Elsevier

Optical Materials

Volume 111, January 2021, 110598
Optical Materials

Bright constant color upconversion based on dual 980 and 1550 nm excitation of SrF2:Yb3+, Er3+ and β-NaYF4:Yb3+, Er3+ micropowders― considerations for persistence of vision displays

https://doi.org/10.1016/j.optmat.2020.110598Get rights and content

Highlights

  • Realistic consideration of requirements for UC displays in ambient light.

  • Dual excitation facilitates monochrome UC displays.

  • NaYF4:Yb3+, Er3+ and SrF2:Yb3+, Er3+ as candidates for greyscale displays.

  • Red to green color tuning in dark environment for SrF2:Yb3+, Er3+.

Abstract

Upconversion phosphors are of interest for 2D head-up and 3D volumetric displays. These exploit persistence of vision; fast scanning of the near-infrared excitation means that the material at each emission point is only excited for a short fraction of the time it takes to trace the whole image. To achieve an average luminance on the order of 100 cd m−2 (necessary for the display to be visible in ambient indoor lighting), the luminance during excitation must be several orders of magnitude higher than its time-averaged value. For this purpose, efficient energy-transfer upconversion materials such as the benchmark β-NaYF4:Yb3+,Er3+ are of obvious interest. However, under 980 nm excitation the perceived color of their emission varies with their luminance, limiting their applicability for a grayscale display. We demonstrate that under dual 980 nm and 1550 nm excitation, a constant green color can be maintained at CIE coordinates (0.31, 0.66), as the luminance is varied. At moderate power densities of 100 W cm-2 at each excitation wavelength, a luminance of 8 × 106 cd m−2 can be achieved. Conservatively considering that the luminance will be reduced by 5 orders of magnitude in a laser scanning display, this material will still be bright enough to be viewed in indoor ambient light. We also investigate SrF2:Yb3+,Er3+, and find color tuning from the red to green is possible under dual wavelength excitation, but only at limited luminance, so color tuning in this manner would only be appropriate for displays viewed in a dark environment.

Introduction

Upconversion (UC) is a process by which photons of lower energy are combined to create photons of higher energy. The phenomenon has been intensively researched for use in potential applications such as head-up or 3D volumetric displays [1], lighting [2], phototherapy [3], optical thermometry [4], solar energy harvesting [5], plastic recycling [6], anti-counterfeiting [7], and bio-imaging and sensing [[8], [9], [10]]. Among these applications, displays and lighting require control of the intensity and perceived color of the UC emission, either to enable a multicolor display, or to hold a constant color whose luminance can be varied for a grayscale display. Herein, we consider color change (due to the change in ratio of the red to green (R/G) emission peaks) in fluoride-based hosts doped with the benchmark Yb3+ and Er3+ ions upon various excitation power densities at both 980 nm (exciting primarily Yb3+) and 1550 nm (exciting Er3+) [[11], [12], [13]].

There are several ways to change the perceived color of the UC emission. Most trivially, the excitation power density can be altered. As UC is a non-linear process, its efficiency changes with excitation power density. The rate of change of the UC emission with excitation power density depends on the number of photons involved in the UC process. For the green UC in Yb3+ and Er3+ co-doped hosts such as hexagonal NaYF4 (β phase) and cubic SrF2 upon 980 nm excitation, the green anti-stokes emission is the result of a two-photon UC process, whereas the red UC emission receives a significant contribution from a three-photon UC channel [[14], [15], [16]]. So the photoluminescence quantum yield (PLQY) of the red UC emission increases more rapidly with increasing excitation power density than the green emission, causing the R/G ratio to increase with increasing excitation power density. Consequently, as the excitation power density is increased, the UC luminance increases, and the perceived color also shifts from green to yellow. This variation of the R/G ratio is shown in the supplementary data in Fig. S1 and Fig. S2 for β-NaYF4:Yb3+,Er3+. Although interesting from the mechanistic standpoint, this color tuning by changing the excitation power density is detrimental from an application point-of-view because one cannot independently control the luminance and the color.

A second approach to tuning the emission color is to develop several different phosphors. Much work has been done on color tuning using NaYF4 as it is renowned as a highly efficient UC material with both Yb3+ and Er3+ doping [12,13]. Chen et al. used different concentrations of dopants and different excitation wavelengths to obtain green (NaYF4:Yb3+,Er3+ (10/1%) @ 976 nm), red (NaYF4:Yb3+,Er3+ (39/1%) @ 976 nm) and yellow (NaYF4:Yb3+,Er3+ (10/5%) @ 1532 nm) UC emission [17]. An alternative approach of increasing the Mn2+ doping from x = 0–30% in NaYF4:Yb3+,Er3+,Mn2+ (20/2/x %) was pursued by Yuan and co-workers to change the UC emission color from green to red upon 980 nm excitation [18]. Similarly, controlled energy migration within different multi-shell nanocrystal structures can lead to phosphors with different emission colors [19,20]. While such work is very attractive, it also introduces a challenge in terms of application in display applications. Namely, that the individual materials should be deposited in individual pixels or voxels for a heads-up screen or a volumetric 3D display, respectively. This approach adds significant technical complexity to the fabrication of the screen or display volume, and also requires focusing and scanning of the excitation lasers. It is therefore highly desirable to control the emission color from a single material.

Looking to multi-shell nanocrystals, there are several approaches and strategies that show color tuning of a single UC material based on energy transfer UC (ETU) [21,22]. In a seminal paper in 2015, Liu and co-workers demonstrated that a single type of core-multi-shell nanocrystal could be made to emit either blue, green or red ETU emission depending on the pulse duration and wavelength of excitation lasers [23]. This elegant structure required a core with four sequential shell depositions with varying doping concentrations of Yb3+, Nd3+, Tm3+, Ho3+ and Ce3+. This work was followed by demonstrations on variable color emission based on steady-state laser excitation at different wavelengths [24,25]. For example, Zhang and co-workers demonstrated a multi-shell structure in which green or blue light could be generated from an outer shell that absorbed the 980 nm radiation [24]. 1550 nm radiation, on the other hand would penetrate to the NaErF4:0.5%Tm core and yield red emission [24]. Luo and co-workers demonstrated that individually controllable blue, green, and red emissions were obtainable from a 4-shell architecture when excited with 808 nm, 980 nm, and 1550 nm, respectively [25]. Interestingly, they use a NaYbF4: 25% Er3+ shell to obtain both the red and green emission, with the red emission dominating upon 980 nm excitation and the green dominating for 1550 nm excitation (the opposite of what we will show below). When Er3+ and Yb3+ are both present at high concentrations, then various cross-relaxation processes start affecting their emission spectrum [26]. The atypical dominance of the red emission over the green after 980 nm excitation is a consequence of the many Yb3+ ions near to the Er3+ activators [20,27].

Although the aforementioned work is excellent, practical challenges are still left open with regard to display phosphors. Firstly, there is the applied question of material production at scale. The multi-shell nanostructures are significantly more complicated to synthesize than single-component sub-micron to microcrystalline phosphors (the latter being readily available in commercial quantities). Secondly, there is the issue of brightness. The PLQY of these multi-shell nanocrystals are generally still below 1% at excitation power densities above 100 W cm−2 [23], whereas microcrystals can easily achieve higher quantum efficiencies (albeit promising work in nanocrystals may change this) [28]. Thirdly, control of interfaces between different layers in a core-shell structure such that the lanthanide ions are in well-defined positions is still an unsolved problem [20,29].

One option for UC display applications – that was originally pursued in the development of UC displays – is to use excited state absorption (ESA) UC [30]. In this approach, two separate excitation lasers are used with differing wavelengths. The first laser is tuned to be absorbed by a ground state molecule and shifts it to an intermediate excited state. The second laser is tuned to be absorbed by the molecule in the intermediate excited-state and shifts it to the high energy excited state from which the UC emission takes place [31]. Such a scheme has the advantage that the UC emission is only visible where the two beams overlap. However, it has the disadvantage of being orders of magnitude weaker than ETU [30,32]. For a display application, the weak luminance of ESA UC imposes significant technical hurdles. Indeed, banks of laser diodes would be needed to enable this approach to display pictures based on persistence of vision in an indoor lit environment, significantly increasing complexity and cost [30].

The reason for the much lower efficiency of ESA than ETU is fundamental. ETU relies on two photons being absorbed by ground-state ions. ESA, on the other hand, depends first on the absorption of a photon from the ground to excited state (similar to ETU) but then the absorption of a second photon by an excited state to a higher excited state. As the density of the intermediate excited-state population will always be much less than the density of ions in the ground state, the rate of absorption for the second step of the ESA process will always be slower than the rate of absorption for the second step of the ETU process (in ETU the rate of absorption contributing to both the first and second step being equal). For this reason, the-state-of-the-art ETU systems have significant benefits in terms of brightness for display applications (with UC quantum yields for commercial NaYF4:Yb3+, Er3+ microcrystals reaching 10% at 10 W cm−2 excitation power density) [33]. Also a relevant example for this work that illustrates both of these points are the submicron-scale SrF2:Yb3+,Er3+ crystals prepared via the scalable synthesis route of precipitation from aqueous solution followed by calcination, which exhibit quantum yields of 2.8% at an excitation power density of 10 W cm−2 [34].

The question then arises, whether the lack of control over intensity and color under single-wavelength excitation of these ETU systems can be overcome via dual-band excitation. Indeed, initial results from Luo and co-workers have shown that Yb3+ and Er3+ co-doped into a Y2O2S microcrystalline host exhibit a varying R/G ratio depending on the excitation power at 980 nm and 1510 nm, respectively [35]. They demonstrated that, under a microscope, red or green figures could be displayed by scanning the 980 nm or 1510 nm excitation spot. In this contribution, we consider in detail the potential in terms of larger scale display application (observable in standard indoor illumination conditions) for such an approach of using 980 nm and 1550 nm excitation to control the emission color and brightness of microcrystals of homogeneous composition.

We start by considering what requirements an UC-based screen would need to achieve an average luminance of at least 100 cd m−2. Then, we investigate the extent to which the emission color can be tuned by adjusting the red and green emission from a single microcrystalline powder host. We demonstrate the Commission Internationale de l'Eclairage (CIE) color tuning range for various brightness output levels achievable by varying 980 nm and 1550 nm excitation power density. Subsequently, we discuss the potential for controlling emission color and brightness at display-appropriate levels based on a single microcrystalline powder. We demonstrate color tuning in the dark for microcrystalline SrF2:Yb3+,Er3+. Furthermore, we demonstrate that a constant color can be maintained over a wide range of luminances for both β-NaYF4:Yb3+,Er3+and SrF2:Yb3+, Er3+ when dual band excitation is used. A monochrome display could be achieved with either material, but favorable CIE coordinates and PLQY of the β-NaYF4:Yb3+,Er3+ material make it the most attractive choice.

Section snippets

Materials and methods

β-NaYF4:Yb3+,Er3+ (18/2) micropowder was synthesized in the same way as described in the work by Krämer et al. [36]. For SrF2: Yb3+,Er3+ (18/2), two stock solutions were initially made. For the first stock solution, Sr(NO3)2 was made by mixing SrCO3 with HNO3 and then dissolved in DI water. The second stock solution was made by mixing Yb2O3 and Er2O3 in HNO3 (65%). The two stock solutions were mixed and treated with HF (40%) four times to precipitate the fluoride. After drying, the solid was

Theory

Before we examine the UC materials, we first establish back-of-the-envelope requirements for a thin-film head-up display application. This calculation also is indicative for the challenges faced by a volumetric display. However, for the volumetric display additional attention must be paid to the absorption, as this must be kept low to allow uniform emission intensities to be created at various depths into the display. Considering indoor applications, a total luminance of at least 100 cd m−2

UC color after pure 980 nm or 1550 nm excitation

In Fig. 1(a), we show the energy level diagrams for Er3+ and Yb3+ and the UC mechanisms under 980 nm and 1550 nm excitation. Here we discuss the possible UC pathways under 1550 nm excitation, as the UC pathways under 980 nm excitation are discussed in detail elsewhere [15,33,41,42]. As illustrated in Fig. 1(a), subsequent photon absorptions and ETU events can lead to the transitions 4I13/24I15/2 followed by 4I9/24I13/2. Now the 4I9/2 and 4I11/2 levels are spaced closely enough that

Conclusion

There is still general commercial and scientific interest in the development of 3D displays. Technologies already with a commercial presence include single-color displays that rely on the generation of plasma in air using femtosecond pulses [51], or projection onto swept surface diffusing surfaces [52]. Scientific progress can certainly expand this application field. For example, holographic projection into volumetric displays using semiconductor nanocrystals has been investigated [53].

CRediT authorship contribution statement

Reetu E. Joseph: Conceptualization, Investigation, Writing - original draft, Writing - review & editing. Damien Hudry: Resources. Dmitry Busko: Writing - review & editing. Daniel Biner: Resources. Andrey Turshatov: Resources, Writing - review & editing. Karl Krämer: Resources, Writing - review & editing. Bryce S. Richards: Supervision, Writing - review & editing, Conceptualization. Ian A. Howard: Conceptualization, Supervision, Writing - original draft, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge the financial support provided by the Helmholtz Association: (i) Recruitment Initiative Fellowship for BSR; (ii) research program Science and Technology of Nanosystems (STN); and (iii) Helmholtz Energy Materials Foundry (HEMF). REJ acknowledges the German Academic Exchange Service (DAAD) for the PhD scholarship and the Karlsruhe School of Optics and Photonics (KSOP) Graduate School for their support

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