1. Introduction

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 ²F_5/2 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 [11]. 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 [19], and will still play important role in future.

2. Experiment

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.

 

Fig. 1. Emission spectra of the crystal sample under femtosecond laser irradiation and 400nm light excitation

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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 [20]:

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.

 

Fig. 2. Luminescent intensity of the ²E-⁴A₂ transition of Cr³⁺ in Al₂O₃ crystal as function of the femtosecond second pump power.

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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.

 

Fig. 3. Absorption spectra of the Cr³⁺: Al₂O₃ crystal and the Al₂O₃ crystal.

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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 [21]. 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 [22]. 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⁺.

4. Conclusions

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.