Light-wave-controlled Haldane model in monolayer hexagonal boron nitride.

In recent years, the stacking and twisting of atom-thin structures with matching crystal symmetry has provided a unique way to create new superlattice structures in which new properties emerge1,2. In parallel, control over the temporal characteristics of strong light fields has allowed researchers to manipulate coherent electron transport in such atom-thin structures on sublaser-cycle timescales3,4. Here we demonstrate a tailored light-wave-driven analogue to twisted layer stacking. Tailoring the spatial symmetry of the light waveform to that of the lattice of a hexagonal boron nitride monolayer and then twisting this waveform result in optical control of time-reversal symmetry breaking5 and the realization of the topological Haldane model6 in a laser-dressed two-dimensional insulating crystal. Further, the parameters of the effective Haldane-type Hamiltonian can be controlled by rotating the light waveform, thus enabling ultrafast switching between band structure configurations and allowing unprecedented control over the magnitude, location and curvature of the bandgap. This results in an asymmetric population between complementary quantum valleys that leads to a measurable valley Hall current7, which can be detected by optical harmonic polarimetry. The universality and robustness of our scheme paves the way to valley-selective bandgap engineering on the fly and unlocks the possibility of creating few-femtosecond switches with quantum degrees of freedom.

Ultra-phase-stable infrared light source at the watt level.

Ultrashort pulses at infrared wavelengths are advantageous when studying light-matter interaction. For the spectral region around 2 µm, multi-stage parametric amplification is the most common method to reach higher pulse energies. Yet it has been a key challenge for such systems to deliver waveform-stable pulses without active stabilization and synchronization systems. Here, we present a different approach for the generation of infrared pulses centered at 1.8 µm with watt-level average power utilizing only a single nonlinear crystal. Our laser system relies on a well-established Yb:YAG thin-disk technology at 1.03 µm wavelength combined with a hybrid two-stage broadening scheme. We show the high-power downconversion process via intra-pulse difference frequency generation, which leads to excellent passive stability of the carrier envelope phase below 20 mrad-comparable to modern oscillators. It also provides simple control over the central wavelength within a broad spectral range. The developed infrared source is employed to generate a multi-octave continuum from 500 nm to 2.5 µm opening the path toward sub-cycle pulse synthesis with extreme waveform stability.

Multi-octave, CEP-stable source for high-energy field synthesis.

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.

Cavity-enhanced noncollinear high-harmonic generation.

Femtosecond enhancement cavities have enabled multi-10-MHz-repetition-rate coherent extreme ultraviolet (XUV) sources with photon energies exceeding 100 eV - albeit with rather severe limitations of the net conversion efficiency and of the duration of the XUV emission. Here, we explore the possibility of circumventing both these limitations by harnessing spatiotemporal couplings in the driving field, similar to the "attosecond lighthouse," in theory and experiment. Our results predict dramatically improved output coupling efficiencies and efficient generation of isolated XUV attosecond pulses.

Large-mode enhancement cavities.

In passive enhancement cavities the achievable power level is limited by mirror damage. Here, we address the design of robust optical resonators with large spot sizes on all mirrors, a measure that promises to mitigate this limitation by decreasing both the intensity and the thermal gradient on the mirror surfaces. We introduce a misalignment sensitivity metric to evaluate the robustness of resonator designs. We identify the standard bow-tie resonator operated close to the inner stability edge as the most robust large-mode cavity and implement this cavity with two spherical mirrors with 600 mm radius of curvature, two plane mirrors and a round trip length of 1.2 m, demonstrating a stable power enhancement of near-infrared laser light by a factor of 2000. Beam radii of 5.7 mm × 2.6 mm (sagittal × tangential 1/e(2) intensity radius) on all mirrors are obtained. We propose a simple all-reflective ellipticity compensation scheme. This will enable a significant increase of the attainable power and intensity levels in enhancement cavities.

Carrier-envelope-phase-stable, 1.2 mJ, 1.5 cycle laser pulses at 2.1 µm.

We produce 1.5 cycle (10.5 fs), 1.2 mJ, 3 kHz carrier-envelope-phase-stable pulses at 2.1 µm carrier wavelength, from a three-stage optical parametric chirped-pulse amplifier system, pumped by an optically synchronized 1.6 ps Yb:YAG thin disk laser. A chirped periodically poled lithium niobate crystal is used to generate the ultrabroad spectrum needed for a 1.5 cycle pulse through difference frequency mixing of spectrally broadened pulse from a Ti:sapphire amplifier. It will be an ideal tool for producing isolated attosecond pulses with high photon energies.

Recent development and new ideas in the field of dispersive multilayer optics.

A dispersive-mirror-based laser permits a dramatic simplification of high-power femtosecond and attosecond systems and affords promise for their further development toward shorter pulse durations, higher peak powers, and higher average powers with user-friendly systems. The result of the continuous development of dispersive mirrors permits pulse compression down to almost single cycle pulses of 3 fs duration. These design approaches together with the existing modern deposition technology pave the way for the manufacture of dielectric multilayer coatings able to compress pulses of tens of picoseconds duration down to a few femtoseconds.

Dispersion management in femtosecond laser oscillators with highly dispersive mirrors.

Recently the manufacture of highly dispersive mirrors with -1300 fs(2) group delay dispersion per reflection was reported. Here we demonstrate the intracavity applicability of these novel mirrors in Ti:sapphire oscillators for the first time, as well as their capability of compensating a substantial amount of material dispersion in the cavity (40 mm fused silica). We also studied the influence of net negative cavity dispersion, realized with these mirrors, on the achievable maximum pulse energy in long-cavity femtosecond oscillators before the onset of anomalous behavior (e.g. multi-pulsing). In addition, we demonstrate a 0.5 GHz Ti:sapphire oscillator the dispersion compensation of which is realized with a single highly dispersive mirror.

Generation of carrier-envelope-phase-stable 2-cycle 740-microJ pulses at 2.1-microm carrier wavelength.

We produce carrier-envelope-phase-stable 15.7-fs (2-cycle) 740-microJ pulses at the 2.1-microm carrier wavelength, from a three-stage optical parametric chirped-pulse amplifier system, pumped by an optically synchronized 49-ps 11-mJ Nd:YLF laser. A novel seed pulse spectral shaping method is used to ascertain the true amplified seed energy and the parametric superfluorescence levels.