Nikolai Lilienfein

Megawatt-scale average-power ultrashort pulses in an enhancement cavity

We investigate power scaling of ultrashort-pulse enhancement cavities. We propose a model for the sensitivity of a cavity design to thermal deformations of the mirrors due to the high circulating powers. Using this model and optimized cavity mirrors, we demonstrate 400 kW of average power with 250 fs pulses and 670 kW with 10 ps pulses at a central wavelength of 1040 nm and a repetition rate of 250 MHz. These results represent an average power improvement of one order of magnitude compared to state-of-the-art systems with similar pulse durations and will thus benefit numerous applications such as the further scaling of tabletop sources of hard x rays (via Thomson scattering of relativistic electrons) and of soft x rays (via high harmonic generation).

Enhancement cavities for zero-offset-frequency pulse trains

The optimal enhancement of broadband optical pulses in a passive resonator requires a seeding pulse train with a specific carrier-envelope-offset frequency. Here, we control the phase of the cavity mirrors to tune the offset frequency for which a given comb is optimally enhanced. This enables the enhancement of a zero-offset-frequency train of sub-30-fs pulses to multi-kW average powers. The combination of pulse duration, power, and zero phase slip constitutes a crucial step toward the generation of attosecond pulses at multi-10-MHz repetition rates. In addition, this control affords the enhancement of pulses generated by difference-frequency mixing, e.g., for mid-infrared spectroscopy.

Balancing of thermal lenses in enhancement cavities with transmissive elements

Thermal lensing poses a serious challenge for the power scaling of enhancement cavities, in particular when these contain transmissive elements. We demonstrate the compensation of the lensing induced by thermal deformations of the cavity mirrors with the thermal lensing in a thin Brewster plate. Using forced convection to fine-tune the lensing in the plate, we achieve average powers of up to 160 kW for 250-MHz-repetition-rate picosecond pulses with a power-independent mode size. Furthermore, we show that the susceptibility of the cavity mode size to thermal lensing allows highly sensitive absorption measurements.

Towards the generation of isolated attosecond pulses with femtosecond enhancement cavities

The generation of extreme-ultraviolet (XUV) isolated attosecond pulses (lAPs) by the process of high harmonic generation (HHG) has enabled the investigation of electronic dynamics in atoms, molecules and solids. Due to limitations in laser technology, the currently available repetition rate of such sources is well below 1 MHz. IAPs at higher repetition rates promise, amongst others, to advance experiments involving the detection of charged particles, where the number of photons per shot is limited by space charge effects. One particularly prominent application is the spatially and temporally resolved investigation of nanoplasmonic fields [1].

Ultrafast optomechanical pulse picking

State-of-the-art optical switches for coupling pulses into and/or out of resonators are based on either the electro-optic or the acousto-optic effect in transmissive elements. In high-power applications, the damage threshold and other nonlinear and thermal effects in these elements impede further improvements in pulse energy, duration, and average power. We propose a new optomechanical switching concept which is based solely on reflective elements and is suitable for switching times down to the ten-nanosecond range. To this end, an isolated section of a beam path is moved in a system comprising mirrors rotating at a high angular velocity and stationary imaging mirrors, without affecting the propagation of the beam thereafter. We discuss three variants of the concept and exemplify practical parameters for its application in regenerative amplifiers and stack-and-dump enhancement cavities. We find that optomechanical pulse picking has the potential to achieve switching rates of up to a few tens of kilohertz while supporting pulse energies of up to several joules.

Cavity-enhanced high-harmonic generation at 250 MHz

We demonstrate a power improvement of two orders of magnitude to the nW-level for multi-MHz-repetition-rate high-harmonic generation in the 100-eV range, driven in an enhancement cavity by 30-fs, 10-kW average-power pulses at 250 MHz.

Repetition-rate scaling of high harmonic generation in gases

With the development of lasers emitting pulses with ever decreasing durations and increasing energies, the importance of high-harmonic generation (HHG) in gases as a tool for precision measurements in the extreme ultraviolet (XUV) has been steadily increasing. Of particular importance is the question of the repetition rate scalability of such sources: on the one hand, applications like XUV frequency comb spectroscopy and attosecond nanoscopy would tremendously profit from efficient HHG at > 100 MHz. On the other hand, at these rates, the HHG interaction volume can neither be replenished with neutral atoms, nor can ionized atoms recombine in the ground state. Fundamental understanding of cumulative effects in HHG is crucial for MHz-HHG applications.

Design, Production and Characterization of Mirrors for Ultra-Broadband, High-Finesse Enhancement Cavities

To enable the enhancement of few-cycle pulses in high-finesse passive optical resonators, a novel complementary-phase approach is considered for the resonator mirrors. The design challenges and first experimental results are presented.

High-Reflectivity Mirrors for Tailoring Carrier-Envelope Phase Properties

A new class of multilayer synthesis problems is addressed. We consider changes of the electric field phase with respect to the pulse envelope after a reflection from a mirror and solve related multilayer optimization problems.