Aktuelle Highlights

1.

A dual-beam 100 kHz OPCPA system has been developed delivering an unprecedented average power at 1.55 μm in 430 μJ, 51 fs, passively CEP-stabilized pulses together with optically synchronized, 125 μJ, 73 fs pulses at 3.1 μm. In contrast to existing few-cycle mid-infrared (i.e., MIR, > 3µm), high repetition rate (i.e., >> 10 kHz) OPCPA systems operating at pulse energies above 100 µJ, our system is based on noncollinear KTA booster amplifiers seeded in the near-infrared at 1.55 µm, and a simple angular dispersion compensation technique [1]. Despite the noncollinear amplifying geometry, KTA can be efficiently used for generating broadband, high-quality MIR pulses at high average power. The resulting OPCPA system is the first ultrafast 100 kHz table-top source delivering two, simultaneously available, optically synchronized infrared beams (i.e., ≥ 1.5 µm) with average powers well above 10 W in each beam and a total average power exceeding 55 W after chirp compensation. Experiments utilizing a reaction microscope have already been started.

Further development will include an upgrade of the 2-branch Yb-fiber pump/seed laser, the implementation of active CEP stabilization, and nonlinear pulse compression of the 1.55 μm beam. The 1.55 μm output of this unique system will serve as the pump of a high-flux soft-X-ray source with a spectrum reaching the water window, while the 3.1 μm beam will provide optically synchronized driver pulses for strong-field interactions.

[1] M. Mero, Z. Heiner, V. Petrov, H. Rottke, F. Branchi, G. M. Thomas, M. J. J. Vrakking, "43 W, 1.55 μm and 12.5 W, 3.1 μm dual-beam, sub-10 cycle, 100 kHz optical parametric chirped pulse amplifier," Opt. Lett. 43, 5246 (2018). [link]

2.

The angular dispersion compensation scheme was first implemented on a small-scale system at the SALSA Photonics Lab at the Humboldt University of Berlin. The infrared optical parametric amplifier (OPA) part of the SALSA system is driven by only 40 μJ pulses at 1.03 μm (i.e., this is the pulse energy measured right at the output of the pump laser) and delivers 7.8 μJ, 38 fs, 1.53 μm and 2.3 μJ, 53 fs, CEP-stable, 3.1 μm pulses at a repetition rate of 100 kHz [2]. One of the remarkable features of this system is the angular-dispersion-compensated 3.1 µm idler beam. Through careful beam and pulse characterization, and high-harmonic generation in YAG (odd orders up to the 9th without much effort), we proved that the corrected idler beam is diffraction-limited, astigmatism-free, and compressible to its transform-limited pulse duration corresponding to only 5 optical cycles. By a direct comparison to our previous SALSA OPA source based entirely on PPLN [3], we also showed that the performance of a noncollinear, KTA-based power amplifier for dual-beam operation at a given broad gain bandwidth is superior to the performance of a collinear, PPLN-based booster stage in terms of conversion efficiency, beam quality, and carrier-envelope phase (CEP) noise. Successful implementation of this simple angular dispersion compensation scheme on the large-scale system at MBI proves its scalability to high average powers.

The OPA source at SALSA is part of the first 100 kHz broadband vibrational sum-frequency generation (BB-VSFG) spectrometer [3]. The early version of the OPA source was based on PPLN amplifier stages and was used to investigate average-power-induced thermal effects in BB-VSFG experiments conducted on molecular layers at an interface between two transparent phases. The paper summarizing the results was Editor's Pick at the Journal of Chemical Physics [4]. Recently, the spectrometer was successfully used to characterize single- and two-component lipid monolayers as a function of surface pressure and mixture ratio [5].

At a pump wavelength of 1 µm, extension of the wavelength range to the MIR range above 5 µm can be achieved by employing novel wide-gap non-oxide crystals. We implemented a small-scale OPA based on LiGaS₂ which was integrated into a BB-VSFG spectrometer. By doing this, we demonstrated (i) the first sub-100 fs, µJ scale pulses in the 7-9 µm range from an LGS OPA pumped at 1 µm, and (ii) the first 100 kHz BB-VSFG measurements in the fingerprint region [6].

[2] Z. Heiner, V. Petrov, G. Steinmeyer, M. J. J. Vrakking, and M. Mero, “100-kHz, dual-beam OPA delivering high-quality, 5-cycle angular-dispersion-compensated mid-infrared idler pulses at 3.1 μm,” Opt. Express 26, 25793 (2018). [link]

[3] Z. Heiner, V. Petrov, and M. Mero, "Compact, high-repetition-rate source for broadband sum-frequency generation spectroscopy," APL Photonics 2, 066102 (2017). [link]

[4] F. Yesudas, M. Mero, J. Kneipp, and Z. Heiner, "Vibrational sum-frequency generation spectroscopy of lipid bilayers at repetition rates up to 100 kHz," J. Chem. Phys. 148, 104702 (2018). [link]

[5] F. Yesudas, M. Mero, J. Kneipp, and Z. Heiner, "High-resolution and high-repetition-rate vibrational sum-frequency generation spectroscopy of one-and two-component phosphatidylcholine monolayers," Anal. Bioanal. Chem. (2019). [link]

[6] Z. Heiner, L. Wang, V. Petrov, and M. Mero, "Broadband vibrational sum-frequency generation spectrometer at 100 kHz in the 950-1750 cm⁻¹ spectral range utilizing a LiGaS₂ optical parametric amplifier," submitted to Optics Express (2019), arXiv:1904.00046 [link]