Mid-infrared Nonlinear Optics Research Articles

Homoatomic Polychalcogenide Nonlinear Optical Anionic Groups with Ultra-Large Optical Anisotropy.

Functional assembly of nonlinear optical (NLO) motifs with a large optical anisotropy is vital to the development of advanced NLO and birefringent materials. In this work, we highlight that, in addition to heteroatomic NLO motifs, homoatomic anionic clusters formed by aggregated anions (S, Se, Te) exhibit diverse chain-, ring-, and cage-like chemical structures as well as one-, two-, and three-dimensional motif alignments. The rich structural chemistry enables homoatomic polychalcogenides (HAPCs) to exhibit asymmetric structural features and anisotropic optical properties, with great potential for NLO and birefringent performance. Focusing on totally 55 binary HAPCs A2Qn (n = 2, 3, 4, 5; A = Na, K, Rb, Cs; Q = S, Se, Te) and their ternary analogues, we employ the state-of-the-art first-principles approach to systematically investigate the modulation evolution of their NLO and birefringent properties. Remarkably, Rb2Te3 and Na2TeSe2 exhibit rarely colossal birefringence (>1.0@10 μm) and NLO effects (>20 × AgGaS2), much larger than conventional NLO chalcogenides. Na2Te3 presents the largest birefringence to date (∼3.48@1, 2.72@2, 2.34@10 μm), indicating the unique structural superiority of HAPC in terms of ultra-large birefringence. By mining the intrinsic mechanism, the HAPC anionic groups are identified as novel mid-infrared NLO "material genes", furnishing unique NLO and birefringent performance for the design of novel optoelectronic materials.

A congruent-melting rubidium zinc vanadium oxychloride exhibiting strong mid-infrared second-harmonic generation and high laser-induced damage threshold

A novel mid-infrared (MIR) nonlinear optical (NLO) crystal Rb4ZnV5O15Cl (RZVC) has been designed and sythesized by the aliovalent substitution strategy based on the parent structure Rb2(VO)(V2O7). Modification of square pyramidal VO5 units through introduction of chlorine ion and zinc metal caion significantly enhanced the dipole moments in a unit cell from 1.653 D for Rb2(VO)(V2O7) to 6.559 D for RZVC. Consequently, polycrystalline RZVC exhibits strong SHG responses of 12.5 × KDP @1064 nm and 1.2 × AGS @2 μm. Furthermore, RZVC features a congruent melting property, an ultra-high LIDT value (62 × AGS @1064 nm), and a deep IR transparency cutoff edge (10 μm). Most importantly, RZVC can achieve full-wavelength phase matching in a very wide MIR transparency range of 1.69–10 μm, completely covering the important atmospheric window (3–5 μm) through NLO processes. Because of its many outstanding properties, RZVC can be a highly desirable candidate for practical MIR NLO applications.

High-quality nanocavities through multimodal confinement of hyperbolic polaritons in hexagonal boron nitride.

Compressing light into nanocavities substantially enhances light-matter interactions, which has been a major driver for nanostructured materials research. However, extreme confinement generally comes at the cost of absorption and low resonator quality factors. Here we suggest an alternative optical multimodal confinement mechanism, unlocking the potential of hyperbolic phonon polaritons in isotopically pure hexagonal boron nitride. We produce deep-subwavelength cavities and demonstrate several orders of magnitude improvement in confinement, with estimated Purcell factors exceeding 108 and quality factors in the 50-480 range, values approaching the intrinsic quality factor of hexagonal boron nitride polaritons. Intriguingly, the quality factors we obtain exceed the maximum predicted by impedance-mismatch considerations, indicating that confinement is boosted by higher-order modes. We expect that our multimodal approach to nanoscale polariton manipulation will have far-reaching implications for ultrastrong light-matter interactions, mid-infrared nonlinear optics and nanoscale sensors.

Structural strategy for advancing nonlinear optical effects in 1D-[MX2]∞ chains: internal distortion and atomic types.

This work aimed to alleviate the limitations of existing mid-infrared nonlinear-optics (MIR-NLO) crystals by conducting theoretical research on 1D-[MX2]∞ (1D = one-dimensional; M = metallic element; X = anionic element) structures in relation to NLO. An analysis was conducted on the electronic structure and optical properties of six selenides (BaZnGeSe4, KxBa1-x/2Ga2Se4, KxBa1-x/2GayIn2-ySe4, KxBa1-x/2In2Se4, BaZnSiSe4, SrZnSiSe4) and three arsenides (Cs2SnAs2, Rb2SnAs2, K2SnAs2) using first principles. Afterwards, the inherent characteristics of these 1D-[MX2]∞ chains were further examined. The calculation results suggest that the presence of internal distortion (non-centrosymmetric) and high electronegativity in M elements can greatly enhance the NLO capability of 1D-[MX2]∞ structures. Furthermore, the first-ever prediction of K2SnAs2's NLO ability has been made. Ultimately, the establishment of a theoretical structure (K2BaSn2As4) provides guidance for the subsequent creation of high-performance MIR-NLO crystals.

Shooting mid-infrared nonlinear optical materials with targeted balance performances in ternary d0-transition metal oxides through first-principles

Heavy metal oxides are emergent candidates for mid-infrared nonlinear optical (NLO) materials, which might achieve a targeted balance property between the idea band gaps and large NLO coefficients. However, the ternary d0-transition metal oxides as mid-infrared NLO materials have not been systematically investigated by the first-principles. Herein, taking ternary d0-transition metal oxides as a research subject, we collected 94 non-centrosymmetric compounds in the inorganic crystal structure database for screening excellent mid-infrared NLO crystals. Among them, seven compounds are highlighted: HT-La2Ti2O7, ZrMo2O8, Rb3V5O14, β-Tl3VO4, La2MoO6, WO2Cl2, and WOCl4. They have a good balance between the wide band gaps 3.01–4.33 eV and large NLO coefficients of 0.5–0.7 × AgGaS2 (d36 = 13.4 p.m./V), simultaneously possess moderate birefringence of 0.055–0.438 at 1064 nm. This study will provide a guideline for exploring new mid-infrared NLO materials with targeted balance performances in ternary d0-transition metal oxides.

Mid-infrared nonlinear optical heptachlorodigallate AGa2Cl7 (A = Ga+, In+) with a unique second harmonic generation mechanism induced by monovalent cations

Nonlinear optical (NLO) anionic group theory holds that the anionic group mainly determines the NLO effect of the materials. The same anionic group basically has similar NLO properties even in different systems, so it is regarded as the “gene” of NLO materials. Isolated monovalent Ga+/In+ cations in the existing AGa2Cl7 (A = Ga+, In+) system carry lone-pair-electronic nonbonding orbitals, which are free between the energy bands near the gap and are not substantially hybridized with the orbitals of anionic [Ga2Cl7]– dimer. Surprisingly, this kind of band structure can significantly enhance the NLO polarizability of both Ga+/In+ cations and [Ga2Cl7]– anionic groups, differing from conventional mechanism of the anionic group theory. First-principles calculations demonstrate that AGa2Cl7 (A = Ga+, In+) indeed exhibits greatly enhanced second harmonic generation (SHG) responses over the analogous KGa2Cl7. The underlying contribution to this unusual enhancement stems not only from the anisotropic distribution of photo-responsive nonbonding electron pairs on Ga+/In+, but also from the synergistic charge transfer effect between cations and anionic groups. Due to the anisotropic induction of Ga+/In+, the same [Ga2Cl7]– anionic group exhibits significantly different SHG effects in different AGa2Cl7 (A = K, Ga, In) systems. This phenomenon is rare and satisfies a unique SHG mechanism, providing insights into NLO crystal physics and novel structural design. Importantly, except for enhanced SHG effects, monovalent Ga+/In+ cations can induce enlarged birefringence, larger than regular tetrahedra-based halides and more conducive to the phase matching. Furthermore, given wide optical transparency and better thermal properties as compared with the infrared NLO benchmark material AgGaS2, the unique AGa2Cl7 (A = Ga+, In+) are predicted as potential mid-infrared NLO materials.

Design, crystal structures, electronic structures and absorption spectra of MnSexTe1−xMoO6: A first principles exploration

MnTeMoO6 is a nonlinear optical material with a potential application in mid-infrared nonlinear optics. But until now, its application is limited for several defects as poor mechanical and thermal performances. To improve the performance of MnTeMoO6, the Se-doping has been employed. MnSexTe1−xMoO6 has been designed, and calculated by using the first principles method. Crystal structures, electronic structures and absorption spectra of MnSen/16Te1−n/16MoO6 (n = 0, 1, 2, 3) have been investigated. The calculation results show that Se4+ ions have a great possibility to substitute Te-sites with equivalent isomorphism in MnTeMoO6. Fortunately, for the Se-doping ratio increase in MnSen/16Te1−n/16MoO6 (n = 1, 2, 3), the structural stability would increase slightly, the laser damage threshold would be improved. Meanwhile, the cleavage fracture of MnSexTe1−xMoO6 crystal would be affected by the c-axis lengthening. Unfortunately, the transmission of MnSexTe1−xMoO6 would become worse in the visible region, thus the laser frequency conversion efficiency would be affected. Taken together, to improve the performance of MnTeMoO6 crystal, Se4+ would be an appropriate dopant to substitute for Te4+. This research results will accelerate the application of MnTeMoO6 crystal, and have reference significance for the exploration and application of new functional materials.

Pb4SeBr6: A Congruently Melting Mid‐Infrared Nonlinear Optical Material with Excellent Comprehensive Performance

AbstractDeveloping mid‐infrared (MIR) nonlinear optical (NLO) crystals is of current research interest, but their rational design is still faced by a great challenge because of the inherent conflict between large second harmonic generation (SHG) response and wide bandgap. To reconcile these conflicting properties, the best way is to combine multiple anions together. Up to now, various MIR NLO crystals with multiple anions are reported, such as oxychalcogenides, oxyhalides, and salt‐inclusion chalcogenide‐halides. However remarkably, the nonsalt‐inclusion chalcohalides with chalcogenide Q2– (Q = S, Se) and halide X– (X = F, Cl, Br) anions combined with the same central cations are still rarely reported. Herein, a lead chalcohalide Pb4SeBr6 is reported, which belongs to the nonsalt‐inclusion chalcohalide and is synthesized by the rational aliovalent substitution of Se2– for Br– anions with PbBr2 as the template. Importantly, this compound can exhibit excellent NLO properties, including large SHG response (1.3 × AgGaS2 (AGS)), wide IR transparent region (0.5–25 µm), and high laser damage thresholds (10 × AGS) as well as the congruently melting performance. These indicate that Pb4SeBr6 is a promising MIR NLO crystal and it will also provide some insights for exploring new MIR NLO crystals based on the rational partial anion substitution.

Ca3(TeO3)2(MO4) (M = Mo, W): Mid-Infrared Nonlinear Optical Tellurates with Ultrawide Transparency Ranges and Superhigh Laser-Induced Damage Thresholds.

Intense interests in mid-infrared (MIR) nonlinear optical (NLO) crystals have erupted in recent years due to the development of optoelectronic applications ranging from remote monitoring to molecular spectroscopy. Here, two polar crystals Ca3(TeO3)2(MO4) (M = Mo, W) were grown from TeO2-MO3 flux by high-temperature solution methods. Ca3(TeO3)2(MoO4) and Ca3(TeO3)2(WO4) are isostructural, which feature novel structures consisting of asymmetric MO4 tetrahedra and TeO3 trigonal pyramids. Optical characterizations show that both crystals display ultrawide transparency ranges (279 nm to 5.78 μm and 290 nm to 5.62 μm), especially high optical transmittance over 80% in the important atmospheric transparent window of 3-5 μm, and superhigh laser damage thresholds (1.63 GW/cm2 and 1.50 GW/cm2), 54.3 and 50 times larger than that of state-of-the-art MIR NLO AgGaS2, respectively. Notably, they exhibit the widest band gaps and the loftiest laser-induced threshold damages among the reported tellurates so far. Moreover, Ca3(TeO3)2(MO4) exhibit type I phase matching at two working wavelengths owing to their large birefringence and strong second-harmonic generation responses from the distorted anions, as further elucidated by the first-principles calculations. The above characteristics indicate that Ca3(TeO3)2(MO4) crystals are high-performance MIR NLO materials, especially applying in high-power MIR laser operations.

Compact OPCPA system seeded by a Cr:ZnS laser for generating tunable femtosecond pulses in the MWIR.

A compact mid-wavelength infrared (MWIR) optical parametric chirped pulse amplification (OPCPA) system generates sub-150 fs pulses at wavelengths from 5.4 to 6.8 µm with >400µJ energy at a 1 kHz repetition rate. A femtosecond Cr:ZnS master oscillator emitting 40 fs pulses at 2.4 µm seeds both a Ho:YLF regenerative amplifier and a two-stage OPCPA based on ZnGeP2 crystals. The 2.05 µm few-picosecond pump pulses from the Ho:YLF amplifier have an energy of 13.4 mJ. Seed pulses for the OPCPA are generated by soliton self-frequency shifting in a fluoride fiber and are tunable between 2.8 and 3.25 µm with a sub-100 fs duration and few-nanojoule energy. The intense MWIR pulses hold strong potential for applications in ultrafast mid-infrared nonlinear optics and spectroscopy.

From Ce(IO3)4 to CeF2(IO3)2: fluorinated homovalent substitution simultaneously enhances SHG response and bandgap for mid-infrared nonlinear optics

An efficient mid-IR NLO material, CeF2(IO3)2, was rationally designed using a fluorinated homovalent substitution strategy.

Rb4Li2TiOGe4O12: A Titanyl Nonlinear Optical Material with the Widest Transparency Range

AbstractPractical mid‐infrared (MIR) coherent light beams generated by frequency conversion in nonlinear optical (NLO) crystals are indispensable in time‐resolved infrared vibrational spectroscopy, remote light detection and ranging, and free‐space communications. Herein, a new titanyl germanate Rb4Li2TiOGe4O12 (RLTG) MIR NLO crystal was obtained by heavier element substitution. It features a complicated structure network composed of compressed TiO5 square pyramids and distorted GeO4 tetrahedra, separated by Rb+ and Li+ cations. More importantly, RLTG exhibits concurrently short ultraviolet (0.28 μm) and long IR (5.58 μm) transmittance cutoffs, fully covering the atmospheric transparent window of 3–5 μm. Related to the short UV cutoff, it shows a higher laser‐induced damage threshold in comparison to commercial MIR NLO crystals, about twice that of KTiOPO4 (KTP) and 50 times that of AgGaS2 (AGS).

Rb4 Li2 TiOGe4 O12 : A Titanyl Nonlinear Optical Material with the Widest Transparency Range.

Practical mid-infrared (MIR) coherent light beams generated by frequency conversion in nonlinear optical (NLO) crystals are indispensable in time-resolved infrared vibrational spectroscopy, remote light detection and ranging, and free-space communications. Herein, a new titanyl germanate Rb4 Li2 TiOGe4 O12 (RLTG) MIR NLO crystal was obtained by heavier element substitution. It features a complicated structure network composed of compressed TiO5 square pyramids and distorted GeO4 tetrahedra, separated by Rb+ and Li+ cations. More importantly, RLTG exhibits concurrently short ultraviolet (0.28 μm) and long IR (5.58 μm) transmittance cutoffs, fully covering the atmospheric transparent window of 3-5 μm. Related to the short UV cutoff, it shows a higher laser-induced damage threshold in comparison to commercial MIR NLO crystals, about twice that of KTiOPO4 (KTP) and 50 times that of AgGaS2 (AGS).

Ba10Zn7M6Q26: Two New Mid-infrared Nonlinear Optical Crystals with T2 Supertetrahedron 3D Framework

Two new mid-infrared (MIR) nonlinear optical (NLO) chalcogenide crystals, Ba10Zn7In6S26 (1) and Ba10Zn7Ga6Se26 (2) were discovered by traditional solid state reactions. Compounds 1 and 2 are isostructural, and they represent a novel three-dimensional (3D) framework made by apex-sharing T2 supertetrahedron 2D layers that are stacked alternately by another kind of T2 supertetrahedra though sharing apexes along the c direction, with T1 tetrahedra inserting in the tunnels and Ba2+ cations locating both in the voids and in the tunnels. Among them, 1 shows attractive MIR NLO properties with a wide band gap of 3.0 eV, a strong second harmonic generation (SHG) signal about 4.1 times that of the commercial AgGaS2 at particle sizes of 30–46 μm under a 2.05 μm laser radiation, and a high laser damage threshold (LDT) of 37.2 times that of benchmark AgGaS2 at particle sizes of 150–210 μm under a 1.064 μm laser radiation. This distinguishes 1 as a new promising MIR NLO material. In addition, the density functional theo...

Experimental study on nonlinear−optical property of Ag4P2Se6

Mid−infrared (MIR) nonlinear optical (NLO) material is critical to laser frequency−conversion technology. Although the main commercial MIR NLO crystals (AgGaS2 AgGaSe2 and ZnGeP2) are practically used, they still have some intrinsic drawbacks, so that it is of great scientific significance to discover new materials with good MIR NLO performances. Among the main systems of chalcogenides to explore MIR NLO crystals, metal thiophosphates and selenophosphates are competitive for their structural diversities and strong covalent P−Q bonds. Here, Ag4P2Se6 (P212121) was selected to evaluate the second−order harmonic generation (SHG) property. As a result, the SHG response with phase−matching ability is about 0.5 times that of AgGaS2 at 2.05 μm in the particle range of 210–270 μm, and the laser damage threshold (LDT) is larger than that of AgGaS2. Remarkably, this compound is of low melt temperature of 642 °C with a congruent melting character, also possesses excellent chemical stability, facilitating its further crystal growth and fabrication.

V-doping in a potential nonlinear optical material Sr2Nb2O7 for increasing the SHG response: A first-principles study

The exploration and improvement of excellent mid-infrared nonlinear optical (NLO) materials is still in great demand. The perovskite-type Sr2Nb2O7 is found to be a good predictor of NLO properties due to the similar band gap and NLO coefficients to KTiOPO4 (KTP). In this paper, we systematically investigated the electronic structures, linear and nonlinear properties of pure and V-doped Sr2Nb2O7 based on DFT. It is found that the V-doped Sr2Nb2O7 doubles the NLO coefficient and exhibits higher birefringence, which is moderate and sufficient for phase-matching. The results are helpful to the further development of the mid-infrared NLO crystals with perovskite structure.

Toward mid-infrared nonlinear optics applications of silicon carbide microdisks engineered by lateral under-etching [Invited

We report the fabrication and characterization of silicon carbide microdisks on top of silicon pillars suited for applications from near- to mid-infrared. We probe 10 μm diameter disks with different under-etching depths, from 4 μm down to 1.4 μm, fabricated by isotropic plasma etching and extract quality factors up to 8400 at telecom wavelength. Our geometry is suited to present high Q single-mode operation. We experimentally demonstrate high-order whispering-gallery mode suppression while preserving the fundamental gallery mode and investigate some requirements for nonlinear optics applications on this platform, specifically in terms of quality factor and dispersion for Kerr frequency comb generation.

New strategy for designing promising mid-infrared nonlinear optical materials: narrowing the band gap for large nonlinear optical efficiencies and reducing the thermal effect for a high laser-induced damage threshold.

To circumvent the incompatibility between large nonlinear optical (NLO) efficiencies and high laser-induced damage thresholds (LIDTs) in mid-infrared NLO materials, a new strategy for designing materials with both excellent properties is proposed. This strategy involves narrowing the band gap for large NLO efficiencies and reducing the thermal effect for a high LIDT. To support these proposals, a series of isostructural chalcogenides with various tetrahedral center cations, Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se), were synthesized and studied in detail. Compared with the benchmark AGS, these chalcogenides exhibit significantly narrower band gaps (1.56-1.73 eV, AGS: 2.62 eV) and high NLO efficiencies (1.6-3.9 times that of AGS at 1910 nm), and also outstanding LIDTs of 8.5-13.3 × those of AGS for potential high-power applications, which are contrary to the conventional band gap view but can be attributed to their small thermal expansion anisotropy, surmounting the NLO-LIDT incompatibility. These results shed light on the search for practical IR NLO materials with excellent performance not restricted by NLO-LIDT incompatibility.

Fabrication and characterization of step-index tellurite fibers with varying numerical aperture for near- and mid-infrared nonlinear optics

We present an overview of the fabrication process and characterization of tellurite, germanate, and germanate–tellurite step-index fibers with different index contrasts. Several compatible core/cladding glass pairs were first explored for fiber manufacturing under ambient atmosphere. The potential of the resulting waveguides for nonlinear optics is revealed by means of supercontinuum generation experiments using a near-infrared femtosecond fiber laser. Fabrication of the glass preforms was also adapted to dehydration procedures, allowing the drawing of low-OH step-index tellurite fibers. The beneficial impact of glass purification on supercontinuum generation toward the mid-infrared region is confirmed.