No abstract

In the aim to get high quality graphene films, with large domains and free from impurities, minimizing also the manufacturing costs, we investigate the graphene grown on copper (Cu) foil by chemical vapor deposition at ambient pressure conditions, by using methane (CH4) as carbon source, diluted in a suitable mixture of argon (Ar) and hydrogen (H2). Several graphene samples were synthesized, for variable exposure times to hydrocarbon precursor, in the range from 1 min to 1 hr. The quality of the graphene films and their structural, morphological, and electronic properties were evaluated by micro‐Raman spectroscopy and other techniques, including, scanning tunneling microscopy, atomic force microscopy, and scanning electronic microscopy. In particular, samples obtained with shorter growth time (less than 10 min) exhibit a non‐uniform coverage of the Cu surface, whereas those synthesized with exposure time between 10 and 30 min show a prevalence of well‐ordered monolayer graphene domains. For longer deposition, the amount of disordered domains increases, as revealed by Raman analysis, and the resulting film shows a nonself‐limiting growth behavior for chemical vapor deposition at atmospheric conditions. In addition, we observed 2 kinds of monolayer graphene, in terms of coupling with the Cu surface, for the samples synthesized between 10 and 30 min. To the best of our knowledge, “coupled” and “decoupled” graphene regions have never been reported at the same time on Cu surface. Furthermore, a Raman statistical analysis has been performed on the G and 2D bands measured in both the kinds of regions, gaining evidence of a bimodal behavior for the graphene spots, corresponding to “coupled” and “decoupled” configurations. This difference, which is appreciable also by the optical microscopy inspection, could be related to the local Cu oxidation and to oxygen intercalation after graphene growth.

Amplified spontaneous emission intensity and gain spectra in polarized light have been measured in a dye doped nematic liquid crystal for different orientation of its optical axis and pump intensity. A possibility for switching the gain of the liquid crystal by an external electric field is shown experimentally. The liquid crystal materials with field controlled gain can be used in microlasers and light micro-amplifiers in both planar and waveguiding geometry.

The polarized spectra of the intensity and gain of the amplified spontaneous emission (ASE) have been measured for two glycerin solutions of a laser dye Rhodamine-640 (RhA and RhB with concentrations of 3 × 10 −4 and 3 × 10 −3 M/L, respectively). The solutions were pumped by the linearly polarized light. The ASE polarization ratio r strongly depends on the pump pulse energy, which is continuously increasing up to one for RhA and saturating at the level of r ≈ 0.8 for RhB. The gain spectra have been measured by variation of the amplification length for ASE. With increasing pump beam energy, the maximum gain continuously grows for RhA and saturates for RhB, reaching the values of 14-15 and 6 cm −1 for both materials for the polarization parallel to the pump beam polarization. At a particular spectral point, the gain coefficient has also been measured by the direct amplification of the probe beam of a microlaser on a cholesteric liquid crystal.

The spectra of amplified spontaneous emission (ASE) intensity and gain have been measured for solutions of laser dye Rhodamine-640 in glycerin with light scattering TiO 2 nanoparticles. Such solutions typically manifest random lasing in backscattering geometry. In this work, the conventional transverse pump geometry is used, and the ASE gain spectra are measured using a variation of the length of the illuminated (pumped) stripe. In this geometry, with increasing concentration of nanoparticles, the "unidirectional" gain along the stripe decreases dramatically. At the same time, strong narrowing of the ASE bandwidth is observed even in the absence of net gain. The experimental results help one to understand the role of the scattering particles in the light amplification. On the one hand, the particles increase a zigzag type of the light path within the amplifying medium, which results in bandwidth narrowing and strong amplification of chaotic light beams. On the other hand, the amplification of a seed beam propagating in a selected direction is dramatically suppressed. The results are discussed using simple modeling of ASE amplification and qualitative arguments.