1. Introduction

Rare earth (RE) doped transparent lanthanum lead zirconate titanate [(Pb,La)(TiZr)O₃, PLZT] ceramics were deemed as a promising candidate in developing ceramic lasers, zero loss optical devices, and other multifunctional photonic components and systems, because of their excellent electrooptic (EO) effects, wide optical transmission window from visible to mid-wave infrared wavelengths, and structural traits in between single-crystal and glass materials [1–4]. Recently, random walks based impact on light emission in Er³⁺/Yb³⁺ and Ho³⁺/Tm³⁺ doped PLZT ceramics was investigated, and enhanced light emission and light-induced scattering have drawn growing attention to generate sensors and light sources [5–7]. Afterwards, broadband light amplification in Er³⁺ doped PLZT and weak localization of light in Nd³⁺ doped PLZT ceramics were controlled by photoinduced scattering and random walks of light [8,9].

To further study the correlation between weak localization of light and photoinduced scattering, in this paper, a 2.0 mol% Ho³⁺ and 5.0 mol% Tm³⁺ codoped PLZT (10/65/35) slab was exposed to a plasma atmosphere. Intriguingly, a striking difference of light emission between natural and polarized light emission in visible band provides some insight into the photoinduced scattering and weak localization processes. Besides, various trailing off times for different wavelengths were obtained, which played an important role behind the experimental findings and could be used to tune the light emission. All these findings lead to a new way in designing and developing multifunctional lasers and EO related devices. We report our results as follows.

2. Experiments and results

The specimens used in this work consisted of 65 mol% lead zirconate plus 35 mol% lead titanate and 10 mol% lanthanum, i.e. PLZT (10/65/35), to which 2.0 mol% Ho³⁺/5.0 mol% Tm³⁺ ions in the forms of Ho₂O₃/Tm₂O₃ were added. The origins of these components were PbO, La₂O₃, ZrO₂, and TiO₂, respectively. The sizes of the specimens were 3.0 mm × 3.0 mm × 6.0 mm, with the 3.0 mm × 3.0 mm surfaces optically polished and coated with antireflection films (with a reflectivity of 0.2% at 790 nm, and 0.5% at 2.0 μm). As shown in Fig. 1(a), a semiconductor laser with center wavelengths of 790 nm (Apollo Instruments, Inc. S30-790-4) was used as the pumping source. The light emission spectra were recorded at the angle of 30° in the spectral region of 200-1100 nm, with an UV-visible-NIR spectrophotometer (Ocean Optics, HR4000CG-UV-NIR). A 10 kV high power supply (Wisman High Power Co., Ltd. 10P-30) was used to apply a voltage to an air gap near the specimen in order to form a plasma atmosphere around it, by setting the distance of two poles at 4.0 mm. A wide band pass filter between 500 and 700 nm was used to separate the light emission from infrared pumping light and ultraviolet light from the electric sparks. Several more lenses and irises used in the setup were omitted purposely in the schematic to emphasize the main light paths. All these experiments were performed at room temperature.

 

Fig. 1 (a) Schematic of experimental apparatus in studying lasing emission in Ho³⁺ and Tm³⁺ doped PLZT ceramics (LD is laser diode at 790 nm; L represents for lens; P represents for polarizer; F represents for filter; PC is personal computer; and HPS is high power supply); (b) Lasing emission in plasma atmosphere for different exposure time; (c) Schematic of level transition in Ho³⁺ and Tm³⁺ doped PLZT ceramics.

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When the pumping source was turned on, strong upconversion emission (UCE) was observed in the Tm³⁺/Ho³⁺ codoped PLZT ceramics, as seen in Fig. 1(b). Two main emission bands peaked at 560 and 690 nm were observed and assigned to the transitions of Ho^{3+ 5}S₂→⁵I₈, and the transitions of Tm^{3+ 3}F_2,3→³H₆, respectively [Referring to Fig. 1(c)]. When setting the applied voltage at 4.5 kV, the air around the specimen was broken down, and a plasma atmosphere was formed. Upon exposing to this plasma atmosphere, the UCE intensity around 690 nm became stronger and stronger with time. While the UCE intensity around 560 nm had a slight decrease at the same time. Noticeably, a broad continuum spectrum between 575 and 675 nm was added to the UCE emission after the specimen was exposed to the plasma atmosphere.

To further study the light propagation and emission in disordered Ho³⁺ and Tm³⁺ codoped PLZT under plasma atmosphere, two polarizers were placed along the pumping and emitting directions, as shown in Fig. 1(a). The polarization angles of these two polarizers were chosen as 45° and 135° with the horizontal plane, respectively. The variation trend of light emission spectrum pumped by polarized light source was with some similarity as well as many differences to that pumped by unpolarized light source. As seen in Fig. 2(a), upon exposing to the plasma atmosphere, the light emission at 690 nm kept rising until 5 minutes. The light emission on the left side of the peak centered at 690 nm was broadened and enhanced, full width at half maximum (FWHM) of the peak increased by 6.5 nm (from 13.2 nm to 19.7 nm). A similar trend was also observed on the left side of the peak at 560 nm, the FWHM increased by 40.0 nm from 19.1 nm to 59.1 nm. In addition, the broad continuum spectrum between 560 and 625 nm grew up and did even surpass the original emission peaks around 560 nm. It is noteworthy that a sharp peak centered at 590 nm with a FWHM of 3.0 nm appeared on the top of the broad continuum spectrum. It was seen that the emission intensity around 560 nm increase under plasma exposure after two mutually perpendicular polarizers were placed between the incident and output directions [Fig. 2(a)], whereas it seems to decrease in Fig. 1(b). This is because that upon exposing to the plasma atmosphere, energy transfer between Ho³⁺ and Tm³⁺ ions were sensitive to the polarization angle of the incident light [10,11], which could present different light emission intensity change as exhibited in Fig. 1(b) and Fig. 2(a). Furthermore, the light propagation length would increase dramatically, and the phase modulation based on high quadratic E-O coefficient of PLZT ceramics [12] could be another reason for the emission intensity difference around 560 nm.

 

Fig. 2 (a) Polarized lasing emission in plasma atmosphere for different exposure time; (b) Dynamic changes of polarized lasing emission under plasma atmosphere.

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When exposing to the plasma atmosphere, all of the light emission pumped by polarized light source would be enhanced immediately. However, the dynamic change curves were quite different. As seen in Fig. 2(b), the typical dynamic changes of five selected waves centered at 550, 590, 620, 650, 698 nm were detected by the spectrometer simultaneously. These light emissions fluctuated drastically and randomly with time, and complicated competitions occurred. Not all these findings could be explained by normal photoluminescence mechanisms. The photoinduced scattering, weak localization, and changed symmetry of luminescence center may all contribute to the experimental results. We will discuss it as follows.

3. Theoretical consideration

3.1 structural feature and scatterers

The XRD data of transparent 2 mol% Ho³⁺ and 5 mol% Tm³⁺ doped PLZT samples was measured by an X-ray diffractometer (D/MAX-rB, Rigaku, Japan), as shown in Fig. 3(a). The strong (110) diffraction peaks corresponds to a cubic perovskite (ABO₃) structure. Other disturbed peaks were very weak, implying less pores and impurity phases formed in the preparation of the specimens. From Fig. 3(b), one can see the schematic diagram of A and B sites in typical perovskite PLZT unit cell, consisting of Pb²⁺, La³⁺ and RE³⁺ cations residing at A sites and Ti⁴⁺ and Zr⁴⁺ occupying B sites [1]. Due to valence difference between La³⁺/Ho³⁺/Tm³⁺ and Pb²⁺ cations, charge balance is maintained by creation of vacancies at A sites, not excluding similar vacancies formed at B sites. In addition, abundant O^2- vacancies could be formed in the material preparation process.

 

Fig. 3 (a) X-ray diffraction intensity of 2 mol% Ho³⁺ and 5 mol% Tm³⁺ codoped PLZT ceramics; (b) The schematic diagram of A and B sites in the ABO₃ perovskite unit cell; (c) A 3-dimensional micrograph of Ho³⁺ and Tm³⁺ codoped PLZT ceramics taken with an atomic force microscope.

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Besides these vacancies, owing to the intrinsically disordered nature of the PLZT ceramics, defects can be formed on the boundaries of grains due to slight variation of the composition and size from grain to grain (refer to the 3-dimensional atomic force microscope graph in Fig. 3(c), showing grain size ranging from 20 to 100 nm). Those vacancies and defects could recurrently trap and scatter free electrons or holes, and result in some novel photoinduced phenomena [9].

3.2 Effect of UV light

In fact, the plasma atmosphere would affect the specimen in two major ways, illumination of the UV light and exertion of the external electric field. Because of the UV light from electrical sparks is close to the energy gap of PLZT materials (3.0 eV) [5], the electrons on the valence band would be stimulated to the conduction band upon exposing to the UV light [13–15]. Due to the short lifetime of the conduction band, a large part of electrons would be captured by the rich deep electron traps (DET), and partly combined with the holes in recombination centers (RC). As shown in Fig. 4(a), both photons from the pumping source and lasing emission would have chance to excite these trapped electrons at the cost of themselves. When these electrons were reexcited to the conduction band, it would recombine with the RC, and radiate optical stimulated luminescence (OSL) randomly both in different wavelength and direction. The rate equation of the electron or hole concentrations of conduction band [n_c(t)], valence band [n_v(t)], DET [n(t)], and HT [h(t)] can be written as [16–18]:

 

Fig. 4 (a) Optical stimulated trapping and illuminating process; (b) Simulation curves of light emission dynamics at 560 and 690 nm after exposing to the plasma atmosphere; (c) Sketch representing recurrent scattering of light in Ho³⁺ and Tm³⁺ codoped PLZT ceramics; (d) Five typical long lasting fading-off time of light emission at 550 nm, 590 nm, 633 nm, 650 nm, and 690 nm.

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All these parameters in 2 mol% Ho³⁺ and 5 mol% Tm³⁺ doped PLZT ceramics could be calculated by using Eqs. (1)–(7) and the changes of measured absorption and luminescence spectra upon exposing to the plasma atmosphere, which were quite different for different wavelengths, and the intensity of I_OSL for different wavelengths would exhibit variable loss or gain. Based on Eqs. (1)–(8), the simulation curves of light emission dynamics at 560 and 690 nm after exposing to the plasma atmosphere were obtained as seen in Fig. 4(b), from which we can see that light emission at both wavelengths increased upon exposing to the plasma atmosphere. These results fitted well with their experimental dynamics in Fig. 2(b), although, there are many differences between the calculated and experimental results, which came from the unstable UV light intensity in experimental process and the reckon without electric field induced symmetry change of Ho³⁺/Tm³⁺ ions in the crystal field. The unstable UV light intensity in the sparks could further increase the fluctuation of the experimental dynamics, and the influence of electric field induced symmetry change of Ho³⁺/Tm³⁺ ions in the crystal field will be discussed in the following section.

In fact, the optical stimulated recurrent trapping and excitation process make the PLZT ceramic extremely disordered for light, as shown in Fig. 4(c). Besides, the defects on the grain boundaries also scatter abundant photons directly. Photons from light emission would occur and walked in the gain medium randomly, consequently, weak localization of light would be formed at this time. From Fig. 4(d) one can see that upon completely shutting off the pumping power at 0 s (marked with a red arrow), the light emission lasted over 2 seconds. The fading-off time of the light emission is defined as the time when the intensity of light emission drops down to 10% of the original value. These trailing-off times at 550 nm, 590 nm, 633 nm, 650 nm and 690 nm were 536 ms, 612 ms, 840 ms, 468 ms, and 710 ms, respectively, which were much longer than the typical 2-4 ms lifetime obtained from Ho³⁺/Tm³⁺ ions doped PLZT specimens [20]. Within such relative long tailing-off times, for one thing, the intensity of lasing emission was reduced by absorption and multiple scattering. For another, lasing emission was also enhanced by stimulated radiation in extremely long path length. The intensity and polarization direction of light would be changed randomly after each time of scattering and reexcitation. When the loss of lasing emission was compensated by the stimulated amplification, enhanced light emission could be detected at the same time. Due to the various long fading-off time for photons at different wavelengths, the output intensity of light emission would be very different even if two photons at neighboring wavelengths, and the intensity of light emission at the left side of the peak (at 698 nm) in the range from 625 nm to 675 nm was enhanced peculiarly along with some fluctuations in light emission intensity. [refer to Fig. 2(a)]. It should be noted that, the light emission change after UV light source illumination is not obvious compared with that under plasma exposure due to the large absorption coefficient and shallow penetration depth in Ho³⁺/Tm³⁺ codoped PLZT ceramics [20].

3.3 Effect of electric field

It is well accepted that the Ti⁴⁺ and Zr⁴⁺ cation is in an equilibrium position without an external electric field, roughly near the center of the octahedron of the unit cell [refer to Fig. 5 (a)] [21,22]. When an external electric field is applied on the PLZT specimen, RE³⁺, Ti⁴⁺, and Zr⁴⁺ cations will shift along the electric field direction, and O^2- anions will shift along the opposite direction. The displacement of those ions along the external electric field would definitely change the symmetry of the unit cell, and hence the resultant crystal field on the RE³⁺ cations. Based on the previous work [5], the expected enhancement ratios of UCE η could be written as:

 

Fig. 5 (a) Configuration of the ABO₃ perovskite unit cell for RE doped PLZT: the left diagram is for zero-field (O group); the right diagrams exhibit electro-induced polarizations (C_4h group); (b) Light emission spectra changes before and after the symmetry of Ho³⁺/Tm³⁺ ions in the crystal filed changed to C_4h group from O group.

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Based on Eq. (9), when the electric field induced symmetry of Ho³⁺/Tm³⁺ ions in the crystal field changed to C_4h group from O group, the emission intensity for ³F₂→³H₆ of Tm³⁺ around 690 nm should be enhanced up to 768% of its original value, as seen in Fig. 5(b) while the emission intensity for ⁵S₂→⁵I₈ of Ho³⁺ should also exhibit an great enhancement up to 316% of its original value. The difference between the theoretical results and experiments data came from the photoinduced recurrent scattering and weak localization of light proposed above.

Moreover, when an external electric field is applied on the PLZT specimen, RE³⁺, Ti⁴⁺, and Zr⁴⁺ cations will shift along the electric field direction [8]. Because of the variable direction of the electric sparks, the Ti⁴⁺ cations would vibrate randomly. Consequently, this plasma atmosphere can stimulate the Ti⁴⁺ cations to its excited states and results in broadband continuum light emission between 570 nm and 620 nm [23]. And through recurrent scattering, trapping, and excitation process, weak localization of light and random lasing action would take place. The sharp spike around 590 nm implied the formation of random lasing emission.

These experimental results may lead to a new way to change the number and distribution of scatterers in optical way, and to control the lasing emission in RE and transitional metal ions doped host materials.

4. Summary

In conclusion, a remarkable light emission enhancement was observed in a Tm³⁺/Ho³⁺ codoped (Pb,La)(Ti,Zr)O₃ ceramic slab upon exposing to a plasma atmosphere. Different results were obtained between unpolarized and polarized light pumping experiments. Dynamical changes of five typical polarized light emissions were detected. The experimental results might be rooted in photoinduced scatterers, random walks, and weak localization of light, along with crystal field variation. In addition, very long fading-off time also played a vital role in the spectra amplification. These findings offer new opportunities in designing and developing of new lasers and sensors, which might take advantages of transitional metal and RE activators in the same host material.