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Biocompatible microlasers based on polyvinyl alcohol microspheres
Journal Pre-proof Biocompatible microlasers based on polyvinyl alcohol microspheres Van Duong Ta, Thiet Van Nguyen, Quan Van Pham, Toan Van Nguyen
PII: DOI: Reference:
S0030-4018(19)31021-1 https://doi.org/10.1016/j.optcom.2019.124925 OPTICS 124925
To appear in:
Optics Communications
Received date : 8 October 2019 Revised date : 9 November 2019 Accepted date : 11 November 2019 Please cite this article as: V.D. Ta, T.V. Nguyen, Q.V. Pham et al., Biocompatible microlasers based on polyvinyl alcohol microspheres, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.124925. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Biocompatible microlasers based on polyvinyl alcohol microspheres Van Duong Taa,*, Thiet Van Nguyen a, Quan Van Pham a, Toan Van Nguyen b,c Department of Optical Devices, Le Quy Don Technical University, Hanoi 100000, Vietnam.
b
Department of Physics, Le Quy Don Technical University, Hanoi 100000, Vietnam
c
Department of Quantum Optics, Faculty of Physics, VNU University of Science, Hanoi 100000,
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a
Vietnam. *
Corresponding author. E-mail address: [email protected]
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ABSTRACT
Biocompatible microlasers, generally made of bio-derived materials, are promising for biosensing and
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cell-tracking. These kinds of lasers offer favourable opportunities like biocompatibility and biodegradability but the materials used often require complicated synthesis and high cost. In this work, we demonstrate that polyvinyl alcohol (PVA), a synthetic water-soluble low-cost polymer, with fascinating properties such as good transparency, biocompatibility, biodegradability is an excellent candidate for making laser cavity. Using a simple and effective technique, dye-doped PVA microspheres can be fabricated with various sizes from 10 to 200 µm. These microspheres can act as excellent lasers under optical excitation with a lasing threshold of ~2 µJ/mm2 and Q factor of lasing modes of ~3000. The lasing mechanism is studied and it is ascribed to WGM. Size-dependent lasing
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characteristics including lasing spectrum, Q factor and lasing threshold are investigated. Owing to the ease of fabrication, the cost-effectiveness, the biocompatibility of the PVA material, our biocompatible microlasers are promising for future biosensing applications. Keywords: Microlaser, Whispering gallery mode, Biocompatible, Polyvinyl alcohol. -1-
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1. Introduction Microlasers are important for on-chip optical communications, data-processing, medical imaging and sensing [1-3]. Recently, miniaturized biocompatible lasers are highly emerging as a new tool for
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biosensing and cell-tracking [4-6]. A number of bio-derived materials have been explored for both gain
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medium such as green fluorescent protein [7], luciferin [8] and laser cavity including proteins [9-11], polysaccharides [11]. In comparison to inorganic materials, these biomaterials offer favourable opportunities like biocompatibility and biodegradability but they generally suffer from high cost and
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complicated synthesis [12]. As a result, exploring a cost-effective material that has similar properties to the above biomaterials is essential for the development of biocompatible lasers.
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Polyvinyl alcohol (PVA) is a synthetic water-soluble polymer with fascinating properties such as good transparency, biocompatibility, biodegradability and finds applications in numerous biomedical fields [13-15]. Despite these excellent properties, PVA has been rarely investigated as an appropriate material for a laser cavity.
Interestingly, PVA microspheres have been reported but they have not used as a cavity for laser generation. For instance, Young et. al. fabricated hydrogel PVA microspheres by combining microfluidic droplet generation with UV photopolymerization [16], Grumezescu et. al. fabricated polylactic-co-glycolic acid-polyvinyl alcohol microspheres using matrix-assisted pulsed laser
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evaporation [17]. It is well known that a spherical microstructure like a microdroplet or a microsphere can serve as a high quality (Q) factor cavity. In this structure, light can be trapped inside by total internal reflections near its interface and amplified by resonant circulation, the so-called, whispering
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gallery mode (WGM). As a result, the investigation of biocompatible PVA microspheres for laser action should be a motivating task. In this work, PVA microspheres are fabricated in a novel way using a simple and effective
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technique. Obtained PVA microspheres can serve as high Q factor resonators. By doping organic dye
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molecules to these structures, lasing emission is obtained under optical pumping. Lasing mechanism and lasing characteristics are studied systematically. 2.
Experimental methods
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2.1. Aqueous dye-doped PVA solution
Firstly, PVA solution, 5 wt%, was prepared by dissolving 0.25 g PVA powder (Mw = 89000-98000,
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from Sigma-Aldrich) in 5 mL deionized water at 70 °C for 3 hours. Secondly, Rhodamine B solution, 1 wt%, was obtained by directly dissolving 20 mg RhB ( >95% dye, from Sigma-Aldrich) in 2 mL deionized water. Finally, 5 mL PVA solution, 5 wt% and 0.5 mL RhB solution, 1 wt% were mixed to obtain the solution that used for making microspheres. The dry ratio of PVA and RhB in microspheres is 98 wt% and 2 wt%, respectively.
2.2. Fabrication of microsphere biolasers
The fabrication process is based on emulsion and dehydration as shown in Figure 1 [11, 18]. Firstly, a droplet of dye-doped PVA solution is injected in an uncured polydimethylsiloxane (PDMS) resin
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(Base of Sylgard 184 Silicon Elastomer, from Dow Corning) (Figure 1a). Secondly, the droplet is dispersed into multiple microdroplets. As water and PDMS are immiscible, the microdroplets are selfassembly formed within several seconds due to surface tension (Figure 1b). Subsequently, the whole mixture is heated at 80 °C for 90 minutes. This heating process completely dehydrates water from the
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microdroplets and solid-state dye-doped microspheres are obtained (Figure 1d). The PDMS was then removed by using a solvent (ethyl acetate). Finally, microspheres were extracted from the solvent and left at room temperature, in ambient conditions for an hour (to completely evaporate the solvent) thus
(a)
Self-assembly microdroplets (b)
Solidification via dehydration (c)
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Droplet injection
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they are ready for optical characterizations.
Solid-state microspheres (d)
Heating (80 C)
RhB-PVA solution
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PDMS
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Water vapour
Fig. 1. (a)-(d) Schematic diagram describing the fabrication process of dye-doped PVA microspheres from an aqueous solution.
2.3. Optical characterizations
Individual microspheres (placed on a glass substrate) were investigated by using a microphotoluminescence (μ-PL) setup. The pumping source was a frequency-doubled, Q-switched Nd:YAG laser (Litron Lasers) with a wavelength of 532 nm, a repetition rate of 10 Hz and a pulse duration of 4-7 ns. Excitation laser was guided (by using a set of mirrors) to the sample under an angle of ~70° to the
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normal of the substrate. Then, the excitation beam was focused on individual microspheres via a typical lens. The beam spot was ~ 350 μm in diameter. Emission was then collected from the top by a 10× objective and subsequently delivered to a spectrometer (AvaSpec-2048L from Avantes) for spectral
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recording. The spectral resolution of the spectrometer is ~ 0.2 nm. All optical measurements were done at room temperature and in ambient conditions. 3.
Results and discussion
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3.1. Dye-doped PVA microspheres
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Using our technique, dye-doped PVA microspheres with a variety of diameters can be produced. Figure 2a shows an optical microscope image of fabricated microspheres indicating clearly spherical structures. Based on the uniform colour, it is suggested that the dye molecules are well distributed
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throughout the microspheres. The spherical shapes of microspheres are even more visible when observing via a scanning electron microscope (SEM) (Figure 2b). In order to get a better view of the
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shape and its surface roughness, high magnification of a single microsphere is presented in Figure 2c. The circular shape and smooth surface can be seen clearly. These two factors are sufficient for high optical confinement and laser generation.
(a)
(c)
(b)
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100 µm
100 µm
30 µm
Fig. 2. (a) Optical image of RhB-doped PVA microspheres. (b) Scanning electron microscope (SEM) image of RhB-doped PVA microspheres with various sizes. (c) A high magnification SEM image of a single microsphere indicating a smooth outer surface.
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3.2. Lasing characteristics of a typical dye-doped PVA microsphere Lasing emission was observed from individual RhB-doped PVA microspheres under laser pulse
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pumping. Figure 3 plots the lasing performance of a typical microsphere which has a diameter of ~100 µm (estimated from the optical microscope image). The cross-over from spontaneous emission, at pump
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pulse energy (PPE) of 1.33 µJ, to lasing emission at PPE of 2.16 µJ can be seen clearly (Figure 3a). Upon higher PPE, the peak intensity of lasing modes increases sharply. When PPE rises two times from 2.16 to 4.21 µJ, the peak intensity increases 7.5 times, from 8000 to 60000 a. u. (Figure 3b). The
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integrated lasing intensity versus PPE is plotted in Figure 3c showing the nonlinear increase of the emission intensity. This result supports the lasing action and the lasing threshold is determined to be
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1.52 µJ. This value is comparable to other kinds of organic microsphere and droplet lasers [19-23]. In order to examine the free spectral range (FSR) and the full width at half maximum (FWHM) of lasing modes, a lasing spectrum at the short wavelength range from 619.5 to 624 nm was plotted in Figure 3d. It can be seen that the lasing modes are well separated. The FSR is about 0.84 nm and the FWHM (denoted as δλ) is around 0.2 nm (limited by our spectral resolution). In addition, lasing modes may exhibit a blue-shift behavior under increasing pump pulse energies. The blue-shift is caused by the thermal effect or heat generated by the pump laser [24]. However, in our
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work, we did not observe the blue-shift of lasing modes (under certain pump pulse energies), which suggests that the PVA based microlasers are relatively resistant to the thermal effect.
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3.3. Lasing mechanism The lasing mechanism is ascribed to WGM. It is well-known that light is confined inside a microsphere by multiple total internal reflections at the microsphere-air interface. Light travels around
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near the equator of the microsphere so the FSR of lasing modes can be estimated as λ2/πnD, where λ is
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resonant wavelengths; n and D are refractive index and diameter of the microsphere, respectively. Based on this equation, the FSR of the 100 µm-diameter microsphere is shown in Figure 3d can be well explained. Assuming, λ = 621.5 nm (close to the experimental measurement), n = 1.48 and D = 100 µm,
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the calculated FSR is 0.83 nm. This value is consistent with the above experimental value of 0.84 nm, which confirms the significant role of WGM on the laser action.
(b) 60000
Pump pulse energy 2.16 J 1.33 J
PL intensity (a. u.)
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8000
6000
4000
PL intensity (a. u.)
(a)
4.21 J
40000
20000
2000
0
560
600
640
0 600
680
610
2x105
1x105
Threshold (1.52 J)
(d)
630
0.84 nm
60000
PL intensity (a. u.)
Intergrated PL intensity (a. u.)
(c)
620
640
Wavelength (nm)
Wavelength (nm)
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40000
4.21 J
nm
20000
0
0
1
2
3
4
5
620
621
622
623
624
Wavelength (nm)
Pump pulse energy (J)
Fig. 3. (a) Emission spectra of the microsphere below and above the lasing threshold. (b) Typical lasing spectrum. (c) Integrated PL intensity of the microsphere versus pump pulse energy exhibits a distinct lasing threshold of 1.52 µJ. The inset shows the optical and PL image of the microsphere. The scale bars are 100 µm. (d) Close-up of the lasing spectrum showing a FSR of 0.84 nm and spectral linewidth of 0.2 nm. -7-
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In addition, it is well-known that emission from a WGM laser can be either transverse electric (TE) or transverse magnetic (TM). Generally, the polarization of the lasing modes can be determined
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experimentally using a polarizer [25]. In many cases, when both TM and TE modes coexist, the polarization of these modes can be determined theoretically via explicit asymptotic formulas [26].
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Herein, by fitting the lasing modes of several microsphere lasers with the explicit asymptotic formulas,
3.4. Size-dependence of lasing spectrum
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it is found that the lasing modes from the PVA microsphere lasers are mainly of TE characteristics.
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Lasing spectrum is related to the gain medium and cavity configuration. In this work, we fixed the dye and its concentration so the lasing spectrum would depend on the microsphere size. As shown in Figures 4a-4c, the number of lasing modes increase and the FSR gradually decreases with increasing microsphere diameter. Particularly, the FSR of 38 µm, 63 µm and 110 µm diameter microspheres are 2.1 nm, 1.3 nm and 0.6 nm, respectively. These values are again highly consistent with the prediction using equation, FSR = λ2/πnD. We investigated several more microspheres and their experimental FSR in comparison with calculation is plotted in Figure 4d. The agreement between the experiment and
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theory indicates that WGM resonance is indeed responsible for the lasing action in dye-doped PVA microspheres.
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(a)
4000
(b)
D = 38 m
D = 63 m
2000
1000
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1000
604 606 608 610 612 614 616 618
622 624 626 628 630 632 634 636
Wavelength (nm)
Wavelength (nm)
25000
(d) 2.5
D = 110 m
20000
Calculation Measurement
FSR (nm)
2.0
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(c)
2000
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3000
PL intensity (a. u.)
PL intensity (a. u.)
3000
PL Intensity (a. u.)
15000
1.5
1.0
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10000
600 602 604 606 608 610 612 614 Wavelength (nm)
0.5 30 40 50 60 70 80 90 100 110 Diameter of microsphere (m)
Fig. 4. (a)-(c) Lasing spectra of dye-doped PVA microspheres with a diameter of 38 µm, 63 µm and 110 µm, respectively. The wavelength range is 14 nm for all figures. The dash lines highlight the positions of typical lasing modes. (d) Comparison between calculated and experimental FSR of various microsphere lasers.
3.5. Size-dependence of Q factor and lasing threshold Q factor is an important parameter of a laser cavity that characterizes the optical confinement [27].
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A high Q factor cavity is desired for many applications such as studying nonlinear optical effects [28] and sensing ultra-small objects (nanoparticles, viruses) [29]. For a microsphere, the Q factor increases (due to low radiation loss) with increasing cavity size [30]. The Q factor of a lasing mode can be calculated as Q = λ/δλ. Figure 5 shows the Q factor of various microsphere lasers with a diameter from 35 µm to 63 µm. As expected, the Q factor tends to increase from 1900 to 3050 with a growing -9-
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microsphere size. A bigger microsphere should exhibit a higher Q factor (>3000) but we do not show the data here due to the limitation of our spectral resolution (~0.2 nm). The obtained Q factor is comparable with microsphere and microdisk lasers using other organic materials [10, 31].
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Lasing threshold is an important parameter of a laser. The lasing threshold depends on various
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factors including the microcavity Q factor, the mode volume and the value of the gain medium. In the microlaser cavity, pumping rate Rp is proportional to the intracavity pump power Pc and inversely proportional to the mode area Am [32]:
Pc Am
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Rp
(1)
In addition, the intracavity power Pc is proportional to the Q factor. We get,
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Rp
Q Am
(2)
From Equation (2), the pumping rate is higher in a higher Q factor microcavity (larger microsphere diameter), leading to a lower lasing threshold as Ith Rp1 . Moreover, the decrease in the mode area is also resulted in increasing the pumping rate and reducing the lasing threshold. However, how exactly the cavity Q factor and the mode area influence on the lasing threshold is still a challenging issue [33]. Previous experimental studies on WGM microdisk lasers have demonstrated that lasing threshold decreases with increasing the microdisk diameter and a 1/D2 relationship was obtained for disk diameter
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and lasing threshold [34, 35]. For microsphere lasers, it is expected that the lasing threshold should decrease with increasing their size. Figure 5b shows a plot of the lasing threshold versus PVA microsphere diameter. Despite some fluctuations, the lasing threshold scales inversely with the power of PVA microsphere diameter.
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Particularly, when the microsphere diameter increases 3 times, from 35 µm to 110 µm, the lasing threshold decreases much more, about 11 times, from 5.7 µJ to 0.5 µJ. Generally, a low lasing threshold
concentration [36]. (b) 6
(a)
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3000
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is more preferred. In this work, the lasing threshold can be reduced further by optimizing the dye
Lasing threshold energy (J)
5
2500
4 3
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Q factor
35
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2000
40
45
50
55
60
65
Diameter of microsphere (m)
2 1
0 30 40 50 60 70 80 90 100 110 Diameter of microsphere (m)
Fig. 5. (a) Q factor and (b) lasing threshold versus microsphere diameter.
4.
Conclusion
We have demonstrated that PVA is an excellent candidate for laser cavities. Dye-doped PVA microspheres with various sizes are fabricated using a novel and effective way. Under optical excitation,
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these microspheres can work as excellent lasers with the lasing threshold of ~2 µJ/mm2. The lasing mechanism is studied and it is ascribed to WGM. Size-dependent lasing characteristics including lasing spectrum, Q factor and lasing threshold are investigated. The results show that the Q factor of microsphere lasers is proportional to its diameter. Q factor values, ranging from 1900 to 3050, are
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obtained for microspheres with a diameter from 35 µm to 63 µm. In addition, the lasing threshold is inversely proportional to the power of the PVA microsphere diameter. Owing to the ease of fabrication and the biocompatibility of the PVA material, our biocompatible microlasers are promising for cell
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imaging and biosensing applications.
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Acknowledgements
This research is funded by Vietnam National Foundation for Science and Technology Development
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(NAFOSTED) under grant number 103.03-2017.318.
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Declaration of interests ☒ 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.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
PII: DOI: Reference:
S0030-4018(19)31021-1 https://doi.org/10.1016/j.optcom.2019.124925 OPTICS 124925
To appear in:
Optics Communications
Received date : 8 October 2019 Revised date : 9 November 2019 Accepted date : 11 November 2019 Please cite this article as: V.D. Ta, T.V. Nguyen, Q.V. Pham et al., Biocompatible microlasers based on polyvinyl alcohol microspheres, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.124925. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Biocompatible microlasers based on polyvinyl alcohol microspheres Van Duong Taa,*, Thiet Van Nguyen a, Quan Van Pham a, Toan Van Nguyen b,c Department of Optical Devices, Le Quy Don Technical University, Hanoi 100000, Vietnam.
b
Department of Physics, Le Quy Don Technical University, Hanoi 100000, Vietnam
c
Department of Quantum Optics, Faculty of Physics, VNU University of Science, Hanoi 100000,
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a
Vietnam. *
Corresponding author. E-mail address: [email protected]
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ABSTRACT
Biocompatible microlasers, generally made of bio-derived materials, are promising for biosensing and
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cell-tracking. These kinds of lasers offer favourable opportunities like biocompatibility and biodegradability but the materials used often require complicated synthesis and high cost. In this work, we demonstrate that polyvinyl alcohol (PVA), a synthetic water-soluble low-cost polymer, with fascinating properties such as good transparency, biocompatibility, biodegradability is an excellent candidate for making laser cavity. Using a simple and effective technique, dye-doped PVA microspheres can be fabricated with various sizes from 10 to 200 µm. These microspheres can act as excellent lasers under optical excitation with a lasing threshold of ~2 µJ/mm2 and Q factor of lasing modes of ~3000. The lasing mechanism is studied and it is ascribed to WGM. Size-dependent lasing
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characteristics including lasing spectrum, Q factor and lasing threshold are investigated. Owing to the ease of fabrication, the cost-effectiveness, the biocompatibility of the PVA material, our biocompatible microlasers are promising for future biosensing applications. Keywords: Microlaser, Whispering gallery mode, Biocompatible, Polyvinyl alcohol. -1-
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1. Introduction Microlasers are important for on-chip optical communications, data-processing, medical imaging and sensing [1-3]. Recently, miniaturized biocompatible lasers are highly emerging as a new tool for
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biosensing and cell-tracking [4-6]. A number of bio-derived materials have been explored for both gain
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medium such as green fluorescent protein [7], luciferin [8] and laser cavity including proteins [9-11], polysaccharides [11]. In comparison to inorganic materials, these biomaterials offer favourable opportunities like biocompatibility and biodegradability but they generally suffer from high cost and
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complicated synthesis [12]. As a result, exploring a cost-effective material that has similar properties to the above biomaterials is essential for the development of biocompatible lasers.
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Polyvinyl alcohol (PVA) is a synthetic water-soluble polymer with fascinating properties such as good transparency, biocompatibility, biodegradability and finds applications in numerous biomedical fields [13-15]. Despite these excellent properties, PVA has been rarely investigated as an appropriate material for a laser cavity.
Interestingly, PVA microspheres have been reported but they have not used as a cavity for laser generation. For instance, Young et. al. fabricated hydrogel PVA microspheres by combining microfluidic droplet generation with UV photopolymerization [16], Grumezescu et. al. fabricated polylactic-co-glycolic acid-polyvinyl alcohol microspheres using matrix-assisted pulsed laser
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evaporation [17]. It is well known that a spherical microstructure like a microdroplet or a microsphere can serve as a high quality (Q) factor cavity. In this structure, light can be trapped inside by total internal reflections near its interface and amplified by resonant circulation, the so-called, whispering
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gallery mode (WGM). As a result, the investigation of biocompatible PVA microspheres for laser action should be a motivating task. In this work, PVA microspheres are fabricated in a novel way using a simple and effective
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technique. Obtained PVA microspheres can serve as high Q factor resonators. By doping organic dye
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molecules to these structures, lasing emission is obtained under optical pumping. Lasing mechanism and lasing characteristics are studied systematically. 2.
Experimental methods
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2.1. Aqueous dye-doped PVA solution
Firstly, PVA solution, 5 wt%, was prepared by dissolving 0.25 g PVA powder (Mw = 89000-98000,
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from Sigma-Aldrich) in 5 mL deionized water at 70 °C for 3 hours. Secondly, Rhodamine B solution, 1 wt%, was obtained by directly dissolving 20 mg RhB ( >95% dye, from Sigma-Aldrich) in 2 mL deionized water. Finally, 5 mL PVA solution, 5 wt% and 0.5 mL RhB solution, 1 wt% were mixed to obtain the solution that used for making microspheres. The dry ratio of PVA and RhB in microspheres is 98 wt% and 2 wt%, respectively.
2.2. Fabrication of microsphere biolasers
The fabrication process is based on emulsion and dehydration as shown in Figure 1 [11, 18]. Firstly, a droplet of dye-doped PVA solution is injected in an uncured polydimethylsiloxane (PDMS) resin
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(Base of Sylgard 184 Silicon Elastomer, from Dow Corning) (Figure 1a). Secondly, the droplet is dispersed into multiple microdroplets. As water and PDMS are immiscible, the microdroplets are selfassembly formed within several seconds due to surface tension (Figure 1b). Subsequently, the whole mixture is heated at 80 °C for 90 minutes. This heating process completely dehydrates water from the
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microdroplets and solid-state dye-doped microspheres are obtained (Figure 1d). The PDMS was then removed by using a solvent (ethyl acetate). Finally, microspheres were extracted from the solvent and left at room temperature, in ambient conditions for an hour (to completely evaporate the solvent) thus
(a)
Self-assembly microdroplets (b)
Solidification via dehydration (c)
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Droplet injection
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they are ready for optical characterizations.
Solid-state microspheres (d)
Heating (80 C)
RhB-PVA solution
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PDMS
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Water vapour
Fig. 1. (a)-(d) Schematic diagram describing the fabrication process of dye-doped PVA microspheres from an aqueous solution.
2.3. Optical characterizations
Individual microspheres (placed on a glass substrate) were investigated by using a microphotoluminescence (μ-PL) setup. The pumping source was a frequency-doubled, Q-switched Nd:YAG laser (Litron Lasers) with a wavelength of 532 nm, a repetition rate of 10 Hz and a pulse duration of 4-7 ns. Excitation laser was guided (by using a set of mirrors) to the sample under an angle of ~70° to the
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normal of the substrate. Then, the excitation beam was focused on individual microspheres via a typical lens. The beam spot was ~ 350 μm in diameter. Emission was then collected from the top by a 10× objective and subsequently delivered to a spectrometer (AvaSpec-2048L from Avantes) for spectral
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recording. The spectral resolution of the spectrometer is ~ 0.2 nm. All optical measurements were done at room temperature and in ambient conditions. 3.
Results and discussion
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3.1. Dye-doped PVA microspheres
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Using our technique, dye-doped PVA microspheres with a variety of diameters can be produced. Figure 2a shows an optical microscope image of fabricated microspheres indicating clearly spherical structures. Based on the uniform colour, it is suggested that the dye molecules are well distributed
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throughout the microspheres. The spherical shapes of microspheres are even more visible when observing via a scanning electron microscope (SEM) (Figure 2b). In order to get a better view of the
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shape and its surface roughness, high magnification of a single microsphere is presented in Figure 2c. The circular shape and smooth surface can be seen clearly. These two factors are sufficient for high optical confinement and laser generation.
(a)
(c)
(b)
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100 µm
100 µm
30 µm
Fig. 2. (a) Optical image of RhB-doped PVA microspheres. (b) Scanning electron microscope (SEM) image of RhB-doped PVA microspheres with various sizes. (c) A high magnification SEM image of a single microsphere indicating a smooth outer surface.
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3.2. Lasing characteristics of a typical dye-doped PVA microsphere Lasing emission was observed from individual RhB-doped PVA microspheres under laser pulse
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pumping. Figure 3 plots the lasing performance of a typical microsphere which has a diameter of ~100 µm (estimated from the optical microscope image). The cross-over from spontaneous emission, at pump
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pulse energy (PPE) of 1.33 µJ, to lasing emission at PPE of 2.16 µJ can be seen clearly (Figure 3a). Upon higher PPE, the peak intensity of lasing modes increases sharply. When PPE rises two times from 2.16 to 4.21 µJ, the peak intensity increases 7.5 times, from 8000 to 60000 a. u. (Figure 3b). The
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integrated lasing intensity versus PPE is plotted in Figure 3c showing the nonlinear increase of the emission intensity. This result supports the lasing action and the lasing threshold is determined to be
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1.52 µJ. This value is comparable to other kinds of organic microsphere and droplet lasers [19-23]. In order to examine the free spectral range (FSR) and the full width at half maximum (FWHM) of lasing modes, a lasing spectrum at the short wavelength range from 619.5 to 624 nm was plotted in Figure 3d. It can be seen that the lasing modes are well separated. The FSR is about 0.84 nm and the FWHM (denoted as δλ) is around 0.2 nm (limited by our spectral resolution). In addition, lasing modes may exhibit a blue-shift behavior under increasing pump pulse energies. The blue-shift is caused by the thermal effect or heat generated by the pump laser [24]. However, in our
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work, we did not observe the blue-shift of lasing modes (under certain pump pulse energies), which suggests that the PVA based microlasers are relatively resistant to the thermal effect.
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3.3. Lasing mechanism The lasing mechanism is ascribed to WGM. It is well-known that light is confined inside a microsphere by multiple total internal reflections at the microsphere-air interface. Light travels around
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near the equator of the microsphere so the FSR of lasing modes can be estimated as λ2/πnD, where λ is
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resonant wavelengths; n and D are refractive index and diameter of the microsphere, respectively. Based on this equation, the FSR of the 100 µm-diameter microsphere is shown in Figure 3d can be well explained. Assuming, λ = 621.5 nm (close to the experimental measurement), n = 1.48 and D = 100 µm,
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the calculated FSR is 0.83 nm. This value is consistent with the above experimental value of 0.84 nm, which confirms the significant role of WGM on the laser action.
(b) 60000
Pump pulse energy 2.16 J 1.33 J
PL intensity (a. u.)
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8000
6000
4000
PL intensity (a. u.)
(a)
4.21 J
40000
20000
2000
0
560
600
640
0 600
680
610
2x105
1x105
Threshold (1.52 J)
(d)
630
0.84 nm
60000
PL intensity (a. u.)
Intergrated PL intensity (a. u.)
(c)
620
640
Wavelength (nm)
Wavelength (nm)
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40000
4.21 J
nm
20000
0
0
1
2
3
4
5
620
621
622
623
624
Wavelength (nm)
Pump pulse energy (J)
Fig. 3. (a) Emission spectra of the microsphere below and above the lasing threshold. (b) Typical lasing spectrum. (c) Integrated PL intensity of the microsphere versus pump pulse energy exhibits a distinct lasing threshold of 1.52 µJ. The inset shows the optical and PL image of the microsphere. The scale bars are 100 µm. (d) Close-up of the lasing spectrum showing a FSR of 0.84 nm and spectral linewidth of 0.2 nm. -7-
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In addition, it is well-known that emission from a WGM laser can be either transverse electric (TE) or transverse magnetic (TM). Generally, the polarization of the lasing modes can be determined
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experimentally using a polarizer [25]. In many cases, when both TM and TE modes coexist, the polarization of these modes can be determined theoretically via explicit asymptotic formulas [26].
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Herein, by fitting the lasing modes of several microsphere lasers with the explicit asymptotic formulas,
3.4. Size-dependence of lasing spectrum
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it is found that the lasing modes from the PVA microsphere lasers are mainly of TE characteristics.
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Lasing spectrum is related to the gain medium and cavity configuration. In this work, we fixed the dye and its concentration so the lasing spectrum would depend on the microsphere size. As shown in Figures 4a-4c, the number of lasing modes increase and the FSR gradually decreases with increasing microsphere diameter. Particularly, the FSR of 38 µm, 63 µm and 110 µm diameter microspheres are 2.1 nm, 1.3 nm and 0.6 nm, respectively. These values are again highly consistent with the prediction using equation, FSR = λ2/πnD. We investigated several more microspheres and their experimental FSR in comparison with calculation is plotted in Figure 4d. The agreement between the experiment and
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theory indicates that WGM resonance is indeed responsible for the lasing action in dye-doped PVA microspheres.
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(a)
4000
(b)
D = 38 m
D = 63 m
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1000
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1000
604 606 608 610 612 614 616 618
622 624 626 628 630 632 634 636
Wavelength (nm)
Wavelength (nm)
25000
(d) 2.5
D = 110 m
20000
Calculation Measurement
FSR (nm)
2.0
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(c)
2000
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3000
PL intensity (a. u.)
PL intensity (a. u.)
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PL Intensity (a. u.)
15000
1.5
1.0
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10000
600 602 604 606 608 610 612 614 Wavelength (nm)
0.5 30 40 50 60 70 80 90 100 110 Diameter of microsphere (m)
Fig. 4. (a)-(c) Lasing spectra of dye-doped PVA microspheres with a diameter of 38 µm, 63 µm and 110 µm, respectively. The wavelength range is 14 nm for all figures. The dash lines highlight the positions of typical lasing modes. (d) Comparison between calculated and experimental FSR of various microsphere lasers.
3.5. Size-dependence of Q factor and lasing threshold Q factor is an important parameter of a laser cavity that characterizes the optical confinement [27].
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A high Q factor cavity is desired for many applications such as studying nonlinear optical effects [28] and sensing ultra-small objects (nanoparticles, viruses) [29]. For a microsphere, the Q factor increases (due to low radiation loss) with increasing cavity size [30]. The Q factor of a lasing mode can be calculated as Q = λ/δλ. Figure 5 shows the Q factor of various microsphere lasers with a diameter from 35 µm to 63 µm. As expected, the Q factor tends to increase from 1900 to 3050 with a growing -9-
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microsphere size. A bigger microsphere should exhibit a higher Q factor (>3000) but we do not show the data here due to the limitation of our spectral resolution (~0.2 nm). The obtained Q factor is comparable with microsphere and microdisk lasers using other organic materials [10, 31].
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Lasing threshold is an important parameter of a laser. The lasing threshold depends on various
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factors including the microcavity Q factor, the mode volume and the value of the gain medium. In the microlaser cavity, pumping rate Rp is proportional to the intracavity pump power Pc and inversely proportional to the mode area Am [32]:
Pc Am
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Rp
(1)
In addition, the intracavity power Pc is proportional to the Q factor. We get,
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Rp
Q Am
(2)
From Equation (2), the pumping rate is higher in a higher Q factor microcavity (larger microsphere diameter), leading to a lower lasing threshold as Ith Rp1 . Moreover, the decrease in the mode area is also resulted in increasing the pumping rate and reducing the lasing threshold. However, how exactly the cavity Q factor and the mode area influence on the lasing threshold is still a challenging issue [33]. Previous experimental studies on WGM microdisk lasers have demonstrated that lasing threshold decreases with increasing the microdisk diameter and a 1/D2 relationship was obtained for disk diameter
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and lasing threshold [34, 35]. For microsphere lasers, it is expected that the lasing threshold should decrease with increasing their size. Figure 5b shows a plot of the lasing threshold versus PVA microsphere diameter. Despite some fluctuations, the lasing threshold scales inversely with the power of PVA microsphere diameter.
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Particularly, when the microsphere diameter increases 3 times, from 35 µm to 110 µm, the lasing threshold decreases much more, about 11 times, from 5.7 µJ to 0.5 µJ. Generally, a low lasing threshold
concentration [36]. (b) 6
(a)
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3000
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is more preferred. In this work, the lasing threshold can be reduced further by optimizing the dye
Lasing threshold energy (J)
5
2500
4 3
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Q factor
35
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2000
40
45
50
55
60
65
Diameter of microsphere (m)
2 1
0 30 40 50 60 70 80 90 100 110 Diameter of microsphere (m)
Fig. 5. (a) Q factor and (b) lasing threshold versus microsphere diameter.
4.
Conclusion
We have demonstrated that PVA is an excellent candidate for laser cavities. Dye-doped PVA microspheres with various sizes are fabricated using a novel and effective way. Under optical excitation,
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these microspheres can work as excellent lasers with the lasing threshold of ~2 µJ/mm2. The lasing mechanism is studied and it is ascribed to WGM. Size-dependent lasing characteristics including lasing spectrum, Q factor and lasing threshold are investigated. The results show that the Q factor of microsphere lasers is proportional to its diameter. Q factor values, ranging from 1900 to 3050, are
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obtained for microspheres with a diameter from 35 µm to 63 µm. In addition, the lasing threshold is inversely proportional to the power of the PVA microsphere diameter. Owing to the ease of fabrication and the biocompatibility of the PVA material, our biocompatible microlasers are promising for cell
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imaging and biosensing applications.
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Acknowledgements
This research is funded by Vietnam National Foundation for Science and Technology Development
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(NAFOSTED) under grant number 103.03-2017.318.
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Journal Pre-proof *Declaration of Interest Statement
Declaration of interests ☒ 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.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: