This paper presents an optically wavelength-tunable and intensity-switchable dye-doped cholesteric liquid crystal (DDCLC) spherical microlaser with an azo-chiral dopant. Experimental results present that two functions of optical control — tunability of lasing wavelength and switchability of lasing intensity — can be obtained for this spherical microlaser at low and high intensity regimes of non-polarized UV irradiation, respectively. If the DDCLC microdroplet is subjected to weak UV irradiation, azo-chiral molecules may transform to the bent cis state at a low concentration rate. The effect can slightly decrease the local order of LCs and thus the helical twisting power of the CLC in the microdroplet. As a result, the CLC pitch may become slightly elongated, which will cause the gradual red-shift of both omnidirectional PBG and lasing emission of the DDCLC spherical microdroplet. In contrast, when the microdroplet is subjected to strong UV irradiation, numerous azo-chiral molecules may simultaneously change to bent cis-isomers to seriously disarrange the helical texture of the CLC, which will quickly deform the PBG and deactivate the lasing emission of the microdroplet. Prolonged irradiation of a blue beam after strong UV irradiation may cause the cis azo-chiral molecules quickly convert back rod-like trans-isomers, which may then regenerate the CLC Bragg onion and PBG structures and reactivate the lasing emission of the microdroplet. Optical control of the DDCLC spherical microlaser is realized on a scale of seconds and minutes when UV irradiation is strong and weak, respectively. The 3D DDCLC spherical microlaser is a highly promising controllable 3D micro-light source or microlaser (e.g., all-optical 3D single photon microlaser) for applications of 3D all-optical integrated photonics, laser displays, and biomedical imaging and therapy, and as a 3D UV microdosagemeter or microsensor.
©2013 Optical Society of America
Dye-doped cholesteric liquid crystal (DDCLC) lasers have attracted considerable attention in recent decade because of their spontaneously self-assembling periodic structure which does not require complex fabrication processing. In addition, DDCLC lasers have superior tunabilities in photonic bandgap (PBG) and lasing threshold or wavelength by external stimuli [1–13]. Given the periodically helical structure of CLC, it can be regarded as a mirrorless one-dimensional (1D) PBG microcavity. Low-threshold lasing emission can occur along the helical axis at the edge(s) of the PBG of the DDCLC as long as the pumped energy is greater than the energy threshold . In respect to the external tunability, DDCLC lasers are far superior to traditional lasers such as gas and solid-state lasers because of the lasing wavelength of their lasing emission can be easily tuned by changing the CLC pitch through heating/cooling, application of voltage or stress, and light irradiation [6–13].
Three-dimensional (3D) microcavities with 3D periodic microstructures have been investigated intensively in recent years because of their good cavity quality and emission characteristics such as ultra-small mode volume, narrow line width, high quality factor, 3D PBG, and potential applications, especially in integrated photonics. To further achieve 3D lasing emission, certain 3D microstructures, such as whispering-gallery mode and Bragg onion microcavities [14–18], should be fabricated. Spherical Bragg onion microcavities are of particular interest because of their perfect 3D rotational symmetry. The PBG in such microcavities is expected to be independent of the propagation direction of the light, and its lasing emission can be omnidirectional. Unfortunately, fabricating such 3D microcavities is very complicated, and no associated lasing is obtained by using solid matter. In 2010, Humar et al. first developed a DDCLC Bragg onion microdroplet laser embedded in an isotropic carrier fluid . Such a spherical microlaser has the following advantages: easy and low cost fabrication of numerous microdroplets, omnidirectional PBG and lasing emission, and high lasing wavelength tunability through temperature changes. The spherical Bragg onion soft-matter microlaser is appropriate for advanced integrated photonics and display applications, and can serve as an omnidirectionally emitted micro-light source or microlaser in flexible/non-flexible 3D laser display, holography, optical computing, biomedical imaging and medical therapy, and as a microsensor.
This paper is the first to present an optically tunable/switchable spherical microlaser with omnidirectional emission; this microlaser is based on a DDCLC microdroplet with an azo-chiral dopant. Experimental results show that two functions of optical control, namely, lasing wavelength tunability and switchability of lasing intensity, can be achieved for this spherical microlaser at low and high intensity regimes of non-polarized UV irradiation, respectively. If the DDCLC microdroplet is subjected to weak UV illumination (472 μW/cm2), azo-chiral molecules may transform to the bent cis state at a low concentration rate. The effect can slightly decrease the local order of LCs and thus the helical twisting power (HTP) of the CLC in the microdroplet. As a result, the CLC pitch may become slightly elongated, which will cause the gradual red-shift of both omnidirectional PBG and lasing emission of the DDCLC spherical microdroplet. In contrast, if the DDCLC microdroplet is subjected to strong UV irradiation (2.8 mW/cm2), numerous azo-chiral molecules may simultaneously change to bent cis-isomers to significantly disturb the helical structure of the CLC, which will quickly deform the PBG and deactivate the lasing emission of the microdroplet. Prolonged irradiation by using a blue beam after strong UV irradiation may cause the cis azo-chiral molecules to revert into rod-like trans-isomers, which may then regenerate the CLC Bragg onion and PBG structures and reactivate the lasing emission of the microdroplet. The DDCLC spherical microlaser can be controlled optically and is capable of 3D lasing, which makes it suitable as a 3D micro-light source or microlaser in 3D applications of all-optical integrated photonics or biomedical imaging and medical therapy, and as a 3D UV microdosagemeter or microsensor.
2. Sample preparation and experimental setups
The DDCLC materials used in this study include 74.3 wt% nematics (MDA-03-3970, Merck), 20.5 wt% left-handed chiral dopant (S811, Merck), 4.7 wt% left-handed azo-chiral dopant (ChAD-2-S, Beam Co.), and 0.5 wt% laser dye (P597, Exciton). The helical pitch of such a DDCLC mixture was pre-adjusted to about 350 nm. The helical twisting power (HTP) values for S811 and azo-chiral dopant (trans state) in MDA-03-3970 host are −11.1 and −11.93 μm−1, respectively. After mixing and stirring the 10 wt% DDCLC and 90 wt% glycerol mixture, many DDCLC microdroplets may simultaneously form and disperse uniformly in the glycerol matrix. The diameter of the formed microdroplets ranges roughly from 20 μm to 50 μm. The DDCLC microdroplet solution was then injected into an empty cell, which was pre-constituted by stacking two clean glass slides with 50 μm-thick spacers between them. To observe the photo-induced variations of the onion-like structure of the DDCLC microdroplet via crossed polarizers by the CCD, a DDCLC cell with long-pitched droplets was fabricated. Long-pitched DDCLC microdroplets with a mixture of 92.2 wt% MDA03-3970, 2.8 wt% S811, and 5 wt% ChAD-2-S were fabricated by using the same procedure as that for fabricating short-pitched DDCLC microdroplets.
An integrated experimental setup in this study was built to measure the lasing spectrum and record the microscopic lasing pattern and the microdroplet structure of the DDCLC spherical microlaser. As shown in Fig. 1, one pumped pulse laser beam, derived from a Q-switched Nd:YAG second-harmonic generation pulse laser (wavelength: 532 nm, pulse duration: 8 ns, repetition rate: 10 Hz), was focused by a 10 × objective lens on the DDCLC microdroplet cell. A fiber optic probe of a fiber-based spectrometer (Jaz-Combo-2, Ocean Optics; resolution: ~1.0 nm) was placed behind the cell to nearly face the cell normally at a 2 cm distance to receive the forward-emitted lasing signal. A half-wave plate and a polarizer were placed in front of the objective lens for adjusting the incident pumped energy. A beam splitter was inserted between the objective lens and the polarizer to split the incident beam with half the energy into the detector of the energy meter to measure the incident pulse energy. The XYZ translation stage was used to adjust the pumped position of the cell, which is pre-fixed on this stage. A CCD camera (1412AC-00-FW, DVC) was used to record the microscopic lasing pattern of the lasing signal which is emitted backward and then passed sequentially through the objective lens, the beam splitter, and the notch filter (for 532 nm). In addition, a polarizing optical microscope (POM) was built to observe the micro-images of the DDCLC microdroplet. The CLC-isotropic phase transition temperature for the DDCLC microdroplet is measured to be around 68 °C under the POM. A white light beam from a mercury lamp was focused by a lens (focal length: 20 cm) on the microdroplet and then collected by the objective lens to the CCD camera, in which two crossed polarizers were inserted before and after the cell, respectively. To perform the optically controllable lasing experiment of the DDCLC spherical microlaser, one UV light source (peak wavelength: 365 nm) and a blue laser (wavelength: 442 nm, He-Cd laser, Kimmon) were used to irradiate the cell to change the structure of the microdroplets and thus modulate the associated lasing emission.
3. Results and discussion
Under the influences of parallel surface anchoring imposed by the glycerol matrix on the CLC and the bulk helical twisting of chiral dopants, many DDCLC microdroplets with a similar Bragg onion structure and different sizes (diameter: 20 μm to 50 μm) can form spontaneously by mixing the DDCLC mixture and glycerol . In the DDCLC microdroplets, the laser dye and the CLC with a radially divergent helical structure act as a gain medium and a Bragg onion optical microcavity, respectively. An omnidirectional PBG can be obtained because of the onion-like helical structure of the microdroplets. As expected, omnidirectional lasing emission may be generated if the DDCLC microdroplet is pumped efficiently under suitable conditions. Figure 2 shows the absorption and fluorescence emission spectra (shown as black and blue curves, respectively) of the DDCLC at isotropic state and the location of the reflection band of the CLC (red curve). The absorption and fluorescence peaks are located at 527 nm and 569 nm, respectively, and their spectral positions are near the wavelengths of pumped pulses (532 nm) and the long wavelength edge (LWE) of the CLC reflection band, respectively, thereby ensuring efficient excitation of the laser dye and the efficient generation of lasing emission at the bandedge.
Before investigating the optical controllability of the DDCLC spherical microlaser, its lasing characteristics (e.g., energy threshold and linewidth) must be identified. The following experiment was conducted by using DDCLC microdroplets with a diameter of ~30 μm. Figures 3(a) and 3(b) show the measured fluorescence emission spectra at various pumped energy values (E = 3.9 μJ/pulse to 13.4 μJ/pulse) and the variations of the measured emission peak and its linewidth (that is, the full-width at half maximum, FWHM) with pumped energy, respectively. A lasing emission peak initially appears at an energy threshold of Eth = 6.7 μJ/pulse, in which the corresponding linewidth of the emission drops sharply from a broad band (~47 nm) to a very narrow spike (~1.0 nm). Over this threshold, the lasing signal is enhanced rapidly by increasing the pumped energy. In addition, the wavelength of the emission spike is located at around 571 nm, a position which corresponds to the LWE of the CLC stop band (Fig. 2). The images of the lasing pattern of the pumped DDCLC microdroplet via the crossed polarizers, which were recorded using a CCD camera, at pumped energies lower, equal to, and higher than Eth (E = 3.9, 6.7, 13.4 μJ/pulse, respectively), are also shown in Fig. 3(c). Clearly, no lasing occurs at a pumped energy (3.9 μJ/pulse) lower than Eth. A weak green lasing spot appears just at Eth (6.7 μJ/pulse) and becomes very bright at a high pumped energy (13.4 μJ/pulse) over Eth.
Generally, optically controlling the structure and the photonic property of a traditional 3D photonic crystal (PC) fabricated by using solid materials is difficult if the PC formation is complete. In this study, the formed 3D PC-like CLC microdroplet is easy to control by changing the structure of the CLC via photo-irradiation of the doped azo-chiral material. With the aid of the azo-chiral dopant, the lasing emission of the DDCLC spherical microlaser may be optically controlled simply by wavelength tuning and intensity switching at the weak and strong intensity regimes of the UV irradiation, respectively. Figures 4(a) illustrates the measured lasing spectra of the DDCLC spherical microlaser pumped at E = 12 μJ/pulse as the microdroplet is irradiated by the UV beam with a weak intensity of IUV = 472 μW/cm2 and at different irradiation times (tUV = 0 min to 20 min), respectively. Apparently, the lasing wavelength of the spherical microlaser can be tuned optically from 563 nm to 586 nm with increasing tUV from 0 min to 20 min (Fig. 4(b)). Thus, the tuning sequence encompasses lasing spectra from green, yellow, to orange region (Fig. 4(c)). Figure 4(b) also shows the variation in the lasing wavelength of the DDCLC microlaser with the relaxation time (trelax) after the weak UV irradiation is turned off. The lasing wavelength of the microlaser can completely recover back to the original state in 20 min. In addition, the DDCLC spherical microlaser can also be controlled by changing the intensity if strong UV irradiation is applied; this method is different from the wavelength tuning method shown in Fig. 4, which is used when weak UV irradiation is applied. The upper sub-figure in Fig. 5(a) and the CCD images in Fig. 5(b) show the measured lasing spectra and corresponding lasing patterns of the intensity-switchable DDCLC spherical microlaser before and after strong UV irradiation with an intensity of 2.8 mW/cm2 for tUV = 5 s (represented by black and red curves, respectively), and then after blue laser irradiation (442 nm) with 2.03 mW/cm2 for tB = 30 s (blue curve) following strong UV irradiation, respectively. Apparently, the lasing emission can be switched off and on through the first cycle of successive irradiation of the strong UV beam and the blue beam. The bottom sub-figure in Fig. 5(a) shows that the spherical microlaser is all-optically switched off and on repeatedly if the spherical microlaser continues to be irradiated through second and third successive irradiation cycles of the strong UV and blue beams following the first irradiation cycle. The experiment proved that the all-optical switching ability of the DDCLC spherical microlaser is reliable for more than tens of irradiation cycles with minimal reduction in lasing performance (not shown herein).
To identify the mechanisms that induce the wavelength tuning and intensity switching features of the DDCLC spherical microlaser, this study fabricated long-pitched CLC microdroplets added with azo-chiral dopant and then observe and examine the structural variations of the microdroplet under weak and strong UV irradiations. The preparation and the cell fabrication for obtaining the long-pitched CLC microdroplets are described in Section 2. Figure 6(a) shows the recorded images of the long-pitched CLC microdroplet with a diameter of ~30 μm before and after weak UV irradiation at 127 μW/cm2 for 20 mins; Fig. 6(b) shows the same images of the microdroplet before and after strong UV irradiation at 3.05 mW/cm2 for 5 s. The dynamic CCD images of the long-pitched CLC microdroplet structure with increasing tUV at weak and strong UV irradiation regimes are shown in the multimedia files (Media 1 and Media 2, respectively). Apparently, the pitch of the CLC microdroplet extends gradually while the tUV increases from 0 min to 20 min at the weak UV irradiation regime. However, the structure of the microdroplet is considerably distorted after 5 s of strong UV irradiation. A model for explaining the mechanisms of the optical tuning and switching DDCLC spherical microlaser at weak and strong UV irradiations, respectively, are shown in Fig. 6(c). The azo-chiral dopant in the microdroplet must play a key role in the optical control of the microdroplet structure shown in Fig. 6. Generally, the azo-chiral molecule exists at either a rod-like trans state or a bent cis state. The absorption spectra of the azo material at the two states are quite distinct. Figure 7(a) (Fig. 7(b)) show the evolution of the measured absorption spectrum of 4.54 wt% azo-chiral dopant dissolved in alcohol if the dopant is irradiated by a strong (weak) UV beam with 2.8 mW/cm2 (472 μW/cm2) at tUV = 0 s to 10 s (60 s). The black curve in the absorption spectra shows that the azo-chiral dopant originally has two absorption peaks at UV (around 365 nm) and visible (around 442 nm) regions in dark (tUV = 0 s). The two peaks are associated with π-π* and n-π* transitions of the azo-chiral molecule, respectively. By increasing tUV from 0 s to 10 s (from 0 s to 60 s), the concentration of cis isomers may quickly (slowly) increase to a saturated state in 10 s (60 s) through the strong (weak) UV beam irradiation-induced trans→cis isomerization of the azo-chiral dopant, which leads to the quick (slow) drop and rise of the two absorption peaks at 365 and 442 nm, respectively. This result implies that the increased concentration rate of the cis isomer under strong UV irradiation is larger than that under weak UV irradiation. The large discrepancy between the increased concentration rates of the cis isomer under weak and strong UV irradiations may cause the difference between the lasing responses of the DDCLC microlaser at the two UV irradiation regimes. The following equation shows the relation between the increased concentration rate of the cis isomer and the optical irradiation intensity:19]. Equation (1) indicates that a higher UV irradiation intensity on the azo-chiral dopants corresponds to an increased cis-isomer concentration rate. The twisting strength of the helical structure of the CLC microdroplet is determined by the sum of the HTP values of the left-handed chiral dopant S811 and the left-handed azo-chiral dopant. When the azo-chiral molecules transform at low concentration rate from a rod-like trans state to a bent cis state under weak UV illumination (I is relatively low in Eq. (1)), the twisting strength of the CLC decays slowly because the low concentration rate of the increased bent cis isomers may cause the slight disturbance of the local order of LCs per unit time [20, 21]. This condition may result in the slight prolongation of the pitch of the CLC microdroplet per unit time (Fig. 6(a)) and the gradually slow red-shift of the PBG and the lasing emission of the DDCLC spherical microlaser with increasing tUV under low UV irradiation (Fig. 4). With increasing tUV, ∂NC/∂t gradually increases because of the slow decrease in NT and the slow increase in NC. A dynamic equilibrium with the relatively lowest trans and the relatively highest cis isomer concentrations can be obtained in the condition of ∂NC/∂t = 0 in Eq. (1). At that moment, both the tunings of the PBG and lasing emission of the DDCLC microspheres stop. By contrast, numerous chiral trans isomers in the CLC microdroplet may rapidly transform to the bent cis state at a high concentration rate under strong UV irradiation (I is relatively high in Eq. (1)) [21, 22]. The production of numerous curve cis isomers within a short time may cause a significant distortion of the CLC structure in the microdroplet (Fig. 6(b)). This phenomenon may deform the PBG and increase the energy threshold of the microlaser beyond the applied pumped energy, leading to the disappearance of the lasing emission of the DDCLC microlaser (Fig. 5). To further monitor the dynamic change of the CLC structure under UV irradiation, the evolution of the obtained reflection spectrum of a homogeneously aligned DDCLC planar cell added with azo-chiral dopant (23 μm thick) is examined under weak and strong UV irradiations. The prescription of the DDCLC mixture used to fabricate the homogeneously aligned DDCLC planar cell is the same as that used to fabricate the short-pitched DDCLC microdroplets. Figures 8(a) and 8(b) present the experimental results. Apparently, the PBG of the DDCLC planar cell slowly red-shifts with increasing tUV with less deformation at a weak UV irradiation regime (Fig. 8(a)). This slow red-shift of non-deformed PBG implies the slow prolongation of the pitch in the CLC, which corresponds to the slow red-shift of the lasing wavelength of DDCLC microspheres. By contrast, PBG of the DDCLC planar cell quickly deforms in a scale of seconds even though a concomitant red-shift occurs at a strong UV irradiation regime. This deformation of PBG implies the distortion of the helical structure of the CLC. Figures 9(a) and 9(b) shows the directly observed CLC textures under the POM with crossed polarizers before and after strong UV irradiation for 30 s. Obviously, the CLC becomes a multi-domain-like structure before transiting to the isotropic state, indicating the occurrence of serious distortion of the CLC structure because the perturbation of strong UV irradiation induces numerous cis isomers within a short time. Consequently, the lasing signal of the DDCLC microlaser under strong UV irradiation quickly disappears.
Figure 7(c) shows the evolution of the measured absorption spectrum of 4.54 wt% azo-chiral dopant dissolved in alcohol after turning off the strong or weak UV irradiation. Apparently, the peak at 442 nm in the absorption spectrum recovers back to almost the original state at trelax = 40 min, which implies that most cis isomers relax back to the trans state within this duration through thermal cis-trans back isomerization. This phenomenon results in the relaxation of the lasing wavelength of the DDCLC microlaser back to the initial state in the same time scale (20 min). Figure 7(d) further presents the evolution of the measured absorption spectrum of the 4.54 wt% azo-chiral dopant dissolved in alcohol under blue-beam irradiation with 2.03 mW/cm2 at increased irradiation time of tB from 0 s to 20 s after turning off the strong UV irradiation. Plainly, the absorption spectrum recovers back to the original state in 20 s under blue-beam irradiation. The speed of recovery of most azo-chiral dopants back to the original state (in 10 s) is significantly quicker than that of natural relaxation (in 40 mins) displayed in Fig. 7(c). The prolonged irradiation using a blue beam following the termination of the strong UV irradiation may induce numerous chiral cis isomers to quickly transform back to the trans state through blue-beam-induced quick cis-trans back isomerization and then cause the distorted microdroplet to quickly revert to its undistorted state, which then quickly recovers the non-deformed PBG and thus reactivates the lasing emission of the DDCLC spherical microlaser (Fig. 5). Notably, under weak UV irradiation, the time taken by the 4.54 wt% azo-chiral dopants dissolved in alcohol to transform to a saturated concentration in a cis state is around 60 s (Fig. 7(b)). This time is much shorter than the total time (20 mins) for the limit of tuning the lasing wavelength of the DDCLC microlaser (Fig. 4). This large discrepancy can be mainly attributed to the fact that both LCs and laser dyes in the system of DDCLC strongly absorbs UV light. The associated absorption spectrum of DDCLC in the isotropic state displayed in Fig. 2 can verify this point. In fact, the strong absorption of LCs and laser dyes can deplete a greater part of incident UV light such that the actual intensity of the UV light absorbed by the azo-chiral dopant is significantly smaller than the apparent intensity (472 μW/cm2) in the DDCLC system. Thus, the total tuning time of the DDCLC microlaser at the weak UV irradiation regime is as long as 20 mins.
This work also measures the temperatures at the DDCLC planar cell added with the azo-chiral dopants using a thermal imager (Fluke, Ti10) after irradiating the cell with weak and strong UV beams for 2 and 20 min, respectively. Experimental results (not shown) indicate that the measured temperatures of the cell at the irradiated location before and after the weak or strong UV irradiation are around 23.4 and 23.7 °C, respectively, which are far from the CLC-isotropic transition temperature of the DDCLC microsphere (~68 °C). This result indicates that the UV irradiation-induced thermal effect is too weak to cause the optical control of the DDCLC spherical microlaser, as shown by the experimental results in Figs. 4 and 5.
This study first demonstrates a spherical microlaser with optical tunability and switchability based on a DDCLC microdroplet with an added azo-chiral dopant. The lasing wavelength of the spherical microlaser can be tuned from 563 nm to 586 nm by increasing the irradiation time of a weak UV beam. This tunability is attributed to the slow trans-cis isomerization of the azo-chiral dopant induced by weak UV irradiation, in which the low concentration rate of the increased cis isomers may slowly disturb the local order of the LCs, gradually decrease the HTP value, and slightly increase the helical pitch of the CLC microdroplet without deforming the CLC structure. These changes may result in the red-shifts of both the PBG and the bandedge lasing emission of the DDCLC microdroplet. In addition, the DDCLC spherical microlaser can be all-optically switched off and on through successive illumination of the strong UV beam and the blue beam. The CLC structure can be seriously distorted by numerous bent cis isomers, which will deform the PBG and deactivate the lasing emission of the DDCLC spherical microlaser because of the high trans-cis isomerization rate of the azo-chiral dopant induced by strong UV irradiation. The microlaser can be switched on by the blue beam irradiation-induced cis-trans back-isomerization of the azo-chiral dopant following the strong UV irradiation. The 3D DDCLC spherical microlaser is a highly promising controllable 3D micro-light source or microlaser for applications of 3D all-optical integrated photonics, laser displays, biomedical imaging and therapy, and as a 3D UV microdosagemeter or microsensor.
The authors would like to thank the National Science Council of Taiwan (Contract number: NSC 100-2112-M-006-012-MY3) and the Advanced Optoelectronic Technology Center, National Cheng Kung University, under the Top University Project from the Ministry of Education, for financially supporting this research.
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