Free-space quasi-phase matching.

We report a new, to the best of our knowledge, approach to phase matching of nonlinear materials based on the free-space multipass cells. This technique is applicable to noncentrosymmetric nonlinear crystals, including crystals that cannot be birefringent phase-matched or quasi-phase matched by periodic poling. Notably, by using this approach, the crystalline quartz is quasi-phase matched with the demonstrated increase of the second harmonic generation efficiency by a factor of 40. The method can be extended toward UV and THz ranges. This promises to revolutionize experimental nonlinear optics and all applications by increasing the number of available crystals for quasi-phase matching by at least one order of magnitude and brings fresh motivation for developing novel nonlinear materials.

Spectral broadening in convex-concave multipass cells.

Since its first demonstration in 2016, the multi-pass spectral broadening technique has covered impressive ranges of pulse energy (3 µJ - 100 mJ) and peak power (4 MW - 100 GW). Energy scaling of this technique into the joule-level is currently limited by phenomena such as optical damage, gas ionization and spatio-spectral beam inhomogeneity. These limitations can be overcome by the novel multi-pass convex-concave arrangement, which exhibits crucial properties such as large mode size and compactness. In a proof-of-principle experiment, 260 fs, 15 µJ and 200 µJ pulses are broadened and subsequently compressed to approximately 50 fs with 90% efficiency and excellent spatio-spectral homogeneity across the beam profile. We simulate the proposed concept for spectral broadening of 40 mJ and 1.3 ps input pulses and discuss the possibility of further scaling.

Mid-Infrared Laser Generation of Zn1-xMnxSe and Zn1-xMgxSe (x ≈ 0.3) Single Crystals Co-Doped by Cr2+ and Fe2+ Ions-Comparison of Different Excitation Wavelengths.

Two different mid-infrared (mid-IR) solid-state crystalline laser active media of Cr2+, Fe2+:Zn1-xMnxSe and Cr2+, Fe2+:Zn1-xMgxSe with similar amounts of manganese or magnesium ions of x ≈ 0.3 were investigated at cryogenic temperatures for three different excitation wavelengths: Q-switched Er:YLF laser at the wavelength of 1.73 µm, Q-switched Er:YAG laser at 2.94 µm, and the gain-switched Fe:ZnSe laser operated at a liquid nitrogen temperature of 78 K at ∼4.05 µm. The temperature dependence of spectral and laser characteristics was measured. Depending on the excitation wavelength and the selected output coupler, both laser systems were able to generate radiation by Cr2+ or by Fe2+ ions under direct excitation or indirectly by the Cr2+→ Fe2+ energy transfer mechanism. Laser generation of Fe2+ ions in Cr2+, Fe2+:Zn1-xMnxSe and Cr2+, Fe2+:Zn1-xMgxSe (x ≈ 0.3) crystals at the wavelengths of ∼4.4 and ∼4.8 µm at a temperature of 78 K was achieved, respectively. The excitation of Fe2+ ions in both samples by direct 2.94 µm as well as ∼4.05 µm radiation or indirectly via the Cr2+→ Fe2+ ions' energy transfer-based mechanism by 1.73 µm radiation was demonstrated. Based on the obtained results, the possibility of developing novel coherent laser systems in mid-IR regions (∼2.3-2.5 and ∼4.4-4.9 µm) based on AIIBVI matrices was presented.

Long-pulse 4.4-4.6 µm laser oscillations of Fe2+ ions in a Zn1-xMnxSe (x = 0.3) crystal pumped by a 1940 nm Tm fiber laser through Cr2+ → Fe2+ energy transfer.

Millisecond-pulse laser operation of Fe2+ ions at 78 K is demonstrated in the Zn1-xMnxSe:Fe2+,Cr2+ (x=0.3) crystal under a Tm fiber 1940 nm laser pumping through a Cr2+→Fe2+ energy transfer process for the first time, to the best of our knowledge. The laser slope efficiency was 1% with respect to absorbed pumping energy at 1940 nm. The laser central wavelength shift from 4450 nm at 78 K up to 4510 nm at 110 K was observed. Tunability from 4350 up to 4670 nm at 78 K was achieved using an intracavity tuning element.