1. Introduction

In the last decade of particular interest is the development of low-threshold microlasers that are completely self-assembled from tunable liquid crystal (LC) microcavities, demonstrating the high flexibility and great capabilities of these structures in creating various instruments for the photonics, spectroscopy, sensing, etc. [1–4]. Because of chemical incompatibility and immiscibility, millions of small droplets of an LC are spontaneously formed, when one mixes the LC and an immiscible fluid. Furthermore, due to the surface tension between two immiscible fluids, these droplets have a perfect spherical shape and the interface is perfectly smooth. In this context, microfluidics technology has drawn much attention for continuously generating large numbers of monodisperse droplets as spherical microresonators, which makes them excellent candidates for optically pumped dye microlasers [5,6]. In the case of chiral anisotropic fluids, like cholesteric liquid crystals (CLCs) the supramolecular arrangement of the fluid within the droplets provides another interesting photonic device, which is a 3D microlaser, based on a “Bragg onion type” microresonator. In recent years, several studies have been published on the CLCs encapsulation in micrometer-sized spherical objects. Inside the microdroplets the CLC helical structure is preserved and, for boundary planar conditions, the helices axes are radially oriented. When the CLCs are doped with fluorescent dyes, 3D omnidirectional laser emission from microdroplets is observed [7–12]. The CLCs can be regarded as one-dimensional photonic crystals, while in many CLCs the transition between the cholesteric and isotropic phases occurs through a cascade of intermediate phases, consisting of self-assembled three-dimensional cubic defect structures, known as blue phases (BPs). The BPs which are believed to consist of double-twist cylinders are classified into three categories, depending on the cylinders’ packing structure: blue phase I (BPI), blue phase II (BPII) and blue phase III (BPIII). BPI has a body-centered unity cell formed of double-twist cylinders, BPII is a simple cubic, and BP III is a foggy phase whose structure is considered as an amorphous network of disclinations. The most noticeable characteristic of the BPs is that they demonstrate the selective reflection of the incident light. Therefore, BPs are particularly attractive for displays, photonics, and real-time thermal imaging applications, where the fast response time is needed [13–15]. Another peculiar interest represents the laser emission in BPs. According to the literature, BPs are in many respects more desirable than CLCs or nematics as a laser host material [16–20].

In this study, we have prepared BPs microspheres formed and emulsified in a Polyvinyl alcohol /water (PVA/water) environment and demonstrated, for the first time, the laser emissions from BPII and BPI microspheres and the temperature-controlled laser tuning from the BPI microspheres.

2. Sample preparation

The experiments were performed using the nematic liquid crystal ZLI-1939, with a birefringence of 0.18, and the chiral dopants CB-15 and MLC-6248 (all materials from Merck). The CLC mixture was prepared with the following percentages: 50wt % ZLI-1939 + 45wt % CB-15 + 5wt % MLC-6248. Here we note, that the low-temperature BPs can be obtained only by a particular selection of both nematic matrix and optically active dopants for a proper concentration ratio. The prepared mixture was stirred in the isotropic phase at ∼95°C for ∼10 minutes to make the constituents uniformly mixed. The reflection spectra were collected by using an optical fiber coupled spectrometer (Ava-Spec AVS-2048-2) having 1 nm resolution. For the optical excitation of the samples, we used a Nitrogen laser (MNL-100) with a pulse duration of 3 ns and a repetition rate of 5 Hz, at a wavelength of 337.1 nm. The energies of the pump beam were measured with optical power/energy meter Nova 7Z01500. To detect the reflection spectra of CLC and BPs phases, as a function of temperature, 99.5wt% Water + 0.05wt% PVA solution was deposited on the glasses substrates by spin-coating and then was rubbed to obtain planar alignment of the liquid crystal material. The CLC mixtures were infiltrated by capillarity inside 12 µm optical cells. To compose the BPs microlasers, two CLC mixtures doped with corresponding laser dyes were prepared. In particular, to obtain lasing in BPII microspheres, a laser dye Coumarin 503 (from Sigma-Aldrich) was added to the CLC host matrix (mixture 1), and to obtain lasing in BPI microspheres, two laser dyes: Uvitex (from NIOPIK), and DCM (from Exciton) were added to the host CLC matrix (mixture 2). The following mixtures were prepared:

99.6wt% (50 wt % ZLI-1939 + 45wt % CB-15 + 5wt % MLC-6248) + 0.4wt% Coumarin 503, (mixture 1),

99.4wt% (50wt % ZLI-1939 + 45wt % CB-15 + 5wt % MLC-6248) + 0.2wt% Uvitex + 0.4 wt% DCM, (mixture 2).

3. Experimental description, results, and discussions

The prepared CLC samples were confined inside a hot stage for precise temperature control. To visualize the CLC, BPI and BPII phases and the phase transitions between them, we took the images under a polarizing optical microscope. The temperature dependences of selective reflection peaks maximums for all phases recorded upon cooling, and the images of BPI, BPII, and CLC phases are shown in Fig. 1(a) and 1(b) respectively. The width of the selective reflection bands for the BPII and BPI phases were respectively 21 and 25 nanometers.

 

Fig. 1. Positions of the central wavelength of the reflection peaks versus temperature for the mixture 50wt % ZLI-1939 + 45wt % CB-15 + 5wt % MLC-6248, during the cooling process (a). Polarized optical microscopy image of the BPII, BPI and CLC phases (b).

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The microdroplets were produced by mechanically mixing 50 microgram of dye-doped CLCs and 50 ml of PVA/Water (in 11/89 wt% ratio) solution. The glass vial (diameter 1.0 cm, height 2.5 cm) containing the PVA/Water blend and the CLC mixtures were then subjected to a shaking process in a laboratory vortex mixer. The solution was mixed at a temperature of about 40 °C for at least 30 min. Since the PVA/Water and liquid crystal are immiscible, this procedure is functional to the formation of the CLC microspheres. The CLC droplet sizes can be manipulated by varying the speed of vortexing. In our experiments (typically between 200-250 rpm), the obtained droplet sizes range between 60 µm and 100 µm. As reported in [11], PVA plays the double role to stabilize the emulsion avoiding the coalescence of droplets and to induce the planar alignment of the liquid crystal molecules at the interface. To confirm the presence of the BPs microspheres in PVA/Water emulsion and to examine their photo-optical behavior, the emulsified mixtures were capillary filled into two 100 µm thickness glass cells. A hot stage with 0.05°C accuracy, was used to control the temperatures of the cells. In Fig. 2, a micrograph of the microspheres in BPI and BPII phases between the crossed polarizers is shown.

 

Fig. 2. Dispersion of BPI (red droplets) and BPII (green droplets) microspheres in PVA/Water solutions. The photo was taken under crossed polarizers.

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To visualize the phase transition between BPI and BPII microspheres, we selected a microsphere with a diameter of about 80 µm. In Fig. 3, the optical image of BP microsphere upon heating, obtained in crossed polarizers and demonstrating the phase transition between BPI and BPII phases, is shown. The temperature varies from 22.5°C to 26.5°C.

 

Fig. 3. the temperature-dependent phase transition in BP microsphere emulsified in PVA/Water solution. The temperature of the left (red) microsphere is 22.5° C and the temperature of the right (green) microsphere is 26.5° C.

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After that, two cuvettes with 3 mm gaps and with the square base (1 cm x 1 cm) were filled with the mixture 1 and mixture 2, respectively. The experimental setup for observing the lasing action in BPII and BPI is shown in Fig. 4. To obtain the desired BP condition and to continuously tune the temperature inside the corresponding emulsion, the cuvette was placed on a heating stage. Each cuvette was pumped with a Nitrogen laser. As illustrated in Fig. 4(a), the light emerging from the pumping laser propagates through the optical lens, focusing the pumping beam into a spot with a diameter of 150 µm, which causes the 3D laser emission from a single BPII or BPI microsphere. The average pumping energy was about 10 mJ/cm². To minimize the detected intensity of the transmitted pumping beam a 300-400 nm band-stop filter was placed perpendicularly to the direction of the light propagation. In Fig. 4(b) are shown the tracks of the pumping laser beam and BPII and BPI microspheres which were optically excited. When the BPII and BPI microspheres, with approximately equal diameters of 80 µm were illuminated with a tightly focused 337.1 nm laser beam, bright green and red dots visible from all sides were appeared showing that we were observing the omnidirectional laser emission from BPII and BPI microspheres, respectively.

 

Fig. 4. Schematic of the experimental setup (a), Nitrogen laser (1), optical lens (2), cuvette filed with BP/PVA/Water emulsion (3), a band-stop filter (4), and spectrometer (5). Images showing the tracks of pumping laser beam and the optically excited BPII and BPI microspheres (b).

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Figure 5(a) demonstrates the laser emission lines from the cuvettes filled with BPII/PVA/Water (left line) and BPI/PVA/Water (right line) emulsions, that were recorded approximately 15 cm away from the samples. The emission from each cuvette excited by the Nitrogen laser shows the laser lines at 517 nm and 629 nm, corresponding to the laser emissions from BPII and BPI phases respectively. Figure 5(b) shows the temperature-dependent lasing tuning in BPI measured by increasing the temperature. Laser emissions were recorded at 629 nm, 620 nm, 608 nm, and 574 nm, sequentially, which corresponds to the laser wavelength tuning range of about 55 nm. The temperature tuning range of the laser emission from BPI was 2.6 °C. Herewith, we note that the laser emission lines were detected in all directions.

 

Fig. 5. Laser lines emitted from BPII and BPI microspheres at 517 nm and 629 nm respectively (a). Lasing spectral tuning by increasing the temperature. Laser emissions from BPI were recorded at 574 nm, 608 nm, 620 nm, and 629 nm, corresponding to a wavelength tuning range of about 55 nm, (b).

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We suppose that the existence of the fluorescence “bell-shaped” curves beside the laser peaks could be an investment of other droplets which are also excited by the pump laser beam, but the threshold is not enough for lasing, and they emit fluorescence. As a next step, we investigated the laser emission intensity as a function of the pump energy. The measurements were performed at λ = 514 nm for a BPII microsphere, and at λ = 629 nm for a BPI microsphere.

As seen from Fig. 6(a, b), we observed a threshold behavior similar to lasing in the CLC phase In particular, bellow the lasing thresholds (2.2 mJ/cm² for BPII microspheres and 2.8 mJ/cm² for BPI microspheres) there is no laser emission and the output light intensities are directly proportional to the input pump energies, while over these thresholds the intensive laser peaks were observed.

 

Fig. 6. Lasing threshold and intensity as a function of pump energy for BPII (a) and BPI samples (b) respectively.

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4. Conclusions

In conclusion, we have demonstrated an omnidirectional lasing from dye-doped liquid crystal BPII and BPI microspheres. Due to the temperature-dependent selective reflection of BPI, the temperature-controlled lasing tuning of about 55 nm was obtained. The temperature guided broad tuning range of BP lasers, coupled with their self-assembly behavior and microscopic sizes provides a number of applications of these materials in areas such as sensing, smart optical filters, diagnostics, thermo-addressable optical filters, displays and optical lenses.