Investigation of a balanced microwave photonic link utilizing a single few-mode fiber.

A balanced microwave photonic link, which utilizes mode-division multiplexing in order to combine both of its optical paths into and out of a single six-mode few-mode fiber, is demonstrated for the first time, to the best of our knowledge. Due to non-negligible modal crosstalk, the link exhibited excess radio-frequency gain fluctuations when compared to an equivalent conventional balanced link. The LP02+LP01 mode pair was the most stable with 2σ (two standard deviations) fluctuations of 0.4 dB about the average link gain, compared to the conventional link's 2σ of 0.16 dB. The excess crosstalk-induced fluctuations may render the link impractical to use outside of the laboratory.

Monte-Carlo simulations of multimode speckle noise in photonic summation devices.

Monte-Carlo simulations of optical speckle noise are performed to predict the range of gain fluctuations for photonic devices which multiplex many single-moded inputs into single multimode waveguides. Here, two waveguides are simulated which bound the cases of interest: a few mode fiber and a standard multimode fiber. When fully-excited and after spatial-filtering by a 10µm photodiode, the former's gain variations can range up to the mean value of the gain itself, ΔG ≈ 〈G〉, whereas for the latter, ΔG ≈ 3.4〈G〉. In certain cases, ΔG can be reduced by offsetting the photodiode relative to the waveguide, results which cannot be predicted using standard analytical speckle noise theories.

Terahertz disorder-localized rotational modes and lattice vibrational modes in the orientationally-disordered and ordered phases of camphor.

The temperature-dependent terahertz spectra of the partially-disordered and ordered phases of camphor (C10H16O) are measured using terahertz time-domain spectroscopy. In its partially-disordered phases, a low-intensity, extremely broad resonance is found and is characterized using both a phenomenological approach and an approach based on ab initio solid-state DFT simulations. These two descriptions are consistent and stem from the same molecular origin for the broad resonance: the disorder-localized rotational correlations of the camphor molecules. In its completely ordered phase(s), multiple lattice phonon modes are measured and are found to be consistent with those predicted using solid-state DFT simulations.

Terahertz vibrational modes of the rigid crystal phase of succinonitrile.

Succinonitrile (N ≡ C-CH2-CH2-C ≡ N), an orientationally disordered molecular plastic crystal at room temperature, exhibits rich phase behavior including a solid-solid phase transition at 238 K. In cooling through this phase transition, the high-temperature rotational disorder of the plastic crystal phase is frozen out, forming a rigid crystal that is both spatially and orientationally ordered. Using temperature-dependent terahertz time-domain spectroscopy, we characterize the vibrational modes of this low-temperature crystalline phase for frequencies from 0.3 to 2.7 THz and temperatures ranging from 20 to 220 K. Vibrational modes are observed at 1.122 and 2.33 THz at 90 K. These modes are assigned by solid-state density functional theory simulations, corresponding respectively to the translation and rotation of the molecules along and about their crystallographic c-axis. In addition, we observe a suppression of the phonon modes as the concentration of dopants, in this case a lithium salt (LiTFSI), increases, indicating the importance of doping-induced disorder in these ionic conductors.

High-contrast terahertz wave modulation by gated graphene enhanced by extraordinary transmission through ring apertures.

Gate-controllable transmission of terahertz (THz) radiation makes graphene a promising material for making high-speed THz wave modulators. However, to date, graphene-based THz modulators have exhibited only small on/off ratios due to small THz absorption in single-layer graphene. Here we demonstrate a ∼50% amplitude modulation of THz waves with gated single-layer graphene by the use of extraordinary transmission through metallic ring apertures placed right above the graphene layer. The extraordinary transmission induced ∼7 times near-filed enhancement of THz absorption in graphene. These results promise complementary metal-oxide-semiconductor compatible THz modulators with tailored operation frequencies, large on/off ratios, and high speeds, ideal for applications in THz communications, imaging, and sensing.