Organic Janus Microspheres: A General Approach to All-Color Dual-Wavelength Microlasers

Article abstract of DOI:10.1021/jacs.9b00362

We propose a general approach for obtaining dual-wavelength organic microlasers in amphiphilic Janus resonators, where hydrophilic and hydrophobic dyes can be spatially separated via polarity-driven encapsulation. Low-threshold dual-wavelength lasing was achieved in a single Janus particle with well-modulated output. This universal approach enables flexibly designing the lasing wavelength of the Janus microlasers in the full visible spectrum by systematically altering the encapsulated laser dyes. Our findings demonstrate a promising route to the photonic integration at the micro-/nanoscale that may lead to the innovation of concepts and device architectures for multifunctional optoelectronic applications.

Full text of DOI:10.1021/jacs.9b00362

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Communication
Organic Janus Microspheres: A General Approach
to All-Color Dual-Wavelength Microlasers
Cong Wei, Yuxiang Du, Yingying Liu, Xianqing Lin, Chuang Zhang, Jiannian Yao, and Yong Sheng Zhao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00362 • Publication Date (Web): 20 Mar 2019
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Organic Janus Microspheres: A General Approach to All-Color Dual-
Wavelength Microlasers
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Cong Wei,†,‡ Yuxiang Du,†,§ Yingying Liu,†,§ Xianqing Lin,†,§ Chuang Zhang,†,§ Jiannian Yao,†,§
and Yong Sheng Zhao*,†,§
†
Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
‡College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
§University of Chinese Academy of Sciences, Beijing 100049, China.
Supporting Information Placeholder
organic laser dyes with distinct polarities can be selectively en-
capsulated into the two hemistructures of a single Janus par-
ticle,22-25 which will result in a typical side-by-side cavity ge-
ometry, providing a versatile approach to spatially separating
the gain materials. Moreover, the high compatibility enable to
spatially incorporate various gain media,26,27 which would in
principle provide a universal and robust strategy to construct
dual-color microlasers with lasing wavelengths tunable within
the entire visible region. Nevertheless, this is still limited by
the synthesis of Janus microparticles with tunable composi-
tions above the diffraction limit that can provide necessary
feedback for lasing.
ABSTRACT: We propose a general approach for the achieve-
ment of dual-wavelength organic microlasers in amphiphilic
Janus resonators, where hydrophilic and hydrophobic dyes
can be spatially separated via the polarity driven encapsula-
tion. Low-threshold dual-wavelength lasing was successfully
obtained in a single Janus particle with well-modulated out-
put. This universal approach enables to flexibly design the las-
ing wavelength of the Janus microlasers in the full visible spec-
trum by systematically altering the encapsulated laser dyes.
Our findings demonstrate a promising route to the photonic
integration at micro/nanoscale that may lead to the innova-
tion of concepts and device architectures for multifunctional
optoelectronic applications.
In this work, we report the general synthesis of fluorescent
Janus microstructures for dual-wavelength lasers, where the
two sides of each isotropic particle can be distinctively doped
with hydrophilic and hydrophobic dyes. Low threshold dual-
wavelength lasing was achieved in a single Janus particle and
the lasing performance was highly dependent on the Janus
structure, providing an effective modulation of the output sig-
nals. This approach is applicable for all hydrophilic and hydro-
phobic organic dyes, and the emission color of the Janus lasers
was preliminarily tuned from blue-pink to green-orange by
systematically altering the encapsulated dyes, showing tre-
mendous potential to the design and fabrication of dual-wave-
length lasers in all-color region. The results are anticipated to
expand the application range of Janus nanomaterials, and pro-
vide helpful enlightenment for the optoelectronic integration
in compact systems.
Generating multicolor laser in a single compact system is cru-
cial for biological labeling,1 full-color laser display,2 multi-
channel optical communication,3 etc. The ever-increasing de-
mand for wide display color gamut and high-throughput pho-
tonic devices calls for nanoscale multi-wavelength coherent
light sources capable of emitting across the full visible spec-
trum.4,5 Till now, multi-wavelength micro/nanolasers were re-
alized mainly through integrating gain media with different
bandgaps on a single device, where the lasing wavelengths
correspond to the bandgaps of the respective materials.6-8 Due
to the absorption of short-wavelength emission by the nar-
row-gap material, lasing in the high-energy region is severely
suppressed.9,10 Spatially separating different gain materials in
side-by-side geometries can minimize the absorption loss,11-13
demonstrating an alternative solution to this problem.14-16
Most of these composite cavities were constructed from inor-
ganic lateral heterostructures, which requires overcoming the
intrinsic difficulties in achieving epitaxial growth of the mis-
matched materials with different emissions, and thus limits
the accessible spectral range of multi-wavelength lasing.
The fabrication of dye doped dual-color fluorescent Janus
particles is illustrated in Figure 1a. According to the minimum
Gibbs free energy criterion, guest laser dye molecules can be
well dispersed in the host matrix if their interfacial energy (γH-
G) is smaller than the entropy increment (TΔS) induced by the
mixing (ΔG=γH-G-TΔS); otherwise they will self-separate at the
thermodynamic equilibrium state.28 Thus, the hydrophilic
part of the amphiphilic Janus microsphere will selectively en-
capsulate hydrophilic dyes, while its hydrophobic counterpart
prefers hydrophobic molecules, forming an ideal heterogene-
ously luminescent microstructure for the achievement of
dual-wavelength lasing via the whispering-gallery-mode
(WGM) resonance.29 Polystyrene (PS) and poly (methyl meth-
acrylate) (PMMA), with different polarities, were selected as
Janus microparticles, whose surfaces have two distinct
physical properties, allow two different properties to occur on
the same particle.17-19 Among them, organic polymer Janus
particles are typical representatives, with one-half of each par-
ticle composed of hydrophilic groups and the other half hy-
drophobic ones.20,21 Based on the principle of like dissolves like,
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the matrix materials to create Janus microlasers. The Janus
obtained from various PS:PMMA weight ratios: (f) 3:4; (g) 1:1;
(h) 4:3. Scale bars: 2 μm.
structures were obtained by inducing phase separation of
PS/PMMA within the micrometer-sized emulsion droplets
(see Supporting Information). 1,4-Bis(α-cyano-4-diphenyla-
minostyryl)-2,5-diphenylbenzene (CNDPASDB, hydrophobic)
and rhodamine-101 (Rh101, hydrophilic), with different emis-
sion and polarity (Figure S1), were adopted to provide optical
gain for the operation of dual-wavelength laser.
The spherical Janus geometry was confirmed by optical mi-
croscopy and SEM images (Figure 1c-d). A clear boundary was
observed between the two hemispheres, which can be distin-
guished by their distinct surface morphologies, with one side
being very smooth while the other slightly rough. The diame-
ter of the Janus spheres is in direct proportion to the size of
the micelles, which should strongly depend on the interfacial
tension between water and CH2Cl2. The interfacial tension
would increase with increasing amount of added polymer
blends, generating larger micelles with smaller specific surface
areas to reduce the interfacial energy of the whole system.
This is exactly what we have observed from the controlled ex-
periments, where the diameter of the Janus spheres was finely
tuned from 3 to 15 μm through increasing the concentration of
PS-PMMA blends (Figure S3). Furthermore, by altering the
weight ratio of PMMA:PS, we were able to modulate the size
ratio between the two hemispheres. As shown in Figure S4,
the rough segment of the obtained Janus spheres became
larger with increasing PS content. Therefore, we can identify
that the side with rough surface is the PS-rich phase, while the
smooth hemispheres are composed of PMMA-rich phase.
In
a
typical preparation (Figure 1b), well mixed
9
CNDPASDB/Rh101/PMMA/PS/CH2Cl2 solution was first
added into the cetyltrimethylammonium bromide (CTAB)
aqueous solution to form an oil-in-water emulsion. Hydro-
phobic CH2Cl2 solution was subsequently encapsulated into
the hydrophobic interior of the CTAB micelles under vigorous
stirring .30,31 With the evaporation of CH2Cl2, spherical drop-
lets consisting of PS and PMMA blends underwent phase sep-
aration to form Janus microspheres. According to the scheme
in Figure 1a, hydrophobic CNDPASDB molecules prefer to be
embedded into the nonpolar PS-rich hemispheres, while the
hydrophilic Rh101 molecules will be dispersed in the polar
PMMA matrix. The spatially selective doping behavior (Figure
S2) drove the formation of the final dual-color Janus micro-
spheres.
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Under UV excitation, yellow and red fluorescence emissions
were respectively observed from the two hemispheres (Figure
1e). Figure S5 shows the spatially resolved PL spectra collected
from the two hemispheres in a single particle. which indicate
the CNDPASDB emission dominates the spectrum collected
from the left part, whereas the Rh101 dominates that from the
right. The relative size of the CNDPASDB section increased
with the increase of added PS, while the Rh101-dominant sec-
tion was positively correlated to the added PMMA (Figure 1f-
h). This further testifies that hydrophobic CNDPASDB mole-
cules prefer to be embedded into the nonpolar PS-rich phase,
while hydrophilic Rh101 molecules prefer to be incorporated
into the polar PMMA. This discriminative doping behavior of
laser dyes provides a general access to the side-by-side cou-
pled optical microcavities.
Hydrophobic polymer
Hydrophilic polymer
a
Hydrophobic dye
Hydrophilic dye
Amphiphilic Janus
Bicolor Janus
microsphere
microsphere
γH-G﹥ T△S
γH-G﹤ T△S
Polarity driven
encapsulation
immiscible
miscible
CTAB
PS
water
PMMA
CNDPASDB
water
Rh101
b
water
evaporation
stirring
CH2Cl2
emulsification
phase separation
CH2Cl2
dye@ Janus
sphere
c
d
e
a
b
Power density (kw/cm2)
5.9
5.2
4.8
4.3
3.6
Eth= 4.1 kw/cm2
Eth= 4.9 kw/cm2
f
g
h
c
d
e
3:4
Δλ1=13.2 nm
Δλ2=12.6 nm
Δλ2=13.1 nm
1:1
4:3
Δλ1=10.4 nm
Δλ1=6.4 nm
Δλ2=10.2 nm
Figure 1. (a) Scheme for the design of Janus microspheres with
dual-color emission. (b) Illustration for the synthesis of the
dual-color Janus microspheres. (c) Optical microscopy images
of the Janus microspheres. Scale bar: 5 μm. (d) SEM image of
the Janus microspheres. Scale bar: 2 μm. (e) PL image of the
CNDPASDB-Rh101 doped Janus microspheres. Scale bar: 5 μm.
PL images of the CNDPASDB-Rh101 doped Janus microspheres
Figure 2. (a) PL spectra of a typical CNDPASDB-Rh101 doped
Janus microsphere as a function of pump energy. Inset: PL im-
ages of the microsphere excited with UV light (top) and fo-
cused laser (bottom). Scale bars: 5 μm. (b) Power dependent
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intensity for the CNDPASDB and Rh101 lasing. (c) PL spectra
of the CNDPASDB-Rh101 doped Janus microspheres with dif-
ferent PS:PMMA ratios. Insets: corresponding PL images.
Scale bars: 5 μm. (d) Relationship between λ12/Δλ1 and the di-
ameter of the PS-rich hemisphere at CNDPASDB emission. (e)
a
b
c
d
2
Relationship between λ2 /Δλ2 and the diameter of the PMMA-
rich hemisphere at Rh101 emission.
520
e
f
540
8
9
When a Janus microsphere was excited with a focused laser
beam (Figure S6), bright emissions with distinct colors were
separately obtained from the two hemispheres (Figure 2a, in-
set), indicating that the Janus geometry can effectively mini-
mize the absorption loss of the short-wavelength emission.
Figure 2a shows the PL spectra of a representative Janus mi-
crosphere pumped with increasing power. At a low pumping
power, two broad bands with peak wavelengths at 560 and 630
nm appeared, consistent with the emissions of CNDPASDB
and Rh101, respectively. When the power was increased to 4.3
kw/cm2, sharp peaks with FWHM of ~0.5 nm started to
emerge at Rh101 emission. When the pumping power was fur-
ther elevated to 5.2 kw/cm2, the intensity of the peaks at Rh101
emission strongly increased; meanwhile, new sharp lines
emerged from the CNDPASDB band, and a dual-color laser
was achieved. Figure 2b depicts the power-dependent inten-
sity for the lasing at 568 and 620 nm, exhibiting two thresh-
olds, 4.1 and 4.9 kw/cm2, respectively. This is resulted from the
transition from spontaneous to stimulated emissions of the
two dyes.
560
b
500
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c
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d
490
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380
Figure 3. (a-d) PL images of the CNDPASDB-Rh101 doped Ja-
nus microsphere excited with UV light (a) and focused laser at
different positions (b-d). Scale bars: 5 μm. (e) Lasing spectra
of the CNDPASDB-Rh101 doped Janus microsphere excited at
different positions shown in b-d. (f) Chromaticity of the lasing
from b-d
This coupled cavity geometry enables to achieve color-tun-
able laser output from the Janus microsphere by pumping dif-
ferent segments. Figure 3a shows the PL image of a typical
dual-color sphere with PS:PMMA ratio of ~1:1. By adjusting the
excitation position, the output color was tuned from red to
yellow (Figure 3b-e). Only the lasing emission from Rh101 was
observed when the excitation was focused on the PMMA part,
while the CNDPASDB lasing was achieved when the excitation
moved onto the PS. Simultaneous dual-color lasing was ob-
tained when the boundary of the two hemispheres was irradi-
ated. Figure 3f shows the calculated chromaticity for the three
lasing spectra, which exhibits a wide tuning range from yellow
(0.42, 0.57) to red (0.71, 0.29).
The lasing spectra of the Janus microspheres with different
diameters were characterized (Figure S7a) to study the micro-
cavity effects. For WGM-type resonance, the mode spacing, Δλ,
and the diameter, D, should satisfy the equation λ2/Δλ=nπD,
where λ is the resonance wavelength and n is the group refrac-
tive index.32 Figure S7b-c plots λ2/Δλ against D at the two
emission bands, which clearly show the linear relationships.
The calculated n (1.59 at CNDPASDB, and 1.58 at Rh101 bands)
are consistent with the intrinsic refractive index of the PS and
PMMA, which indicates that the PL modulation in the Janus
microsphere is attributed to the WGM-type resonance. The
measured Q factors are on the order of 103 (Figure S8), which
is pretty high for organic resonators. The size ratio between
the two hemispherical parts was modulated to figure out the
contribution of each part to the WGM resonance (Figure 2c).
As shown in Figure S9, the value of λ2/Δλ measured from the
CNDPASDB band are strikingly different from that at the
Rh101 band, unless the size ratio is 1:1 (note that the refraction
indices are approximately identical). Furthermore, as shown
in Figure 2d-e, the λ2/Δλ values are linearly correlated to the
diameter of corresponding hemispheres, rather than to that of
the whole Janus particle. This demonstrates that the parallelly
aligned hemispheres in the Janus structure form two inde-
pendent WGM cavities, which is further confirmed by the nu-
merical simulation (Figure S10).
The polarity driven encapsulation is universal to the hydro-
philic and hydrophobic dye pairs; therefore, we can freely de-
sign the lasing color of the two hemispheres by doping with
various dyes. As shown in Figure 4a-b, blue-pink colored Janus
microspheres were obtained if we replace the doped dyes with
cyano-substituted oligo(p-phenylenevinylene) (CNDPDSB)
and sulforhodamine 101. The non-polar CNDPDSB molecules
prefer to be embedded into the PS-rich hemispheres, while the
ionic sulforhodamine 101 prefer to be incorporated into the
PMMA. Under optical pumping, two separated sets of lasing
were observed in the range of 449-474 and 606-631 nm (Figure
4c), consistent with the emissions of CNDPDSB and sulforho-
damine 101, respectively (Figure S11). In a similar way, cyan-red
and green-orange dual-color lasers were also successfully
achieved from the coumarin 153-rhodamine 6G (Figure 4d-f),
and coumarin 6-rhodamine B (Figure 4g-i) double-doped Ja-
nus microspheres. The corresponding chromaticity of the
three dual-color lasing spectra was illustrated in Figure S12,
displaying a wide tunable range covering the whole visible col-
ors. This provides a general strategy for the programmable de-
sign and construction of compact all-color dual-wavelength
lasers.
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CNDPDSB
N
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N+
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c1
c2
-
SO3
CN
CN
-
Na+
SO3
Sulforhodamine 101
d
f
f1
O
O
f2
Coumarin 153
HCl
CF3
N
O
N
H
9
N
O
O
Rhodamine 6G
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g
i
i1
OH
O
i2
Coumarin 6
N
O
N+
Cl-
N
Rhodamine B
S
N
O
O
Figure 4. Molecular structures of sulforhodamine 101 and
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microspheres doped with corresponding dyes. (c, f, i) Normal-
ized lasing spectra of the Janus microspheres shown in the in-
set. Scale bars: 5 μm.
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In summary, we developed a general approach to the syn-
thesis of binary fluorescent organic Janus microspheres, which
serve as side-by-side coupled WGM cavities for the achieve-
ment of dual-color microlasers. The connected WGM cavities
can be separately modulated, offering the possibility to
achieve novel photonic functionalities in the compact Janus
structures. Moreover, the flexibility and compatibility of the
Janus microspheres enable to flexibly design the lasing wave-
length from the two hemispheres by doping with various dyes,
demonstrating a general synthetic strategy for the dual-color
microlasers across the full visible spectrum. We anticipate
that our results will provide beneficial enlightenment for the
design of multifunctional nanophotonic materials with novel
performances and applications.
ASSOCIATED CONTENT
Supporting Information
Experimental details and additional data. The Supporting In-
formation is available free of charge on the ACS Publications
website.
AUTHOR INFORMATION
Corresponding Author
yszhao@iccas.ac.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported financially by the Ministry of Science
and Technology of China (Grant No. 2017YFA0204502), and
the National Natural Science Foundation of China (Grant Nos.
21790364, 21805246 and 21533013).
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Amphiphilic Janus
microsphere
Bicolor Janus
microsphere
γH-G﹥ T△S
γH-G﹤ T△S
Polarity driven
encapsulation
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immiscible
miscible
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