U. Keller

Self-compression of ultra-short laser pulses down to one optical cycle by filamentation

Theoretical studies of filamentation of ultra-short near-IR laser pulses propagating in a noble gas predict near single-cycle pulses with the intensity being clamped to the field ionization threshold. Experimental results show that this method is carrier envelope offset phase preserving and provides a very simple source for generating few-cycle intense laser pulses. This suggests a very simple design for the generation of ultra-short, sub-femtosecond XUV optical pulses.

Spatio-temporal characterization of few-cycle pulses obtained by filamentation

Intense sub-5-fs pulses were generated by filamentation in a noble gas and subsequent chirped-mirror pulse compression. The transversal spatial dependence of the temporal pulse profile was investigated by spatial selection of parts of the output beam. Selecting the central core of the beam is required for obtaining the shortest possible pulses. Higher energy efficiency is only obtained at the expense of pulse contrast since towards the outer parts of the beam the energy is spread into satellite structures leading to a double-pulse profile on the very off-axis part of the beam. Depending on the requirements for a particular application, a trade-off between the pulse duration and the pulse energy has to be done. The energy of the sub-5-fs pulses produced was sufficient for the generation of high order harmonics in Argon. In addition, full simulation is performed in space and time on pulse propagation through filamentation that explains the double-pulse structure observed as part of a conical emission enhanced by the plasma defocusing.

Femtosecond Yb:Lu(2)O(3) thin disk laser with 63 W of average power.

We present successful power-scaling of an Yb:Lu(2)O(3) thin disk laser to record high-power levels both in cw and mode-locked operation. In a simple multimode resonator we achieved 149 W of output power in cw operation with 73% optical-to-optical efficiency (eta(opt)). Building an 81 MHz fundamental transverse mode resonator with dispersion compensation and a semiconductor saturable absorber mirror (SESAM) for passive mode locking we achieved 63 W of average power in 535 fs pulses (eta(opt)=35%). The output beam is nearly diffraction limited (M(2)<1.2). The 0.78 microJ pulses with a peak power of 1.28 MW had a central wavelength of 1034 nm and were close to the Fourier transform limit. With an SESAM with a larger modulation depth we obtained pulses as short as 329 fs at 40 W average power corresponding to a pulse energy of 0.49 microJ and a peak power of 1.32 MW.

Sub-four-cycle laser pulses directly from a high-repetition-rate optical parametric chirped-pulse amplifier at 3.4 μm.

We generate sub-four-cycle pulses (41.6 fs) with 12 μJ of pulse energy in the mid-infrared spectral range (center wavelength 3.4 μm) from a high-repetition-rate, collinear three-stage optical parametric chirped-pulse amplifier (OPCPA) operating at 50 kHz. Apodized aperiodically poled MgO:LiNbO3 crystals with a negative chirp rate are employed as gain media to achieve ultrabroadband phase-matching while minimizing optical parametric generation. The seed pulses are obtained via a 1.56 μm femtosecond fiber laser, which is spectrally broadened in a dispersion-shifted telecom fiber to support 1000 nm bandwidth idler pulses in the mid-infrared.

Versatile attosecond beamline in a two-foci configuration for simultaneous time-resolved measurements

We present our attoline which is a versatile attosecond beamline at the Ultrafast Laser Physics Group at ETH Zurich for attosecond spectroscopy in a variety of targets. High-harmonic generation (HHG) in noble gases with an infrared (IR) driving field is employed to generate pulses in the extreme ultraviolet (XUV) spectral regime for XUV-IR cross-correlation measurements. The IR pulse driving the HHG and the pulse involved in the measurements are used in a non-collinear set-up that gives independent access to the different beams. Single attosecond pulses are generated with the polarization gating technique and temporally characterized with attosecond streaking. This attoline contains two target chambers that can be operated simultaneously. A toroidal mirror relay-images the focus from the first chamber into the second one. In the first interaction region a dedicated double-target allows for a simple change between photoelectron/photoion measurements with a time-of-flight spectrometer and transient absorption experiments. Any end station can occupy the second interaction chamber. A surface analysis chamber containing a hemispherical electron analyzer was employed to demonstrate successful operation. Simultaneous RABBITT measurements in two argon jets were recorded for this purpose.

SESAM modelocked Yb:CaGdAlO 4 laser in the soliton modelocking regime with positive intracavity dispersion

We demonstrate femtosecond SESAM modelocking in the near-infrared by using cascaded quadratic nonlinearities (phase-mismatched second-harmonic generation, SHG), enabling soliton modelocking in the normal dispersion regime without any dispersion compensating elements. To obtain large and negative self-phase modulation (SPM) we use an intracavity LBO crystal, whose temperature and angles are optimized with respect to SPM, nonlinear losses, and self-starting characteristics. To support femtosecond pulses, we use the very promising Yb:CaGdAlO(4) (CALGO) gain material, operated in a bulk configuration. The LBO crystal provides sufficient negative SPM to compensate for its own GDD as well as the positive GDD and SPM from the gain crystal. The modelocked laser produces pulses of 114 fs at 1050 nm, with a repetition rate of 113 MHz (average output power 1.1 W). We perform a detailed theoretical study of this soliton modelocking regime with positive GDD, which clearly indicates the important design constraints in an intuitive and systematic way. In particular, due to its importance in avoiding multi-pulsed modelocking, we examine the nonlinear loss associated with the cascading process carefully and show how it can be suppressed in practice. With this modelocking regime, it should be possible to overcome the limits faced by current state of the art modelocked lasers in terms of dispersion compensation and nonlinearity management at high powers, suppression of Q-switching in compact GHz lasers, and enabling femtosecond soliton modelocking at very high repetition rates due to the high nonlinearities accessible via cascading combined with eliminating the need for intracavity dispersion compensation.

Light-Matter Interaction at Surfaces in the Spatiotemporal Limit of Macroscopic Models.

What is the spatiotemporal limit of a macroscopic model that describes the optoelectronic interaction at the interface between different media? This fundamental question has become relevant for time-dependent photoemission from solid surfaces using probes that resolve attosecond electron dynamics on an atomic length scale. We address this fundamental question by investigating how ultrafast electron screening affects the infrared field distribution for a noble metal such as Cu(111) at the solid-vacuum interface. Attosecond photoemission delay measurements performed at different angles of incidence of the light allow us to study the detailed spatiotemporal dependence of the electromagnetic field distribution. Surprisingly, comparison with Monte Carlo semiclassical calculations reveals that the macroscopic Fresnel equations still properly describe the observed phase of the IR field on the Cu(111) surface on an atomic length and an attosecond time scale.

227-fs pulses from a mode-locked Yb:LuScO 3 thin disk laser

The highest average output power and pulse energy from mode-locked laser oscillators are currently achieved using semiconductor saturable absorber mirror (SESAM) [1] mode-locked thin disk lasers (TDL) [2]. Up to now the shortest pulses of 240 fs at 22 W average output power in this configuration were demonstrated with the monoclinic Yb:KYW [3], which suffers from strong anisotropy in its thermal properties, which prevents further power scaling. Using the cubic sesquioxide Yb:Lu 2 O 3 as gain medium, pulses as short as 370 fs could be obtained with a pump power limited output power of 20.5 W at 1034 nm [4]. The new mixed sesquioxide Yb:LuScO 3 [5] provides a nearly doubled gain bandwidth compared to Yb:Lu 2 O 3 of more than 20 nm and supports the generation of significantly shorter pulses.

Passively mode-locked thin disk lasers reach 10 microjoules pulse energy at megahertz repetition rate and drive high field physics experiments

The direct generation of high energy pulses with a diode-pumped solid-state laser oscillator is a promising approach to enable the multi-megahertz operation of numerous applications in science and industry. Thin disk lasers enable the required average power with excellent beam quality, such that passive mode locking with a semiconductor saturable absorber mirror (SESAM) can be achieved. Using this concept, the authors have recently been able to reach a pulse energy as high as 5.1 muJ at 12 MHz repetition rate from an Yb:YAG thin disk oscillator. This significant increase over previous results was made possible by realizing the importance of the nonlinearity of air inside a thin disk laser cavity. To eliminate the airs contribution to the nonlinearity, the laser was covered with a box which then was flooded with helium. Here a further increase in pulse energy to 11 muJ is presented. The existing 40.7 MHz laser cavity presented was extended to a total of 37 m, by inserting a 23.4 m long multiple-pass cavity (MPC) and a simple 4f extension using curved mirrors with 5000 mm radius of curvature. This resulted in a repetition rate of 4 MHz, at which we achieved an average power of 45 W when operated in the box flooded with helium. The sech2-shaped pulses had a FWHM-duration of 791 fs and a spectral bandwidth of 1.56 nm, resulting in a time-bandwidth product of 0.35 (Fourier-limit: 0.315). The beam quality was nearly diffraction-limited with an M2-value of 1.1 (measured at 9.4 muJ).

Frequency comb generation with 50-GHz channel spacing in the telecom C-band

We present a frequency comb with a spectral width of 43 nm at -20 dB. This comb was generated by spectrally broadening the output of an amplified 50-GHz Er:Yb:glass laser with a highly nonlinear fiber.