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Published in Vladimir Mitin, Taiichi Otsuji, Victor Ryzhii, Graphene-Based Terahertz Electronics and Plasmonics, 2020
D. Yadav, G. Tamamushi, T. Watanabe, J. Mitsushio, Y. Tobah, K. Sugawara, A. A. Dubinov, A. Satou, M. Ryzhii, V. Ryzhii, T. Otsuji
Optical and/or injection pumping of graphene can enable negative-dynamic conductivity in the THz spectral range, which may lead to new types of THz lasers [36–39]. The THz gain in optically pumped graphene has been experimentally confirmed [40, 41]. However, optical pumping suffers from carrier heating, preventing from obtaining carrier population inversion and eventual gain [38, 39]. On the other hand, in the graphene structures with p-i-n junctions, the injected electrons and holes have relatively low energies compared with those in optical pumping, so that the effect of carrier cooling can be rather pronounced, providing a significant advantage of the injection pumping in realization of graphene THz lasers [28, 42–44]. Recent extensive studies on Auger processes reveal the difficulty of carrier population inversion to be obtained [45–49], which could be dominated in carrier heating regime with breaking the linear dispersive conical band profiles and/or band broadening due to many body effects of the inter-carrier Coulomb scattering [50, 51]. In this regard, current-injection pumping is the best way to substantially suppress the carrier heating and the Auger processes towards the lasing operation.
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Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
optical proximity correction (OPC) a method of selectively changing the shapes of patterns on the mask in order to more exactly obtain the desired printed patterns on the wafer. optical proximity effect proximity effect that occurs during optical lithography. optical pumping excitation of an atom or molecule resulting from absorption of optical frequency electromagnetic radiation; the electromagnetically assisted accumulation of population into or out of one or more states of a quantum mechanical system. In practice, this generally involves selective absorption of the electromagnetic field to populate an excited state, followed by a less selective decay into more than one ground state. For example, a system having ground state spin sublevels can be optically pumped by circularly polarized light into a single ground state spin sublevel. In a multilevel system, more selective transfer of population from one state to another can be achieved by adiabatic passage. optical rectification the second-order nonlinear optical process in which a material develops a static electric field in response to and proportional to the square of the strength of an applied optical field. optical repeater optoelectric device that receives a signal and amplifies it and retransmits it. In digital systems, the signal is regenerated. optical representation of binary numbers the representation of binary numbers 0 and 1 using light. Since an optical detector is sensitive to light intensity, it is a very logical choice to represent
Basic Physics and Recent Developments of Organic Random Lasers
Published in Marco Anni, Sandro Lattante, Organic Lasers, 2018
Ilenia Viola, Luca Leuzzi, Claudio Conti, Neda Ghofraniha
The gain material has to be excited to reach a population inversion, achieved through optical pumping. Nearly all random lasers are excited by a high-power pulsed pump laser. Although it is relatively easy to synthesize a random-laser material that supports extended modes, it is extremely difficult to realize one in which the modes are localized. This requires very strong scattering and, therefore, scattering elements with a size comparable to the wavelength and very high refractive index. At the same time, absorption has to be avoided because it would be counterproductive for lasing. One possible approach is to try to induce a population inversion in disordered gallium arsenide or other semiconductor structures, which has a refractive index as high as 3.5 [5,45]. For visible light, signatures of localized modes with long lifetime have been found in titanium dioxide powders, which have a refractive index of about 2.7 [46–48].
Deformed lying helix transition and lasing effect in cholesteric LC layers at spatially periodic boundary conditions
Published in Liquid Crystals, 2020
S. P. Palto, N. M. Shtykov, I. V. Kasyanova, B. A. Umanskii, A. R. Geivandov, D. O. Rybakov, I. V. Simdyankin, V. V. Artemov, M. V. Gorkunov
The spatial modulation of the refractive index in oriented CLC layers, due to the helicoidal structure of the director field, allows us to consider their optical properties in terms of one-dimensional photonic crystals characterised by a spectral stop-band, within which the light propagation along the axis of the cholesteric spiral is forbidden for one of the circular polarisations [1,5]. It is well known that at the edges of the stop-band there is an increase in the density of states, which leads to a series of photonic effects associated with both the features of luminescence and the possibility of low-threshold lasing at the edges [5,7]. It was the desire to improve the lasing properties of CLC systems that stimulated this work. The fact is that both in the first liquid-crystal microlasers [7], and recently [2,3,8–13], CLC layers with the helix axis perpendicular to the layer normal are usually used. For such geometry of the helical axis orientation, the optical pumping is longitudinal (performed along a direction close to the normal to the CLC layer). According to [17] the threshold lasing gain is inversely proportional to the cube of the length of the distributed feedback. This cubic law for the lasing gain was also confirmed numerically for CLCs helicoidal structures [14]. Thus, increasing the length of the CLC layer is favourable for decreasing the lasing threshold. However, with the longitudinal optical pumping, an increase in the layer thickness inevitably leads to a strong drop in the pump efficiency due to the non-uniform absorption of the pump energy over the layer thickness. The situation at the root can be changed if we use the geometry, when the axis of the helix lies in the plane of the CLC layer. This helix geometry is known as Uniform Lying Helix (ULH). In [16] it was shown that the lasing threshold can indeed be significantly decreased for CLCs in the ULH geometry.