Information on forest structure is vital for sustainable forest management. Currently, airborne LiDAR remote sensing has been well established as an effective tool to characterize the structure of ...canopies and forest inventory variables. Radiometry and geometry are highly intertwined in LiDAR remote sensing of forest vegetation and phenology influences the geometric-optical properties of deciduous and evergreen trees causing seasonal variation in LiDAR observations. This variation may be considered as a nuisance or exploited in for example tree species identification. Airborne LiDAR data are also influenced by sensor functioning, acquisition settings, scan geometry and the atmosphere. Reliable estimation of subtle phenological effects calls for data in which the impact of the external factors is minimal. We experimented with such data and explored LIDAR waveforms (WFs) in boreal trees in winter, early summer and late summer. Our objectives were to i) assess the match of the multitemporal LiDAR data for observing true changes in vegetation; ii) quantify the influence of phenology in deciduous and evergreen trees; iii) study the effect of varying scan zenith angle (SZA) and canopy age on WF features in different phenostates; iv) assess the temporal feature correlation in individual living and dead standing trees. A WF-recording pulsed LiDAR sensor unit operating at the wavelength of 1550 nm was used in repeated acquisitions. WF attributes such as energy, peak amplitude and echo width were derived for each pulse and were localized vertically to crown, understory and ground components. Silver and downy birch, black alder, European aspen, Siberian larch, Scots pine, Norway spruce and dead standing spruce formed our strata. Results showed that phenology caused more variation in WF features of deciduous trees compared to evergreen conifers. Deciduous trees displayed substantial between-species variation that was linked with differences in branching pattern, leaf orientation and bark reflectance. Pine displayed a possible winter-early summer anomaly in canopy backscattering that may be linked with changes in foliage clumping or with the role of stamens in early summer trees. Trees displayed positive temporal correlation in WF features and correlations were the strongest in evergreen and deciduous conifers and decreased with time. SZA had minor influence on WF features whereas age exercised a strong effect on many features with parallel variation between species and phenostates. Structural changes following death, i.e. ‘aging’ changed the geometric WF features of dead standing trees. Our results provide new insights for enhancing tree species identification by using WF LiDAR and for LiDAR time-series analysis of vegetation.
•Multitemporal waveform (WF) LiDAR data with high radiometric and geometric match.•Effects of phenology on WF features at 1550 nm investigated.•Phenological effects are larger in deciduous than in evergreen trees.•WFs vary seasonally also in evergreen trees, but geometric WF features are stable.•WF features of dead standing trees change over time.
A 1550 nm‐band InAs/InGaAlAs quantum dot (QD) distributed feedback (DFB) laser is fabricated, in which the side‐wall grating is formed in a simultaneous process with the ridge waveguide, and ...room‐temperature continuous‐wave single‐mode oscillation is achieved. A rather low‐threshold current density of about 2.2 kA cm−2 is obtained. Side‐mode suppression ratios (SMSRs) are 28 dB just above the threshold current Ith and 44 dB at I/Ith = 2.0. Antireflection (AR) coating suppresses the amplified spontaneous emission (ASE) and increases the slope efficiency by a factor of 1.3.
1550 nm‐band InAs/InGaAlAs quantum dot (QD) distributed feedback (DFB) laser grown on InP(311)B substrate is fabricated with side‐wall gratings formed in a simultaneous process with a ridge waveguide. Room‐temperature (RT) continuous‐wave (CW) single‐mode oscillation is achieved. Antireflection (AR) coating suppresses the amplified spontaneous emission (ASE) and increases the slope efficiency by a factor of 1.3.
•The ternary sulfide of NaYS2 was adopted as a novel host for high-efficient UCL.•Er3+ single-doped NaYS2 was designed to avoid luminescence quenching via CTS.•Breakthrough upconversion brightness ...and efficiency were achieved in the NaYS2:Er3+.•The mechanisms for high-efficiency UCL and color stability were studied.
Optimizing the host lattice is the most efficient strategy to break through the intrinsic characteristic of low efficiency up-conversion luminescence (UCL). In principle, ternary sulfide, e.g. NaYS2, is an ideal UCL host matrix due to its low phonon energy and highly asymmetrical crystal structure. However, it was found that the low lying S2-→Yb3+ charge transfer state (CTS) existing in the ternary sulfide material severely quenches the UCL under 980 nm excitation, which generally employs Yb3+ ion as the conventional sensitizer. Herein, we design Er3+ single-doped NaYS2 phosphors for highly efficient UCL under 1550 nm excitation. This novel NaYS2:Er3+ phosphor excited at 1550 nm exhibits a 94-fold enhancement of UCL intensities compared to that of conventional NaYS2:Yb3+,Er3+ under 980 nm excitation. More encouragingly, compared to the commercial β-NaYF4:Yb3+,Er3+ phosphors, the UCL efficiency and brightness of NaYS2:Er3+ excited at 1550 nm are twice that of β-NaYF4:Yb3+,Er3+ excited at 980 nm. Such extraordinary efficient UCL is accomplished by taking advantages of the intrinsic low phonon energy (279 cm−1) of the NaYS2 host and the highly asymmetrical trigonal crystal system. Furthermore, in the Er3+ single-doped NaYS2 system, the S2-→Er3+ CTS lies at a high state of ≈34500 cm−1, which will not quench the efficient UCL from Er3+ ions. The large absorption cross-section at 4I13/2 energy level (high response to 1550 nm radiation) and long lifetime of the metastable state of 4I9/2 also contribute to the population of the emission levels (2H11/2/4S3/2). These findings provide a new class of host materials for achieving highly efficient UCL.
We investigate experimentally the state bistability driven by current or feedback in a 1550 nm vertical-cavity surface-emitting laser (1550 nm-VCSEL) with negative optoelectronic feedback (NOEF). The ...results show that through continuously varying the controlled parameters such as the bias current or the feedback strength along different routes, under a set of given system parameters, there may exist two kinds of output states, i.e., state bistability, which are critically dependent on the history of how the controlled parameters are varied to reach the given values. Within the varied range of bias current or feedback strength, the state bistability is shown via the time series, power spectrum, and phase portrait of the output signal, and the physical mechanisms for the state bistability appearing in the NOEF 1550 nm-VCSEL are analyzed. Furthermore, the complexities of the corresponding states are evaluated by calculating the characteristic value of the normalized permutation entropy of the output from the NOEF 1550 nm-VCSEL. Additionally, by revealing the evolution of the dynamical states under the increase and decrease in the controlled parameters, the ranges of the parameters required for generating current-induced or feedback-induced state bistability can be specified to certain degree.
The polarization bistability (PB) features in a 1550 nm vertical-cavity surface-emitting laser (1550 nm-VCSEL) subject to variable polarization optical injection (VPOI) are experimentally ...investigated. We have investigated the influence of polarization angle θ, of optical injection on PBs induced by scanning the power (power-induced PB) and the frequency of injected light (frequency-induced PB) along with different routes. The experimental results show that, for the fixed frequency of the injection laser and by scanning the injection power within a certain region, there exists a determined range of θ, in which power-induced PB in the 1550 nm-VCSEL subject to VPOI can be observed. For VPOI with θ, located within the determined range, the power-induced PB possesses larger hysteresis width, as compared with orthogonally polarized optical injection. The hysteresis width will be larger for a smaller value of θ,. Under the condition of fixed injection power and continuously varying the frequency of the injection laser, two frequency-induced hysteresis cycles can be observed, of which one is located at negative frequency detuning, and the other is located at positive frequency detuning. The variations of the two hysteresis width with θ, exhibit different trends. Also, when the bias current of the 1550 nm-VCSEL increases, the hysteresis width can be further broadened for the power-induced PB, and the broadening effect is relatively weak for the frequency-induced PB.
1550 nm near-infrared (NIR) light excited lanthanide ion (Ln3+) doped up-conversion (UC) materials with tunable emission color have great application potential in optical anti-counterfeiting and it ...is essential to explore new stable phosphors and expand the emission color gamut for multicolor security printing. Herein, a new type of double perovskite tungstate UC phosphors LiYMgWO6: Er3+ with green and red double color emissions under 1550 nm excitation were synthesized by high temperature solid-state method. The emission color output of LiYMgWO6: Er3+ was adjusted from yellow-green to red by introducing Ho3+ ion to enhance the red emission and suppress the green emission. Fluorescent lifetime measurements were carried out to illustrate the color modulation mechanism. As a proof of concept, the fluorescent printing inks based on as-prepared phosphors were synthesized and fluorescent labels with special patterns were designed to demonstrate the anti-counterfeiting application.
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•1550 nm pumped upconversion emission color of LiYMgWO6: Er3+ was modulated by Ho3+ doping.•The Chromaticity Modulation mechanism was investigated.•Anti-counterfeiting application based on the as-prepared phosphors was displayed. .
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•The enhancement mechanism of Tm3+ ions as intrinsic luminescence centers and transient energy trapping centers for the transition luminescence at 1550 nm has been investigated.•The ...enhancement mechanism of the conversion luminescence at 1550 nm by Yb3+ ions as mutual sensing ions and reverse energy transfer centers was investigated.•The equilibrium relationship between quenching and enhancement of the intensity of the conversion luminescence on the samples by the introduction of alkali metal ions has been investigated.
Upconversion fluoride phosphors Na1-xMxY1-a-b-cF4:Er3+a, Tm3+b, Yb3+c (M = Li+/K+) have been synthesized by low-temperature combustion method. The optimal doping ratios of ions in the matrix lattice were determined by orthogonal experiments with the control variable method. It was found that when a certain amount of Tm3+ ions were doped into the lattice of Er3+ ions, the upconversion fluorescence intensity and red-to-green ratio of the samples were significantly enhanced. When a small amount of Yb3+ ions was introduced into the Er3+-Tm3 + ions co-doped samples, the upconversion fluorescence intensity of the samples was continued to be enhanced, but the red-to-green ratio was slightly decreased. The mechanism of the influence of the upconversion fluorescence intensity and the red-to-green ratio of the multidoped samples with lanthanide ions was also systematically investigated. Based on the results of orthogonal experiments, the optimal component formulations were determined and alkali metal ions were further introduced. The upconversion fluorescence enhancement mechanism of the samples after the introduction of alkali metal ions was systematically investigated. In this work, the upconversion fluorescence intensity of the prepared samples was significantly enhanced by synergistic sensitization between the ions. In addition, by adjusting the red-to-green ratio of the fluorescence of the samples, a new idea is provided for the preparation of upconversion phosphors with high color purity.
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•Novel Na3Y(VO4)2: Yb3+, Er3+ up-conversion phosphors are synthesized for the first time.•Up-conversion properties and mechanisms of Na3Y(VO4)2: Yb3+, Er3+ excited by 980 and 1550 nm ...are analyzed.•Green and red dual-color luminescence are successfully achieved in the single Na3Y(VO4)2:Yb3+, Er3+ phosphor under 980 and 1550 nm excitation.•High color purity red luminescence is achieved under 1550 nm excitation.•Temperature sensing properties of novel Na3Y(VO4)2: Yb3+, Er3+ phosphors are investigated.
Multi-color up-conversion luminescence materials with broad emission range are scarce extremely due to the finite excitation wavelength, thin types of activators, and inappropriate emission levels. Yb3+, Er3+ dual-sensitized system is established in the text, green and red dual-color luminescence are successfully achieved in novel Na3Y(VO4)2:Yb3+, Er3+ phosphor. The phosphors show green luminescence under 980 nm excitation and high color purity red luminescence under 1550 nm excitation, a wide range of emission colors can be tuned by changing various concentration of Yb3+. The up-conversion luminescence properties and mechanisms of Yb3+, Er3+ co-doping phosphor under 980 and 1550 nm excitation are discussed in detail. Moreover, the temperature performance of the Na3Y(VO4)2:Yb3+, Er3+ phosphors are investigated based on Boltzmann-type population distribution theory. Temperature dependence of the two green emissions ratio of Er3+ is obtained and the maximum temperature sensing sensitivity is 6.7×10−3 K-1 at 406 K.
Oxy-fluoride glass-ceramics (GC) has advantages such as low phonon energy of fluorides and the excellent physical and chemical properties of oxides. Here we report the synthesis of a series of ...transparent oxy-fluoride GCs with various doping contents of Er3+ (0.2 mol%-10 mol%) synthesized under different temperatures, and their UCL properties under the excitation of 1550 nm laser. We found that Er3+-doped glass matrix with NaYF4 fluoride nanocrystals demonstrated greatly enhanced UCL compared with as-prepared glass. The samples showed red light emission with 3 mol% Er3+ as the optimal doping content. The wavelength of UCL could be fine-tuned by the temperature of heat treatment, and 893 K seems to be the optimal temperature. We also studied the photoelectric conversion characteristics of the optimal Er3+ doped oxy-fluoride GCs with a polycrystalline silicon solar cell under the excitation of a 1550 nm laser as a possible strategy to effectively improve the photo-electric conversion efficiency of solar cells.
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