## 辐照导致的稀土离子变价

Eu2þ/Eu3þ reduction and Tb3þ/Tb4þ transformation in SrAl2Si2O8:Eu/Tb system

McKeever, S. W. S., et al. Radiation Measurements 133 (2020): 106278.

Laser Photochemistry of Lanthanide Ions

Photoreduction of Ytterbium and Samarium

### 价态的表征

The determination of dopant ion valence distributions in insulating crystals using XANES measurements

### 飞秒激光系列(中国/日本)

(1) the Eu3+ ions at sites with higher covalence degree can be more easily reduced than those at sites with lower covalence degree via femtosecond laser pulses. 通过Fluorescence line-narrowing spectra得到证实(激发源单色可调激光、8 K温度)。
(2) 辐照前后吸收峰变化，200 to 410 nm处的增强来自Eu2+的吸收，410 to 500 nm处的增强来自positive hole centers。后者的归属是基于别人文献报道的the gamma irradiation in some borate and silicate glasses导致了positive hole centers在类似位置(410 to 500 nm)的吸收。
(3) 飞秒激光还原的机理：It is well-known that femtosecond laser pulses can create electrons and holes in the materials throughout multiphoton excitation. The irradiation of the glass by femtosecond laser creates electron-hole pairs by band-to-band transition via multiphoton excitation. This means that the electrons are delocalized from the valence band (VB) and transferred to the conduction band (CB). The electrons and holes are redistributed throughout the conduction and valence bands. As a result, some electrons were trapped by the Eu3+ ions which led to the formation of the Eu2+ ions. Some holes were trapped by the trapped centers that gave rise to the appearance of the positive hole centers which induced unique absorption in the range from 400 to 500 nm.

4.
Bi3+在800 nm飞秒激光辐照下，部分地变为Bi2+或Bi+，而材料本身是不吸收800 nm (non-resonant wavelength)光的，所以这个photoreduction是非线性过程。It is suggested that the multiphoton absorption process and photoreduction of Bi3+ ions are involved in the experimental phenomenon. Through Joule heating, multiphoton ionization, and collisional ionization, free electrons are created in the fs laser-focused area. After that, Bi3+ ions capture the free electrons and hence transform into Bi2+, and even Bi+ ions. The holes would be trapped at nonbridging oxygens in the SiO4 polyhedrons. Note that Bi3+ + e− → Bi2+ and Bi2+ + e− → Bi+ are very similar to the conventional chemical processes. It is suggested that control of the laser parameters can tune the active centers for the luminescence tuning of main group metal ion-doped phosphors.

5. Sm3+掺杂钠铝硼酸盐
[Permanent photoreduction of Sm3+ to Sm2+ inside a sodium aluminoborate glass by an infrared femtosecond pulsed laser-APL-1999] (1) Recently, it was found that direct excitation of the CTS and 5d – 4 f transition bands can induce photoreduction in some rare-earth ions.(溶液中)
(2) 800 nm fs pulsed laser辐照后吸收谱的变化：通过对比还原气氛下得到的Sm2+掺杂玻璃的吸收谱，醉着认为吸收谱的差异对应于辐照后得到的Sm2+的吸收。
(3) Sm3+在700-900 nm范围没有吸收，所以光还原是一个多光子还原的过程。
(4) EPR：对于没有辐照的样品，没有看到apparent signals，但是对于辐照后的样品，出现了一个在g =2.0的峰，可以归属于defect centers of holes trapped by nonbridging oxygen ions and tetrahedral coordinated boron atoms, and electrons trapped by the quasi-F centers as in gamma-ray-irradiated sodium borate glasses.
(5) 机理：Following the irradiation of the focused femtosecond laser, active electrons and holes were created in the glass through multiphoton ionization, Joule heating, and collisional ionization processes. Holes were trapped by nonbridging oxygen ions as well as by tetrahedral coordinated boron atoms, while some of the electrons were trapped by the Sm3+.
(6) 热稳定性：The trap levels of defect centers may be deep, resulting in stable Sm2+ at room temperature. 作者做实验研究了光还原得到的Sm2+的热稳定性，发现室温的时候Sm2+很稳定，但是一旦温度超过Tg(玻璃转化温度)，就被氧化为Sm3+。作者这里并没有说明thermal bleaching的气氛。

### 日本

#### 日本-固体

• PLAL caused not only the micronization but also the reduction of Eu ions.
• SHG是second harmonic generation of Nd:YAG，对应于532 nm，THG是三阶倍频对应355 nm，FHG四阶倍频对应266 nm。266 nm的最有效。
• 对KBaPO4来说，SHG和THG似乎还让Eu2+发光变弱了。
• Considering the PLE spectra and the laser wavelength dependence of reduction by the photo excitation, the photon energy of wavelength corresponding to [Eu3+ − O2−] CT is probably the threshold to induce the reduction by the photo excitation.
• 还原过程：
(1) 由于电荷迁移态Eu2+ (Eu3+ + e) and O (O2− + h+)。However, they usually recombine to their original state and release the energy as the luminescence at the same time. Therefore, the hole generated at O2− must be trapped somewhere in order to stabilize Eu ions in divalent. PO4 groups in phosphate phosphor can probably trap the hole because SiO4 groups in Sr2SiO4:Eu are reported to be able to trap the hole.
(2) Sr或者Ba的空位捕获了hole，作者观察到了Ba和Sr空位的发光(和还原气氛的Eu2+发射波长不同)，但是没有直接证据。(我：可能是得到占据不同格位的Eu2+)
• 激光作用范围：In the case of the reduction by the photo excitation, only Eu ions existing near the particle surface within a penetration depth of pulsed laser are reduced unlike in the case of H2 reduction process.

3. SrAl2O4:Eu3+
[Radiation-induced reduction of Eu3+ doped in SrAl2O4 — Materials Letters – 2021] Ionizing radiation的坏处：it may cause a risk to harm biological cells as it damages DNA.

Ionizing radiation的计量：One of the techniques is to monitor and record the dose history of radiation workers by using a so-called dosimeter. In a dosimeter, a luminescent material is used as a sensing element, which typically shows thermoluminescence (TL) , optically stimulated luminescence (OSL), or radio-photoluminescence (RPL)  phenomenon.

PRL材料的典型代表：Ag-doped phosphate glass

Emission-color change in Eu-doped high-symmetry glaserite-type silicates

Emission-color change caused by photoredox reactions of europium ions in Ba3MgSi2O8

The pump-probe measurement verifies that the excitations above the band gap energy induce the formation of the transient Eu3+ ions with a recovery life time of 11.0 ms.

6. Eu3+ acting as electron trap in glass
[Eu3+ Ion as Electron Trap in Glass-Journal of the Physical Society of Japan-1967] [Color Centers in X-Irradiated Soda-Silica Glasses-JACS-1964]

7. [X-ray and UV irradiation effects on Ce3+ ion doped in UV sensitive glass-1999] When UV radiation irradiated on this glass, Ag+ ion is reduced to Ag atom. The role of Ce3+ ion in this glass is considered to be a sensitizer which sensitize the reduction of Ag+ ion.

Possible applications of FEL in UV photochemistry and nuclear waste reprocessing-1993

#### 日本-溶液

1. EuCl3 in organic solvent (photochemcial reduction)

2. Eu3+掺杂aqueous solution和alcoholic solution (Eu(ClO4)3)
[Higher yield of photoreduction from Eu3+ to Eu2+ with shorter wavelength irradiation-CPL-1992]
(1) 如果只激发f-f的可见光区，那么只有PL，没有redox reaction。Photoredox reactions have been known to occur when a charge transfer band, in other words, a 5d←4f transition in the UV region, is excited。
(2) Eu2+稳定性：溶液中光还原得到的Eu2+不稳定(所以测瞬态吸收谱)：Although Eu2+ is not unstable in an aqueous solution, flash photolysis is a convenient means of measuring it before the second reactions (discussed later) and its eventual diffusional escape from the observing volum.
(3) 波长依赖光还原作者只选取了两个波长，分别对aqueous solution和alcoholic solution的样品进行测试，计算四种情形下的光还原的quantum yield，发现都是短波长更有效。
(4) 波长依赖光还原的原因：激发CT态，形成了molecule Eu2+RHOH+，然后弛豫到发射能级，但是对应的fluorescence的QY很低。大部分的Eu2+RHOH+会存在一个dissociation和recombination的过程(两个过程竞争)，前者对应于光还原。
(5) 光还原的quantum yield计算：利用消光系数还有参比物，计算光还原的效率。

3.Eu3+掺杂aqueous solution和alcoholic solution
[High photoreduction yield of Eu3+ to Eu2+ in alcoholic solutions and its wavelength dependence-JAC-1993]

(1) 应用：Photoredox reactions of lanthanide and actinide ions are expected to be utilized for nuclear fuel reprocessing.
(2) 辐照源：193 nm (ArF)、222 nm (KrCl)、248 nm ((KrF)、308 nm ((XeCl)
(3) alcoholic solution中Eu3+更容易被光还原，相比水溶液。
(4) 概述：The photoreduction yield of Eu3+ to Eu2+ depends on the kind of Eu3+ counter-ions and solvents and on the excitation wavelengths. The photoreduction yield can be explained in terms of the dissociation efficiency of the geminate pair (Eu2+ and oxidized ligand). Similar chemistry may be observed for other lanthanide as well as actinide ions in solution.

4.Sm3+掺杂methanol solution

(1) 在甲醇溶液中，Sm3+到Cl的CTB在220 nm有最大值。作者这里用的是KrF excimer laser波长为248 nm，和CTB匹配。
(2) 作者似乎并没有说光还原之后的Sm2+是否稳定，应该是不稳定，就像之前Eu2+

6. EuCl3 in in methanol in the presence of 15-crown-5-ether

• White-light laser：the femtosecond white-light laser was generated during the irradiation.
• 摘要：Europium 3+ ions in methanol were found to be reduced to the corresponding 2+ ions upon irradiation with intense femtosecond laser pulses. The excitation wavelength of 800 nm was nonresonant with their electronic transitions of Eu3+. It is notable that femtosecond white-light laser was generated when the reactions occurred. The mechanisms can be explained in terms of solvated electron formation followed by the reduction. The electron ejection in a focused beam in solution has been known to be accompanied by white-light laser.
• 上图：The Eu2+ fluorescence intensity increases nonlinearly with increases in the irradiation energy. The solid line of the log–log plots has a slope of 2.9. The two right inserted figures are pictures of white-light laser on a paper 20 cm after laser passing through the sample cell for the cases of the highest and lowest irradiation energies. The size of the central white portion is approximately 40 mm in diameter. When the Eu2+ fluorescence was observed, the femtosecond white-light laser was emitted. The top picture is a side view of the cell, showing luminescence and bubbles during irradiation. The black arrow on the picture indicates the incident laser direction.

7. SmCl3 in in methanol in the presence of 15-crown-5-ether

• 4f electronic excited states反应活性低的原因：The lanthanide ions have 4f-4f (ff) transitions in UV-vis-IR regions with atomic-like line spectra because f-orbital electrons have high electron densities near the nuclei and have weak interactions with the surrounding solvent. As a result, the 4f electronic excited states show low reaction activities; in fact, no photoredox reaction has been reported from the lowest emissive states in the 4f electronic excited state in solution.
• 三种方法实现Eu3+的photo-reduction:
• CT state: excitation to the CT state by one-photon absorption, and the reaction quantum yields are determined to be relatively high (0.1-0.9) dependentonthesolventandtheexcitationwavelengths.
• resonant multiphoton reaction：The first laser light was tuned to one of the ff transitions  (394 nm, from 7F0 to 5L6), and the second and/or third photon was pumped up to the CT states using nano- and picosecond laser pulses. One of the important findings is that a short duration pulse excitation dramatically improves the reaction efficiency.
• 飞秒激光(应该也是multiphoton reaction，不过是nonresonant multiphoton reaction)：focused femtosecond laser pulses at a nonresonant wavelength. The scheme is explained in terms of a reduction to Eu2+ in response to solvent ionization, followed by electron capture by Eu3+
• Sm2+ in in methanol in the presence of 15-crown-5-ether有宽带发光(750 nm)，543 nm可以有效激发Sm2+到4f55d1态。通过检测Sm2+的发光强度，可以反映Sm2+的生成量。
• 403 nm levels work as intermediates of the multiphoton reaction.
• 飞秒实验：作者这里也采用了非共振的飞秒激光来还原Sm3+，作者认为其机理和邱建荣的类似。
• 上面右侧图中偏移是因为光的波长不纯。

8. Yb3+ (YbCl3·6H2O) in MeOH and EtOH

• Absorption spectrum of Yb2+ significantly depends on solvents. The absorption and emission spectra of Ln2+ (Ln = Eu or Sm) are known to be sensitive to solvents. 作者在室温下并没有观察到Yb2+的发光，虽然有明显的吸收。
• 电荷迁移态：The CT levels for YbCl3·6H2O in MeOH and EtOH have peaks at 210 and 240−245 nm and had a tail that extended over 270 nm; therefore, the tail can be excited by the fourth harmonics of the Nd3+:YAG laser (266 nm).
• 还原氧化的双重作用：266 nm开始的时候可以还原Yb3+，但是Yb2+在266 nm或者355 nm处也有吸收，所以266 nm也可以氧化Yb2+。因此用266 nm的光作用在初始样品上，会观察到Yb2+随着辐照增加而增加，随后达到平衡状态，即单位时间还原得到的Yb2+ = 单位时间氧化得到的Yb3+
• 在近红外区，作者观察到了很弱的Yb3+的降低，说明Yb2+的出现是以消耗部分Yb3+作为代价的。
• 波长依赖实验：[Higher yield of photoreduction from Eu3+ to Eu2+ with shorter wavelength irradiation-CPL-1992]报道了波长依赖的UV light-induced photo-reduction，发现波长越短越有效。对于作者的实验来说，The CT state with higher energy may give higher reduction yields for this Yb3+ system, but it is not clear at the present stage. To discuss precisely the UV wavelength dependency, the present experimental errors were too large.
• 误差来源：would contain several tens of percent uncertainty due to the lack of precise measurements of the focused beam size and intensity distributions in the spot。

9. Eu3+ in aerated water in the presence of 2-propanol.

• 第一种方法：reduce Eu3+ by exciting the charge-transfer (CT) band in degassed alcohol with nanosecond laser pulses of VUV or UV wavelength. This procedure has been shown to be effective for not only lanthanoid ions but also noble metal ions dissolved in radioactive waste solution.
• 第二种方法：resonant two-photon excitation to CT state via the f-f transition using visible laser pulses. We have found two photon reduction of Eu3+ to Eu2+ in nano- and picosecond pulse excitations. The reduction efficiencies of Eu3+ were largely improved from on the order of 10−5 to 1 by picosecond pulse excitation. The improvement would be largely reinterpreted by the simple reasons of pulse shortening. The laser intensity should have increased 1 × 104 times by the shortening pulses from 20 ns to 2 ps under the same focusing conditions.
• 第三种方法：reduction of lanthanoid ions with electrons ejected from solvent by nonresonant multiphoton ionization using focused near-infrared (NIR) femtosecond laser pulses. Lanthanoid ions were successfully reduced in degassed alcohol using this approach.
• Highlights：
(1) Near-infrared femtosecond laser pulses cause nonresonant multiphoton ionization of water.
(2) Trivalent europium is reduced by hydrated electrons generated in aerated water in the presence of 2-propanol.
(3) Precipitation of europium sulfate is investigated by absorption/extinction spectroscopy and transmittance measurements.
• 结论部分：This work reveals that the reactive species generated in water by femtosecond laser pulses can reduce europium ions even under an aerated condition. The absence of oxygen is not the critical factor, but hydroxyl radical scavengers are indispensable to achieve the reduction of europium ions.
注：Hydroxyl radicals are highly reactive species that attack most of the organic molecules. They are highly oxidizing in nature which is attributed to their oxidation potential
• The single UV photon reaction showed a tendency toward saturation in the product absorbance of Eu2+ as the number of laser shots. This was simply because of the back photo-reaction of  and an approach to a photo-stationary equilibrium. The back reaction was in fact observed with a quantum yield of 0.35 at 308 nm laser irradiation starting from a fresh Eu2+ sample.

### 韩国

• Sm2+ ions show a high persistent spectral hole burning efficiency. 但是一般来说Sm3+更稳定，所以it is thus needed to reduce Sm3+ to Sm2+ for use PSHB in frequency domain optical storage.
• 空气中Sm3+自还原的基质BaB8O13  or SrB4O7.
• 存在不同的方法得到Sm2+，不同的方法可能导致不同的光学特性，比如photo-bleaching effect。本文repared SrB4O7 and SrB6O10 crystals by using two different reduction methods, i.e., thermal treatment in air and X-ray irradiation.
• xx

### 中国

#### 黄彦林

1.BaBPO5:Sm3+
[Structural defects and luminescence properties of Sm2+ ions doped in BaBPO5 phosphor by X-ray irradiation-JAC-2009] (1) 激发谱的差异：不同方法制备出来的Sm2+掺杂BaBPO5的发光特点不同，作者这里是通过X射线辐照得到Sm2+，别人通过还原气氛得到的Sm2+，激发谱有些许差异。This difference may be due to the different preparation method.The X-ray-irradiated sample may have different defect structures compared to those samples heated in H2-gas.
(2) 热稳定性：To check the thermal stability of Sm2+ ions, the X-ray-irradiated sample was heated at several temperatures in air and then rapidly cooled down to room temperature. Then the emission spectra were measured at RT and are shown. It is found that the Sm2+ ions were stable up to a temperature of 450 °C. At the temperature 250 °C, the intensity of Sm2+ emission keeps stable. At the temperature from 250 to 450 °C, the intensity of Sm2+ fluorescence decreased. This could be understood that the Sm2+ ions reduced by X-ray irradiation were oxidized by heat treatment in air. The oxidation of Sm2+ was constantly increased with increasing the thermal treatment temperature. It can be explained that the electrons or holes in the trapped center are released by thermal energy and then recombined with Sm2+ ions resulting in the Sm3+ ions.

• Sm2+也可以作为probe，就像Eu3+，也存在$${ }^{5} \mathrm{D}_{0} \rightarrow{ }^{7} \mathrm{~F}_{0}$$的nondegenerate的emission。变温实验发现了存在三种Sm3+ crystallographic sites have different temporal and spectral behavior from each other，但是只有一种是在室温下能发光的。
• 488 nm是常用的实现Sm2+的photobleaching的激光波长(光一方面减少Sm2+，一方面激发Sm2+使其发光)，Sm2+发光强度和激光的作用时间可以用一个双指数来拟合，This indicates that there are at least two kinds of decay times during the photobleaching.
• It is well known that, under X-ray irradiation, the O components are easy to be ionized to release an electron that could be captured at Sm3+ sites, which results in the reduction of Sm2+.
• Actually, the real mechanism of photobleaching effect and hole burning of Sm2+ ions are not clear at present; however, it can be widely accepted that the photobleaching process is charge transfers from the Sm2+ ions to nearby defects.
• The photobleaching experiments show that the luminescence intensity decreases quickly as the 488-nm laser irradiation time is increased. Usually, this photobleaching effect is ascribed to the photoionization of Sm2+ to Sm3+.  This shows that the Sm2+ ions formed by X-ray irradiation are not stable. It is reasonable to say that these defects induced by X-ray irradiation play an important role in the photobleaching.

3. Sm3+掺杂Li2O–SrO–B2O3玻璃
[Huang Y, Jiang C, Jang K, et al. Luminescence and microstructure of Sm2+ ions reduced by x-ray irradiation in Li2O–SrO–B2O3 glass-JAP-2008] (1) 光学烧孔：Luminescence of Sm2+ has been paid an intense attention because this ion is known to have a property of persistent spectral hole burning (PSHB). Sm2+掺杂玻璃对PSHB来说很有吸引力，因为glasses are the wellknown host in which the inhomogeneous linewidth is significantly broadened，而且, PSHB observed in glass matrices containing Sm2+ ions showed relatively high thermal stability。之前也有报道通过X射线辐照得到的Sm2+掺杂玻璃表现出比还原其分析得到的样品更好的光学烧孔的性能。
(2) 5D07F0 (Sm2+)：两个能级都是非简并的，所以不能在CF下劈裂，因此图中该峰的宽化reflect the bonding environment of the Sm2+ ions with large inhomogeneities.
(3) 弱宽峰PL：可能来自Sm2+的d-f跃迁，也可能是来自defect centers的发射。

5. Radiation-Induced Effects and Valence Changes of Samarium Ions in Sr3Sm ( PO4 ) 3

5. Temperature-Dependent 5D0-7F0 Luminescence of Sm21 Ions Doped in Alkaline Earth Borophosphate Glass

#### 非黄彦林

Photoreduction of active ions with ultrashort-pulse lasers is one remarkable way to modulate the luminescence of phosphors. Once the valence state of the activator centre changes, the luminescence of phosphors will be modulated.

Compared to photons, ionizing radiation possesses much higher energy. Ionizing radiation is capable of liberating electrons from atoms or molecules and breaking chemical bonds, thereby ionizing them. Utilizing ionizing radiation with appropriate doses, the luminescence of phosphors can be modified due to the radiation-induced defects or radiation-induced tuning of dopant valence states, or radiation-induced matrix phase evolution.

In general, optical excitation is widely regarded as the excitation source for the observation of PL. Furthermore, the bombardment by short-pulsed lasers can cause photoactivated chemical reactions. It is known that some divalent Ln ions (e.g., Eu2+) are unstable in an oxidizing atmosphere. Annealing these phosphors at relative high temperatures for hours in a reducing atmosphere is normally required for the reduction of Eu3+ to Eu2+, unless the host has no specific structure groups.122 In contrast, some Ln3+ ions, such as Eu3+ and Sm3+ ions, can be reduced to their divalent counterparts in ambient conditions with the aid of a photon-assisted technique. The mechanism is due to the charge transfer (CT) states in the deep UV regime, and these CT states are photochemically active.123 After reduction, the PL property of these ions shows substantial changes. In early studies, Eu3+ and Sm3+ ions were found to be reduced to Eu2+ and Sm2+ ions by a nanosecond excimer laser.124 The reduction processes were achieved by a one-photon absorption process. Further to the photoreduction study in Eu3+ ions, the results indicated that shorter wavelengths and using methanol as a medium could enhance the reduction yield of Eu2+ ions.125 On account of advances in laser technology, pico/femto-second-pulsed lasers are readily available. Nobuaki et al. harvested Eu2+ from Eu3+ ions in methanol by using a picosecond Ti:Sapphire laser operating at 394 nm.126 The excitation wavelength can match with the transition 7F05L6 and hence the electrons in Eu3+ ions can be effectively excited to the CT states for the reduction. The concentration of Eu2+ ions increased with each pulse and eventually leveled off. Upon the formation of Eu2+ ions, the PL colour of the solution changed from pink to white, accounted for by the mixture of pink colour from the PL of Eu3+ and the blue colour from Eu2+ ions. A multiphoton ionization process can also be used to perform photoreduction.127 For example, Nishida et al. adopted a similar method to reduce Eu3+ to Eu2+ ions by using pulsed fs laser at 800 nm.128 As shown in Figure 9a, the blue PL of Eu2+ increased along with the decreasing PL of Eu3+, and this effect was more pronounced at higher pulse energies.

Moreover, external sources beyond the UV, visible, and infrared regimes, such as X-rays129 and γ-rays,130 have previously been considered for tuning the PL of Ln3+ ions. Huang et al. reported the reduction of Sm3+ to Sm2+ ions by using X-ray irradiation on Sm-doped BaBPO5 (Figure 9b).131 In their experiments, the thermal stability of the reduced Sm2+ ions was examined by exposing the sample at various temperatures. The reduced sample was able to remain stable up to a temperature of 450 °C. Meanwhile, the effect of γ radiation on the PL, absorption, and decay characteristics of Y2O3:Eu3+ and SrAl2O4:Eu2+ phosphors has been illustrated.132 Interestingly, the PL spectra and decay characteristics of Y2O3:Eu3+ phosphor remained intact after irradiation with γ-ray (595 kGy). This result indicates the absence of a reduction of Eu3+ to Eu2+ in the sample, because the reduction process is a host-sensitive process. In addition, the SrAl2O4:Eu2+ phosphor showed a slight decrease in PL intensity after γ-ray irradiation. The reduction in PL was the consequence of γ-ray-induced defects, which could favor extra nonradiative relaxation.

The photon is a fundamental particle in physics, exhibiting wave–particle duality. Over the past several decades, laser technologies have been widely used to process various materials, particularly using ultrashort lasers with a high resolution in time and space. The electric field strength of a focused ultrashort laser pulse can be up to 100 TW·cm−2, which is sufficient for inducing many nonlinear optical effects in photonic materials, when the pulse energy is 1 μJ and the width is 100 fs. Hence, focused ultrashort-pulse lasers can be employed to induce various microstructural changes in transparent materials.70 Femtosecond ultrashort pulse lasers have been utilized to fabricate many types of functional optical devices, including photonic crystals, couplers, optical waveguides, and 3D optical storage devices.71 For instance, using the ultrashort-pulsed light, microscopic modifications of luminescence have been successfully achieved in photonic glasses (Figure 6a).72

Photoreduction of active ions with ultrashort-pulse lasers is one remarkable way to modulate the luminescence of phosphors. Once the valence state of the activator centre changes, the luminescence of phosphors will be modulated (Figure 6b).73 Using space-selective irradiation by an ultrashort-pulse laser, simultaneous tuning of the dopant distribution and matrix phase evolution has also been presented in the glassy phase. Using an 800-nm mode-locked pulse laser and an XYZ stage, parallel waveguide structures can be written inside the glass bulk (Figure 6c).74 With the benefit of the ultrashort pulse duration, the orderly precipitation of nanocrystals Ga2O3 and LaF3 and the selective incorporation of metal ions into them can be achieved.

In addition, the employment of a filtered light has proven to be a simple and effective method for tuning PL.75 Through changing the cutoff filter and controlling the illumination time, the emission wavelength can be modulated, owing to self-regulated arrangement of the absorption edge and the Si crystalline size in porous silicon.76 Moreover, UV irradiation can increase the luminescence quantum efficiency of a nanophosphor coated with polymers, resulting in cured passivating polymers.77

Compared to photons, ionizing radiation possesses much higher energy. Ionizing radiation is capable of liberating electrons from atoms or molecules and breaking chemical bonds, thereby ionizing them. Utilizing ionizing radiation with appropriate doses, the luminescence of phosphors can be modified due to the radiation-induced defects or radiation-induced tuning of dopant valence states, or radiation-induced matrix phase evolution.78 In general, the most common forms of ionizing radiation include α-, β-, γ-, and X-rays.79

Besides tuning by fs laser irradiation, luminescence of main group metal ion-doped phosphors can be modulated by γ-ray irradiation. Bi-doped α-BaB2O4 (α-BBO) single crystals were prepared through the Czochralski method in an N2 atmosphere.160 The absorption and PL spectra of as-grown crystals indicate that Bi2+ and Bi3+ centers occur in the hosts. There was no NIR luminescence in the as-grown Bi-doped crystals under 808 nm or 980 nm laser diode excitation. In contrast, intense NIR emission was observed in the γ-ray-irradiated Bi-doped α-BBO (Figure 11c). Bi+ ions were considered NIR luminescent centres. Under γ-ray irradiation, the electrons can be easily released from the Ba vacancies (VBa″), and would freely displace in the lattice. Then, the Bi2+ and Bi3+ ions would capture the free electrons and transform to Bi+ centres. Interestingly, the γ-ray-irradiated Bi-doped α-BBO can be bleached by heat annealing, due to the processes of Bi+ − 2e− → Bi3+ and VBa + 2e− → VBa″. Similarly, NIR luminescence from γ-ray-irradiated Bi-doped Y4GeO8 crystals was investigated.161

4.3.1 Femtosecond Laser-Induced Tuning

2. SrAl2O4:Eu2+和Y2O3:Eu3+
[Photoluminescence and gamma-ray irradiation of SrAl2O4:Eu2+ and Y2O3:Eu3+ phosphors-JPCS-2007] In irradiated samples, the brightness is decreased without sensible change in the wavelength distribution of the luminescence spectrum and in the decay kinetic upon gamma exposure. Moreover, the emission due to Eu3+→Eu2+ conversion in Y2O3:Eu3+ phosphors was not observed in our sample after irradiation to high exposure. Also the brightness of SrAl2O4:Eu2+ phosphor turned out to decrease after the exposition to ionizing radiation while the luminescence wavelength distribution remained unchanged.

4. Y2O3:Eu3+
[Ultraviolet light-induced spectral change in cubic nanocrystalline Y2O3:Eu3+-CPL-2003]