† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 51132004, 11474096, 11604199, U1704145, and 11747101), the Fund from the Science and Technology Commission of Shanghai Municipality, China (Grant No. 14JC1401500), the Henan Provincial Natural Science Foundation, China (Grant No. 182102210117), and the Higher Educational Key Program of Henan Province of China (Gant Nos. 17A140025 and 16A140030).
The up-conversion luminescence tuning of rare-earth ions is an important research topic for understanding luminescence mechanisms and promoting related applications. In this paper, we experimentally study the up-conversion luminescence tuning of Er3+-doped ceramic glass excited by the unshaped, V-shaped and cosine-shaped femtosecond laser field with different laser powers. The results show that green and red up-conversion luminescence can be effectively tuned by varying the power or spectral phase of the femtosecond laser field. We further analyze the up-conversion luminescence tuning mechanism by considering different excitation processes, including single-photon absorption (SPA), two-photon absorption (TPA), excited state absorption (ESA), and energy transfer up-conversion (ETU). The relative weight of TPA in the whole excitation process can increase with the increase of the laser power, thereby enhancing the intensity ratio between green and red luminescence (I547/I656). However, the second ETU (ETU2) process can generate red luminescence and reduce the green and red luminescence intensity ratio I547/I656, while the third ESA (ESA3) process can produce green luminescence and enhance its control efficiency. Moreover, the up-conversion luminescence tuning mechanism is further validated by observing the up-conversion luminescence intensity, depending on the laser power and the down-conversion luminescence spectrum under the excitation of 400-nm femtosecond laser pulse. These studies can present a clear physical picture that enables us to understand the up-conversion luminescence tuning mechanism in rare-earth ions, and can also provide an opportunity to tune up-conversion luminescence to promote its related applications.
The up-conversion luminescence of rare-earth ions is a nonlinear optical phenomenon, which can absorb low-energy excitation light and produce high-energy ultraviolet or visible emission. This phenomenon has been extensively explored because of its distinct optical properties, such as near infrared excitation, photostability, narrow spectrum, large Stokes shift, long luminescence lifetime, and well-defined emission bands.[1,2] Recently, the up-conversion luminescence of rare-earth ions has widely applied in many fields, such as laser sources,[3,4] fiber-optic communications,[5,6] light-emitting diodes,[7] solar cells,[8] color displays,[9,10] biolabeling and biomedical sensors.[11–15] The up-conversion luminescence mechanism of rare-earth ions can be changed by exciting different light sources. When the rare-earth ions are excited by the continuous wave laser, the ground state absorption (GSA) or excited state absorption (ESA),[16,17] energy transfer up-conversion (ETU),[18] and photon avalanche (PA)[19] are important up-conversion luminescence processes among numerous fundamental mechanisms. A femtosecond laser field has a high peak intensity and a short pulse duration. It also easily excites the up-conversion luminescence of rare-earth ions through two-photon absorption (TPA) or multi-photon absorption (MPA).[20,21]
In recent years, realizing the up-conversion luminescence tuning of rare-earth ions has become a popular research topic and is very important for promoting its relevant applications. For example, controllable red, green and blue emission can be used in the bright white emission and color display. Single-band up-conversion emission is applied to biomedical sensing images, and multiple color tuning is important for anti-counterfeiting applications. The relevant studies have proposed some methods to tune the up-conversion luminescence of rare-earth ions. For example, conventional chemical method can be utilized to effectively tune the up-conversion luminescence of rare-earth ions by changing chemical composition,[22] crystal structure,[23] nanoparticle size,[24] and surface groups.[25] Physical methods could also be used to tune up-conversion luminescence of rare-earth ions, such as applying electric field,[26] magnetic field,[27] plasmon,[28] temperature,[29] and laser parameters.[30–33] In addition, our group has realized luminescence modulation by modulating the phase or polarization of the femtosecond laser field.[34,35] For example, the single-photon and two-photon luminescence in Er3+ ion can be enhanced or suppressed by π or square phase modulation.[34] The green and red up-conversion luminescence in Er3+-doped NaYF4 nanocrystals can be tuned by the square phase modulation.[35] However, up-conversion luminescence tuning by varying laser power or performing multiply spectral phase modulation is rarely studied because of the complexity and the coexistence of the multiple excitation processes.
In this paper, we propose a new scheme to tune the up-conversion luminescence in Er3+doped ceramic glass by varying the power or spectral phase of the femtosecond pulse. The experimental results show that green and red up-conversion luminescence can be effectively tuned by varying the laser power of the unshaped or V-shaped femtosecond pulse, but cannot be modulated by modifying the laser power of the cosine-shaped femtosecond pulse. We explain the experimental observations by analyzing different excitation processes. The relative weight of TPA in the whole excitation processes can increase as the laser power increases, thereby increasing the intensity ratio between green and red luminescence (I547/I656). Moreover, the second ETU process suppresses the intensity ratio between green and red luminescence I547/I656, while the third ESA process can enhance the intensity ratio of I547/I656. The up-conversion luminescence tuning mechanism in Er3+-doped ceramic glass is further validated by observing the up-conversion luminescence intensity at different laser powers and the down-conversion luminescence under 400-nm femtosecond laser pulse excitation.
In our experiment, 5%Er3+-doped NaYF4 nanocrystals dispersed in silicate glass were used as our sample and were prepared with the composition of 40SiO2–25Al2O3–18NaCO310YF3–7NaF–5ErF3 (in unit mol%). The mixed raw materials were placed in a platinum crucible with a lid, and were treated for 45 min at 1450 °C in an ambient atmosphere and successively heated at 450 °C for 10 h. Glass products were further processed by incising and polishing, and the sample with a size of 7 mm×12 mm×2.5 mm was used in our optical experiment. The x-ray diffraction (XRD) patterns and transition electron microscopy (TEM) images were obtained in accordance with previously described methods[36] to reveal the existence of a cubic α-NaYF4 crystal. Moreover, the spherical nanocrystals were distributed densely and homogeneously in a glass matrix with an average size of 20 nm–30 nm. Figure
The femtosecond pulse shaping technique has been widely used to control multi-photon absorption in atomic and molecular systems, and the V-style and cosine phase modulation have been utilized in actual experiments.[37,38] Here, we further utilize the V-style and cosine phase modulation to tune up-conversion luminescence of the Er3+-doped ceramic glass. We show the laser spectrum (black solid line) and the V-style (red dashed line) and cosine phase (blue dotted line) modulation in Fig.
The UV-Vis-NIR absorption spectrum of the Er3+-doped ceramic glass is shown in Fig.
In the experiment, we use the unshaped, V-shaped and cosine-shaped femtosecond laser pulse to excite the Er3+-doped ceramic glass, and we collect the up-conversion luminescence signals by a spectrometer, and further observe and analyze the intensity ratio between green and red up-conversion luminescence under different laser powers. Figure
To demonstrate the physical mechanism of the green and red up-conversion luminescence tuning by varying the laser power or spectral phase, we show the energy level diagram of Er3+ ion and the possible excitation and emission processes in Fig.
Since the generation of red and green luminescence can be attributed to different excited pathways for the unshaped and shaped femtosecond pulses, we can explain the green and red up-conversion luminescence tuning in Fig.
We first demonstrate the physical mechanism of green and red up-conversion luminescence tuned by varying the laser power of the unshaped and V-shaped femtosecond pulse. For the unshaped femtosecond laser pulse, under the lower laser power, the TPA cannot effectively occur. The green luminescence is mainly generated by the ETU1 process, while the red luminescence is mainly contributed by the ETU1 and ETU2 process, which leads to the lower intensity ratio of I547/I656. However, under the higher laser power, the TPA can be effectively excited, which plays an important role in generating the green and red luminescence. With the increase of the laser power, the relative weight of the TPA in the whole excitation process can be increased, which induces the intensity ratio of I547/I656 to rise. Comparing with the unshaped femtosecond laser pulse, the peak intensity of the V-shaped femtosecond pulse is weak but it is still larger than that of the cosine-shaped femtosecond pulse. The relative weight of TPA in the whole excitation process for the V-shaped femtosecond pulse is lower than that for unshaped femtosecond pulse under the same laser power. Consequently, the intensity ratio of I547/I656 for the V-shaped femtosecond pulse is lower than that of unshaped femtosecond laser pulse. Therefore, the intensity ratio tuning of I547/I656 by varying the laser power for the unshaped or V-shaped femtosecond pulse can be attributed mainly to the change in the relative weight of TPA in the whole excitation process.
We further analyze the influence of laser power on the intensity ratio of I547/I656 for the cosine-shaped femtosecond pulse. Compared with the unshaped femtosecond pulse and V-shaped femtosecond pulses, the cosine-shaped femtosecond pulse has multiple subpulses, and its peak intensity is very weak. Even when the laser power is increased, the TPA still cannot be effectively excited. The ESA and ETU process can occur but their relatively weight remains unchanged as the laser power varies. Moreover, it is clear that the ESA3 process can enhance the green luminescence, while the ETU2 process can enhance the red luminescence. By varying the laser power of the cosine-shaped femtosecond pulse, the green and red luminescence generation can simultaneously change, resulting in a nearly constant intensity ratio of I547/I656. Therefore, the intensity ratio tuning of I547/I656 by varying the laser power for cosine-shaped femtosecond pulse can be attributed to the stable relative weight of ESA and ETU process in the whole process.
To further verify our findings about the mechanisms for the green up-conversion luminescence and the red up-conversion luminescence, respectively, we show the up-conversion luminescence intensity at wavelengths of 547 nm (blue squares) and 656 nm (red circles) by varying the laser power of the unshaped (Fig.
To further verify the important role of TPA in green and red up-conversion luminescence, we use a 400-nm femtosecond pulse to excite the Er3+-doped ceramic glass to simulate the TPA. The down-conversion luminescence spectrum in the visible light region is shown in Fig.
According to these studies, we can find that the unshaped or the V-shaped pulse has an ultrashort pulse duration, and does not induce the ESA process, while the cos-shaped pulse with multiple subpulses can effectively excite the higher excited state absorption by the ESA process. By increasing the laser power, the relative weight of the TPA in the whole excited process can increase for the unshaped and V-shaped femtosecond pulse, while the TPA cannot be effective excited for cos-shaped femtosecond pulse. Therefore, the multiple time-delayed subpulses can create the ESA process and the larger laser power can induce the effective TPA in rare-earth ions. These femtosecond pulse shaping techniques can be further extended to tune the up-conversion luminescence of other rare-earth doped nanomaterials
In this work, we have tuned the intensity ratio between the green and red up-conversion luminescence of Er3+-doped ceramic glass by varying the laser power of unshaped, V-shaped and cosine-shaped femtosecond laser pulse. The experimental results show that the intensity ratio between green and red up-conversion luminescence can be increased by increasing the laser power of the unshaped and V-shaped femtosecond field, while it is nearly unchanged by varying the laser power of the cosine-shaped femtosecond field. Our analysis indicates that the increase of the relative weight of TPA in the whole excitation process can increase the intensity ratio of I547/I656, the ETU2 process can generate the red up-conversion luminescence and reduce the intensity ratio of I547/I656, while the ESA3 process can produce the green up-conversion luminescence and enhance its control efficiency. These studies present a clear physical picture of the green and red up-conversion luminescence tuning in rare-earth ions and they can also provide a new way to control the up-conversion luminescence tuning of rare-earth ions. Future studies can design the experiment from two aspects to tune the intensity ratio of green and red up-conversion luminescence: the first is to control the power of a femtosecond laser field, and the second is to modulate the spectral phase of a femtoseocond laser pulse.
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