Infrared to ultraviolet and visible upconversion luminescence was demonstrated in trivalent cerium doped YAlO3 crystal (Ce3+: YAP) under focused infrared femtosecond laser irradiation. The fluorescence spectra show that the upconverted luminescence comes from the 5d-4f transitions of trivalent cerium ions. The dependence of luminescence intensity of trivalent cerium on infrared pumping power reveals that the conversion of infrared radiation is dominated by three-photon excitation process. It is suggested that the simultaneous absorption of three infrared photons pumps the Ce3+ ion into upper 5d level, which quickly nonradiatively relax to lowest 5d level. Thereafter, the ions radiatively return to the ground states, leading to the characteristic emission of Ce3+.
© 2006 Optical Society of America
Recently, upconversion luminescence based on two- and three-photon simultaneous excitation has received considerable attention because of advances of technology in infrared femtosecond laser and development of highly efficient multiphoton-sensitive organic materials [1–4]. As a new irradiation source, infrared femtosecond laser has several merits over other lasers [1, 5]. First, the infrared femtosecond laser can penetrate deeply into bulk materials because the laser wavelength is out of the intrinsic ultraviolet absorption band of most transparent materials. In addition, the interaction between tightly focused femtosecond laser and materials can be dominated by multiphoton processes, and these processes can be confined within a femtolitre volume [1, 5]. For upconversion luminescence based on multiphoton absorption, electrons at the ground states of active ions are directly promoted to the upper excited states without assistance of intermediate states [6–8]. This pump process is quite different from other upconversion mechanisms such as energy transfer, excited-state absorption, cooperative upconversion, and photon avalanche [9–11]. Currently, almost all reports on upconverted luminescence by multiphoton simultaneous absorption have been concentrated on organic materials. There is little attention paid to the similar phenomenon in inorganic solid-state materials [6–8], especially in crystals. Recently, a visible upconversion luminescence based on two-photon simultaneous absorption in Cr3+: Al2O3 crystal has been experimentally demonstrated by us . In this paper, we report a three-photon-excited upconversion luminescence in Ce3+: YAP crystal via femtosecond laser pumping. Ce3+: YAP is one kind of efficient scintillators, and has been widely used into a variety of application fields, such as nuclear medical imaging (PET), high-energy physics and nuclear physics, security control system and else industrial applications [13–16].This observation can be useful in exploiting new applications of Ce3+: YAP, and will provide a promising method to obtain upconversion luminescence in other crystals.
Crystal of Ce: YAP with Ce3+ concentration of 0.5at% was grown by the Czochralski technique in an argon atmosphere. The high-purity Y2O3 (>99.999), CeO2 (>99.99) and Al2O3 (99.99%) powders were used as starting materials for growth. The apparatus and detailed crystal-growth procedure have been described elsewhere . The crystal obtained was cut and polished into crystal samples with thickness of 1.5mm for subsequent femtosecond laser irradiation and spectral measurements.
A regeneratively amplified 800 nm Ti: sapphire laser that emits 120 femtosecond, 1 kHz, mode-locked pulses was used as pump source. The laser beam was focused into sample by an optical lens with focal length of 100mm. The position of the focal point was beneath the sample surface. The average power at the focused area was controlled below 19mW. The fluorescence spectra excited by focused femtosecond laser and 267nm monochromatic light from a xenon lamp were recorded by a spectrophotometer of ZOLIX SBP300 and JASCO FP6500, respectively. In addition, the absorption spectrum was measured with a JASCO V-570 spectrophotometer. All the measurements are preformed under room temperature.
3. Results and discussion
Under focused femtosecond laser irradiation, strong purple emission light was seen on the focused spot. Figure 1 shows the emission spectrum of the Ce3+: YAP irradiated by focused femtosecond laser. The Fig. 1 also contains a spectrum of Ce3+: YAP irradiated by 267nm monochromatic light from a xenon lamp.
The spectrum of Ce3+: YAP crystal irradiated by femtosecond laser is similar to that irradiated by 267nm monochromatic light. They both exhibit a broad emission band centered on 380nm. These results indicate that the emission of the Ce3+: YAP excited by femtosecond laser can be ascribed to the 5d-4f transitions of the Ce3+ ions. In fact, Ce3+ ions in YAP crystal reside at lattice sites having Clh symmetry. The 5d levels are extremely host sensitive and located in ultraviolet region. The lowest 5d state is 33 000 cm-1 above the 4F7/2 ground state. The transitions from ground states to 5d states are spin-allowed, corresponding to several strong absorption bands in Ce3+: YAP crystal. The measured absorption spectrum of Ce3+: YAP is shown in Fig. 2.
These strong absorption bands usually act as efficient pumping bands. Excitation at ultraviolet bands can pump the Ce3+ ion into upper 5d levels, which quickly cascade down to the lowest 5d level. Thereafter, the ions radiatively relax to the ground states, creating broad 5d-4f emissions. At room temperature, the fluorescence appears as a broad band from 330nm to beyond 450nm with a peak at 380nm. From the above demonstration, it seems as if the focused infrared femtosecond laser can act as an ultraviolet source, producing a spatially confined irradiation in the interior of bulk of Ce3+: YAP crystal.
Generally, conversion of infrared radiation to the visible or ultraviolet emission involves multiphoton absorption process. The multiphoton absorption probability depends strongly on the laser intensity, leading to an exponential dependence of fluorescence intensity on the pumping power. The relationship between the pumping power and the fluorescence intensity can be described as :
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 logarithmic transformation of the pumping power and fluorescence intensity is plotted in Fig. 3.
The number of photons n can be determined from the slope coefficient of the linear fitted line. The slope coefficient of the fitted line is 2.95, which indicates that the upconversion luminescence originates from three-photon absorption of 800nm infrared laser.
Upconversion processes of the solid materials containing rare earth ions with complicated 4f energy levels have been extensively investigated to clarify actual upconversion pump processes. Generally, the main upconversion mechanisms can be classified as energy transfer, excited-state absorption, cooperative upconversion, and photon avalanche [9–11]. Since there is no any absorption band from 380 to 850nm in the absorption spectrum of the Ce3+: YAP crystal, and no optical breakdown and no detectable change of the absorption before and after femtosecond laser irradiation on this irradiation condition, the upconversion luminescence is not due to energy transfer, excited-state absorption, cooperative upconversion and photon avalanche. It is also not a femtosecond laser induced defect-assisted process.
Three-photon simultaneous absorption can be responsible for the upconversion luminescence. There are two possible processes. One is the two-photon simultaneous excitation to a real level followed by one-photon absorption. In this case, the Ce3+: YAP should have an intermediate state corresponding to absorption at 400nm. However, the Ce3+ ion is characterized by lack of intermediate state between the ground states and the lowest 5d excited states. At the same time, no absorption band at 400nm was found in the absorption spectrum of the Ce3+: YAP. Therefore, there is only one possible process responsible for the three-photon absorption, the three-photon simultaneous absorption. Upconversion luminescence dominated by multiphoton simultaneous absorption has been investigated in many organic materials and several solid-state inorganic materials [1–4, 6, 7]. These processes have the advantage of utilizing some active ions with simple energy levels. One requirement for efficient three-photon simultaneous absorption is that the active ions have excited states that can be populated by simultaneously absorbing three pump photons. Another important condition is that the pumping photon density must be high, which can be overcome by using focused femtosecond laser. In this study, the Ce3+: YAP has strong absorption bands spanning from 200nm to 300nm, just corresponding to the energy of three pump photons. Pumped with focused 800nm femtosecond laser, the Ce3+ ions can be directly promoted from ground states to upper excited states by three photons, producing population of electrons in the excited state. The electrons then nonradiatively relax to the lowest 5d state, and then radiatively to terminal state, creating the characteristic emission of Ce3+ in YAP crystal.
It is worth noting that the three-photon excited upconversion luminescence is not limited to Ce3+: YAP crystal. We have also examined the upconversion behavior of several other Ce3+ doped crystals such as Ce3+: GSO, Ce3+: YSO. These crystals have strong emission in blue region by 800nm femtosecond laser irradiation. The quantitative research on the conversion efficiency of upconversion emission in these crystals is under investigation.
In summary, the ultraviolet and visible upconversion luminescence in Ce3+ YAP has been observed under focused infrared femtosecond laser irradiation. The dependence of the fluorescence intensity of the Ce3+ on the pump power shows that the upconversion luminescence is dominated by three- photon absorption process. The analysis reveals that the absorption of three photons is simultaneous rather than sequential. This result provides a promising method to obtain ultraviolet or visible emission from infrared pumping light in solid-state crystals, and has potential applications in visible lasers, high-resolution optical data storage, three-dimensional displays, etc.
The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China under the grant number of 60377040. This work has also been supported by Shanghai Committee of Science and Technology (04XD14018 and 0352nm042).
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