1. Introduction

A significant amount of interest has been focused on the luminescent properties of rare-earth (RE) ion doped crystals for a wide variety of applications including eye-safe lasers, phosphors, IR-countermeasures, gas sensing, and scintillators [1–7]. Recent results have indicated that PbCl₂ and PbBr₂ based ternary lead halides (e.g. KPb₂Cl₅, RbPb₂Cl₅, KPb₂Br₅, RbPb₂Br₅) are promising novel host materials for infrared (IR) solid-state lasers [7–10]. The low maximum phonon energies of these ternary lead halides lead to reduced nonradiative decay rates and efficient emission from RE ions at IR wavelengths. IR-to-visible upconversion luminescence in RE-doped low-energy phonon crystals has been studied for potential applications in displays, sensors, and upconversion lasers [11,12]. In addition, great effort on the investigation of pure and RE activated halide crystals were made to develop efficient scintillators [6,13–16]. Recently, the development of high performing halide scintillator materials including Ce³⁺-doped LaBr₃ and LaCl₃ as well as Eu²⁺-doped SrI₂ and BaI₂ has led to significant improvements in the applications of radiation detectors [13–16].

Lead based binary and ternary lead halides (PbCl₂, PbBr₂, KPb₂Cl₅ and KPb₂Br₅) are regarded as wide-gap semiconductors with band gap energies ranging from ~2.5 eV to 4.8 eV [17,18]. They are also considered non-hygroscopic crystals with narrow phonon spectra of <140 cm⁻¹ to ~203 cm⁻¹ [4,7,19]. They are monoclinic crystals and transparent in the 0.3-20 μm for the chlorides and 0.4-30 μm spectral range for the bromides [7–12]. When ternary lead halides (e.g KPb₂Br₅) are doped with RE ions, it is assumed that the RE³⁺ ion substitute into the Pb²⁺ lattice sites, while the vacancy of K⁺ provides for the charge compensation. In this work, near-IR emission properties of Pr³⁺- and Er³⁺-doped KPb₂Br₅ (KPB) crystals were evaluated for possible applications in 1.5 μm light source development for eye-safe lasers and optical communications. Moreover, preliminary spectroscopic characterization on the UV-Vis luminescence of Ce³⁺-doped KPb₂Cl₅ (KPC), Eu²⁺-doped KPC and KPB crystals are evaluated for potential radiation detection applications.

2. Materials and experimental details

Ternary potassium lead chloride and potassium lead bromide were synthesized by reacting stoichiometric proportions of using ultra-dry and high purity (99.999%, Alfa Aesar) starting materials of PbCl₂, KCl, PbBr₂, and KBr beads. The synthesized material was subsequently purified through a horizontal zone-refinement. The purified host materials were then loaded with 0.5-2 wt.% of ultra-dry RE-halide powders (PrBr₃, ErBr₃, CeCl₃, EuCl₂, EuBr₂) (99.99%, Alfa Aesar). Following halogenation of the synthesized RE doped KPb₂X₅ (X = Cl, Br) materials, crystal growth experiments were performed using a two-zone vertical or a modified horizontal Bridgman technique. More details of the purification and material preparation of RE doped KPC and KPB were described elsewhere [10]. Samples of good optical quality with dimensions 3 × 5 × 5 mm³ were selected and polished for optical spectroscopy measurements.

The room temperature absorption spectra were recorded using a Cary 5000 spectrophotometer. Infrared emission studies were carried out using a Tm-fiber laser operating at 1.907 µm, an Er-fiber laser operating at approximately 1532 nm or diode lasers operating at 1.45 µm. The ultraviolet (UV) luminescence spectra were obtained using Cary Eclipse Fluorescence Spectrophotometer, PTI TimeMaster Fluorometer or Jobin Yvon Fluorog-3 Spectrofluorometer. The visible/near-IR emissions were dispersed in a 0.3 m monochromator in conjunction with a photomultiplier tube (PMT)/InGaAs detector. The emission spectra were recorded using a standard lock-in technique. Excitation spectra and emission decay measurements were performed using the output (5 ns pulses, 10 Hz) of a Nd:yttrium aluminium garnet (YAG) pumped Optical Parametric Oscillator (OPO) system. Radioluminescence was obtained by X-ray irradiation through a Be window, using a Phillips 2274 X-ray tube with a tungsten target operated at 50 kV and producing an effective X-ray energy of ~25 keV with a dose rate of 1.75 Gy/s for 30 s. The luminescence decay time for UV-visible spectral region was obtained using a HORIBA Jobin Yvon Fluorolog-3 Spectrofluorometer paired with a time-correlated single photon counting (TCSPC) module. The excitation source was a NanoLED pulsed at 314 nm, each pulse was <1.5 ns in length.

3. Results and discussion

3.1 Comparative studies of Pr³⁺- and Er³⁺- doped potassium lead bromide for eye-safe laser applications

Efficient laser emission in the 1.5 – 1.6 µm eye-safe wavelength window continues to be of significant interest for civilian and military applications [10–12]. Resonantly pumped Er:KPB and Pr:KPB were explored for eye-safe laser applications given that Er³⁺ and Pr³⁺ have the metastable emission transitions ⁴I_13/2 → ⁴I_15/2 and ³F₃,³F₄ → ³H_4, respectively, that fall within the eye-safe wavelength regime. Moreover, the low phonon-energy characteristic of KPB (~138 cm⁻¹) implies that additional thermal loads via multiphonon relaxation between levels is weaker [20,21]. Fig. 1 depicts the emission cross-section spectrum of the ³F₃,³F₄ → ³H₄ transition of Pr³⁺. This spectrum was generated by exciting the ³F₃,³F₄ band of Pr³⁺ with a 1440 nm diode laser. Figure 1 also shows the ⁴I_13/2 → ⁴I_15/2 emission band of Er³⁺, which was produced by exciting the ⁴I_13/2 with a 1532 nm Er-fiber laser. To visualize the entire ⁴I_13/2 → ⁴I_15/2 Er³⁺emission band free from laser scattering, the emission was measured under both 970 nm and 1532 nm laser excitation and then the two spectra were joined together. This is necessary because the 1532 nm pump wavelength is within the ⁴I_13/2 → ⁴I_15/2 emission band of Er³⁺, as was displayed in the inset of Fig. 1. The peak emission cross-sections of Er:KPB and Pr:KPB were calculated to be ∼1.6 x 10⁻²⁰ cm² at 1540 nm and ∼6.1 x 10⁻²⁰ cm² at 1667 nm, respectively, using the well- known Fuchtbauer-Ladenburg relation [2]. It is clear from Fig. 1 that the emission cross-section arising from Pr³⁺ is spectrally broad relative to Er³⁺possessing a full width at half maximum of ∼87 nm which provides a possibility of larger laser tunability in the eye-safe wavelength regime. Moreover, Pr³⁺ has an overall larger peak emission cross-section relative to Er³⁺-doped in KPB as well as the more traditional host YAG [12].

 

Fig. 1 Room temperature emission cross-section spectra and schematic energy level diagrams for Pr:KPb₂Br₅ and Er:KPb₂Br₅. The inset shows the ⁴I_13/2→⁴I_15/2 Er³⁺ emission band under 1532 nm pumping.

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As a byproduct of resonant excitation of Pr:KPB at 1550 nm, visible upconversion emission was observed. Shown in Fig. 2(a) is the visible upconversion emission from Pr:KPB when excited at ∼1550 nm. For comparison, the visible emission was also generated by direct 474 nm and 590 nm laser excitation in Figs. 2(b) and 2(c), respectively. Similar to exciting Pr³⁺ at 474 nm, emissions from the ³P₀ → ³H₄, ³H₆, ³F₂, ³F₃ and ³P₁ → ³H₅, ³F₂ transitions were observed. An additional emission band not seen under 474 nm excitation corresponding to the ¹D₂ → ³H₄ transition was observed and can be also be generated directly by exciting the ¹D₂ absorption band of Pr³⁺ (Fig. 2(c)). According to reference [9], the visible upconversion in a similar system, Pr:Rb₂Pb₂Cl₅, is suggested to be attributed to the following scheme: (1) upconversion to the ¹G₄ level via this process ³F₃ + ³F₃ → ¹G₄ + ³H₅ which contributes to the ³H₅ population and (2) a series of excited state absorption (ESA) of the pump which leads to populating the ¹D₂ and ³P_1,2,3 states via the following processes: Pump + ³H₅ → ¹G₄, Pump + ¹G₄ → ¹D₂ and Pump + ¹D₂ → ³P_1,2,3. However in Pr:KPB, the decay transients of the emission coming from the ¹D₂ state (600 nm) and ³P₀ states (650 nm) suggests that energy transfer upconversion (ETU) is responsible for populating the visible emitting states under 1.5 μm excitation. The transients show a significant rise time after the laser pulse followed by a decay time of τ ~213 µs and τ ~140 µs for the ¹D₂ and ³P₀ states, respectively (Figs. 2(d) and 2(e)). The emission decay times are significantly longer relative to direct excitation of the ¹D₂ (τ ~25 µs) and ³P₀ (τ ~17 µs) states (Figs. 2(d) and 2(e)).

 

Fig. 2 Visible emission from Pr: KPb₂Br₅ via (a) 1550 nm, (b) 474 nm, and (c) 590 nm pulsed laser excitation. (d) Visible emission decay transients from the ¹D₂ state under 1550 nm and 590 nm excitation and (e) Visible emission decay transients from the ³P₀ state under 1550 nm and 590 nm excitation.

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This feature is indicative of nonradiative ETU process taking place [22]. To obtain population within the ¹D₂ and ³P_0,1 levels, the ¹G₄ level must be populated. Initially, the 1.5 µm pump populates the ³F₃,³F₂ levels and, due to the resonance conditions amongst the ³F₃ → ³H₅ and ³F₃ → ¹G₄ transitions, the ETU process ³F₃ + ³F₃ → ³H₅ + ¹G₄ is possible [8] which provides a channel for populating the ¹G₄. Once there is population within the ¹G₄ state, the ¹D₂ state is populated through the ETU process ¹G₄ + ¹G₄ → ¹D₂ + ³H₅ [13]. Finally, once there is concurrent population within the ¹G₄ and ¹D₂ levels, the ³P₀ and ³P₁ levels may be populated via the ETU process ¹G₄ + ¹D₂ → ³H₅ + ³P₁ [13]. In addition, if there is significant population in the ³F₂ level, the ³P₀ level could be populated via the ETU process ¹D₂ + ³F₂ → ³P₀ + ³H₄ [13] (Fig. 5).

Visible and infrared upconversion luminescence was observed in Er:KPB under resonant, 1532 nm laser excitation of the ⁴I_13/2 excited state (Fig. 3). The dominant upconversion emission from Er:KPB is in the infrared and corresponds to the ⁴I_9/2 → ⁴I_15/2 transition with a peak at 807 nm (Fig. 3). A weaker infrared upconversion emission band at 858 nm corresponds to the ⁴S_3/2 → ⁴I_13/2 transition of Er³⁺. The dominant ⁴I_9/2 → ⁴I_15/2 infrared upconversion is typical in low phonon-energy hosts. For example, under similar pumping conditions, the dominant upconversion transition in Er-doped yttria (Er:Y₂O₃) is not the ⁴I_9/2 →⁴I_15/2 but rather the visible transitions. This difference is due to the fact that yttria has a larger phonon energy (450-550 cm⁻¹) relative to KPB (∼138 cm⁻¹), and therefore multiphonon emission from the ⁴I_9/2 is more competitive. In even larger phonon energy hosts such as YAG the ⁴I_9/2 →⁴I_15/2 upconversion emission is negligible. The most intense visible upconversion emission arising from Er:KPB is green (~548 nm) and is due to the ⁴S_3/2→⁴I_15/2 transition of Er³⁺. A second, weaker green upconversion emission band ²H_11/2→⁴I_15/2 was also observed at 528 nm. The ²H_11/2 excited state is only ~800 cm⁻¹ above ⁴S_3/2, an energy difference that can be bridged by high frequency phonons. Consequently, the ²H_11/2 and ⁴S_3/2 excited states are thermally coupled at room temperature. Thus, at 300 K these two states will be treated as one state and will be labeled as ²H_11/2 + ⁴S_3/2. A technique commonly used to decipher if ETU or ESA is populating upper energy levels involves comparing the excitation spectrum of the upconverted emission with the corresponding ground state absorption spectrum [23–25]. An indicator that ETU is active occurs when the excitation spectrum of the upconverted emission looks comparable to the corresponding ground state absorption spectrum [23–25]. Evidence of ESA occurs when the excitation spectrum of the upconverted emission possesses additional absorption peaks not seen in the corresponding ground state absorption band. These peaks represent additional absorption transitions that occur after the initial ground state absorption [23–25]. Depicted in Figs. 4(a) and 4(b) are the 300 K excitation spectrum of the ⁴I_9/2 → ⁴I_15/2 and ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 upconverted emissions from Er:KPB along with the ⁴I_15/2 → ⁴I_13/2 ground state absorption spectrum of Er:KPB. It is evident that the excitation spectra in Figs. 4(a) and 4(b) are very similar to the ⁴I_15/2 → ⁴I_13/2 ground state absorption spectrum of Er:KPb₂Br₅, indicating that ETU is likely responsible for the population of ⁴I_9/2 and the thermally coupled ²H_11/2 + ⁴S_3/2 levels rather than ESA.

 

Fig. 3 Room temperature emission from Er: KPb₂Br₅ stimulated by 1532 nm CW laser excitation.

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Fig. 4 (a) Room temperature excitation spectra of the upconverted emission from ⁴I_9/2→⁴I_15/2 of Er³⁺ overlaid with the 300K ⁴I_15/2 →⁴I_13/2 ground state absorption band for Er: KPb₂Br₅. (b) Room temperature excitation spectra of the upconverted emission from ²H_11/2 + ⁴S_3/2→⁴I_15/2 of Er³⁺ overlaid with the 300 K ⁴I_15/2 →⁴I_13/2 ground state absorption band for Er: KPb₂Br₅. (c) Room temperature upconverted emission decay transients of the ⁴I_9/2→⁴I_15/2 and ²H_11/2 + ⁴S_3/2→⁴I_15/2 from Er: KPb₂Br₅.

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To further confirm the nature of the upconversion mechanisms, the decay transients of the upconverted emissions were collected. Figure 4(c) displays the ⁴I_9/2 → ⁴I_15/2 decay transients of Er:KPB after 1532 nm pulsed laser excitation (~5 ns). The transients show a rise time after the laser pulse followed by a decay time of τ ~4.2 ms. The emission decay time is significantly longer relative to direct excitation of the ⁴I_9/2 (τ ~1.9 ms [14]). This feature is indicative of nonradiative ETU process taking place [26]. Furthermore, another characteristic of ETU is that the decay time of the upconverted emission corresponds to the decay time of the intermediate state [26]. In the present case, the decay time of the upconverted ⁴I_9/2 → ⁴I_15/2 emission from Er:KPb₂Br₅ is 4.2 ms, which is similar to the 4.6 ms decay time of the intermediate state (⁴I_13/2) [14]. In the case of the radiative ESA process, the decay occurs immediately after the laser pulse given that ESA occurs only during the laser pulse. The decay time of the upconverted emission via ESA is similar to the decay time after direct laser excitation [24,26]. Displayed in Fig. 4(c) is the decay transient of the ²H_11/2 + ⁴S_3/2→⁴I_15/2 emission, where there is a rise time after the laser pulse followed by a decay times of τ ~1.9 ms. The emission decay time is significantly longer relative to direct excitation of the ²H_11/2 + ⁴S_3/2, which has a value of τ ~180 µs [14]. It has been reported that excited state absorption is likely the mechanism responsible for the green upconversion in Er:KPb₂Cl₅ under excitation of the ⁴I_13/2 [27]. However, the decay transient in Fig. 4(c) along with the excitation spectrum of the ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 upconverted emission from Er:KPB (Fig. 4(b)) suggests that ETU is responsible for populating the ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 levels. In addition, the decay time of the upconverted ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 emission is 1.9 ms, which is identical to the 1.9 ms decay time of the intermediate state (⁴I_9/2) [14]. Consequently, population within the ⁴I_9/2 and ²H_11/2 + ⁴S_3/2 levels are assigned to the energy transfer upconversion processes (⁴I_13/2, ⁴I_13/2) → (⁴I_15/2, ⁴I_9/2) and (⁴I_13/2, ⁴I_9/2) → (⁴I_15/2,²H_11/2 + ⁴S_3/2), respectively (Fig. 5).

 

Fig. 5 Schematic energy level diagram of (a) Pr³⁺ ETU processes (i) ³F₃ + ³F₃ → ³H₅ + ¹G₄, (ii) ¹G₄ + ¹G₄ → ¹D₂ + ³H₅, (iii) ¹G₄ + ¹D₂ → ³H₅ + ³P₁, (iv) ¹D₂ + ³F₂→ ³P₀ + ³H₄. (b) Er³⁺ ETU processes (i) ⁴I_13/2 + ⁴I_13/2→ ⁴I_15/2 + ⁴I_9/2 (ii) ⁴I_13/2 + ⁴I_9/2→⁴I_15/2 + ²H_11/2,⁴S_3/2.

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3.2 Studies of Ce³⁺- and Eu²⁺- doped potassium lead halides for scintillator applications

Scintillator research has attracted much attention over the past years for various applications such as monitoring nuclear materials, medical imaging, high-energy particle physics, industrial inspections, and gas exploration [6, 13–16]. Several rare-earth activated halide scintillators have been established and showed highly favorable scintillation materials [6, 13–16]. Divalent Europium and trivalent Cerium have been widely exploited as activators in several solid-state phosphor and scintillator materials [13–16, 28]. In recent years, allowed 5d-4f transitions of Pr³⁺ and Nd³⁺ ions giving rise to luminescence in the UV to visible wavelength range have also gained attention owing to their promising scintillation characteristics [29,30]. Thus, the search for new materials as well as the optimization of current scintillators has continued to be an active area of research with important implications for various civilian and homeland security related applications. In the following, the spectroscopic characterization of Ce³⁺-doped KPC as well as Eu²⁺-doped KPC and KPB were explored for possible applications in radiation detection.

3.2.1 Ce³⁺-doped KPb₂Cl₅

In this work, 2 wt. % Ce³⁺-doped KPC was grown using a self-seeded Bridgman technique employing a two-zone furnace. Additional chlorination (bubbled through high-purity HCl gas for further removal of oxide impurities) was carried out for the investigated 2 wt.% Ce:KPC. In the previous study on 1 wt.% Ce:KPC [31], a modified horizontal Bridgman growth technique was employed and no further purification was performed. A commercial Ce:YAP crystal (Scionix Holland) was included for a comparative study on the luminescence spectroscopy. The excitation and emission spectra of Ce:YAP and Ce:KPC crystals were obtained from room temperature luminescence measurements, as displayed in Fig. 6 and Fig. 7, respectively. The emission spectra of Ce:YAP were collected in the spectral range of 300-450 nm under 250 nm, 270 nm, and 300 nm excitations. The strongest Ce³⁺ emission centered ~365 nm under 300 nm pumping was observed from Ce:YAP crystal (Fig. 6). A broad excitation band covering 250-340 nm for the Ce³⁺ emission at 365 nm was observed from Ce:YAP as reported by other studies of this material [30,32,33]. In the case of Ce:KPC, the broad emission spectrum centered ~375 nm emission was obtained under 310 nm excitation (Fig. 7). The excitation spectrum of the Ce³⁺ emission monitored at 375 nm revealed a strong excitation peak at ~310 nm along with weaker excitation bands between 220 and 270 nm. On the other hand, the previous study on 1 wt.% Ce:KPC showed strong excitation bands between 200 and 270 nm along with very weak excitation bands between 300 and 330 nm (inset of Fig. 7, top) [31]. Similar absorption features were also observed from Ce³⁺-doped LaCl₃ hosts and, reference [15] reported one broad band centered at ~200 nm and one saturated band between 237 and 294 nm. It was suggested that the excitation bands at higher energy are most likely related to exciton absorption (host absorptions), and that the other broad band can be ascribed to the 4f → 5d absorptions of Ce³⁺ [15]. The Ce³⁺ emission results from the electron transition from the lowest 5d crystal-field level to the 4f ground state of the Ce³⁺ ion. It was realized that a relatively narrow emission spectrum of 1 wt.% Ce:KPC was obtained under 250 nm pumping where the host absorption was dominant (inset of Fig. 7, top). It seems likely that the existence of different Ce³⁺ centers could probably be involved in the previous study of 1wt.% Ce:KPC.

 

Fig. 6 The excitation (black dotted line) and emission spectra of Ce:YAP crystal at room temperature. The excitation and emission wavelengths are also indicated. The inset shows a schematic energy-level diagram of tentatively assigned optical excitation and emission transitions of Ce³⁺ in YAP and KPC.

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Fig. 7 The excitation (black dotted line) and emission (black solid line) spectra of Ce³⁺-doped KPb₂Cl₅ crystal at room temperature. The emission spectrum of undoped KPb₂Cl₅ is also shown (dashed line). The top of inset shows the excitation and emission spectra of previous study 1wt.% Ce: KPb₂Cl₅ crystal. The inset (bottom) shows the transmittance of Ce: KPb₂Cl₅ (right scale).

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The optical excitation and emission transitions of Ce:YAP are illustrated in the energy-level diagram as well as tentatively assigned 4f-5d transition for Ce:KPC (inset of Fig. 6) [29–33]. No Ce³⁺ emission was observed from the present sample 2 wt.% Ce:KPC under excitations between 220 and 270 nm suggesting that these are host-related excitation bands. Furthermore, the observed 5d-4f Ce³⁺ emission was confirmed by measuring the emission spectrum of undoped KPC under 310 nm pumping, which did not show any features between 350 and 550 nm (Fig. 7, dashed line). The 5d electrons are not efficiently shielded by other electrons, and the energy levels of the 4f-5d electronic configuration are therefore strongly influenced by the crystal field. Consequently, the energy levels in the 4f-5d electronic configuration can vary significantly between different hosts. It can be noted that the transmittance spectrum of the Ce:KPC crystal partially overlaps with the emission spectrum, indicating that some self-absorption is likely to occur (inset of Fig. 7, bottom). Studies have shown for several crystal scintillators (Ce:LaCl₃, Ce:LaBr₃, CeF₃) that the light output of the crystals is reduced when the absorption edge overlaps with the emission spectrum [34].

The photoluminescence lifetime of Ce³⁺-doped KPC was determined to be ~30-50 ns, which is comparable to other Ce-doped halide crystals [15–18]. Figure 8 shows the X-ray excited emission spectra of Ce-doped KPC and undoped KPC recorded at room temperature. Radioluminescence (RL) shows a broad band centered at ~530 nm for Ce:KPC and ~600 nm for undoped KPC. The radioluminescence of Ce: KPC appears to be in a different spectral region as the photoluminescence (~375 nm) of Ce³⁺ ions. The radioluminescence of Ce-doped chloride based materials is typically obtained in the 300-400 nm spectral region [6,15,16]. This suggests that X-ray excitation of Ce:KPC is inefficient in exciting Ce³⁺ but rather leads to KPC host emission. Possible reasons include inefficient incorporation of Ce³⁺ into KPC, introduction of impurities, or defects-related trapping levels which then hinder the energy transfer and capture processes. In order to gain more insight into the Ce³⁺ incorporation in the KPC crystal, detailed studies of the temperature dependence of the excitation and emission spectra as well as the decay dynamics are required.

 

Fig. 8 Radioluminescence spectra of undoped KPb₂Cl₅ and Ce³⁺ doped KPb₂Cl_5.

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3.2.2 Eu²⁺-doped KPb₂Cl₅ and KPb₂Br₅

The transmission spectra of undoped KPC and Eu²⁺-doped KPC as well as and undoped KPB and Eu²⁺-doped KPB were recorded in the 200-800 nm wavelength range at room temperature as shown in Fig. 9(a) and 9(b), respectively. The transmittance of undoped KPC and KPB was ~40% with absorption edges of ~330 nm and ~380 nm, respectively. A reduced transmission was observed for the Eu-doped crystals with an onset of the optical absorption band starting at ~500 nm, indicating the incorporation of Eu ions into the host materials [31]. The optical absorption band below 430 nm can be tentatively attributed to the 4f → 5d transitions of Eu²⁺ ions [18], as shown in Fig. 9(c). Under xenon lamp excitation, no blue luminescence was observed from the investigated crystals probably due to concentration quenching of the Eu²⁺ ions and/or the presence of Eu-related defects which can lead to quenching [35]. It was reported that Eu²⁺ concentrations of greater than ~0.5 mol% exhibited concentration quenching effects in Eu²⁺-doped fluoride and chloride crystals [35,36]. The Eu²⁺ emissions from Eu-doped scintillators are typically obtained in the spectral range of ~400-450 nm arising from the 5d → 4f transition to the ground state (⁸S_7/2) of the 4f⁷ configuration [35–37]. As described in Fig. 9(d), the ground state 4f⁷ electronic configuration of the free Eu²⁺ ion is ⁸S_7/2, and the next 4f⁷ manifold (⁶P_J) lies roughly around 28000 cm⁻¹ above the ground state [38,39]. It can be noticed that the Eu²⁺ 4f⁷ levels are lower than the lowest-lying 4f⁶-5d levels for a free Eu²⁺ion that starts near ~34000 cm⁻¹. A tentative energy level scheme of the Eu doped potassium lead halides is shown in Fig. 9(d), which indicates the onset of absorption at ~430 nm (~23255 cm⁻¹).

 

Fig. 9 Transmittance spectra of (a) undoped KPb₂Cl₅ and Eu²⁺ doped KPb₂Cl₅ (b) undoped KPb₂Br₅ and Eu²⁺ doped KPb₂Br₅ crystals at room temperature in the region of 200-800 nm. (c) Absorption spectra of both crystals are also depicted. (d) A schematic diagram of tentative energy level of Eu²⁺ doped potassium lead halides is also shown.

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4. Conclusions

The optical properties of Pr³⁺- and Er³⁺-doped in the low phonon-energy halide host KPb₂Br₅ were explored for their potential as an eye-safe wavelength laser material. Measurements show that the peak emission cross-section for Pr:KPb₂Br₅ is determined to be ∼6.1x10⁻²⁰ cm² at 1667 nm, which is greater than the peak emission cross-section of Er: KPb₂Br₅ (1540 nm, ∼1.6x10⁻²⁰ cm²). Moreover, emission cross-section spectra of Pr³⁺ are much broader relative to Er:KPb₂Br₅, providing an opportunity for greater laser tunability in the eye-safe wavelength regime. While Pr³⁺ possesses attractive absorption and emission cross-sections, it has been observed that resonant pumping of the ³F₃,³F₄ band of Pr³⁺ provides a channel for visible upconversion which accompanies the 1.5-1.6 µm infrared emission. These upconversion processes depopulate the ³F₃,³F₄ levels and lead to population of the higher lying ³P₀,³P₁, and ¹D₂ states. The upconversion processes populating these states have been identified as energy transfer upconversion (ETU). They appear to be less efficient in Pr³⁺ relative to the ETU processing taking place in Er³⁺ systems under resonant pumping of the ⁴I_13/2 state. However, cross relaxation was identified as an additional quenching process of the ³F₃,³F₄ levels, indicating that a lower Pr³⁺ concentration is necessary in order to suppress cross relaxation and maximize the radiative decay. In the case of Er:KPb₂Br₅, ⁴I_9/2 → ⁴I_15/2 infrared upconversion and the ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 are the most dominant upconversion channels in this low phonon energy host. The excitation spectra along with the emission decay transients of the infrared and green upconversion were collected and strongly suggests that ETU is the mechanism responsible for the ⁴I_9/2 → ⁴I_15/2 and ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 upconversion emissions in Er:KPb₂Br₅.

A spectroscopic characterization of UV-induced luminescence from Ce³⁺-doped KPb₂Cl₅, as well as Eu²⁺-doped KPb₂Cl₅ and KPb₂Br₅ crystals was presented. Upon Xenon lamp excitation, allowed 5d-4f emission centered at ~375 nm was observed in Ce³⁺-doped KPb₂Cl₅. The luminescence excitation measurements have provided the 4f-5d absorption band of Ce³⁺ centered ~310 nm along with the host related absorptions at ~220-270 nm. A fast luminescence decay time of ~30-50 ns was obtained from Ce:KPb₂Cl₅, however, radioluminescence at ~530 nm appeared to originate from the host KPb₂Cl₅. The onset of the optical absorption below 430 nm can be tentatively attributed to the 4f-5d transitions of the Eu²⁺ ions in Eu:KPb₂Cl₅ and Eu:KPb₂Br₅ crystals, however, 5d-4f luminescence was not observed. Additional studies of the time-resolved excitation and emission as well as the decay dynamics of the investigated crystals are needed in order to understand the luminescence characteristics and to evaluate the scintillation properties of these materials.