Nematicon-driven injection of amplified spontaneous emission into an optical fiber

Abstract

We investigate experimentally the interaction between amplified spontaneous emission (ASE) and a soliton, which are both generated in a dye-doped nematic liquid crystal (LC) cell. A light beam is injected through an optical fiber slid into the cell to form a soliton beam. ASE is then automatically collected by this self-induced waveguide and efficiently coupled into the same optical fiber, in the backward direction. We demonstrate that the presence of the soliton improves the ASE collection by one order of magnitude. We also show that the ASE is highly polarized in the plane of the LC cell, and that the ASE spectrum depends on the pump stripe orientation with respect to the LC director. The origin of the spectral anisotropy of the gain curves is determined with the help of femtosecond pump-probe spectroscopy. © Amplified spontaneous emission (ASE) arises in gain media that are excited with a stripe-shaped beam, like in fiber amplifiers or bulk amplifying materials. Photons that are emitted spontaneously along the long axis of the excited area are amplified more than those emitted in other directions. The ASE is therefore directional and narrow-band, since during the propagation the wavelength with the highest optical gain emerges from the photo-luminescence broadband spectrum [1][2][3][4]. The absence of feedback simplifies the architecture of the device with respect to lasers and is also responsible for a low spatial coherence. ASE is therefore a better candidate for illumination purposes than broadband and narrow-band lasers, which are impaired by speckle [5, 6].ASE has been widely studied in solid state films [7][8][9][10][11], microfluidic devices [12, 13] and only recently in dye-doped nematic liquid crystals (LCs) [14, 15]. Nematic LCs are characterized by a mean orientation of the molecules, called director. Rodlike LCs exhibit anisotropic permittivity, with a different value parallel (ε ) and orthogonal (ε ⊥ ) to the long axis of the molecules. When the anisotropy is positive (ε > ε ⊥ ), LC molecules are reoriented parallel to an applied external electric field. In particular, a light beam can reorient the LC director locally, which induces a local increase of the refractive index. The beam propagates without diffraction as it is confined by the self-induced waveguide. This soliton-like beam is called nematicon, as it occurs in nematic LCs [16][17][18]. Once the nematicon is formed, it can be used as waveguide to transmit other beams [16]. In our case the nematicon is generated in a dye-doped LC cell. Elongated molecules, like dyes or polymers, which are dispersed in nematic liquid crystal host show the tendency to align themselves along the director [19, 20]. The ordering imprinted by the LC also increases the pumping efficiency and the polarization ratio of the emission. The main drawback of LC as host for photoluminescent materials is their need to be integrated into glass cells, in order to have a macroscopic order of the LC. The edges of the cell are either sealed with glue or left open, in both ...

Soliton-assisted random lasing in optically-pumped liquid crystals

“…2(b) displays the corresponding results for an ordinary-polarized pump: as before, the nematicon increases the lasing efficiency. For o-polarized pump light the threshold is slightly lower and a marked increase in the output-vsinput slope and lasing efficiency is clearly visible, consistently with previous observations in the same material [10,43] In the latter geometry, the o-pump is not confined but interacts more efficiently with the guest-host, θ0 is the bulk orientation due to the boundary conditions. (e) Experimental setup: SM, spectrometer; OBJ, microscope objective; Laser1, pulsed pump laser at 532nm; Laser2, cw laser at 1064nm.…”

“…Aided by nematicons, reorientational and electronic nonlinear responses, characterized by distinct time-and power-scales, can synergystically be combined [39,40]. Owing to their large numerical aperture [41], nematicon waveguides solitons have also been employed in experiments involving incoherent light generation by fluorescence [42] or amplified spontaneous emission [43], offering a means to better collect and couple the emitted light into optical fibers.In this Letter we demonstrate a novel example of synergy between diverse nonlinear responses: the combination of spatial solitons and random lasing into a "nematicon random laser", whereby a low-power self-confined beam provides a guided-wave landscape for the stimu- * assanto@uniroma3.it lated emission induced by collinear optical pumping of dye-doped nematic liquid crystals (NLC).Several benefits can be expected from adopting such light-induced guided-wave configuration for random lasing. At variance with standard thin film geometries, the thick NLC cell provides an extended volume where optical pumping can produce fluorescence and, in turn, stimulated emission and lasing action via random scattering and feedback.…”

“…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.…”

“…Aided by nematicons, reorientational and electronic nonlinear responses, characterized by distinct time-and power-scales, can synergystically be combined [39,40]. Owing to their large numerical aperture [41], nematicon waveguides solitons have also been employed in experiments involving incoherent light generation by fluorescence [42] or amplified spontaneous emission [43], offering a means to better collect and couple the emitted light into optical fibers.…”

“…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].…”