Controlling the Two-Photon-Induced Photon Cascade Emission in a Gd³⁺/Tb³⁺-Codoped Glass for Multicolor Display

Abstract

We reported the first observation of the two-photon-induced quantum cutting phenomenon in a Gd³⁺/Tb³⁺-codoped glass in which two photons at ~400 nm are simultaneously absorbed, leading to the cascade emission of three photons in the visible spectral region. The two-photon absorption induced by femtosecond laser pulses allows the excitation of the energy states in Gd³⁺ which are inactive for single-photon excitation and enables the observation of many new electric transitions which are invisible in the single-photon-induced luminescence. The competition between the two-photon-induced photon cascade emission and the single-photon-induced emission was manipulated to control the luminescence color of the glass. We demonstrated the change of the luminescence color from red to yellow and eventually to green by varying either the excitation wavelength or the excitation power density.

Introduction

The phenomenon of photon cascade emission or the so called “quantum cutting”, in which a photon of high energy is absorbed and converted to two or more photons with lower energies, has been studied intensively in the past few decades because of its potential applications in mercury-free lamps and plasma display panels^{1,2,3,4,5,6,7,8,9}. In recent years, this phenomenon has drawn great attention in the research and development of high-efficiency solar cells because it can significantly improve the conversion efficiency of photon to electricity and reduce heat generation^10,11,12,13. Owing to their unique energy states, rare-earth ions, especially the lanthanide ions, are considered as promising candidates not only for photon up-conversion but also for photon down-conversion^{13,14,15,16,17,18,19,20,21,22}. For example, solid state full color display²³ has been demonstrated by exploiting the photon up-conversion in three lanthanide ions of Pr³⁺, Er³⁺ and Tm³⁺. In addition, the lanthanide ions have exhibited fascinating luminescent properties such as intense narrow-band emission, high conversion efficiency, broad emission peaks, much different lifetimes and good thermal stability^{8,21,24,25,26,27}. Therefore, rare-earth-ion-doped materials have been widely studied and exhibited potential application in the fields of illumination, imaging, display, solar cells and medical radiology because such materials can be fabricated at a low cost and in large quantities^{23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44}.

In rare-earth-ion-doped materials, Tb³⁺-doped glasses have been the focus of many studies because of their high luminescence efficiency at around 550 nm which is convenient for direct coupling with silicon detectors⁴⁵. More interestingly, it has been shown that the luminescence can be further enhanced by adding Gd³⁺ into Tb³⁺-doped glasses because of the energy transfer (ET) from Gd³⁺ to Tb³⁺, as schematically shown in Fig. 1. In fact, the ET between Gd³⁺ and Tb³⁺ has been extensively investigated in many other different host materials^{46,47,48,49,50}. Although electrons can be generated in Gd³⁺, the luminescence from Gd³⁺/Tb³⁺-codoped glasses arises mainly from the transitions from the level ⁵D₄ to the levels ⁷F_0–6 in Tb³⁺ which give rise to four emission bands in the visible light region^{46,47,48,49,50}. In Fig. 1, it is noticed that the levels ⁶G_J in Gd³⁺ are located well above the high-energy levels in Tb³⁺ (⁵K₇ etc.) while the levels ⁶D_J, ⁶I_J and ⁶P_J in Gd³⁺ have similar energies with some energy levels in Tb³⁺. If the population of the levels ⁶G_J in Gd³⁺ is induced, one can expect the transitions of electrons to the low-energy levels (⁶D_J, ⁶I_J and ⁶P_J) of Gd³⁺, the ET of electrons from Gd³⁺ to Tb³⁺, the transitions of electrons to the level ⁵D₄ and finally to the levels ⁷F_0–6. Such a cascade transition process may result in the cascade emission of photons with different energies. In practice, the population of the levels ⁶G_J in Gd³⁺ can be realized by using femtosecond (fs) laser light at ~400 nm through two-photon-induced absorption (TPA). The high peak power and wide linewidth of fs laser light are highly suitable for effectively exciting the levels ⁶G_J in Gd³⁺. Actually, fs laser light at 800 nm has been used to excite the three-photon-induced luminescence in rare-earth-ion-doped glasses³⁷. When fs laser light at 400 nm is used to excite the levels ⁶G_J in Gd³⁺, the level ⁵D₃ in Tb³⁺ with a wavenumber of ~26336 cm⁻¹ (corresponding to a wavelength of ~381 nm) can also be populated through Rabi oscillation or phonon-assisted transition⁵¹, leading to the conventional emission from Tb³⁺. For excitation wavelengths (λ_ex) shorter than 400 nm, the population probability for the levels ⁶G_J is reduced while that for the level ⁵D₃ is increased. It implies the existence of a competition between the cascade emission and conventional emission that depends strongly on λ_ex. On the other hand, the population of the level ⁵D₃, which is caused mainly by Rabi oscillation⁵¹, will exhibit a strong dependence on the excitation power density (P_ex). Therefore, it is expected that one can manipulate the competition between the cascade emission and conventional emission and thus control the luminescence color by varying λ_ex or P_ex, exploring its applications in color display⁵².

Figure 1

Energy level diagram of Gd³⁺ and Tb³⁺ in which the two-photon-induced absorption, the photon cascade emission and the single photon absorption assisted by Rabi oscillation or phonons and the possible electronic transitions between the energy levels are illustrated.

Results and Discussion

The proposed scheme was examined by using different glasses codoped with Gd³⁺ and Tb³⁺ and the dependence of luminescence color on λ_ex and P_ex was found to be a popular phenomenon (details in Supplementary Information, see Fig. S1). However, this behavior also exhibits a dependence on the concentration of Gd³⁺ and glass matrix. While the concentration of Gd³⁺ determines the photon cascade emission, the glass matrix affects the phonon-assisted processes such as the nonradiative decay or the relaxation of electrons. In this work, we show a pronounced phenomenon observed in a silicate glass with a composition of 56SiO₂-10Al₂O₃-12Li₂O-20Gd₂O₃-2Tb₂O₃ (mol%). The ET from Gd³⁺ to Tb³⁺ was also observed in this glass (details in Supplementary Information, see Section 4).

In order to see the λ_ex-dependent competition, we varied λ_ex from 375 to 405 nm and examined the luminescence of the glass. For λ_ex ≤ 390 nm, the luminescence always appeared to be green. However, the luminescence was changed from green to yellow when λ_ex was slightly shifted from 390 to 392 nm. More surprisingly, the luminescence turned to be red when λ_ex was further shifted to 394 nm. A comparison of the emission spectra under different λ_ex of 390, 392 and 394 nm is presented in Fig. 2. The photos for the excitation spot are shown in the insets. It can be seen that the emission spectrum at λ_ex = 390 nm is dominated by the emission band at ~540 nm which corresponds to green color. For λ_ex = 392 nm, the relative intensities of the emission bands at ~580 nm and ~622 nm, which correspond to yellow and red colors, increase rapidly. A close inspection reveals that the peak of the emission band at ~622 nm is blue-shifted to ~613 nm. In addition, two new emission bands emerge at ~654 nm and ~704 nm, contributing to red color. For λ_ex = 394 nm, the intensity of the emission band at ~613 nm exceeds that of the emission band at ~550 nm and the relative intensities of the emission bands at ~654 and 704 nm are further increased, turning the color of the luminescence into red. In previous reports, the luminescence of Tb³⁺-doped and Gd³⁺/Tb³⁺-codoped glasses originates mainly from the electronic transitions from the level ⁵D₄ to the levels ⁷F_0–6 and appears to be green. There is no report on the observation of yellow or red luminescence. Here, it is interesting that the color change in the luminescence occurs in a narrow wavelength region of 390−394 nm, which is comparable to the linewidth of the fs laser pulses (~4.0 nm).

Figure 2

Comparison of the emission spectra of the glass under different λ_ex of 390, 392 and 394 nm.

The insets show the photos of the excitation spots and the luminescence colors.

In order to understand the underlying physical mechanism, we examined the evolution of the emission spectrum and the luminescence color with increasing P_ex for different λ_ex, as shown in Fig. 3a–c. The dependence of the luminescence intensities for different emission bands on P_ex was also extracted, as shown in Fig. 3d–f. For λ_ex = 390 nm, the luminescence color appeared to be green and remained unchanged with increasing P_ex. It can be seen that the intensities of the four major emission bands, which are centered at 488, 544, 585 and 622 nm, increased almost with the same rate. Consequently, the emission spectrum remained nearly unchanged except the absolute intensity. The fitting of the P_ex dependence of the luminescence intensity gives nearly the same slope of ~1.0 for all the emission bands, indicating that the emission is governed by single-photon process. The situation was changed for λ_ex = 392 nm. Two new emission bands emerged at 654 and 704 nm and the emission band peaking originally at 622 nm was blue-shifted to 613 nm. The electronic transitions related to the new emission bands (details in Supplementary Information, see Table I), which are attributed to the transitions between the high-energy levels of Gd³⁺, were previously observed in other materials^53,54 doped with Gd³⁺ under single-photon excitation. For comparison, the glasses doped with only Gd³⁺ or Tb³⁺ were also examined under the same condition (details in Supplementary Information, see Figs S3 and S5). The slopes for the emissions bands at 488, 544 and 585 nm remained unchanged while smaller slopes were observed for the two new emission bands and the emission band at 613 nm. The change in the emission spectrum was negligible and the luminescence color remained to be yellow with increasing P_ex. For λ_ex = 394 nm, the dependence of the emission spectrum on P_ex became significant. It is noticed that the intensities of the emission bands at 488 and 544 nm increased in greater rates and the center of the excitation spot became yellow at high P_ex, as evidenced in the larger slopes of ~1.40. This behavior indicates that two-photon process has been involved in the emission process.

Figure 3

Evolution of the emission spectrum and the luminescence color of the glass with increasing P_ex under different λ_ex of (a) 390 nm, (b) 392 nm and (c) 394 nm. The dependence of the luminescence intensities for different emission bands on P_ex and the fitting for the experimental data are presented in (d–f) for λ_ex of 390, 392 and 394 nm, respectively.

As mentioned above, the effective population of the levels ⁶G_J can be realized by simultaneously absorbing two photons at 400 nm. For λ_ex = 390 nm, the population of the levels ⁶G_J through TPA can be neglected because of the large energy mismatch. As λ_ex is red-shifted toward 400 nm, an increase in the population probability is expected. In Fig. 4, we present the evolution of the emission spectrum and the luminescence color with increasing P_ex measured for λ_ex = 400 and 405 nm. For λ_ex = 400 nm, the emission spectrum was similar to that observed at λ_ex = 394 nm. However, it is noticed that the intensity of the emission band at 613 nm became stronger than that of the emission band at 544 nm and the luminescence appeared to be red at low P_ex. At high P_ex, the intensity of the latter exceeded that of the former, changing the luminescence color to green, as shown in the inset of Fig. 4a. This behavior is also reflected in the P_ex dependence of the luminescence intensity shown in Fig. 4c. The slopes for the emission bands at 488 and 544 nm obtained by fitting the experimental data (1.59 and 1.64) were much larger than those for the emission bands at 590, 613, 654 and 704 nm whose slopes are reduced by nonradiative decay (details in Supplementary Information, see Fig. S6). For λ_ex = 405 nm, the emission spectrum was changed remarkably because of the emergence of many new emission bands, as shown in Fig. 4b. In this case, the red luminescence observed at low P_ex evolved gradually into green one with increasing P_ex, similar to that observed at λ_ex = 400 nm. In comparison, the slopes for the emission bands at 488 and 544 nm were reduced slightly to ~1.50 because the TPA process at λ_ex = 405 nm is not as efficient as that at λ_ex = 400 nm.

Figure 4

Evolution of the emission spectrum and the luminescence color of the glass with increasing P_ex under different λ_ex of (a) 400 nm and (b) 405 nm. The dependence of the luminescence intensities for different emission bands on P_ex and the fitting for the experimental data are presented in (c,d) for λ_ex of 400 and 405 nm, respectively.

Based on the theory of colorimetry (details in the Supplementary Information, see Section 7), one can easily deduce the chromaticity coordinates for the luminescence observed under different excitation conditions and the results are shown in Fig. 5. From the Fig. 5a, under low P_ex, we can see that the chromaticity coordinates for λ_ex = 390, 392 and 394 nm appear in the green, yellow and red regions, respectively. In all cases, a shift of the chromaticity coordinate with increasing P_ex is found and it becomes larger for longer λ_ex. The largest shift of the chromaticity coordinate is observed at λ_ex = 400 nm, as shown in Fig. 5b. In this case, the luminescence color is changed from red to yellow and eventually to green with increasing P_ex.

Figure 5

CIE chromaticity coordinates for the luminescence of the glass under different λ_ex and P_ex.

(a) λ_ex = 390, 392 and 394 nm and (b) λ_ex = 400 nm.

When the levels ⁶G_J in Gd³⁺ are effectively populated, the photon cascade emission is expected to dominate the emission process. It is clearly reflected in the new emission bands which correspond to the photons emitted in the cascade transition of electrons between the energy levels of Gd³⁺ and Tb³⁺. In Fig. 6, we present a comparison of the emission spectra obtained at different λ_ex of 390, 392, 394, 400 and 405 nm in a logarithmic coordinate where the new emission bands originating from the photon cascade emission can be readily identified at λ_ex = 400 and 405 nm and indicated by arrows. In most cases, the cascade emission of three photons is observed, as shown in Fig. 1. The first photon is generated by the transition of electrons between the levels of ⁶G_J and ⁶I_J (or ⁶P_J) in Gd³⁺. The wavelengths of the emission photons range from 571 to 704 nm, contributing yellow or red color. The transition from the levels ⁶G_J to ⁶D_J in Gd³⁺ is thought to be nonradiative^53,54. The emission of the first photon is followed by an ET process of electrons from Gd³⁺ to Tb³⁺. Then, the emission of the second photon occurs through the transitions of electrons between the levels ⁵H₃, ⁵F_4,2,1, ⁵I_6,4, ⁵K₈ and the level ⁵D₃ or ⁵D₄. During this process, the wavelengths of the emission photons cover a broad wavelength range of 495 to 690 nm, contributing mainly yellow and red colors. The emission of the last photon occurs mainly between the levels ⁵D₄ (⁵D₃) and ⁷F_6,5,4,3, contributing to green color. Although the intensities are quite weak, one can identify the new emission bands at 421, 438 and 460 which can be assigned to the transitions between the levels ⁵D₃ and ⁷F_5,4,3.

Figure 6

Comparison of the emission spectra of the glass plotted in a logarithmic coordinate under different λ_ex of 390, 392, 394, 400 and 405 nm.

The new emission bands observed in the photon cascade emission are indicated by arrows.

Having understood the photon cascade emission, one can easily understand the luminescence color change induced by varying λ_ex and P_ex. For λ_ex < 392 nm, the levels ⁶G_J are not populated effectively and the photon cascade emission is not initiated. In this case, the population of the levels ⁵D₃ and ⁵D₄ in Tb³⁺ through Rabi oscillation or phonon assistance is dominant, giving rise to the four emission bands and green luminescence. For λ_ex > 392 nm, the electrons begin to occupy the levels ⁶G_J, initiating the photon cascade emission which competes with the conventional emission. When the emission process becomes dominated by the photon cascade emission, the luminescence appears to be yellow or red. With increasing P_ex, the population of the level ⁵D₃ becomes significant because of two reasons. First, the population probability due to Rabi oscillation increases with increasing P_ex. Second, more electrons relax from the levels ⁶G_J to the level ⁵D₃ after emitting two photons. Therefore, a rapid increase in the intensities of the emission bands at 488 and 544 nm is observed, turning the luminescence into yellow and green at high P_ex. In our case, it was found that the efficiency of the two-photon-induced transitions and ET process from Gd³⁺ to Tb³⁺ depends strongly on the concentrations of Gd³⁺ and Tb³⁺ because it is proportional to the inverse sixth power of the distance between the two types of ions (details in the Supplementary Information, see Section 5). In this work, we have compared the tunable range of the luminescence color by varying λ_ex and P_ex for several glass samples with different concentrations of Gd³⁺ and found that the best performance was achieved in the glass with the largest concentration of Gd³⁺.

Conclusion

In conclusion, we have observed the two-photon-induced photon cascade emission in a glass codoped with Gd³⁺ and Tb³⁺ by using fs laser pulses. By varying either λ_ex or P_ex, we have demonstrated the manipulation of the luminescence color through the competition between the photon cascade emission and conventional emission. Many new emission bands, which are not usually observed in the traditional luminescence of Tb³⁺-doped or Gd³⁺/Tb³⁺-codoped materials have been clearly revealed in the emission spectrum when the photon cascade emission is dominant. More importantly, the dependence of the luminescence on both λ_ex and P_ex implies potential applications in laser-induced color display.

Methods

Materials Preparation

The silicate glass was prepared through melting the mixture of analytical reagents of SiO₂, Al(OH)₃, Li₂CO₃, Gd₂O₃ and Tb₄O₇ at 1600 °C for 30 minutes. The melt was then poured onto a preheated (200 °C) stainless steel plate and annealed at 500 °C for 2 hours. The synthetized glass was cut into 10 × 10 × 1 mm³ sheets and polished for optical measurements.

Photonluminescence Measurements

In our experiments, the fs laser light with a repetition rate of 76 MHz and a pulse duration of ~130 fs delivered by a Ti: sapphire oscillator (Mira 900S, Coherent) was introduced into a harmonic generator (Harmonics 9300, Coherent) and the output light with a tunable wavelength from 375 to 405 nm was employed to excite the glass. The excitation intensity was characterized by the peak power density of the fs laser pulses. It was introduced into an inverted microscope (Observer A1, Zeiss) and focused on the glass by using a 60 × objective lens (NA = 0.85). The diameter of the excitation spot was estimated to be ~4.0 μm. The luminescence generated by the glass was collected by the same objective lens and delivered to a spectrometer (SR-500I-B1, Andor) equipped with a charge-coupled device (CCD) (DU970N, Andor) for analysis. The photos of the excitation spot were taken by using a camera from the eyepiece of the microscope.