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 Physics , 2010, DOI: 10.1103/PhysRevLett.105.127401 Abstract: On the basis of a full-vectorial three-dimensional Maxwell-Bloch approach we investigate the possibility of using gain to overcome losses in a negative refractive index fishnet metamaterial. We show that appropriate placing of optically pumped laser dyes (gain) into the metamaterial structure results in a frequency band where the nonbianisotropic metamaterial becomes amplifying. In that region both the real and the imaginary part of the effective refractive index become simultaneously negative and the figure of merit diverges at two distinct frequency points.
 Physics , 2014, DOI: 10.1103/PhysRevB.92.085407 Abstract: In contrast to conventional structures, efficient non-radiative carrier recombination counteracts the appearance of optical gain in graphene. Based on a microscopic and fully quantum-mechanical study of the coupled carrier, phonon, and photon dynamics in graphene, we present a strategy to obtain a long-lived gain: Integrating graphene into a photonic crystal nanocavity and applying a high-dielectric substrate gives rise to pronounced coherent light emission suggesting the design of graphene-based laser devices covering a broad spectral range.
 Journal of Science and Technology of Agriculture and Natural Resources , 2008, Abstract: In this research, after optimization of sterilizing cyst and larvae of second stage of Heterodera schachtii, the possibility of using nematode on seedlings of sugar beet (Beta vulgaris L.) in in vitro condition was studied for developing larvae to cyst. For this purpose, non sterile cysts were extracted from infected soil and hatched into zinc chloride solution with concentration of 0.5gl-1. Then, for preparation of sterile second stage larvae, several sterilizing treatments were used . Mean comparisons were performed between sterilized live larvae number by Duncan's method. Results showed that 70% ethanol for 1 minute followed by 5% sodium hypochlorite for 5 minutes and 0.1% sodium hypochlorite for 20 minutes were the best treatments for disinfecting cysts and larvae, respectively. In the next step, two nematode susceptible sugar beet varieties were applied to produce cyst from the larvae in in vitro culture. PGoB medium containing different hormonal compositions was used to produce hairy roots and inoculation of seedling with sterilized larvae. After nematode inoculation tests, were the stained cysts were observed under stereomicroscope and counted 40 days later. Five to twelve cysts were formed on the roots of each seedling from two varieties. As a result, it seems that this technique can be used for sugar beet germplasm evaluation to screen nematode resistant genotypes in in vitro controlled condition.
 Physics , 2008, DOI: 10.1103/PhysRevLett.101.093002 Abstract: We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding respectively to Mollow, Raman and Four Wave Mixing mechanisms. We measure an output power of up to 300 $\mu$W and present the main properties of these different lasers in each regime.
 Physics , 2011, DOI: 10.1364/OL.36.001200 Abstract: We present a variety of dissipative solitons and breathing modes in a medium with localized gain and homogeneous linear dissipation. The system posses a number of unusual properties, like exponentially localized modes in both focusing and defocusing media, the existence of modes in focusing media at negative propagation constant values, the simultaneous existence of stable symmetric and anti-symmetric localized modes when the gain landscape possesses two local maxima, as well as the existence of stable breathing solutions.
 Physics , 2005, DOI: 10.1209/epl/i2004-10450-8 Abstract: We investigate the propagation of a coherent probe light pulse through a three-level atomic medium (in the $\Lambda$--configuration) in the presence of a pump laser under the conditions for gain without inversion. When the carrier frequency of the probe pulse and the pump laser are in a Raman configuration, we show that it is possible to amplify a slow propagating pulse. We also analyze the regime in which the probe pulse is slightly detuned from resonance where we observe anomalous light propagation.
 Adrian C. Selden Physics , 2010, DOI: 10.1016/j.optcom.2011.06.021 Abstract: The diffuse reflectance (albedo) and transmittance of a Raman random gain medium are calculated via semi-analytic two-stream equations with power-dependent coefficients. The results show good agreement with the experimental data for barium nitrate powder. Both the Raman albedo AR and Raman transmittance TR diverge at a critical gain gc, interpreted as the threshold for diffusive Raman laser generation. However, the ratio TR/AR approaches a finite limiting value dependent on particle scattering albedo v and scattering asymmetry g. The dependence of the generation threshold on the scattering parameters is analysed and the feedback effect of Fresnel reflection at the gain boundaries evaluated. The addition of external mirrors, particularly at the pumped surface, significantly reduces the threshold gain.
 Physics , 2013, DOI: 10.1103/PhysRevA.88.033810 Abstract: We study the surface modes of a homogeneous spherical gain medium and provide a comprehensive analytic treatment of a special class of these modes that support spectral singularities. Because the latter have a divergent quality factor, we call them the singular gallery modes. We show that they can be excited using arbitrarily small amounts of gain, and as a result, the system lacks a lasing threshold, effectively. This shows that we can realize spectral singularities in the surface modes of extremely small spherical samples with modest amounts of gain. We also examine the possibility of exciting singular gallery modes with different wavelengths using the same amount of gain. This corresponds to the situation where the system undergoes simultaneous lasing at different wavelengths.
 Physics , 2012, DOI: 10.1103/PhysRevLett.108.187402 Abstract: We establish a new approach for pump-probe simulations of metallic metamaterials coupled to the gain materials. It is of vital importance to understand the mechanism of the coupling of metamaterials with the gain medium. Using a four-level gain system, we have studied light amplification of arrays of metallic split-ring resonators (SRRs) with a gain layer underneath. We find that that the differential transmittance \Delta T/T can be negative for SRRs on the top of the gain substrate, which is not expected, and \Delta T/T is positive for the gain substrate alone. These simulations agree with pump-probe experiments and can help to design new experiments to compensate the losses of metamaterials.
 J. Heinrichs Physics , 2005, DOI: 10.1088/0953-8984/18/20/002 Abstract: We study reflection and transmission of waves in a random tight-binding system with absorption or gain for weak disorder, using a scattering matrix formalism. Our aim is to discuss analytically the effects of absorption or gain on the statistics of wave transport. Treating the effects of absorption or gain exactly in the limit of no disorder, allows us to identify short- and long lengths regimes relative to absorption- or gain lengths, where the effects of absorption/gain on statistical properties are essentially different. In the long-lengths regime we find that a weak absorption or a weak gain induce identical statistical corrections in the inverse localization length, but lead to different corrections in the mean reflection coefficient. In contrast, a strong absorption or a strong gain strongly suppress the effect of disorder in identical ways (to leading order), both in the localization length and in the mean reflection coefficient.
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