1. Introduction

Rare earth dopants, such as Er³⁺, Eu³⁺ and Ho³⁺ doped materials, have attracted extra attentions since they have rich emission lines, strong luminescence intensity and narrow excitation energy bands in the visible (VI) and near-infrared region. Er³⁺ doped luminescence materials are of particular interests because they have excellent luminescent properties in both VI and middle infrared (MIR) wavelength regions, which have wide applications in lasers, displays, biosensors and telecommunications. [1–9]

Recently, benefit from the development of magneto-optical measurement facilities under high magnetic field, [10–28] people observed that PL intensities of Er³⁺ doped luminescent materials could be modulated greatly under the application of external magnetic fields. For examples, Moshchalkov et al. [12] observed two orders of PL suppression in Er³⁺-doped nano-glass-ceramics under the pulsed magnetic field up to 50 T, Zhang et al. [25] observed significantly PL suppression in Er³⁺: YVO₄ crystal under the magnetic field up to 47 T, and Wang et al. [27] observed color change of Mn²⁺ doped (Er³⁺, Yb³⁺): NaYF₄ nanocrystals with external magnetic field of 40 T. All of these observations are very significant, because the magneto-optical properties obtained here could be used as magneto-optical bifunctional materials in magnetic field detection, high magnetic field calibration, and magnetic field controlled PL. But a challenge is still existing in utilizing these materials since the working magnetic field to achieve remarkable effect is still a little high. Selecting suitable excitation laser energy and tailoring electron energy levels of materials by chemical or materials methods are two effective ways to lower down the working magnetic field of magneto-optics materials. Therefore, it is important to seek for suitable materials with matchable excitation light sources for achieving significant magneto-optics effects with moderate low magnetic field. In 2015, Zhang et al. obtained large PL intensity modulations at 7.7 T in Er³⁺: YVO₄ crystal by using 488.0 nm argon laser as the excitation light source rather than 407 nm. [25] But it is hard to decrease the working magnetic field further more by simply change the laser wavelength, because there is no suitable tunable laser with narrow laser line around 490 nm. However, modulate the electron energy levels by changing the neighboring atoms of Er³⁺ in the host matrix may work well.

In this paper, Er³⁺: GdVO₄ crystal was prepared for the investigation of magnetic field induced PL modulations, where Gd³⁺ was used to substitute Y³⁺ in Er³⁺: YVO₄ crystal to change the energy levels of Er³⁺ luminescent center through crystal field effect. The magnetic field- and temperature- dependent PL were investigated under pulsed magnetic field, and the results show that the PL intensities of both green and MIR emissions could be increased 25 times under 4 T, whose working magnetic field is much smaller than that reported in other materials. Comparison between the enhancement factors of the two PL emission bands demonstrate that both excitation and emission energy transitions have made contributions to the PL enhancement.

2. Experimental section

The Er³⁺: GdVO₄ crystal was prepared from Er³⁺: GdVO₄ polycrystalline by optical floating zone method, and the Er atom content was 5 mol% with respect to the total number of Er and Gd atoms. [19] The host matrix GdVO₄ crystal is tetragonal, and the Gd site symmetry is D_2d. [19] The measured sample disk with diameter of 5 mm and thickness of 1 mm was cut from the as prepared crystal rod with disk plane perpendicular to the rod growth direction (longitudinal direction of the rod). The PL spectra under magnetic field were measured using a PL measurement setup under pulsed magnetic field in Wuhan National High Magnetic Field Center. [19, 21, 22, 25]

Fig. 1 shows the schematic of the experimental setup for magneto-optical PL measurement. The magnetic field was generated by a liquid nitrogen-cooled resistive coil magnet with the pulse duration of 290 millisecond (ms) and the falling side of 270 ms, and the homogeneity of the magnetic field at the center is better than 99% within a volume of 20 mm × ϕ10 mm space region. The sample was mounted on a probe and placed into the center of the magnet, where a pickup coil was used to measure the magnetic field at the sample location. From the bottom-right inset of Fig. 1, one can see that our measurement here is in Faraday configuration where the wave vector of the laser is nearly parallel to the magnetic field. A 488.0 nm argon laser line was launched into the probe using a fiber and focused onto the sample by a pair of quartz lenses. The PL signal was collected by the same optical components and recorded by two CCD spectrometers, one is silicon EM-CCD spectrometer (Andor DU970P, SR500) with spectral resolution of 0.025 nm for green PL, another one is InGaAs CCD spectrometer (Princeton SPI-300, OMV-5) with spectral resolution of 0.6 nm for MIR PL. It is worth noticing that the PL signal collected from the sample was divided into two branches by a beam splitter, so that both green and MIR PL could be recorded synchronously within one magnetic pulse. The exposure time of each PL is about 1 ms, during which the magnetic field varies less than 0.2 T at the falling side of the magnetic pulse.

 

Fig. 1 Schematic of the experimental setup for PL measurement under pulsed magnetic field. The bottom-left inset shows the time relation of the magnetic pulse and CCD synchronization output pulse sequence, while the bottom-right one gives the zoom in picture of the optics probe inside the magnet.

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3. Results and discussion

Figure 2 gives the absorption and PL spectra of Er³⁺: GdVO₄ crystal. Fig. 2(a) is the absorption spectrum of Er³⁺: GdVO₄ crystal and GdVO₄ host matrix taken by a UV–Vis–NIR spectrophotometer (Lambda 950, PerkinElmer) with the scanning resolution of 0.1 nm per step at room temperature. The solid line is the spectrum of Er³⁺: GdVO₄ crystal, from which at least seven different absorption bands could be observed clearly, which are ⁴I_15/2 → ⁴H_9/2 (∼407 nm), ⁴I_15/2 → ⁴F_3/2 (∼445 nm), ⁴I_15/2 → ⁴F_5/2 (~454 nm), ⁴I_15/2→⁴F_7/2 (∼490 nm), ⁴I_15/2→⁴H1_1/2 (∼522 nm), ⁴I_15/2→⁴S_3/2 (∼549 nm), and ⁴I_15/2→⁴H_9/2 (∼656 nm), respectively. [4] Among these absorption peaks, the one locates at 490 nm covers the 488.0 nm argon laser line, therefore we will select the 490 nm absorption band as the excitation wavelength band. The inset presents the detailed structure of the absorption spectrum around 490 nm taken by an EM-CCD camera with spectra resolution of 0.025 nm at 80 K. Four sub-absorption peaks induced by the crystal field effect could be recognized clearly, and the doublets of each sub-peak indicate that the location of Er³⁺ ions in the crystal have two different sites caused by the impurity of host matrix which will not be discussed here. It is worth noticing that two sub-peaks located around 488.1 nm and 488.3 nm are right close to the 488.0 argon laser line, which are much closer to the laser than that of Er³⁺ in YVO₄ crystal. [25] The red dashed line is absorption spectrum of GdVO₄ crystal, the curve is very smooth and no absorption peak could be observed obviously. This confirms that all the absorption peaks observed from the Er³⁺: GdVO₄ crystal come from the transitions of Er³⁺. Fig. 2(b) is the PL spectra of Er³⁺: GdVO₄ crystal taken at 80 K, and the green emission band comes from the ⁴S_3/2→⁴I_15/2 transition, while the MIR emission band comes from the ⁴I_13/2→⁴I_15/2 transition. Under the exciting of a 488.0 nm argon laser line, both green and MIR strong emissions could be obtained by using a VI CCD spectrometer and a MIR spectrometer at the same time (see Fig. 1).

 

Fig. 2 Absorption and PL spectra of Er³⁺: GdVO₄ crystal. (a) Absorption spectrum of Er³⁺: GdVO₄ crystal taken at room temperature, several absorption peaks including a 490 nm absorption band could be observed. The inset is the detailed spectrum of the 490 absorption band taken at 80 K, at least four sub-absorption peaks could be clearly recognized. (b) PL spectra of green emission band (⁴S_3/2→⁴I_15/2 transition) and MIR emission band (⁴I_13/2→⁴I_15/2 transition) from Er³⁺: GdVO₄ crystal taken at 80 K.

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Fig. 3(a) and (b) demonstrate the mapping of the green PL emission and the MIR PL emission from the Er³⁺: GdVO₄ crystal versus the PL emission wavelength and magnetic field, respectively. The spectra were recorded simultaneously under a single magnetic field pulse up to 7 T at 80 K. This means that the green spectrum and MIR ones obtained here are completely comparable. From the Fig. 3 one can see that the intensity of all the PL peaks in the green PL spectra and MIR ones were enhanced synchronously under the sweeping of the applied magnetic field, and the enhancement tracks of the two PL bands are consistent with each other very well. Which means that the dominant origination of the magnetic field induced PL modulations comes from the excitation transition process, since it is the only same part of the two PL emission processes.

 

Fig. 3 Magnetic field dependent PL spectra Er³⁺: GdVO₄ crystal obtained at 80 K. (a) is the green PL emissions, (b) is the MIR PL emissions. Both of the two series of PL spectra were taken under a single pulsed magnetic field up to 7 T at the same time by using a VI CCD spectrometer and a MIR spectrometer.

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Fig. 4 demonstrates the PL enhancement behaviors of Er³⁺: GdVO₄ at different temperatures. Fig. 4(a) and (b) are the data for the VI emission band and the MIR emission band, respectively. To clearly characterized the PL enhancement of the emissions, an enhancement factor f was defined as f = I_B/I₀, where I_B and I₀ are the integrated PL intensities under the magnetic field intensity of B and zero at the same temperature, respectively. From the figures, one can see that there are two main enhancement peaks (peak1 and peak2) for both of the two emission bands at the magnetic field range of 0–7 T. Peak1 locates around 2.5 T at the temperature range of 10–80 K, while peak2 locates around 4 T with a shoulder at 3.5 T. At the temperature above 200 K the enhancements are very weak, as the sample temperature goes down to 4.2 K, the enhancement factor f increases monotonically. As for peak2, the values of f for the green PL and the MIR PL increase from nearly 1 to 25.6 and 26.6, respectively. While for peak1, f increases monotonically and reaches the maximum value around 20 K, then decreases rapidly to a very small value as the temperature goes down further. These temperature- and magnetic field-dependent enhancement are similar to that of the Er³⁺: YVO₄ crystal, [25] which was controlled by both the thermal effect and the Zeeman effect. For the PL enhancement at the temperature larger than 200 K, the thermal effect is dominant and the magnetic field effect is disturbed. As the temperature goes down, the thermalized electrons come back to the lower branches of the energy levels and result in the increase of the electron populations which will lead to the increase of the PL enhancement. With the temperature goes down further to liquid helium temperature, the electrons will be restricted to the lowest energy branches, and result in the disappearing of some enhancement peaks corresponding to the transitions from the upper branches of the ground states to the excited states.

 

Fig. 4 Magnetic field dependent VI and MIR PL emission behaviors. (a) PL enhancement factor as functions of magnetic field for the green PL emissions at the temperature from 4.2 K to 300 K. (b) PL enhancement factor as functions of magnetic field for the MIR PL emissions at the temperature from 4.2 K to 300 K. (c) Positions of the enhancement peaks versus temperature.

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To observe the details of the PL enhancement for both VI and MIR emissions clearer, the magnetic field dependent PL enhancement curves, as well as the peak area ratios of the PL enhancement factors for the MIR emission and the VI ones have been demonstrated in Fig. 5. Fig. 5(a) is the normalized PL enhancement factor as functions of magnetic field at the temperature from 4.2 K to 300 K. The solid lines represent the data of MIR PL, while the dashed lines are the green ones. When we put the VI and MIR enhancement curves together, it is easy to see that the value of the enhancement factor varies a little around the enhancement peaks. This is because Zeeman splitting of the radiative energy levels changes the population distributions of the electrons in sub-energy levels. Which implies that, except for the excitation energy levels, radiative energy levels also have some contribution to the magnetic field induce PL modulations. Fig. 5(b) is the full width at half maximum (FWHM) of the enhancement peaks versus temperature, one can see that the FWHM of the enhancement peaks become narrower as the temperature decreases until it close to 10 K. This is because the lowering down of the temperature suppresses the vibrations of the atoms within the crystal which decreases the transition bandwidths. When the temperature goes down further, the FWHM begins to increase slightly (see Fig. 5(b)). This is due to the moving of electrons from the energy level of peak1 to that of peak2. Fig. 5(c) presents the area ratios of the two enhancement peaks versus temperature. The dots are data extracted from the experiment, where the smooth solid lines are calculated population ratios (P_r) according to the temperature dependent Boltzmann distribution formula. [8]. As the temperature goes down, the area ratio of peak2 for both emission bands increase, which means that more and more electrons were concentrated to the lowest energy level (corresponding to peak2), and lead to the increase of the excitation efficiency of the ground state.

 

Fig. 5 (a) Normalized PL enhancement factor (VI and MIR enhancement factor both divided the maximum value of f with VI emission band at corresponding temperatures) as functions of magnetic field at the temperature from 4.2 K to 300 K. The solid lines represent the data of MIR PL, while the dash line is the one for green PL. (b) Full width at half maximum (FWHM) of the enhancement peaks versus temperature. (c) Temperature dependent peak area ratios of f for MIR and VI emissions, the dots are experimental data, while the solid lines are calculated ones (P_r) for energy levels of 0 cm⁻¹ and 40 cm⁻¹, respectively.

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To better understand the PL enhancement processes of Er³⁺: GdVO₄ crystal under magnetic field, Fig. 6 presents the energy levels of Er³⁺ involved. From the picture, one can see that there are four energy levels (⁴I_15/2, ⁴I_13/2, ⁴S_3/2 and ⁴F_7/2) and three transitions (⁴I_15/2→⁴F_7/2, ⁴S_3/2→⁴I_15/2, and ⁴I_13/2→⁴I_15/2) involved in the VI and MIR emission processes. When the 488.0 nm laser is irradiated onto the sample, the electrons were stimulated from the ground states (⁴I_15/2) to the excited states (⁴F_7/2). Under the effect of magnetic field, both ⁴I_15/2 and ⁴F_7/2 splitted into several sub energy levels which make the excitation energy gaps change with the varying of the magnetic field, and then result in several PL enhancement peaks for both VI and MIR PL (see Fig. 4). [25] While for the emission process of VI and MIR PL, the splitting of ⁴I_13/2 and ⁴S_3/2 also change the electron populations of each energy level, and cause the differences of enhancement factors between them (see Fig. 5(a)). Therefore, we can conclude that, both the excitation process and the emission processes have made contributions of the PL enhancement, where the excitation process is the dominant one here. Based on these results, it is easy to understand that, the selecting of excitation laser energy and the tailoring of the excitation energy levels are two efficient ways to design magnetic field induced PL modulations in down conversion luminescent materials like Er³⁺: GdVO₄ crystals.

 

Fig. 6 Schematic diagram of energy level and transition for Er³⁺ in GdVO₄ crystal. Wathet arrows indicate exciting transitions. Green and dark red arrows indicate the radiative transitions. Dark black and gray wave arrows indicate assistant exciting transitions and nonradiative transitions, respectively.

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4. Conclusion

Er³⁺: GdVO₄ crystal was prepared and nearly 25 times of magnitude increase of PL intensity has been obtained for both green and MIR emissions under moderate low magnetic field of 4 T. Comparative studies of magnetic field induced VI and MIR PL enhancements demonstrate that both the excitation process and the emission processes have made contributions to the PL enhancement, and the excitation process is the dominant one. Besides, the successful lower down of the working magnetic field of PL modulation in Er³⁺: GdVO₄ crystal compared to that of Er³⁺: YVO₄ as reported, proved that the selecting of the excitation laser energy and the tailoring of the excitation energy levels are two feasible ways to design magnetic field induced PL modulation materials, which are very important for realizing magneto-optical devices with even lower working magnetic field.