Advanced Solid-State Lasers: feature issue introduction.

This joint issue of Optics Express and Optical Materials Express features 23 peer-reviewed articles primarily by authors who participated in the Advanced Solid State Lasers conference, which was part of the Optica Laser Congress held in Tacoma, Washington, USA on October 8-12, 2023. This review provides a brief summary of these articles covering the latest developments in laser gain materials and nonlinear crystals, mode-locked lasers, fiber lasers and amplifiers, nonlinear optical sources, and novel concepts in laser design.

Widely tunable near-infrared optical parametric oscillator based on a 5%MgO:PPLN partial cylinder pumped at 1064†nm by a 1-kHz sub-nanosecond microchip laser.

We report the implementation of a singly resonant optical parametric oscillator using a 5%MgO:PPLN partial cylinder pumped by a sub-nanosecond microchip laser emitting 1064 nm at a repetition rate of 1 kHz. It is continuously tunable from 1410 nm up to 4330 nm by rotating the cylinder and a total energy of several microjoules is emitted with a beam quality factor M2 lower than 3.

Second-order nonlinear optical coefficients of the monoclinic crystal BaGa4Se7.

We characterize the magnitude and relative signs of the second-order nonlinear optical coefficients of the biaxial crystal BaGa4Se7 (BGSe) combining measurements of phase-matched and non-phase-matched second-harmonic generation at different wavelengths using slabs and a sphere.

Development and experimental demonstration of negative first-order quasi-phase matching in a periodically poled Rb-doped KTiOPO4 crystal.

We worked on a new scheme of quasi-phase matching (QPM) based on the negative first order of the spatial modulation of the sign of the second-order nonlinearity. Applying this scheme in the case of angular-QPM (AQPM) in a biaxial crystal reveals new directions of propagation for efficient parametric frequency conversion as well as "giant" spectral acceptances. The experimental validation is performed in a periodically poled rubidium-doped KTiOPO4 biaxial crystal. This new approach naturally extends to other periodically poled uniaxial crystals such as periodically poled LiNbO3.

Validation of the angular quasi-phase-matching theory for the biaxial optical class using PPRKTP.

We report the first experimental validation of angular quasi-phase-matching (AQPM) theory in a biaxial crystal by performing second-harmonic generation (SHG) in the periodically-poled Rb-doped KTiOPO4 (PPRKTP) crystal cut as a sphere. Both AQPM and birefringence phase-matching (BPM) angles were measured thanks to a Kappa circle.

Quadratic nonlinear optical properties of the organic N-benzyl-2-methyl-4-nitroaniline (BNA) biaxial crystal.

We performed the direct measurement of second harmonic generation and sum frequency generation phase-matching directions in the organic N-benzyl-2-methyl-4-nitroaniline crystal over its visible and near-infrared transparency range. The fit of these data allowed us to refine the Sellmeier equations of the three principal refractive indices in this range. With these equations, we improved the calculated tuning curves of terahertz emission from a phase-matched difference frequency process. We also determined the absolute magnitude of the d24 nonlinear coefficient.

Study of middle infrared difference frequency generation using a femtosecond laser source in LGT.

We demonstrate phase-matched difference frequency generation in the emerging nonlinear crystal La3Ga5.5Ta0.5O14. Tunable wavelengths between 1.4 and 4.7 µm are generated by using femtosecond sources. We also report on the measurements of the optical damage threshold in the femtosecond regime and on the nonlinear refractive index n2.

Dielectric frame, Sellmeier equations, and phase-matching properties of the monoclinic acentric crystal GdCa4O(BO3)3.

We directly measured the phase-matching properties of the biaxial GdCa4O(BO3)3 (GdCOB) crystal using the sphere method. We studied second-harmonic generation and difference frequency generation in two principal planes of the crystal. All these data allowed us to refine the Sellmeier equations of the three principal refractive indices. These equations are valid over the entire transparency range of GdCOB and then could be used to calculate the tuning curves of infrared optical parametric generation.

Acentric langanite La3Ga5.5Nb0.5O14 crystal: a new nonlinear crystal for the generation of mid-infrared parametric light.

The mid-infrared spectral range extending from 2 to 6 µm is significant for scientific and technological applications. A promising nonlinear oxide crystal La3Ga5.5Nb0.5O14 (LGN) is proposed and fully characterized for the first time to our knowledge. The transparency range extends between 0.28 and 7.4 µm. The two principal refractive indices were measured and we found that the nonlinear coefficient d11 = 3.0 ± 0.1 pm/V at 0.532 µm. The simultaneous fit of data allowed us to refine the Sellmeier equations of LGN and to calculate the tuning curves for optical parametric generation (OPG) pumped at 1.064 µm. Calculations are consistent with recorded data and also show the generation of a supercontinuum between 1.5 and 3.5 µm when pumped at 0.98 µm by a Ti:Sapphire laser.

Phase-matching directions and refined Sellmeier equations of the monoclinic acentric crystal BaGa4Se7.

We directly measured the phase-matching directions of second-harmonic and difference frequency generation in the principal planes of the biaxial BaGa4Se7 (BGSe) crystal using the sphere method. The simultaneous fit of the data allowed us to refine the Sellmeier equations of the three principal refractive indices of BGSe, and to calculate the tuning curves for infrared optical parametric generation.

Angle noncritical phase-matched second-harmonic generation in the monoclinic crystal BaGa4Se7.

We performed a complete study of angle noncritical phase-matched second-harmonic generation in BaGa4Se7 in the mid-infrared region. We measured the dielectric frame orientation and showed that it does not rotate as a function of wavelength. We determined the phase-matching wavelengths and the associated spectral and angular acceptances along the x and y axes of the dielectric frame. We also estimated the magnitude of the nonlinear coefficients involved.

Anisotropy analysis of third-harmonic generation in a germanium-doped silica optical fiber.

We performed an intermodal third-harmonic generation around 516 nm in a germanium-doped silica optical fiber. The analysis of the complex polarization behavior that was observed allowed us to determine the orientation symmetry group of the fiber and the relative values of the independent coefficients of the third-order electric susceptibility tensor.

Phase-matching directions, refined Sellmeier equations, and second-order nonlinear coefficient of the infrared Langatate crystal La3Ga(5.5)Ta(0.5)O14.

We directly measured phase-matching directions of second harmonic, sum, and difference frequency generations in the Langatate La3Ga(5.5)Ta(0.5)O14 (LGT) uniaxial crystal. The simultaneous fit of the data enabled us to refine the Sellmeier equations of the ordinary and extraordinary principal refractive indices over the entire transparency range of the crystal, and to calculate the phase-matching curves and efficiencies of LGT for infrared optical parametric generation.

Widely tunable optical parametric oscillator in a 5 mm thick 5% MgO:PPLN partial cylinder.

A 5 mm thick and 38 mm long 5% MgO-doped periodically poled lithium niobate (MgO:PPLN) cylinder with a grating period of 28 µm has been engineered as a partial cylinder with an angular aperture of 45°. An optical parametric oscillator based on this crystal and pumped at 1.064 µm is reported. The output energy is comparable to that generated in the same kind of sample cut as a slab, but the wavelength tunability from 1.41 to 4.3 µm is much broader and continuous.

Dual-wavelength source from 5%MgO:PPLN cylinders for the characterization of nonlinear infrared crystals.

We conceived a unique fully parametric source based on two independent cylindrical OPOs simultaneously pumped by the same Nd:YAG laser. Each OPO delivers more than 2 mJ and is continuously tunable between 1.41 µm and 4.3 µm. This source is of particular interest for the study of the generation of infrared parametric light in nonlinear crystals. It was validated by performing difference frequency generation experiments in CdSe crystals with output in the range 8 - 10 µm.

Energetic and spectral properties of triple photon downconversion in a phase-matched KTiOPO4 crystal.

We measured the spectrum and energy of infrared triple photons generated in a phase-matched KTiOPO4 crystal pumped by picosecond beams at 532 and 1662 nm. The experimental data are in good agreement with our model, taking into account the spectral linewidths of the incident beams as well as a parasitic Kerr effect.

Phase-matching properties and refined Sellmeier equations of the new nonlinear infrared crystal CdSiP2.

We directly measured the second harmonic generation and difference frequency generation phase-matching directions of the nonlinear crystal CdSiP(2) until 9.5 µm using the sphere method. The simultaneous fit of the experimental angular data allowed the Sellmeier equations of the ordinary and extraordinary principal refractive indices to be refined over the entire transparency range of the crystal.

Rotation of the absorption frame as a function of the electronic transition in the Nd3+:YCa4O(BO3)3 monoclinic crystal.

We report the first measurements of the angular distribution of absorption under polarized light corresponding to seven electronic transitions of Nd(3+) ions in the monoclinic crystal Nd:YCOB, revealing a rotation of the symmetry axes of the different patterns.

Singular topology of optical absorption in biaxial crystals.

We show for the first time that biaxial crystals exhibit continua of directions of propagation where the absorption coefficient is the same for the two associated polarization modes. This statement is supported by both calculations and experimental data obtained in Nd:YCOB.

Angular quasi-phase-matching experiments and determination of accurate Sellmeier equations for 5%MgO:PPLN.

We validated the theory of angular quasi-phase-matching (AQPM) by performing measurements of second-harmonic generation and difference-frequency generation. A nonlinear least-squares fitting of these experimental data led to refine the Sellmeier equations of 5%MgO:PPLN that are now valid over the complete transparency range of the crystal. We also showed that AQPM exhibits complementary spectral ranges and acceptances compared with birefringence phase matching.