Visible upconversion luminescence was observed in Cr3+: Al2O3 crystal under focused femtosecond laser irradiation. The luminescence spectra show that the upconversion luminescence originates from the 2E-4A2 transition of Cr3+. The dependence of the fluorescence intensity of Cr3+ on the pump power reveals that a two-photon absorption process dominates in the conversion of infrared radiation to the visible emission. It is suggested that the simultaneous absorption of two infrared photons produces the population of upper excited states, which leads to the characteristic visible emission from 2E state of Cr3+.
©2005 Optical Society of America
Upconversion luminescence has been deeply and exhaustively investigated for several decades. The upconversion process offers a simple but powerful alternative to convert infrared laser output to visible and ultraviolet emission, and has generated much interest in applications such as visible lasers, optical storage, three dimensional display, infrared detection, and medical imaging [1–4]. Blossom in two fields accelerates the development in upconversion. One is the progress in the technology of solid-state lasers and semiconductor laser diodes operating at infrared region. Another is the advancement in upconversion materials containing rare earth ions. The predominated upconversion mechanisms by using normal pumping sources are energy transfer, excited-state absorption, cooperative upconversion, and photon avalanche [5–7]. A condition for these upconversion processes is that the absorbing center has a metastable state that is intermediate in energy between the ground state and the emitting state. The rare earth ion-doped materials are extensively used as upconversion materials in that these ions generally have plenty of 4f states, and transitions within the 4f core are insensitive to outer influences. The transition-metal ions are also excellent alternatives, and they are widely used in laser crystals. In contrast to the full-blown exploring of rare earth ions in upconversion, transition-metal ions have been paid less attention. There are a few reports about the transition-metal upconversions concerning several ions such as Ti2+-, Ni2+-, Cr3+-, Mo3+-, Re4+-, and Os4+- doped systems [8–14]. Among these researches, the Cr3+ is noticeable, because the 2E-4A2 emission of Cr3+ is one of the best representative transitions in the optical spectroscopy of solids. This transition of 2E-4A2 has been deeply studied in hundreds of Cr3+ doped crystals and glasses to utilize the visible emission and study the crystal field. These reports about the upconversion in Cr3+ -doped crystals concern energy transfer from rare earth ions. Reference (10) exhibits a conversion of infrared pumping to 2E-4A2 emission of Cr3+ in Cr3+, Yb3+: Y3Ga5O12 crystals. In this process, the 2F5/2 states of Yb3+ act as intermediate state, absorbing the infrared photons and then transferring energy to the excited states of Cr3+. Upconversion excitation of Cr3+ broadband luminescence (4T2-4A2) through the energy transfer from rare-earth ions was also proposed in Cr3+, Er3+ codoped YSGG crystals . There are no reports about 2E-4A2 luminescence of Cr3+ by direct pumping Cr3+ using infrared light, to the authors’ knowledge. The main reason can be attributed to the lack of intermediate state between the 2E state and 4A2 state of Cr3+. So, in order to directly obtain 2E-4A2 luminescence of Cr3+ from infrared pumping, a new mechanism of upconversion must be considered. Recently, advances of technology in femtosecond laser make multiphoton simultaneous absorption to be a considerable upconversion mechanism [15–18]. The existence of intermediate level between the ground states and excited states is not a prerequisite for this mechanism. The absorbing centers can simultaneously absorb two or more photons to excited states. This mechanism greatly extends the upconversion processes. In this paper, we report an upconversion luminescence in Cr3+: Al2O3 crystal (ruby) via infrared femtosecond laser pumping. The Cr3+: Al2O3 crystal was used for this demonstration because it has no absorption band in the near infrared range, and has ever acted as a landmark in development of laser history , and will still play important role in future.
Ruby Samples with dimensions of 10×10×2mm3 were taken from a high purity single crystal of Cr3+: Al2O3. All the six surfaces of the sample were optically polished to facilitate subsequent femtosecond laser irradiation and spectral measurements.
An regeneratively amplified 800 nm Ti:sapphire laser that emits 120 femtosecond, 1 kHz, mode-locked pulses was used as the irradiation source. The laser beam was focused into sample by objective lens or optical lens. When using 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. In fact, the laser beam can be focused into any place within the sample. 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, an optical lens with a focal length of 100mm was used to focus the femtosecond laser beam, and the power density at the focused area was controlled below 170W/cm2. The fluorescence spectra excited by focused femtosecond laser were recorded by a spectrophotometer of ZOLIX SBP300. The fluorescence spectra excited by a 400nm 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 preformed at room temperature.
3. Results and discussion
When the Cr3+: Al2O3 crystal was irradiated by focused femtosecond laser, strong red emission light was seen on the focused spot. Figure 1 shows the emission spectrum of the Cr3+: Al2O3 crystal irradiated by focused femtosecond laser. For comparison, the emission spectrum of the Cr3+: Al2O3 crystal under excitation of the 400nm monochromatic light from a xenon lamp is shown in Fig. 1.
The spectrum of Cr3+: Al2O3 crystal exhibits a sharp emission band peaked at 694nm, which is similar to the spectrum of Cr3+: Al2O3 crystal excited by 400nm monochromatic light from a xenon lamp, and is a characteristic emission of Cr3+. This result indicates that the emission of the Cr3+: Al2O3 crystal excited by femtosecond laser can be attributed to the 2E-4A2 transition of Cr3+ ions. For Cr3+ in Al2O3 crystal, Cr3+ substitutes for some of Al3+, and adopts octahedral ligand coordination. The 3d levels are extremely host sensitive. The strong crystal field in Al2O3 leads to the splitting of 3d electron orbits of Cr3+ and produces the ground level: 4A2, and the excited states : 2E, 4T2, and 4T1, etc. the transitions from 4A2 to 4T2, and 4T1 are spin-allowed, so these energy levels act as broad pumping levels. The 2E is the narrow lowest excited band, acting as emitting level. The unusual magnitude of this crystal field splitting extends the lowest 2E state 14 400cm-1 above the ground state. Thus the 2E-4A2 transition of Cr3+: Al2O3 crystal lies in visible spectral region. Exciting any of the pumping bands of 4T2, and 4T1 results in fast relaxation to lowest 2E excited state. At room temperature, the fluorescence emitting from 2E state appears as a sharp band with a peak at 694nm corresponding to the transition to the 2E terminal state
Generally, it is impossible to obtain visible emission from an infrared pumping via a single photon process. So, in order to excite electrons to the higher 3d level of Cr3+ that can produce a visible emission, it must be considered that the upconversion luminescence involves a multiphoton process. The multiphoton process depends strongly on the laser intensity. A relationship between the pumping power and the fluorescence intensity can be used to describe the multiphoton process :
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, we can obtain a series of fluorescence spectra. Here, the number of photons n can be determined from the 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 Cr3+: Al2O3 crystal is shown in Fig. 2.
It can be seen that the slope coefficient of the fitted line is 2.05, which indicates that the upconversion is a two-photon excitation process.
To elucidate the upconversion mechanisms, the absorption spectra must be considered carefully. Figure 3 shows the absorption spectra of Cr3+: Al2O3 crystal. It can be seen that the Cr3+: Al2O3 crystal exhibits several strong absorption bands. The two broad bands centered on 409nm, and 549nm, correspond to the spin-allowed 4A2-4T1, and 4T2 transitions, respectively. The strongest absorption bands peaked at 209nm and 222nm can be assigned to the absorption of F and F+ centers in Al2O3 crystal . In addition, there are two weak sharp absorption peaks centered at 692nm and 694nm, which correspond to the R1 and R2 lines, respectively.
In order to reveal the actual upconversion processes, it is necessary to clarify whether the upconversion process is a sequential or simultaneous two-photon excitation process. As we known, two-photon-excited upconversion processes involve mechanisms of energy transfer, excited-state absorption, cooperative upconversion, and photon avalanche. For these mechanisms a rigorous condition is that active ion must have a metastable state that is intermediate in energy between the ground state and the emitting state. These mechanisms require the absorption of two or more photons, and the photons are absorbed sequentially rather than simultaneously. The mechanisms of energy transfer, excited-state absorption, cooperative upconversion operate only on a condition that the energy separation between the intermediate state and the excited state must correspond to the photon energy of pumping laser. This means that the Cr3+: Al2O3 crystal should have an absorption band near 800nm if one of them dominated the upconversion processes. However, there is no obvious absorption band near 800nm, which can be seen from Fig. 3. This implies a new mechanism dominating the upconversion process. As for photon avalanche mechanism, the pump photon is resonant with a transition from the intermediate metastable level to a more highly excited state. However, there is no intermediate metastable state between the 4A2 ground state and the 2E excited state. Furthermore, a characteristic power threshold for an avalanche process has not been observed. So, the photon avalanche can also be ruled out. In addition, we confirmed that, on this irradiation condition, there was no detectable change of the absorption of this crystal in the 200-800nm wavelength region after the femtosecond laser irradiation. Therefore, the upconversion luminescence is not a femtosecond laser induced defect-assisted process.
A mechanism of two-photon simultaneous absorption can be considered here. The two-photon simultaneous absorption is not a novel phenomenon, and had been observed in Eu3+: CaF2 crystal in 1961 . From then on, the process was for a long time considered only as an academic question. Though this process has received considerable attention in recent years because of its extensive applications in organic materials. There is little attention paid to the upconversion process in inorganic solid-state materials. Although it is hard to obtain upconversion luminescence by multiphoton absorption in inorganic solid-state materials, it can be overcome by using femtosecond laser as pumping source. The requirement for efficient two-photon simultaneous absorption is that the active ions have excited states that can simultaneously absorb two pumping photons. At the same time, the pumping photon density must be high, which can be reached by using focused femtosecond laser. In this study, the Cr3+: Al2O3 crystal has an absorption band spanning from 324nm to 473nm, which implies that the energy of two photons of infrared pumping laser can be efficient simultaneously absorbed. Pumping the Cr3+ by using focused 800nm femtosecond laser produces population of electrons in the excited state in blue band, the excited electrons nonradiatively relax to the lowest 2E state, and then radiatively return to the 4A2 ground states, leading to the characteristic optical emission of Cr3+.
In conclusion, the visible upconversion luminescence in Cr3+: Al2O3 has been experimentally demonstrated by focused infrared femtosecond laser pumping. The relationship between the fluorescence intensity and the pumping power shows that the upconversion luminescence is a two- photon excitation process. The analysis reveals that the absorption of two photons is simultaneous rather than sequential. This result provides a new method to produce the upconversion luminescence in crystals, and has potential applications in visible lasers, 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 numbers of 50125258 and 60377040. This work has also been supported by Shanghai Nanotecnology Promote Center (0352nm042).
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