Second-order Nonlinear Optical Effects Research Articles

From Cd(SCN)2(CH4N2S)2 to Cd(SCN)2(C4H6N2)2: Controlling Sulfur Content in Thiocyanate Systems Significantly Improves the Overall Performance of UV Nonlinear Optical Materials.

Combining a strong second-order nonlinear optical (NLO) effect (>1×KH2PO4 (KDP)), a large band gap (>4.2 eV), and a moderate birefringence in ultraviolet (UV) NLO crystals remains a formidable challenge. Herein, Cd(SCN)2(C4H6N2)2, the first example of a thiocyanate capable of realizing a phase-matched UV NLO crystal material, is obtained by reducing the sulfur (S) content in the centrosymmetric (CS) structure of Cd(SCN)2(CH4N2S)2. Compared to the "shoulder-to-shoulder" one-dimensional (1D) chain of Cd(SCN)2(CH4N2S)2, Cd(SCN)2(C4H6N2)2 has a different sawtooth 1D chain structure. Cd(SCN)2(CH4N2S)2 has second harmonic generation (SHG) inertia with a band gap of 3.90 eV and a UV cutoff edge of 342 nm, however, it possesses a large birefringence (0.35@546 nm). In contrast, the symmetry center breaking of Cd(SCN)2(C4H6N2)2 leads to remarkably strong SHG intensity (10 times that of KDP). Furthermore, it has a wide band gap (4.74 eV), short UV cutoff edge (234 nm), and moderate birefringence capable of phase matching (0.17@546 nm). This research indicates that thiocyanates are a promising class of UV NLO crystal materials, and that modulation of the sulfur content of CS thiocyanates is an effective strategy for the development of UV NLO crystals with excellent overall performances.

Kerr-Enhanced Optical Spring.

We propose and experimentally demonstrate the generation of enhanced optical springs using the optical Kerr effect. A nonlinear optical crystal is inserted into a Fabry-Perot cavity with a movable mirror, and a chain of second-order nonlinear optical effects in the phase-mismatched condition induces the Kerr effect. The optical spring constant is enhanced by a factor of 1.6±0.1 over linear theory. To our knowledge, this is the first realization of optomechanical coupling enhancement using a nonlinear optical effect, which has been theoretically investigated to overcome the performance limitations of linear optomechanical systems. The tunable nonlinearity of demonstrated system has a wide range of potential applications, from observing gravitational waves emitted by binary neutron star postmerger remnants to cooling macroscopic oscillators to their quantum ground state.

Two Acentric Zn-Based l-Tartrates with Moderate Second Harmonic Generation Responses and Large Birefringence.

The discovery of novel organic-inorganic hybrid nonlinear optical (NLO) crystal materials holds great potential in advancing laser science and technology as they offer a wide range of benefits including easy synthesis, structural versatility, and high hyperpolarizability. Herein, the integration of l-tartaric acid (L-C4H6O6) and ZnSO4 gave rise to two acentric Zn-based organic-inorganic hybrid crystals, namely, Zn2(H2O)2(C4H4O6)2·3H2O (Zn-LT) and Zn2(H2O)(C4H4O6)(C4H6O6)(SO4)·4H2O (Zn-LTS). They both feature layered structures constructed by [ZnO6] octahedron, l-C4H6O6/l-C4H4O62-, water molecule, or [SO4] tetrahedron. Interestingly, they possess moderate second-order NLO effects of 0.28 × KH2PO4 (Zn-LT) and 0.57 × KH2PO4 (Zn-LTS), large birefringence of 0.075 (Zn-LT) and 0.069 (Zn-LTS), suggesting that the introduction of [SO4] groups with intrinsically weak polarizability and weak optical anisotropy induces the enhancement of the NLO effect while without reducing birefringence much. In addition, both of them display UV cutoff edges near 210 nm, indicating their potential as NLO crystals applied in the UV and solar-blind region.

From C4H7N2Ge0.4Sn0.6Br3 to C6H11N2Ge0.4Sn0.6Br3: Effective Modulation of the Second Harmonic Generation Effect and Optical Band Gap by Planar π-Conjugated Organic Cation Size.

In the search for nonlinear optical (NLO) materials with excellent overall performance, we have devoted ourselves to organic-inorganic hybrids consisting of anionic groups containing stereochemically active lone-pair (SCALP) electron cations and organic planar π-conjugated group cations. Accordingly, in this paper, two novel organic-inorganic hybrid metal halides, C4H7N2Ge0.4Sn0.6Br3 (I) and C6H11N2Ge0.4Sn0.6Br3 (II), have been synthesized. The powder second-harmonic technique shows that both C4H7N2Ge0.4Sn0.6Br3 and C6H11N2Ge0.4Sn0.6Br3 have moderately strong second-order nonlinear optical effects, which are about 2.03 (I) and 1.16 (II) times that of KH2PO4 (KDP), respectively. They also have different optical band gaps of 2.75 (I) and 2.88 eV (II) due to the different sizes of the organic cations, and their photoluminescent and thermal properties were also investigated. This work provides new structural insights for the design and modulation of organic-inorganic hybrid halide materials with multiple excellent optical properties.

Nonreciprocal Transport and Optical Phenomena in Quantum Materials

In noncentrosymmetric materials, the responses (for example, electrical and optical) generally depend on the direction of the external stimuli, called nonreciprocal phenomena. In quantum materials, these nonreciprocal responses are governed by the quantum geometric properties and symmetries of the electronic states. In particular, spatial inversion ([Formula: see text]) and time-reversal ([Formula: see text]) symmetries play crucial roles, which are also relevant to the geometric Berry phase. Here, we give a comprehensive review of the nonreciprocal transport and optical responses including ( a) the magnetochiral anisotropy, i.e., the nonlinear resistivity with respect to the electric field, in semiconductors and metals, ( b) the nonreciprocal transport in superconductors such as the nonreciprocal paraconductivity and the superconducting diode effect in bulk and Josephson junctions, and ( c) the second-order nonlinear optical effects in the electric field of light, including the geometric shift current in nonmagnetic systems, magnetic systems, and superconductors.

Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides

Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbOX2 (X = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl2 can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl2 and a reversible ferroelectric-to-paraelectric phase transition in NbOI2 under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbOX2 for nonlinear optical device applications.

A two-fold interpenetrated mixed-ligands acentric 3D coordination polymer Zn(OBA)(DIB)·0.5H2O: Topological analysis, fluorescent and second-order NLO properties

Second-order nonlinear optical (NLO) materials have important applications in the realms of photoelectronics and photonics whose microstructures need to be organized into a noncentrosymmetric (NCS) structure. In this work, a mixed-ligands three-dimensional (3D) noncentrosymmetric coordination polymer Zn(OBA)(DIB)·0.5H2O (1) was hydrothermally prepared from 4, 4′-oxydibenzoic acid (H2OBA), 1, 3-di(1H-imidazol-1-yl)benzene (DIB) and Zn(NO3)2·6H2O via spontaneous resolution approach, which crystallizes in a noncentrosymmetric space group Fdd2. Compound 1 was characterized by powder X-ray diffraction (XRD), thermogravimetric (TGA) and IR analyses. The structural analysis shows that 1 is a two-fold self-interpenetrated three-dimensional (3D) network material. Single crystal X-ray diffraction and the topological analysis manifests that the topological structure of 1 belongs to a cds topological net. The measurement of the fluorescent properties of 1 and the free DIB ligand show that the emission peak at 460 nm of 1 can be attributed to the charge transition from the п bond electron to the п anti-bond orbital of the DIB ligand. The second-harmonic generation (SHG) measurement shows that the second-order NLO effect of compound 1 is approximately equivalent to that of potassium dihydrogen phosphate (KDP). Compound 1 is high thermal stable and insoluble in water and common organic solvents, making it to be a potential candidate as a second-order NLO material.

Direct generation of a terahertz vector beam from a ZnTe crystal excited by a focused circular polarized pulse.

A vector beam is a type of topological beam in which the polarization direction of light rotates around a singularity on the wavefront. This paper proposes a method to generate a vector beam by tightly focusing a pump beam in the crystalline direction such that the second-order nonlinear optical effect is forbidden. The directional dependence of the effective nonlinearity in zincblende crystals, such as ZnTe, was analytically investigated. Two types of nonlinear polarization singularities were found in [111] and [100] directions. Their polarization topological charge ℓ was +1 and -1, respectively. To experimentally demonstrate the proposed method, a (111) cut ZnTe crystal was selected as the nonlinear crystal. The polarization state of the generated terahertz (THz) beams was measured with a custom-built THz spectroscopic polarization imaging system. Radially polarized distributions were observed within the entire generated spectral region. Such a broadband feature of the generated vector beam is likely due to the topological nature of the focused pump beam, where the wavevectors are winding once about the optical axis. This simple method for generating THz vector beams will accelerate its applications.

Strong bulk photovoltaic effect in engineered edge-embedded van der Waals structures

Bulk photovoltaic effect (BPVE), a second-order nonlinear optical effect governed by the quantum geometric properties of materials, offers a promising approach to overcome the Shockley-Quiesser limit of traditional photovoltaic effect and further improve the efficiency of energy harvesting. Here, we propose an effective platform, the nano edges embedded in assembled van der Waals (vdW) homo- or hetero-structures with strong symmetry breaking, low dimensionality and abundant species, for BPVE investigations. The BPVE-induced photocurrents strongly depend on the orientation of edge-embedded structures and polarization of incident light. Reversed photocurrent polarity can be observed at left and right edge-embedded structures. Our work not only visualizes the unique optoelectronic effect in vdW nano edges, but also provides an effective strategy for achieving BPVE in engineered vdW structures.

Nonlinear Optical Glass-Ceramic From a New Polar Phase-Transition Organic-Inorganic Hybrid Crystal.

Melt-quenched glasses of organic-inorganic hybrid crystals, i.e., hybrid glasses, have attracted increasing attention as an emerging class of hybrid materials with beneficial processability and formability in the past years. Herein, we present a new hybrid crystal, (Ph3 PEt)3 [Ni(NCS)5 ] (1, Ph3 PEt+ =ethyl(triphenyl)phosphonium), crystallizing in a polar space group P1 and exhibiting thermal-induced reversible crystal-liquid-glass-crystal transitions with relatively low melting temperature of 132 °C, glass-transition temperature of 40 °C, and recrystallization on-set temperature of 78 °C, respectively. Taking advantage of such mild conditions, we fabricated an unprecedented hybrid glass-ceramic thin film, i.e., a thin glass uniformly embedding inner polar micro-crystals, which exhibits a much enhanced intrinsic second-order nonlinear optical effect, being ca. 25.6 and 3.1 times those of poly-crystalline 1 and KH2 PO4 , respectively, without any poling treatments.

Achieving enhanced second-harmonic generation in ferroelectric nematics by doping D–π–A chromophores

A fluidic ferroelectric nematic liquid crystal was deployed as a matrix to achieve spontaneous polarization of D–π–A chromophores and realize a dramatic increase in second-harmonic generation.

Promotion of the second-order nonlinear optical effect by introducing ether linkage into polymer main chains

The lower rotational barrier of ether bond than that of single carbon–carbon one resulted in enhanced d33 values from 94 to 166 pm V−1, providing a new strategy for adjustment of molecular alignment of nonlinear optical polymers under electric field.

RE(OH)2NO3 (RE = Tb, Dy, Ho, and Er): Synthesis and Nonlinear Optical Properties of Water-Resistant Lanthanide Hydroxide Nitrates.

A series of lanthanide-containing water-resistant nitrates, namely RE(OH)2NO3 (RE = Tb(1), Dy(2), Ho(3), and Er(4)), was obtained through the hydrothermal process. As possible nonlinear optical materials, they feature a layered isomorph composed of an [REO3(OH)6] polyhedron and an [NO3] triangle, and the synergistic arrangement of the [REO3(OH)6] and [NO3] groups in their structures leads to their obvious second-order nonlinear optical effect. Nevertheless, the unique optical absorption caused by the electronic transitions on 4f-4f orbitals of lanthanides results in their second harmonic generation responses of different strengths, with 1 exhibiting 5.07 times that of KDP, but 2-4 showing less than half of KDP. In addition, 1 possesses an outstanding water-resistant capacity and a transparent cut-off edge around 300 nm, foreshadowing its potential value as a nonlinear optical crystal. Moreover, 1 is found to emit characteristic green fluorescence due to the typical 5D4 → 7F5 transitions of the excited Tb3+ ions.

铌酸锂微纳波导中的二阶非线性光学效应研究进展

<p indent="0mm">Lithium niobate (LiNbO₃, LN) crystal has a large second-order nonlinear coefficient (<italic>d</italic>₃₃=25.2 pm/V@1064 nm), wide transparent window (0.35–5 μm), and stable periodic microdomain structure preparation, which is a good platform for the study and application of second-order nonlinear optical effects. Lithium niobate thin film (LNTF) is regarded as a promising integrated photonics platform due to its excellent linear and nonlinear optical properties. As the basic unit of an integrated optical system, lithium niobate micro/nano waveguide has been studied as a transmission and control device, and it shows excellent second-order nonlinear optical characteristics. The second-order nonlinearity plays an important role in modern optics, including second-harmonic generation (SHG), sum- and difference-frequency generation, parametric down conversion, and parametric oscillation. Efficient and compact wavelength converters based on second-order nonlinearity are key components for a wide range of applications, including entangled photon sources, optical parametric oscillators, and optical parametric amplifiers. The second-order nonlinear polarization intensity is proportional to the square of the electric field intensity. Micro/nano optical waveguides can improve the light intensity in the device through the local characteristics of the spatial light field, to improve the conversion efficiency of the nonlinear processes. And significantly enhanced electric field strength makes the normalized nonlinear conversion efficiency of LNTF waveguides exceed that of the reverse proton<bold> </bold>exchange lithium niobate waveguides (frequency conversion efficiency 150% W^–1 cm^–2@1550 nm) by one order of magnitude. The nonlinear conversion efficiency of LNTF micro/nano waveguides can be further improved through mode phase matching (MPM) or quasi-phase matching (QPM) to optimize the spatial overlap between the eigenmodes involved in the nonlinear process. The detailed theory, fabrication and application were demonstrated in the manuscript as follows: (1) The theory of second harmonic generation. As an example of second-order nonlinear process, the factors affecting the conversion efficiency of second harmonic generation were analyzed, and can be summarized as nonlinear coefficient, mode special overlap, mode area and phase-matching condition. (2) The fabrication of periodically poled lithium niobate waveguides and periodically poled technics. Based on the micro fabrication processes such as electron beam lithography, dry etching and chemico-mechanical polishing, the propagation loss could be reduced to <sc>0.027 dB/cm.</sc> With the combination of low loss waveguides and the electrode polarization process, periodically poled lithium niobate microstructures and the detailed fabrication processes were discussed in the manuscript. (3) Efficient second harmonic generation based on MPM and QPM. It was reported that the normalized efficiency can reach 1000% W^–1 cm^–2, and we described broadband and tunable SHG in detail. (4) Spontaneous parametric down conversion. With the development of quantum information technology and its application in the fields of secure communication and precision measurement, quantum photon sources with high speed, high brightness and high purity have attracted more and more attention. Due to the strong optical confinement, the micro/nano waveguides reduce the pump power and improve the quality of generated photon pairs, which is helpful to realize the integrated optical path of quantum information processing. In summary, this paper reviews the recent development of second-order nonlinear optical effects in LNTF micro/nano waveguides. Firstly, the theory of second harmonic conversion is introduced, and then the fabrication and periodically poled techniques of LNTF micro/nano waveguides are discussed. As an example, the efficient SHG and parametric down conversion of LNTF waveguides were detailedly demonstrated. The processes of mode phase matching and quasi-phase matching technics, tunable SHG and wideband frequency doubling effect were emphatically investigated.

Broadband second-harmonic generation in step-chirped periodically poled lithium niobate waveguides.

Periodically poled lithium niobate (PPLN) structures on a chip enable efficient second-order nonlinear optical effects, benefiting from the tight light confinement and the utilization of d33. Here, we report a broadband second-harmonic (SH) generation in a step-chirped PPLN waveguide on X-cut lithium niobate on insulator (LNOI). The high fidelity of the poling period is demonstrated over the whole length of 7 mm using a non-destructive technique of piezoresponse force microscopy. The SH signal was continuously observed in the step-chirped PPLN waveguides while scanning the wavelength of the pump laser from 1550 nm to 1660 nm. The SH conversion efficiency was measured to be 9.6 % W-1 cm-2 at 1642 nm. This work will benefit wavelength conversions of light sources with wideband spectra.

Boltzmann vs. non-Boltzmann (non-linear) thermometry - Yb3+-Er3+ activated dual-mode thermometer and phase transition sensor via second harmonic generation

Second harmonic generation (SHG) and upconversion luminescence (UCL), as second-order nonlinear optical (NLO) effects, are crucial in modern optics and optoelectronics. Here, through the chemical engineering process we obtained a series of optically active BaTiO3: Er3+, Yb3+ perovskite materials, exhibiting temperature-dependent NLO responses. Upon 976 nm laser light irradiation, the micron-sized, polycrystalline samples show simultaneously intense UCL and SHG effects. Comparison of the thermometric features based on luminescence thermometry, i.e., band intensity ratio based on thermally-coupled levels of Er3+, and NLO thermometry, i.e., SHG/UCL intensity ratio, shows that the superior thermal sensitivity (≈ 4%/K, at ~400 K) and excellent temperature resolution (δT = 0.07 K) is achieved for the NLO thermometry approach. Furthermore, the combination of NLO spectroscopy with UCL allows rapid, more precise and clear phase transition detection (from tetragonal to cubic phase at around 386 K), compared to conventional structural methods, such as X-ray diffraction and Raman spectroscopy. Our work suggests that some non-Boltzmann, non-linear thermometers can provide much more effective temperature readouts (compared to the commonly studied Boltzmann thermometers), simultaneously working as accurate, rapid and non-invasive phase transition sensors.

Thermally crosslinkable second-order nonlinear optical polymer networks: high stability, good transparency, and large second-order nonlinear optical effects

A new in situ thermal crosslinking approach was proposed to design second-order nonlinear optical polymer networks through a simple synthetic route, with the combination of the “isolate chromophore” concept.

Two host-guest grown ether supramolecules show switchable phase transition, dielectric and second-harmonic generation effect.

The excellent properties of host-guest crown ether inclusions in phase transition, dielectric and second-order nonlinear optical properties have attracted much attention. In this paper, we successfully designed and prepared two novel host-guest crown ether supramolecules [(DFBA)(15-crown-5)]X (X = ClO4-, 1; ReO4-, 2) by reactions of 2,6-difluorobenzylamine (DFBA) with 1,4,10,13-pentaoxacyclopentadecane (15-crown-5) in HClO4, or HReO4 aqueous solution. By the introduction of difluoro-substituted benzylamine as a guest cation, the phase transition temperatures are greatly increased to 377 K for 1 and 391 K for 2. More importantly, the space group of 1 has changed from centrosymmetric (CS) P2/c to the non-centrosymmetric (NCS) Pca21 in 2 when substituting perchlorate (ClO4-) with the larger and heavier perrhenate (ReO4-), which leads to 2 showing a switchable and stable second-harmonic generation (SHG) effect. According to the principle of momentum matching between a cation and anion, the perrhenate group increases the energy barrier of the molecular thermal motion, which not only significantly increases the phase transition temperature of 2 but also causes it to be frozen and crystallized in a NCS space group at room temperature. This research demonstrates that a polar molecule can adjust the suitability of anions and cations inside the crystal by practical chemical means.

Halogen-Ion-Induced Structural Phase Transition Giving a Polymorph of HgBr2 with Balanced Nonlinear Optical Properties.

The key to developing new infrared nonlinear optical (NLO) materials is balancing second-order nonlinear optical effects and the laser-induced damage threshold (LIDT). In this paper, a new polymorph of HgBr2 (P212121) was synthesized by a "halogen-ion-induced effect" in solution, which features a crystal structure different from that of the original phase (Cmc21) and exhibits better NLO properties. Its powders show a strong SHG effect (9 × KDP), a high LIDT (30 × AgGaS2), a wide infrared transparent range, and stability in air, making it a prospective NLO material in the IR region. In addition, the above excellent NLO characteristics are well illustrated in DFT theoretical calculations. More importantly, experimental results show that the new infrared NLO polymorph with excellent comprehensive properties could be controllably obtained by using the halogen-ion-induced strategy.

Second Harmonic Generation in Germanium Quantum Wells for Nonlinear Silicon Photonics

Second-harmonic generation (SHG) is a direct measure of the strength of second-order nonlinear optical effects, which also include frequency mixing and parametric oscillations. Natural and artificial materials with broken center-of-inversion symmetry in their unit cell display high SHG efficiency, however, the silicon-foundry compatible group IV semiconductors (Si, Ge) are centrosymmetric, thereby preventing full integration of second-order nonlinearity in silicon photonics platforms. Here we demonstrate strong SHG in Ge-rich quantum wells grown on Si wafers. Unlike Si-rich epilayers, Ge-rich epilayers allow for waveguiding on a Si substrate. The symmetry breaking is artificially realized with a pair of asymmetric coupled quantum wells (ACQW), in which three of the quantum-confined states are equidistant in energy, resulting in a double resonance for SHG. Laser spectroscopy experiments demonstrate a giant second-order nonlinearity at mid-infrared pump wavelengths between 9 and 12 μm. Leveraging on the strong intersubband dipoles, the nonlinear susceptibility χ(2) almost reaches 105 pm/V, 4 orders of magnitude larger than bulk nonlinear materials for which, by the Miller’s rule, the range of 10 pm/V is the norm.