In this paper we presents the near infrared to visible upconversion luminescence in a Tb3+-doped ZnO-B2O3-SiO2 glass excited with 800nm femtosecond laser irradiation. The upconversion luminescence is attributed to 5D4 to 7Fj (j=3, 4, 5, 6) transitions of Tb3+.The relationship between upconversion luminescence intensity and the pump power indicates that a three-photon simultaneous absorption process is dominant in this upconversion luminescence. The calculated value of the three-photon absorption cross section σ3 of the glass is 1.832×10-81cm6s2. Also, three-dimensional display is demonstrated based on the multiphoton absorption upconversion luminescence for the first time.
©2007 Optical Society of America
In recent times a great deal of attention has been made on the upconversion luminescence of materials containing rare earth and transition metal ions due to the possibility of near-infrared-pumped visible lasers. Recently, upconversion luminescence based on multiphoton excitation has received more attention due to the advancement in the technology of infrared femtosecond lasers [1–3]. As a new excitation source, the infrared femtosecond laser has more advantages compared to other lasers [1, 3]. First, the femtosecond laser provides intense pulses at higher repetition rate and offers shorter pulse duration. Second, the high intensity of the femtosecond laser pulses could cause simultaneous multiphoton absorption more easily than other lasers. Third, radiation of the infrared femtosecond laser can penetrate through solid materials such as glasses and crystals. Therefore, the infrared femtosecond laser has been used to analyze the mechanisms of the multiphoton absorption upconversion luminescence and offers a possibility of near-infrared-pumped visible lasers and has promising applications in optical communication, high-density optical storage, three dimensional display, infrared detection and medical imaging [1–8]. In the recent years there are several reports on the multiphoton absorption upconversion luminescence in organic materials, but little attention has been paid to the multiphoton absorption upconversion luminescence in glasses and other inorganic materials [9–10]. Various luminescence phenomena have been observed during and after the femtosecond laser irradiation [9, 11–14]. It is considered that multiphoton absorption, multiphoton ionization and avalanche ionization might occur during the femtosecond laser irradiation. However, the luminescence mechanisms are complicated and further investigations are necessary.
In this paper, we have been reported the upconversion luminescence in a Tb3+-doped ZnO-B2O3-SiO2 glass under femtosecond laser irradiation. We have selected this glass because of its bright luminescence when excited with ultraviolet source at 267nm (which is one third of the wavelength of Ti: sapphire femtosecond laser) compared with other Tb3+-doped oxide glasses. In addition, it is important to study the multiphoton absorption upconversion luminescence in glass for the realization of three dimensional displays because glass can be fabricated as a plate with large size. The value of the three-photon absorption cross section is calculated. We also demonstrated the three-dimensional display based on multiphoton absorption upconversion luminescence.
The chemical composition of the glass sample prepared was 65ZnO-20B2O3-15SiO2-0.5Tb2O3 (mol %). Reagent grade ZnO, B2O3, SiO2 and Tb4O7 were used as the starting materials. Tb3+ was doped as the luminescent center. Approximately 30g batch was mixed and melted in Pt crucibles in an electronic furnace at 1300 °C for 2 h in an ambient atmosphere. The melt was then quenched to obtain transparent glass. The glass was cut and polished for optical measurements.
A regeneratively amplified 800nm Ti: sapphire laser system with 1 kHz repetition rate and approximately 120fs pulse duration was used as an irradiation source. The laser beam was focused onto samples through optical lens in order to obtain higher power density. The focal point can be monitored by a confocal microscope system linked to a charge coupled device system when objective lens is used. The position of the focal point was on the interior of the glass. By choosing appropriate objective lens or optical lens and adjusting the power density of laser beam, we can control the spot size below several microns. The fluorescence spectra excited with femtosecond laser were measured from the side of the glass sample by a spectrophotometer of ZOLIX SBP300. The scanning rate of this spectrophotometer was 100nm/min.The photoluminescence spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as an excitation source. In addition, the absorption spectrum was measured on Hitachi UV-4100 spectrophotometer. All the measurements were carried out at room temperature.
3. Results and discussion
An intense green emission has been observed near the focal point during the femtosecond laser irradiation. Fig. 1 presents the emission spectra of the glass irradiated with the femtosecond laser focused by an optical lens with focal length of 10cm.and excited with 267nm from a xenon lamp. This spectra consist of four emission peaks at about 490nm, 545nm, 580nm and 620nm wavelengths and are assigned to the transitions from 5D4 to 7Fj (j=3, 4, 5, 6) of Tb3+ions, respectively. The emission spectra appear to be similar for both excitation wavelengths. The excitation spectrum of the glass is also shown in Fig. 1. All the peaks can be assigned to the 4f-4f transitions of Tb3+ and the strongest excitation peak at 382nm could be assigned to the 7F0→5D3 transition of Tb3+. The inset of Fig. 1 is the photograph of emission state of the glass irradiated with the focused femtosecond laser. Highly localized green emission is observed in the glass.
Generally, it is impossible to obtain visible emissions with near-infrared excitation wavelengths in oxide glasses. It must be a multiphoton related process when we observed green emission of Tb3+ excited at 800nm during the femtosecond laser irradiation. Multiphoton absorption, multiphoton ionization and avalanche ionization may occur during the femtosecond laser irradiation. Juodkazis et al. studied dependences of the probabilities of multiphoton and impact ionization which are relevant to femtosecond dielectric breakdown . For silica glass when irradiated with an 800nm, 200fs laser at the laser intensity of 6TW/cm2, the probability of impact ionization is much larger than the probability of multiphoton ionization. In the present case, we couldn’t observe any detectable structural change in the glass by optical microscope after the femtosecond laser irradiation. Thus we could suggest that the upconversion luminescence is not due to multiphoton ionization and avalanche ionization which are relevant to dielectric breakdown. It is a phenomenon, typically related to multiphoton absorption upconversion luminescence. The multiphoton absorption upconversion luminescence process depends strongly on the excitation intensity.
As far as the multiphoton process is concerned, the relationship between the emission intensity and the exciting pump power can be expressed as follows:
where I is the integrated intensity of the upconversion luminescence, P is the pump power of the femtosecond laser, and n is the photon number. The n can be experimentally determined. Fig. 2 presents a series of luminescence spectra by changing the excitation pump power of femtosecond laser at a fixed focused point. 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. Fig. 3 presents the relationship of the femtosecond laser pumping power and luminescent intensity of the glass. The slope of the logarithmic fitted line is about 3.093 and the correlation coefficient is 0.998, which indicates that the upconversion process is a three-photon excitation process.
The absorption spectrum of the Tb3+ doped ZnO-B2O3-SiO2 glass is shown in Fig. 4.This spectrum has shown a strong absorption band at about 267nm,which could be due to 4f-5d transition of Tb3+ in the ZnO-B2O3-SiO2 glass.
It is well known that upconversion processes involve mechanisms of excited-state absorption, energy transfer, cooperative upconversion and photon avalanche [16, 17]. The mechanisms of energy transfer, excited-state absorption and cooperative upconversion could be considered only when the energy separation between the intermediate state and the excited state correspond to the photon energy of pumping laser. This means the glass should have an absorption band near 800nm, if one of them dominated the upconversion processes. However, there is no obvious absorption band near 800nm for the studied glass (see Fig. 4). Therefore, the multiphoton absorption upconversion luminescence process can not be accounted for these mechanisms. The photon avalanche as a mechanism of the upconversion can also be ruled out because there is no intermediate metastable state between the ground state and the excited state. Moreover, the upconversion luminescence is not a femtosecond laser induced defect-assisted process because there was no detectable change in the absorption of the glass in the 200-800nm wavelength range, after the femtosecond laser irradiation under our experimental conditions. Another upconversion mechanism is that the Tb3+ absorbs two-photons simultaneously to a level first and then absorbs the last photon. In this case, the Tb3+ doped ZnO-B2O3-SiO2 glass should have an intermediate state corresponding to absorption at 400nm. However, the Tb3+ions are lack of intermediate state between the ground states and the excited states and thus there is no absorption band at 400nm (see Fig.4).
Therefore, the only upconversion mechanism is a three-photon simultaneous absorption process. The active ions have excited states that can simultaneously absorb three pumping photons and also the pumping photon density is high that leads to efficient three-photon simultaneous absorption. Femtosecond laser used in our experiment could obtain high pumping photon density for efficient three-photon simultaneous absorption. In our study, the glass has an absorption band at about 267nm, which implies that the energy of three photons of infrared pumping laser can be efficiently and simultaneously absorbed. Pumping the Tb3+ by using focused 800nm femtosecond laser causes population of electrons in the excited state, the excited electrons nonradiatively relaxes to the 5D4 state, and then radiatively return to the 7Fj (j=3, 4, 5, 6) states, leading to the characteristic green emission of Tb3+.
We have also measured the three-photon absorption coefficient of Tb3+-doped ZnO-B2O3-SiO2 glass with an excitation wavelength 800nm by using the intensity-dependent transmission measurement. The transmitted intensity of the laser beam as a function of incident intensity can be expressed as :
where γ is the three-photon absorption coefficient of Tb3+-doped ZnO-B2O3-SiO2 glass, z is the optical propagation path and I0 is the incident intensity of excitation beam. Equation (1) is adequate for a pulsed beam with a rectangular temporal profile. In our case the incident pulses have a quasi-Gaussian temporal profile. The measured transmitted intensity as a function of incident intensity is shown in Fig 5. The best-fit parameter of γ is 8.24×10- 24cm3W-2 .
Based on the known γ value of the measured glass, the absorption cross section for the Tb3+-doped ZnO-B2O3-SiO2 glass is obtained as 
where NA is the Avagadro constant, d0 is the concentration of doped Tb3+ ions(moles per cm3), and (hc/λ) is the energy of an incident photon at 800nm.The calculated value of the three-photon cross section of Tb3+-doped ZnO-B2O3-SiO2 glass σ3=1.832×10-81cm6s2.
As the Tb3+-doped glass has shown highly localized green emission when excited with focused femtosecond laser, this observed phenomenon can be used for three dimensional solid state displays. Fig. 6 presents the photograph of a simple mark through scanning the 800nm femtosecond laser into the Tb3+-doped glass at a scanning rate of 300pixes/s via computer controlled scanning type galvanometer. The femtosecond laser beam was focused onto the the glass by a lens with focal length of 2cm. We have observed a three dimensional green elliposoidal shape image with 13 horizontal lines inside the glass. It should be possible to realize three-dimensional, solid state, three color display based on multiphoton absorption upconversion luminescence if we could design and control the combination of doping species and microstructure of the glass.
In summary, it is concluded that an intense green upconversion luminescence in the Tb3+ doped ZnO-B2O3-SiO2 glass has been observed under infrared femtosecond laser irradiation. The upconversion luminescence is a three photon excitation process from the relationship between the fluorescence intensity and the pump power where the upconversion mechanism is a three-photon simultaneous absorption process. The value of the three-photon absorption cross section σ3 of Tb3+-doped ZnO-B2O3-SiO2 glass is about 1.832×10-81cm6s2. We also demonstrated the three dimension display based on multiphoton upconversion luminescence. There results have potential applications in the three-color, solid state, three dimensional display systems.
This work was financially supported by National Natural Science Foundation of China (Grant No.50672087) and High Technology Research and Development Program of China (G20060914), and was partially supported by the China Postdoctoral Science Foundation (2005038020).
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