Blue frequency-upconversion fluorescence emission has been observed in Ce3+-doped Gd2SiO5 single crystals, pumped with 120-fs 800 nm IR laser pulses. The observed fluorescence emission peaks at about 440nm is due to 5d→4f transition of Ce3+ ions. The intensity dependence of the blue fluorescence emission on the IR excitation laser power obeys the cubic law, demonstrating three-photon absorption process. Analysis suggested that three-photon simultaneous absorption induced population inversion should be the predominant frequency upconversion mechanism.
©2006 Optical Society of America
Considerable interest has recently been focused on the conversion of infrared radiation into visible or ultraviolet light through frequency upconversion in rare-earth-doped materials, for a wide variety of applications including infrared-pumped visible laser [1, 2], optical power limiting [3, 4], high-density 3D optical data storage, display , infrared quantum counters , biological nanolables , 3D lithographic microfabrication , photodynamic therapy , and (3D) fluorescence imaging . Among doped trivalent rare-earth ions, praseodymium , neodymium , holmium, erbium, and thulium  have been paid much attention and a wide range of upconversion wavelengths between 380 and 750 nm involving the transition within the 4f n levels were produced in different hosts. In all these cases, upconversion fluorescence have been pumped using different excitation mechanisms such as energy transfer upconversion (ETU), excited-state absorption (ESA), cooperative upconversion, and photon avalanche, all of which involve the use of metastable intermediate levels which act as a storage reservoir for the pump energy. The trivalent cerium ions, Ce3+, which involving a single ground 4f and a single excited state 5d, exhibit broad ultraviolet or visible luminescence in many crystals hosts [16–18] due to the allowed 5d→4f electric dipole transition. It is, therefore, a suitable candidate for ultraviolet upconversion luminescence applications. However, there are few published reports of infrared-pumped upconversion luminescence based on Ce3+ ions due to the lacking of metastable intermediate levels and efficient upconversion mechanisms.
Multiphoton absorption is a well-known phenomenon and it can involve a variety of mechanisms, such as simultaneous multiphoton absorption and two or more photon absorption followed by successive absorption of photons with real intermediary excited states. Upconversion fluorescence induced by multiphoton absorption has been shown in organic compound [19 20], semiconductors , nanocrystals , and inorganic glasses [23, 24]. Multiphoton absorption mechanism can contribute to the absorption of light at irradiance levels of interest and makes it possible the realization of upconversion luminescence of Ce3+ doped materials based on the direct absorption of multiphoton. Despite many recent progresses on multiphoton absorption studies in numerous materials, the upconversion luminescence in Ce3+ doped crystals via direct multiphoton absorption has been paid little attention.
Due to its potential application in multiphoton optical data storage , three dimensional microfabrication , and multiphoton excited upconverted fluorescence [27, 28], research in multiphoton absorption processes generated by IR femtosecond lasers has been very active in recent years. More recently, we demonstrated that the use of femtosecond laser irradiation to obtain upconversion luminescence in Cr3+ doped Al2O3 and YVO4 crystals [29, 30]. In addition, femtosecond laser induced upconversion luminescence has been shown in Ce3+ ions doped glass . Combining the promising advantages of multiphoton absorption and femtosecond laser, the study of the frequency upconversion processes of Ce3+ ions doped crystal is important. Here, we report a strong simultaneous three-photon absorption blue upconversion luminescence of Ce3+-doped Gd2SiO5 single crystals excited by ultrashort femtosecond laser pulses at 800 nm.
Single crystals of Ce3+-doped Gd2SiO5 were grown by the Czochralski method in inductively heated iridium crucible under a high purity nitrogen atmosphere. The high purity powders of Gd2O3 (≥99.99%), Ce2O3 (≥99.99%) and SiO2 (≥99.999%) were used as starting materials. The raw materials were fired at 1000°C for more than 10 h prior to weighing and mixing to remove moisture, then pressed into pellets and sintered at 1200°C before loading into the iridium crucible. A pulling rate of 2 mm/h and rotation rate of 12 rpm were adopted in the growth experiments. Samples with thickness of 1mm for femtosecond laser irradiation and spectral measurements were cut from the as-grown crystals perpendicularly to the growth axis and polished on both sides.
We used an regeneratively amplified 800 nm Ti:Al2O3 laser that emits 120 femtosecond, 1 kHz, mode-locked pulses as the irradiation source. A chirped pulse with 10Hz, ~ 220 picosecond, and central wavelength of ~ 800nm from Ti:Al2O3 laser was used for comparison. The chirped pulse is generated from the preamplifier chain of an optical parametric amplification system. Laser beam was focused into sample by objective lens. The focal point can be monitored by a confocal microscope system linked to a charge coupled device system. The position of the focal point was beneath the sample surface. The spot size can be controlled at least below several microns by choosing appropriate objective lens or optical lens and adjusting the average power of laser beam. In this study, the femtosecond and picosecond laser beam was focused by an optical lens with a focal length of 100 mm. The fluorescence spectra excited by focused femtosecond laser were recorded by a spectrophotometer of ZOLIX. The fluorescence spectra excited by a 266 nm monochromatic light from a xenon lamp were measured by JASCO FP6500 spectrophotometer. In addition, the absorption spectra were measured with a JASCO V-570 spectrophotometer. All the measurements were taken at room temperature.
3. Results and discussion
Figure 1 shows the emission spectra of Ce3+:Gd2SiO5 irradiated by focused femtosecond laser at 800 nm. Strong blue light emission was observed clearly by the naked eye on the focused spot when focusing the femtosecond laser on the sample. The luminescence spectrum exhibits a typical broad emission band peaking at about 440 nm. For comparison, we also show the emission spectrum of Ce3+:Gd2SiO5 crystals excited at 267 nm (800/3) by monochromatic light from a xenon lamp. The emission spectra show that the corresponding spectral distributions of the measured samples are basically the same for both single-photon and multiphoton excitations. The structure of Gd2SiO5 belongs to the monoclinic space group P21/c, and in Ce3+:Gd2SiO5 crystal, Ce3+ is substituted for Gd3+. Ce3+ ion has a 4f 1 configuration, the ground state consists of a doublet (2F5/2 and 2F7/2). Since the radial wave function for the excited 5d electron of Ce3+ ions extends spatially well beyond the closed 5s 25p 6 shells, 5d states are strongly perturbed by the ligand field of the host, and the lower excited states are the crystal-field components of the 5d configuration. Above results indicates that the blue luminescence of Ce3+:Gd2SiO5 crystals induced by femtosecond laser irradiation is due to interconfiguration transitions from the lowest level of the 5d configuration to the 4f 1 ground state of Ce3+ ions.
It should be noted that a slightly red shift of the near IR excited fluorescence was observed. This can be explained by the reabsorption effect induced by multiphoton absorption. In one-photon spectroscopy, the luminescence emission is characteristic of the surface rather than of the crystalline volume because of the drastic attenuation of the radiation as it propagates into the sample. Therefore, the reabsorption effect can be neglected. Due to the significantly smaller values of the multiphoton absorption coefficients, the reabsorption effect plays its roles during the femtosecond laser irradiation process and red shift occurred. In fact, similar phenomena have also been observed in organic compound .
Up to now, it has been shown that the predominant mechanisms of the upconversion on rare-earth doped materials are energy transfer upconversion (ETU), excited-state absorption (ESA), cooperative upconversion, and photon avalanche. It was believed that these mechanisms were not involved in Ce3+:Gd2SiO5 crystals under femtosecond laser irradiation due to the lacking of two excited states or no transmittance decrease and upconversion signal grows nonlinearly with increasing pump power. Apart from these predominant mechanisms, multiphoton simultaneous absorption is also a mechanism for upconversion luminescence although little attention has been paid to this mechanism in the solid-state materials containing rare earth ions.
According to the theoretical consideration of multi-photon absorption process, the relationship between the pumping power and the fluorescence intensity can be described as a power law: I ∝ Pn. Where, I is the integrated intensity of the upconversion luminescence, P is the average power of the pumping laser, and n is the photon number. The number of photons must satisfy that the total energy of n photons exceeds or equals to the excitation energy required by excited states. The n can be experimentally determined by varying the pumping power of femtosecond laser at fixed focused point. Here, the number of photons n can be determined as a slope coefficient of the linear fitted line by plotting the logarithmic transformation of the pumping power and fluorescence intensity. The log-log relationship of pumping power of femtosecond laser and fluorescence intensity of Ce3+:Gd2SiO5 is shown in Fig. 2. It can be seen that the slope coefficient of the fitted line is 2.97, which indicates that the upconversion luminescence emission is generated by the three-photon absorption process.
The three photon absorption, which includes two mechanisms, one of which is direct three photon absorption and the other is two photon absorption followed by one photon absorption from the excited level, are also represented by a cubic relationship between the pumping power and the fluorescence intensity. In our case, the direct three-photon absorption can be considered to be responsible for the upconversion emission in Ce3+:Gd2SiO5 crystals. A more detailed understanding of the three-photon simultaneous absorption upconversion process can be gained by examination of the linear absorption and excitation spectra of the Ce3+:Gd2SiO5 crystals. The measured one-photon absorption spectrum of the Ce3+:Gd2SiO5 crystal is shown in Fig. 3. For the 4f→5d transition of Ce3+, the electron-phonon interaction is strong because the 5d electrons are outermost, so that the broad absorption bands attributed to Ce3+ ions appear in the absorption spectra. One can find that the two strong broad absorption bands located at 285 and 345 nm in the 190–400 nm range are due to the Ce3+ ions. But the absorption is also included the absorption peaks of Gd3+ ions. In order to identify others absorption peaks of Ce3+ ions, we shown the excitation spectra of Ce3+:Gd2SiO5 crystal when emission at 345 nm, as that shown in the inset of Fig. 3. It is clear that the 248 nm band is also associated with Ce3+ ions. We can see that there is no linear absorption at the one-photon energy or the two-photon energy of 800-mm radiation. Here, the processes of simultaneous absorption of photons in Ce3+:Gd2SiO5 crystals only involving a single ground and a single excited state of Ce3+ ions. Therefore, it is very unlikely that two photon absorption followed by one photon absorption from real intermediary excited states. In addition, the three-photon energy of the near-IR radiation at 800 nm falls within the strong absorption band of the Ce3+ ions, and therefore direct three-photon absorption in the sample may be expected.
To form efficient three-photon simultaneous absorption, the photon density should be high. This is confirmed by the picosecond laser experiments. We performed the experiments using a chirped pulse with 10Hz, 220 picosecond, and 800nm Ti:Al2O3 laser as irradiation source. It is interesting to note that no any visible luminescence emission was observed on the Ce3+:Gd2SiO5 crystal when focused the 800nm picosecond laser with the same pulse power density before focus as that of femtosecond laser irradiation experiments. Above results indicate that the high photon density generated by focused femtosecond laser may make three-photon simultaneous absorption possible.
The process of the direct three photon induced fluorescence of Ce3+:Gd2SiO5 by femtosecond laser irradiation may be as follows: first, the simultaneous absorption of three photons by Ce3+:Gd2SiO5 leading to population of the upper 5d excited state. Second, the excited state relaxes nonradiatively to the zeroth vibronic level-the bottom of the 5d state before returning to the 2F5/2 ground state via emission of a single photon. The population inversion is created between the lower emitting state, 5d, and the ground state, 2F5/2.
In conclusion, the blue upconversion luminescence in Ce3+ ions doped Gd2SiO5 single crystal has been experimentally demonstrated by infrared femtosecond laser irradiation. No blue luminescence was observed when focused the 800nm picosecond laser with different pulse power density on the Ce3+:Gd2SiO5 crystal. The relationship between the fluorescence intensity and the pumping power shows that the upconversion luminescence is a three-photon excitation process. The analysis reveals that direct three photons absorption should responsible for the upconversion luminescence. Femtosecond three-photon generation of blue upconversion luminescence in Ce3+-doped Gd2SiO5 single crystals reveals potential applications in ultraviolet upconversion laser, date storage, display, and imaging.
References and links
1. R. Scheps, “Upconversion laser processes,” Prog. Quant. Electron. 20, 271–358 (1996). [CrossRef]
2. G. S. He, L. Yuan, Y. Cui, M. Li, and P. N. Prasad, “Studies of two-photon pumped frequency-upconverted lasing properties of a new dye material,” J. Appl. Phys. 81, 2529–2537 (1997). [CrossRef]
3. W. Tutt and T. F. Boggess, “A review of optical limiting mechanisms and devices using organics, fullerenes, semiconductors and other materials,” Prog. Quantum Electron. 17, 299–338 (1993). [CrossRef]
4. J. E. Ehrlich, X. L. Wu, I.-Y. S. Lee, Z.-Y. Hu, H. Rockel, S. R. Marder, and J. W. Perry, “Two-photon absorption and broadband optical limiting with bis-donor stilbenes,” Opt. Lett. 22, 1843–1845 (1997). [CrossRef]
5. E. Downing, L. Hesselink, J. Raltson, and R. Macfarlane, “A three-color, solid-state, three-dimensional display,” Science 273, 1185–1189 (1996). [CrossRef]
6. J. S. Chivian, W. E. Case, and D. D. Edden, “The photon avalanche: A new phenomenon in Pr3+ -based infrared quantum counters,” Appl. Phys. Lett. 35, 124–125 (1979). [CrossRef]
7. R. S. Niedbala, H. Feindt, K. Kardos, T. Vail, J. Burton, B. Bielska, S. Li, D. Milunic, P. Bourdelle, and R. Vallejo, “Detection of Analytes by Immunoassay using up-converting phosphor technology,” Anal. Biochem. 293, 22–30 (2001). [CrossRef] [PubMed]
9. A. M. R. Fisher, A. L. Murphree, and C. J. Gomer, “Clinical and preclinical photodynamic therapy,” Laser Surg. Med. 17, 2–31 (1995) [CrossRef]
11. M. E. Koch, A. W. Kueny, and W. E. Case, “Photon avalanche upconversion laser at 644 nm,” Appl. Phys. Lett. 56, 1083–1085 (1990). [CrossRef]
12. S. C. Goh, R. Pattie, C. Byrne, and D. Coulson, “Blue and red laser action in Nd3+:Pr3+ co-doped fluorozirconate glass,” Appl. Phys. Lett. 67, 768–770 (1995). [CrossRef]
13. F. Lahoz, I. R. Martin, and J. M. Calvilla-Quintero, “Ultraviolet and white photon avalanche upconversion in Ho3+-doped nanophase glass ceramics,” Appl. Phys. Lett. 86, 051106–051108 (2005). [CrossRef]
14. S. Q. Man, E. Y. B. Pun, and P. S. Chung, “Upconversion luminescence of Er3+ in alkali bismuth gallate glasses,” Appl. Phys. Lett. 77, 483–485 (2000). [CrossRef]
15. D. C. Nguyen, G. E. Faulkner, M. E. Weber, and M. Dulick, “Blue upconversion thulium laser,” in Solid State Lasers, George Dube Ed., Proc. SPIE 1223, 54–63 (1990). [CrossRef]
16. R. R. Jacobs, W. F. Krupke, and M. J. Weber, “Measurement of excited-state-absorption loss for Ce3+ in Y3Al5O12 and implications for tunable 5d→4f rare-earth lasers,” Appl. Phys. Lett. 33, 410–412 (1978). [CrossRef]
17. N. Sarukura, Z. Liu, and Y. Segawa, “Ultraviolet subnanosecond pulse train generation from an all-solid-state Ce:LiCAF laser,” Appl. Phys. Lett. 67, 602–604 (1995). [CrossRef]
18. J. Qiu, Y. Shimizugawa, Y. Iwabuchi, and K. Hirao, “Photostimulated luminescence of Ce3+ -doped alkali borate glasses,” Appl. Phys. Lett. 71, 43–45 (1997). [CrossRef]
19. A. P. Davey, E. Bourdin, F. Henari, and W. Blau, “Three photon induced fluorescence from a conjugated organic polymer for infrared frequency upconversion,” Appl. Phys. Lett. 67, 884–885 (1995) [CrossRef]
20. G. S. He, J. Dai, T.-C. Lin, P. P. Markowicz, and P. N. Prasad, “Ultrashort 1.5-μm laser excited up converted stimulated emission based on simultaneous three-photon absorption,” Opt. Lett. 28, 719–721 (2003). [CrossRef] [PubMed]
21. L. Wang, Z. Cheng, Q. Ping, and X. Hou, “Three-photon photoemission from GaAs-O-Cs negative electron affinity surfaces induced by 2.06 μm nanosecond laser pulses,” Appl. Phys. Lett. 67, 91–93 (1995). [CrossRef]
22. J. W. M. Chon, M. Gu, C. Bullen, and P. Mulvaney, “Three-photon excited band edge and trap emission of CdS semiconductor nanocrystals,” Appl. Phys. Lett. 84, 4472–4474 (2004). [CrossRef]
23. K. S. Bindra, H. T. Bookey, A. K. Kar, B. S. Wherrett, X. Liu, and A. Jha, “Nonlinear optical properties of chalcogenide glasses: Observation of multiphoton absorption,” Appl. Phys. Lett. 79, 1939–1941 (2001). [CrossRef]
24. H. You and M. Nogami, “Three-photon-excited fluorescence of Al2O3-SiO2 glass containing Eu3+ ions by femtosecond laser irradiation,” Appl. Phys. Lett. 84, 2076–2078 (2004). [CrossRef]
25. M. Watanabe, S. Juodkazis, H. B. Sun, S. Matsuo, and H. Misawa, “Two-photon readout of three-dimensional memory in silica,” Appl. Phys. Lett. 77, 13–15 (2000). [CrossRef]
26. W. H. Zhou, S. M. Kuebler, K. L. Braun, T. Y. Yu, J. K. Cammack, C. K. Ober, J. W. Perry, and S. R. Marder, “An efficient two-photon-generated photoacid applied to positive-tone 3D microfabrication,” science 296, 1106–1109(2002). [CrossRef] [PubMed]
28. J. Bewersdorf, R. Pick, and S. W. Hell, “Multifocal multiphoton microscopy,” Opt. Lett. 23, 655–657 (1998). [CrossRef]
29. L.-Y. Yang, Y.-J. Dong, D.-P. Chen, C. Wang, N. Da, X. W. Jiang, C. Zhu, and J.-R. Qiu, “Upconversion luminescence from 2E state of Cr3+ in Al2O3 crystal by infrared femtosecond laser irradiation,” Opt. Express. 13, 7893–7898 (2005). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-20-7893. [CrossRef] [PubMed]
30. L. Yang, C. Wang, Y. Dong, N. Da, X. Hu, D. Chen, and J. Qiu, “Three-photon-excited upconversionluminescence of YVO4 single crystal by infrared femtosecond laser irradiation,” Opt. Express. 13, 10157–10162 (2005). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-25-10157. [CrossRef] [PubMed]
31. H. You and M. Nogami, “Upconversion luminescence of Al2O3-SiO2:Ce3+ glass by femtosecond laser irradiation,” Appl. Phys. Lett. 85, 3432–3434 (2004). [CrossRef]
32. R. P. Chin, Y. R. Shen, and V. Petrova-koch, “Photouminescence from Porous Silicon by Infrared Multiphoton Excitation,” Science 270, 776–778 (1995). [CrossRef]