RESUMEN
In this paper, a mid-infrared perfect absorber based on the dual gratings-coupled graphene-dielectric multilayer structures (DGC-GDM) is proposed, in which GDM is sandwiched between two Au gratings. The DGC-GDM absorber shows advantages of dual-band and tunable absorption, insensitive to polarization, ultrathin thickness and wide angle range absorption. Two kinds of SPPs in the GDM layer can be excited by the upper and lower Au gratings, respectively, which confine the incident light into the GDM and thus contribute to the dual-band absorption. The wavelength of the absorption peak can be effectively changed by varying the Fermi level of graphene. Most importantly, an analytic formulas describing the relationships between the parameters of the absorber and the absorption spectra is derived. And the accuracy of the theoretical formulas is verified by comparing the simulation results with the theoretically calculated ones. Therefore, the exact values of parameters of the structure for an absorption peak as required can be obtained. The proposed structure can be applied to absorbers that are working at other frequencies.
RESUMEN
Two (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices with different superlattice period but the same total thickness were deposited on LaAlO3 substrates by pulsed laser deposition. Dielectric properties of these samples were investigated by means of terahertz time-domain spectroscopy (THz-TDS) under external continuous wave green laser excitation and optical-pump terahertz-probe spectroscopy (OPTP) at room temperature. Experimental results show that the real part of the permittivity for both superlattices increases significantly with increasing green laser pump power, which indicates the decrease of the plasma frequency, along with the increase of the electron scattering rate, soft mode eigenfrequency and oscillator strength in the Drude-Lorentz model. Furthermore, it's observed that the insulating superlattice exhibits a more significant dielectric tunability than the metallic superlattice. Besides, the carrier lifetime of superlattices is much shorter than the La0.7Sr0.3MnO3 thin film in the OPTP measurements, indicating that the electrons excited in the La0.7Sr0.3MnO3 layers may be trapped by the defects located in the interfaces of La0.7Sr0.3MnO3 and SrTiO3 or the SrTiO3 layers. With the optical field-induced tunability of dielectric properties, (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices show great potential in the actively tunable devices in the THz range.
RESUMEN
Graphene possesses a unique Landau level system that is non-equidistantly spaced in energy, as thus a large amount of optical transitions may become possible. Here, by utilizing this unique feature, we propose a novel dual field method which combines both external magnetic field and gate electric field together to control the optical response of the graphene-based devices. The key principle of this method is to selectively allow different optical transitions in graphene among Landau levels via an electric gate tuning of the Fermi level. By applying this method to a graphene based amplitude modulator and through an implementation based on transfer matrix method, we numerically demonstrated the well characteristics of switchable modulation on four individual channels, a huge modulation depth up to 80 dB and an extremely low required energy of tuning Fermi level down to 10 meV. Such excellent frequency tunability and gate controlling ability of this dual field method may open up the potential for applications in active optoelectronics, spin optics, ultrafast optics and etc.
RESUMEN
A structure composed of a nanosheet array may trap photons, which could be used to enhance the optical response via resonances; this may be highly useful in ultraviolet (UV) detection, photocatalysis, solar cells, etc. Moreover, anatase nanosheets exposed with the active {001}, {111}, and {116} facets are promising in applications such as Li-ion battery, photocatalysis, and electrochemistry. Therefore, in this study, the {116} faceted single-crystalline anatase nanosheet array with the sheet spacing in the range of several hundred nanometers was directly grown on the transparent conductive substrates. A photo-detector was fabricated by transferring a graphene electrode to the top of the anatase nanosheet array. The resultant device is inactive to visible-light irradiation and instantly responds to the UV light, which is due to the unique UV absorption property of the anatase nanosheet array and the advantage of the Schottky junction between the interfaces of graphene and anatase. The energy barrier between the two materials is 0.122 eV. We have provided a thorough research on graphene/monocrystal anatase-NSs.