AllnN Films Display Negative Imaginary Conductivity at Terahertz Frequencies
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a 980 nm laser diode show two main emission bands centered at 479 nm and 653 nm which correspond to the strong 1 G 3 H transition and weak 1 G 3 F 4 6 4 4 transition, respectively. The γ-AlON: 1.2 mol% Yb, 0.5 mol% Tm samples reach the maximal upconversion intensity. Concentration quenching was observed
for samples with larger concentrations of Tm 3+ . The researchers said that the dependence of upconversion intensity on pumping power intensity demonstrates a two-photon process. Based on the energy matching conditions, the researchers concluded that two 980 nm photons are absorbed by Yb3+ to generate 2F7/22F5/2
transitions and then induce the excitation of Tm3+ from the ground 3H6 level to the 1G level by an energy transfer upconver4 sion process. The researchers reported that the 1G43H6 and 1G43F4 transitions generate upconversion luminescence at around 479 nm and 653 nm, respectively. ZHAOYONG SUN
AlInN Films Display Negative Imaginary Conductivity at Terahertz Frequencies
with the 1300–1550 nm laser sources used in fiber-based communications systems. They deposited 250 nm films of indiumrich alloys directly on C-plane sapphire using metalorganic chemical vapor deposition with no buffer layers that would complicate the analysis of the experiment. As reported in the August 15 issue of Optics Letters (DOI: 10.1364/OL.34.002507; p. 2507), the researchers tuned the bandgap of the indium-rich nitride films for use with 1.3–1.5 μm lasers, and reported on results of a film with an aluminum content of 20%. To probe the film’s optical properties, the research group transmitted electromagnetic waves with frequencies from 200 GHz to 2 THz through the sapphire and the film. After some data processing to remove noise and taking a fast Fourier transform to find the frequency dependence of the response, they used the measured complex dielectric constant to find the complex conductivity.
The imaginary part of the conductivity of the film is slightly below zero at the lowest frequencies measured, and has a linear response with a negative slope up to 2 THz. The size of this effect is significant, with the imaginary part at 2 THz making up approximately a third of the real part. The researchers discuss the origins of the negative imaginary part of the conductivity in terms of the confinement of electrons due to the nanoscale dimensions of the samples, enhanced backscattering, and the compositional inhomogeneity of the AlInN alloy that enhances carrier localization in the film. Potential fluctuations produce the same confinement of the electrons that nanostructure edges do, causing similar backscattering that produces transient current reversal and a negative imaginary conductivity. JIM RANTSCHLER
causes transport of material, especially at high current density. In on-chip interconnects, this eventually leads to a break in the wire, which results in chip failure. “In addition to the high current carrying capacity, graphene nanoribbons also have excellent thermal conductivity,” Murali said. Because heat generation is a s
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