1. Introduction

Lasers have immensely influenced development of modern science and technology in a broad scope of roles ranging from highly specialized scientific light sources, characterization instruments, robust engineering and miniature communication devices, to components of mass produced household electronics. Liquid crystal (LC) lasers are a special kind of mirrorless dye lasers in which fluorescent-dye-containing LC material acts as a gain medium and at the same time provides a distributed feedback cavity [1]. When excited with an optical pump, gain in LC medium overcomes losses and produces lasing emission at a wavelength defined by the resonant cavity. Strongly scattering optical systems with introduced gain can exhibit spectral and temporal properties of a multi-mode laser oscillator and a laser-like behavior even without external cavity [2]. Dielectric media with a random light scattering structure produce random lasing action when gain exceed the losses of the system [3]. Such material systems are known as random lasers (RLs). Random lasing has been anticipated in media with strong scattering which produces recurring scattering events which with the presence of gain can produce random lasing [2,4]. Multiple scattering results in an enhanced photon mean free path and laser amplification [4,5]. Random scattering defines optical modes with a central frequency and bandwidth, lifetime, and enables the light to stay within the gain medium long enough for amplification to become efficient. Lasing threshold in a material is met when gain exceeds the losses. Random recurrent scattering forms multiple cavities distributed in the bulk of a material which oscillate at frequencies slightly different from each other. As the pump energy exceeds the threshold, multiple discrete lasing modes often become visible in the emission spectrum. Modes with the longest lifetime that have the lowest lasing threshold acquire the highest intensity [6]. The angular distribution of the output of RLs is spread over the solid angle of $4\pi \Omega$. Investigation of RLs is motivated by fascination and fundamental interest to understand interaction of light with complex structures of scattering systems as well by potential scientific and technological applications [3]. Strong scattering with gain and localized lasing modes are not trivial to achieve in material systems [2,4,7,8]. Early random laser experiments were based on powdered materials and colloidal suspensions of micro or nano sized solid inorganic particles in a fluorescent dye containing media [2,8]. Light-scattering supramolecular structures with fluorescent dye doped liquid crystal materials can satisfy these requirements.

Internal topological structure and anisotropy makes LCs effective host material system for random lasers. Random lasing has been achieved in variety of dye doped LC host materials and phases including nematic LCs [9–12], cholesteric liquid crystals (ChLC) [13–15], blue-phase LC (BPLC) [16,17], and smectic A$^{*}$ LCs [18]. Light scattering needed for random lasing in a LC host material can be increased by spontaneously distributing uniform ordering of host LC material using polymer domains [19] or nanoparticle inclusions such as Ag [14], Au [20], Pt [21]. Lasing action can be controlled by external electric or magnetic fields [1,12,22]. Intrinsic simplicity of manufacturing allows creation of LC RL in complex geometries including sphere phase LC [23], LC dispersed in solid polymer [9,10,24], LC confinement in capillary channels [21,25–27], polydispersed LC droplets suspended in optically inactive carrier fluid [15,17], and LC elastomer soft solids [28].

When decreasing temperature from the isotropic phase to ChLC phase, there are three distinct BPLC phases: BPIII, BPII, and BPI. The director axes of blue phases are self-assembled in double twisted helix cylinder. BPIII is amorphous fog phase, whereas BPII and BPI is simple cubic and body-centered cubic crystalline structure. The BPLC are optically isotropic on a macro scale but contain irregular grain boundaries between domains with different molecular orientations. Delicate balance between periodicity of BP domains and diffused light scattering on the grain boundaries allows band edge lasing enabled by Bragg selective reflection [29] or light-scattering-induced random lasing [16,17], both in a very similar host material. Monodisperse BPLC droplets can be generated using mechanical agitation but the size is difficult to control [30]. Microfluidics enabled the miniaturisation of flow geometries [31]. Production of highly controlled and monodisperse microspheres is enabled by microfluidics device design based on coaxially nested glass capillaries with circular and square cross sections [32]. The droplet dimensions can be adjusted by tuning the production parameters: orifice size, flow rates, interfacial tension and viscosities of the fluids involved [33,34]. The common choice for emulsifier or stabilizer is either an ionic surfactant or a water-soluble polymer [35].

In this work we present a random laser achieved in BPI LC material encapsulated into a free-standing spherical polymer microshell. Encapsulation enables delivery of optically active BP spheres to a variety of locations and surfaces of arbitrary shape which are not restricted by a traditional LC layer confined into a cell made of two substrates. Micro-shell encapsulation geometry allows previously impossible multilayer stacking or large length scale periodic or random assemblies of optically active spheres. In contrast to a high-precision manufacturing required to build ultra-precise cavities of other lasers, reported RL benefits from typical low cost of LC based RL lasers, simplicity of fabrication, and abundance of raw materials required for synthesis of LCs. Topologically and structurally enabled omnidirectional profile allows to consider reported RL droplets as a light source as well as an actuator in sensing applications in complex geometries [20,36].

2. Materials and methods

2.1 Preparation of mixtures

The LC studied in this work was a mixture of nematic LC: E31 from BDH, now Merck ($\Delta$n = 0.22, $\Delta \varepsilon =16.2$, clearing point = 61.5 $^{\circ }C$), and right handed chiral dopant (CD) R811 (4-$\{$ [(1- methylheptyl)oxy]carbonyl$\}$ phenyl-4- (hexyloxy)benzoate) from Fusol Materials, China, with a helical twisting power (HTP) = 10.3 $\mu m^{-1}$. The nematic and CD concentrations were 64.5 wt.$\%$ and 35.5 wt.$\%$, respectively. The phase temperature of LC mixture was identified by placing the sample on a thermal stage connected to a high-precision temperature controller (Instec, Inc.). The phase transition of dye-free mixture was observed at a series of temperatures using a Nikon polarized optical microscope (POM) as: ISO - 29$^{\circ }C$ - BPII - 26$^{\circ }C$ - BPI - 17$^{\circ } C$ - ChLC. The respective photos of phase transition as shown in appendix Fig. 5. The dye-doped BPLC mixture was composed of E31 (63.75 wt.$\%$), R811 (35.5$\%$ wt.$\%$) and PM597 (0.75 wt.$\%$). The PM597 (1,3,5,7,8-pentamethyl-2,6-di-t-butyl-pyrromethene-difluoroborate, CAS 137829-79-9, from Exciton) is a high quantum efficiency laser dye which is commonly used in ChLC band edge [37] and random lasers [22]. The resultant mixtures were kept at 60$^{\circ }$C in an ultrasonic mixer for 10 minutes to completely dissolve components into homogeneous isotropic fluid and left to cool under ambient conditions to a room temperature BPLC state. The dispersed phase to incubate dye-doped BPLC microcapsules in the microfluidic chip were prepared by dissolving 2 wt.$\%$ of 2,4-tolylene di-isocyanate (TDI, Sigma-Aldrich) in the dye doped BPLC mixture (E31-R811-PM597) mixture at 40$^{\circ }$C, and followed by incubating at 25$^{\circ }$C for 3 hours to form stable BPLC solution. The continuous phase and collecting phase were aqueous solution containing 5 wt.$\%$ of sodium dodecyl sulphate (SDS, Sigma-Aldrich) and 2 wt.$\%$ of tetraethylenepentamine (TEPA, Sigma-Aldrich). The dispersed phase of LC solutions containing 2 wt.$\%$ of TDI was injected into the left capillary, the continuous phase of aqueous solution containing 5 wt.$\%$ of SDS and 2 wt.$\%$ of TEPA was injected into the interstices between the left capillary and the square capillary. Oil-in-water (O/W) emulsion droplets having innermost oil of the LC and TDI mixture and outer layer of SDS were generated as templates to incubate the dye doped BPLC microcapsules (DBPLCMs) with double-twist structure. The SDS layer could stabilize the O/W interface and create a 3D spherical exterior. A polyurethane (PU) layer rapidly generated via polymerization between TDI and TEPA at the droplet interface can further confine the encapsulated LCs. Then the liquid crystal microspheres were collected in an aqueous solution containing 5 wt.$\%$ SDS and 2 wt.$\%$ TEPA, followed by heating the LCMs to 40$^{\circ }$C to form isotropic microcapsules. The unreacted TDI in the isotropic fluid with a low viscosity migrated toward the microcapsule surface to carry out the further polymerization and the shell ripened. Finally, the mesogens in isotropic microcapsules gradually formed BPLCs via spontaneous self-assembly when decreasing the temperature.

2.2 Sample fabrication

A microfluidic device with combination of co-flow and flow-focusing geometry [38] was created for encapsulation of a blue phase LC mixure. Double emulsification of LC in a single step was achieved using interfacial polymerization method [39].The capillary microfluidic device was designed to have two tapered cylindrical capillaries (each with ID: 0.70 × OD: 0.87, Cat$\#$ CV7087-B-100 from VitroTubes, USA) assembled inside a square capillary (with ID: 0.900 × OD: 0.180, Cat$\#$ 8290-050 from VitroTubes, USA). The left & right cylindrical capillaries were tapered using a micropipette puller: Sutter P-97 (Sutter Instruments, USA) and then carefully sanded using a ceramic tile to have desired orifice diameter as shown in appendix Fig. 8. The droplet size is dependent on the interfacial tension, viscosity of fluid (inner and outer) [32]. To decrease the size of the droplets the capillary carrying dispersed phase was inserted inside a parallel round capillary at the output end, as shown in Fig. 8(a) and 8(b). This increases the flow of continuous phase in agreement with Bernoulli’s principle and reduces the diameter of the double emulsion droplets. The device was interfaced with syringe pumps (NE-300, New Era Inc.) using PTFE tubing (ID: 0.38mm × OD: 0.09mm, Cat $\#$ BB31695-PE/2, Scientific Commodities Inc.). During droplet generation, flow rates were controlled by two syringe pumps. Flow rate used for the production of dye-doped BPLC microsphere: dispersed phase was 20 $\mu$l/hr and continuous phase was 200 $\mu$l/min. The setup conditions defined above were used to generate BP droplets. The droplets were carefully collected on the microscope slide which was rendered hydrophilic by spin-coating a solution of 5 wt.$\%$ lecithin and 95 wt.$\%$ hexane. BP droplets with PU shell were allowed to be ripened for 1 hour and the residual continuous phase was carefully removed to allow the droplet to self-assemble in a single layer, as shown in Fig. 1(b)–1(c). Following procedure was performed to prepare the sample for scanning electron microscope (SEM) analysis: (1) LC capsules were fixed in UV-cured adhesive on a glass slide. (2) Capsules and cured adhesive were cut with blade. (3) Liquid crystal material was washed away with ethanol. (4) Let to dry. (5) The glass slide was cut along the same cutting line in step 2. (6) A thin Au layer was sputter coated on the cross-section. (7) Imaging with high resolution-SEM. A reference cell sample was made using two 1 mm thick soda lime glass as substrate. No additional alignment layer was coated on cell inner surfaces. Cell gap thickness was maintained at 100 $\mu$m using DuPont Kapton film as spacer.

2.3 Lasing setup

Final part of the project was to setup the pump laser, spectrometer and the sample consisting of monodispersed, single layered close-packed BP droplets. The schematic of pumping laser and detection unit used in the experiment is shown in Fig. 3(a). It had a small footprint pump laser assembly as prior reported in literature as well as those adopted by a similar method [19,37]. Laser assembly generated short, linearly polarized pulses at 532 nm for the excitation of the laser dye dissolved in the encapsulated dye-doped blue phase droplets. Lasing experiment was performed by external pumping test sample using a pulsed Nd:YAG laser assembly based on SSY-1 laser head emitting linearly polarized, low repetition rate, 10 ns beam at 532 nm [37]. Energy of the pump beam was controlled by using absorptive polarizer with manually adjustable orientation. Polarizer was placed in front of non-polarizing beam splitter cube providing a reference of a pump beam allowing to record energy of every pulse. Energy of laser beam was measured using Molectron Optimum 4001 pulse energy meter. Preceding the experiment, energy calibration of a pump beam with respect to a reference beam was performed at different orientation of energy controlling polarizer. Calibration allowed to minimize polarization related inaccuracy of pump energy readings. The sample was placed approximately in the focal plane of lens with a focal length of 120 mm. The full-width at half maximum (FWHM) of a beam waist on a sample was $\sim$ 200 $\mu$m, measured using DataRay WinCamD-LCM beam profiling camera and DataRay 8.0 C35 software. Angle of incidence of a pump beam on a sample was kept constant at 135$^\circ$ with respect to the plane of sample’s substrate, as shown in Fig. 3(a) (only the position of detector was changed to establish omnidirectionality). Lasing emission spectra were measured using high-resolution Jobin Yvon-Spex TRIAX 550 spectrometer. Emission from optically pumped sample was collected at an angle of +90$^\circ$ for measurement of a lasing threshold. Omnidirectionality of random lasing emission was confirmed by comparing emission spectra in the range of collection angles [-90$^\circ$; +90$^\circ$].

3. Results

The BPLC mixture was encapsulated with the polymer shells of spherical shape by combination of interfacial polymerization and droplet microfluidics. Polymer shells with encapsulated BP were transferred on a 1 mm thick soda lime microscope glass slide which was rendered hydrophilic. Images of densely packed assembly of PU encapsulated BPLC (See Fig. 6) and dye-doped BPLC droplets (Fig. 7) are viewed with a polarizing optical microscope in reflection and transmission modes with crossed polarizers as shown in Fig. 1. The dye free BP droplets in Fig. 1(b) were created using device (with cappilary tip diameters of 58 $\mu$m and 187 $\mu$m) shown in Fig. 8(a) and flow rates of 40 $\mu$l/hr (dispersed phase) and 200 $\mu$l/min (continuous phase). The PM597 doped BP droplets in Fig. 1(c) and 1(d) were created using device (with capillary tip diameters of 35 $\mu$m and 93 $\mu$m) as shown in Fig. 8(b) and flow rates of 20 $\mu$l/hr (dispersed phase) and 200 $\mu$l/min (continuous phase).

 

Fig. 1. POM micrographs of (a) pure BPI in a cell of 15 $\mu$m thickness (b) dye free BPI droplets viewed in reflection mode, (c) PM597 doped BPI droplets viewed in reflection mode, (d) PM597 doped BPI droplets viewed in transmission mode. Scale bars are 100 $\mu$m. Images acquired with crossed polarizers at 22$^{\circ }$ C.

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Fig. 2. Droplet morphology: (a) SEM sample showing cross section of closely packed droplets cut with a blade and washed with ethanol, (b) top view of an isolated droplet with LC inside, and (c) thickness of encapsulating shell as measured under SEM.

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Polymer shells have nearly spherical shape and droplets are monodisperse with average diameter of 99.0 $\mu$m and standard deviation of 5.7 $\mu$m. The morphology of the encapsulating shell was verified using SEM, as shown in Fig. 2. The shell thickness was found to be within the range of 3-6 $\mu$m as measured at various sites of different shells. Encapsulating PU has refractive index n $\approx$ 1.48 and is transparent in a visible range of spectrum [40,41]. Polymer shells with encapsulated BPLC were compared with a reference LC cell. Studies have shown that the droplet morphology extends the temperature range of the BP in droplets [42,43] which was experimentally verified for wide temperature BP droplets with following transitions: ISO - 29$^\circ C$ - BPII - 25$^\circ C$ - BPI - 10$^\circ C$ - ChLC as compared to BP in cell, ISO - 29$^\circ C$ - BPII - 26$^\circ C$ - BPI - 17$^\circ C$ - ChLC.

Dye-doped samples were excited with a Nd:YAG, 532 nm laser in a setup shown in Fig. 3(a). Appearance of a pumped sample during lasing experiment is shown in Fig. 3(c) & 3(d). Pump beam focused on a sample excited multiple droplets. Both pump beam transmitted through the sample and emission from the sample can be directly observed on a white screen placed parallel to the sample, as shown in Fig. 3(c).

 

Fig. 3. (a) Schematic of the optical setup for BPLC microsphere lasing, (b) spectra of the pump beam and 0.75 wt.$\%$ PM597 doped nematic mixture of E31 with a 100 $\mu$m optical path, (c)-(d) appearance of a sample when excited with pump beam.

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Emission from the pumped sample during lasing threshold measurement was collected at +90$^{\circ }$. The lasing threshold for the PM597 doped BPI in a 100 $\mu$m cell without any alignment layer was determined to be $\sim$ 7.8 $\mu$J/pulse, while the lasing threshold for the PM597 doped BPI in an thin PU shell was lower at $\sim$ 3.7 $\mu$J/pulse, as shown in Fig. 4(a). Random lasing emission spectra from both polymer encapsulated samples and a cell are shown in Fig. 4(b) and 4(c), respectively. The omnidirectional nature of random laser emission from the droplets is confirmed by moving the detector within a range of collection angles from +90$^{\circ }$ to -90$^{\circ }$, as shown in Fig. 4(d).

 

Fig. 4. Plot of laser emission intensity versus pump energy for (a) lasing thresholds for PM597 doped BPI in 100 $\mu$m droplets (blue) and PM597 doped BPI in a 100 $\mu$m cell (red), and emission spectra of: (b) PM597 doped BPI in a 100 $\mu$m cell, (c) PU encapsulated BPI droplets of 100 $\mu$m diameter, (d) emitted spectra collected across the plane of optical pump confirming omnidirectionality in BP droplets.

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Furthermore, laser emission spectra where chosen each for droplet and cell such that the number of random lasing modes were equal, green spectra in Fig. 4(b) and blue spectra in Fig. 4(c), each with six peaks (or random modes). These spectra were used to characterize the resonant frequencies, Q-factors and average mode spacing of the random modes in the cell and the droplet configuration, respectively and elaborated in Table 1.

Table 1. Comparative quantitative metrics of random lasing modes in a cell and droplet.

View Table

4. Discussion

Random lasing is often demonstrated by observation line narrowing of emission spectrum above the pump energy threshold of non-collimated and highly diffused light output [13]. Lasing threshold of a dye doped LC laser is empirically determined using commonly accepted technique monitoring of emission intensity as a function of pump energy [44]. The threshold is defined as an intersection of two linear parts of emission intensity curve. Observation of a lasing threshold is supplemented by rapidly decreasing FWHM of the emission peak from the sample.

The 532 nm pump beam is strongly absorbed by PM597 dye which has absorption band centered at 529 nm with FWHM of 35 nm [45]. When excited with a 532 nm pump beam, the sample experiences a broad fluorescent emission with maximum wavelength centered at 572 nm and FWHM of 58 nm, as shown in Fig. 3(b). At a larger pump energy emission spectrum becomes narrower and at the same time emission intensity increases rapidly. For lasing threshold measurement, the detector was placed at angle of +90$^{\circ }$ relative to the sample as shown in Fig. 3(a). As the pump energy reaches the lasing threshold, laser emission emerges along a solid angle of 4$\pi \Omega$. The emission spectra acquired from droplets at increasing pump energy are shown in Fig. 4(c). The threshold lasing values for PM597 doped BPI in 100 $\mu$m cell and 100 $\mu$m droplets are shown in Fig. 4(a). Following this method, the lasing threshold for 100 $\mu$m droplets was determined as 3.7 $\mu$J/pulse whereas the lasing threshold for dye doped BP in 100 $\mu$m cell with no alignment layer was determined to be 7.8 $\mu$J/pulse. There is almost 50$\%$ reduction in the threshold value inside the droplets due to the lensing effect inside the droplets. The spherical surface of the droplets acts like a focusing element thereby increasing photon density inside the droplets and making them more efficient sources of liquid crystal lasers.

Random lasing emission spectra appear equivalent when measured from polymer encapsulated sample at collection angles from -90$^\circ$ to +90$^\circ$ with respect to the plane of substrate holding polymer capsules. Omnidirectionality of random lasing emission is enabled by the topological structure of a BPLC and enhanced by the geometrical shape of a polymer encapsulating vessel. Lasing modes appearing stochastically from pulse to pulse which is characteristic of RLs [5] based on ChLC [13] and BP [16]. Narrow emission lines representing spatial resonances in a LC material [22] with pulse-to-pulse fluctuations in emission spectra due to spontaneous emission within randomly scattering gain media [3]. Emission of described RL is in good agreement with LC RLs using the same fluorescent dye PM597 [11,12,25,46].

Emission spectra are acquired for single pump pulse. At higher pump intensity, discrete narrow peaks emerge in the emission spectrum. There is a probability that a photon is scattered back to the same scattering point from which it is scattered earlier. Thus, the pump intensity first reaches the threshold where the gain length is near the maximum. A significant spectral narrowing and an increase of peak emission intensity occurs. Then, the pump intensity reaches threshold where the gain exceeds the loss in some random cavities. Lasing oscillation occurs in these cavities, adding discrete peaks to the emission spectrum. The presence of discrete peaks proves random lasing with resonant feedback within the droplets [47]. When the gain length and excitation volume are the same, a fewer number of scatterers leads to weaker optical scattering. Hence, the number of random cavities where the lasing threshold can be reached is smaller. The lasing-wavelengths varied from pulse to pulse, indirectly confirming that the laser spikes did not result from the whispering gallery effect of the microspheres.

To make an even comparison, laser emission bands were chosen for cell and droplet such that the number of random emission peaks were equal, implying the same number of resonant feedback paths. The quantitative metrics of the random lasing modes from these emission bands are tabulated in Table 1. It can be observed that the random lasing modes from the PU encapsulated BPI droplets consistently provide higher Q-factor. Q-factor value above 4000 was observed for random lasing modes from the 100 $\mu$m droplets. Whereas highest Q-factor value of 2200 was observed for random lasing modes from the cell. The emission band or the random lasing modes for cell and droplets were in the range of 575-585 nm and 577-585 nm respectively, while the average mode spacing for the droplet was smaller at 0.97 nm compared to 1.56 nm from the cell.

In summary, coherent random lasing is observed to take place in dye-doped free-standing blue phase microspheres, in which the randomly distributed BPI crystals contribute to the resonant feedback. Random lasing emission is evidenced by empirically observed lasing threshold and narrowing of the emission spectrum. Lasing threshold in polymer encapsulated samples is 3.7 $\mu$J/pulse which is smaller compared to lasing threshold measured with the same LC mixture in a reference cell. Polymer encapsulation enables free-standing random laser vessels which do not require traditional LC cell. Free-standing random lasing capsules can be assembled in aggregates on a carrier surface to be embedded into a host medium. The randomness and the number of spikes in the lasing emission peak are related to the number of closed loop light-paths and the size of BPI crystal domain size. The size of crystal domain can be controlled by the cooling rate during the BPLC fabrication process [16]. Compared to other LC mesophases (nematic, ChLC, ferroelectric), BPLC clearly stand out as a promising laser host as they are optically isotropic. They are relatively easy to fabricate as no surface alignment layer is needed. These droplets can be attached on top of a fiber laser to create omnidirectional laser sources with potential to be used in medical diagnostics or can be coupled evanescently with fiber laser. We envisage that this work will open new avenues for applications in the burgeoning field of fiber-lasers for medical application.

Appendix

5. Textures of blue-phase liquid crystal in a cell and in freestanding droplets

5.1. Reflection microscopy images of BP transition in a 15 $\mu$m cell

 

Fig. 5. POM study with LC mixture in a 15 $\mu$m cell with no alignment layer. The cell was studied under reflection mode with crossed polarizers (Scale bars are 100 $\mu$m).

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5.2. Reflection microscopy images of encapsulated pure blue-phase droplets

 

Fig. 6. Transition temperature study of pure BP droplets. The droplets were observed under reflection mode with crossed polarizers (Scale bars are 100 $\mu$m).

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5.3. Reflection microscopy images showing dye-doped blue-phase droplets

 

Fig. 7. Transition temperature study of dye-doped BP droplets. The droplets were observed under reflection mode with crossed polarizers (Scale bars are 100 $\mu$m).

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6. Microfluidic device

 

Fig. 8. (a)-(b) Microfluidic devices with combination of flow-focusing and co-flow geometry (Scale bars: 100 $\mu$m) (a) device to create pure BP droplets, (b) device to create dye-doped BP droplets, (c) photo of the nested glass capillary microfluidic device, (d) cartoon of the device interfacial polymerization method used to stabilize droplets.

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Funding

Ohio Development Services Agency (TECG 2015-0128).

Acknowledgments

All lasing experiments were performed in Prof. Peter Palffy-Muhoray’s laboratory. The SEM imaging was done at the characterization facility in the Advanced Materials and Liquid Crystal Institute at Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials.

Disclosures

The authors declare no conflict of interests.

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