1. Introduction

Microfluidic and droplet lasers exhibit unique optical functions that are unattainable with solid lasers [1–5]. From the optical viewpoint, the advantages of liquids are deformability (fluidity), tunability, and dissolvability for various dyes. Although lots of microlasers have been fabricated in recent years, photo- or thermal-degradation of organic dyes occasionally cause serious problems concerning device lifetime or high-power adaptability [1, 2, 6, 7]. These problems are particularly serious in microdevices, since a high optical energy is concentrated in a small volume for enhancement of the device performance. Inorganic materials generally exhibit a superior durability than organic materials. This holds true for fluorescent materials; i.e., lanthanide ions provide more resistant light emitters than organic dyes [8]. Since the first lanthanide laser emission at cryogenic temperature in 1960s [9], various solid-state microlasers have been fabricated by the use of lanthanide elements (Nd, Sm, Eu, Er, Tm, etc.) [10–12]. It has been pointed out that the absorption enhancement or the efficient excitation is an important subject for creating a high-performance laser [9]. Fiber-optic or capillary structures are usually employed in lanthanide-based lasers and amplifiers to attain a sufficient optical length for the pump beam absorption [13–16]. Another approach for increasing the pump beam absorption is to use a random medium that induces a strong scattering. Random lasers have been studied keenly since the stimulated emission was observed in a dye solution containing microparticles [17]. The stimulated emission takes place due to confinement of fluorescent rays in the gain medium (photon localization). At the same time, extension of the pump beam path, i.e., enhancement of the absorbance in a small volume, plays an important role for generating a population inversion in the medium. The absorbance enhancement in random media seems effective for increasing the fluorescence intensity of lanthanide ions.

In a previous study, the authors proposed to use polyethylene glycol (PEG) as a random medium, and demonstrated random lasing by dispersing organic dye (rhodamine 6G) in PEG [18]. In comparison with conventional random media, PEG has the following advantages [19]; i.e.,

The notation like PEG 300 is usually used to distinguish PEG types by denoting the molecular weight (200‒35,000). In the chemical science field, ethylene glycol (EG) or diethylene glycol, which can be regarded as a PEG type with a small molecular weight (62 or 106), is commonly used as a bipolar solvent that dissolves both polar and nonpolar molecules. In the photonics technology, they are occasionally used as an optical fluid for waveguiding or gap filling, since they are nonvolatile, exhibit a suitable index of refraction (1.43‒1.46), and dissolve various dye molecules [2, 20–23]. PEG also possesses these physical, chemical, and optical characteristics in addition to the above advantages 1)–4). When creating microfluidic or droplet lasers, therefore, PEG will be a suitable solvent for the lanthanide ions. In this study, we examined the fluorescence characteristics of europium ions (Eu³⁺) in PEG 300 (liquid) or PEG 1000 (solid), and compared them with those of the aqueous solution.

2. Experiments

Sample solutions were prepared by dissolving europium trichloride (Mitsuwa Chemicals) in water or PEG 300 (Kishida Chemical) at a concentration of 0.01 mol/l. Figure 1 shows the transmission spectra of the aqueous or PEG solution in a glass cell (50 mm thickness). The blank for the transmittance evaluation was pure water or PEG containing no Eu ions. As Fig. 1(a) shows, Eu³⁺ originally has an absorption line at 395–396 nm wavelength, which corresponds to the electronic transition of ⁷F₀→⁵L₆ [8]. As Fig. 1(b) shows, the PEG solution exhibits a broad absorption band in the range below 400 nm, which is probably caused by the ion-induced deformation of PEG molecules [24, 25]. (Note that the blank is PEG before Eu ion dissolution.) Since this absorption band was expected to enhance the excitation efficiency through the energy transfer process [25–28], we excited the solution by using an ultraviolet light emitting diode (Nichia Corporation, NVSU333A, 365 nm). Actually this ultraviolet radiation induced fluorescence of Eu³⁺ in the PEG solution. The fluorescence intensity was, however, no stronger than that of the direct excitation process at 396 nm. We therefore used a violet laser diode of 396 nm (Shanghai Dream, SDL-395-LM-100T, 100mW) in the following experiment. (A strong emission may be attainable by excitation at a wavelength below 350 nm.) Other lanthanide ions were also examined by using several pump light sources that we have in our laboratory, but no strong emission was visible due to their unsuitable wavelengths. We selected Eu³⁺ in the following experiments for this reason.

 

Fig. 1 Transmission spectra of the Eu³⁺ ions that dissolved in (a) water or (b) PEG 300 (liquid). The ionic concentration was 0.01 mol/l and the sample thickness was 50 mm. The blank for the transmittance measurement was water or PEG that contained no Eu ions.

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Fluorescence spectra were measured by the use of the optical system shown in Fig. 2(a). A short-pass filter (transmission range: <475 nm) was placed in front of the violet laser diode, since the laser emitted unnecessary radiation in the long wavelength range. The laser beam was focused into the sample cell by using a silica-glass lens (focal length: 50 mm). As Fig. 2(b) shows, a strong fluorescence was visible along the laser beam path in the PEG solution, whereas the fluorescence was faint in the aqueous solution. The fluorescence was collected at the opposite side of the sample cell through a long-pass filter (transmission: >450 nm) that blocked the unabsorbed pump beam. The fluorescent beam was picked up by an optical fiber (core diameter: 400 μm) and measured by a multichannel spectrometer (B & W Tek, BTC112E) with a spectral resolution of 0.5 nm.

 

Fig. 2 (a) Optical system for the fluorescence measurement. (b) Photograph of the water and PEG solutions containing Eu³⁺ (0.01 mol/l). The pump laser beam (396 nm) passed through the solutions from the left to the right, inducing a strong fluorescence in the PEG solution.

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3. Results

Figure 3 shows the fluorescence spectra of Eu³⁺ ions (0.01 mol/l) that were dissolved in mixed solvents of water and PEG. As Fig. 3(a) shows, the aqueous solution exhibits the strongest peak at 592 nm, which corresponds to the ⁵D₀→⁷F₁ transition [8]. The other two peaks at 613 and 698 nm correspond to ⁵D₀→⁷F₂ and ⁵D₀→⁷F₄ transitions, respectively. As Fig. 3(b) shows, addition of PEG causes no spectral change until its volume ratio exceeds 50%. When the ratio increases to 90%, however, the fluorescent peaks start to grow. Particularly, the peak at 613 nm, which accompanies a side peak at 616 nm, exhibits a notable growth, and becomes higher than the 592-nm peak when the PEG ratio exceeds 95%. At the same time, other peaks emerge at 579 and 652 nm.

 

Fig. 3 Fluorescence spectra of Eu³⁺ that dissolved in the mixture of water and PEG 300. The volume ratio of the PEG varied (a)‒(i) between 0 and 100%. The Eu concentration was 0.01 mol/l. The sample cell thickness was 10 mm.

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Figures 4(a)‒4(f) show the dependence of the peak height on the PEG ratio. A rapid increase in the fluorescence intensity is visible in the range over 90%. The fluorescence at 613 nm is 80-fold higher in PEG (100%) than water. Figure 4(g) shows the energy levels of the Eu³⁺ ion [8]. Although these fluorescent peaks correspond to the transitions from the same energy level (⁵D₀), their intensity is affected quite differently by the increase of the PEG ratio.

 

Fig. 4 (a)‒(f) Dependence of the fluorescent peak height on the volume ratio of PEG. (g) Energy levels of the Eu³⁺ ion that are related to the excitation (396 nm) and the fluorescence emission at various wavelengths.

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The measurements above were conducted with a sample cell of 10 mm thickness. As the transmission spectra in Fig. 1 show, a large portion of the pump light energy (396 nm) passed through the sample being unused for the Eu³⁺ excitation. This inefficiency became more notable in small samples. Figures 5(a)‒5(c) show the fluorescence spectra that were measured with a sample cell of 1, 2, or 5 mm thickness. As the sample thickness decreased, the fluorescence became weak whereas the unabsorbed pump light became stronger at the sample exit. To attain a strong pump energy absorption in a small sample, we replaced PEG 300 by PEG 1000, which took the solid phase at room temperature and accordingly induced a strong scattering. Europium chloride was dissolved in the solvent at 70 °C, at which temperature PEG 1000 took the liquid phase. Then the solution was cooled down to room temperature for solidification (freezing point: 36 °C) [19]. Fluorescence spectra were measured in the same manner as conducted with the liquid samples. Before freezing (>36 °C), the solution exhibited a similar fluorescence spectrum as those in Figs. 5(a)‒5(c). The fluorescence intensity, however, changed gradually during the freezing process (36‒40 °C). As Figs. 5(d) and 5(e) show, the solid sample exhibited a stronger fluorescence than the liquid sample when the thickness was 1 or 2 mm. This fact verified the usefulness of the scattering matrix. As Fig. 5(f) shows, however, the fluorescence decreased gradually as the sample thickness increased to 5 mm, since the fluorescence was also scattered as it passed through the thick sample.

 

Fig. 5 Fluorescence spectra of (a)‒(c) the liquid (PEG 300) or (d)‒(f) solid (PEG 1000) samples. The Eu concentration was 0.01 mol/l. The sample thickness was 1, 2, or 5 mm.

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Figure 6(a) shows the transmittances of the liquid and solid PEGs. Whereas PEG 300 (liquid) is transparent over the entire visible spectral range, PEG 1000 (solid) scatters (attenuates) visible light completely within 1‒2 mm thickness. As the circles in Fig. 6(b) show, the pump light does not reach the region beyond 2 mm distance, and hence, cannot excite dye molecules in that region. This is the reason that the fluorescence becomes weak in the thick solid sample. By contrast, the pump light propagates with a small attenuation in the liquid sample (the triangles), and hence, the fluorescence increases in proportion to the sample thickness. Although the solid PEG was effective to confine the pump light within the 2 mm thickness, the width and height of the current sample cell (10 × 20 mm²) were too large to restrict the pump light divergence in the lateral direction. Consequently, no stimulated emission was visible with the current samples. A waveguide or droplet has to be created with the PEG solution to induce the laser emission.

 

Fig. 6 (a) Transmission spectra of PEG 300 (liquid) and PEG 1000 (solid). The samples were contained in a glass cell with a thickness of 10 or 1.2 mm. A xenon lamp was used as a light source in this experiment. The blank for the transmittance measurement was an empty glass cell. (b) Intensities of the scattered pump light (396 nm). As Fig. 2(b) shows, the violet laser beam was put into the sample cell, and the scattered light intensity was measured from the cell side, i.e., in the direction perpendicular to the pump beam path. The horizontal axis shows the distance from the cell entrance. The samples were PEG 300 (∆) and PEG 1000 (○).

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Finally, the resistivity of the solution was examined. Figure 7(a) shows fluorescence spectra that were measured before and after the continuous laser irradiation process (396 nm, 100 mW). In contrast to the organic dye solution [1, 2, 6, 7], no degradation was visible after the 2 h irradiation. Figures 7(b) and 7(c) show the result of a preservation test, in which the fluorescence spectra of the sample solution were measured occasionally during a period of 55 days. The spectral change was negligible, which indicated the stability of the PEG solution.

 

Fig. 7 (a) Fluorescence spectra that were measured before (0 h, the thick gray line) and after (2 h, the thin line) the continuous irradiation of the pump laser beam (396 nm wavelength, 100 mW, 1 mm diameter). (b) Fluorescence spectra that were measured after preservation of 1 or 55 days. (c) Temporal change of the fluorescent peak heights (592 or 613 nm wavelength) during the preservation process.

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4. Discussion

As mentioned in Section 2, the energy transfer efficiency from PEG to the Eu³⁺ ion was insufficient to enhance the fluorescence intensity. Recently chemists have synthesized various organic-inorganic composites (sol-gel glasses or polymers) that absorb ultraviolet light energy to excite the lanthanide ions [14‒16, 26‒29]. It may be possible on the basis of these researches to modify PEG molecules for enhancement of the energy transfer efficiency. The process of energy collection and transfer, however, usually needs ultraviolet light whose photon energy is higher than the excitation energy of lanthanide ions (down conversion). It should be examined carefully whether those high-energy photons accelerate photo-degradation of the chromophores and other organic materials.

As Figs. 3 and 4 show, the fluorescence intensity increased notably by replacing water with PEG. This phenomenon is assumed to be caused by removal of water molecules from the vicinities of Eu³⁺ ions. It is known that the excited state of Eu³⁺ ion (⁵D₀) loses energy through the resonance with the third harmonic oscillation (phonon) of water molecules [30, 31]. Suitable ligands, i.e., molecules that are attached to or located around the central ion, protect lanthanide ions from being accessed by solvent molecules [32, 33]. Liquid with a small phonon energy, e.g., heavy water (D₂O), is also useful as a solvent that induces no fluorescence quenching [30, 33–35]. In the current experiment, water molecules that surrounded Eu³⁺ ions seem to have decreased in number when the PEG ratio exceeded 90%.

The results in Figs. 3 and 4 also indicate that the fluorescence enhancement takes place more notably at 613 nm than 592 nm. The selection rule in quantum mechanics predicts that the symmetry of the surrounding fields forbids some fluorescent transitions of lanthanide ions [8]. This is the case for the ⁵D₀→⁷F₂ transition (the electric dipole transition) that induces the emission at 613 nm. It is known that the electric and magnetic fields at the ion site are heavily influenced by ligands [36, 37]. An asymmetric field that is induced by ligands activates those forbidden transition and accordingly, enhances the fluorescence intensity [26, 33, 38]. It has been reported for various lanthanide ions that the fluorescence is enhanced by addition of a suitable agent, e.g., polyethylene oxide or aluminum oxide, to a solid matrix [34, 36, 39]. PEG also acts as an effective ligand for creating an asymmetric electric field; e.g., PEG 200 was added to Eu³⁺-doped glass to enhance the fluorescence [40]. In another experiment, PEG 400 was used as a solvent for NiBr₂ to study the ligand field of Ni ions [24]. These experiments suggest that PEG is a suitable solvent for activating lanthanide fluorescence.

The current experiment demonstrated 80-fold fluorescence enhancement in the PEG solvent. As mentioned in Section 1, PEG has an advantage that the phase transition takes place at around room temperature [18, 19]. That is, the liquid and solid phases can be controlled reversibly in microfluidic devices. For example, one can freeze a droplet resonator or a flowing solution, and then melt it again for deformation or sample change. The phase transition also changes the optical properties of samples (Fig. 5). These characteristics will provide additional functions to microfluidic devices.

5. Conclusion

An organic solution containing lanthanide ions has been examined as a substitute for organic dye solutions that are currently used in microfluidic and droplet lasers. Eu³⁺ ions emit 80-fold stronger fluorescence in PEG than water. This fluorescence enhancement is attributed to both reduction of the phonon-induced quenching and generation of an asymmetric ligand field. In addition, a strong scattering in the solid PEG matrix increases the pump light absorbance, and consequently doubles the fluorescence intensity. This absorption enhancement is particularly useful to improve the excitation efficiency of micrometer-sized weakly-absorbing samples. These advantages together with the bistability and nonvolatility render PEG a suitable solvent for creating lanthanide-based microfluidic devices.