NaK5La2(SO4)6: Enhanced Birefringence of Multiple-Alkali Metal Sulfate Systems via Rare Earth Metal-Centered Polyhedra.

Polarization modulation of ultraviolet (UV) birefringent crystals is crucial for various applications. Here, we introduce distorted La-O polyhedra into alkali metal sulfates to synthesize a novel birefringent material with excellent UV transmission and birefringence. The incorporation of distorted La-O polyhedra significantly increases the birefringence to 0.0255 at 550 nm, surpassing that of many alkali metal sulfates while maintaining excellent UV transparency. The material exhibits excellent thermal stability up to 450 °C. Theoretical calculations show the connection between the crystal structure and optical functionality, confirming that the incorporation of La-O polyhedra enhances birefringence. This research provides novel insights into the discovery and design of outstanding birefringence materials.

SESAM mode-locked Yb:Sr3Y2(BO3)4 laser.

We demonstrate the first sub-40 fs soliton pulse generation from a diode-pumped Yb:Sr3Y2(BO3)4 laser passively mode-locked by a semiconductor saturable absorber mirror. Pulses as short as 38 fs at a central wavelength of 1051.7 nm were achieved with an average output power of 115 mW and a pulse repetition rate of 67.7 MHz. The maximum average output power reached 303 mW at 1057.8 nm with a slightly longer pulse duration of 52 fs, which corresponded to a peak power of 76.9 kW and an optical efficiency of 25.3%.

[C(NH2)3]2Zn(CO3)2: A Guanidinium-Templated Ultraviolet Nonlinear Optical Material.

A novel guanidinium-templated ultraviolet (UV) nonlinear optical zinc carbonate crystal, [C(NH2)3]2Zn(CO3)2 (GZCO), has been synthesized in a closed system at low temperatures. GZCO crystallizing in the tetragonal noncentrosymmetric nonpolar space group, P41212 exhibits a three-dimensional anionic framework constructed by interconnected [Zn6C6O32] 12-membered ring channels with inorganic CO3 triangles and ZnO4 tetrahedra. Notably, the anhydrous GZCO shows a very high thermal stability among guanidine-based hybrid NLO materials benefiting from the confinement effect of the organic cations within inorganic channels. The UV-visible transmittance spectrum reveals that GZCO has a short UV cutoff edge of 210 nm, corresponding to the large band gap of 5.9 eV. GZCO exhibits a mild second-harmonic generation efficiency of 0.5 × KH2PO4 with type-I phase-matching behavior.

Enhanced Electron-Phonon Coupling Effect in Rare-Earth Borate Crystals Containing a "Quasi-Free-Oxygen" Motif.

Revealing the interaction between electrons and phonons, e.g., electron-phonon coupling or decoupling, is a great challenge for physics and functional material communities. For rare-earth single crystals, the electron-phonon coupling and fluorescence behaviors strongly depend on the crystal structure and constituent motifs. Here, we proposed a universal "quasi-free O" as an effective structural motif to enhance phonon-assisted electronic transitions and photoluminescence. Using Gd3+ ion as a probe, we studied Gd:La2CaB10O19 (Gd:LCB) and GdMgB5O10 (GdMB) crystals composed of double B-O layers and dangling "quasi-free O", respectively, which enable strengthened phonon-involved luminescence. Especially, a GdMB crystal features an infinite [O-Gd-O-Gd-O] chain (O represents quasi-free oxygen), thus greatly promoting the energy transfer and electron-phonon coupling effect. As a result, its Huang-Rhys S factor is two times larger than that of a Gd:LCB crystal under room temperature. These results put forward "quasi-free O" to improve the electron-phonon coupling intensity and allow LCB and GdMB crystals to serve as potential hosts for phonon-terminated vibronic lasers.

Diode-pumped sub-50-fs Kerr-lens mode-locked Yb:GdYCOB laser.

We present a sub-50-fs diode-pumped Kerr-lens mode-locked laser employing a novel "mixed" monoclinic Yb:Ca4(Gd,Y)O(BO3)3 (Yb:GdYCOB) crystal as a gain medium. Nearly Fourier-limited pulses as short as 43 fs at 1036.7 nm are generated with an average power of 84 mW corresponding to a pulse repetition rate of ∼70.8 MHz. A higher average power of 300 mW was achieved at the expense of the pulse duration (113 fs) corresponding to an optical-to-optical efficiency of 35.8% representing a record-high value for any Yb-doped borate crystal. Non-phase-matched self-frequency doubling is observed in the mode-locked regime with pronounced strong spectral fringes which originate from two delayed green replicas of the fundamental femtosecond pulses in the time domain.

Struvite-type AMgPO4·6H2O (A = NH4, K): Two Natural Deep-Ultraviolet Transparent Nonlinear Optical Crystals.

Deep-ultraviolet (DUV) nonlinear optical (NLO) materials play vital roles in diverse fields. Unfortunately, only the KBe2BO3F2 crystal has found commercial applications so far. Therefore, the discovery of new DUV NLO crystals is still urgent. As we all know, digging into the properties of existing crystals is also an effective way to obtain new NLO crystals. Herein, two natural asymmetric orthophosphates AMgPO4·6H2O (A = NH4, K) are proposed. Although their structures and some properties such as infrared spectra, thermal properties, and dielectric properties have been previously characterized, their NLO properties have not been reported. Thus, in this work, these two natural DUV transparent orthophosphates NH4MgPO4·6H2O (NMP) and KMgPO4·6H2O (KMP) were successfully acquired by a simple slow evaporation method. The single-crystal X-ray diffraction data indicate that NMP and KMP are isomorphic and that both belong to the Pmn21 space group of the orthorhombic system. Remarkably, NMP and KMP possess short cutoff edges below 190 nm, and their second-harmonic generation (SHG) efficiencies are 0.62 and 0.80 times that of KH2PO4(KDP), respectively; furthermore, they can achieve type-I phase matching at 1064 nm.

Single-walled carbon-nanotube saturable absorber assisted Kerr-lens mode-locked Tm:MgWO4 laser.

We demonstrate sub-100-fs Kerr-lens mode-locking of a Tm:MgWO4 laser emitting at ∼2µm assisted by a single-walled carbon-nanotube saturable absorber. A maximum average output power of 100 mW is achieved with pulse duration of 89 fs at a pulse repetition rate of ∼86MHz. The shortest pulse duration derived from frequency-resolved optical gating amounts to 76 fs at 2037 nm, corresponding to nearly bandwidth-limited pulses. To the best of our knowledge, these are the shortest pulses generated from any Tm-doped tungstate crystal and the first report on saturable absorber assisted Kerr-lens mode-locking of a Tm laser at ∼2µm.

Low-loss fs-laser-written surface waveguide lasers at >2 µm in monoclinic Tm3+:MgWO4.

Surface channel waveguides (WGs) based on a half-ring (40-60-µm-diameter) depressed-index cladding (type III) geometry are fabricated in monoclinic Tm3+:MgWO4 by femtosecond (fs) laser writing at a repetition rate of 1 kHz. The WGs are characterized by confocal laser microscopy and µ-Raman spectroscopy. A Tm3+:MgWO4 WG laser generates 320 mW at ∼2.02µm with a slope efficiency of 64.4%. The WG emits a transverse single-mode and linear polarization (E||Nm). A remarkable low loss of <0.1dB/cm is measured for the WG. Vibronic laser emission at ∼2.08µm is also achieved.

Spectroscopy and high-power laser operation of a monoclinic Yb3+:MgWO4 crystal.

Monoclinic (wolframite-type) monotungstate crystals are promising for rare-earth doping. We report polarized room- and low-temperature spectroscopy and efficient high-power laser operation of such a ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}_4}$Yb3+:MgWO4 crystal featuring high stimulated emission cross section (${\sigma _{\rm SE}}\; = \;{6}.{2}\; \times \;{{10}^{ - 20}}\;{{\rm cm}^2}$σSE=6.2×10-20cm2 at 1056.7 nm for light polarization ${\rm E}\;||\;{N_m}$E||Nm), large Stark splitting of the ground state (${765}\;{{\rm cm}^{ - 1}}$765cm-1), large gain bandwidth (26.1 nm for ${\rm E}\;||\;{N_g}$E||Ng), and strong Raman response (most intense mode at ${916}\;{{\rm cm}^{ - 1}}$916cm-1). A diode-pumped ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}_4}$Yb3+:MgWO4 laser generated 18.2 W at ${\sim}{1056}\;{\rm nm}$∼1056nm with a slope efficiency of ${\sim}{89}\% $∼89% and a linearly polarized laser output.

Polarization switching realized in the continuous-wave and acousto-optic Q-switched pulse Er:Yb:LaMgB5O10 lasers at 1556 and 1568 nm.

By using the natural birefringence of crystalline quartz, the switching of a 1568 nm laser with polarization parallel to the Z optical indicatrix axis to a 1556 nm laser with polarization parallel to the Y optical indicatrix axis was observed in both continuous-wave and acousto-optic Q-switched pulse lasers based on an X-cut Er:Yb:LaMgB5O10 crystal, when the alignment of the output mirror in a 976 nm diode-end-pumped plano-concave resonator was precisely tilted. For the continuous-wave regime, a 1568 nm laser with a maximum output power of 500 mW and slope efficiency of 16%, as well as a 1556 nm laser with a maximum output power of 400 mW and slope efficiency of 13.5% were realized, respectively. For the Q-switched regime, a 1568 nm pulse laser with an energy of 144 µJ and width of 300 ns, as well as a 1556 nm pulse laser with an energy of 168 µJ and width of 270 ns, were obtained respectively by precisely tilting the alignment of the output mirror, when the absorbed pump power was 4.0 W and pulse repetition frequency was 0.5 kHz.

Continuous-wave and SESAM mode-locked femtosecond operation of a Yb:MgWO4 laser.

We present a detailed continuous-wave regime characterization and, for the first time to the best of our knowledge, SESAM mode-locked femtosecond operation with a monoclinic, Yb3+-doped, MgWO4 crystal. Pumping with a low-power, single-mode fiber-coupled laser diode emitting at 976 nm, we demonstrate threshold for continuous-wave (cw) operation as low as 50 mW (absorbed pump power) and slope efficiency up to ~60% (with respect to the absorbed pump power) for two of the principal emission polarizations. The output wavelength in the cw regime is continuously tunable over a ∼50 nm broad range. Mode-locking the laser with a SESAM, we achieve almost Fourier-transform limited pulses with a duration of 132 and 125 fs depending on the polarization.

Efficient continuous-wave diode-pumped Er3+:Yb3+:LaMgB5O10 laser with sapphire cooling at 1.57 µm.

Efficient 1.57 µm continuous-wave laser was demonstrated in an X-cut, 2.0 mm-thick Er3+:Yb3+:LaMgB5O10 crystal with sapphire cooling end-pumped by a 976 nm laser diode. In a plano-concave cavity, a laser with a maximum output power of 0.61 W and a slope efficiency of 23% was realized at an absorbed pump power of 4.0 W. A continuous-wave 1566 nm micro-laser with a maximum output power of 0.47 W and a slope efficiency of 16% was also obtained. The lasers were totally linear polarization parallel to the crystalline Z axis. The results show that the Er3+:Yb3+:LaMgB5O10 crystal with high thermal conductivity may be a good gain medium for laser around 1.55 µm.

Crystal growth, optical spectroscopy and laser action of Tm3+-doped monoclinic magnesium tungstate.

We report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 µm spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.

Sub-100 fs Tm:MgWO4 laser at 2017 nm mode locked by a graphene saturable absorber.

We present the first sub-100 fs bulk solid-state laser in the 2-µm spectral range employing the monoclinic Tm3+-dopedMgWO4 crystal as an active medium. By applying a graphene-based saturable absorber and chirped mirrors for dispersion management, stable self-starting mode-locked operation at 2017 nm was achieved. Nearly Fourier-limited pulses as short as 86 fs featuring a bandwidth of 53 nm were generated at a repetition rate of 76 MHz. A pulse energy of 1.1 nJ was achieved at 87 MHz for a pulse duration of 96 fs. The mode-locked Tm3+:MgWO4 laser exhibits excellent stability with a fundamental beat note extinction ratio of 80 dBc above noise level.

Monoclinic Tm3+:MgWO4: a promising crystal for continuous-wave and passively Q-switched lasers at ∼2 µm.

Monoclinic thulium-doped magnesium monotungstate, Tm3+:MgWO4, is promising for efficient power-scalable continuous-wave (CW) and passively Q-switched lasers at >2 µm. Under diode pumping at 802 nm, a compact CW laser based on Z-cut Tm:MgWO4 generated 3.09 W at 2022-2034 nm with a slope efficiency of 50% which represents the highest output power ever achieved with this type of laser host. Stable passive Q-switching of the Tm:MgWO4 laser is demonstrated for the first time, to the best of our knowledge, using single-walled carbon nanotubes, graphene, and Cr2+:ZnS saturable absorbers. Using the latter, the best performance is achieved with 16.1 µJ/13.6 ns pulses at 2017.8 nm with a maximum average output power of 0.87 W and a peak power of 1.18 kW.

Efficient orthogonally polarized dual-wavelength Nd:LaMgB5O10 laser.

We reported an efficient, continuous-wave, diode-pumped, orthogonally polarized dual-wavelength laser working at 1051.8 and 1081.4 nm in a new neodymium-doped borate crystal, Nd:LaMgB5O10, which was grown by the top-seeded solution growth method. A maximum output power of 5.1 W was obtained with an absorbed pump power of 14.8 W, corresponding to an optical-to-optical conversion efficiency of 34.5% and a slope efficiency of 42.5%. By slightly tilting the laser cavity output mirror, the balanced dual-wavelength emissions were obtained with the total output power as high as 4.2 W. This new efficient dual-wavelength laser may be a promising light source for terahertz generation with a rarely large frequency difference of 7.8 THz through difference frequency generation.

Two UV optical crystals with strong optical anisotropy, large band gaps and an α-BBO type structure.

The [B3O6] group has attracted a lot of attention as α-BaB2O4 (α-BBO) is composed of the [B3O6] group showing a short UV cut-off edge and a large birefringence. However, it is difficult to synthesize novel crystals with properties beyond α-BBO using [B3O6] as a fundamental building block (FBB). Hence, we successfully synthesized two cases of α-BBO-type organic-inorganic hybrid compounds using α-BBO as the template and melamine [C3N6H6] as the fundamental building block. The coplanar and parallel arrangement of π-conjugated [C3N6H6] groups makes them exhibit excellent optical properties, including improved birefringence responses (0.264 and 0.243@546 nm) and wide bandgaps (4.12 and 4.20 eV).

Hybrid organic-inorganic triguanidine arsenate dihydrate for ultraviolet nonlinear optical application.

Introduction of the planar organic cation [C(NH2)3]+ into tetrahedronal inorganic arsenate generates a new ultraviolet nonlinear optical crystal triguanidine arsenate dihydrate, [C(NH2)3]3AsO4·2H2O, which exhibits a short ultraviolet absorption edge (210 nm), suitable second-harmonic generation response (0.9 × KDP), and moderate birefringene (0.0387 at 546 nm). Notably, by optimizing the growth conditions, transparent and wafer well-developed bulk single crystals can be easily prepared.

To improve the key properties of nonlinear optical crystals assembled with tetrahedral functional building units.

Nonlinear optical (NLO) crystals assembled with conventional non-π-conjugated tetrahedral functional building units (FBUs), generally referring to [PO4] and [BO4], usually exhibit weak nonlinearity and poor birefringence. It is currently proposed that partially substituting oxygen atoms with fluoride atoms in these FBUs could enhance these crucial properties. Hence, we investigated for the first time the NLO-related properties of NH4BAsO4F (ABAF), which was constructed from tetrahedral [BO3F] and [AsO4] FBUs, and enhancements of these properties were observed in this material, that is large second-harmonic generation (SHG) response (2 × KDP) and improved birefringence (0.03 at 1064 nm). Notably, both SHG coefficient and birefringence of ABAF exceeded those of a great majority of phosphates, sulfates, or boron phosphates and achieved a preferable balance. It is interesting that ABAF shows vast structural similarities to the typical NLO crystals Sr2Be2B2O7 (SBBO) and KBe2BO3F2 (KBBF), which might be the partial reason why it showed improvement in these vital properties. This work may afford some inspiration for enhancing the key performances of NLO crystals assembled with non-π-conjugated tetrahedra.

2(C3H7N6)+·2Cl-·H2O: an ultraviolet nonlinear optical crystal with large birefrigence and strong second-harmonic generation.

Large birefringence (Δn = 0.277 at 546 nm) and strong second-harmonic generation (SHG) intensity (4.3 × KDP) achieved a remarkable balance in a potential ultraviolet nonlinear optical crystal: 2(C3H7N6)+·2Cl-·H2O. Theoretical calculations showed that planar benzene-like π-conjugated [C3N6] groups had important optical properties.