P4 Ultrafast tailored light

The ambition of P4 is to explore the parameter space that lies between conventional femtosecond frequency combs obtained from continuous wave (CW) mode-locked lasers, on the one side, and high-energy pulsed laser amplifiers, on the opposite side.

Whereas such fs frequency combs with an inter-mode spacing fr in the MHz-GHz offer outstanding spectral resolution, the pulse intensity and the peak spectral brightness are moderate because of the high repetition rate fr. Consequently, excitation of low-probability optical transitions, driving of multiphoton processes and application of nonlinear-optical spectroscopies is difficult because of the necessity to boost the source output power, which requires optical amplification at the full repetition rate fr of the master oscillator.

Tailoring the time- and frequency-domain formats of pulse amplification at moderate kHz repetition rates, pursued in this SFB project part P4, enables us to drastically improve the brightness of spectral modes and the peak power without raising the average power of the laser source to excessive levels.

One-photon and two-photon excitation schemes of the 8.4-eV (148.7-nm) VUV transition in the 229Th nucleus, pursued in P5, pose dramatic challenges for any established type of laser technology— single-frequency CW, long-pulse narrowband, fs frequency comb, or ultrashort-pulse broadband— because of the extremely narrow transition linewidth and an exceptionally low absorption cross-section.

Based on the long-standing experience of the team of P4 in the development of diode-pumped high-energy fs Yb amplifiers, parametric frequency conversion in crystals and gases, and introducing novel techniques for linear- and nonlinear-optical spectral reshaping, in the project part P4 we focus on three laser development tasks that correspond to three nuclear excitation schemes pursued in P5:

  1. The team will build a 148.7-nm VUV source, corresponding to the 7th harmonic of the fundamental laser frequency, with the effective linewidth below 0.2 GHz.
  2. The team will produce energetic 297-nm broadband femtosecond pulses for a symmetric two-photon excitation of the 229Th nucleus by frequency quadrupling a Raman-shifted Yb amplifier pulse.
  3. The team will extend the optical frequency conversion chain to realize two broadly tunable outputs, with 210—297 nm and 297—500 nm wavelength tuning ranges, respectively, to enable an asymmetric two-photon excitation of the 8.4 eV transition via an intermediate energy level engineered, by a joint effort between experiment P5 and theory P6, in the crystalline material hosting 229Th ions

Currently, there are several ongoing efforts to realize high-brightness VUV sources around 148.7 nm, reviewed in detail in the project part description of P5, 229Th nuclear spectroscopy, for direct laser excitation of the nuclear isomer state. To date, none has achieved a confirmed result.

The structure and the sequence of the laser technology objectives in P4 has a one-to-one correspondence to the spectroscopy objectives in P5, i.e., each of the objectives in P4 provides a laser source matching the designated nuclear excitation process studied by the P5 partner. The three source development objectives are grouped in two methodologies and are pursued around two different amplifiers. Each of the two methodologies also corresponds to one of the two envisaged PhD projects.

Team of P4

The sub-project P4 “Ultrafast tailored light” is led by Andrius Baltuska at the Photonics Institute at TU Wien. The ultrafast laser group that he is leading has extensive experience (about 20 years) in the development of ultrafast high-peak power and high-energy laser systems and application of them to ultrafast spectroscopy. Currently there are about 10 laser systems (mostly Yb-based, but also Ti:Sa) running or under development at the ultrafast laser labs.

The existing personnel in the ultrafast laser group support the SFB with the following tasks:

  • Audrius Pugzlys (Senior staff scientist): Specialist in laser development and nonlinear frequency conversion, will in this project be supervising – together with the PI – the laser development and frequency conversion
  • Paolo Carpeggiani (Senior staff scientist): Specialist in ultrafast and XUV spectroscopy, currently working on high-harmonic generation using frequency shifted and compressed Yb-pulses, will contribute to this project with supervision of the work on frequency shifting the 1030nm-pulses using stimulated Raman scattering in hollow-core fibers
  • Markus Zeiler (Senior staff scientist): Specialist in experimental ultrafast laser-matter interaction, currently working on THz-spectroscopy with an Yb-burst-laser, will contribute to the present project with supervision of the four-wave mixing process and the Th-spectroscopy experiments
  • Fabian Dona (Technical staff): Mechanical engineer, will contribute with drawing and manufacturing the laser head for the Yb:CALGO laser to be built during this project

 

The key expertise that is brought in by project P4 into the SFB consortium of COMB.AT is the development and construction of highly specialized amplified laser systems and their use for ultrashort pulse generation in various spectral ranges from the mid-IR to the VUV. Two PhD students have joined the existing team to create the desired two different types of CW-pumped Yb amplifiers.