Emission properties of random laser media with a bubble structure

Summary form only given. In recent years, several novel structures of amplifying random media have been reported for controlling the spectrum and efficiency of random laser emission [1-3]. For example, monodisperse scattering particles have shown to allow us to control the emission spectrum [1], and when there is a defect in the distribution of monodisperse particles, light is found to be concentrated and enhanced in the defect region [2]. These two studies have revealed that the laser emission can be controlled by adding some regularity to the random structure. Another variation in the structure is the shape of the scatterers, which has been found to affect the efficiency of random lasing [3]. In this study, we investigate the random laser emission from random media with a “bubble structure” which has non-scattering regions distributed randomly. Although this structure seems to be similar to superdiffusive media [4], the non-scattering regions are not controlled to realize a Lévy flight for light.Figure 1 shows a schematic of the random structures we used. Random media with a bubble structure are fabricated by using pulverized photopolymer (Gluelabo GLX19-117) doped with rhodamine 6G (R6G) laser dye as spacer particles (SPs). The size of SPs ranges from 53 to 300 μm. TiO2 scattering particles with a diameter of 180 nm and the SPs are dispersed in light curable photopolymer (Adell K40) which is doped with R6G with a density of 6.7 × 103 mol/l. The volume filling fraction of scattering particles is 10%. K40 and GLX polymers have the same refractive index of 1.5, and are doped with the same density of R6G. The liquid polymer medium is then put on a slide glass with a cover glass on top and cured by illuminating it with ultraviolet and visible light for 10 min. The thickness of the fabricated random media is 0.2 mm. A typical emission spectrum obtained from a bubble structure is shown in Fig. 2, together with one for a homogene- us medium for comparison. The bubble medium is seen to exhibit higher peak intensity than the homogeneous medium. We performed experiments with various ranges of the size of SPs: 53-300 μm, 53-106 μm, 106-180 μm, and 180-300 μm. Results show that bubble structures present up to 1.5 times higher emission intensities than the homogeneous one for the SPs of 53-300 μm. The filling fraction of SPs in which strong light emission occurs was found to be 5 to 10%. It is interesting to note that amplifying random media with R6G doped GLX as a host medium show no laser emission so that the emission efficiency of R6G in GLX is much lower than R6G in K40.We investigated the mode properties of the bubble medium to examine the reason for the strong emission. The intensity correlation between different pairs of spikes in the emission spectrum was evaluated [5] for a homogeneous medium and a bubble medium containing 5% of the SPs with sizes of 53-300 μm. The average number of independent resonant modes was found to be about 2.7 for both cases, whereas the average number of spikes per mode was about 1.5 for the bubble medium and 2 for the homogeneous medium. The result shows that addition of SPs in an amplifying random medium leads to a reduction in the number of resonant frequency in each random lasing mode. Numerical simulations are being planned to elucidate this phenomenon.