On-chip erbium-ytterbium-co-doped lithium niobate microdisk laser with an ultralow threshold.

Erbium-ion-doped lithium niobate (LN) microcavity lasers working in the communication band have attracted extensive attention recently. However, their conversion efficiencies and laser thresholds still have significant room to improve. Here, we prepared microdisk cavities based on erbium-ytterbium-co-doped LN thin film by using ultraviolet lithography, argon ion etching, and a chemical-mechanical polishing process. Benefiting from the erbium-ytterbium co-doping-induced gain coefficient improvement, laser emission with an ultralow threshold (∼1 µW) and high conversion efficiency (1.8 × 10-3%) was observed in the fabricated microdisks under a 980-nm-band optical pump. This study provides an effective reference for improving the performance of LN thin-film lasers.

On-chip ytterbium-doped lithium niobate waveguide amplifiers with high net internal gain.

Integrated optical systems based on lithium niobate on insulator (LNOI) have shown great potential in recent years. However, the LNOI platform is facing a shortage of active devices. Considering the significant progress made in rare-earth-doped LNOI lasers and amplifiers, the fabrication of on-chip ytterbium-doped LNOI waveguide amplifiers based on electron-beam lithography and inductively coupled plasma reactive ion etching was investigated. The signal amplification at lower pump power (<1 mW) was achieved by the fabricated waveguide amplifiers. A net internal gain of ∼18 dB/cm in the 1064 nm band was also achieved in the waveguide amplifiers under a pump power of 10 mW at 974 nm. This work proposes a new, to the best of our knowledge, active device for the LNOI integrated optical system. It may become an important basic component for lithium niobate thin-film integrated photonics in the future.

Integrated ytterbium-doped lithium niobate microring lasers.

Integrated and stable microlasers are indispensable building blocks of micro-photonics. Here, we report the realization of an ytterbium-doped lithium niobate microring laser operating in the 1060-nm band under the pump of a 980-nm-band laser. The monolithic laser has a low threshold of 59.32 µW and relatively high output power of 6.44 µW, a state-of-the-art value for rare-earth ions-doped lithium niobate thin-film lasers. The monolithic laser with desirable performance and attractive scalability may find many applications in lithium niobite photonics.

Intense Luminescence and Good Thermal Stability in a Mn2+-Activated Mg-Based Phosphor with Self-Reduction.

White light-emitting diodes provide widespread applications in lighting, electronic equipment, and high-tech displays. However, thermal quenching effect severely limits their practical application. Here, we developed an orange-red phosphor ß-KMg(PO3)3:Mn2+, which emits bright orange-red light when excited by ultraviolet light without the energy transfer of sensitizer, owing to the strong crystal field provided by ß-KMg(PO3)3 for Mn2+. The self-reduction of Mn4+ → Mn2+ and good thermal stability have been realized in an ambient atmosphere. The defect types were verified by X-ray photoelectron spectroscopy, and cationic vacancy plays a significant role in the self-reduction of Mn4+ → Mn2+. Furthermore, the properties of the trap energy levels were studied by thermoluminescence. The recombination luminescence of the detrapped carriers released from the deep trap levels at high temperatures suppresses the luminescence loss of thermal quenching. Moreover, the trap energy levels play an important role in the mechanoluminescence of ß-KMg(PO3)3:Mn2+. This work emphasizes the significance of the defects in the luminescent characteristics and opens up a new approach for the development of advanced optical functional materials.

On-chip ytterbium-doped lithium niobate microdisk lasers with high conversion efficiency.

Integrated optical systems based on lithium niobate on insulator (LNOI) have attracted the interest of researchers. Recently, erbium-doped LNOI lasers have been realized. However, the reported lasers have a relatively lower conversion efficiency and only operate in the 1550 nm band. In this paper, we demonstrate an LNOI laser operating in the 1060 nm band based on a high Q factor ytterbium-doped LNOI microdisk cavity. The threshold and the conversion efficiency of the laser are 21.19 µW and 1.36%, respectively. To our knowledge, the conversion efficiency is the highest among the reported rare-earth-doped LNOI lasers. This research extends the operating band of LNOI lasers and shows the potential in realizing high-power LNOI lasers.

Self-Reduction-Related Defects, Long Afterglow, and Mechanoluminescence in Centrosymmetric Li2ZnGeO4:Mn2.

Mechanoluminescent materials have shown great application potential in the fields of stress detection, anti-counterfeiting, and optical storage; however, its development is hindered by the unclear mechanism. Different from the mainstream exploration of new mechanoluminescent materials in non-centrosymmetric structures, a centrosymmetric mechanoluminescent material Li2ZnGeO4:Mn2+ is synthesized by a standard high-temperature solid-state reaction in an ambient atmosphere. Combined with the Rietveld refinement, photoluminescence, electron spin resonance, and X-ray photoelectron spectroscopy, it is proved that the increase in oxygen vacancies is accompanied by the self-reduction process from Mn4+ to Mn2+, and the mechanism of mechanoluminescence is clarified through the afterglow and thermoluminescence spectra. The carriers trapped by the shallow traps participate in the mechanoluminescence process through the tunneling effect, while the carriers trapped by the deep traps take part in the mechanoluminescence process via conduction band or tunneling. A signature anti-counterfeiting application is designed using the new mechanoluminescent material Li2ZnGeO4:0.004Mn2+. Utilizing the afterglow characteristics of Li2ZnGeO4:xMn2+ phosphors, we designed an intelligent long-persistent luminescence quick response code (QR-code) and visualized information encoding/decoding model, which provides a fast, simple, and effective method for information encryption, transformation, and dynamic anti-counterfeiting. This study not only analyzes the self-reduction and mechanoluminescence processes in detail but also breaks the limitation of crystal symmetry and provides a new strategy for the exploration of novel mechanoluminescent materials.

On-chip erbium-doped lithium niobate microring lasers.

Lithium niobate on insulator (LNOI), regarded as an important candidate platform for optical integration due to its excellent nonlinear, electro-optic, and other physical properties, has become a research hotspot. A light source, as an essential component for an integrated optical system, is urgently needed. In this Letter, we reported the realization of 1550 nm band on-chip LNOI microlasers based on erbium-doped LNOI ring cavities with loaded quality factors higher than 1 million at ∼970nm, which were fabricated by using electron beam lithography and inductively coupled plasma reactive ion etching processes. These microlasers demonstrated a low pump threshold of ∼20µW and stable performance under the pump of a 980 nm band continuous laser. Comb-like laser spectra spanning from 1510 to 1580 nm were observed in a high pump power regime, which lays the foundation of the realization of pulsed laser and frequency combs on a rare-earth ion-doped LNOI platform. This Letter effectively promotes the development of on-chip integrated active LNOI devices.

Construction of a novel mechanoluminescent phosphor Li2MgGeO4:xMn2+ by defect control.

Lattice defect plays a significant role in the optical properties of elastic mechanoluminescent materials, which could be modulated by cationic non-equivalent replacement. Here, a series of novel mechanoluminescent phosphors Li2-xMgGeO4:xMn2+ (0 ≤ x ≤ 0.025) were synthesized via a high-temperature solid-state reaction method in an ambient atmosphere. The defect type and its relationship with optical perfomance were clarified via X-ray photoelectron spectroscopy, electron spin resonance, and thermoluminescent spectroscopy. Along with the introduction of Mn ions, the trap levels of oxygen vacancies become shallow, which are beneficial to produce long afterglow and mechanoluminescence. This study offers a feasible approach for developing new functional materials via defect control in self-reduction systems.

Correction: Saeed, S., et al. Enhancement of Photorefraction in Vanadium-Doped Lithium Niobate through Iron and Zirconium Co-Doping. Materials 2019, Vol. 12, 3143.

The authors wish to make the following corrections to this paper [...].

Interaction between Mo and intrinsic or extrinsic defects of Mo doped LiNbO3 from first-principles calculations.

Lithium niobate (LiNbO3, LN) plays an important role in holographic storage, and molybdenum doped LiNbO3 (LN:Mo) is an excellent candidate for holographic data storage. In this paper, the basic features of Mo doped LiNbO3, such as the site preference, electronic structure, and the lattice distortions have been explored from first-principles calculations. Mo substituting Nb with its highest charge state +6 is found to be the most stable point defect form. The energy levels formed by Mo with different charge states are distributed in the band gap, which are responsible for the absorption in the visible region. The transition of Mo in different charge states implies molybdenum can serve as a photorefractive center in LN:Mo. In addition, the interactions between Mo and intrinsic or extrinsic point defects are also investigated in this work. Intrinsic defects [Formula: see text] could cause the movement of the [Formula: see text] energy levels. The exploration of Mo, Mg co-doped LiNbO3 reveals that although Mg ion could not shift the energy level of Mo, it can change the distribution of electrons in Mo and Mg co-doped LN (LN:Mo,Mg) which help with the photorefractive phenomenon.

Correction: p-Type conductivity mechanism and defect structure of nitrogen-doped LiNbO3 from first-principles calculations.

Correction for 'p-Type conductivity mechanism and defect structure of nitrogen-doped LiNbO3 from first-principles calculations' by Weiwei Wang et al., Phys. Chem. Chem. Phys., 2020, 22, 20-27.

Recent Progress in Lithium Niobate: Optical Damage, Defect Simulation, and On-Chip Devices.

Lithium niobate (LN) is one of the most important synthetic crystals. In the past two decades, many breakthroughs have been made in material technology, theoretical understanding, and application of LN crystals. Recent progress in optical damage, defect simulation, and on-chip devices of LN are explored. Optical damage is one of the main obstacles for the practical usage of LN crystals. Recent results reveal that doping with ZrO2 not only leads to better optical damage resistance in the visible but also improves resistance in the ultraviolet region. It is still awkward to extract defect characteristics and their relationship with the physical properties of LN crystals directly from experimental investigations. Recent simulations provide detailed descriptions of intrinsic defect models, the site occupation of dopants and the variation of energy levels due to extrinsic defects. LN is considered to be one of the most promising platforms for integrated photonics. Benefiting from advances in smart-cut, direct wafer bonding and layer transfer techniques, great progress has been made in the past decade for LNs on insulators. Recent progress on on-chip LN micro-photonic devices and nonlinear optical effects, in particular photorefractive effects, are briefly reviewed.

Microdisk resonators with lithium-niobate film on silicon substrate.

In this paper, we report the fabrication of lithium niobate (LN) microdisk resonators on a pulsed-laser deposited polycrystalline LN film on a silicon substrate rather than commercially provide LN film on insulator. The quality factor of these polycrystalline LN microdisks were measured above 3.4×104 in the 1550-nm band. Second harmonic generation was demonstrated in the fabricated microresonators. Because the properties of homemade LN film can be easily tuned by doping various ions, LN devices on homemade LN film may have more flexible functions and broad applications.

Linear Tuning of Phase-Matching Temperature in LiNbO3:Zr Crystals by MgO Co-Doping.

We grew a series of co-doped LiNbO3 crystals with fixed 1.5 mol % ZrO2 and various MgO concentrations (1.0, 3.0, 4.0, 6.0 mol %), and investigated their optical properties and defect structures. By 3.0 mol % MgO co-doping, the optical damage resistance at 532 nm reached 6.5 × 106 W/cm2, while the phase-matching temperature for doubling 1064 nm was only 29.3 °C-close to room temperature-which was conducive to realizing the 90° phase matching at room temperature by slightly modulating the incident angle of the fundamental beam. Notably, we found that the phase-matching temperature increased linearly with the increase of MgO doping, and this linear dependence helped us to grow the high-quality crystal for room temperature 90° phase matching. Moreover, the defect analysis indicated that the linear tuning of phase-matching temperature might be attributed to Mg Li + + Zr Nb - neutral pairs in crystals.

p-Type conductivity mechanism and defect structure of nitrogen-doped LiNbO3 from first-principles calculations.

Most metal-doped lithium niobates (LiNbO3, LN) exhibit n-type conductivity. The absence of p-type conductive LiNbO3 limits its application. Based on the finding that p-type conductive LiNbO3 can be realized by doping with a non-metallic element N, we investigate the most stable defect configurations and formation energies of LiNbO3 doped with non-metal nitrogen (LN:N) by first-principles calculations. Nitrogen substitution, interstitial and quasi-substitution point defects in different sites and their effects were explored. The results show that N prefers to occupy the oxygen site with only little lattice distortion. Ab initio molecular dynamics (AIMD) simulations confirm the structural stability of an N ion occupying the O site. The charge-state transition level ε(0/-1) slightly above the valence band maximum (VBM) indicates that N point defects would contribute to p-type conductivity of LiNbO3. The analysis of the band structure reveals that the partially filled impurity levels can accommodate electrons that jump from valence bands and result in holes to become the main charge carriers. The calculation not only explains the occurrence of p-type conductivity in LN:N but also provides a simple and efficient way to discover p-type conductive candidates in numerous doped LiNbO3 crystals.

Enhancement of Photorefraction in Vanadium-Doped Lithium Niobate through Iron and Zirconium Co-Doping.

A series of mono-, double-, and tri-doped LiNbO3 crystals with vanadium were grown by Czochralski method, and their photorefractive properties were investigated. The response time for 0.1 mol% vanadium, 4.0 mol% zirconium, and 0.03 wt.% iron co-doped lithium niobate crystal at 488 nm was shortened to 0.53 s, which is three orders of magnitude shorter than the mono-iron-doped lithium niobate, with a maintained high diffraction efficiency of 57% and an excellent sensitivity of 9.2 cm/J. The Ultraviolet-visible (UV-Vis) and OH- absorption spectra were studied for all crystals tested. The defect structure is discussed, and a defect energy level diagram is proposed. The results show that vanadium, zirconium, and iron co-doped lithium niobate crystals with fast response and a moderately large diffraction efficiency can become another good candidate material for 3D-holographic storage and dynamic holography applications.

Fabrication and Characteristics of Heavily Fe-Doped LiNbO3/Si Heterojunction.

A series of heavily Fe-doped LiNbO3 (LN:Fe) crystals were grown via the Czochralski method. The dark- and photo-conductivity of the 5.0 wt.% Fe-doped LiNbO3 crystal reached 3.30 × 10-8 Ω-1 cm-1 and 1.46 × 10-7 Ω-1 cm-1 at 473 nm, which are about 7 and 5 orders of magnitude higher than that of congruent LiNbO3, respectively. Then, a p-n heterojunction was fabricated by depositing the heavily Fe-doped LiNbO3 on a p-type Si substrate using the pulsed laser deposition. The current-voltage curve of the LN:Fe/Si heterojunction presents a well-defined behavior with a turn-on voltage of 2.9 V. This LN:Fe/Si heterojunction gives an excellent prototype device for integrated optics and electro-photonics.

Synthesis, structure and characterization of M(IO3)2(HIO3) (M = Ca, Sr) as new anhydrous alkaline earth metal bis-iodate hydrogeniodate compounds.

Two new metal iodates, namely, M(IO3)2(HIO3) (M = Ca, Sr), were successfully synthesized by the hydrothermal method, and their structures were determined by single-crystal X-ray diffraction. They were found to be isostructural, and crystallized in the monoclinic space group P21/n with the lattice parameters: a = 6.9647(14) Å, b = 15.719(3) Å, c = 7.2042(14) Å, ß = 92.76(3)°, Z = 4 for Ca(IO3)2(HIO3), and a = 7.0697(14) Å, b = 15.986(3) Å, c = 7.3802(15) Å, ß = 93.13(3)°, Z = 4 for Sr(IO3)2(HIO3). The structure featured a [HIO3]·[IO3]- complex with strong intermolecular interactions, and it was composed of [IO3]- anions and [HIO3]·[IO3]- complexes interconnected by Ca2+ or Sr2+ cations. The two compounds were characterized by X-ray diffraction, IR and Raman spectroscopies, UV-Vis diffuse reflectance spectroscopy, thermogravimetry and differential scanning calorimetry as well as by theoretical calculations.

P-Type Lithium Niobate Thin Films Fabricated by Nitrogen-Doping.

Nitrogen-doped lithium niobate (LiNbO3:N) thin films were successfully fabricated on a Si-substrate using a nitrogen plasma beam supplied through a radio-frequency plasma apparatus as a dopant source via a pulsed laser deposition (PLD). The films were then characterized using X-Ray Diffraction (XRD) as polycrystalline with the predominant orientations of (012) and (104). The perfect surface appearance of the film was investigated by atomic force microscopy and Hall-effect measurements revealed a rare p-type conductivity in the LiNbO3:N thin film. The hole concentration was 7.31 × 1015 cm-3 with a field-effect mobility of 266 cm²V-1s-1. X-ray Photoelectron Spectroscopy (XPS) indicated that the atom content of nitrogen was 0.87%; N atoms were probably substituted for O sites, which contributed to the p-type conductivity. The realization of p-type LiNbO3:N thin films grown on the Si substrate lead to improvements in the manufacturing of novel optoelectronic devices.

Analysis of the structure and abnormal photoluminescence of a red-emitting LiMgBO3:Mn2+ phosphor.

Mn2+ is attracting increasing attention due to its potential application as an activated ion in the phosphors. In this study, a rare-earth free phosphor, LiMgBO3:Mn2+, was synthesized using a high temperature solid-state reaction. Combined with X-ray diffraction and PL/PLE as well as first-principles calculation, the coordination environment of Mn2+ was disclosed, revealing that Mn2+ ions were inclined to occupy two partially filled Li sites. Under 427 nm-excitation, an abnormal deep red emission band centered at 705 nm was observed. This was due to the strong crystal field environment of Mn2+ induced by the extremely distorted five-coordinated sites, which was proved by the electronic band structure obtained from the density functional theory calculations. In addition, the luminescence intensity and thermal stability of LiMgBO3:Mn2+ was significantly enhanced when Li+ was replaced by Na+ and K+. This study disclosed the origin of the abnormal luminescence of Mn2+ in LiMgBO3 and further enriched our understanding of the abnormal luminescence of Mn2+ in special crystal structures (five-coordinated configuration).