Deep-ultraviolet Nonlinear Optical Materials Research Articles

A perspective on deep-ultraviolet nonlinear optical materials

Deep ultraviolet (DUV) nonlinear optical (NLO) crystals have important applications in lithography, microfabrication, and high-resolution photoelectric spectrometer. In recent years, significant progress has been made in both the theoretical design and experimental research of DUV NLO crystals. This review aims to offer a comprehensive perspective on exploring a new generation of DUV NLO materials. First, we summarize various computer-aided strategies for crystal structure design and emphasize their significant role in advancing the discovery of DUV NLO materials. Then, we outline several representative DUV crystals of experimental synthesis. Finally, we discuss the future prospects for exploring new generations of DUV NLO materials. We believe that employing data-driven, computer-aided methods to explore DUV NLO materials will help address the current challenges in the field of DUV NLO materials research. The close integration of calculation and experimentation will unlock new opportunities.

Ba4B14O25: A Deep Ultraviolet Transparent Nonlinear Optical Crystal with Strong Second Harmonic Generation Response Achieved by a Boron-Rich Closed-Loop Strategy.

Discovering new deep ultraviolet (DUV) nonlinear optical (NLO) materials is the current research hotspot. However, how to perfectly integrate several stringent performances into a crystal is a great challenge because of the natural incompatibility among them, particularly wide band gap and large NLO coefficient. To tackle the challenge, a boron-rich closed-loop strategy is supposed, based on which a new barium borate, Ba4B14O25, is designed and synthesized successfully via the high-temperature solid-state melting method. It features a highly polymeric 3D geometry with the closed-loop anionic framework [B14O25]8- constructed by the fundamental building blocks [B14O33]24-. The high-density π-conjugated [BO3]3- groups and the fully closed-loop B-O-B connections make Ba4B14O25 possess excellent NLO properties, including short UV cutoff edge (<200nm), large second harmonic generation response (3.0 × KDP) and phase-matching capability, being a promising DUV-transparent NLO candidate material. The work provides a creative design strategy for the exploration of DUV NLO crystals.

Enlargement of Bandgap and Birefringence in Nonlinear Optical Alkali‐Metal Sulfate Crystals by the Substitution of Asymmetrical Non‐π‐Conjugated Cation

AbstractWide bandgap and large birefringence, as well as strong second harmonic effect, are important but often mutually restraint prerequisites for deep‐ultraviolet (DUV) nonlinear optical (NLO) crystals, a type of photoelectronic materials that play an indispensable role in the DUV laser generation. For the long term, the search for new DUV NLO crystals has focused on the alkali‐metal inorganic acid salts where the variations of anionic groups are considered. Herein, another design strategy is reported, in which the alkali‐metal cations are substituted by the asymmetrical non‐π‐conjugated cations to harmonize the above mutual restrictions. Accordingly, LiN2H5SO4 (LNHSO) crystal is obtained by the substitution of [N2H5]+ for alkali‐metal ions in LiMSO4 (M = Na, K, and Rb). Compared with LiMSO4 (M = Na, K, and Rb), LNHSO exhibits a moderate phase‐matching second‐harmonic generation response (0.6 × KH2PO4), increased birefringence (two to three times that of other sulfates), and the widest bandgap (7.69 eV) among all the reported NLO sulfates. The structural analysis and theoretical calculation reveal that the structural anisotropy, arrangement, and packing configuration of the novel NLO‐active [N2H5]+ cation in LNHSO effectively promotes the optical properties and enlarges the bandgap. This work provides an innovative perspective for the rational design and synthesis of DUV NLO materials with excellent performance.

Theoretical investigations on ternary defective diamond-like infrared nonlinear optical materials in Be-Ga-Se system

Beryllium (Be) has made great success in developing Be-based oxide deep-ultraviolet (DUV) nonlinear optical (NLO) materials, but the development of Be-based chalcogenide infrared (IR) NLO materials is still tardy because of the lack of rational material design strategy and in-depth understanding on the composition-structure-propertiy relationships in the compounds. In this work, by coupling [BeSe4] with NLO-active [GaSe4] tetrahedral groups, one thermodynamically stable and three metastable defective diamond-like (DL) BeGa2Se4 (BeGSe) compounds, BeGSe-I (I4¯ 2 m), BeGSe-II (I4¯), BeGSe-III (Pc) and BeGSe-IV (P4¯ 2 m), have been predicted by Universal Structure Predictor: Evolutionary Xtallography (USPEX) code and phonon dispersion spectra. Notably, BeGSe-II and BeGSe-III show high thermal conductivities (6.520 W/m/K for BeGSe-II, 3.835 W/m/K for BeGSe-III), and achieve a good balance between wide band gap (Eg ≥ 3.0 eV) and large NLO effect (≥ 1.0 × AGS) for an excellent IR NLO material. Meanwhile, the charge transfer enhanced band gap mechanism, and covalent radius, bulk modulus effected lattice thermal conductivity in the defective DL-AGa2Se4 (A = Be, Mg, Zn, Cd, Hg) were also uncovered. The results give insights into the design of new defective DL compounds with wide band gap and high thermal conductivity based on [BeSe4] tetrahedron.

Strategy for a Rational Design of Deep-Ultraviolet Nonlinear Optical Materials from Zeolites.

Deep-ultraviolet (DUV) nonlinear optical (NLO) materials play a crucial role in cutting-edge laser technology. In order to solve the serious layered growth tendency of the sole commercial DUV NLO crystal KBe2BO3F2 (KBBF), developing alternative systems of compounds with bulk crystal habits has become an urgent task for practical applications. Herein, a novel strategy was developed by applying non-centrosymmetric (NCS) cancrinite (CAN)-type zincophosphates {Na6(OH)2(H2O)2}Cs2[ZnPO4]6 with bulk-crystal habits as the prototype to design new DUV NLO crystals. Two new anhydrous alkali zincophosphates, namely, {(Li6-xNaxO)A2}[(ZnPO4)6] (A = Cs, Rb; x = 2-3) crystallizing in the NCS hexagonal space group P63 (no. 173) with a CAN-type framework, were successfully synthesized via a modified fluoro-solvo-hydrothermal method by applying triethylamine (TEA) and concentrated NaF solution as a co-solvent. Interestingly, the rigidity of the NCS CAN-type framework acting as the host ensures the non-centrosymmetry of the resulting new compounds. Meanwhile, the replacement of water molecules by guest cationic species in the channels or cages can greatly improve the thermal stability of the resultant crystal and tune its NLO properties. The synergetic effect of the host framework and the guest species makes the two compounds transparent down to the DUV region (<200 nm) and exhibit SHG effects. Therefore, the proposed rational design strategy of applying the known zeotype NCS frameworks as prototypes together with the modified fluoro-solvo-hydrothermal method opens a great avenue for highly effectively exploring new DUV NLO materials.

Na4[B6O9 (OH)3](H2O) Cl: A Deep-Ultraviolet Transparent Nonlinear Optical Borate Chloride with {[B6O9 (OH)3]3-}∞ Chains.

The development of nonlinear-optical (NLO) crystals with short ultraviolet cutoff edges is significant and challenging. Here, a new sodium borate chloride, Na4[B6O9 (OH)3](H2O) Cl, was successfully obtained by the mild hydrothermal method, which crystallizes in a polar space group Pca21. The structure of the compound is characterized by {[B6O9 (OH)3]3-}∞ chains. Measurements of optical properties indicate the compound exhibits a deep-ultraviolet (DUV) cutoff edge (≤200 nm) and moderate second harmonic generation response (0.4 × KH2PO4). It presents the first DUV hydrous sodium borate chloride NLO crystal and the first sodium borate chloride possessing a one-dimensional B-O anion framework. Probing into the connection of structure and optical properties has been performed based on theoretical calculations. These results are instructive for designing and obtaining new DUV NLO materials.

Prediction of Promising Deep Ultraviolet Nonlinear Optical Materials in the Li2O–B2O3 System

Deep ultraviolet (UV) nonlinear optical (NLO) materials are of intense interest owing to their broad technological applications from laser photolithography to semiconductor manufacturing, biomedicine, and attosecond pulse generation. To date, deep UV NLO materials are still scarce due to the multiple rigorous criteria that should be fulfilled. Using evolutionary crystal structure prediction combined with first-principles calculations, we systematically explored the pseudobinary Li2O–B2O3 system and predicted twenty-four metastable structures in the Li2O–B2O3 system. We found that four structures of LiBO2, e.g., oC16, oP16-1, oP16-2, and mP16, are promising deep UV NLO crystals. Among them, oC16-LiBO2, oP16-2-LiBO2, and mP16-LiBO2 exhibit significantly large SHG coefficients (>6 KDP), moderate birefringence (∼0.12@1064 nm), and suitable band gaps (∼7 eV). Importantly, the oC16-LiBO2 structure is expected to be synthesized experimentally as its formation energy is lower than that of γ-LiBO2.

A Cation-Driven Approach toward Deep-Ultraviolet Nonlinear Optical Materials.

The design of new materials with special performances is still a great challenge, especially for the deep-ultraviolet nonlinear optical materials in which it is difficult to balance large bandgaps and strong second harmonic generation responses due to their inverse relationship. Cation variation not only influences the whole structure frameworks but also directly participates in the formation of electronic structures, both of which could lead to the uncontrollability of the properties of the designed materials. Here, a novel approach, aiming at purposeful and foreseeable material designs, is proposed to characterize the role of cations. By the verification of several series of borates, the influences of cation variation on property changes are explored systematically. Accordingly, a feasible strategy of designing deep-ultraviolet nonlinear optical materials by substituting barium for lead has been concluded, which could obviously blue-shift the ultraviolet cutoff edge and maintain the relatively strong second harmonic generation response (more than 2 times of KH2PO4), achieving the property optimization, and especially works efficiently in fluorooxoborates. The property optimization design strategy and the cation characterization method are not only helpful in exploring nonlinear optical materials but also enlightening in material design and selection.

Design of deep-ultraviolet nonlinear optical materials based on fundamental building blocks with excellent microscopic properties

Here, the [B3O6F] group is considered to possess excellent microscopic properties. Accordingly, a deep-ultraviolet nonlinear optical material: Li2B3O5F is designed.

Defect engineering of second-harmonic generation in nonlinear optical semiconductors

Second-harmonic generation (SHG) plays a key role in developing modern optical devices and quantum optics, as they can expand the wavelength range provided by common laser sources. Conventionally, the modulation of SHG in nonlinear optical (NLO) materials heavily depends on various external effects. In this article, by developing the basic rules that comprise the interplay between defects and the SHG effect, we propose an alternative strategy to intrinsically regulate the SHG of NLO materials via defect engineering, which is feasible via controlling the growth conditions. Using first-principles calculations, we demonstrate that the SHG susceptibility of KBe 2 BO 3 F 2 (KBBF), the only available deep-ultraviolet NLO material with a weak SHG effect, can be dramatically enhanced by nearly one order of magnitude under specific growth conditions. This is comparable to many other famous narrow-band gap NLO crystals. In general, our developed basic rules and material demonstration could be extended to other NLO systems. • The SHG of NLO materials can be intrinsically regulated via defect engineering • The SHG is affected by the in-gap defect level and the concentration of defects • The SHG of KBBF could be dramatically enhanced by nearly one order of magnitude Conventionally, the modulation of second-harmonic generation (SHG) in nonlinear optical (NLO) materials heavily depends on various external factors. By developing basic rules that consider the interplay between defects and the SHG effect, Li et al. propose a strategy to intrinsically regulate the SHG of NLO materials via defect engineering.

Design of novel deep-UV nonlinear optical materials with one-dimensional functional module [BO2]∞ chain and fluorine-driven short phase-matching

Design of deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials is urgently needed owing to the lack of high-performance crystals to meet the contradictory criteria of deep-UV transparency, strong NLO response and moderate birefringence to reach deep-UV phase-matching. In this work, a “highly-polymerized units optimizing electron delocalization” strategy was proposed to explore novel types of deep-UV NLO anionic functional module and design new materials. Two novel NLO systems with new NLO anionic functional module-one dimensional [BO2]∞ chains were successfully designed. In total, four LiBO2 (I-IV) and five Li2BO2F (I–V) structures with good thermodynamically and dynamically stable/metastable were obtained by using evolutionary algorithm crystal structure prediction techniques. All these structures contain the [BO2]∞ infinite chains and have deep-UV transparency (< 177.3 nm). LiBO2 (II, IV) and Li2BO2F (I, IV-V) exhibit large NLO response (2.0–2.3 × KH2PO4) and large birefringence (0.093–0.146 @1064 nm). Remarkably, Li2BO2F–I possesses an extremely short phase-matching wavelength about 159 nm, which is beneficial from the introduction of fluorine to maintain the superior functional module [BO2]∞ and optimize the optical properties. The results indicate that [BO2]∞ can be as a new deep-UV NLO functional module, this work provides an insight into the design of new deep-UV compound with outstanding NLO performances.

The Microscopic Origin of Second Harmonic Generation Response: The Spatial Structure of Instantaneous Dipole Moments in Electron Excitation.

A deep understanding on the crucial factors of the enhanced macroscopic second harmonic generation (SHG) in some deep-ultraviolet nonlinear optical (NLO) materials is needed to design new NLO materials. Since an optical process relates to the electron excitation and polarization simultaneously, the instantaneous dipole moments and their structures in excitation should be seriously taken account to seek the principal factor in SHG response. In this work, we study the Ba4 B11 O20 F (BBOF), a NLO material, by using the orbital projection technique. From the projected SHG of our theoretic calculation, we recognize the principal dipole moment of the dominant influence on SHG and the relevant atom groups between which the dipole moment is accommodated. With the conclusion that the dipole moment with the most significant influence on SHG is the one between the oxygen-boron polyhedral anion group and barium cation, we predict that Ba4 Al11 O20 F (BAOF) has a comparable SHG response.

An Optimized KBe2BO3F2‐Like Structure: The Unity of Deep‐Ultraviolet Transparency, Nonlinear Optical Property, and Ferroelectricity

AbstractAt present, only KBe2BO3F2 (KBBF) can effectively realize a deep‐ultraviolet (DUV) coherent output, while the layered growth habit seriously hinders its application. Here, a new design strategy with an optimized KBBF‐like structure based on the quasi‐phase matching principle is proposed. Thus, the first KBBF‐like ferroelectric, Li2KRb(SO4)2, is designed. It exhibits a short DUV absorption cutoff edge of 163 nm, a large nonlinear optical (NLO) effect of 1.7 × KH2PO4 under a 1064 nm laser, and typical ferroelectricity. More importantly, it completely eliminates the layered structure by replacing K‐F bonds with strong ‐Li‐O‐S‐ ones as the interlayer force. Therefore, this work finds a new promising DUV NLO candidate, a unity of the DUV transparency, NLO response, and ferroelectricity, which is advantageous to explore new DUV NLO materials.

Alkali metal sulfate: A new non-π-conjugated deep-ultraviolet quasi-phase matching crystal

Nonlinear optical crystals with ferroelectricity could realize frequency conversion based on the quasi-phase matching principal, which could effectively break the limitation of the inherent birefringence. Here, an alkali metal sulfate crystal, LiKSO4, is found, which combines a short absorption cutoff edge of 160 nm, a large second harmonic generation response of 4 × KH2PO4 and the typical ferroelectric polarization reversal into a single structure. Besides, it possesses an optimized Sr2Be2B2O7-like structure with -S-O-Li- bonds in the adjacent layers, which effectively avoids the layer growth tendency. Therefore, this manuscript provides the first sulfate deep-ultraviolet quasi-phase matching crystal, which opens a new horizon for the development of deep-ultraviolet nonlinear optical materials.

Sulfamide: A Promising Deep-Ultraviolet Nonlinear Optical Crystal Assembled from Polar Covalent [SO2 (NH2 )2 ] Tetrahedra.

For the first time, the polar covalent tetrahedron [SO2 (NH2 )2 ] is revealed as a new deep-ultraviolet (DUV) nonlinear optical (NLO)-active unit according to theoretical calculations. Furthermore, sulfamide consisting of polar [SO2 (NH2 )2 ] units was confirmed as an excellent candidate as a DUV NLO crystal. Sulfamide provides the optimal balance between composition, structure, and properties, in addition to a very short absorption of 160 nm. It achieves multiple optical performance records for non-π-conjugated DUV NLO materials, including the strongest second harmonic generation (SHG) efficiency (about 4 times that of KDP), the largest birefringence (obv.: 0.07@589.3 nm) and the shortest SHG wavelength predicted as 188 nm.

Sulfamide: A Promising Deep‐Ultraviolet Nonlinear Optical Crystal Assembled from Polar Covalent [SO2(NH2)2] Tetrahedra

AbstractFor the first time, the polar covalent tetrahedron [SO2(NH2)2] is revealed as a new deep‐ultraviolet (DUV) nonlinear optical (NLO)‐active unit according to theoretical calculations. Furthermore, sulfamide consisting of polar [SO2(NH2)2] units was confirmed as an excellent candidate as a DUV NLO crystal. Sulfamide provides the optimal balance between composition, structure, and properties, in addition to a very short absorption of 160 nm. It achieves multiple optical performance records for non‐π‐conjugated DUV NLO materials, including the strongest second harmonic generation (SHG) efficiency (about 4 times that of KDP), the largest birefringence (obv.: 0.07@589.3 nm) and the shortest SHG wavelength predicted as 188 nm.

The dependence of crystal structure and ultraviolet nonlinear optical property of KBe&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;BO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;F&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; by density functional theory and &amp;pi;-conjugate confinement

<p indent="0mm">KBe₂BO₃F₂ (KBBF) is currently the only commercialized deep-ultraviolet (DUV) nonlinear optical (NLO) crystal, and its crystal structure is one of the most-utilized templates for the rational design; therefore, the understanding and elaboration of its structure–property relationship has an important significance. Herein, on the basis of density-functional theory (DFT), the bonding mode and the electronic structure of the KBBF are deeply investigated, and the contributions of the component groups, <italic>i</italic>.<italic>e</italic>., the BO₃ π-conjugated group and the BeO₃F non-π-conjugated group, to the DUV–NLO properties are explored and explained. The “π-conjugated confinement” feature of the KBBF structure allows it to achieve the highly desired balance between the wide band-gap and the large birefringence, and eventually enables KBBF to achieve the second harmonic generation in the high-energy DUV-spectral region. These understanding and elaboration of the “π-conjugated confinement” may provide useful insights for the future design of the DUV–NLO materials.

Recent Development of Non-π-Conjugated Deep Ultraviolet Nonlinear Optical Materials

Recent Development of Non-π-Conjugated Deep Ultraviolet Nonlinear Optical Materials

A Lanthanum Ammonium Sulfate Double Salt with a Strong SHG Response and Wide Deep-UV Transparency.

The targeted synthesis of deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials, especially those with non-π-conjugated sulfates, has experienced considerable difficulties due to the need to reconcile the oft-competing requirements for deep-UV transparency and strong second-harmonic generation (SHG). We report herein the designed synthesis of the first rare-earth metal-based deep-UV sulfate La(NH4 )(SO4 )2 by a double-salt strategy involving introduction of complementary cations, together with optical studies that reveal a short-wavelength deep-UV absorption edge (below 190 nm) and the strongest SHG response among deep-UV NLO sulfates (2.4×KDP). Theoretical calculations and crystal structure analysis suggest that the excellent balance between SHG response and deep-UV transparency can be attributed to a synergistic interaction of the hetero-cations La3+ and [NH4 ]+ , which optimize alignment of the [SO4 ] tetrahedra and highly polarizable [LaO8 ] polyhedra.

Structure and Origin of the Second-Harmonic Generation Response of Nonlinear Optical Material Sr2Be2B2O7.

Sr2Be2B2O7 (SBBO) has long been considered as one of the most promising deep-ultraviolet nonlinear optical materials, but its crystal structure described by space group P6̅c2 in previous studies has remained questionable. On the basis of first-principles calculations coupled with the high-throughput crystal structure prediction method, we found three energetically favorable structures for SBBO with space groups Cm, Pm, and P6̅. These structures and a superstructure of space group Pm-S derived from the Cm structure were refined by the Rietveld method using the available powder X-ray diffraction data. These analyses show that the Pm-S structure is the best one, but its parent Cm structure is almost equally good and has the advantage of having higher symmetry. Via atom response theory analysis, we resolved the cause for the second-harmonic generation (SHG) responses of SBBO at the atomic and orbital level to elucidate the importance of local inversion symmetry in reducing the SHG response.