Optically band-tunable color cone lasing emission in a dye-doped cholesteric liquid crystal with a photoisomerizable chiral dopant

Article abstract of DOI:10.1063/1.3364942

This work demonstrates the feasibility of an optically band-tunable color cone lasing emission (CCLE) based on a dye-doped cholesteric liquid crystal with a photoisomerizable chiral dopant. Experimental results indicate that the lasing band of the formed CCLE can be tuned optically among various color regions by adjusting the UV irradiated fluence. The optical band tunability of the laser is attributed to the presence of two chiral agents with twisting powers of opposite signs in the cell and the UV-irradiation-induced decrease of the right-handed twisting power of the photoisomerizable chiral dopant via t_ra_ns→c_is isomerization, subsequently inducing the other chiral agent to reduce the structural pitch of the cell. Total tunable wavelength range of the laser exceeds 100 nm. Moreover, the band-tunable laser exhibits a high spectral stability under illumination of a visible light or thermal treatment.

Full text of DOI:10.1063/1.3364942

Optically band-tunable color cone lasing emission in a dye-doped cholesteric liquid
crystal with a photoisomerizable chiral dopant
C.-R. Lee, S.-H. Lin, H.-S. Ku, J.-H. Liu, P.-C. Yang, C.-Y. Huang, H.-C. Yeh, and T.-D. Ji
Citation: Applied Physics Letters 96, 111105 (2010); doi: 10.1063/1.3364942
View online: http://dx.doi.org/10.1063/1.3364942
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APPLIED PHYSICS LETTERS 96, 111105 ͑2010͒
Optically band-tunable color cone lasing emission in a dye-doped
cholesteric liquid crystal with a photoisomerizable chiral dopant
C.-R. Lee,1,a͒ S.-H. Lin,1 H.-S. Ku,1 J.-H. Liu,2 P.-C. Yang,3 C.-Y. Huang,1 H.-C. Yeh,4 and
T.-D. Ji1
1Institute of Electro-Optical Science and Engineering and Advanced Optoelectronic Technology Center,
National Cheng Kung University, Tainan 701, Taiwan
2Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
3Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 320, Taiwan
4Graduate Institute of Electro-Optical Engineering, National Kaohsiung First University of Science
and Technology, Kaohsiung 811, Taiwan
͑Received 31 December 2009; accepted 24 February 2010; published online 16 March 2010͒
This work demonstrates the feasibility of an optically band-tunable color cone lasing emission
͑CCLE͒ based on a dye-doped cholesteric liquid crystal with a photoisomerizable chiral dopant.
Experimental results indicate that the lasing band of the formed CCLE can be tuned optically
among various color regions by adjusting the UV irradiated fluence. The optical band tunability of
the laser is attributed to the presence of two chiral agents with twisting powers of opposite signs in
the cell and the UV-irradiation-induced decrease of the right-handed twisting power of the
photoisomerizable chiral dopant via trans→cis isomerization, subsequently inducing the other
chiral agent to reduce the structural pitch of the cell. Total tunable wavelength range of the laser
exceeds 100 nm. Moreover, the band-tunable laser exhibits a high spectral stability under
illumination of a visible light or thermal treatment. © 2010 American Institute of Physics.
͓doi:10.1063/1.3364942͔
Dye-doped cholesteric liquid crystal ͑DDCLC͒ lasers
have been thoroughly studied owing to their ease of fabrica-
tion, their interesting distributed feedback ͑DFB͒ lasing
mechanism associated with the unique photonic crystal-like
band structure of the cholesteric liquid crystal ͑CLC͒, and
related potential applications.1–6,8 As the fluorescence dyes
doped in the CLC are excited, the spontaneously emitted
fluorescence within the stop bands is inhibited to propagate
but enhanced at the band edges. The fluorescence at band
edges can propagate via a multireflection,7 subsequently pro-
ducing an extremely small group velocity and an extremely
high density of photonic state. Based on the DFB effect of
the active DDCLC multilayer in the multireflection process,
the rates of spontaneous and stimulated emissions at band
edges can both be amplified, enabling a high gain to exceed
the loss for a low-threshold lasing emission.1
Our recent study demonstrated that a “single pitched”
DDCLC laser can simultaneously emit a wide-band lasing
emission with an angular dependence on the wavelength.8
This emission is called color cone lasing emission ͑CCLE͒,
which differs from the conventional scenario that, at most,
two lasing peaks at the band edges can simultaneously occur
along the helical axis in a DDCLC.1–6 To implement such a
CCLE in practice, this study elucidates the optical band tun-
ability of a color cone laser based on a DDCLC cell with a
photoisomerizable chiral dopant. Experimental results indi-
cate that the lasing band of the DDCLC laser can be tuned
optically among various color regions by varying the UV
irradiated fluence. The basic mechanism responsible for in-
ducing the band tunability of the laser is attributed to the
presence of two chiral agents with twisting powers of oppo-
site signs in the cell and the decay of the right-handed twist-
ing power of the photoisomerizable chiral dopant via trans
→cis isomerization, subsequently causing the other chiral
agent to reduce the structural pitch of the cell. The overall
tunable wavelength range for the tunable laser exceeds 100
nm. Furthermore, the band-tunable laser displays a high
spectral stability under illumination of a visible light or ther-
mal treatment.
The nematic liquid crystal ͑NLC͒, left-handed chiral
agent, two laser dyes, and photoisomerizable chiral dopant
͑right-handed͒ used herein are ZLI2293 ͑from Merck͒,
S811 ͑from Merck͒, 4-Dicyanmethylene-2-methyl-6-͑p-
dimethylaminostyryl͒-4H-pyran ͑DCM͒ and Pyrromethene
567 ͑P567͒ ͑from Exciton͒, and Isosorbide 2,5-bis͑4-
methoxy-cinnamate͒ ͑IBM͒, respectively. The homogeneous
mixture, with
a
mixing ratio of ZLI2293:S811:
IBM:DCM:P567 of 66.29:31.42:1.47:0.57:0.25 wt %, is
prepared. The helical twisting power ͑HTP͒ values of S811
and IBM in ZLI2293 are Ϫ10.358 and 39.8 ␮m−1, respec-
tively. The empty cell is prefabricated by integrating two
indium-tin-oxide-coated glass slides separated with two
23 ␮m thick plastic spacers. Both glass slides in the empty
cell are precoated with a polyvinyl alcohol ͑PVA͒ film, and
prerubbed in a parallel direction. The above mixture is then
injected into the empty cell in order to form a DDCLC cell
via capillary effect. Next, the DDCLC cell is placed in a
clean and opaque specimen box at room temperature for
about 7 days, enabling CLC in the cell to self-organize
slowly to a perfect planar structure. The determined absorp-
tion and fluorescence emission spectra ͑not shown herein͒ of
the DDCLC cell in the isotropic phase show that the maxima
of the absorption and fluorescence emission of the cell are
located at about 526 and 574 nm, respectively. When the
wavelength exceeds 575 ͑690͒ nm, the absorption ͑the fluo-
rescence emission͒ is extremely weak and can be neglected.
a͒
Electronic mail: crlee@mail.ncku.edu.tw.
0003-6951/2010/96͑11͒/111105/3/$30.00
96, 111105-1
© 2010 American Institute of Physics
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111105-2
Lee et al.
Appl. Phys. Lett. 96, 111105 ͑2010͒
FIG. 1. ͑Color online͒ Four obtained CCLE patterns distributed from long-
to short-wavelength region based on the DDCLC cell under preillumination
of one UV light with IUV=3 mW/cm2 and tUV=͑a͒ 0 s, ͑b͒ 20 s, ͑c͒ 80 s,
and ͑d͒ 160 s, respectively.
FIG. 3. ͑Color online͒ Variations in the wavelength of the lasing signals at
the LWE of CLCRB in the obtained CCLE patterns at ␪=0° –45° with ͑a͒
the irradiated time ͑0–160 s͒ and ͑b͒ the irradiated intensity ͑0–3 mW/cm2͒
of the UV light based on the DDCLC cell.
Because the wavelength ͑532 nm͒ of the pumped pulses is
near 526 nm, the cell can be excited efficiently.
This work employs two experimental setups for detect-
ing the reflection and lasing spectra of the DDCLC cell,
respectively, at each oblique angle ␪ ͑relative to the cell nor-
mal N͒. Our previous study includes the same setups and
the same method and parameters in the measurements of
reflection and lasing spectra of the cell and, therefore, are
not repeated here for brevity.8 The optically band-tunable
lasing experiments are performed using one UV light source
͑Ͻ400 nm͒ with a changeable intensity ͑IUV͒ to preirradiate
or not the cell for a variable time ͑tUV͒ before the reflection
and lasing spectra measurements.
Figures 1͑a͒–1͑d͒ display four obtained CCLE patterns
on the screen ͑behind the cell͒ by the excitation of the
pumped pulses ͑E=15 ␮J/pulse͒, if the DDCLC cell
with the photoisomerizable IBM is preirradiated by one
uniform UV beam with a fixed irradiated intensity of IUV
=3 mW/cm2 and a variable irradiated time of tUV=0 s,
20 s, 80 s, and 160 s, respectively. Figures 2͑a͒–2͑d͒ further
display the detailed lasing spectra of the four obtained
CCLEs in Figs. 1͑a͒–1͑d͒, respectively, as well as the corre-
sponding reflection spectra at various oblique angles ͑␪
=0° –50°͒. Obviously, the wavelength of the lasing signal in
each CCLE decreases with an increasing oblique angle and is
located close to the long wavelength edge ͑LWE͒ of the CLC
reflection band ͑CLCRB͒ at each oblique angle. Our previ-
ous study includes both the explanation of the mechanism for
the formation of the CCLE and its related features and, there-
fore, are not repeated here for brevity.8 In particular, the red,
orange, yellow, and green lasing rings at nearly 35° ͓as dis-
played in Figs. 1͑a͒–1͑d͒, respectively͔ are similar in that
their wavelength at 35° is close to the value at which the
LWE of CLCRB, measured at nearly 35°, overlaps the short
wavelength edge of CLCRB, measured at 0°; briefly,
␭
͑35°͒=␭ ͑0°͒ ͓as evident in the reflection spectra
LWE
SWE
measured at 35° and 0° ͑green and black, respectively͒ in
Figs. 2͑a͒–2͑d͔͒. As is widely assumed, beams of fluores-
cence with the same wavelength ͓␭
͑35°͒=␭ ͑0°͔͒ and
LWE
SWE
propagating at nearly 35° and 0° may reinforce indirectly
each other owing to the enhancements of the related respec-
tive rates of spontaneous emission.8 Efforts are currently un-
derway to thoroughly elucidate the formation of the unique
lasing ring at nearly 35° in each CCLE. Instead, this study
investigates the optical band-tunable feature of such a
DDCLC color cone laser.
Next, rather than the irradiated time, the irradiated inten-
sity of the UV light on the cell is changed to examine the
optical band-tunability of the CCLE in the DDCLC. Four
CCLE patterns and their detailed lasing spectra and the
corresponding reflection spectra of the cell measured at
␪=0° –50° ͑not shown herein͒, which resemble those lasing
patterns displayed in Figs. 1͑a͒–1͑d͒ and those spectra in
Figs. 2͑a͒–2͑d͒, respectively, can also be obtained if the
cell is preilluminated by the UV light with a fixed tUV
=160 s and a variable IUV=0 mW/cm2, 0.59 mW/cm2,
1.43 mW/cm2, and 3 mW/cm2, respectively. Figures 3͑a͒
and 3͑b͒ plot similar experimental results with respect to the
detailed lasing bands within 0°–45° in the tunable CCLE by
varying tUV and IUV, respectively. Such a similarity between
the variations of the lasing wavelength of the CCLE with tUV
and IUV implies that the UV irradiated fluence ͑i.e., the prod-
uct of tUV and IUV͒ is responsible for the band-tunable effect
of the color cone laser. Therefore, experimental results in
Fig. 3 clearly indicate that the lasing band of the laser can be
tuned optically from the long-wavelength region of 599.35–
671.11 nm ͑orange-reddish͒ to the short-wavelength region
of 564.27–604.42 nm ͑greenish-orange͒ by increasing the
UV irradiated fluence from 0 to 480 mJ/cm2. This optical
band tunability of the DDCLC color cone laser differs from
the conventional scenario in which normally only a wave-
length of edge lasing signal alone emitted at 0° is optically
tunable in common DDCLC lasers described elsewhere.2–5
Because a separate experiment ͑data not shown͒ has con-
firmed the lack of such a band-tunable feature of the CCLE
based on a similar DDCLC cell without IBM chiral agent,
the optical band tunability of the CCLE illustrated above
͑Figs. 1–3͒ is certainly attributed to the modification of the
structural pitch of the cell via the UV-irradiation-induced
trans→cis isomerization of the isomerizable IBM. Details
are as follows. The chiral dopant, IBM ͑synthesized and of-
fered by our colleagues, J.-H. Liu et al.͒, is a right-handed
FIG. 2. ͑Color online͒ Detailed lasing spectra of the four obtained CCLE
patterns and corresponding reflection spectra measured at ␪=0–50° based
on the DDCLC cell under preillumination of one UV light with IUV
=3 mW cm−2 and tUV=͑a͒ 0 s, ͑b͒ 20 s, ͑c͒ 80 s, and ͑d͒ 160 s,
respectively.
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111105-3
Lee et al.
Appl. Phys. Lett. 96, 111105 ͑2010͒
This study also examines the spectral stability for the
tunable DDCLC color cone laser. Experimental results indi-
cate that, under the post-illumination of a visible light or
post-thermal treatment ͑i.e., several heating-cooling cycles of
the cell͒, the lasing band of the tunable color cone laser
remains unchanged in the spectrum. This finding suggests
that the trans→cis isomerization for IBM is irreversible,
which is in contrast with the unstable features of usual azo-
materials, in which the cis-dyes can reverse to trans-state
by slow thermal or rapid optical ͑under the illumination
of a light in long-wavelength region͒ cis-trans back
isomerization.2,4,5 Efforts are currently underway in our
laboratory to demonstrate that neither a CCLE effect nor an
associated band-tunable feature are found in a pumped
pulses-induced unstable and, thus, an imperfect CLC struc-
ture based on a DDCLC with a reversibly photoisomerizable
azo-material. Consequently, the irreversibility of the IBM
significantly increases the spectral stability of such a tunable
color cone laser described in this study.
FIG. 4. ͑Color online͒ ͑a͒ Chemical structures of the photoisomerizable
chiral dopant IBM at rodlike trans- and bent cis-states. ͑b͒ Variation in the
measured absorption spectra ͑250–500 nm͒ of the IBM in ZLI2293 ͑without
doping laser dyes͒ in the isotropic state with the UV irradiated time
͑0–600 s͒ at a fixed intensity of 3 mW/cm2.
In summary, this work develops an optically band-
tunable color cone laser based on a DFB DDCLC cell with a
photoisomerizable chiral dopant. Experimental results indi-
cate that the lasing band of the formed CCLE based on the
DDCLC cell can be optically tuned among various color
regions by varying the UV irradiated fluence. The total opti-
cally tunable range of the lasing band for the DDCLC laser
exceeds 100 nm. Such a laser with a wide and stable opti-
cally band tunability presented herein has unique advantages
beyond those unstable single-wavelength-tunable lasers
based on azo-associated DDCLC cells reported previously.
chiral compound with two cinnamoyl ͑CvC͒ photochro-
matic group.9 Figure 4͑a͒ shows the chemical structures for
the trans- and cis-isomers of IBM. Generally, this isomeriz-
able chiral agent remains stable at a rodlike trans-state in the
dark. Once irradiated by the UV light, the trans-IBM con-
verts into the cis-state with two possible bent structures
͓Fig. 4͑a͔͒ via trans→cis isomerization. Figure 4͑b͒ shows
the decrease in the absorption of IBM in ZLI2293 ͑without
doping laser dyes͒ in the isotropic state within the spectrum
region of 300–340 nm when increasing the irradiated time
of the UV light from 0 to 600 s at a fixed intensity of
3 mW/cm2. Notably, a similar finding ͑data not shown͒ is
also obtained if the UV irradiated intensity is increased at a
fixed irradiated time. Experimental results in Fig. 4͑b͒ are
because the concentration of cis-isomer via the continued
trans→cis isomerization can increase when increasing the
UV irradiated time/intensity. The cis-isomers in the DDCLC
cell may disorganize the local order of the NLC molecules
due to their bent structures, resulting in the subsequent decay
of the right-handed twisting power of IBM in NLC. Owing
to the presence of the two chiral agents ͑S811 and IBM͒ with
twisting powers of opposite signs in the cell and the UV-
irradiation-induced decay of the right-handed twisting power
of IBM in the cell via trans→cis isomerization, the power
of the other chiral agent ͑S811͒ to twist NLC can rise to
decrease the structural pitch of the cell. Accordingly, as the
UV irradiated time or intensity gradually increases, the pitch
can gradually contract in the cell. This explains why CLCRB
and the band edges, and thus, the lasing band of the obtained
CCLE can gradually blueshift with increasing the UV irradi-
ated fluence ͑Fig. 3͒. However, as the LWE of the CLCRB
for large oblique angles ͑Ͼ45°͒ enters the absorption region
of the laser dyes ͑Յ575 nm͒, the strong reabsorption effect
of the fluorescence significantly diminishes the likelihood of
lasing emission at those large angles. Consequently, the
angle range and thus the band of lasing emission decrease
with the increase of the UV irradiated fluence ͑Fig. 3͒.
The authors would like to thank the National Science
Council of the Republic of China, Taiwan ͑Contract No.
NSC 97-2112-M-006-013-MY3͒ and the Advanced Opto-
electronic Technology Center, National Cheng Kung Univer-
sity, under projects from the Ministry of Education for finan-
cially supporting this research. Ted Knoy is appreciated for
his editorial assistance.
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