Nonlinear absorption in lithium triborate frequency converters for high-power ultrafast lasers.

We report on an analysis of the nonlinear absorption in lithium triborate (LBO) used for second and third harmonic generation of ultrashort laser pulses at average powers in the order of kW and with sub-picosecond pulse duration. Thermographic imaging of the LBO crystals together with a simple analytical model revealed the presence of nonlinear absorption in both harmonic generation processes. Subsequent processing with a numerical model considering the nonlinear mixing, the absorption, and the heat conduction was used to estimate the absorption coefficients. Average powers exceeding 100 W in the ultraviolet and 400 W in the visible spectral range were obtained while maintaining a good beam quality by avoiding excessive nonlinear absorption.

Ultrafast green thin-disk laser exceeding 1.4 kW of average power.

We present an ultrafast laser with a near-diffraction-limited beam quality delivering more than 1.4 kW of average power in the visible spectral range. The laser is based on second harmonic generation in a lithium triborate crystal of a Yb:YAG thin-disk multipass amplifier emitting more than 2 kW of average power in the infrared.

Highly-efficient continuous-wave intra-cavity frequency-doubled Yb:LuAG thin-disk laser with 1 kW of output power.

We report on the generation of continuous-wave, intra-cavity frequency-doubled, multi-mode laser radiation in an Yb:LuAG thin-disk laser. Output powers of up to 1 kW at a wavelength of 515 nm were achieved at an unprecedented optical efficiency of 51.6% with respect to the pumping power of the thin-disk laser. The wavelength stabilization and spectral narrowing as well as the polarization selection, which is necessary for a stable and efficient second-harmonic generation, was achieved by the integration of a diffraction grating into the dielectric end mirror of the cavity, which exhibits a diffraction efficiency of 99.8%. At a frequency-doubled output power of 820 W the peak-to-valley power fluctuations measured during 100 minutes of laser operation amounted to only 8.2 W (1.0%). The beam parameter product of the frequency-doubled output was 3.4 mm·mrad (M2 ≈ 20), which is suitable for standard beam delivery using fibers with a core diameter of 100 µm and a NA of 0.2.

Mechanical phase matching of birefringent non-linear crystals.

Second-order nonlinear processes such as second harmonic generation or parametric amplification have found numerous applications in the scientific and industrial world, from micromachining to petawatt laser facilities. These nonlinear interactions are mostly carried out in birefringent crystals because of their low cost and the possibility to operate at high powers Phase-matching configurations in birefringent crystals are determined by their refractive indexes. Here, we show that an important mechanical stress can be used to significantly change the phase-matching properties of a birefringent crystal. As an example, we demonstrate the shift of second harmonic non-critical phase matching wavelength of LiB3O5 (LBO) crystal at room temperature from 1200 nm to 1120 nm by applying compressive forces up to 100 MPa. We believe that this mechanical phase matching can be used as an additional degree of freedom to optimize nonlinear optical frequency mixing geometries.

Psychiatric trainees as second victims after exposure to patient suicide: a French qualitative study.

Background: The exposure to patient suicide (PS) has been identified as one of the most frequent and troubling professional experience for psychiatric trainees. Further studies are needed to better understand how residents cope with these experiences and the association between perceived support and the impacts of PS. Method: In this qualitative study, we aimed to assess the impact of exposure to PS during psychiatric residency on trainees' professional career and practical experience. A total of 19 French psychiatric residents participated in 4 focus-groups performed between November 2017 and May 2019. Results: A total of 4 thematic clusters were identified through a five-step content analysis, namely: (a) reactions to the exposure; (b) coping strategies; (c) professional impact; and (d) prevention and postvention proposals. All participants described the critical impact of the support provided after PS, especially by their senior staff. Those who felt supported by their superior reported less negative impact, both in emotional and professional dimensions. Participants also shared proposals to improve the prevention and postvention issues related to the exposure to PS. Conclusion: We performed the first qualitative study based on focus groups on the impact of PS on psychiatric residents, which allowed for an in-depth understanding of the participants' lived experiences of the exposure to PS. The narratives inform the need and means to implement prevention and postvention strategies designed to buffer the negative impact of the exposure to PS in psychiatric trainees.