Fluorescence Enhancement of Europium Ions in a Scattering Matrix

Mitsunori Saito and Takahiro Koketsu

Department of Electronics and Informatics, Ryukoku University, Seta, Otsu 520-2194, Japan

Keywords: Fluorescence, Europium, Polyethylene Glycol, Scattering, Droplet Laser.

Abstract: Microlasers are usually composed of organic dyes that emit fluorescence with a high efficiency. Those dyes,

however, lose their fluorescence function in a short time because of optically- or thermally-induced bleaching.

This degradation is particularly serious with microdevices, since a high-powered beam is focused into a small

volume of the device. The problem of the device degradation can be solved, if organic dyes are replaced by

fluorescent lanthanide ions (europium, erbium, neodymium, etc.) that have a superior durability against

optical and thermal hazards. The lanthanide ions, however, have a smaller absorption cross-section than

organic dyes, and hence, a pump light for exciting the ions is absorbed insufficiently inside a microdevice. A

long optical path is therefore required to enhance the excitation efficiency. Polyethylene glycol is a useful

solvent for dispersing europium ions, since it turns to a translucent matrix by solidification. In this translucent

matrix, pump light (396 nm wavelength) is scattered heavily, which leads to extension of the optical path and

enhancement of the absorbance. Consequently, fluorescence of the europium ions (613 nm) becomes twofold

stronger in the solid phase than the liquid phase.

1 INTRODUCTION

Microfluidic devices are attracting interests in

various technical fields including chemical analysis,

biological sensing, and material synthesis (Hawkins

and Schmidt, 2010). Organic dyes are usually used

for yielding a fluorescence function in the liquid.

Microdroplet laser is a unique fluidic device in

which fluorescence circulates numerous times indu-

cing a stimulated emission. A droplet of dye solution

self-forms a perfect sphere with a smooth surface,

which facilitates the light circulation. In addition,

deformable spheres realize the emission wavelength

tuning. Since these unique features are unattainable

with solids, droplet lasers have been studied

extensively in the last three decades (Tzeng et al,

1984). Disadvantages of the droplet lasers include

handling difficulty (fluidity) and instability

(volatility), which are essential features of liquids.

These problems are avoidable by enclosing a droplet

in a silicone rubber (Saito et al, 2008); i.e., whereas

an enclosed droplet can be handled like a solid, its

fluidity is preserved in a flexible silicone rubber.

In spite of the improvement above, microdroplet

lasers still have another problem; i.e., organic dye

molecules degrade rapidly because of photochemical

or thermochemical reactions during excitation and

emission processes. This degradation generally takes

place in organic dye lasers (Gersborg-Hansen et al,

2007). Conventional dye lasers, therefore, circulate a

dye solution to replace bleached molecules by fresh

ones. As regards dye-doped polymer lasers, disk

rotation (Kytina et al, 2004) or molecular diffusion

(Yoshioka et al, 2012) are achieved to extend the

device lifetime. Unfortunately these methods are

unadoptable for microdroplet lasers, although the

dye bleaching is even more serious in a small droplet

that is exposed to a strong laser beam (Barnes et al,

1993).

A possible solution for the degradation problem

is replacement of organic dyes by lanthanide

elements, e.g., europium (Eu), erbium (Er),

neodymium (Nd), thulium (Tm), samarium (Sm), etc.

Photoluminescence of these ions are well known

(Shionoya and Yen, 1999) and widely used in lasers,

amplifiers, and illuminators. Many researchers

fabricated microspherical lasers by doping

lanthanide ions in crystals and glasses, e.g., Sm-

doped fluorite (Garret et al, 1961), Nd-doped glass

(Miura et al, 1996), and Er-doped glass (Klitzing et

al, 2000). A droplet laser was also fabricated by

using a Eu chelate (Lin et al, 1992).

The lanthanide ions, however, have a disadvanta-

ge that an absorbance is too low to absorb pump

light efficiently in a small droplet. In this respect,

organic dyes are superior to the lanthanide ions. It

Saito M. and Koketsu T.

Fluorescence Enhancement of Europium Ions in a Scattering Matrix.

DOI: 10.5220/0006089600150021

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 15-21

ISBN: 978-989-758-223-3

has been reported, for example, that the pump-light

absorbance of rhodamine is 10,000-fold stronger

than that of Er (Tagaya et al, 1997). The optical path

has to be extended to absorb pump light efficiently.

Dispersion of scattering particles is effective to

extend the optical path (Watanabe et al, 2005).

Turbid media are also attracting interests for

development of random lasers (Wiersma et al, 1995).

Polyethylene glycol (PEG) is a useful matrix for

dispersing fluorescent materials, since it dissolves

various dyes and ions in the liquid phase and

becomes translucent in the solid phase (Saito and

Nishimura, 2012). Random lasing has been realized

by the use of scattering in the solid PEG that

contains rhodamine 6G (Saito and Nishimura, 2016).

In this study, we prepared a PEG solution of

europium chloride (EuCl

) and examined the

enhancement of the pump-light absorbance and

fluorescence in this scattering solid.

2 SAMPLE PREPARATION

Figure 1(a) shows transmission spectra of aqueous

solutions of rhodamine 6G (typical organic dye) in a

sample cell of 10 mm thickness. A strong absorption

band emerged when the concentration exceeded 10

‒5

mol/l. As the black line in Fig. 1(b) shows, however,

Figure 1: Transmission spectra of the aqueous solutions

containing (a) rhodamine 6G (10

-6

‒10

-4

mol/l) or (b) EuCl

(10

-2

mol/l). The solutions were put in a sample cell of 10

or 50 mm thickness. Pure water in the same sample cell

was used as a blank for the transmittance evaluation.

an aqueous solution of EuCl

in the same sample cell

(10 mm) exhibited only a trace of absorption (394

nm) in spite of 1000-fold increase in the

concentration (10

‒2

mol/l). The gray line shows the

absorption band that became clear by extension of

the sample thickness (50 mm). However, both the

depth and width of the absorption band were still

smaller in a Eu solution than a rhodamine solution.

These facts indicate that europium needs a long

optical path to absorb pump light. In addition, the

pump light wavelength has to be tuned accurately to

meet the absorption wavelength.

To extend the optical path, we used PEG as a

solvent. As Fig. 2(a) shows, the melting point of

PEG changes notably depending on the molecular

weight. PEG with a molecular weight of 300 (PEG

300), for example, takes a transparent liquid phase at

room temperature, whereas PEG 1000 takes a

translucent solid phase. Figure 2(b) shows

transmission spectra of PEG 300 and PEG 1000 in a

sample cell of 10 mm thickness. PEG 300 is

transparent over the visible spectral range, whereas

PEG 1000 scatters visible light too strongly to allow

transmission. In the sample preparation process,

PEG 1000 was heated to ~50 °C for dissolving

EuCl

, and then cooled to room temperature. No

precipitation took place in the solidification process,

and Eu ions were dispersed uniformly in the solid.

Figure 2: (a) Melting points of PEGs with various

molecular weights. (b) Transmission spectra of PEGs at

room temperature. The photograph shows the PEGs in the

liquid and solid phases.

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

3 ABSORBANCE

Spectral measurement was conducted for the PEG

300 that contained EuCl

of 10

‒2

mol/l. The solution

was put into a glass cell of 10‒50 mm thickness. The

PEG containing no salts [the black line in Fig. 2(b)]

was used as a blank for evaluation of the

transmittance. Figure 3(a) shows the transmission

spectra that were measured for the samples of 10 or

50 mm thickness. The absorption band at 394 nm is

slightly weaker in the PEG than water [Fig. 1(b)]. In

the ultraviolet region (<380 nm), however, the

transmittance decreases more notably in the PEG

than water. We used the absorption band at 394 nm

in the current experiment, since no suitable

ultraviolet laser was available in our laboratory.

Figure 3: (a) Transmission spectra of EuCl3 in PEG 300

(liquid). The concentration was 10‒2 mol/l and the sample

thickness was 10 or 50 mm. (b, c) The optical densities of

EuCl3 in PEG 300 or water. The numerals beside the

marks denote wavelengths. The concentration was 10‒2

mol/l.

Figure 3(b) shows the absorbance or the optical

densities (‒logT) that were evaluated from the

transmittances (T

) at several wavelengths around the

absorption band. The optical density increases in

proportion to the sample thickness, although some

deviation is visible at 30 mm probably due to

insufficient optical alignment in the measurement

process. The highest absorbance is attainable at 394

nm, and it is halved at 396 nm. Figure 3(c) shows

the result for the aqueous solution [Fig. 1(b)].

Although the absorbance at 394 nm is slightly higher

in water than PEG 300, these solutions will exhibit

similar absorption efficiency for the pump light. As

regards the PEG 1000 (solid), the transmission

spectrum was not measurable, since no light passed

through the sample [the gray line in Fig. 2(b)].

4 FLUORESCENCE

The results in Fig. 3 indicate that efficient excitation

is achievable at around 394 nm wavelength. We

used a laser diode of 396 nm in the current

experiment. The laser power was 100 mW. Figure 4

shows an optical setup for fluorescence measure-

ments. Samples were put into a glass cell of 10 or 50

mm thickness. The laser beam was focused on the

input side of the glass cell. Fluorescence was picked

up in the direction perpendicular to the laser beam

axis, since no strong light emerged from the output

side of the glass cell when the sample was

translucent (PEG 1000). As the photograph in Fig. 4

shows, the transparent sample (PEG 300) also

emitted fluorescence in the perpendicular direction.

The collection lens and the probe fiber (core

diameter: 400 μm) were moved to change the pickup

position, i.e., the distance z from the input side. The

fluorescence spectrum was measured by using a

multichannel spectrometer (B & W Tek, BTC112E).

Figure 4: Optical setup for the fluorescence measurement.

Fluorescence was picked up from the sample side or the

exit end by using a probe fiber of the multichannel

spectrometer. The photograph shows fluorescence that is

emitted from the laser beam path inside the PEG solution

of EuCl3.

Fluorescence Enhancement of Europium Ions in a Scattering Matrix

Figure 5 shows the fluorescence spectra of the

solid sample (PEG 1000, Eu concentration: 10

-2

mol/l). Measurement position was 0, 1, 3, or 10 mm

apart from the input side. Fluorescence peaks are

visible at 592, 613, and 698 nm. These peaks

correspond to the electronic transitions of

→

, and

→

, respectively. The peak at

396 nm is attributed to the scattered pump light. The

pump light attenuates gradually as it propagates in

the sample because of scattering by the matrix and

the absorption by Eu ions. As Fig. 5(d) shows, the

pump light penetrates only ~10 mm depth from the

input end. According to the pump light attenuation,

the fluorescence peaks become lower as the position

becomes distant from the input side.

Figure 5: Fluorescence spectra of the solid sample (PEG

1000) that were measured from the sample side. The peak

at 396 nm was caused by scattering of the pump laser.

Measurement position was (a) 0, (b) 1, (c) 3, or (d) 10 mm

from the input side.

Spectral measurements for the liquid sample

(PEG 300, Eu concentration: 10

‒2

mol/l) were

conducted in the same manner. Figure 6 shows the

spectra that were measured at different positions.

The peak at 396 nm is weak and its height changed

little with the measurement position. This fact

indicates that the pump light propagates with a

negligible absorption loss in the transparent solution.

Accordingly, fluorescence peaks at 592, 613, and

698 nm exhibit no position dependence. Although

the fluorescence in the transparent solution is

apparently stronger than that in the translucent solid,

the actual fluorescence intensity is thought to be

stronger in the solid, since fluorescence also suffers

scattering before it is picked up by the probe fiber.

Figure 6: Fluorescence spectra of the liquid sample (PEG

300) that were measured from the sample side. The peak

at 396 nm was caused by scattering of the pump laser.

Measurement position was (a) 0, (b) 1, (c) 3, or (d) 10 mm

from the input side.

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

Figure 7(a) shows the position dependence of the

pump light intensity, i.e., the peak height at 396 nm,

in the solid and liquid phases of PEG. The

attenuation of the pump light is negligible in the

liquid phase. By contrast, the pump light attenuates

rapidly in the solid phase. The attenuation length is

about 1 mm; i.e., the pump light intensity becomes

1/e of the original value within 1 mm. This fact

indicates that the pump light is confined efficiently

in a microdroplet if it is composed of the solid PEG.

Figure 7(b) shows the position dependence of the

fluorescence intensity in the solid phase. All peaks

become lower with the attenuation of the pump

power. The attenuation length, however, extends to

~7 mm, which is longer than that of the pump light.

This difference is explained by the scattering of the

fluorescence rays. That is, fluorescence rays are

emitted in all directions, and some of them emerge

form a distant position due to the scattering.

Figure 7: Position dependence of the measured peak

height. (a) shows the pump light intensities in the solid or

liquid samples. (b) and (c) shows the fluorescence

intensities of the solid and liquid samples, respectively.

The numerals beside the lines denote peak wavelengths.

By contrast to the solid phase, the liquid phase

yielded no position dependence, as shown in Fig.

7(c). The intensity ratio of the 613-nm peak to the

other two peaks is higher in the liquid phase than the

solid phase. This fact is related to the width of the

613-nm peak; i.e., as Figs. 5 and 6 show, the peak is

narrower in the liquid phase than the solid phase.

The ligand field around Eu ions possibly changes

through the phase transition process (Wong et al,

2002).

Finally, fluorescence emission was measured in

the forward direction to confirm the effectiveness of

the scattering. As Fig. 7(a) shows, the pump light

excites the solid sample efficiently only within its

penetration depth. The samples were therefore put in

a thin glass cell in this experiment. As the dotted

lines in Fig. 4 illustrate, fluorescence was measured

at the exit side of the sample cell. Figures 8(a) and

8(b) show the spectra that were measured for the

solid and liquid samples of 1 mm thickness. The

fluorescence peak height was twice higher in the

solid than the liquid. The effect of the absorption

enhancement was notable in these thin samples. As

Figure 8: Fluorescence spectra that were measured in the

forward direction. The samples were PEG 1000 (solid) or

PEG 300 (liquid) that contained EuCl3. The sample

thickness was (a, b) 1, (c, d) 2, or (e, f) 5 mm.

Fluorescence Enhancement of Europium Ions in a Scattering Matrix

Figs. 8(c) and 8(d) show, however, the peak height

difference became less notable for the samples of 2

mm thickness. When the thickness extended to 5

mm, the fluorescence peaks shrank in the solid phase,

as shown in Fig. 8(e). By contrast, the fluorescence

peaks of the liquid grew further with the increase of

the sample thickness, as shown in Fig. 8(f).

5 DISCUSSION

In the transparent matrix, e.g., glasses or liquids,

fluorescence of lanthanide ions increases in

proportion to the sample thickness, since the pump

light reaches the exit end with a small optical

attenuation. Optical amplifiers or lasers, therefore,

use a long fiber to attain a strong fluorescence

intensity. If a high-powered laser beam is required, a

thick crystal (solution) or a long fiber is a better

choice than the scattering PEG. The advantage of the

microlaser is an efficient light emission from a small

volume. From this viewpoint, PEG is a suitable

matrix for creating a fluorescent device whose size

is smaller than 1 mm.

As mentioned earlier, the fluorescence peak at

613 nm exhibited different shapes depending on the

phase (solid or liquid). We also observed different

spectra in the aqueous and PEG solutions. This

phenomenon seems to be related to the surrounding

ligand field. Further investigation on this

phenomenon possibly leads to improvement of the

fluorescence efficiency.

In the current experiment, a laser diode of 396

nm was used as a pump light source, since other

suitable sources were not available in our laboratory.

As Fig. 3(a) shows, the transmittance of the sample

solution decreases in the ultraviolet range below 380

nm. If this transmittance decrease originates from

the absorption by the Eu ions, ultraviolet pump light

will induce more efficient fluorescence emission.

Note that this transmittance decrease was certainly

induced by addition of EuCl

since the pure PEG

300 was used as a blank for the transmittance evalu-

ation. It possibly happens, however, that the addition

of EuCl

promotes the ultraviolet absorption of PEG

[Fig. 2(b)]. Further experiments are needed to clarify

the origin of this transmittance decrease.

As Fig. 2(a) shows, the phase transition of PEGs

takes place at around room temperature. In addition,

PEGs exhibit a bistable behavior during the phase

transition process; i.e., both the solid and liquid

phases are stable at a certain temperature. This

phenomenon seems useful to create a bistable

microlaser. We are currently thinking of creating a

random droplet laser on the basis of the current

experimental results.

6 CONCLUSIONS

Polyethylene glycol is a suitable matrix for creating

a micro optical device that uses lanthanide ions as a

fluorescence emitter. It exhibits a strong scattering

in the solid phase, and extends an optical path of

pump light, leading to efficient excitation of the

fluorescent ions. This enhanced fluorescence was

demonstrated experimentally by exciting a solution

of EuCl

with a laser diode of 396 nm wavelength.

The fluorescence peak at 613 nm became twofold

higher in this scattering matrix than the original

liquid.

ACKNOWLEDGEMENTS

This research was supported by Japan Society for

the Promotion of Science (15K04642).

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Fluorescence Enhancement of Europium Ions in a Scattering Matrix