1. Introduction

Oxyorthosilicates of rare earths are known for a long time. In early investigation it has been ascertained that they form monoclinic crystals belonging to P2/c₁ or C2/c space groups depending on radius of rare earth ions [1]. Among these compounds the Gd₂SiO₅ (GSO) and Lu₂SiO₅ (LSO) have proved to be good hosts for the design of luminescent materials since they have desirable physicochemical properties, show large transparency region and are able to accept high concentration of luminescent rare earth ions. It has been demonstrated that GSO and LSO hosts doped with trivalent cerium are efficient scintillating materials able to convert γ-ray or X-ray excitation into visible emission [2–5]. LSO crystal doped with ytterbium proved to be promising laser active material emitting in near infrared [6, 7].

Dissimilarity in P2/c₁ crystal structure of GSO and C2/c crystal structure of LSO results in differences of nearest surrounding of rare earth ions. In the GSO lattice gadolinium ions reside in two non-equivalent sites Gd1 (coordination number CN = 9, C_3v local symmetry) and Gd2 (CN = 7, C_s local symmetry) whereas in the LSO lattice two non-equivalent Lu1 and Lu2 ions have coordination numbers 6 and 7, respectively but the same C_s local symmetry. Reported investigation of GSO host doped with Ce³⁺, Eu³⁺, Er³⁺ or Yb³⁺ [8–10] indicates that incorporated luminescent rare earth ions substitute both the Gd1 and Gd2 sites, irrespective of their ionic radius. Ionic radius of Lu³⁺ is smaller than that of Gd³⁺. Therefore in LSO lattice bigger ions from the beginning of rare earth series tend to enter larger LuO₇ polyhedra than smaller sixfold coordinated LuO₆ sites. In fact, it has been found that for LSO:Ce³⁺ system about 5-10% of Ce³⁺ ions is located in LuO₆ polyhedra [11–13]. Location of cerium ions in GSO and LSO hosts has been considered in detail because occurrence of two different Ce³⁺ sites results in broadening of luminescence band and lengthening luminescence decay thereby affecting adversely scintillation performance of these systems.

Interest in Lu₂SiO₅ – Gd₂SiO₅ (LGSO) solid solution hosts arises mainly from practical reasons. Manufacturing of LSO crystals is expensive as a consequence of high cost of lutetium oxide combined with a very high temperature, ca 2050 °C, of the crystal growth process. Melting point of GSO is advantageously lower by about 300 °C but the crystals grown show strong tendency to cleave making thereby cutting and polishing difficult. Early works on the crystal growth have revealed that LGSO compound with a general chemical formula (Lu_xGd_1-x)₂SiO₅ forms crystals with a structure C2/c inherent to LSO even when x is as low as 0.2 [14,15]. More recent investigation has ascertained that the crystal structure of LGSO changes from C2/c (LSO-type) to P2₁/c (GSO-type) when 0.15 < x < 0.17 [16]. It has been shown also that melting point of LGSO crystals for 0.15 < x ≤ 0.20 is close to 1770 °C [16], a value slightly higher than 1800 °C determined for GSO.

In the present work we deal with LGSO:Ce³⁺ crystals co-doped with Sm³⁺ or Dy³⁺. Trivalent samarium and dysprosium deserve some attention since they are able to show efficient visible emission distributed in several bands between blue and red region of the spectrum. Owing to Hund’s rule there is a close similarity of their energy level structures. The 4f⁵ configuration of Sm³⁺ gives rise to numerous excited levels with the ⁴G_5/2 metastable level located at around 17500 cm⁻¹and separated from next lower energy level by about 7000 cm⁻¹. Owing to this large energy separation the contribution of multiphonon relaxation to the ⁴G_5/2 decay is small and radiative downward transitions ⁴G_5/2 – ⁶H_J (J = 5/2, 7/2, 9/2 and 11/2) give rise to intense luminescence bands located at about 570, 600, 645 and 710 nm, respectively. In the energy level structure of Dy³⁺ ions the metastable ⁴F_9/2 level at about 21500 cm⁻¹ is separated from the next lower energy level also by about 7000 cm⁻¹ and radiative downward transitions ⁴F_9/2 – ⁶H_J (J = 15/2, 13/2, 11/2 and 9/2) give rise to intense luminescence bands at about 480, 575, 660 and 750 nm, respectively. For the two ions the yellow luminescence bands related to transitions ending on the first excited multiplets (⁶H_7/2 for Sm³⁺ and ⁶H_13/2 for Dy³⁺) have the highest intensities, thereby offering a potential of four-level-laser operation. Despite these advantages, crystals doped with Sm³⁺ or Dy³⁺ have not been considered in the past as promising laser active media or useful visible phosphors because absorption bands above metastable levels are too weak to provide sufficient efficiency of excitation by classical light sources. Recent progress in the field of semiconductor sources emitting in the UV and blue region has renewed interest in these systems, however. In fact, it has been pointed out in numerous papers that crystals and glasses doped with samarium or dysprosium are promising phosphors for the design of UV LED excited white-light sources [17–19]. Also, the availability of GaN/InGaN blue diode lasers makes much closer the development of all-solid-state yellow /orange lasers employing Sm³⁺ or Dy³⁺ –doped crystals. In fact, first report on laser-diode pumped YAG:Dy³⁺ laser emitting in the yellow region of the spectrum has been published recently [20].

Enhancement of excitation efficiency in these systems by energy transfer from co-doped cerium ions is a tempting issue. Interconfigurational 4f-5d transitions of Ce³⁺ ions provide broad and intense absorption band in UV-blue region and the emission band related to their downward 5d-4f transition overlaps weak albeit numerous absorption lines of Sm³⁺ and Dy³⁺ ions in the region of optical pumping. Preliminary investigation of LGSO:Ce,Sm [21] and LGSO:Ce,Dy [22] revealed that the band related to the 4f-5d transition of Ce³⁺ contributes to the excitation spectra of Sm³⁺ and Dy³⁺ luminescence in these systems, thus corroborating the occurrence of Ce³⁺ - Sm³⁺ and Ce³⁺ - Dy³⁺ energy transfer.

Intention of the present work is to determine peculiarities of excitation energy flow that govern luminescence phenomena in LGSO:Ce,Sm and LGSO:Ce,Dy crystals. Use of femtosecond light pulse excitation gives opportunity to observe time resolved spectra and dynamics of excitation and relaxation processes involved, thereby facilitating a fundamental understanding of luminescence phenomena and possibly assessing practical potential of systems under study.

2. Experimental

Investigated single crystals with nominal stoichiometries (Lu_0.2Gd_0.78Ce_0.01Sm_0.01)₂SiO₅ and (Lu_0.2Gd_0.78Ce_0.01Dy_0.01)₂SiO₅ were grown by the Czochralski technique. Details of the crystal growth procedure were described elsewhere [16]. X-ray examination of fabricated crystals revealed that the structure of LGSO:1at%Ce,1at%Sm belongs to the C2/c space group inherent to LSO, whereas the structure of LGSO:1at%Ce,1at%Dy is consistent with the P2/c₁space group inherent to GSO. Results of ICP-ES measurement that were performed employing a Thermo Scientific ICAP 7000 ICP Spectrometer revealed a significant dissimilarity between intentional and actual chemical compositions of the crystals. In particular, Ce³⁺ concentrations of 0.6 at% was measured for samples with nominal cerium doping levels of 1at% indicating that big Ce³⁺ ions are preferentially retained in the melt. In addition, incorporation of bigger ions Ce³⁺ and Sm³⁺ brings about the increase of the Lu/Gd ratio in the crystal whereas smaller Dy³⁺ ions induce opposite effect and this finding helps to understand why the crystal structures of two samples differ.

Experiments were accomplished using a source of incident light pulses consisting of femtosecond laser (Coherent Model “Libra”) that delivers a train of 100 fs pulses at a centre wavelength of 800 nm and pulse energy of 1 mJ with a repetition rate regulated up to 1kHz. To obtain light pulses at different wavelengths the laser was coupled with an optical parametric amplifier (Light Conversion Model “OPerA”). In this configuration the pulse wavelength can be varied between 230 – 2800 nm with the pulse energy comprised between 6 and 150 μJ, depending on the spectral region.

When investigating down and up-conversion phenomena a luminescence emerging from samples was observed in the direction perpendicular to the excitation beam. Appropriate long- and short-pass filters have been used to eliminate unwanted radiation. To record luminescence spectra and luminescence decay curves a grating spectrograph (Princeton Instr. Model Acton 2500i) coupled to a streak camera (Hamamatsu Model C5680) operating in the 200 – 1100 nm spectral region with a temporal resolution of 20 ps was employed. All measurements were made at room temperature.

3. Results and discussion

3.1. Effect of wavelength of femtosecond pulse excitation on down-converted luminescence spectra of LGSO:Ce,Sm

Luminescence spectra in the 380 nm – 630 nm spectral region were observed when changing excitation wavelength between 305 nm and 405 nm with steps of 5 nm. All spectra recorded at wavelengths below 395 nm consist of very broad luminescence stretching from about 390 nm to 600 nm that we assign to the 5d-4f transitions of Ce³⁺ ions and narrow luminescence bands related to the ⁴G_5/2 – ⁶H_5/2 and ⁴G_5/2 – ⁶H_7/2 transitions of Sm³⁺ located in the 550 - 575 nm and 590 – 620 nm spectral regions, respectively. Further increase of excitation wavelength results in a steep decrease of Ce³⁺ luminescence intensity which finally disappears for 405 nm excitation. Figure 1 compares selected luminescence spectra derived from a streak camera images. It should be noticed that an up-converted spectrum is included in this figure. It will be considered later on in Section 3.5. It can be seen that in contrast to Sm³⁺ luminescence the intensity distribution of Ce³⁺ luminescence is markedly affected by excitation wavelength. When exciting at 305 nm the spectrum is dominated by a broad band component in the short wavelength part with a maximum near 430 nm. With increasing excitation wavelength the intensity of this component decreases steadily and the Ce³⁺ luminescence excited at 325 nm acquires a form of large symmetric band with a maximum around 500 nm. Next, for excitation wavelength going from 330 nm to 385 nm the band component peaking at 430 nm initially grows forming the spectrum very similar to that excited at 305 nm, then decreases and finally spectra excited at 325 nm and 385 nm become identical. Further increase of excitation wavelength to 395 nm results in red shift and slight narrowing of this symmetric luminescence band. To account for these findings we will refer to results obtained during study of LSO:Ce³⁺ scintillator reported by Bo Liu et al in [11]. Based on low temperature excitation spectra recorded employing a synchrotron facility and numerical decomposition of emission spectra the authors were able to reveal the contribution of transitions in two different cerium sites.In, particular, bands in excitation spectrum peaking at 210, 262 and 294 nm have been attributed to cerium site emitting around 393 nm and 422 nm and denoted following Suzuki et al [2, 5] by Ce1. Similarly, bands peaking at 323 and 376 nm in the excitation spectrum have been attributed to Ce2 site showing rather weak emission with a maximum near 462 nm [11]. Examination of spectra shown in Fig. 1 reveals that bands related to cerium emission in LGSO are red-shifted by several tens of nanometers as compared to those in LSO but the assumption of selective excitation of Ce1 and Ce2 sites explains well close similarity of luminescence spectra excited at wavelengths 325 nm and 385 nm. Identity of spectra excited at 305 nm and those excited between 345 nm and 365 nm indicates strongly that the latter excitation gives rise to transitions ending on one or two remaining 5d levels of Ce1 site, not identified in [11]. Observed distribution of luminescence intensity implies that incorporation ratio Ce2/Ce1 in LGSO is markedly higher than that in LSO, unless radiative transition rate of Ce2 ions in LSO is much smaller. In addition, dependence of luminescencespectra on excitation wavelength implies that the transfer of excitation energy between Ce1 and Ce2 in LGSO is rather weak. In Fig. 2 time resolved luminescence spectra excited at 305 nm and 385 nm and recorded during a different times after excitation pulse are compared to verify this implication. Incident light pulses with wavelength 305 nm excite mainly Ce1 ions that show a luminescence band centred at about 430 nm. Nevertheless, at room temperature vibronic bands related to the 4f-5d transitions of Ce³⁺ are broad enough to enable weaker, albeit discernible excitation of Ce2 ions that contribute to the luminescence spectrum forming a wing stretching to about 600 nm. This contribution increases with time elapsed after the excitation pulse. Similarly, incident light with wavelength of 385 nm excites mainly Ce2 ions that show a luminescence band centred at about 500 nm. The spectrum recorded during 20 ns after excitation pulse contains an additional maximum around 450 nm that we attribute to the Ce1 emission. It can be seen that its intensity diminishes with increasing time delay. The findings described above imply that a nonradiative Ce1→ Ce2 energy transfer occurs and/or intrinsic lifetime of the Ce1 luminescence is markedly shorter than that of the Ce2 luminescence. The latter supposition arises since room temperature lifetimes amounting to 34 ns and 42 ns has been measured for Ce1 and Ce2 sites in LSO:Ce³⁺ [2]. Therefore, in the following luminescence decay curves for the system under study will be examined.

 

Fig. 1 UV-excited luminescence spectra of LGSO: Ce, Sm. Excitation wavelengths are indicated on the right side of the figure. For a comparison, an up-converted spectrum excited at 1300 nm is shown in the bottom.

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Fig. 2 Luminescence spectra for LGSO: Ce, Sm recorded at different times after femtosecond pulse excitation at 305 nm (left) and 385 nm (right). For a comparison an up-converted spectrum excited at 1300 nm is included in the left graph.

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3.2. Effect of wavelength of femtosecond pulse excitation on luminescence decay curves of LGSO:Ce,Sm

Decay curves of luminescence at wavelengths 430, 480, 530 and 570 nm were recorded for different wavelengths of excitation pulse between 305 nm and 385 nm with steps of 5 nm. In this measurement the spectral width of luminescence observed employing a monochromator coupled with a streak camera was set to 5 nm. Irrespective of excitation wavelength the decay curves of luminescence at 430 nm and 480 nm showed non exponential time dependence. Deviation from a single exponential time dependence for decay curves of luminescence at 530 nm was less significant and those observed at 570 nm were nearly single exponential. Figure 3 compares decay curves of luminescence at 430, 480, 530 and 570 nm excited at 305 nm. Based on numerical analysis it was found that decay curves of luminescence observed at 430 and 480 nm can be approximated assuming an initial fast single exponential decay with a time constant ca 5 ns (spread of actual values ranging from 4.7 ns to 6.1 ns for particular measurements is believed to be due to an experimental incertitude) followed by a much slower exponential decay with time constants between 20 ns and 28 ns. For luminescence observed at 530 and 570 nm the contribution of the fast decay stage is very small and corresponding decay curves are nearly consistent with a single exponential time dependence with time constants between 28 and 34 ns. Both the faster and slower decays that we attribute to Ce1 and Ce2 relaxation, respectively, are characterized by lifetime values considerably smaller than those reported for LSO:Ce³⁺ [2]. This dissimilarity may be due to several reasons. First, in contrast to the LSO host the nearest surrounding of cerium ions in LGSO shows a certain structural disorder resulting from partial substitution of lutetium ions by gadolinium thereby potentially affecting radiative transition rates of Ce³⁺ ions. Second, the luminescence quenching of Ce³⁺ luminescence by defect centres in our sample is likely to be enhanced by markedly higher cerium concentration as compared to nominal 0.25at% in LSO [2]. Third, a nonradiative energy transfer to samarium ions may accelerate the relaxation of excited cerium ions. The occurrence of the latter process can be inferred from spectra shown in Fig. 1. In particular, the contribution of Sm³⁺ luminescence does not change when excitation wavelength changes in the region 305 nm – 385 nm indicating that samarium ions are excited by energy transfer from cerium ions rather than by direct excitation into narrow absorption lines of Sm³⁺ ions. The higher contribution of Sm³⁺ luminescence in the spectrum excited at 395 nm stems from the fact that in this spectral region a vanishing intensity of long wavelength wing of Ce³⁺ absorption coincides with a strong Sm³⁺ absorption band.

 

Fig. 3 Decay curves of LGSO: Ce, Sm luminescence excited at 305 nm and observed at wavelengths 430nm, 480 nm, 530 nm and 570 nm.

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3.3. Effect of wavelength of femtosecond pulse excitation on down-converted luminescence spectra of LGSO:Ce,Dy

Luminescence spectra in the 390 nm – 630 nm spectral region were observed when changing excitation wavelength between 325 nm and 405 nm with steps of 5 nm. All spectra recorded contain a very broad luminescence stretching from about 390 nm to 600 nm that we assign to the 5d-4f transitions of Ce³⁺ ions and much narrow luminescence bands related to the ⁴F_9/2 – ⁶H_15/2 and ⁴F_9/2 – ⁶H_13/2 transitions of Dy³⁺ located near 480 nm and 575 nm spectral regions, respectively. Figure 4 compares selected luminescence spectra derived from streak camera images. Observed effect of excitation wavelength on luminescence spectra for LGSO:Ce,Dydepicted in Fig. 4 and for LGSO:Ce,Dy considered above is substantially different. In contrast to the latter system the spectrum of LGSO:Ce,Dy does not change when excitation wavelength grows from 315 nm to 365 nm. Further increase of excitation wavelengths up to 405 nm results in a gentle decrease of intensity of the short wavelength wing between 400 nm and ca 475 nm whereas the part of luminescence band between about 500 nm and 560 nm show a slight increase. It can be seen also that the Sm³⁺ luminescence band centred at about 575 nm is not affected by the change of excitation wavelength. To interpret these data we will refer to results of investigation of GSO:Ce³⁺ system reported by Suzuki et al [2]. Based on excitation and luminescence spectra recorded at 11 K Authors were able to distinguish Ce1 site emitting near 425 nm and Ce2 site emitting near 480 nm. They showed that the excitation spectrum of Ce1 emission at 11 K contains a nearly symmetric band stretching from ca 310 nm to 370 nm whereas that of the Ce2 emission has a form of a double band stretching from about 310 nm to 410 nm. Also, it has been found that at 296 K the excitation spectra of the Ce1 and Ce2 emissions consist of very similar bands between 310 nm and 370 nm but a weak wing stretching to 410 nm appears in the excitation spectrum of the Ce2 emission. Examination of spectra shown in Fig. 4 reveals that bands related to cerium emission in LGSO:Ce,Dy are red-shifted by several tens of nanometers and much broader as compared to those in GSO:Ce [2]. Nevertheless, their dependence on excitation wavelength can be accounted for based on peculiarities of room temperature excitation spectra mentioned above for cerium-doped GSO. In fact, Ce³⁺ luminescence spectra excited at different wavelengths between 310 nm and 370 nm do not differ because the excitation spectra of Ce1 and Ce2 sites in this spectral region are nearly the same. When wavelength of incident light increases from 370 nm to 405 nm, the contribution of Ce1 luminescence diminishes monotonously and the Ce2 sites are excited mainly. Recorded time resolved spectra of Ce³⁺ luminescence inLGSO:Ce,Dy corroborate this interpretation. Figure 5 compares spectra excited at selected wavelengths and recorded during a time of 200 ns after an excitation pulse. It can be seen that bands of luminescence excited at 235 and 355 nm are identical. Spectral position of their maxima is consistent with the Ce1 emission but their asymmetrical shape with a long wavelength wing point at the contribution of the Ce2 luminescence. Excitation at 405 nm gives rise to a broad, nearly symmetric band with a maximum around 500 nm indicating thereby that resulting luminescence is due essentially to Ce2 sites.

 

Fig. 4 UV-excited luminescence spectra of LGSO: Ce, Dy. Excitation wavelengths are indicated on the right side of the figure. For a comparison, an up-converted spectrum excited at 1300 nm is shown in the bottom.

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Fig. 5 Luminescence spectra for LGSO: Ce, Dy recorded during 200 ns after femtosecond pulse excitation at wavelengths 325 nm, 355 nm and 405 nm. For a comparison an up-converted spectrum excited at 1300 nm is included.

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3.4. Effect of wavelength of femtosecond pulse excitation on luminescence decay curves of LGSO:Ce,Dy

Decay curves of luminescence at wavelengths 430, 480, 530 and 570 nm were recorded for different wavelengths of excitation pulse between 325 nm and 405 nm with steps of 10 nm. In this measurement the spectral width of luminescence that was observed employing a monochromator coupled with a streak camera was set to 5 nm. It has been found that decay curves of luminescence monitored at 430 and 480 nm followed a single exponential time dependence with time constant values between 20 ns and 22 ns irrespective of excitation wavelength between 325 nm and 370 nm. Those monitored at 530 nm and 570 nm exhibit thesame characteristics when excited below 370 nm. However, longer excitation wavelengths give rise to an initial fast non exponential stage as exemplified in Fig. 6 and the time dependence of luminescence in this case can be approximated assuming a double exponential decay with lifetimes of 5 ns and 25 ns. Close similarity of lifetime values for single exponential decays monitored at 430 nm or 480 nm and for the longer component of a double decay implies that related luminescence originates in the Ce1 site. Wavelengths of excitation and of resulting luminescence characterised by a double decay indicate that ions in Ce2 site are responsible for the fast decay component. Results presented above do not furnish information regarding the occurrence of energy transfer between cerium ions in LGSO:Ce,Dy system, however. Although incorporated cerium ions enter distorted lattice sites in this disordered host crystal the decay of Ce1 luminescence shows a single exponential time dependence. This observation implies that distortion of local surrounding does not affect radiative transition rates of cerium ions, or alternatively, an efficient energy migration over cerium ions occurs. On the other hand observed luminescence decay characteristics point at negligible energy transfer from Ce1 to Ce2 sites. It is not clear whether the inverse Ce2 → Ce1 energy transfer takes place, however. Careful examination of initial stages of luminescence decay excited between 325 and 370 nm revealed, that in contrast to decay curves of luminescence monitored within a short wavelength part of the band, those monitored at longer wavelengths show rising parts as exemplified in Fig. 7. The rising partsappear in a time region consistent with the decay time of the Ce2 luminescence mentioned above pointing thereby at the contribution of the Ce2 → Ce1 energy transfer.

 

Fig. 6 Decay curves of LGSO: Ce, Dy luminescence observed at 530 nm, excited at 325 nm, 355 nm and 405 nm.

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Fig. 7 Initial parts of decay curves of LGSO: Ce, Dy luminescence excited at 325 nm, and observed at 430 nm, 530 nm and 570 nm.

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It should be noticed here that time resolved spectra shown in Fig. 5 for LGSO:Ce,Dy, as well as those shown earlier in Fig. 3 for LGSO:Ce,Sm do not contain luminescence bands of Dy³⁺ and Sm³⁺ ions, respectively. This is due to the fact that their luminescence is long-lived, with experimental lifetime values of 0.36 ms and 1.80 ms, respectively, and its contribution to spectra recorded at short times after excitation pulses is negligible small.

3.5. Up-conversion of femtosecond infrared light pulses into visible luminescence in LGSO:Ce,Sm and LGSO:Ce,Dy

Up-converted luminescence spectra for systems mentioned above, recorded upon femtosecond light pulse excitation at wavelength 1300 nm, are included in Figs. 1, 4, and 5. This excitation wavelength was chosen since it does not coincide with Sm³⁺ absorption and corresponds to a weak wing of Dy³⁺ absorption band. As a matter of fact, it was observed that spectral characteristics the up-converted luminescence in the two systems are weakly affected by the change of excitation wavelength in the infrared region. As an example, Fig. 8 compares up-converted spectra in LGSO:Ce,Sm recorded for several different excitation wavelengths. It can be seen that the intensity of up-converted luminescence diminishes with increasing excitation wavelength, despite the fact that the change of energy of incident infrared pulses is within ca 10% in the 1200 nm – 1400 nm spectral region. A striking dissimilarity of upconverted spectra in Fig. 8 and down converted spectra in Fig. 1 is related to the contribution of Sm³⁺ luminescence. Markedly higher contribution of Sm³⁺ luminescence to spectra shown in Fig. 8 implies that another efficient excitation mechanism takes place in addition to the Ce³⁺ → Sm³⁺ energy transfer. Significant dissimilarity can be easily seen alsoin time resolved spectra of up-converted and-down converted Ce³⁺ luminescence shown in Figs. 4 and 5. For the two systems the up-converted spectra consist of luminescence bands related to Ce1 sites, similar to those reported for X-ray excited LGSO:Ce³⁺ system [15].

 

Fig. 8 Comparison of up-converted luminescence spectra for LGSO: Ce, Sm recorded upon infrared excitation at several different wavelengths indicated.

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To interpret these data we will refer to the knowledge about phenomena that accompany the propagation of ultrashort light pulses in transparent media, documented in numerous published papers, e.g. in excellent work by Brodeur and Chin [23]. In particular, the supercontinuum (or white light generation) induced by infrared femtosecond pulses is attributed to the interplay between the effect of self-phase modulation (SPM) and the effect of self-focusing. Self-focusing brings about a creation of filaments - regions with transversal diameters within tens of micrometers encompassing a significant part of the pulse energy. The effect of self-focusing is counterbalanced by a defocusing effect of multiphoton excitation (MPE) which promotes electrons from valence band VB to conduction band CB thereby producing free electrons in the filament region. This process depletes the field energy on one hand and a plasma created induces a negative change in the index of refraction on the other hand. As a consequence, the self-focusing is arrested and a stable continuum generation can be obtained. A photograph in Fig. 9 evidences the creation of light filaments in our samples. A camera coupled with a microscope was employed to record spacial characteristics of the light beam in the crystal, observed perpendicular to the infrared pulse propagation. It can be seen in Fig. 9 that filaments created have different lengths. Their diameters are between 3 and 10 µm. Based on information summarized above we will consider mechanisms that may be responsible for up-converted spectra observed. In principle, a mechanism of intra-ion multiphoton excitation of Ce^3+, Sm³⁺ or Dy³⁺ ions, involving simultaneous absorption of several infrared photons may occur because of high instantaneous power of incident pulses. As a consequence of large bandwidth of cerium absorption the multiphoton intra-ion excitation mechanisms of different orders, depending on the wavelength of infrared pulses, would take place. Absorption lines related to transitions to high energy levels of Sm³⁺ and Dy³⁺ are narrow and weak but numerous, thereby this way of excitation cannot be excluded. However, this hypothesis is not able to explain why the lower energy Ce2 sites are not excited. Another mechanism, likely to occur, consists of absorption of visible photons from a supercontinuum light. Direct, one-photon excitation mechanism is excluded since the cut-off wavelength of supercontinuum generation in orthosilicate crystals is close to 500 nm [24] but the two-photon intra-ion excitation mechanism would be able to populate metastable levels of luminescent ions in question. Unfortunately, this kind of excitation cannot account for absence of Ce2 luminescence, too. In view of this, we suppose that the mechanism governing the excitation of luminescent ions in LGSO consists of energy transfer from free electrons created in the conduction band of the host by multiphoton excitation. All these ions have excited states with energies higher than the bottom of conduction band of the host, making this mechanism feasible.

 

Fig. 9 A photograph showing spatial features of a femtosecond infrared pulse in an oxyorthosilicate crystal.

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4. Conclusions

Incorporation of Sm³⁺ or Dy³⁺ ions endows the LGSO:Ce³⁺ system with the ability to show a broad-band luminescence stretching from 400 nm to ca 650 nm, offering thereby a potential utility of these phosphors for the design of UV LED-excited light sources. Fundamental processes of excitation energy flow, consisting of Ce³⁺ excitation, energy transfer between luminescent rare earth ions and radiative relaxation of their metastable levels occur irrespective of dissimilarity of the crystal lattices in the P2/c₁ and C2/c crystal structures of LGSO host. Nevertheless, employment of short pulse excitation and fast detection technique made it possible to reveal a substantial effect of the host crystal symmetry on spectral characteristics of systems studied, resulting from the dissimilarity of local surrounding of luminescent ions. Marked effect of the excitation wavelength variation in the UV region on Ce³⁺ luminescence spectra in LGSO crystal with C2/c crystal structure inherent to LSO is attributed to selective excitation of two available Ce1 and Ce2 sites differing in energies of 5d states of respective Ce³⁺ ions. Examination of recorded luminescence decay curves reveals also that selective excitation of one type of cerium site is not able to discriminate completely the contribution of the second one, presumably because the structural disorder of the host brings about a partial spectral overlap of their absorption bands. A negligible effect of the excitation wavelength changing below ca 385 nm on the Ce³⁺ luminescence in LGSO crystal with P2/c₁ crystal structure inherent to GSO is attributed to the coincidence of energies of 5d levels for Ce1 and Ce2 sites, similar to those reported for GSO:Ce³⁺ system. Based on examination of luminescence decay curves it is supposed that the Ce2 → Ce1 energy transfer occurs in this system. Up-converted spectra excited in IR region are not affected by dissimilarity of two crystal structures of LGSO but differ strongly as compared to down-converted spectra excited in UV region. Careful consideration of experimental data leads to the conclusion that mechanism governing the excitation of luminescent ions in LGSO consists of energy transfer from free electrons created in the conduction band of the host by multiphoton excitation.