Random lasing and weak localization of light in dye-doped nematic liquid crystals

Abstract

The first observation of random laser action in a partially ordered, optically anisotropic nematic liquid crystal with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve collapses and the typical narrowing and explosion effect leads to discrete sharp peaks. The unexpected surviving of interference effects in recurrent multiple scattering provide the required optical feedback for lasing in nematics. Coherent backscattering of light waves in orientationally ordered nematic liquid crystals manifests a weak localization of light which strongly supports diffusive laser action in presence of gain medium. Intensity fluctuations of the speckle-like emission pattern indicate the typical spatio-temporal randomness of diffusive laser emission. A comparison of the laser action is reported for systems with different order degree: fully disordered semiconductor powders, self-ordered cholesterics and partially ordered nematic liquid crystals.

Soliton-assisted random lasing in optically-pumped liquid crystals

“…In recent years they have attracted a great deal of attention, mainly due to the versatility stemming from cavity-less geometries and the ease of realization [1][2][3][4][5][6][7][8][9]. In liquid crystals, suitable dopants can provide the gain action through optical pumping, while optical birefringence in conjunction with intense fluctuations of the dielectric tensor yield the required recurrent multiple scattering for random resonances to occur [10][11][12][13][14][15][16][17]. In the nematic phase, moreover, liquid crystals are positive uniaxial materials subject to optic axis reorientation under the action of electric fields, either at low or optical frequencies [18].…”

“…The large interaction volume can exploit several feedback paths through scattering, support many coupled lasing modes [44] and their competition/thermalization [45], producing smoother spectrum and spatial profile as compared to "standard" random lasers. Furthermore, owing to anisotropic light scattering in uniaxials [46], the spontaneous as well as the stimulated emissions in NLC doped with pyrromethene-dye tend to be polarized in the plane of the director alignment [10,43]; hence, the generated photons are co-polarized with the reorientational soliton and can be trapped in the light-induced waveguide, the nematicon. At variance with a standard one-beam configuration (see Fig.…”

“…Such guesthost mixture, when resonantly pumped by nanosecond laser pulses at 532nm, is able to provide fluorescence and optical amplification in the visible spectrum around 570-580nm [10,43]. When excited by mW-power e-wave beams, through non-resonant reorientation it can also support the formation and propagation of self-guided spatial optical solitons of arbitrary wavelength [20].…”

Lowering the excitation threshold of a random laser using the dynamic scattering states of an organosiloxane smectic A liquid crystal

“…7 The most extensively studied LC phase for random lasers, however, is the nematic phase. [8][9][10][11][12][13] In random lasers, an important parameter is the transport mean free path l, which is defined as the distance a photon travels before its direction is randomized, because it influences the excitation threshold for laser emission. This property, which is also inversely proportional to the scattering cross-section, is sensitive to external fields in nematic LCs.…”

“…This property, which is also inversely proportional to the scattering cross-section, is sensitive to external fields in nematic LCs. Consequently, reports have typically focused on thermally, [8][9][10] optically, 12 and electrically 13 driven changes in the scattering properties and the subsequent effect they have on the laser characteristics. There have also been studies carried out on the statistical properties of the emission from dye-doped nematic random lasers.…”