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.

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 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.

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 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.

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.

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.

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.

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.

Crystal Structure of High-Temperature Phase ß-NaSrBO3 and Photoluminescence of ß-NaSrBO3:Ce(3.).

α-NaSrBO3 is an excellent phosphor host for phosphor-converted white light-emitting diode (w-LED) application with very interesting properties. However, it undergoes a phase transformation to ß-NaSrBO3 at the LED working temperature. In this study, the high-temperature phase ß-NaSrBO3 was stabilized to room temperature by introducing Na(+) and Ce(3+) via a high-temperature solid-state reaction method. The crystal structure of ß-NaSrBO3 was determined from the powder X-ray diffraction data. It crystallizes in space group P21/c with the following lattice parameters: a = 6.06214(8) Å, b = 5.41005(7) Å, c = 9.1468(1) Å, ß = 102.116(1)°, and V = 293.301(7) Å(3). Na and Sr sites are found to be mixed occupied by each other, and the isolated [BO3](3-) anionic groups are distributed in parallel. Ce(3+)-activated ß-NaSrBO3:Ce(3+) blue-emitting phosphors were synthesized. The temperature-dependent photoluminescence spectra indicate that the thermal stability of ß-NaSrBO3:Ce(3+) is better than that of α-NaSrBO3:Ce(3+) at the same temperature. A near-ultraviolet pumped warm w-LED with a ß-NaSrBO3:0.05Ce(3+) phosphor as the blue component was fabricated. The w-LED lamp after illumination at 250 mA gives chromaticity coordinates, a color rendering index, and a correlated color temperature of (0.3821, 0.3430), 92.8, and 3654 K, respectively.

Fast UV-Vis photorefractive response of Zr and Mg codoped LiNbO3:Mo.

A series of LN:Mo,Zr and LN:Mo,Mg crystals with different doping concentrations were grown and their holographic properties were investigated from UV to the visible range. Each crystal allows for holographic storage from UV to the visible as LN:Mo. When the concentration of MgO is enhanced to 6.5 mol%, the response time can be dramatically shortened to 0.22 s, 0.33 s, 0.37 s and 1.2 s for 351, 488, 532, and 671 nm laser, respectively. The results show that LN:Mo,Mg is a promising candidate for all-color holographic volume storage with fast response.

Pyroelectric effect in green light-assisted domain reversal of Mg-doped LiNbO3 crystals.

We developed a real-time imaging system to probe the light-assisted domain reversal process of Mg-doped LiNbO(3). An interesting phenomenon was observed where the domain appeared to reverse just after the laser was obscured. An exclusive electric field of about 350 V/mm was measured at 532 nm of light irradiation at an intensity of 6.6 × 10(4) W/cm(2). The exclusive electric field was considered to be produced by a pyroelectric effect owing to a temperature change in the region of irradiation. The temperature change in the light-irradiated region was calculated to be about 2.3°C. Our experimental results indicate that a change of the electric field caused by the pyroelectric effect may play a significant role when LiNbO(3) or other ferroelectric crystals are used under strong light.

Photorefraction of molybdenum-doped lithium niobate crystals.

Molybdenum-doped lithium niobate crystals were grown under different polarization conditions and their holographic properties were investigated. In contrast to current dopants, hexavalent molybdenum prefers niobium sites. Thereby, holographic storage becomes possible from the ultraviolet to the visible with considerably lower response time. The response time of 0.5 mol. % Mo-doped LiNbO(3) can be especially shortened to as small as 0.35 s with a still high saturation diffraction efficiency of about 60% at 351 nm. Molybdenum-doped lithium niobate thus is a promising candidate for all-color holographic storage applications.

Fast photorefractive response of vanadium-doped lithium niobate in the visible region.

A series of vanadium-doped lithium niobate crystals was grown and their photorefractive properties were investigated with a 532 nm laser. At a total light intensity of 471 mW/cm(2), a short response time of only 0.57 s was achieved for 0.1 mol.% vanadium in LiNbO(3). The photorefractive process is dominated by the diffusion field instead of the photovoltaic field. The dominant charge carriers are electrons. The possible mechanism for the fast photorefractive response is discussed.

An excellent crystal for high resistance against optical damage in visible-UV range: near-stoichiometric zirconium-doped lithium niobate.

Near-stoichiometric zirconium-doped lithium niobate crystals were fabricated and their optical damage resistance was investigated. It was found that these crystals can withstand a light intensity of 20 MW/cm2 at 514.5 nm cw laser, 80 GW/cm2 at 532 nm pulse laser, and 120 kW/cm2 at 351 nm cw laser. The minimum switching field is only 1.00 kV/mm for 0.5 mol% zirconium-doped lithium niobate crystal. These properties suggest that the near-stoichiometric zirconium-doped lithium niobate crystals will be an excellent candidate for quasi-phase matching technique.

Improved ultraviolet photorefractive properties of vanadium-doped lithium niobate crystals.

A series of vanadium-doped lithium niobate (LN:V) crystals has been grown and their photorefractive properties were investigated. For 0.1 mol. % V-doped LiNbO(3), a fast photorefractive response of 160 ms was obtained with a 351 nm laser and a total light intensity of 583 mW/cm(2). The measurements of the x-ray photoelectron spectrum and electron paramagnetic resonance show that V(3+), V(4+), and V(5+) ions exist in these LN:V crystals. V(3+) and V(4+) ions correspond to the 420 and 475 nm absorption peaks, respectively. The fast photorefractive response and high sensitivity indicate that LN:V is a suitable candidate for UV photorefractive applications.

Ultraviolet band edge photorefractivity in LiNbO3:Sn crystals.

The ultraviolet (UV) band edge photorefractivity of Sn-doped LiNbO(3) (LN:Sn) at 325 nm has been investigated. A sharp decrease of beam distortion, which is accompanied by a significant increase in the photoconductivity, is observed in LN:Sn crystals with Sn-doping concentrations at or above 2.0 mol%. The diffraction efficiency, the holographic recording sensitivity and response rate, and the two-wave coupling gain coefficient are greatly enhanced when the Sn-doping concentration reaches 2.0 mol% or more. Unlike LiNbO(3) doped with Hf in which the UV gratings can be erased easily by a red beam, the UV gratings in LN:Sn can withstand long-term red beam illumination. Electrons are determined to be the dominant light-induced charge carriers responsible for the UV band edge photorefraction. The observed enhancement on the UV band edge photorefractivity is found to be associated with the showup of an absorption band around 325 nm in LN:Sn crystals with Sn-doping concentrations at or above 2.0 mol%.

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.

Light scattering induced by opposite microdomains in LiNbO3:Fe:Hf crystals.

We report on the light scattering phenomenon in annealed multidomain LiNbO3:Fe:Hf crystals. The scattering sources are found to be some fog-like "defects", which cause the polarization-dependent scattering of the light, and can be removed completely by the illumination of visible light. Based on these results and the etch patterns, these "defects" are suggested to be refractive index fluctuations induced by the space charges accumulated at the boundary of opposite microdomains. The influence of quick heating-up on the "defects" is also studied and the results firmly support our suggestion about the nature of the "defects". At last, the temporal curves of the transmitted intensity during the light scattering are explained. The mechanism for the opposite microdomain formation is also explained from the view of crystal growth.