, providing evidence for the microscopic origin of the phenomenon.

Cascaded interactions mediated by terahertz radiation.

We investigate a regime of parametric amplification in which the pump and signal waves are spectrally separated by only a few hundreds of GHz frequency - therefore resulting in a sub-THz frequency idler wave. Operating in this regime we find an optical parametric amplifier (OPA) behavior which is highly dissimilar to conventional OPAs. In this regime, we observe multiple three-wave mixing processes occurring simultaneously which results in spectral cascading around the pump and signal wave. Via numerical simulations, we elucidate the processes at work and show that cascaded optical parametric amplification offers a pathway toward THz-wave generation beyond the Manly-Rowe limit and toward the generation of high-energy, sparse frequency-combs.

AXSIS: Exploring the frontiers in attosecond X-ray science, imaging and spectroscopy.

X-ray crystallography is one of the main methods to determine atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and unravelling the structure and dynamics of the most important reactions in chemistry and biology is essential to uncover their mechanism. Many of these reactions, including photosynthesis which drives our biosphere, are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates. In the AXSIS project at CFEL in Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes. For that purpose, we develop a compact, fully coherent, THz-driven atto-second X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer. A multidisciplinary team formed by laser-, accelerator,- X-ray scientists as well as spectroscopists and biochemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.

Sagnac interferometric multipass loop amplifier.

We propose and investigate experimentally an interferometrically stable, polarization-selective pulse multiplexing scheme for direct laser amplification of picosecond pulses. The basic building block of this scheme is a Sagnac loop which allows for a straightforward scaling of the pulse-multiplexing scheme. Switching the amplifier from single-pulse amplification to burst mode increases extraction efficiency, reduces parasitic non-linearities in the gain medium and allows for higher output energies. Time-frequency analysis of the amplified output pulses demonstrates the viability of this approach.

Highly stable ultrabroadband mid-IR optical parametric chirped-pulse amplifier optimized for superfluorescence suppression.

We present a 9 GW peak power, three-cycle, 2.2 microm optical parametric chirped-pulse amplification source with 1.5% rms energy and 150 mrad carrier envelope phase fluctuations. These characteristics, in addition to excellent beam, wavefront, and pulse quality, make the source suitable for long-wavelength-driven high-harmonic generation. High stability is achieved by careful optimization of superfluorescence suppression, enabling energy scaling.

Scalable Yb-MOPA-driven carrier-envelope phase-stable few-cycle parametric amplifier at 1.5 microm.

Carrier-envelope phase-stable 4 microJ pulses at approximately 1.5 microm are obtained from a femtosecond Yb:KGW-MOPA-pumped two-stage optical parametric amplifier. This novel technology represents a highly attractive alternative to traditional Ti:sapphire front-ends for seeding multimillijoule-level optical parametric chirped-pulse amplifiers. For this task, we demonstrate stretching of the OPA output to approximately 40 ps and recompression to 33 fs pulse duration. As a stand-alone system, our tunable two-stage OPA might find numerous applications in time-resolved spectroscopy and micromachining.

Experimental implementation of optical clockwork without carrier-envelope phase control.

We demonstrate optical clockwork without the need for carrier-envelope phase control by use of sum-frequency generation between a continuous-wave optical parametric oscillator at 3.39 microm and a femtosecond mode-locked Ti:sapphire laser with two strong spectral peaks at 834 and 670 nm, a spectral difference matched by the 3.39-microm radiation.

Light-induced gaps in semiconductor band-to-band transitions.

We observe a triplet around the third harmonic of the semiconductor band gap when exciting 50-100 nm thin GaAs films with 5 fs pulses at 3 x 10(12) W/cm(2). The comparison with solutions of the semiconductor Bloch equations allows us to interpret the observed peak structure as being due to a two-band Mollow triplet. This triplet in the optical spectrum is a result of light-induced gaps in the band structure, which arise from coherent band mixing. The theory is formulated for full tight-binding bands and uses no rotating-wave approximation.

Evidence for third-harmonic generation in disguise of second-harmonic generation in extreme nonlinear optics.

In contrast with traditional nonlinear optics, a peak at the spectral position of the second harmonic of a laser can also be generated in an inversion-symmetric medium in the regime of extreme nonlinear optics. We describe the underlying mechanism of such third-harmonic generation in disguise of second-harmonic generation and compare theory with the optical as well as the radio-frequency spectra measured in recent experiments on thin ZnO films. The peak at twice the carrier-envelope offset frequency in the radio-frequency spectra is shown to be an unambiguous signature of such a process.

Role of the carrier-envelope offset phase of few-cycle pulses in nonperturbative resonant nonlinear optics.

We study the influence of the carrier-envelope offset phase of few-cycle pulses on nonperturbative resonant extreme nonlinear optics in a semiconductor. If the Rabi frequency becomes comparable to the light frequency, the different Rabi sidebands interfere around twice the laser center frequency, giving rise to a signal which strongly depends on the carrier-envelope offset phase. This signature should be measurable in GaAs samples.

Determining the carrier-envelope offset frequency of 5-fs pulses with extreme nonlinear optics in ZnO.

We excite ZnO samples with two-cycle optical pulses directly from a mode-locked oscillator with average powers of several tens of milliwatts. The emitted light reveals peaks at the carrier-envelope offset frequency f(ø) and at 2f(ø) in the radio-frequency spectra. These peaks can still be detected in layers as thin as 350 nm-a step toward determining the carrier-envelope offset phase itself.

Signatures of carrier-wave Rabi flopping in GaAs.

For excitation of the model semiconductor GaAs with optical pulses which are both extremely short ( 5 fs) and extremely intense ( approximately 10(12) W cm(-2)), we can meet the condition that the Rabi frequency becomes comparable to the band gap frequency-a highly unusual and previously inaccessible situation. Specifically, in this regime, we observe carrier-wave Rabi flopping, a novel effect of nonlinear optics which has been predicted theoretically and which is related to the failure of the area theorem.