1. Introduction

The white emitting rare earth phosphors are usually designed based upon Stokes emission of Ce³⁺ and Eu²⁺ ions under UV/blue excitation [1–3] or anti-Stokes mixture phosphors composed of activating sensitizer Yb³⁺ ions and emitting Er³⁺, Tm³⁺, Ho³⁺ [4–7]. Recently Wang et al. [8,9] reported bright anti-Stokes white emission in fully concentrated single rare earth oxides. This process occurs as a result of multiphoton up-conversion (UC) with near infrared laser diode excitation (976nm) in crude lanthanide oxide powders placed in vacuum (several mbar). Long, excitation intensity dependent rise times, high orders of the UPC process (N>4) and saturation at higher laser excitation have been demonstrated. Moreover, the absolute UPC intensity was found to decrease with the pressure by several orders of magnitude above 1mbar. The most appealing result was a high efficiency of white emission: about 10%. Such a high efficiency suggests a possibility of application in white light systems.

The efficient yellow anti-Stokes upconversion emission was recently reported by us for nanocrystalline lithium neodymium tetraphosphate (LiNdP₄O₁₂) powders [10, 11]. In the present work we demonstrate the efficient anti-Stokes white emission (ASWE) in nanocrystalline lithium ytterbium tetraphosphate (LiYbP₄O₁₂) observed at normal atmospheric pressure. The influence of pressure, temperature and excitation power have been investigated. Following the photocurrent measurements the mechanism of ASWE based on CT emission of Yb³⁺ ion associated with the transient interconversion of Yb³⁺ to Yb²⁺ is discussed.

2. Experimental

The LiYbP₄O₁₂ nanocrystalline powders were synthesized according to the method described by Wiglusz et al. [12]. The studies were conducted on LiYbP₄O₁₂ nanocrystalline powder with average grain diameters 40 nm. The emission spectra were measured using 976 nm CW LD, 808 nm CW LD and 458 nm CW from argon laser as excitation sources and an AVS-USB2000 Spectrometer from Avantes. The UC rise times were recorded using a LeCroy WaveSurfer 400 oscilloscope and a mechanical chopper. The photoconductivity measurements were measured using a Keithley 2400 Sourcemeter with 50V voltage applied to a pellet with the diameter 5mm and thickness 2mm obtained from LiYbP₄O₁₂ nanocrystaline powder pressed under 70kN and with silver contacts. Temperature of the sample was controlled using an A40 M thermovision camera from Flir Systems. The electrooptical efficiency has been estimated using digital images acquired with a Canon EOS 400D camera. The images have been taken using the exposure time t_exp = 1/4000 s and f/32 aperture with ISO 100.

3. Results and discussion

3.1 Luminescence measurements

The anti-Stokes emission in LiYbP₄O₁₂ nanocrystalline powder was observed upon illumination with a 976 nm laser diode (see Fig. 1a ). We have found that after switching on the laser beam onto a small part of the powder sample there appeared a blue emission peaked at 475 nm which after 0.67 s changed the color to yellow and after 1 s all the volume of sample emitted white luminescence whose spectral characteristics are shown in Fig. 1b. The experiment was carried out in ambient atmospheric conditions at room temperature. The observed white emission was very intense, changed in time (see Fig. 1b) and became stable after 2.33 s. It consisted of an inhomogenously broadened band in the range 400 – 800 nm with the maximum centered at 610 nm. The spectrum of the blue emission is shown in the inset of Fig. 1b. The ASWE intensity slowly grows in time reaching the plateau after 2s. The time evolution of the total ASWE intensity relative to the blue emission is shown in Fig. 1c.

 

Fig. 1 The time evolution of up-conversion emission color (a) and the spectral characteristics (1) of LiYbP₄O₁₂ nanocrystalline powder. Figure 1c shows the time increase of white emission. Figure 1d illustrates the temporal changes of blue emission. Figure 1e shows the dependence of the decay time of blue emission on incident power. The spot diameter was 140 μm.

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The time evolution of the up-converted signal is composed of fast appearing and ‘short’ living blue 472nm emission of Yb³⁺-Yb³⁺ pair followed by long building-up broad-band white emission (a, d). The decay time of the blue emission τ_D@472 nm decreases (a, b), while the white upconversion intensity (a, P>1600mW) increases with the increase of excitation intensity. The rise time τ_R of the white emission depends on the delay time (t_D) elapsed since switching off the excitation source.

The visual inspection of the temporal appearance of the up-converted emission revealed very long build-up times of ASWE preceded by blue emission at ~475 nm (Fig. 1b) immediately after the NIR excitation was switched on. The blue emission intensity decays with a ~19.5 ms rate constant and is followed by a very slow rise time of the white emission. The quantitative studies are presented in Fig. 1d, e. As one may note the intensity of blue emission is measured as the intensity of the spike preceding the white emission, (Fig. 1c). This homology indicates the observed emission to originate from cooperative Yb³⁺-Yb³⁺ pairs. It is interesting to note that the rise times t_R depend on the delay time elapsed since the switching off the excitation source t_R was equal to 2.74, 3.37 and 5.25 seconds for the 5.5, 10.5 and 15.8 seconds of resting time (τ_D), respectively.

In the course of experiments we have found that the growth of white emission intensity was dependent on excitation laser power density. This effect was observed as changes in the threshold of white emission and in the slope of the power dependence. Therefore we performed the power dependence experiments with a focused laser beam. The power dependence of the overall intensity of white light emission from LiYbP₄O₁₂ nanocrystalline powder is shown in Fig. 2a . The intensity profile demonstrates the characteristic onset with a threshold at ~0.7 W followed by rapid growth by almost three orders of magnitude and final saturation above 1.1 W of the optical excitation power.

 

Fig. 2 The incident laser power impact on intensity of white emission (a), blue emission (b) and white emission at simultaneous two colour excitation- 976 nm LD and 458 nm argon laser (c) of LiYbP₄O₁₂ nanocrystalline powder.

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The dependence of anti-Stokes emission intensity I on the incident pumping power P is usually discussed in terms of the power law I ~P^N, where the N exponent is related to the number of photon transitions involved in the process. In our experiment N was calculated to be 15.2. Such a high value together with the sigmoidal in-out shape and long intensity rise times support the photon avalanche-like mechanism. It is important to note that this fast increase of intensity of emission occurred only within a relatively narrow power range 0.78-1.1W. The observed saturation of ASWE intensity above 1.1W points towards a phase transition of the system which is in agreement with the photon avalanche model of Guy, Joubert and Jacquier [13,14] within the Landau theory of phase transitions [15].

The effect of excitation power on the blue emission of LiYbP₄O₁₂ nanocrystalline powders is shown in Fig. 2b. This dependence was well fitted with the power law formula taking the effective number of photons N = 1.68 and indicates the double photon absorption process to be responsible for (Yb³, Yb³⁺) pair emission. The white emission intensity obtained with a focused 976 nm excitation increased significantly when the system was pumped with additional excitation beam in the visible range of argon laser operating at 458 nm (2c). The ASWE intensity increased linearly with the power of the incident argon laser beam.

The effect of temperature on ASWE intensity of LiYbP₄O₁₂ nanocrystalline powders in air is presented in Fig. 3 . The experiment has been carried out in the following way: The temperature of white emission of the sample placed in the cryostat was directly measured by a thermovision camera. The temperature inside the cryostat was gradually decreased from 300 to 200K. We have found that the white emission occurred at 550 °C. This result is close to the results of Wang et al. [7] who have estimated the temperature of white emission of the sample in vacuum to be 500-1000°C. It was found that with lowering the temperature in the cryostat by 60 K the white emission was completely quenched. The temperature dependence of white emission is plotted in Fig. 3a. One can see that it increases rapidly with temperature above the threshold value ~810 K reaching the maximum at 870 K where the intensity is enhanced by five orders of magnitude.

 

Fig. 3 The effect of excitation power (a, b) at room temperature on the intensity of the anti-Stokes emission of LiYbP4O12 nanocrystalline powders (λ_exc = 976 nm CW, P = 1.65 W focused with a f = 40mm lens).

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The dependence of temperature of white emission on incident light power is shown in Fig. 3b. It can be seen that in a range 0.1 −1.5 W the temperature of white emission increases from 50°C to 550°C. It is interesting to note that the temperature increases sublinearly with power approaching some limit magnitude.

The temperature dependence of ASWE intensity may be discussed within the Auzel model [16,17] which is described by the exponential relation of multiphonon anti-Stokes emission

The IR induced white emission of concentrated Yb³⁺ compounds was reported recently in Yb₂O₃ and Yb₃Al₅O₁₂ crystalline powders placed in vacuum by Wang et al. [8,9]. The authors have demonstrated that the ASWE emission in YbAG, (Yb_0.3Y_0.7)₂O₃ and Yb₂O₃ increases under vacuum conditions. For the pressure over 0.5 mbar, the ASWE intensity decreased exponentially by up to 4 orders of magnitude. This behavior was dependent evidently on concentration of Yb³⁺ ions because diluted oxides (Yb_0.3Y_0.7)₂O₃) demonstrated less significant dependence than fully concentrated Yb₂O₃.

We have carried out the measurements of white emission intensity of LiYbP₄O₁₂ nanocrystalline powder in dependence on the surroundings of the sample. The dependence of ASWE intensity on pressure is shown in Fig. 4a . With decreasing the pressure the overall intensity is seen to increase by two orders of magnitude (see Fig. 3) compared to the normal atmospheric conditions. Our results (⋄) demonstrate that ASWE is very efficient at atmospheric pressure and leads to over one order of magnitude stronger emission at pressures over 1mbar, unlike for the Yb doped oxides.

 

Fig. 4 The impact of ambient pressure on white emission of LiYbP₄O₁₂ nanocrystalline powder (a) and the dependence of its intensity on surrounding atmosphere pressure (⋄) (b). For comparison purposes the similar dependences for YbAG (◻), Yb₂O₃ (O) and (Yb_0.3Y_0.7)₂O₃ (Δ) from Wang et al [9] are also presented.

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Wang and associates [8,9] have discussed the impact of vacuum on the WE process in terms of heat dissipation model, which predicts enhancement of photon emission in vacuum due to reduced thermal convection. The reduced thermal convection is associated with thermal conductivity which is extremely low in nanocrystalline systems [19] (thermal conductivity coefficient of single crystals of LiYbP₄O₁₂ is K = 0.033 W/cm·K [20] and YbAG is K = 0.0072 W/cm·K [21]). It seems that low thermal conductivity may be responsible for observation of white emission in LiYbP₄O₁₂ nanocrystalline powder under normal atmospheric pressure.

3.2 Photoconductivity measurements

The photoconductivity experiments were carried out on the powder of LiYbP₄O₁₂ nanocrystallites compressed into a pellet with the diameter 5mm and thickness 2mm coated with silver paste by illuminating with 976 nm LD. The time evolution of the electric resistivity of LiYbP₄O₁₂ nanocrystalline powders was measured as a function of the varying power of the LD light (see Fig. 5a ). It can be seen that the measured resistivity changes by four orders of magnitude.

 

Fig. 5 Photoconductivity studies of LiYbP₄O₁₂ nanocrystalline powders reveal up to 4 orders of resistance variability of photocurrent with 976 nm excitation (a-b). c. Photoresistivity measured for focused (white emission) and unfocused (blue emission) pumping laser beam.

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The dependence of the photocurrent on the excitation power is presented in Fig. 5b. One can see that the photocurrent increases nonlinearly with incident power. It was found that the power dependence of photocurrent may be well approximated according to the power law I_PC ~I^N, where N ≈4. Such order of the process means that the photocurrent is due to an avalanche process induced by a four-photon transition corresponding to 5.4 eV. This magnitude is rather far from the maximum of the CT band in LYP as reported by Dorenbos et al. [22] (6.59 eV) however the energy minimum of CT state must be localized much lower and the CT band may be effectively populated to create electron transfer from O^2- to Yb³⁺ resulting in Yb²⁺-CT cluster.

The multiphoton induced photoconductivity was already reported by Idrish Miah [23], however, only two- and three photon processes were evidenced. It is interesting to see that the nonlinear increase of photocurrent with applied incident light power is similar to phototransistor-like behavior. We note that the photocurrent grows up quite slowly after switching on the incident laser light. The rise time of the photocurrent maximum was determined to be 17 s. The quenching time of the photocurrent was slightly lower 15 s.

The time evolution of the resistivity due to photoconductivity was measured under the focused laser beam condition (1.299 MW/cm², diameter of spot 140 μm) for ASWE and unfocused beam condition (1.819 W/cm², diameter of spot 7 mm) for the blue emission as showed in Fig. 5c. One can see that the resistivity under the focused beam was characterized by one-order smaller value than for the unfocused beam. It is important to note that the photoresistivity reached some minimum magnitude after 60 s and did not change with time for both blue and white emissions. It may be concluded that the efficient photoconductivity starts before the white emission. Our experiments proved that the second process occurring via two coupled [(Yb³⁺, Yb³⁺)- (Yb³⁺, Yb³⁺)] pairs is most probably dominating in our system. It was suggested by Wang et al. [8] that four-photon excitation of Yb³⁺ systems is capable to reach the conduction band. This may occur via a direct multiphoton absorption or cooperative absorption of two Yb³⁺-Yb³ pairs. A direct excitation of conduction band should result in photoconductivity of our powder. Since the photocurrent PC is related to the number of free electrons in CT band the photoconductivity electrons are proportional to the number of Yb²⁺ ions. Recently Brandt et al. [24] reported photoconductivity of Yb-doped YAG and Lu₂O₃ crystals at high excitation densities. They have observed an unexpectedly high photocurrent, up to several hundred nanoamperes at applied incident power of 10 W and 500V voltage. The photocurrent measured in LiYbP₄O₁₂ nanocrystalline powder was much higher, close to 10 microamperes at much weaker incident power (1.5W) and lower voltage (50V).

3.3 Origin of white emission

The broad band emission of Yb³⁺ doped MO-Al₂O₃ (M = Ca, Sr, Ba) phosphors was observed in up-conversion experiments by Verma et al. [25] and discussed in terms of inter-conversion between the 3 + and 2 + valence states of ytterbium ions. The broadband emission in Yb:Y₂O₃ nanocrystalline powders after illumination with laser tuned to absorption band of Yb³⁺ion was reported by Redmond et al. [26] and ascribed to black body radiation. It was showed that with 976nm excitation the LiYbP₄O₁₂ nanocrystalline powder demonstrates the cooperative blue emission from ytterbium pair (Yb³⁺-Yb³⁺) which is then dominated by delayed and strong broadband white emission. It was also found that at low temperature only blue cooperative emission was observed and no sign of white emission was detected. The intense white emission was recently reported by Joshi et al. [27] for Ca₁₂Al₁₄O₂₃:Yb³⁺/Yb²⁺ nanophosphor.

Recently Stryganyuk et al. [28] reported luminescence spectra of Yb³⁺ doped LiY_1-xYb_xP₄O₁₂ using synchrotron radiation for high energy excitation in the 5-17 eV range at temperature T = 8-320 K. The broad band emission at 2.8 eV resulted from charge transfer luminescence (CTL) due to charge transfer cluster formed by Yb²⁺ ion and a hole localized over the surrounding ligands. The CT luminescence of Yb²⁺ may be realized through recombination of an electron with a hole localized on the ligand.

Quite recently Dorenbos et al. [22] reported vacuum ultra-violet spectroscopy of Yb³⁺ ion in LiYP₄O₁₂ and identified the observed broadband emission in a range 2-4.5 eV with two CTL transitions to ²F_7/2 and ²F_5/2 terms of Yb³⁺ ion. Following the excitation spectrum of LiYbP₄O₁₂ the authors have assigned the maximum of CT band at 10 K to be located at 6.49 eV (≈52300 cm⁻¹) and the respective CT emission band CT →²F_7/2 at 3.59 eV and CT→²F_5/2 at 2.57 eV. The CT starts from the top of the valence band and ends in the ground state of the divalent Yb²⁺. After the charge transfer, the Yb²⁺ ion radius becomes around 18 pm larger than the trivalent Yb³⁺ ion [28]. This radius increase is followed by strong lattice relaxation and large offset in the configuration diagrams, leading to a broad ~0.91 eV (7338cm⁻¹) wide CT band.

The white emission in our experiment consists of a single broad band which is located ~2.03 eV (~16400 cm⁻¹). Therefore that emission is characterized by an enormously large Stokes shift of 4.46 eV (~35900 cm⁻¹). As concluded by Dorenbos [22], the CT starts from the top of the valence band and ends in the ground state of the divalent Yb²⁺. Since the CT band in our system, which is responsible for white emission, is much stronger deformed it must lead to a larger width of the white emission band (~13500 cm⁻¹) than in the experiment of Dorenbos et al. [22] (~2000-3000 cm⁻¹). From the Stokes shift magnitude we can predict the position 0-0’ transition to be located at 23 800 cm⁻¹. To verify these estimations we have carried the experiments with simultaneous two color excitation using the 458 nm blue laser line (21834 cm⁻¹) of argon laser and 808 nm (12376 cm⁻¹) line of laser diode. One can suppose that the first line excites directly Yb²⁺ cluster from the state of two excited (Yb³⁺,Yb³⁺) pairs at 6 eV whereas the second laser line with much lower energy cannot access the Yb²⁺ -CT cluster.

To discuss the mechanism of anti-Stokes white emission in LiYbP₄O₁₂ nanocrystalline powders we have to summarize its principal characteristics. The white emission was observed as a broad band centered at 600 nm (~16660 cm⁻¹ ≈2.2eV). It occurs at high temperature (550°C) and is accompanied by efficient photoconductivity. This means that the created free carriers (electrons) are responsible for creation of Yb²⁺ ions. It is interesting to note that the 3 + to 2 + valence change has been obtained reversibly under photoexcitation. To confirm an origin of the broadband emission as 4f¹³5d→⁴f¹⁴ transition of Yb²⁺ ion we have carried the experiments with additional pumping of Yb²⁺ directly into the 4f¹³5d band by using excitation at λ_exc = 458nm with an argon laser. We have observed a linear increase of the white emission (see Fig. 2c). We did not observe an enhancement of the ASWE intensity when we pumped with a lower energy excitation high power laser line of 808 nm LD being below the 0-0’ transition.

The mechanism of anti-Stokes white emission of LiYbP₄O₁₂ nanocrystalline powders is complex. A threshold and saturation of intensities at high power limit suggest the avalanche-like mechanism to be responsible for the observed behavior, however, it is preceded by a sequence of mutually combined phenomena. The following sequence of events leading to white emission in LYbP₄O₁₂ is observed (see Fig. 6 ). In the initial phase, after switching on the CW excitation (up to ~0.5 s), Yb³⁺ ions are continuously excited (²F_7/2→²F_5/2) and due to high concentration of active ions the efficient cooperative (²F_5/2,²F_5/2)→ (²F_7/2,²F_7/2) blue emission of Yb³⁺-Yb³⁺ pairs occurs at 2.5 eV. Then, the other higher order cooperative processes occur, associated with interaction of incident photons with the double coupled pairs [(Yb³⁺-Yb³⁺)- (Yb³⁺-Yb³⁺)] leading to blue emission and violet emission. In the second step, processes leading to creation of electrons responsible for the photocurrent are commenced. Through the excitation ladder of Yb³⁺ pairs it is possible to populate the CT states of Yb³⁺ with creation of free electrons responsible for photocurrent. The phenomenon is temperature dependent but the laser induced heating is not the sole origin of white emission. On one hand, bridging of non-resonant transitions is facilitated at higher temperature, on the other hand, the presence of electronic levels in Yb³⁺ seems to be a prerequisite condition for ASWE to occur. The photon avalanche conductivity antecedents the ASWE process. In the third step, the created free electrons become responsible for creation of Yb²⁺ CT clusters following the reducing reaction Yb³⁺ + e → Yb²⁺. In the fourth step, the broadband emission of the 4f¹³5d→⁴f¹⁴of Yb²⁺ CT cluster appears. The appearance of Yb²⁺ ions is reflected in gradual increasing of the f-d absorption band accessible via energy transfer from (Yb³⁺, Yb³⁺) pair in excited state (²F_5/2,²F_5/2) directly into 4f¹³5d excited state of Yb²⁺ ion. Since the f-d and CT transitions are parity allowed transitions the white emission is very intense.

 

Fig. 6 Scheme of electronic relaxation leading to IR induced anti-Stokes white emission in LiYbP₄O₁₂ nanocrystalline powders. The energy levels scheme is taken from Dorenbos et al. [22].

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4. Conclusion

The IR induced anti-Stokes white emission of LiYbP₄O₁₂ nanocrystallite powder was observed under ambient atmosphere. It was characterized by a threshold behavior vs. the excitation laser diode power and temperature which is typical to avalanche processes. Moreover, the rise time of the white emission were very slow: close to 2 s and shortened with the increasing excitation power. The slope of power dependence was described by an extremely high order parameter N = 16. The efficiency of white emission increased by two orders of magnitude in vacuum. It was found that the ASWE process was critically dependent on temperature and occurred after crossing the threshold at 510° C . The temperature of white emitting sample was relatively low 510-550° C and did not increase linearly with the increasing excitation power. Such behavior suggests that some part of excitation power is not converted directly into heat. It seems that it may be due to laser cooling effects as suggested by Happek et al. [29]. The efficient photoconductivity observed in our experiment is proportional to P⁴ and is similar to phototransistor-like behavior. Although the photocurrent signal increases rather slowly with the excitation power and its dropping down after switching off the laser is also slow, the observed phenomenon may find some new applications.

The results presented in this work demonstrate that the origin of the white emission of LiYbP₄O₁₂ nanocrystals is evidently associated with emission of Yb²⁺ - CT cluster. This emission can be utilized for sensing the atmospheric pressure and temperature as well as measurements of laser diode power. We suppose that low thermal conductivity due to the phonon confinement drastically reduces thermal dissipation processes via radiationless transitions. The efficiency of ASWE seems to be very high. According to Wang and Tanner [8] estimation it was about 10% for Yb₂O₃ powder measured in vacuum as a result of high efficiency of pumping laser diode yield, 50-60%. The high electroptical efficiency of the ASWE process suggests its possible application as new white light source.