Influence of liquid crystalline phases on the tunability of a random laser

In this paper, we report the temperature behavior of an optimized disordered photonic system-based liquid crystal by means of heat capacity and refractive index measurements. The scattering system is formed by a porous borosilicate glass random matrix (about 60%) infiltrated with a smectogenic liquid crystal (about 16%) and a small amount of laser dye (0.1%). The rest of the scattering system is about 24% air, giving rise to a high refractive index contrast scattering system. Such a system has the functionality to change the refractive index contrast with temperature due to the liquid crystal temperature behavior. The system, optically pumped by the second harmonic of a $Q$-switched Nd:YAG pulsed laser working at 532 nm, exhibits random laser action, the threshold of which depends upon the liquid crystalline mesophase. Temperatures of existence of the smectic-$B$ phase correspond to the most optimized random laser. In such a mesophase, the transport mean free path has been determined as about 16 μm in a coherent backscattering experiment.

Figure 1

SEM images of the pressed colorless borosilicate glass as disks of 1 cm diameter and 1.5 mm thickness. View of the disk-surface and lateral-surface at two magnifications.

Figure 2

Schematic setup for random lasing measurements. L and F stand for lens and filter, respectively. The filter was used to block the pump radiation at 532 nm.

Figure 3

Experimental configuration for coherent backscattering measurements. BS stands for a nonpolarized cube beam splitter; Pol stands for a linear polarizer; PM stands for a photomultiplier mounted on a translating stage, and the Chopper is used at a frequency of 20 Hz.

Figure 4

Heat capacity data as a function of temperature for the confined $X_1$ sample. Black symbols and continuous curve account for heating and cooling runs, respectively. The inset shows refractive index data as a function of temperature for both $X_1$ (empty symbols) and borosilicate glass (BG) (full symbols). Arrows in the inset correspond to three representative temperatures of the Sm-$B$, Sm-$A$, and $N$ mesophases.

Figure 5

Emission intensity peaks for the $X_1$ sample confined to the BG disk at several excitation energies at 313 K with the sample in the Sm-$B$ mesophase [A: -Δ- ($0.3\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$); $\mathrm{B}:-\square -\left(2.6\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}\right)$; C: -ο- ($5.3\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$); D: -◊- ($15.8\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$)]. The inset shows the intensity at the peak as a function of the excitation intensity.

Figure 6

Emission intensity peaks for the $X_1$ sample confined to the BG disk at several excitation energies at 325 K with the sample in the Sm-$A$ mesophase [A: -Δ- ($0.5\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$); $\mathrm{B}:-\square -\left(3.2\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}\right)$; C: -ο- ($8.4\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$); D: -◊- ($15.8\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$)]. The inset shows the intensity at the peak as a function of the excitation intensity.

Figure 7

Emission intensity peaks for the $X_1$ sample confined to the BG disk at several excitation energies at 337 K with the sample in the $N$ mesophase [A: -Δ- ($1.6\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$); $\mathrm{B}:-\square -\left(6.3\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}\right)$; C: -ο- ($10.5\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$); D: -◊- ($15.8\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$)]. The inset shows the intensity at the peak as a function of the excitation intensity.

Figure 8

Linewidth of the emission intensity peaks as a function of the excitation intensity for the sample $X_1$ at 313 K (filled circles), 325 K (filled squares), and 337 K (filled diamonds). The inset shows the linewidth of the emission intensity peaks as a function of the excitation intensity normalized by that of the laser threshold.

Figure 9

Laser threshold excitation intensity as a function of the refractive index contrast ratio ($w$) for sample $X_1$ at 313, 325, and 337 K. Also included is the symbol corresponding to a closely related system in Ref. [22].

Figure 10

(a) Emission intensity peaks for the $X_1$ sample confined to the BG disk at an excitation intensity of $15.8\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$ at several temperatures. The inset shows the linewidth of both the emission intensity peaks (circles) and the peak intensity (squares) as a function of temperature. Empty and full symbols correspond to heating and cooling experiments, respectively. (b) Emission intensity peaks at several excitation energies at 357 K $\mathrm{A}:-\mathrm{\Delta }-\left(15.8\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}\right);\mathrm{B}:-\square -\left(47\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}\right);\mathrm{C}:-\mathrm{o}-\left(63\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}\right);$ D: -◊- ($84\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$)]. The inset shows the linewidth of the emission intensity peaks as a function of the excitation intenisty.

Figure 11

Coherent backscattering cone for the $X_1$ sample confined to the BG disk. The sample thickness is 5 mm and was measured at room temperature in the superccoled Sm-$B$ phase $\left(\mathrm{Sm}-B_{\mathrm{spc}}\right)$.