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White and full-color displays based on upconversion (UC) processes in multilayered NaLu1-x-yYbxTmy(WO4)2/NaLu1-x-zYbxHoz(WO4)2 films deposited on 20 × 20 mm2 Pyrex glass substrates are demonstrated by scanning with a 978 nm focused beam from a diode laser. Moreover, spatially resolved red, green and blue pixels are selected by focusing the excitation light at different depths on three stacked films with compositions individually optimized for UC emission of each fundamental color. The highest temperature used in synthesis/deposition process was 580 °C allowing the use of glass substrates.
© 2014 Optical Society of America
1. Introduction
Upconversion (UC) [1] processes in lanthanide (Ln) doped phosphors produce light of different colors, with energy conversion ≤ 2%, when the proper Ln, host compound and excitation wavelength are selected. The intensity balance of these emissions produces white light. Pollack proposed full color UC displays based on in-plane deposition of pixels with red (R), green (G) and blue (B) UC emissions [2]. Dispersion of micro or nanopowders in a transparent resin and printing on glass substrate was proposed, but this has several drawbacks: i) the organic resins are prone to thermal degradation under the light power density used for excitation of bright UC, ii) particle dispersion decreases the density of the UC centers and therefore the irradiance of the display decreases, and iii) the in-plane separation of R, G, B pixels requires carefully aligned masks, making difficult the application to large areas.
Downing [3] made another proposal for three-dimensional full color UC displays. Three ZBLAN glass sheets, each one doped with Pr3+ (R), Er3+ (G) or Tm3+ (B) were laminated together and simultaneously excited. Color selection was based on the use of two specific wavelengths for each Ln, thus operation required the use of six different light sources, which is unpractical for applications. Moreover, the incorporation of lanthanides to glasses is limited by glass devitrification, therefore the UC efficiency of such glasses defined as the ratio between emitted and absorbed light power is low, ~0.1% [4].
Despite that UC displays have not been widely demonstrated, they are still attractive for application in radiofrequency free environments, as passive security labels, or as incoherent infrared to visible image conversion [5]. Typically, robust InGaAs LEDs with emission near to 978 nm are used for excitation of Yb3+ sensitizer and visible UC is obtained from Tm, Er or Ho activators. White and color-tuned UC emissions have been described in several halide and oxide host materials, for instance LaF3 (melting point m.p. = 1493 °C) [6], BaF2 (m.p. 1368 °C) [7], hexagonal β-NaYF4 (incongruent m.p. 984 °C but with transition to a cubic phase at 665 °C) [8,9], Lu2O3 (m.p. 2487 °C) [10], Lu3Ga5O12 (m.p. >1500 °C) [11], GdVO4 (m.p. 1780 °C) [12], CaMoO4 (m.p. 1430 °C) [13], monoclinic KLu(WO4)2 (incongruent m.p. 1090 °C with polymorphic phase transition at 1057 °C) [14], Y2SiO5 (m.p. 1950 °C) [15], and CaSnO3 (m.p. ≈1000 °C) [16], among others, either as micro and nanopowders [9–16], nanocrystals embedded in transparent glassy ceramics [4–7], or Ln-doped glasses [17].
Direct deposition of ceramic films onto glass is generally prevented by the high temperatures required for phase synthesis and crystallization. Only silica films with LaF3 nanoparticles were prepared for white light emission purposes, but the processing required annealing to 800 °C, thus quartz substrates were used [6]. Films prepared at low (<600°C) temperature are generally porous and have poor crystallinity [18], which degrades the radiative properties of lanthanides. In this work, we selected NaLu(WO4)2 (incongruent m.p. 1130 °C) as host of Ln ions because in it Ln ions have large radiative cross sections and it has been synthesized by solid state reaction at rather low temperature (≥ 500° C) [19].
2. Experimental methods
NaLu1-x-y-zYbxTmyHoz(WO4)2 was synthesized by the sol-gel method [20] to facilitate low temperature processing and to allow film formation. Proper amounts of raw Ln2O3 (Ln = Ho, Er, Tm, Yb and Lu, 99.99%) were treated, under vigorous stirring and heating at 95 °C, in diluted HNO3 and mixed with required amounts of Na2CO3 (99.5%) and (NH4)10W12O41∙7H2O. Citric acid (CA) and ethylene glycol (EG) were added in the molar ratio [CA]:[METAL] = 4:1 and [CA]:[EG] = 1:10. Ammonia(30 wt%)was used for pH = 4-5 adjustment. Films were formed with a spin coater from Chemat Technology, model KW-4A. Each film layer was dried at 120 °C for 10 min and annealed in air at 500-550 °C for 10 min. For elimination of organic components the ðlm was annealed at 500-550 °C for 10-24 h.
UC was excited with a LIMO 25 W diode laser (DL) module whose peak emission shifts in the range 972.5-978 nm depending on the used diode current, here we use 978 nm as its nominal emission wavelength. The DL output was focused on the sample to a spot of 0.945 mm of diameter. Emission was collected at 90° with a condenser lens and focused to the optical fiber entrance of a Spectral Products spectrometer, model SM440. Film luminance was measured with a Konica-Minolta, model CS2000, spectral radiometer.
Images were created with a galvo system incorporating a telecentric lens (f = 56 mm, optimized for λ = 1064 nm) enabling to sweep the focused beam over an area of ≈50x50 mm2 at the focal plane. The cw DL beam was focused onto the sample surface, with a beam waist of ≈400 μm (1/e2 diameter). The scan speed used was in the range 60 to 580 mm/s. A Canon EOS 600D camera with exposure time 10-30 s was used for image acquisition.
3. Results
The XRD patterns of NaLu1-x-yYbxTmyHoz(WO4)2 films deposited on Pyrex glass and annealed to 580 °C for 12 h show that the films are single phase and polycrystalline with a weak preferential orientation along the c axis. Figure 1(a) shows that they grow densely packed with a deposition rate of ≈20 nm/layer. Figures 1(b) and 1(c) show the high film transparency, which is desired to minimize UC optical losses and to prevent defocusing of the pump beam.
Fig. 1 NaLu1-xYbx(WO4)2 films on Pyrex. (a) Cross section SEM image of a Pyrex / 46 layers of NaLu0.88Yb0.02Ho0.10(WO4)2/18 layers of NaLu0.965Yb0.03Tm0.005(WO4)2 sample annealed to 580 °C. (b) Image of a Pyrex / 15 layers of NaLu0.845Yb0.15Er0.005(WO4)2 sample annealed to 580 °C. (c) Comparison of the optical transmission of a Pyrex substrate and the above film.
Figure 2 shows the UC emissions of NaLu1-x-y-zYbxTmyHoz(WO4)2 compositions and typical paths for their UC excitation and emission. The DL emission excites the Yb sensitizer and blue UC bands arise from Tm3+ activator (1G4→3H6) while green (5F4,5S2→5I8) and red (5F5→5I8) bands correspond to Ho3+ activator emissions. The contribution to the color of 1G4→3F4 Tm3+ transition can be neglected because of its weak intensity.
Fig. 2 Upconversion of NaLu1-x-y-zYbxTmyHoz(WO4)2 for 2.4 kW/cm2 excitation (λEXC = 978 nm, Yb3+, 2F7/2→2F5/2) power density. (a) NaLu0.84Yb0.15Tm0.01(WO4)2 and (b) NaLu0.80Yb0.15Ho0.05(WO4)2. The corresponding excitation and luminescence paths are shown for reference, (c) Yb3+-Tm3+ and (d) Yb3+-Ho3+.
From the study of the Yb3+ lifetime we conclude that x≤ 0.2 (20 at% Yb) and a Yb2O3 purity ≥ 99.99% are required for efficient UC. For higher concentrations or lower purities, τ/τRAD < 0.8, where τRAD = 368 µs is the radiative lifetime of Yb3+ in NaLu1-xYbx(WO4)2 [19].
Attempts to produce white UC with a single NaLu1-x-y-zYbxTmyHoz(WO4)2 composition failed because a large R/G intensity ratio (required to compensate the low red response of the human eye) is only observed at low Yb concentration (x<0.02) but this induces a low efficiency in the energy transfer processes from Yb to Tm. To circumvent this situation we used multilayered NaLu1-x-yYbxTmy(WO4)2 / NaLu1-x-zYbxHoz(WO4)2 films with spatially separated Yb/Tm and Yb/Ho ratios. Purest white light was obtained with a Pyrex / 40 layers of NaLu0.88Yb0.02Ho0.10(WO4)2 / 18 layers of NaLu0.96Yb0.03Tm0.01(WO4)2 film. This film showed white UC with color coordinates (x = 0.332, y = 0.326) and a luminance of Lv = 54 cd/m2, when pumped with 2.4 kW/cm2 of 978 nm DL light. The image rise time is determined by the energy transfer time between the sensitizer and activator, typically in the ms range or below, depending on the dopand concentrations, while the image fall time is equal the Yb lifetime, i.e. ≈325 μs for the 2 at% Yb used concentration. Image latency or persistence in longer time scale was not observed. Figures 3(a) and 3(b) show two white images formed on similar samples. In order to show that these images contain the three basic colors Figs. 3(c) 3(d) and 3(e) show further images collected using proper R, G or B filters between the sample and the camera.
Fig. 3 (a,b) Images created by scanning (420 nm/s) with a focused cw DL beam a Pyrex / 46 layers of NaLu0.88Yb0.02Ho0.10(WO4)2 / 18 layers of NaLu0.965Yb0.03Tm0.005(WO4)2 sample. The laser excitation power density was ≈800 W/cm2. Images created in the latter sample and observed through different band-pass optical filters: (c) Blue filter. (d) Green filter. (e) Red filter. The central bright point visible in all cases corresponds to the starting and final positions of the scanning beam.
Previous work describing full color UC displays considered the device implementation on the basis of in-plane spatially separated RGB pixels of the same material by sequential depositions through masks [2]. In the following, we show an alternative approach based on the excitation power dependence of the UC response.
The efficiency of UC processes is governed by a IUC = k × IEXCn law, where IEXC and IUC are the excitation and UC light power densities and n is equal or lower than the number of photons required to excite the activator [21]. Figure 4 shows this behavior for a Pyrex / 46 layers of NaLu0.88Yb0.02Ho0.10(WO4)2 / 18 layers of NaLu0.965Yb0.03Tm0.005(WO4)2 multilayered sample. The n exponents calculated for R, G and B UC are 1.75, 1.66 and 1, respectively, see Fig. 4(b). When a Gaussian laser beam is highly focused (with short depth of focus) on the active film, most of the UC intensity comes from the beam waist position with large optical power density. As the beam propagates out of the waist position its cross section increases rapidly and the power density decreases with the inverse of the beam diameter square. According to Fig. 4(a) this reduces the UC response and limits the emission to the excitation beam waist position. Such device can be implemented in an integrated manner by using a microlens array with a pixel including three lenses with different focal lengths in such a way that each lens focuses the laser beam on a different optically active layer separated in depth. Figure 5 shows a schematic version of this proposal.
Fig. 4 Evolution of the integrated UC intensity (IUC) versus 978 nm excitation light power density (IEXC) (a), and its logarithmic representation (b) for a Pyrex / 46 layers of NaLu0.88Yb0.02Ho0.10(WO4)2 / 18 layers of NaLu0.965Yb0.03Tm0.005(WO4)2 sample. (red ▲) 725-600 nm UC. (green ●) 600-515 nm UC. (blue ■) 515-425 nm UC.
Fig. 5 Excitation scheme for selection of red, green and blue UC emissions from three stacked films. The left side shows the implementation in an integrated device. The right image shows the implemented demonstration and the bottom image shows the result obtained under stationary cw excitation.
As the availability of a specific microlens arrays requires design and fabrication which are beyond the scope of the present work, here we just demonstrate the principle and viability of the proposed approach. For this purpose we have used three films with UC emissions to produce red, Pyrex / 15 layers of NaLu0.88Yb0.02Ho0.10(WO4)2; green, Pyrex / 15 layers of NaLu0.845Yb0.15Er0.005(WO4)2; and blue, Pyrex / 16 layers of NaLu0.84Yb0.15Tm0.01(WO4)2, colors, respectively. The films were spaced with 1 mm thick glass slides, therefore, the distance between consecutive films was 2 mm. The DL beam was split into three secondary beams and each beam was focused with a microscope objective on one of the above mentioned films. For a constant excitation intensity (≈0.6 kW/cm2) the luminance of the films were in the ratio 1:100:15 for R, G, and B channels, respectively. In order to obtain similar output luminance (≈2 cd/m2) in each channel the excitation beam intensity of G and B channels were modulated with optical filters. Figure 5 shows the excitation scheme. The resulting image produced by UC contains three spatially separated points each one with a basic color, see also Fig. 5. Output luminance above 50 cd/m2 seems feasible once thickness and composition of the red UC film would have been optimized. The cross section of the focused beam waist was measured with a beam profiler as 0.78 × 10−4 cm2, while this area 2 mm far from the beam waist was 44 × 10−4 cm2, i.e. 56 times larger. According to the results of Fig. 4(a), it is clear that the UC output 2 mm far from the beam waist is negligible, and the excited film (and therefore the UC color) can be selected using the proper focus distance.
4. Conclusions
Multilayered NaLu1-x-y-zYbxTmyHoz(WO4)2 film structures synthesized by spin coating the sol-gel viscous phases on Pyrex followed by annealing to 550-580 °C enable the production of upconversion optical displays. A new approach for full color upconversion displays based on the focusing at different depths of a laser beam incident on a stack of films optimized for individual red, green and blue upconversion, respectively, has been demonstrated.
Acknowledgment
Work supported by MAT2011-29255-C02-01 and TEC2011-22422 projects of the Spanish Ministry of Economy and Competitiveness. EC-H is supported by BES-2012-060296 grant.
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