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Random lasing actions have been observed in optically isotropic pure blue-phase and polymer-stabilized blue-phase liquid crystals containing laser dyes. Scattering, interferences and recurrent multiple scatterings arising from disordered platelet texture as well as index mismatch between polymer and mesogen in these materials provide the optical feedbacks for lasing action. In polymer stabilized blue-phase liquid crystals, coherent random lasing could occur in the ordered blue phase with an extended temperature interval as well as in the isotropic liquid state. The dependence of lasing wavelength range, mode characteristics, excitation threshold and other pertinent properties on temperature and detailed make-up of the crystals platelets were obtained. Specifically, lasing wavelengths and mode-stability were found to be determined by platelet size, which can be set by controlling the cooling rate; lasing thresholds and emission spectrum are highly dependent on, and therefore can be tuned by temperature.
©2012 Optical Society of America
1. Introduction
For many years, random lasing, whose feedback mechanism is based on multiple scattering and interference effects in a chaotic amplifying medium, has attracted substantial interest from a variety of scientific fields [1, 2]. Random lasers have been observed in both inorganic and organic scattering systems, including grated laser crystals [3], semiconductor nanostructures [4, 5], polymer films [6] and biological tissues [7]. Such lasers possess many useful characteristics such as low spatial coherence, multiple lasing wavelengths, broad solid angle of laser output directions, compact mirrorless cavity sizes/dimensions and are finding an ever increasing application in some of these speckle-free imaging [8], medical diagnostics [7] and document coding [1]. Random lasing actions may be generally classified into two types corresponding to coherent (resonant) or incoherent (non-resonant) feedback [9]. Owing to their extraordinarily large index birefringence and linear and nonlinear light scattering abilities, coupled to the large susceptibilities of crystalline reorientation or disorder by external fields, liquid crystals in their mesophases are ideal host mediums for lasing actions [10–22] and offer a wide variety of tuning mechanisms. For such liquid crystals based random lasing actions, nematic mesogens offer resonant feedback [10], whereas smectic A* mesogens provide non-resonant feedback [11]. In cholesterics (chiral nematics), resonant feedback is enabled by the planar texture [12] or ñuctuations in a low-frequency driven sample [14], whereas non-resonant feedback arises from a highly scattering focal conic texture [14]. Both coherent and incoherent random lasing have been observed in pure cholesteric (chiral nematic) [14] and polymer-based liquid crystals [15–18].
In this paper, we present the results of our recent studies of resonant feedback-type random lasing action in Blue-Phase liquid crystals (BPLC). Blue phases (BPs) are chiral mesophases with three-dimensional cubic defect structures; they exist between the isotropic (ISO) phase and the cholesteric phase (N*) with self-assembled polycrystalline texture. As a result of their 3-D photonic crystalline make-up, optical isotropy (polarization independence), much faster electro-optics response [23, 24] than nematic, and ease of fabrication as they generally do not require surface alignment, they are in many respects more desirable than cholesterics or nematics as a laser host material. Even though the BPLCs are optically isotropic on a macro scale, discontinuous grain boundaries among platelets gives rise to diffuse light scattering. In polymer stabilized blue-phase liquid crystals (PS-BPLC), index mismatch between the polymer and the mesogen also contribute to light scattering. These scattering mechanisms, coupled to the gain provided by a laser dye dopant, enabled random lasing actions to occur in BPLC and PS-BPLC as reported here. In the next sections, the emission characteristics of different phases of pure as well as PS-BPLC are reported. This is followed by a discussion on how platelet size and polymer stabilization affect the randomness of lasing wavelengths, and thermal tuning of the random laser output characteristics in PS-BPLC and its relationship to the fluorescence of the laser dye.
2. Sample preparation
To formulate BPLC at room temperature, a suitable amount of chiral dopant S-811 was dissolved in the nematic host mixture consisting of E48 and 5CB (all from Merck). This material exhibited the following phase sequence during cooling; ISO-(31.3 þC)-BPI-(22.5 þC)-N*. For PS-BPLC, the precursor was prepared by blending the photo curable prepolymers, 7.1 wt% reactive diacrylate mesogen RM-257 and 5.4 wt% trimethylolpropane triacrylate (TMPTA), and 0.4 wt% photoinitiator 2,2-dimethoxy-2-phenyl acetophenone (DMPAP), into the chiral nematic material consisting of the chiral dopant ZLI-4572 and the nematic mesogens, JC-1041XX and 5CB. The phase sequence of the precursor was ISO-(41.5 þC)-BPI-(37.2 þC)-N*. It is important to note here that the emission band of the random lasers (from 605 to 635 nm) is clearly outside the band-edge (around 560nm) of the PS-BPLC and so the observed effects are not related to band-edge lasing.
Both mixtures were doped with 1 wt% laser dye [2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile (DCM, Exciton). Quartz capillary tubes of ~2 cm in length and internal diameter of 100 µm were then filled with the uniformly mixed BPLCs. Because of their relatively large volume in any direction, these cylindrical BPLC cores possessed sufficient space for the emitted light to follow closed-loop paths leading to resonant coherent feedback. The samples with precursors were then polymerized at 39 þC for 20 min with a UV intensity of 8 mW/cm2, resulting in a polymer stabilized blue-phase liquid crystal with a phase sequence of ISO-(56.2 þC)-BPI-(<0 þC)-N* [Note: The BP-N* transition is below the freezing point outside the range of our temperature stabilization system. These samples were examined with a polarized optical microscope in reflected light mode (R-POM) confirmed the development of the platelet texture. With repeated thermal cyclings between the Isotropic- and the blue- phases, the platelets of the polymer-stabilized blue-phase liquid crystals (PS-BPLC) eventually adopted the same stable configuration that facilitated the scattered light to follow fixed trajectories for resonant feedback.
3. Experimental results and discussion
The experimental setup for observing random lasing action in BPLCs is shown in Fig. 1 . The sample was optically excited using the second harmonic (λ = 532 nm) of an Q-switched Nd:YAG laser with pulse duration of 8 ns and a repetition rate of 1 Hz. The pump beam passed through an iris, a half-wave plate and a polarizing beam splitter and was then focused by a plano-convex cylindrical lens (f = 10 cm on x-axis) before reaching the sample. The measured spot size (elliptical in shape) was about 0.177 mm2 (~90 µm on x-axis and ~2.5 mm on y-axis). The emission signals were collected by an optical fiber that was connected to a spectrometer (USB 4000, Ocean optics) at the endpoint of the capillary tube. All BPLC samples tested have been placed on a hot stage for temperature control of the lasing behavior and also the cooling rate that determined the size of the BP platelets.
As illustrated in Fig. 2 , light waves that are emitted from the excited gain medium travel among the self-assembled BP platelets while undergoing multiple scattering. When some of the waves return to their starting position under constructive interference, closed-loop paths are realized and enable coherent (resonant feedback). Along these paths, the light simultaneously experience scattering loss and amplification; when gain exceeds the loss, random lasing will occur.
Figure 3 compares the emission spectra of a pure BPLC and a PS-BPLC. The emission profile of the pure BPLC that comprises discrete lasing modes (Δλ ≈1.2 nm) results from interference associated with multiple scattering by randomly distributed BP platelets, as presented in Fig. 3(a). Cooling the sample to the cholesteric phase causes the platelets to disappear, and the formation of a focal conic texture. The scattering system becomes even more disordered, so the scattered light undergoes a diffusive random walk, rather than resonant closed loops, yielding a relatively even profile (Δλ ≈8 nm).The thermal hysteresis and bistable behavior in the same BPLCs investigated by Wang et al. [25] suggest the possibility of switching between these two states and therefore the emission characteristics using different temperature control processes.
Fig. 3 Emission spectra of (a) pure BPLC and (b) PS-BPLC systems in different phases. And, (c) R-POM images of a PS-BPLC sample in Blue-Phase (BP) and isotropic phase (ISO).
Above the clearing point of BPLC (~31.3 þC) the BP platelets vanished and the emission spectrum is dominated by spontaneous emissions. However, in polymer-stabilized BPLC, the mismatch between the refractive indices of the polymer and the mesogen also contributes to multiple scatterings and enabled random laser action. Figure 3(b) shows the spectra of the random lasers observed in the isotropic phase (above 56.2 þC) as well as the blue-phase of PS-BPLCs. As shown in Fig. 3(c), the photographs of the PS-BPLC under R-POM observation reveal that the shape [defined by the polymer network] of the platelets still persists in the isotropic phase.
Unlike other scattering media such as solid powder [4] or nanorods [5], BPLC-based random lasers allow one to vary the scattering platelet size (of the same sample) by changing the cooling rate after first heating it to the isotropic phase; this allow one to alter the lasing characteristics. Figures 4(a-c) compare the emission behavior and the R-POM images of pure BPLC with platelets ranging from small (< 3 μm) through intermediate (10-20 μm) to large (> 30 μm) sizes. Upon rapid cooling at roughly 50þC/min, small platelets with diameters of under 3μm were formed. Discrete lasing modes appeared stochastically from pulse to pulse [Fig. 4(a)]. The lasing-wavelengths varied from pulse to pulse, indirectly confirming that the laser spikes did not result from the whispering gallery effect of the capillary walls. Reducing the cooling rate to 0.1 þC/min enabled larger platelets with diameters of 10 to 20 μm to form. The increase in platelet sizes altered the lasing behavior; several modes emerged in most of the pulses, as revealed by, the peaks at 612, 614, 617, 620 nm [Fig. 4(b)]. Under the limitations of the temperature controller, the largest platelets with diameters of over 30 μm were grown at a rate of 0.01 þC/min. In that case, specific modes, with peak wavelengths of 609, 611, 613, 617 nm, were observed in almost every pulse [Fig. 4(c)].
Fig. 4 R-POM images and emission spectra (different colored curves represent different pulses) of the samples that under the conditions of (a) small (< 3 µm), (b) intermediate (10-20 µm) and (c) large (> 30 µm) platelet size of pure BPLC, and (d) PS-BPLC with platelets of intermediate size (6-15 µm).
The change in the lasing modes profile may be attributed to the following mechanisms: (i) Abundance of closed-loop optical paths: In a medium that contains smaller platelets, there are more closed-loop paths to provide coherent feedback. As a result, emitted light could follow a different set of paths from pulse to pulse. By contrast, the lasing modes of the medium that contains large platelets are more stable because the number of optical paths is more limited. (ii) Stability of BP platelets: Upon illumination by the intense pump laser pulses, the platelets were significantly perturbed by laser-field-induced optical and thermal disturbances [26]. Smaller platelets, which contain less complete crystal lattices, result in a weaker structure, and so are more easily affected by the pump laser. Hence, successive optical excitation changes the structure of the system and therefore the optical paths for resonance.
On the other hand, the polymer network in PS-BPLC provides a strong and stable structure for multiple scatterings; the emitted light follows almost the same paths under different pump pulses. Figure 4(d) displays the corresponding random lasing spectra of the PS-BPLC sample. The diameters of the platelets range from 6 to 15 µm, resembling the intermediate platelet size of the pure BPLC. Yet, the lasing modes of 612, 617, 621 nm are constant throughout repeated identical pulses. We also observed similar phenomena in other BPLCs materials with different reflected colors, possessing band-edges out of the emission band, to clarify that the action observed was not multimode band-edge lasing.
To further characterize the emission properties, both the peak intensity and the linewidth of the emission spectrum were recorded as a function of the excitation energy density. As depicted in Fig. 5(a) , there is a clearly defined excitation threshold above which spectral narrowing and intensified emission were observed. At room temperature, the threshold in PS-BPLC is ~401 µJ/mm2 per pulse. The excitation threshold is dependent on the operating temperature, which we attributed to the dependence of the fluorescence intensity of DCM, c.f. Fig. 5(b) which shows that the threshold fell (rose) as the dye-emission intensity increased (decreased). The lowest threshold of 277 µJ/mm2 per pulse occurred at around 54 þC.
Fig. 5 (a) The peak intensity and the linewidth of the emission from a PS-BPLC sample as a function of the excitation energy density, and (b) Temperature dependence of the lasing threshold [excitation pulse energy density] and the peak fluorescence intensity of DCM. (These graphs relate to the mode with the highest intensity.)
Figure 6 shows the broadening of the laser emission spectrum when the PS-BPLC sample was heated. As the temperature rose from 32 to 117 þC (well above the clearing point), the laser emission profile was altered along with the fluorescence band of DCM, and the lasing modes covered the range 608 to 643 nm. Based on the above experiments, the laser characteristics did not change evidently with the BP-ISO transition but were dominated by the fluorescence of the laser dye. In a separate experiment on pure BPLC containing platelet size of 3-10 µm at 27 þC, the threshold was found to be 525 μJ/mm2 per pulse, i.e. the threshold in PS-BPLC was ~31% lower than in pure BPLC. The difference may have resulted from the fact that multiple scattering that was caused by platelet-boundary discontinuity (in BPLC) was much weaker than that caused by the mismatch between the refractive indices of the polymer and the mesogen (in PS-BPLC); this accounts for lasing action above the clearing point. As previously mentioned that the dye-doped pure BPLC is applicable as a thermally switchable BP-N* (coherent-incoherent) random laser, we have further checked the threshold difference between these two phases. The threshold in the cholesteric state was found to be ~24% lower than in the blue phase. The decrease in excitation threshold is because that the focal conic texture is with stronger scattering, comparing to the platelet texture.
Fig. 6 Thermal tuning of PS-BPLC random lasing (blue) versus DCM fluorescence (red) from 32 to 117þC (top to bottom).
4. Conclusions
In summary, coherent random lasing is observed to take place in dye-doped BPLCs, in which the random distributed micrometer-size platelets contribute to resonant feedback. Owing to the mismatch between refractive indices of the polymer and the mesogen, laser action also occurs in the isotropic (liquid) phase of PS-BPLC. The degree of lasing-spike randomness is related to the number of closed loop light-paths and the platelet stability which can be controlled by the cooling rate during the BPLC fabrication process or adding a polymer network. In a PS-BPLC random laser, the thermal tunability of the both excitation threshold and emission spectrum was dominated by DCM fluorescence. In the un-optimized set up, the lowest lasing threshold was ~277 µJ/mm2 per pulse and the achieved temperature controlled lasing wavelengths tuning range was ~35 nm. By judicious choice of the optical configuration and material properties, we expect to greatly improve on these performance characteristics in the near future. Compared to other mesophases (e.g. nematic, cholesteric, ferroelectric) of liquid crystals blue-phase liquid crystals clearly stand out as a promising laser host as they are optically isotropic, i.e. polarization independent; also they are relatively easy to fabricate as no surface alignment layer is needed.
Acknowledgment
The authors gratefully acknowledge the National Science Council of Taiwan, for financial support under Contract No: NSC99-2119-M-110-006-MY3, and from the US Air Force Office of Scientific Research.
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