Research Overview 2006-2012

Ultrafast Lasers

Pushed pulse energy and average power of ultrafast laser oscillators by four orders of magnitude from typically 1 nJ to >10 µJ and from ≈100 mW to >270 W [1, 2], directly from laser oscillators without additional amplifiers (see Figure). Damage on SESAMs was better understood and improved designs enabled multi-kW intracavity power. We demonstrated carrier envelope offset (CEO) beat signal which opens the path towards intracavity high harmonic generation and frequency comb generation in the VUV to XUV spectral region. We expect to reach energies in the 100 µJ range and average output power of 500 W with better dispersive mirrors.
Pushed pulse repetition rate and frequency combs from diode-pumped solid-state lasers with excellent noise properties. Demonstrated SESAM modelocked diode-pumped Er:Yb:glass lasers operating at a center wavelength of 1.5 µm with up to 100 GHz pulse repetition rates in 2008, demonstrated close to quantum noise limited timing jitter and frequency combs with superior noise performance at around 100 MHz pulse repetition rates. Industrial transfer enabled world-record results in advanced telecom systems. More recently in 2011, femtosecond pulses from Yb-doped solid-state lasers at gigahertz repetition rates generated several watts of average output power which should enable compact and stable gigahertz frequency combs.
First high-power femtosecond VECSELs and inventor of the MIXSEL concept: The next step towards even more compact and less expensive ultrafast lasers can be achieved with ultrafast semiconductor lasers. Fast progress in output power and pulse duration after the first demonstration of passively modelocked external cavity surface emitting lasers (VECSELs) in collaboration with Prof. Tropper in 2000. This progress was recognized with an invited review in Physics Reports in 2006 [3]. A more recent highlight is a femtosecond VECSEL with more than 1 W average output power [4]. First demonstration of a wafer-scale integrated modelocked VECSEL where both the gain and the absorber are integrated into the same wafer was demonstrated in 2007 – referred to as the MIXSEL (Modelocked Integrated eXternal-cavity Surface Emitting Laser). An optically pumped MIXSEL has recently demonstrated picosecond pulses with more than 6 W average power [5]. We demonstrated excellent noise performance of these lasers very similar to ultafast solid-state lasers.
Mid-IR OPCPA: The generation of intense, ultrashort optical pulses in the mid-infrared spectral region still represents a challenge, but is very relevant for attosecond science. In 2009, we demonstrated the first femtosecond optical parametric chirped-pulse amplifier (OPCPA) operating in the mid-infared and being pumped by diode-pumped solid-state lasers directly. Work in progress pushes towards higher pulse energies and shorter pulses using novel aperiodically poled MgO:LiNbO3 (APPLN).

Attoclock

The carrier envelope offset phase (CEP) stabilization enabled Prof. Keller to invent the attoclock [6], which has been used to determine the timing of strong field ionization. It was found that tunneling delay time is effectively zero for helium and argon atoms within the experimental uncertainties of a few tens of attoseconds [6, 7]. A precise measurement of the electron emission angle reveals that the standard model of strong field ionization, where the initial step consists of ionization by tunneling and the second step is Newtonian movement in the laser field, needs to be refined: the larger the atomic polarizability, the more important it becomes to account for the Stark shift of the energy levels and the deformation of the ion potential by the induced dipole [7]. For double ionization of argon we found that the ionization time of the first electron is in good agreement with the model predictions, whereas the ionization of the second electron occurs significantly earlier than predicted [8] and the two electrons exhibit some unexpected correlation.

Attoline

Starting in 2001, we have invested a significant amount of time and resources in developing an “attoline” to generate attosecond pulses in the VUV and XUV using high harmonic generation (HHG) with intense few-cycle pulses in the near infrared. We used a home-built kHz Ti:sapphire laser system with which we demonstrated high energy filament compressors in the one to two optical cycle regime for the first time. With a major opto-mechanical upgrade of our attoline (2006 to 2008, see Figure) and a commercial upgrade of our laser system to support CEP stability (2009 to 2011), we have made sufficient improvements to characterize both attosecond pulse trains and single attosecond pulses (see Figure). Initial scientific highlights include the first experimental observation of the theoretically predicted quantum path interference in HHG [9] and one of the first attosecond transient absorption measurements [10]. Within the NCCR MUST collaboration, the attoline has been recently expanded to accommodate attosecond surface science experiments.
Most recent attosecond transient absorption measurements revealed new insight in virtual dipole transitions. Virtual single-photon transitions can be interrupted and time-gated optical gain and loss of equal magnitude can be observed. The observed emission features are evidence for a new optical gain mechanism, which is of particular interest in the spectral region of our experiment (extreme ultraviolet).