Tunable broadband near-infrared luminescence in glass realized by defect-engineering.

Tunable broadband near-infrared (NIR)-luminescent materials play a crucial role as light sources and tunable fiber lasers in modern technologies such as high-capacity telecommunication, imaging, and remote sensing. Despite considerable effort in studying the luminescent materials doped with rare-earth or transition metal ions, it is still challenging to achieve tunable broadband emission in photonic materials, especially in glasses, for active-fiber applications. In the present work, such NIR emission is achieved by modifying oxygen-deficient structural defects (i.e., singly ionized oxygen vacancies (VO∙) in tellurium (Te)-doped germanate glass). The local glass chemistry around Te is controlled by engineering singly ionized oxygen vacancies (VO∙) in alkali-alumino-germanate glass. This enables fine-tuning of the configurations and chemical states of Te centers over a wide range of chemical states, from ionic states to neutrally charged clusters and to positively charged clusters, resulting in various intriguing luminescent behaviors (e.g., wavelength-tunable emission, great emission enhancement, bandwidth extension).

Visible and Near-Infrared Emission in Ba3Sc4O9:Bi Phosphor: An Investigation on Bismuth Valence Modification.

Bismuth (Bi)-activated luminescence materials have attracted much attention for their tunable broad emissions ranging from a visible to near-infrared (NIR) region. However, it remains a challenge to regulate the Bi valence state and achieve NIR emission via a facile way. Here, we report the design and preparation of Ba3Sc4O9:Bi phosphors, which emit visible and NIR emissions simultaneously even prepared in the air condition. The self-reduction mechanism of Bi3+ species in Ba3Sc4O9 with a rigid crystal structure is illustrated based on the charge compensation model, and the coexistence of different Bi-active centers, Bi3+ for visible emission, while Bi+ and Bi0 for NIR emission, is confirmed by the spectroscopic data and X-ray photoelectron spectroscopy (XPS) analysis. The enhanced NIR emission was further achieved through controlled reducing treatment and the related mechanism has also been clarified. This work paves a new way to control bismuth valence and tune the emission of Bi-based luminescence materials for emerging photonics applications.

Single-frequency DBR Nd-doped fiber laser at 1120 nm with a narrow linewidth and low threshold.

We report a narrow linewidth and low threshold single-frequency distributed Bragg reflector (DBR) fiber laser at 1120 nm based on a short 1.5 cm long Nd-doped silica fiber which, to the best of our knowledge, is the first demonstration of a Nd-doped fiber-based single-frequency fiber laser with a wavelength greater than 1100 nm. A stable single-longitudinal-mode laser operation with a signal-to-noise ratio greater than 67 dB was verified by a scanning Fabry-Perot interferometer. The laser threshold is as low as 10 mW. The DBR fiber laser has a maximum output power of 15 mW and optical-to-optical efficiency for the launched pump power reaches more than 8%. The narrow linewidth of 71.5 kHz is obtained in such a single-frequency fiber laser (SFFL). Our result is expected to offer an exciting new opportunity to realize high-performance SFFLs above 1100 nm.

Ultraviolet-A Persistent Luminescence of a Bi3+-Activated LiScGeO4 Material.

Long persistent phosphors (LPPs) with ultraviolet (UV) luminescence have great potential for application in the fields of biomedicine, environmental, and catalysis. However, it is currently limited by the design and development of remarkable UV LPPs with a suitable spectral region and an ultralong afterglow decay time. Herein, we develop a new type of Bi3+-activated LiScGeO4 LPP, which exhibits bright ultraviolet-A (UVA) persistent luminescence (PersL). Because of the existence of numerous stabilized effective traps, the as-synthesized phosphors can undergo an ultralong PersL decay time far longer than 12 h. The PersL properties, effective trap depths, distributions, and types, as well as the possible mechanism for the PersL behavior of LiScGeO4:Bi3+, are comprehensively surveyed utilizing PersL excitation spectra, PersL decay analyses, thermoluminescence experiments, and X-ray photoelectron spectroscopy. This work can cover the shortage of LPPs in the UV region and also can lay the foundation for the development of more excellent UV LPPs toward versatile novel applications.

Ultra-broadband red to NIR photoemission from multiple bismuth centers in Sr2B5O9Cl:Bi crystal.

Bismuth (Bi)-doped materials are a new family of laser materials, and they usually exhibit extremely broad near-infrared (NIR) luminescence in 1000-1700 nm. Therefore, they can be utilized for a new generation of ultra-broadband tunable laser sources and ultra-broadband fiber amplifier. The broadband characteristics of Bi-active NIR luminescence can meet the needs of special wavelength laser sources that rare-earth-doped lasers cannot provide. However, at present, the Bi-doped NIR luminescence materials are mainly concentrated on glass, while Bi-doped NIR luminescence laser crystals are rarely reported. In this work, a novel Bi-doped crystal Sr2B5O9Cl:Bi is reported with NIR luminescence, which exhibits broadband absorption in ultraviolet and visible regions, and can produce ultra-broadband from red to NIR luminescence covering 600-1600 nm. The results of excitation, emission spectra, and fluorescence lifetime show that the Sr2B5O9Cl:Bi crystal contains three different Bi-active NIR emission centers. This work could enrich our understanding on Bi NIR emission behaviors in crystals. And this material provides a possibility for the development of a new laser source.

915 nm all-fiber laser based on novel Nd-doped high alumina and yttria glass @ silica glass hybrid fiber for the pure blue fiber laser.

The fiber laser in the range of 900-1000 nm is essential to generate the blue fiber laser through frequency doubling for the laser display, laser underwater communications, and laser lighting. Yet, the well-developed three-level Yb-doped fiber laser can only realize the blue-green fiber laser at around 490 nm, which is far from the pure blue area (450 nm). To further achieve the pure blue fiber laser, the Nd-doped fiber has emerged as a proper choice to realize a shorter wavelength laser (<920 nm) through the F3/24→I9/24 transition of Nd3+. Here, based on the facile "melt-in-tube" (MIT) method, a novel Nd-doped high alumina and yttria glass @ silica glass hybrid fiber was successfully prepared using the Nd:YAG crystal as the precursor core. The crystal core converts to the amorphous glass state after the drawing process, as evidenced by Raman spectra. The gain coefficient at 915 nm of the hybrid fiber reaches 0.4 dB/cm. Further, the laser oscillation at 915 nm with over 50 dB signal-to-noise ratio was realized by a short 3.5 cm gain fiber. Our results indicate that MIT is a feasible strategy to produce novel fiber for generating fiber laser at special wavelengths.

Actively Targeted Deep Tissue Imaging and Photothermal-Chemo Therapy of Breast Cancer by Antibody-Functionalized Drug-Loaded X-Ray-Responsive Bismuth Sulfide@Mesoporous Silica Core-Shell Nanoparticles.

A theranostic platform combining synergistic therapy and real-time imaging attracts enormous attention but still faces great challenges, such as tedious modifications and lack of efficient accumulation in tumor. Here, a novel type of theranostic agent, bismuth sulfide@mesoporous silica (Bi2S3@ mPS) core-shell nanoparticles (NPs), for targeted image-guided therapy of human epidermal growth factor receptor-2 (HER-2) positive breast cancer is developed. To generate such NPs, polyvinylpyrrolidone decorated rod-like Bi2S3 NPs are chemically encapsulated with a mesoporous silica (mPS) layer and loaded with an anticancer drug, doxorubicin. The resultant NPs are then chemically conjugated with trastuzumab (Tam, a monoclonal antibody targeting HER-2 overexpressed breast cancer cells) to form Tam-Bi2S3@mPS NPs. By in vitro and in vivo studies, it is demonstrated that the Tam-Bi2S3@mPS bear multiple desired features for cancer theranostics, including good biocompatibility and drug loading ability as well as precise and active tumor targeting and accumulation (with a bismuth content in tumor being ≈16 times that of nontargeted group). They can simultaneously serve both as an excellent contrast enhancement probe (due to the presence of strong X-ray-attenuating bismuth element) for computed tomography deep tissue tumor imaging and as a therapeutic agent to destruct tumors and prevent metastasis by synergistic photothermalchemo therapy.

Unusual thermal response of tellurium near-infrared luminescence in phosphate laser glass.

We report an unusual thermal response of tellurium (Te) near-infrared (NIR) luminescence in phosphate laser glass, where the luminescence first increases and then decreases with heat-treatment temperatures increasing from 250°C to the glass transition temperature (Tg). This is followed by a distinct revival of Te NIR luminescence at temperatures above Tg. This result differs from the scenario in conventional rare-earth (Er3+, Nd3+, and Yb3+)-doped phosphate glasses, where the rare-earth NIR emission decreases with increasing heat-treatment temperature. The difference may originate from conversion between Te4 and other Te species, which depends on the evolution of the glass structure and molecular motion during the reheating processes, leading to unusual thermal response of Te NIR luminescence. The increase in Te4 clusters enhances Te NIR emission, indicating that Te NIR luminescence is assigned to the Te4 cluster, in contrast to previous studies. Heating and cooling cycles between 50°C and 250°C reveal strong dependence of the thermal degradation on glass structure. Te-doped phosphate laser glass with zero thermal degradation can be realized by stabilizing NIR luminescence center Te4 by adjusting the glass structure with reduced network crosslinking. The superior optical performance has been confirmed in our previous study that the NIR luminescence properties can be well maintained in Te-doped fiber. The findings indicate that Te-doped phosphate glass with unusual thermal responses can potentially be used in fiber laser devices.

Near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser.

The Earth's magnetic field has significant effects that protect us from cosmic radiation and provide navigation for biological migration. However, slow temporal variations originating in the liquid outer core invariably exist. To understand the working mechanism of the geomagnetic field and improve accuracy of navigation systems, a high-precision magnetometer is essential to measure the absolute magnetic field. A helium optically pumping magnetometer is an advanced approach, but its sensitivity and accuracy are directly limited by the low-frequency relative intensity noise and frequency stability characteristics of a light source. Here, we demonstrate a near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser. The relative intensity noise is only 5 dB higher than the quantum-noise limit, and the root mean square of frequency fluctuation is ∼17 kHz after locked. This fiber laser could suppress the fluctuation of magnetic resonant frequency and improve the signal-to-noise ratio of the magnetic resonance signal detection.

Mn4+-Doped Heterodialkaline Fluorogermanate Red Phosphor with High Quantum Yield and Spectral Luminous Efficacy for Warm-White-Light-Emitting Device Application.

Narrow band red-emitting Mn4+-doped fluoride phosphor is an essential red component of modern white-light-emitting-diode (WLED) devices. Its luminescence has sensitivity to structure and influences the performance of WLED. In this paper, we report a high-performance Mn4+ phosphor based on a new heterodialkaline fluorogermanate, CsNaGeF6:Mn4+. As determined by the single-crystal X-ray diffraction analysis, the CsNaGeF6 compound crystallizes in the orthorhombic crystal system with space group Pbcm (No. 57). Under excitation by 360 and 470 nm photons, CsNaGeF6:Mn4+ emits intense red light near 630 nm with a high quantum yield of 95.6%. The electronic energy levels of the Mn4+ ion in Cs2GeF6, Na2GeF6, and CsNaGeF6 are calculated using the exchange charge model of crystal-field theory. The local Mn4+ environment inducing different zero-phonon-line emissions in the structures is probed by electron paramagnetic resonance. The Mn4+-doped heterodialkaline fluorogermanate CsNaGeF6:Mn4+ exhibits broader emission as a result of the lowest symmetry. It has higher quantum yield than Na2GeF6:Mn4+ and higher spectral luminous efficacy than Cs2GeF6:Mn4+. Given the good thermal stability and efficient luminescence, a prototype warm-WLED device with a color rendering index of 92.5, a correlated color temperature of 3783 K, and a luminous efficacy of 176.3 lm/W has been fabricated by employing the CsNaGeF6:Mn4+ phosphor as the red component. Our results not only reveal that a high-performance Mn4+ red phosphor is achieved through cationic substitutions but also construct a relationship of performance-structure to guide the design of Mn4+ phosphors in the future.

Site Occupation of Eu2+ in Ba2- xSr xSiO4 ( x = 0-1.9) and Origin of Improved Luminescence Thermal Stability in the Intermediate Composition.

Knowledge of site occupation of activators in phosphors is of essential importance for understanding and tailoring their luminescence properties by modifying the host composition. Relative site preference of Eu2+ for the two distinct types of alkaline earth (AE) sites in Ba1.9995- xSr xEu0.0005SiO4 ( x = 0-1.9) is investigated based on photoluminescence measurements at low temperature. We found that Eu2+ prefers to be at the 9-coordinated AE2 site at x = 0, 0.5, and 1.0, while at x = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with comparable preference, which is verified by density functional theory (DFT) calculations. Moreover, by combining low-temperature measurements of the heat capacity, the host band gap, and the Eu2+ 4f7 ground level position, the improved thermal stability of Eu2+ luminescence in the intermediate composition ( x = 1.0) is interpreted as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB), which results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Radioluminescence properties of the samples under X-ray excitation are finally evaluated, suggesting a great potential scintillator application of the compound in the intermediate composition.

Frequency noise of distributed Bragg reflector single-frequency fiber laser.

We investigated the frequency noise in the distributed Bragg reflector single-frequency fiber laser (DBR-SFFL) theoretically and experimentally. A complete theoretical analysis is demonstrated by considering the energy-transfer upconversion (ETU) process and establishing linkages between the frequency noise and the relative intensity noise (RIN) of the DBR-SFFL. The experimental results of the diverse DBR-SFFLs in different working conditions are in good agreement with the theoretical analyses. These investigations are beneficial to optimizing frequency noise property to promote the wide application of the DBR-SFFLs. The proposed results can be generally applicable to the short-linear-cavity SFFL with centimeters order of the cavity length.

High-power and near-shot-noise-limited intensity noise all-fiber single-frequency 1.5 µm MOPA laser.

An all-fiber high-power and broad-frequency-band near-shot-noise-limited kHz-linewidth (Δν ~1.7 kHz) single-frequency master-oscillator power amplifier (MOPA) laser at 1.5 µm is demonstrated. To significantly suppress the intensity noise of seed laser and mitigate the detrimental effects of amplified spontaneous emission and stimulated Brillouin scattering in fiber amplifiers, more than 23 W of a stable low noise single-frequency laser output is achieved with a relative intensity noise of < -150 dB/Hz @0.5 mW (near to the shot-noise limit: -152.9 dB/Hz) in the frequency band from 0.1 to 50 MHz. It is believed that the achieved laser performance of ultra-low intensity noise and high-power output make the laser source become a promising candidate in further applications, such as cold atom optical lattice, quantum key distribution, and gravitational wave detection.

Self-injection locked and semiconductor amplified ultrashort cavity single-frequency Yb3+-doped phosphate fiber laser at 978 nm.

Based on a self-injection locking scheme and the nonlinear amplification effect of a semiconductor optical amplifier, a low intensity noise amplified ultrashort cavity single-frequency fiber laser at 978 nm is demonstrated with a final output power of > 230 mW and a broad temperature range of > 15 °C for single-longitudinal-mode operation. The effective cavity length of the fiber oscillator is less than 6 mm, comprising a 3.5-mm-long highly Yb3+-doped phosphate fiber and a pair of fiber Bragg gratings. For the frequency range from 1.8 to 10 MHz, the relative intensity noise close to -150 dB/Hz is achieved. The signal-to-noise ratio of > 68 dB and the laser linewidth of < 10 kHz are obtained. Such narrow linewidth low noise 978 nm laser is promising, as the high-performance pump source or the efficient blue and UV light sources after nonlinear frequency conversion.

kHz-order linewidth controllable 1550 nm single-frequency fiber laser for coherent optical communication.

A kHz-order linewidth controllable 1550 nm single-frequency fiber laser (SFFL) is demonstrated for the first time to our best knowledge. The control of the linewidth is realized by using a low-pass filtered white Gaussian noise (WGN) signal applied on a fiber stretcher in an optical feedback loop. Utilizing WGN signals with different signal amplitudes An and different cutoff frequencies fc, the linewidths are availably controlled in a wide range from 0.8 to 353 kHz. The obtained optical signal-to-noise ratio (OSNR) is more than 72.0 dB, and the relative intensity noise (RIN) at frequency greater than 40 MHz reaches -148.5 dB/Hz which approaches the shot noise limit (-152.9 dB/Hz). This kHz-order linewidth controllable SFFL is meaningful and valuable, for optimizing the receiver sensitivity and bit error rate (BER) performance of the coherent optical communication system based on high-order quadrature amplitude modulation (QAM).

Site Occupancy Preference and Antithermal Quenching of the Bi2+ Deep Red Emission in ß-Ca2P2O7:Bi2.

The resistance to thermal quenching is an essential factor in evaluating the performance of luminescent materials for application in white light emitting diodes (WLEDs). In this work, we studied the site occupancy preference and thermal quenching of luminescence in ß-Ca2P2O7:Bi2+ red phosphor at low (10-300 K) and high temperatures (303-573 K). In ß-Ca2P2O7, the host lattice has four different calcium sites, at which Bi2+dopant can be located. After comparing the change of bond energy when the Bi2+ ions are incorporated into the four calcium sites, we found out that Bi2+ would preferentially occupy the smaller energy variation sites Ci(2) and Ci(1) in this compound, which can be assigned to Bi(2) and Bi(1), respectively. Surprisingly, we noticed that the variation of emission intensity is different under different excitations when the temperature changes from 10 to 300 K. When exciting into the typical absorption of Bi(1) sites at 419 nm, the emission intensity at 300 K remains only 38% as compared to that at 10 K, while exciting into typical Bi(2) absorption at 460 nm, the emission intensity increases to 110%. When further increasing the temperature from 303 to 573 K, we observed a similar phenomenon, and the emission at 460 nm excitation starts to quench at 453 K. The emission intensity at 573 K still remains 86.1% of that at 303 K. This might be attributed to the Bi(2) → Bi(1) energy transfer. It is also evidenced by the time-resolved emission spectra and lifetime values. This work gives new insights into better understanding luminescent behaviors of Bi2+-doped materials with multiple cation sites. This should be helpful in the future when designing the bismuth doped phosphor for WLEDs with better resistance to thermal quenching.

Superbroad visible to NIR photoluminescence from Bi+ evidenced in Ba2B5O9Cl: Bi crystal.

The nature of bismuth NIR luminescence is essential to develop the bismuth doped laser materials with high efficiency and desirable emission wavelength, and it, thereby, receives rising interests. Our previous work reported the Bi(0) luminescence from Ba2B5O9Cl: Bi with a lifetime of ~30 µs and the conversion of Bi(2+) to Bi(0). This work found indeed the conversion could be enabled in the compound by an in situ reduction technique and it, however, happens via an intermediate state of Bi(+). Once the ion of Bi(+) is stabilized and built into the compound, it can luminesce in a super broad spectral range from 600 to 1200 nm with a lifetime longer than 1 ms, due to the cascade transitions from (3)P2 and (3)P1 to (3)P0. This is completely different from Bi(0) and Bi(2+) in the compound, and it has never been noticed before. We believe this work can help us better understand the complex nature of bismuth luminescence.

Unusual anti-thermal degradation of bismuth NIR luminescence in bismuth doped lithium tantalum silicate laser glasses.

For application of bismuth laser glasses in either fiber amplifier or laser, their performance stability in long run should be understood especially in extreme conditions. However, so far, there are few reports on it. Here, we found, after the cycle experiments on heating and cooling, that the proper increase of lithium content in lithium tantalum silicate laser glass can lead to unusual anti-thermal degradation of bismuth NIR luminescence, which completely differs from the scenario in germanate glass. FTIR, 29Si MAS NMR spectra, absorption and dynamic photoluminescence spectra are employed to unravel how this happens. The results illustrate that it should be due to the decrease of polymerization of silicate glass network, which in turn allows the regeneration at 250°C, and therefore, the content increase of bismuth NIR emission centers. In the meanwhile, we noticed though Bi luminescence can be thermally quenched its peak does not shift along with temperature, which seldom appears in laser materials. The unique property might guarantee the unshift of Bi fiber laser wavelength once such glass was made into fiber devices even as the environmental temperature changes. The role of lithium is discussed in the evolution of glass structures, the suppression of glass heterogeneity, and the thermal stability of Bi luminescence, and it should be helpful to design homogeneous silicate laser glass with outstanding thermal stability.

1120 nm kHz-linewidth single-polarization single-frequency Yb-doped phosphate fiber laser.

A spectrally clean kHz-linewidth single-polarization single-frequency distributed Bragg reflector Yb-doped phosphate fiber (YPF) laser at 1120 nm (> 1100 nm) for the first time is demonstrated. By enhancing the reflectivity of output fiber Bragg grating and optimizing the length of YPF to implement the effective ASE suppression and single-longitudinal-mode long-wavelength lasing, a stable output power of over 62 mW is achieved from a 31-mm-long highly YPF with a linewidth of 5.7 kHz. The signal to noise ratio of > 67 dB, the polarization extinction ratio of > 25 dB, and the relative intensity noise of < -150 dB/Hz for the frequencies above 10.0 MHz are obtained in such single-frequency fiber laser. This narrow linewidth fiber laser is an ideal laser source to generate the coherent single-frequency 560 nm light via frequency doubling for biochemical analysis application.

Dual-wavelength passively q-switched single-frequency fiber laser.

We propose a compact dual-wavelength Q-switched single-frequency fiber laser based on a 17-mm-long home-made highly Er3+/Yb3+ co-doped phosphate fiber (EYDPF) and a semiconductor saturable absorber mirror (SESAM). The short cavity length and a polarization-maintaining fiber Bragg grating (PM-FBG) ensure that only one longitudinal mode is supported by each reflection peak. The maximum pulse energy of more than 34.5 nJ was realized with the shortest pulse duration of 110.5 ns and the Q-switched fiber laser has a repetition rate reaching over 700 kHz with a temporal synchronization of pulses at two wavelengths. Besides, the optical signal-to-noise ratio (OSNR) of larger than 64.5 dB was achieved.